Atmos. Chem. Phys., 9, 6611–6632, 2009
www.atmos-chem-phys.net/9/6611/2009/
© Author(s) 2009. This work is distributed under
the Creative Commons Attribution 3.0 License.
Atmospheric
Chemistry
and Physics
The impact of MM5 and WRF meteorology over complex terrain on
CHIMERE model calculations
A. de Meij1,*, A. Gzella1, C. Cuvelier1, P. Thunis1, B. Bessagnet2, J. F. Vinuesa1, L. Menut3, and H. M. Kelder4
1European Commission – DG Joint Research Centre, Institute for Environment and Sustainability, 21020 Ispra, Italy
2INERIS, Institut National de l’Environnement industriel et des Risques, Parc Technologique ALATA, 60550
Verneuil-en-Halatte, France
3Laboratoire de Me´te´orologie Dynamique, Institut Pierre-Simon Laplace, Ecole Polytechnique, Palaiseau, France
4Department of Applied Physics, Eindhoven University of Technology, Eindhoven, The Netherlands
*now at: Energy, Environment and Water Research Centre, The Cyprus Institute, 20 Kavafi Street, 1645, Nicosia, Cyprus
Received: 17 November 2008 – Published in Atmos. Chem. Phys. Discuss.: 26 January 2009
Revised: 22 June 2009 – Accepted: 23 August 2009 – Published: 11 September 2009
Abstract. The objective of this study is to evaluate the
impact of meteorological input data on calculated gas and
aerosol concentrations. We use two different meteorological
models (MM5 and WRF) together with the chemistry trans-
port model CHIMERE. We focus on the Po valley area (Italy)
for January and June 2005.
Firstly we evaluate the meteorological parameters with ob-
servations. The analysis shows that the performance of both
models in calculating surface parameters is similar, however
differences are still observed.
Secondly, we analyze the impact of using MM5 and
WRF on calculated PM10 and O3 concentrations. In gen-
eral CHIMERE/MM5 and CHIMERE/WRF underestimate
the PM10 concentrations for January. The difference in PM10
concentrations for January between CHIMERE/MM5 and
CHIMERE/WRF is around a factor 1.6 (PM10 higher for
CHIMERE/MM5). This difference and the larger underes-
timation in PM10 concentrations by CHIMERE/WRF are re-
lated to the differences in heat fluxes and the resulting PBL
heights calculated by WRF. In general the PBL height by
WRF meteorology is a factor 2.8 higher at noon in January
than calculated by MM5. This study showed that the differ-
ence in microphysics scheme has an impact on the profile of
cloud liquid water (CLW) calculated by the meteorological
driver and therefore on the production of SO4 aerosol.
A sensitivity analysis shows that changing the Noah Land
Surface Model (LSM) in our WRF pre-processing for the
5-layer soil temperature model, calculated monthly mean
Correspondence to: A. de Meij
(a.demeij@cyi.ac.cy)
PM10 concentrations increase by 30%, due to the change in
the heat fluxes and the resulting PBL heights.
For June, PM10 calculated concentrations by
CHIMERE/MM5 and CHIMERE/WRF are similar and
agree with the observations. Calculated O3 values for
June are in general overestimated by a factor 1.3 by
CHIMERE/MM5 and CHIMERE/WRF. High temporal
correlations are found between modeled and observed O3
concentrations.
1 Introduction
Aerosols play an important role in health effects (respira-
tory and cardiovascular disease, Moshammer and Neuberger,
2003), pollution, eutrophication/acidification of aquatic and
terrestrial ecosystems and radiative forcing (absorbing and
scattering of solar radiation, Kaufman et al., 2002). Ground-
based measurement networks provide information about the
atmospheric conditions at a particular time and location and
can not be used alone for policymaking to establish effec-
tive strategies for air emissions reduction policy. The atmo-
spheric chemistry-transport-dispersion models (CTMs) have
the advantage that they can be used to complement monitor-
ing data, assess the effects of future changes in gas, aerosol
and aerosol precursor emissions and to study the impact of
source pollutants on air quality elsewhere.
Each atmospheric chemistry transport model includes a
specific sequence of operations, with specific input data, such
as emissions and meteorology to calculate gas and aerosol
concentrations. Uncertainties in the estimation of gases and
primary aerosols in the emission inventories (De Meij et
Published by Copernicus Publications on behalf of the European Geosciences Union.

6612 A. de Meij et al.: Study aerosol with two meteorological models (MM5 and WRF)
al., 2006), aerosol dynamics (physical transformations, dry
and wet removal, transport), meteorological factors (tem-
perature, humidity, wind speed and direction, precipitation,
cloud chemistry, vertical mixing), the impact of orography
on meteorological parameters (Carvalho et al., 2006), the im-
pact of horizontal resolution of meteorology on model cal-
culations (Baertsch-Ritter et al., 2004; Menut et al., 2005)
and the fact that the formation of aerosols are known to be
nonlinearly dependent on meteorological parameters such as
temperature, humidity and vertical mixing (Haywood and
Ramaswamy, 1998; Penner et al., 1998; Easter and Peters,
1994) and the concentrations of precursor gases (West et al.,
1998), all contribute to uncertainties in the calculated gas and
aerosol concentrations. A good estimate of meteorological
variables in the meteorological datasets is therefore crucial
for calculating gas and aerosol impacts on air quality and cli-
mate change, and evaluating coherent reduction strategies.
The main objective of this study is to evaluate the impact
of meteorological input data on calculated aerosol concentra-
tions. We study the central Po valley (northern Italy), which
has been identified as one of the two main areas (together
with Benelux) where pollutant levels will remain problem-
atic by 2020, according to the different scenarios carried
out in the frame of the Clean Air for Europe (CAFE) pro-
gramme by the International Institute for Applied System
Analysis (IIASA). We focused our analysis on the year 2005
and particularly on a winter month (January 2005) and a
summer month (June 2005), to highlight the impact of dif-
ferent meteorological conditions prevailing in winter and
summer on the calculated gas and aerosol concentrations.
To this end we performed simulations with the CHIMERE
model (http://www.lmd.polytechnique.fr/CHIMERE/), using
two different meteorological models, the Mesoscale Meteo-
rological model (MM5, Grell et al., 1994) and the Weather
Research and Forecasting model (WRF, (http://wrf-model.
org/index.php). So far, work has been done in comparing
MM5 and WRF simulated meteorological parameters with
observations (Zhong et al., 2007; Michelson and Bao, 2006),
and the impact of MM5 and WRF on ozone calculated val-
ues (Soong et al., 2006). To our knowledge, no studies have
been performed in evaluating the impact of MM5 and WRF
on calculated aerosol species.
Section 2 deals with the description of the simulations,
the air chemistry transport model, the meteorological mod-
els and the emission inventory. In Sect. 3 a description of
the measurement data is given. In Sect. 4 the results are pre-
sented. We discuss the results in Sect. 5 and we finish with
conclusions in Sect. 6.
2 Methodology
The CHIMERE model (Bessagnet et al., 2004) is used to
simulate air quality over the Po valley area for January and
June 2005 based on the meteorological data sets provided
by MM5 and WRF. More details regarding the atmospheric
chemistry and meteorological models are given in Sect. 2.1
and 2.2, respectively.
We start our study by evaluating the meteorological pa-
rameters temperature, relative humidity, wind direction and
wind speed, calculated by both weather prediction models.
The modelling results were compared with meteorological
observations for the year 2005, given by the monitoring net-
work of the Regional Agencies for Environment Protection
in Lombardy (Agenzia Regionale per la Protezione dell’ Am-
biente, ARPA Lombardia, http://www.arpalombardia.it, last
accessed 12 March 2009).
Then we evaluate the calculated aerosol (PM10) and ozone
(O3) concentrations, using the CHIMERE model with MM5
and WRF results as input data, by comparing the model cal-
culated concentrations with measurements from the EMEP
station and measurements from the ARPA networks (Lom-
bardy and Veneto). We focus on PM10 and O3 because these
pollutants have more adverse health effects than other pol-
lutants and are therefore commonly measured at most of the
air quality monitoring stations. More details regarding the
measurement networks are given in Sect. 3.
Four simulations are performed with CHIMERE, two sim-
ulations with MM5 meteorology (CHIMERE/MM5) for Jan-
uary 2005 and June 2005, and two simulations with WRF
meteorology (CHIMERE/WRF) for January and June 2005.
The meteorology has been created for the whole year
2005, with no nudging to the observations of the meteoro-
logical stations.
For the four simulations, a spin-up time of 4 days is ap-
plied in order to initialize the model.
2.1 Description CHIMERE model
CHIMERE is an off-line chemistry transport model, driven
by a meteorological driver, such as MM5 (Grell et al., 1994)
or WRF (http://wrf-model.org/index.php, last accessed 12
March 2009).
The complete chemical mechanism in CHIMERE is called
MELCHIOR1 (Lattuati, 1997, adapted from the original
EMEP mechanism, Hov et al., 1985), which describes more
than 300 reactions of 80 species. The reduced mechanism
MELCHIOR2 includes 44 species and about 120 reactions,
derived from MELCHIOR1 (Derognat et al., 2003).
Processes like chemistry, transport, vertical diffusion, pho-
tochemistry, dry deposition, in-cloud and below cloud scav-
enging and SO2 oxidation in clouds are included in the
model. The thermodynamic equilibrium model ISORROPIA
(Nenes et al., 1998) is used to calculate the equilibrium
partitioning of the gas-liquid-solid aerosol phase of various
aerosols compounds (e.g. SO=4 , NO−3 , NH+4 , Na+, Cl−). An
overview of the processes and references is given in Table 1.
More details regarding the parameterizations of the above
mentioned processes are described in Bessagnet et al. (2004)
and references therein.
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A. de Meij et al.: Study aerosol with two meteorological models (MM5 and WRF) 6613
Table 1. Overview of the chemical and physical processes which are included in the air chemistry transport model CHIMERE. For a more
detailed description of the processes in CHIMERE, see Bessagnet et al. (2004).
Process type Reference
Chemistry MELCHIOR2, based on Lattuati (1997)
Dry deposition Seinfeld and Pandis (1998)
Photolysis rate constants Tropospheric Ultraviolet Visible module (TUV), Madronich and
Flocke (1998)
Wet deposition
In cloud and below cloud scavenging of gases and
aerosols:
Guelle et al. (1998) and Tsyro (2002)
Aerosols ISORROPIA, Nenes et al. (1998)
Coagulation Fuchs (1964)
Nucleation Kulmala et al. (1998)
Condensation/evaporation Yes
Cloud effects on photolysis rates Yes, see Bessagnet et al. (2004)
Transport Parabolic Piecewise Method (PPM), Colella and Woodward (1984)
Vertical diffusion Troen and Mahrt (1986)
Turbulent transport Stull (1988)
Cloud chemistry of SO2 oxidation by H2O2 and
O3
Yes
Anthropogenic and Biogenic aerosol formation Yes, Anthropogenic yields come from Grosjean and Seinfeld (1989),
Moucheron and Milford (1996), Odum et al. (1996, 1997) and Schell
et al. (2001).
Biogenic aerosol yields for terpene oxidation according to Pankow et
al. (1994, 2001)
Vertical structure 8 hybrid sigma pressure levels up to ±5500 m
The lateral boundary conditions of gas species are monthly
average values and are taken from the INCA model (http://
www-lsceinca.cea.fr/welcome real time.html, last accessed
12 March 2009). The boundaries conditions of aerosols are
taken from the monthly mean aerosol concentrations pro-
vided by the larger scale model GOCART (Ginoux et al.,
2001, 2004).
CHIMERE consists of 8 hybrid sigma pressure levels, up
to 500 hPa (±5500 m).
The domain (approximately 300×300 km, centred at
45.0◦ N, 10.0◦ E) covers most of the Po Valley, Italy, includ-
ing southern part of the Alps, see Fig. 1.
2.2 Description meteorological input
The PSU/NCAR mesoscale model MM5 (3.7.4) is a limited-
area, non-hydrostatic or hydrostatic, terrain following sigma-
coordinate model designed to simulate or predict mesoscale
and regional scale atmospheric circulations (Grell et al.,
1994).
The Advanced Research WRF system (WRF-ARW V2.2)
can be used as an alternative meteorological driver for MM5
in the air quality modelling. It is considered by NCAR as the
successor of MM5, since further development of MM5 will
come to an end in favour of WRF (see NCAR websites).
Fig. 1. Map of the location of the model domain in North Italy
(centred at 45.0◦ N, 10.0◦ E), which covers most of the Po valley,
including southern part of the Alps.
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6614 A. de Meij et al.: Study aerosol with two meteorological models (MM5 and WRF)
The meteorological data sets used for the study were cre-
ated within the Po valley air quality Model Inter-comparison
(POMI) exercise, which is coordinated by the Institute of En-
vironment and Sustainability, JRC, Ispra, Italy (http://aqm.
jrc.it/POMI/, last accessed 12 March 2009). The POMI ex-
ercise is focused on the area of the Northern Italy and two
nested domains are set up there for meteorological data.
WRF operates on the 5 km and 2.5 km resolution domains
(one-way nested) and MM5 – on the 6 km and 2 km resolu-
tion domains (two-way nested).
Both MM5 and WRF use meteorological initial conditions
and lateral boundary conditions from 6 h analyses from the
NCEP Global Final (FNL) Analyses. Data produced during
pre-processing and modelling simulations of MM5 and WRF
are in the Lambert conformal projection. Both models have
been set up to compute Sea Surface Temperature (SST) vary-
ing in time with 1-h output time resolution. The time step of
output data has been set to 1 h as well in both cases.
However, it should be noticed that the choice of the pa-
rameterization in MM5 and WRF is not always the same.
The choice of the model setup in MM5 and WRF is based on
previous studies (i.e. for WRF: Kesarkar et al., 2007; Guer-
rero et al., 2008) and recommendations by NCAR. The main
differences between the MM5 and WRF parametrization are
related to PBL schemes and microphysics. The settings of
the meteorological models are given in Table 2.
2.3 Emission data
In this study we use the City Delta III project (http://aqm.
jrc.it/citydelta, last accessed 12 March 2009) emission in-
ventory, which has been used in recent studies Vautard et
al. (2007) and Thunis et al. (2007). A detailed description of
the emission inventory can be found in Cuvelier et al. (2007).
3 Description measurement data sets
The meteorological parameters provided by MM5 and WRF
are compared with the observations from the EMEP mea-
surement station Ispra (Italy) and from monitoring stations of
the ARPA Lombardia network. The aerosol concentrations
calculated by CHIMERE are compared with the aerosol mea-
surements from the same or closely located air quality mon-
itoring sites of the EMEP (Ispra, Italy) and ARPA networks
(Lombardy, Veneto). The names of the stations for which we
have meteorological data and PM10 data available are: Ispra
(45.48◦ lat, 8.63◦ lon), Cantu (45.74◦ lat, 9.13◦ lon), Erba
(45.79◦ lat, 9.20◦ lon), Mantova (45.16◦ lat, 10.80◦ lon) and
Castelnovo Bariano (45.03◦ lat, 11.29◦ lon), Sermide (45.01◦
lat, 11.29◦ lon).
To have a broader view on measured ozone concentra-
tions for comparison purposes, additional air quality moni-
toring sites (not collocated with meteorological stations) are
taken into account from ARPA network (Lombardy). The
names of the stations are Osio Sotto (45.63◦ lat, 9.60◦ lon),
Gambara (45.25◦ lat, 10.29◦ lon), Corte de Cortesi (45.27◦
lat, 10.00◦ lon), Marmirolo Fontana (45.12◦ lat, 10.44◦ lon),
Lecco (46.00◦ lat, 9.28◦ lon), Varese (45.63◦ lat, 8.88◦ lon),
Chiavenna (46.32◦ lat, 9.40◦ lon) and Milano (45.49◦ lat,
9.24◦ lon). All air quality monitoring sites are characterized
as background stations (including urban and suburban back-
ground), which is essential for comparison with the regional
scale modelling results. More details regarding the different
networks are given below.
3.1 EMEP measurement site Ispra
The EMEP measurement station at Ispra, Italy (8.6◦ E,
45.8◦ N) belongs to the Co-operative Programme for Mon-
itoring and Evaluation of the Long-range Transmission of
Air Pollutants in Europe (EMEP), which evaluates air qual-
ity in Europe by operating a measurement network, as well
as performing model assessments (http://www.emep.int/, last
accessed 12 March 2009). This EMEP station, situated at
the eastern side of the Lago Maggiore at the foothills of the
Alps, is located on the premises of the Joint Research Cen-
tre, Ispra (Italy). Concentrations of carbon monoxide (CO),
ozone (O3) and secondary aerosol precursors (SO2, NOx) are
continuously monitored (http://ccu.jrc.it/, last accessed 12
March 2009). Daily aerosol samples are collected on quartz
fibre filters to determine PM10 and PM2.5 concentrations and
chemical compositions (SO=4 , NH+4 , NO−3 , black carbon).
Rain water samples are also collected to assess the aerosol
wet deposition. In addition, PM10 concentration, aerosol size
distribution in the range 8 nm–10µm, and aerosol absorption
coefficient are continuously monitored.
One of the artefacts occurring with the main filter type
(quartz) used by the Ispra EMEP station, is the evapora-
tion of ammonium nitrate at higher temperatures. Temper-
atures exceeding 20◦C cause complete NH4NO3 evaporation
from the quartz filter, a loss of 100%; and a loss of about
25% for NH+4 , depending on the (NH4)2SO4/NH4NO3 ratio
measured on the filter. Temperatures between 20 and 25◦C
could lead to a loss of 50% of the nitrate aerosol (Schaap
et al., 2003, 2004a). Therefore almost all reported summer
NH4NO3 and NH+4 concentrations present only a lower limit,
rather than a realistic concentration.
3.2 ARPA
Monitoring data of the ARPA networks (Agenzia Regionale
per la Protezione dell’ Ambiente) in Lombardy (http://
ita.arpalombardia.it/ita/index.asp, last accessed 12 March
2009) and Veneto (http://www.arpa.veneto.it, last accessed
12 March 2009) are used for comparison of meteorological
variables (temperature, relative humidity, precipitation, wind
speed and wind direction) with MM5 and WRF calculated
meteorological parameters, as well as PM10 and O3 mea-
sured values with calculated model concentrations.
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A. de Meij et al.: Study aerosol with two meteorological models (MM5 and WRF) 6615
Table 2. Overview of the WRF and MM5 parameterisations, which are used to create the meteorological input for CHIMERE.
Parameter WRF MM5
Integration time step [s] 30 18
Radiation calculation frequency [min] 5 30
Snow cover effects Yes (Noah) Yes (Noah)
Cloud effect on radiation Yes Yes
Microphysics WSM6 (mix phase) (Hong and Lim, 2006) 4 (simple ice) (Dudhia, 1989)
Cumulus scheme None None
PBL YSU (MRF successor) (Hong et al., 2006) MRF (Hong and Pan, 1996)
Radiation RRTM (Mlawer et al., 1997) RRTM (Mlawer et al., 1997)
LSM Noah (Chen and Dudhia, 2001) Noah (Chen and Dudhia, 2001)
Surface Layer Monin-Obukhov Monin-Obukhov
Air quality data from four monitoring stations of ARPA
networks (three from Lombardy and one from Veneto) col-
located with meteorological monitoring stations are used in
this work: Erba, Cantu, Mantova and Castelnovo Bariano.
On the monitoring site of Erba concentrations of carbon
monoxide (CO), ozone (O3) and secondary aerosol precur-
sors (SO2, NOx) are continuously measured as well as PM10
levels (using TEOM with correction factors). In Cantu the
PM10 concentrations are measured using beta absorption
method and apart from this continuous data about CO, O3
and NOx are being collected. In Mantova (S. Agnese) only
NO2, NO, CO and PM10 (using TEOM with correction fac-
tors) are measured.
On the monitoring station of Castelnovo Bariano (ARPA
Veneto) concentrations of secondary aerosol precursors
(SO2, NOx) as well as PM10 are continuously measured, us-
ing respectively fluorescence, chemiluminescence and gravi-
metric methods. Hourly meteorological data (for validation
purposes) for this monitoring station are not available on the
website of ARPA Veneto. Therefore the supporting meteoro-
logical data were taken from the monitoring station Sermide
(ARPA Lombardia) which is located in the distance of about
2.5 km from Castelnovo Bariano.
All of the stations used for the comparison of modelled O3
concentrations with measurements are located in Lombardy.
They operate within ARPA network and measure ozone con-
centrations using the UV absorption method.
4 Results
Firstly we evaluated the two meteorological datasets by com-
paring the calculated meteorological parameters with obser-
vations. Secondly we performed an evaluation of the impact
of using two meteorological models in the CHIMERE model
on calculated PM10 and O3 concentrations.
4.1 Meteorology
The evaluation of the modelled meteorological datasets is
based on the observations from five monitoring stations lo-
cated in Lombardy, Italy: Ispra, Mantova, Cantu, Erba and
Sermide, using data given by ARPA Lombardia network.
The following meteorological parameters were evaluated:
temperature on the 2 meters level (data available for all sta-
tions), wind speed and direction (data available for two sta-
tions), as well as relative humidity and rain (data for four sta-
tions). The calculated statistics are: BIAS error, root mean
square error (RMSE), standard deviation (SD) and the co-
efficient of determination (R squared). For the wind direc-
tion data the mean absolute error (MAE) was calculated and
the wind roses were analyzed. For the precipitation data the
sums of observed and modelled amount of rain were calcu-
lated for each of the analyzed periods. Apart from this a ca-
pability of capturing the precipitation events by the models
was evaluated using following hit rate statistics: probabil-
ity of detection (POD), false alarm (FA), frequency BIAS
(FBI), Hansen-Kuipers score (HKS) and odds ratio (OR)
(Stephenson, 2000; Goeber and Milton, 2001). For de-
tailed description of the formulas used to calculate statistics
see Appendix. The analysis was performed for the annual
means (year 2005) with the focus on winter (January 2005)
and summer (June 2005) mean. In Sect. 4.1.5 we evaluate
the vertical profile of the potential temperature calculated by
WRF and MM5 by comparing the results with observations
from Linate airport.
4.1.1 Surface meteorology statistics
The analysis of the annual averaged statistics shows that the
temperatures are mainly underestimated, up to 3.6◦C (for
MM5 in Mantova), however WRF model gives higher tem-
peratures than MM5, Table 3a. The RMSE is within the
range of 2.3 to 4.3◦C for both models. The values of rela-
tive humidity are in general overestimated by WRF and un-
derestimated by MM5 but the differences between models
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6616 A. de Meij et al.: Study aerosol with two meteorological models (MM5 and WRF)
Table 3a. Statistics for the temperature at the 2 m height.
Parameter/model BIAS [◦C] RMSE [◦C] SD [◦C] R squared NR MEAN OBS
Time period/station WRF MM5 WRF MM5 OBS WRF MM5 WRF MM5 OBS [◦C]
YEAR
ISPRA 0.4 −2.4 3.1 3.9 8.8 8.0 8.4 0.9 0.9 7958 13.1
ERBA −1.2 −1.7 3.6 3.7 8.9 8.1 7.9 0.9 0.9 6929 10.7
CANTU 0.5 −0.4 3.1 2.9 9.6 8.6 8.6 0.9 0.9 8521 11.2
SERMIDE −1.2 −1.5 2.3 2.5 9.0 9.4 9.1 1.0 1.0 8724 13.6
MANTOVA −3.2 −3.6 3.9 4.3 9.8 9.5 9.1 1.0 1.0 8285 15.4
JANUARY
ISPRA 1.7 −1.3 4.3 4.1 4.9 3.5 3.3 0.4 0.4 742 1.9
ERBA 0.7 0.3 3.0 3.1 4.1 3.7 3.6 0.5 0.4 742 2.2
CANTU 1.8 1.0 4.4 4.2 5.2 3.6 3.2 0.4 0.4 742 0.6
SERMIDE −0.7 −1.2 2.0 2.3 2.6 2.5 2.5 0.5 0.5 742 2.2
MANTOVA −1.8 −2.1 2.6 2.9 2.7 2.7 2.7 0.6 0.5 610 3.1
JUNE
ISPRA −0.4 −2.2 3.3 3.8 5.8 4.0 4.8 0.7 0.7 720 21.6
ERBA −3.3 −3.7 4.0 4.2 5.6 4.5 4.4 0.9 0.9 720 22.8
CANTU −0.3 −0.8 2.4 2.2 5.9 4.7 4.9 0.9 0.9 648 21.2
SERMIDE −1.4 −1.5 2.0 2.4 5.0 5.6 5.9 0.9 0.9 696 23.3
MANTOVA −4.0 −4.5 4.4 4.9 5.8 5.5 5.5 0.9 0.9 720 26.1
Table 3b. Statistics for the relative humidity at the 2 m height.
Parameter/model BIAS [%] RMSE [%] SD [%] R squared NR MEAN OBS
Time period/station WRF MM5 WRF MM5 OBS WRF MM5 WRF MM5 OBS [%]
YEAR
ISPRA −5 −2 18 20 24 17 18 1 0 7957 73
ERBA 2 −2 15 17 21 17 19 1 0 7001 64
CANTU 3 −1 16 19 24 18 19 1 0 7758 68
MANTOVA 3 4 13 13 21 19 18 1 1 7215 67
JANUARY
ISPRA −10 −10 26 31 28 21 21 0 0 742 76
ERBA −6 −11 19 24 23 17 19 0 0 742 66
CANTU 1 −6 18 23 26 20 22 1 0 407 67
MANTOVA −3 −8 10 15 15 18 18 1 0 198 84
JUNE
ISPRA −6 −3 17 16 23 13 15 1 1 720 71
ERBA 8 6 14 14 17 15 17 1 0 720 57
CANTU 11 7 17 16 19 15 17 1 0 720 56
MANTOVA 7 9 12 14 17 16 17 1 1 720 53
and observations are comparable to the uncertainty of mea-
surements (3–5%), Table 3b. WRF output follows better the
hourly pattern of relative humidity.
The results for wind speed and wind direction can be eval-
uated only for 2 monitoring sites i.e. Ispra and Mantova,
Fig. 2a and Table 3c–d. Moreover, the wind data are largely
missing for Mantova for the winter period (January–March)
and for Ispra for the first half of the year. Therefore the reli-
able statistically analysis of the results is ensured mainly an-
nually and for the summer period (in Mantova). The Po val-
ley area is characterized by low wind speeds (stagnant condi-
tions), which makes the wind field difficult to simulate with
the prognostic meteorological models such as MM5 (Dosio
et al., 2002; Baertsch-Ritter et al., 2003; Minguzzi et al.
2005; Carvalho et al., 2006; Stern et al., 2008). This has been
confirmed also by the results described in this work. The
wind speed is overestimated up to 1.7 m/s (less by MM5).
The prevailing annual wind direction is well reproduced by
both models (especially for the Ispra location). The annual
amount of rain is overestimated by WRF and in general un-
derestimated by MM5, Fig. 2b. The analysis of the hit rate
for precipitation events over the whole year 2005 was per-
formed using 6 threshold values for the rain amount accu-
mulated over the day: 0.1 mm/day, 0.2 mm/day, 0.5 mm/day,
1 mm/day, 2 mm/day and 5 mm/day (see Table 3e). The hit
rate statistics are in general better for WRF.
For the winter period WRF gives higher temperatures than
MM5. The RMSE values for both models are also similar
as for the annual means. The relative humidity is underesti-
mated by both models, however more by MM5 (8–11%). For
both of these parameters WRF results show generally higher
R squared values than MM5 results.
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A. de Meij et al.: Study aerosol with two meteorological models (MM5 and WRF) 6617
Table 3c. Statistics for the wind speed.
Parameter/model BIAS [m/s] RMSE [m/s] SD [m/s] R squared NR MEAN OBS
Time period/station WRF MM5 WRF MM5 OBS WRF MM5 WRF MM5 OBS [m/s]
YEAR ISPRA 1.7 1.0 2.4 1.5 0.8 2.3 1.7 0.0 0.0 8757 2.5MANTOVA 2.2 2.0 2.8 2.6 0.3 1.8 1.7 0.2 0.1 6479 0.4
JANUARY ISPRA – – – – – 3.1 2.0 – – – –MANTOVA 2.2 2.7 2.7 3.3 0.2 1.7 1.8 0.0 0.0 127 0.4
JUNE ISPRA – – – – – 1.8 1.2 0.0 – – –MANTOVA 2.2 1.8 2.7 2.2 0.2 1.7 1.3 0.1 0.1 719 0.5
There was not enough data available to perform a ro-
bust comparison of the MM5 and WRF results on wind
speed and direction with observations. The data for Man-
tova (only available for the first week of January 2005) show
that the wind speed is largely overestimated by both models,
however, BIAS and RMSE values are lower for WRF than
for MM5. The MAE values calculated for wind direction
data are comparable for both models (see Table 3c and d).
The WRF model overestimates the rainfall and shows
in general more precipitation than MM5 for January 2005,
Fig. 2b. The very high value of rain amount given by WRF
for Sermide is caused mainly by the rainfall forecasted by
WRF, which is 2.39 cm on 1 January, at hour 02:00 LST
and then, about the same amount of rain between the 18
(17:00 LST) and 19 (09:00 LST) January. Observational data
show the first rainfall on 5 January (hour 09:00) which is
0.02 cm and reach the amount of only 1 cm by the end of
the month. WRF output calculates 5.82 cm of rain and MM5
about 2.6 cm of cumulated rainfall for January. The hit rate
statistics were not analyzed for January, because there is not
enough data in this period (to less and very small precipita-
tion events).
In the summer period both models underestimate the tem-
perature, up to 4.5◦C (for MM5 in Mantova) and have similar
R squared values, although WRF gives smaller error values.
The relative humidity is mainly overestimated. WRF results
show higher R squared values than MM5 but the BIAS val-
ues are generally lower for MM5 for this parameter. For
the summer period the comparison between modelled and
observed wind speed and wind direction was possible only
for the monitoring station in Mantova. The wind speeds are
overestimated by both models of about 2 m/s, although the
error values are lower for MM5. The wind direction is poorly
reproduced. The daily values of the hit rate statistics for
June 2005 did not give enough observed occurrences of the
events and the hit rate statistics are for that reason unsound.
Therefore the analysis was done using 4 thresholds of the
rain amount accumulated over 6 h: 0.1 mm/6 h, 0.2 mm/6 h,
0.5 mm/6 h, 1 mm/6 h (see Table 3f). WRF catches the pre-
cipitation events better than MM5. However, the amount of
the rain is overestimated by both models for June 2005.
Table 3d. Statistics for the wind direction.
Parameter/model MAE [◦] NR
Time period/station WRF MM5 OBS
YEAR
ISPRA 93.1 77.6 8757
MANTOVA 84.0 81.6 6479
JANUARY ISPRA – – –MANTOVA 76.7 77.5 127
JUNE ISPRA – – –MANTOVA 82.5 80.0 719
4.1.2 Sounding data statistics
In this section we evaluate the vertical profile of the potential
temperature gradient calculated by WRF and MM5 by com-
paring the results with observations from the Linate airport
location.
In Fig. 3a–e we compare the potential temperature gra-
dient (ptg) profile between 10 m and 200 m at the hours
00:00 h, 06:00 h, 12:00 h and 18:00 h for the whole year. Pos-
itive values in Fig. 3a indicate that the atmospheric layer be-
tween 10 m and 200 m is stable, negative values indicates that
the layer is unstable, values around 0 indicates neutral con-
ditions of the atmosphere (Stull, 1988). We see that the ptg
profile by MM5 and WRF is in good agreement with the ob-
servations. At 00:00 h the ptg profile by MM5 is in general
higher than by WRF. At 06:00 h the ptg profile by WRF and
MM5 are similar and correspond well with the observations.
At 12:00 h we see that from spring time (day 60) to autumn
(day 280) the ptg profiles are negative, indicating unstable
conditions in the first 200 m. These instable conditions are
well captured by both MM5 and WRF. During winter time
both models calculate stable conditions, which corresponds
to the observations. At 18.00 h we have limited observational
data available. However, the ptg profile by WRF agrees well
with the observations.
In Fig. 3b–e Taylor diagrams are shown (one per ana-
lyzed time) which integrate three statistical measures on one
plot (Taylor, 2001). The black star represents observations
and coloured stars – the models. Apart from the standard
deviation and correlation coefficient, the diagram shows also
the RMSEC (centered RMSE), which is measured on the
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6618 A. de Meij et al.: Study aerosol with two meteorological models (MM5 and WRF)
Table 3e. Hit rate statistics for the rain data, the whole of the year 2005.
WRF MM5
ISPRA ERBA CANTU SERMIDE ISPRA ERBA CANTU SERMIDE
>0.1 mm/day
POD 0.67 0.75 0.73 0.70 0.62 0.74 0.71 0.79
FA 0.07 0.12 0.08 0.07 0.07 0.10 0.11 0.12
FBI 0.91 1.18 1.04 1.01 0.90 1.11 1.14 1.30
HKS 0.60 0.63 0.64 0.63 0.55 0.64 0.60 0.67
OR 28.47 22.18 29.28 29.10 20.89 24.90 19.33 27.50
>0.2 mm/day
POD 0.60 0.79 0.70 0.71 0.52 0.67 0.69 0.73
FA 0.05 0.10 0.07 0.06 0.07 0.09 0.08 0.08
FBI 0.85 1.28 1.03 1.06 0.87 1.11 1.05 1.20
HKS 0.55 0.68 0.63 0.65 0.45 0.58 0.61 0.64
OR 29.41 32.49 29.74 37.20 15.24 20.27 24.82 30.06
>0.5 mm/day
POD 0.60 0.70 0.58 0.59 0.54 0.50 0.58 0.53
FA 0.04 0.07 0.06 0.06 0.03 0.05 0.06 0.06
FBI 0.94 1.15 1.00 1.19 0.83 0.83 1.00 1.16
HKS 0.56 0.63 0.52 0.53 0.51 0.45 0.52 0.47
OR 35.38 31.24 20.99 23.00 33.84 19.53 20.99 16.89
>1 mm/day
POD 0.71 0.67 0.54 0.43 0.58 0.48 0.46 0.43
FA 0.02 0.05 0.04 0.04 0.01 0.02 0.03 0.04
FBI 0.92 1.26 1.00 1.10 0.67 0.74 0.77 1.00
HKS 0.69 0.62 0.50 0.39 0.58 0.46 0.44 0.39
OR 146.20 39.00 28.68 16.88 212.80 42.58 32.04 19.81
>2 mm/day
POD 0.83 0.46 0.73 0.20 0.67 0.46 0.27 0.40
FA 0.00 0.02 0.02 0.02 0.01 0.02 0.02 0.01
FBI 0.92 1.08 1.27 0.90 0.83 0.92 0.82 0.70
HKS 0.83 0.44 0.71 0.18 0.66 0.44 0.25 0.39
OR 1585.00 35.79 140.44 11.93 316.00 48.00 19.75 75.11
>5 mm/day
POD 0.00 0.67 1.00 0.00 0.00 0.00 0.00 0.00
FA 0.01 0.01 0.00 0.01 0.00 0.00 0.00 0.00
FBI 3.00 1.33 1.50 4.00 0.00 0.33 0.50 1.00
HKS −0.01 0.66 1.00 −0.01 0.00 0.00 0.00 0.00
OR 0.00 350.00 – 0.00 – 0.00 0.00 0.00
plot as the distance between the observed and modelled val-
ues. For the hour 00:00 and 06:00 WRF performs better than
MM5 in the sense that its results give higher correlation val-
ues, standard deviation closer to the observed one and also
the RMSEC in this case is smaller than for MM5. At the
hour 12:00 both models perform with similar quality, how-
ever MM5 shows the standard deviation which is closer to
observed values. At the hour 18:00 MM5 also reproduces the
observed standard deviation values better than WRF. How-
ever, WRF gives higher correlation value and lower RMSEC
than MM5 for this time. In general we can say that the po-
tential temperature gradient by WRF is better than by MM5.
4.2 Aerosols and ozone
In this section the impact of using two different meteorolog-
ical models, MM5 and WRF in the CHIMERE model, on
calculated PM10 and O3 (ozone) concentrations is presented
for January and June 2005.
4.2.1 Calculated PM10 concentrations with MM5
and WRF meteorology for January 2005
Aerosols formation is non-linear dependent on meteorolog-
ical parameters, such as relative humidity, temperature, and
removal processes (e.g. precipitation), which determine how
aerosols are dispersed and transported over distance. There-
fore for the comparison of calculated PM10 concentrations
we selected those stations for which we have also meteo-
rological data available. The combination of having PM10
measurement data together with meteorological data, allows
us to understand better the PM10 profile.
For both simulations, using MM5 and WRF meteorology
(CHIMERE/MM5 and CHIMERE/WRF), the model under-
estimates on average the observed PM10 concentrations for
the five stations by a factor 2 and 3.2 for January respec-
tively, see Table 4. Analyzing the calculated PM10 con-
centrations for the stations, we find that CHIMERE/MM5
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A. de Meij et al.: Study aerosol with two meteorological models (MM5 and WRF) 6619
Table 3f. Hit rate statistics for the rain data, June 2005.
WRF MM5
ISPRA ERBA CANTU SERMIDE ISPRA ERBA CANTU SERMIDE
>0.1 mm/6 h
POD 0.40 0.56 0.63 0.33 0.70 0.56 0.63 0.33
FA 0.05 0.07 0.05 0.03 0.05 0.15 0.15 0.06
FBI 0.90 1.44 1.25 1.33 1.30 2.44 2.38 2.67
HKS 0.35 0.48 0.57 0.31 0.65 0.40 0.48 0.27
OR 14.00 16.09 30.33 17.50 40.44 6.91 9.76 7.21
>0.2 mm/6 h
POD 0.43 0.57 0.57 0.00 0.43 0.43 0.57 0.00
FA 0.04 0.06 0.03 0.03 0.05 0.10 0.08 0.03
FBI 1.00 1.57 1.00 1.50 1.29 2.00 1.71 1.50
HKS 0.39 0.51 0.54 −0.03 0.38 0.33 0.49 −0.03
OR 20.44 20.19 41.78 0.00 13.38 6.95 14.83 0.00
>0.5 mm/6 h
POD 0.40 0.67 0.50 0.00 0.40 0.33 0.25 0.00
FA 0.02 0.03 0.02 0.02 0.01 0.03 0.06 0.00
FBI 0.80 2.00 1.00 2.00 0.60 1.33 1.75 0.00
HKS 0.38 0.63 0.48 −0.02 0.39 0.31 0.19 0.00
OR 37.67 56.00 49.00 0.00 76.00 18.83 5.22 –
>1 mm/6 h
POD 0.00 1.00 0.50 – 0.50 1.00 0.50 –
FA 0.02 0.03 0.02 0.02 0.00 0.02 0.02 0.00
FBI 1.00 4.00 1.50 - 0.50 3.00 1.50 –
HKS −0.02 0.97 0.48 – 0.50 0.98 0.48 –
OR 0.00 – 50.00 – – – 50.00 –
shows an underestimation in PM10 for the Ispra station
by a factor 1.3. Very high PM10 concentrations are ob-
served at the beginning of the month for Mantova, lead-
ing to a monthly mean measured value of 207µg/m3, re-
sulting to an underestimation of the model by a factor 3
(CHIMERE/MM5) and 6 (CHIMERE/WRF) for this sta-
tion. These values are caused by fireworks at the begin-
ning of the month (ARPA Lombardy, personal communi-
cation). Emissions from fireworks are not included in our
emission inventory. However, from the second half of the
week onwards for Mantova, we find that the model underes-
timates PM10 by a factor 1.1 to 2.1 for both the simulations
(CHIMERE/MM5 and CHIMERE/WRF). Excluding Man-
tova form the analysis shows a significant improvement of
the results. PM10 concentrations are for the four stations un-
derestimated on average by a factor 1.4 (CHIMERE/MM5)
and 2 (CHIMERE/WRF).
As shown above, differences in calculated and observed
PM10 concentrations are also found for the EMEP measure-
ment station at Ispra (I). For this station we have to our dis-
posal surface concentrations of SO=4 , NO
−
3 , NH
+
4 , organic
carbon and black carbon, which allows us to compare these
aerosol species with model calculated values and allows us to
determine which of the aerosol species is responsible for the
discrepancy between observed and calculated aerosol con-
centrations.
Comparing NO−3 aerosol (9.31µg/m3) and NH+4
(4.21µg/m3) for Ispra, we found that CHIMERE/WRF is in
good agreement with the observations, see Table 5.
CHIMERE/MM5 overestimates the observed NO−3
aerosol concentrations by a factor of 1.4, while NH+4
calculated concentrations are in good agreement with the
observations. The latter could be related to the underestima-
tion by the model of SO=4 and overestimation of NO
−
3 . The
temporal correlation coefficients by CHIMERE/WRF are
higher than by CHIMERE/MM5. SO=4 is underestimated
by a factor 2 (CHIMERE/MM5) and 1.5 (CHIMERE/WRF)
when compared to the monthly mean observed value
(3.83µg/m3). Calculated SO2 concentrations are in gen-
eral overestimated by a factor 1.3 when compared to the
measurements. The wintertime underestimation of sulphate
concentrations has been reported by previous studies and is
possibly due to the insufficient of oxidation chemistry in the
model (Jeuken, 2000; Kasibhatla et al., 1997).
The large underestimation of PM10 could be related to
the underestimation of black carbon and organic carbon.
Our model gives the sum of organic carbon (OC), elemen-
tal carbon (EC) and anthropogenic dust. Analysing the sum
of OC, EC and anthropogenic dust, denoted as PPM, we
see that the model underestimates for January the measured
PPM by a factor of around 3 and 4 for CHIMERE/MM5
and CHIMERE/WRF respectively, see Table 5. A possible
explanation for this large underestimation is related to the
frequent wood burning for heating purposes in northern Italy
in winter time and the secondary organic aerosol formation,
which can contribute to around 55% of the organic aerosol
mass in winter time (Lanz et al., 2007). Uncertainties in the
emission factors for EC and OC in the emission inventory
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6620 A. de Meij et al.: Study aerosol with two meteorological models (MM5 and WRF)
Table 3f. Continued.
Definition of the statistical parameters used for the comparison be-
tween modelled and observed values.
SD =
√
√
√
√
√
n∑
i=1
(y − y)2
n
Standard deviation: a measure of the dispersion of the observed
(calculated) values around the mean.
RMSE=
√
√
√
√
√
n∑
i=1
(x−y)2
n
Root mean square error: a measure of difference between the model
and the observations (measure of accuracy).
BIAS=
n∑
i=1
x−y
n
R2 = (CORR)2
Square of the correlation coefficient (indicates the linear relation-
ship between model and observations).
MAE=
n∑
i=1
DWD
n
,
where DWD – difference of the wind direction calculated from:
min(x−y, y−x+360), for x>y
DWD=min(y−x, x−y+360), for y>x
Mean absolute error calculated to indicate the error in wind direc-
tion prediction.
y – observed value
y – mean of observed values
x – modelled value
n – number of observations
Rain specific hit rate statistics:
For the hit rate statistics the following symbolic representation was
used:
Observation
Yes No
Forecast
Yes A B
No C D
A – correct hits,
B – false hits (false alarm),
C – false rejections (misses),
D – correct rejections.
including unaccounted sources, which contribute to the un-
derestimation of EC and OC in the inventory could be held
responsible for the underestimation of PM10 in a winter pe-
Table 3f. Continued.
Based on this the following categorical statistics were calculated:
POD =
A
A+ C
,
probability of detection of the rain event;
FA =
B
B +D
,
false alarm (probability of false detection of the rain event);
FBI =
A+ C
A+ B
,
frequency BIAS (the measure of over – or underestimation of the
events number; FBI=1 indicates that the event is forecasted exactly
as often as it is observed);
HKS=
AD−BC
(A+ C)(B +D)
,
Hansen-Kuipers score (indicates the ability of the model to give
correct forecast of the event as well as to avoid the false alarms);
OR =
AD
BC
,
odds ratio (OR>1 indicates that the POD>FA), (Stephenson, 2000;
Goeber and Milton, 2001).
riod, as discussed by Schaap et al. (2004b). Another source
for the underestimation can be related to the additional pro-
cesses of SOA formation from traffic and wood burning as
described in Robinson (2007), which are not included in our
SOA formation scheme. Observations show that organic car-
bon has a significant contribution to the PM10 mass for Ispra
(46%), with 29.8µg/m3. Elemental carbon contributes with
10% to PM10 mass (5.1µg/m3), and dust contributes with
2.5% to the total PM10 mass (1.4µg/m3).
4.2.2 Differences in calculated PM10 concentrations
between CHIMERE/MM5 and CHIMERE/WRF
for January
Our analysis of calculated PM10 concentrations for the five
stations in January shows that modelled mean PM10 values
between CHIMERE/MM5 and CHIMERE/WRF are differ-
ent. The calculated PM10 values for CHIMERE/MM5 are
on average a factor 1.6 higher than CHIMERE/WRF. An-
alyzing the monthly mean PM10 concentration for January
for Ispra (CHIMERE/MM5), we find a concentration around
43.2µg/m3. CHIMERE/WRF calculates a monthly mean
PM10 concentration of 26.6µg/m3 for Ispra, see Table 4.
The temporal correlation coefficients by CHIMERE/WRF
are higher than by CHIMERE/MM5, indicating that the spa-
tial gradients of the daily mean concentrations are relatively
well reproduced by the model using the WRF meteorology.
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A. de Meij et al.: Study aerosol with two meteorological models (MM5 and WRF) 6621
Table 4. Monthly mean PM10 concentrations calculated by the CHIMERE model using MM5 and WRF meteorology for January 2005,
including the standard deviation and the temporal correlation coefficient, together with the measurements.
Name station Monthly mean January model with
MM5 µg/m3±stdev.; correlation
coeff.
Monthly mean January model with
WRF µg/m3±stdev.; correlation
coeff.
Monthly mean January obser-
vations µg/m3±stdev.
Ispra 43.2±22.3; 0.55 26.9±13.3; 0.73 57.4±31.7
Cantu 43.7±21.3; 0.40 28.7±15.9; 0.74 78.8±40.6
Erba 39.5±19.9; 0.42 29.0±13.9; 0.70 67.5±20.8
Mantova 64.2±42.3; 0.70 36.7±16.3; 0.82 207
Castelnovo Bariano 51.9±40.1; 0.27 28.6±13.2; 0.47 70.7±20.8
Average 48.5 30.0 96.3
Table 5. Monthly mean measured concentrations for Ispra of SO=4 , NO
−
3 and NH
+
4 , together with the model calculated mean concentrations
using MM5 and WRF, for January 2005, including the standard deviation and the temporal correlation coefficient.
Mean January 2005, Ispra EMEP measurement CHIMERE MM5 CHIMERE WRF
µg/m3±stdev. µg/m3±stdev.; µg/m3±stdev.;
correlation coeff. correlation coeff.
SO=4 3.83±3.20 1.93±0.62; 0.20 2.57±1.78; 0.77
NO−3 9.31±8.84 13.4±9.94; 0.69 7.88±5.55; 0.84
NH+4 4.21±3.93 4.43±2.85; 0.70 3.23±2.25; 0.88
Sum EC,OC, dust 36.3±20.1 12.9±9.68; 0.49 8.23±6.06; 0.58
In general, the standard deviations by CHIMERE/MM5 are
larger than by CHIMERE/WRF. The reason for this is that
for CHIMERE/MM5 higher PM10 peak values are calculated
than by CHIMERE/WRF.
The differences in PM10 concentrations for January are
on average around 10µg/m3 (not shown), with the excep-
tion of the period 14–18 January, where a large difference in
calculated PM10 between the two simulations is found, see
Sect. 4.2.3 for a detailed the explanation.
To understand the differences in PM10 between
CHIMERE/MM5 and CHIMERE/WRF, we analyse the
PBL heights and the related sensible and latent heat fluxes
(SHF and LHF respectively) for the five different locations,
for which we compare the PM10 calculated concentrations.
The sensible heat flux (dry) and latent heat flux (moist)
are provided by the land surface model. The reason why
we analyze first the SHF and LHF is that these parameters
provide the heat fluxes to the PBL scheme which stimulates
the turbulence in the boundary layer and determines the
height and temporal profile of the PBL and the resulting
vertical aerosol distribution.
The LSM model applied in MM5 and WRF is Noah, there-
fore sensible and latent surface heat fluxes should be similar.
For the five different locations we observe similar SHF.
On average the monthly mean SHF with MM5 is −8.0 W/m2
and with WRF −6.9 W/m2. However, for the LHF larger
differences are observed between MM5 and WRF, which is
in general 10.2 W/m2 for WRF and 5.7 W/m2 for MM5.
The underlying reason for these differences in LHF, is
that the shortwave incoming radiation at the surface between
MM5 and WRF is different. Overall more shortwave in-
coming radiation is estimated by MM5. On average the
amount of shortwave incoming radiation for the five stations,
between 07:00 LT–16:00 LT, is 124 W/m2 for MM5 and for
116 W/m2 for WRF. The downward shortwave radiation is
a source of energy for the soil. More incoming shortwave
radiation and the availability of moisture at the surface will
stimulate the heat and moisture transport away upward from
the surface (Stull, 1988). The difference in shortwave radi-
ation between MM5 and WRF is a result of the difference
in cloud cover. The cloud cover is diagnosed with the pre-
processor in CHIMERE, which allows us to determine cloud
cover and compare the amount of cloud liquid water between
the two meteorological models.
Analyzing the cloud attenuation between the two meteoro-
logical models, we observe that in general MM5 shows less
cloud attenuation than WRF does, which results in more in-
coming radiation by MM5. This is due to the difference in
microphysics scheme. The number of hydrometer categories
in WSM6 (vapour, cloud water, cloud ice, rain, snow, grau-
pel) is larger than in the Simple Ice scheme (vapour, cloud
water/ice, rain/snow), this leads to more cloud liquid water
and more rain fall (Hong et al., 2006).
More cloud liquid water content by WRF, result in more
cloud attenuation by WRF (and more rain by WRF as de-
scribed in Sect. 4.1.1). This has an impact on the latent heat
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6622 A. de Meij et al.: Study aerosol with two meteorological models (MM5 and WRF)




Fig. 2a. Wind roses for Ispra (left) and Mantova (right) monitoring stations, for the whole of the year 2005. The scale indicates the frequency
of the wind direction.

Sums of rain from observations and models,
for the whole year 2005
0.0
20.0
40.0
60.0
80.0
100.0
120.0
ISPRA ERBA CA NTU SERMIDE
su
m
o
f r
ain
[c
m
]
OB S
W RF
M M 5

Sums of rain from observations and models,
for January 2005
0.0
1.0
2.0
3.0
4.0
5.0
6.0
7.0
ISPRA ERBA CA NTU SERMIDE
su
m
o
f r
ain
[c
m
]
OB S
W RF
M M 5

Sums of rain from observations and models,
for June 2005
0.0
2.0
4.0
6.0
8.0
10.0
12.0
14.0
16.0
ISPRA ERBA CA NTU SERMIDE
su
m
o
f r
ain
[c
m
]
OB S
W RF
M M 5




Sums of rain from observations and models,
for the whole year 2005
0.0
20.0
40.0
60.0
80.0
100.0
120.0
ISPRA ERBA CA NTU SERMIDE
su
m
o
f r
ain
[c
m
]
OB S
W RF
M M 5

Sums of rain from observations and models,
for January 2005
0.0
1.0
2.0
3.0
4.0
5.0
6.0
7.0
ISPRA ERBA CA NTU SERMIDE
su
m
o
f r
ain
[c
m
]
OB S
W RF
M M 5

Sums of rain from observations and models,
for June 2005
0.0
2.0
4.0
6.0
8.0
10.0
12.0
14.0
16.0
ISPRA ERBA CA NTU SERMIDE
su
m
o
f r
ain
[c
m
]
OB S
W RF
M M 5




Sums of rain from observations and models,
for the whole year 2005
0.0
20.0
40.0
60.0
80.0
100.0
120.0
ISPRA ERBA CA NTU SERMIDE
su
m
o
f r
ain
[c
m
]
OB S
W RF
M M 5

Sums of rain from observations and models,
for January 2005
0.0
1.0
2.0
3.0
4.0
5.0
6.0
7.0
ISPRA ERBA CA NTU SERMIDE
su
m
o
f r
ain
[c
m
]
OB S
W RF
M M 5

Sums of rain from observations and models,
for June 2005
0.0
2.0
4.0
6.0
8.0
10.0
12.0
14.0
16.0
ISPRA ERBA CA NTU SERMIDE
su
m
o
f r
ain
[c
m
]
OB S
W RF
M M 5

Fig. 2b. The quantities of rain observed and predicted by the mod-
els, respectively from top to bottom: for the whole of the year 2005,
for January and for June.
flux by WRF, which is in general almost a factor 2 higher
as mentioned earlier. The difference in CLW between WRF
and MM5 for the five stations is on average around a fac-
tor 1.2, up to a maximum of 4 (WRF higher). For some days
the differences in CLW between WRF and MM5 are larger,
because for some days MM5 does not calculate CLW while
WRF does (see Sect. 4.2.3 for more details of this difference
and the impact on the aerosol calculations).
This larger flux of latent heat by WRF is responsible for
the higher PBL heights.
Another reason for the lower PBL heights by MM5 could
be related to a stronger inversion effect by MM5. The tem-
peratures at 2 m level for the five stations by MM5 are lower
(bias −3.3◦C) than by WRF (bias 1.7◦C) when compared to
the observations, which indicate a stronger inversion effect
by MM5 than by WRF. Analyzing the vertical temperature
gradient profiles for the five stations for MM5 and WRF,
we see indeed that MM5 has a stronger inversion gradient
(2.2◦C) than WRF (0.7◦C) over the first 150 m.
On average, the PBL height by WRF for the five stations
at noon is around 270 m, while by MM5 97 m. This is more
than a factor 2.8 difference. This difference in PBL height
is responsible for the differences in aerosol concentrations
between CHIMERE/WRF and CHIMERE/MM5. The verti-
cal mixing with WRF meteorology is better, because of the
higher PBL height, which leads to lower aerosol concentra-
tions at ground level than with MM5 meteorology as men-
tioned before.
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A. de Meij et al.: Study aerosol with two meteorological models (MM5 and WRF) 6623
Fig. 3a. Vertical potential temperature gradient profiles between 10 m–200 m by WRF, MM5 for the Linate airport, together with the
observations for 00:00 h, 06:00 h, 12:00 h and 18:00 h for the whole year.

Fig. 3b. Statistics (Taylor plot, Taylor, 2001) for vertical potential
temperature gradient profiles (10 m–200 m) by WRF, MM5 for the
Linate airport, together with the observations for 00:00 h for the
whole year.
4.2.3 Episode of large difference in PM10
concentrations between CHIMERE/MM5
and CHIMERE/WRF
In Sect. 4.2.2 is mentioned that a large difference in cal-
culated PM10 concentrations between CHIMERE/MM5 and
CHIMERE/WRF is observed for the period 14–18 January
for Ispra. In this section we give the explanation for this
large difference in PM10.

Fig. 3c. Statistics (Taylor plot, Taylor, 2001) for vertical potential
temperature gradient profiles (10 m–200 m) by WRF, MM5 for the
Linate airport, together with the observations for 06:00 h for the
whole year.
Analyzing the temporal profile of PM10 concentrations
for January for CHIMERE/MM5 and CHIMERE/WRF,
we observe maximum PM10 values of 90µg/m3 by
CHIMERE/MM5, whereas CHIMERE/WRF calculates a
maximum of 45µg/m3. This large difference in calculated
PM10 concentrations cannot be explained by the difference
in PBL scheme alone.
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6624 A. de Meij et al.: Study aerosol with two meteorological models (MM5 and WRF)

Fig. 3d. Statistics (Taylor plot, Taylor, 2001) for vertical potential
temperature gradient profiles (10 m–200 m) by WRF, MM5 for the
Linate airport, together with the observations for 12:00 h for the
whole year.

Fig. 3e. Statistics (Taylor plot, Taylor, 2001) for vertical potential
temperature gradient profiles (10 m–200 m) by WRF, MM5 for the
Linate airport, together with the observations for 18:00 h for the
whole year.
This large difference in PM10 calculated values is re-
lated to the difference in calculated NO−3 concentra-
tions by CHIMERE/MM5 (33µg/m3) and CHIMERE/WRF
(16µg/m3) for this period. The underlying reason for the
higher NO−3 aerosol concentrations by CHIMERE/MM5 can
be explained by the absence of cloud liquid water (CLW)
in MM5 for that period (observed in WRF). As described
before (Sect. 4.2.2) the microphysics scheme in WRF pro-
duces more CLW than in the Simple Ice scheme, because of
the number of hydrometer categories in WSM6 (Hong et al.,
2006). The oxidation of SO2 in cloud liquid water by H2O2
is very fast and is an important source of sulphate aerosol
formation (Pandis and Seinfeld, 1989; Seinfeld and Pandis,
and references herein), see reactions below:
HSO−3 + H2O2 → SO2OOH
−
+ H2O (R1)
SO2OOH− + H+ → H2SO4 (R2)
SO2 concentrations during this period with CHIMERE/WRF
drop to an average of 0.75 ppb while CHIMERE/MM5 cal-
culates an average of 5.0 ppb during this period. Mean H2O2
concentration for the CHIMERE/WRF is around 0.02 ppb,
whereas CHIMERE/MM5 a mean of 0.07ppb is calculated
for that period. CHIMERE/WRF calculates a mean concen-
tration of SO=4 of 5.5µg/m3, while CHIMERE/MM5 calcu-
lates a mean of 2.0µg/m3 SO=4 for that 5 days period. Mea-
surements show an average of 9.3µg/m3 for SO=4 for that
period, with a maximum of 12.5µg/m3 on 17 January. Ob-
servations show that clouds were present for that period (http:
//iamest.jrc.it/meteo/meteo.php?). CHIMERE/MM5 calcu-
lates lower SO=4 concentration, because SO2 is not oxidized
by H2O2 into SO=4 as there is no CLW observed by MM5
for that period. Due to the presence of CLW in the WRF
meteorology, SO2 is oxidized by H2O2 into SO4 aerosol.
As mentioned before, CHIMERE/WRF calculates a mean
NO−3 concentration of 16µg/m3 for the period 14–18th,
whereas for CHIMERE/MM5 a mean concentration of
33µg/m3 is calculated. These large differences in NO−3
aerosol contribute to the differences in PM10.
The difference between the two simulations in NO−3 cal-
culations can be explained by the reaction of the sulphate
aerosol with ammonia. If sufficient ammonia is available to
neutralize all sulphate, the residual amount of ammonia can
neutralize nitric acid to form the ammonium nitrate aerosol.
We have seen that CHIMERE/MM5 does not produce much
SO=4 as CHIMERE/WRF does. This means that the ammonia
can react with the nitric acid to form the nitrate aerosol, lead-
ing to a higher NO−3 concentration than CHIMERE/WRF,
causing higher PM10 values between 14 and 18 January than
CHIMERE/WRF. On days when no CLW is found for both
MM5 and WRF, the difference in calculated aerosol concen-
trations between CHIMERE/MM5 and CHIMERE/WRF are
smaller, around 10µg/m3.
4.2.4 Spatial distribution of PM10 calculated
concentrations by CHIMERE/MM5 and
CHIMERE/WRF for January
Figure 4 shows the monthly mean spatial distribution of
the PM10. Large differences between the model simula-
tions using MM5 and WRF are found. For CHIMERE/MM5
(Fig. 4a) the model calculates a PM10 concentration around
40–50µg/m3 for a large part over the Po valley, with elevated
levels for the Milan city, up to 105µg/m3.
In Fig. 4b, CHIMERE/WRF shows a much lower PM10
concentration over the Po valley area than CHIMERE/MM5
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A. de Meij et al.: Study aerosol with two meteorological models (MM5 and WRF) 6625
(on average a factor 2 lower) and a concentration for the Mi-
lan city of 59µg/m3. These differences are due to the dif-
ference in LHF and the resulting PBL heights caused by mi-
crophysics as described in Sect. 4.2.2. A sensitivity analysis
showed that changing only the PBL scheme in WRF from
YSU into MRF, does not improve the calculated PM10 con-
centrations for January.
Another important parameter responsible for the surface
heat fluxes could be related to the choice of the land surface
model.
We performed a sensitivity analysis by changing the Noah
LSM scheme in WRF by the 5-layer soil temperature model
and the YSU PBL with the MRF.
The PM10 spatial distribution and concentrations for this
simulation improve in Fig. 4c. For the Po valley area PM10
concentrations are on average around 35–40µg/m3, which is
up to a factor of 1.6 higher than the simulation using WRF
meteorology with the Noah land surface model and closer
to the concentrations of CHIMERE/MM5 (CHIMERE/MM5
20% higher) and correspond better to the observations in the
Lombardy region. For the Milan city a monthly mean con-
centration of 79µg/m3 is found, which is a factor 1.3 higher
than with Noah LSM and is closer to CHIMERE/MM5. For
the five stations, the PM10 concentrations are on average 41%
higher than with Noah LSM and YSU PBL.
As described above, the choice of LSM has an impact on
the heat fluxes and the resulting PBL heights, the vertical
mixing and therefore in the aerosol concentration. The un-
derlying reason for the improvement in PM10 concentrations
is related to the change in PBL height with the 5 layer soil
temperature LSM+MRF PBL scheme in respect to the PBL
height with the Noah LSM. When we analyze for the stations
the heat fluxes we see that the SHF with the 5-layer soil mois-
ture LSM are on average a factor 2 lower than with the Noah
LSM; on average −13.6 W/m2 with WRF 5-layer soil tem-
perature and MRF PBL, while with Noah LSM an average of
−6.9 W/m2 is calculated. However, LH fluxes are on average
2 W/m2 higher using the 5-layer soil temperature LSM than
with Noah LSM.
Analyzing the resulting PBL heights for the five stations
using the 5-layer soil temperature LSM, we see that the PBL
height at noon for Ispra, Erba and Cantu are a factor 2 lower
than when the Noah LSM is used and are closer to the PBL
heights calculated by MM5. This results in reducing the ver-
tical mixing in the first layers, leading to higher aerosol con-
centrations at ground level.
When we change the Noah LSM scheme in our WRF pre-
processing for the 5-layer soil temperature model and keep
the YSU PBL scheme, calculated PM10 concentrations for
January 2005 increase by 30% in respect to the simulation
using Noah LSM.
(a)
(b)
(c)
Fig. 4. Monthly mean PM10 concentrations for January by
CHIMERE using the MM5 meteorology (a), WRF meteorology (b)
and WRF meteorology using the 5-layer soil temperature model +
MRF PBL scheme (c).
4.2.5 Calculated PM10 concentrations with MM5 and
WRF for June
In Table 6 we analyse the model results of the calculated
monthly PM10 concentrations for June 2005 and compare
them with observations for five stations in the Lombardy re-
gion.
For both model simulations the PM10 concentrations are in
better agreement with the observations than in January. The
model mean calculated concentrations by CHIMERE/MM5
(on average 29.9µg/m3) and CHIMERE/WRF (on average
30µg/m3) agree well with the observations (29.2µg/m3).
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6626 A. de Meij et al.: Study aerosol with two meteorological models (MM5 and WRF)
Table 6. Monthly mean PM10 concentrations calculated by the CHIMERE model using MM5 and WRF meteorology for June 2005,
including the standard deviation and the temporal correlation coefficient, together with the measurements.
Name station Monthly mean June model with
MM5 µg/m3±stdev.; correlation
coeff.
Monthly mean June model with
WRF µg/m3±stdev.; correlation co-
eff.
Monthly mean June observa-
tions µg/m3±stdev.
Ispra 27.0±7.07; 0.16 25.7±7.94; 0.43 20.1±8.29
Cantu 26.9±6.67; 0.23 28.9±8.02; 0.38 31.8±11.7
Erba 25.4±6.46; 0.33 30.8±7.45; 0.44 32.7±10.3
Mantova 40.6±7.64; 0.31 37.4±5.99; 0.26 39.8±10.7
Castelnovo Bariano 29.4±5.01; 0.11 27.4±4.35; 0.34 21.7±8.92
Average 29.9 30.0 29.2
Table 7. Monthly mean measured concentrations for Ispra of SO=4 , NO
−
3 and NH
+
4 , together with the model calculated mean concentrations
using MM5 and WRF, for June 2005, including the standard deviation and the temporal correlation coefficient.
Mean June 2005, Ispra EMEP measurement
µg/m3±stdev.
CHIMERE MM5 µg/m3±stdev.;
correlation coeff.
CHIMERE WRF
µg/m3±stdev.; correla-
tion coeff.
SO=4 5.38±2.78 5.00±1.64; 0.16 5.65±1.56; 0.46
NO−3 1.31±1.09 1.73±2.16; 0.22 2.19±2.27; 0.22
NH+4 2.33±1.10 2.07±1.07; 0.01 2.46±1.07; 0.33
Sum EC,OC, dust 10.5±4.92 3.73±1.11; 0.15 4.16±1.43; 0.38
The temporal correlation coefficients by CHIMERE/WRF
are larger than by CHIMERE/MM5. Calculated SO=4 , and
NH+4 concentrations are in good agreement with the obser-
vations, see Table 7. SO=4 CHIMERE/MM5 (5.00µg/m3)
and CHIMERE/WRF (5.65µg/m3) are in a good agree-
ment with the observations (5.38µg/m3). NO−3 aerosol by
CHIMERE/WRF is overestimated by a factor 1.7 and the
monthly mean concentration by CHIMERE/MM5 is over-
estimated by a factor 1.3 when compared to the observa-
tions. The calculated monthly mean NH+4 concentrations by
CHIMERE/MM5 and CHIMERE/WRF are in good agree-
ment with the observations. However, as daily tempera-
tures exceed 20◦C in June, these measured concentrations
should be considered as lower limit values, due to evapora-
tion from the quartz filter, see Sect. 3.1 for the explanation.
Analysing the PPM (sum of EC, BC and dust), we see that
the model underestimates the measured PPM by a factor 2.8
(CHIMERE/MM5) and 2.5 (CHIMERE/WRF). A possible
explanation for this is related to the emissions factors applied
for OC and EC in the emission inventories and the underes-
timation of SOA formation as described before.
The differences in PM10 concentrations between the two
model simulations are small, which is not the case for Jan-
uary as described before. The underlying reason for this is
that difference in the heat fluxes between MM5 and WRF are
not that large as seen for January; SHF by WRF is 7% higher,
LHF by WRF is 9% lower when compared to the heat fluxes
calculated by MM5.
These smaller differences in the heat fluxes result in the
small differences in PBL heights for the five different sta-
tions. The PBL heights, using MM5 and WRF, both with
Noah LSM scheme, are on average ±1407 m (MM5) and
±1464 m (WRF) for June for the five stations at 2 p.m. These
small variations in the PBL heights will not affect the verti-
cal mixing in the first layers of the model and therefore not
invoke a large difference in aerosol distribution between the
two model simulations.
4.2.6 Sensitivity analysis of PM10 calculations
for January
Our model simulations using MM5 and WRF meteorology
showed underestimations in PM10 concentrations for January
2005. These could be related to the uncertainties in the emis-
sion inventories and the lack of natural and anthropogenic
sources of PM. However, we observed also large differences
in calculated aerosol concentrations between model simula-
tions using MM5 and WRF meteorology, while the emission
input does not change.
In this section we explain that the latter difference is
related to the parameterizations in the meteorological pre-
processing.
In Sect. 4.2.4 we have seen that changing the LSM in WRF
from Noah to the 5-layer soil temperature model and the PBL
scheme from YSU into MRF, increase the calculated PM10
concentrations on average to 41% for the five stations.
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A. de Meij et al.: Study aerosol with two meteorological models (MM5 and WRF) 6627
Table 8. Monthly mean ozone values calculated by the CHIMERE model using MM5 and WRF meteorology for June 2005, together with
the measurements and the correlation coefficients (based on hourly values), including the standard deviation and the temporal correlation
coefficient.
Name station Monthly mean
model with MM5
(ppb)±stdev.
Monthly mean
model with WRF
(ppb)±stdev
Monthly mean
observations
(ppb)±stdev
Correlation
MM5 vs Obs
Correlation WRF
vs. Obs
Ispra 46.4±8.34 52.4±8.26 35.3±5.83 0.77 0.75
Erba 54.3±9.31 56.8±9.79 27.6±11.5 0.60 0.51
Osio Sotto 42.2±8.76 45.8±9.10 50.1±11.9 0.71 0.57
Gambara 50.1±7.70 50.2±5.21 49.5±9.24 0.47 0.40
Corte de Cortesi 49.5±7.73 50.1±6.26 41.3±5.46 0.75 0.65
Marmirolo Fontana 48.7±6.77 49.8±4.67 36.6±5.98 0.70 0.57
Lecco 52.7±8.66 63.5±9.78 56.6±15.8 0.46 0.63
Varese 41.3±6.85 45.9±4.62 53.6±13.2 0.50 0.35
Chiavenna 49.3±4.19 55.8±2.91 49.3±12.4 0.17 0.45
Milano 31.5±8.21 29.5±5.90 39.8±8.65 0.68 0.41
Average 46.6 50.0 40.0 0.58 0.53
A sensitivity analysis showed that changing only the PBL
scheme in WRF from YSU into MRF, does not improve the
calculated PM10 concentrations for January.
Another sensitivity analysis showed that changing the
LSM model in MM5 from Noah to the 5-layer soil temper-
ature model, sensible heat and latent heat fluxes change and
to some extent the resulting PBL heights.
On average, the SHF for the five stations using the 5-layer
soil temperature model is almost a factor 2 lower, i.e.
−14.6 W/m2 (which corresponds with the average SHF us-
ing 5-layer soil temperature model in WRF, −13.6 W/m2),
while with the Noah LSM, SHF is on average −8.0 W/m2,
as described in Sect. 4.2.2. However, LHF goes up from
5.7 W/m2 (Noah) to 11.2 W/m2. This results in that the PBL
height does not change as much as seen between MM5 and
WRF and therefore aerosol concentrations does not change
much (on average 2µg/m3 for the Po valley area).
When the Simple Ice microphysics scheme in the MM5
simulation is changed for the Mixed Phase microphysics
scheme, we see that the monthly mean PM10 concentrations
are lower, up to 20%. The underlying reason for this is that
with the Mixed Phase scheme, more cloud liquid water is
calculated by the model than with the Simple Ice scheme,
which is responsible for lower NO3 aerosol peak values and
the resulting PM10 values as described in Sect. 4.2.3.
4.2.7 Calculated O3 concentrations with CHIMERE/
MM5 and CHIMERE/WRF for June
In Table 8 the monthly mean O3 calculated values by
CHIMERE/MM5 and CHIMERE/WRF are given for nine
background stations, together with the observations and the
correlation coefficients.
(a)
(b)
Fig. 5. Monthly mean O3 concentrations for June by CHIMERE us-
ing the MM5 meteorology (a) and WRF meteorology, Noah LSM
and YSU PBL (b). Var = Varese, Lec = Lecco, Chi = Chiavenna,
O S = Osio Sotto, C C = Corte di Cortesi, Gam = Gambara,
Mar = Marmirolo Fontana, Mil = Milan.
Overall the monthly mean O3 values by CHIMERE are
overestimated on average by a factor 1.3 for both using MM5
and WRF meteorology and the correlation coefficients are in
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6628 A. de Meij et al.: Study aerosol with two meteorological models (MM5 and WRF)
general higher by CHIMERE/MM5. In Fig. 5a and 5b, the
monthly (June) mean O3 concentrations by the CHIMERE
model are shown, using MM5 and WRF meteorology. In
general the concentrations in the Po Valley area are simi-
lar. However we observe differences in O3 values in the
mountain regions, of around 6–9 ppb with a maximum up
to 14 ppb. Analyzing the monthly mean wind direction and
wind speed, we see that WRF monthly mean wind speed is
3 m/s, with a larger daily amplitude and frequency from south
to north direction. The monthly mean wind speed by MM5 is
2 m/s, with lower daily velocity amplitude and a lower south–
north frequency. The larger wind speed by WRF transports
the O3 from the Po valley area higher up over the mountains,
resulting in higher O3 concentrations over this area. A simi-
lar effect of larger wind speeds on O3 concentrations over the
Pre Alps has been observed earlier by Minguzzi et al. (2005).
Figure 6a–c presents hourly average surface concentra-
tions of O3, NO and NO2 for the complete month of June
2005. Due to its large-scale spatial representativity, the aver-
age of ozone concentration gives very good correlations be-
tween the model and observations (CHIMERE/MM5 0.96,
CHIMERE/WRF 0.97). The diurnal cycle is well repre-
sented compared to the measurements. Before the sunrise
(07:00 LT), the two models give different estimations: MM5
slightly underestimates the measurements (∼5 ppb) when
WRF slightly overestimates (∼3 ppb). During the convec-
tive period (from 07:00 to 16:00 LT), the two models over-
estimate the ozone concentrations. After 16:00 LT, when the
boundary layer collapses, the models again underestimates
the surface concentrations.
In average, this may be explained by analyzing the sur-
face NO and NO2 time series. Contrarily to ozone (sec-
ondary specie), these species are primary sources, depend-
ing on several activity sectors and are less spatially homo-
geneous. NO represents mainly the traffic source and this
is explained by the morning peak (around 07:00 LT) when
the nocturnal boundary layer remains thick: sources are not
well mixed and the differences between models and measure-
ments (∼10 ppb) represent in the same time the uncertainty
on the stable boundary layer estimation, the uncertainty on
the emissions inventories knowledge, the uncertainty of the
morning wind field and the subsequent advection and the
spatial heterogeneity of these sources. The fact that the NO
with MM5 is higher than measurements expresses the direct
impact on the low underestimation of ozone for the same
time period. At the end of the day, after 16:00 LT, the over-
estimation of modelled NO2 represents the end of the activ-
ity period for the traffic and probably a boundary layer cer-
tainly too low in average with the two meteorological mod-
els. These differences are often observed in CTM modeling
and are the result of the uncertainty of meso-scale modeling
to estimate accurately the unstable to stable boundary layer
transition (including its time length and amplitude).
(a)
O3 diurnal average June 05
0
10
20
30
40
50
60
70
80
90
0-24h
pp
b
Obs
MM5
WRF

(b)
NO2 diurnal average June 05
0.00
5.00
10.00
15.00
20.00
25.00
30.00
35.00
0-24h
pp
b
Obs
MM5
WRF

(c)
NO diurnal average June 05
0.00
2.00
4.00
6.00
8.00
10.00
12.00
14.00
0-24h
pp
b
Obs
MM5
WRF

Fig. 6. Diurnal average of ozone (a), NO2 (b) and NO (c) for the
observations (blue line) of the stations Ispra, Osio Sotto, Corte di
Cortesi, Gambara and Varese, together with the calculated values
by CHIMERE/MM5 (red line) and CHIMERE/WRF (green line),
for June 2005.
5 Summary and concluding remarks
The impact of two different meteorological models (MM5
and WRF) on PM10, aerosols and O3 calculations over the
Po valley region (Italy) for January and June 2005 is investi-
gated.
First we evaluate for January, June and annually the cal-
culated meteorological parameters by MM5 and WRF (tem-
perature, wind speed, wind direction, relative humidity and
precipitation) with observations.
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A. de Meij et al.: Study aerosol with two meteorological models (MM5 and WRF) 6629
Overall we can say that the analysis of the meteorolog-
ical modelling results shows that the performance of both
models in calculating surface parameters is similar in all
tested periods however differences are still observed. The
temperatures are usually underestimated but in the most of
cases within a BIAS range of −3◦C. RMSE varies from 2
to 4.4◦C and is lower than the SD calculated from observa-
tions. WRF usually produces higher temperature averages
than MM5. The relative humidity is mainly overestimated
but the BIAS values in most cases do not reach the level of
10% of RH. RMSE changes from about 12 to 20% (only for
January the range is larger: from ∼10 to 31%) and the con-
dition of RMSEmod<SDobs, is fulfilled in most of cases also
for this parameter. WRF produces higher averages of rela-
tive humidity than MM5 during the winter period. The wind
field is not well reproduced due to difficulties caused by very
low wind speeds occurring in the Po Valley area (average
observed wind speeds over all analyzed periods were below
1 m/s). Both models overestimate largely the wind speed val-
ues with the BIAS higher than 2 m/s and RMSE varying from
1.5 up to 3.3 m/s. The WRF model usually produces higher
wind velocities than MM5. The observed prevailing wind
direction is well reflected by the models for Ispra location,
however, poorly reproduced for Mantova. The quantity of
precipitation, according to statistics for the whole year, is
overestimated by WRF and underestimated by MM5. The
analysis of the hit rate statistics shows that WRF catches bet-
ter the rain events.
The vertical potential temperature gradient profiles by
WRF and MM5 correspond well to the observations from the
Linate airport location for the whole year. This indicates that
for this location both MM5 and WRF are able to reproduce
the stability/instability of the atmosphere.
This study evaluates the impact of using two different me-
teorological models with the CHIMERE model on aerosol
and O3 calculations for January and June 2005.
In general the model underestimates the observed PM10
concentrations by a factor 2 (with MM5 meteorology) and 3
(with WRF meteorology) for January 2005. NH+4 is in good
agreement with the observations for the Ispra EMEP station
for both the models, whereas NO−3 using the MM5 meteorol-
ogy is underestimated by a factor 1.4, but is in good agree-
ment with observations using WRF. SO=4 is underestimated
by a factor 2 and 1.5 by the model using MM5 and WRF re-
spectively. However, the sum of EC, OM and anthropogenic
dust is underestimated from the observations by the simula-
tion using MM5 (by a factor 3) and WRF (by a factor 4).
The difference in PM10 concentrations for January be-
tween CHIMERE/MM5 and CHIMERE/WRF is around a
factor 1.6 (PM10 higher with MM5 meteorology). This
difference and the larger underestimation in PM10 concen-
trations by CHIMERE/WRF are related to the differences
in PBL heights calculated by WRF meteorology. In gen-
eral the PBL height by WRF meteorology is a factor 2.8
higher at noon in January than calculated by MM5. This
could result in a better vertical mixing of the aerosols than
CHIMERE/MM5, causing lower aerosol concentrations at
the surface.
The underlying reason for the differences in PBL heights
can be explained by the differences found in the latent heat
flux, which is responsible for the profile of the PBL, and
the stronger temperature inversion effect by MM5. The
WRF meteorology calculates a monthly mean latent heat flux
which is a factor two larger than MM5.
The explanation for these differences in LHF is that the
shortwave incoming radiation at the surface between MM5
and WRF is somehow different. In general more shortwave
incoming radiation is observed by MM5 as a result of less
cloud cover by MM5, which is caused by the difference in
the microphysics scheme in MM5 and WRF.
This difference in microphysics scheme helps us to explain
also the difference in PM10 peak values, which are observed
between 14 and 18 January, as described in Sect. 4.2.3. In
that section we explain that the presence of cloud liquid wa-
ter (CLW) leads to the oxidation of SO2 into SO=4 aerosol.
The absence of CLW at certain periods by MM5 (when WRF
calculates CLW) leads to the production of higher NO−3 con-
centrations, and the resulting higher PM10 concentrations.
Changing the Noah LSM scheme in our WRF pre-
processing for the 5-layer soil temperature model, calculated
PM10 concentrations for January 2005 increase by 30% in
respect to the simulation using Noah LSM.
For June the differences in PM10 concentrations between
the model simulations using MM5 and WRF are small. Com-
pared to the observations, the model simulation using MM5
and WRF meteorology corresponds well with the observa-
tions (29.2µg/m3). Analyzing the heat fluxes, the PBL
height and PBL profile we observe small differences between
the two meteorological models.
Analyzing the calculated O3 values for June, we see that
for both the simulations the model overestimates on average
by a factor 1.3 the measured O3 concentrations and the cor-
relation coefficients are high. The higher O3 concentrations
over the mountains with WRF meteorology could be related
to the higher daily and more frequent south to north wind
speed during day time than by MM5, bringing the O3 from
the Milan area up to the mountains. Similar differences in
calculated O3 concentrations were observed by Minguzzi et
al. (2005). In this study the wind fields were varied, leading
to higher ozone concentrations over the foothills of the Alps.
Underestimation of PM10 calculations is a common prob-
lem in air quality modelling (Van Loon et al., 2004; Schaap et
al., 2007; Vautard et al., 2007; Stern et al., 2008). The under-
lying reason for this could be related to different factors con-
tributing to the uncertainties in air quality modelling, such as
uncertainties in the emission inventories, including the tem-
poral and vertical distribution of the emissions (De Meij et
al., 2006), the lack of natural and anthropogenic sources of
PM (Schaap et al., 2004b), the role of the gas and aerosol
boundary conditions on calculated aerosol concentrations in
www.atmos-chem-phys.net/9/6611/2009/ Atmos. Chem. Phys., 9, 6611–6632, 2009

6630 A. de Meij et al.: Study aerosol with two meteorological models (MM5 and WRF)
de model domain (De Meij et al., 2007) and the uncertainties
in the meteorological parameters, such as mixing height and
temperature (Hongisto, 2005) and wind fields (Minguzzi et
al., 2005).
In the Po valley, especially during winter time, stagnant
weather conditions are observed. These meteorological con-
ditions are responsible for high PM concentrations. Low
wind speeds and weak vertical mixing are responsible for
these stagnant conditions, which are difficult to simulate
with the meteorological models such as MM5 (Dosio et al.,
2002; Minguzzi et al., 2005; Carvalho et al., 2006; Stern
et al., 2008). This phenomenon was also encountered for
the Milan city by the models in the Citydelta exercise (http:
//aqm.jrc.it/citydelta, last accessed 12 March 2009; Cuvelier
et al., 2006; Vautard et al., 2006).
This study showed the differences in meteorological pa-
rameters between two meteorological models over complex
areas, especially during winter time periods. It shows how
this affects the calculated gas and aerosol concentrations,
which are non-linear dependent on meteorological condi-
tions (Haywood and Ramaswamy, 1998; Penner et al., 1998;
Easter and Peters, 1994).
The challenging task for the future is to improve the mod-
els’ capability to simulate meteorological parameters, such
as wind speed, wind direction, heat fluxes over complex ter-
rain with a higher accuracy. This will improve, together
with a more accurate emission inventory and better chemical
mechanisms, the calculated gas and aerosol concentrations,
which are necessary for scientific studies and for policy mak-
ing.
Acknowledgements. The authors would like to thank E. Chaxel for
the WRF interface in CHIMERE, J. L. Monge and S. Potempski
for the additional support of the code, S. Galmarini for the valuable
discussions. Also thanks to ARPA Lombardy and Veneto for the
PM10 and meteorological measurements. The authors would like
to thank the anonymous reviewers for the constructive comments.
This work has been performed on the Beowulf Linux cluster of the
Global Environment Monitoring Unit, Institute of Environment and
Sustainability, JRC, Ispra (I).
Edited by: A. S. H. Prevot
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