ATMOSPHERIC SCIENCE LETTERS
Atmos. Sci. Let. 11: 33–38 (2010)
Published online 12 January 2010 in Wiley InterScience
(www.interscience.wiley.com) DOI: 10.1002/asl.251
Impacts of formaldehyde photolysis rates on tropospheric
chemistry
M. C. Cooke,1* S. R. Utembe,1 P. Gorrotxategi Carbajo,1 A. T. Archibald,1 A. J. Orr-Ewing,1 M. E. Jenkin,1,2
R. G. Derwent,3 D. J. Lary4 and D. E. Shallcross1
1School of Chemistry, University of Bristol, Cantocks Close, Bristol BS8 1TS, UK
2Atmospheric Chemistry Services, Okehampton, Devon EX20 1FB, UK
3Rdscientific, Newbury, Berkshire, UK
4Goddard Earth Sciences and Technology Center, University of Maryland Baltimore County, Baltimore, MD, USA
*Correspondence to:
M. C. Cooke, School of Chemistry,
University of Bristol, Cantocks
Close, Bristol BS8 1TS, UK.
E-mail: chmcc@bristol.ac.uk
Received: 7 July 2009
Revised: 3 November 2009
Accepted: 11 November 2009
Abstract
A global chemistry transport model is employed to investigate the impact of recent
laboratory determinations of photolysis parameters for formaldehyde on concentrations
of tropospheric trace gases. Using the new laboratory data, the photolysis of formaldehyde
is a more significant removal pathway. HOx levels are increased with the greatest changes
towards the top of the troposphere and the poles, making formaldehyde a more significant
source of upper tropospheric HOx than previously thought. Global totals of ozone and
secondary organic aerosol increase with the rise in ozone being more significant at higher
solar zenith angles. Copyright  2010 Royal Meteorological Society
Keywords: formaldehyde; photolysis; global; modelling; HOx
1. Introduction
Formaldehyde is a ubiquitous constituent of the tro-
posphere that is produced by direct emission from
fossil fuel consumption and biomass burning and as
a product in the oxidation of volatile organic com-
pounds. Removal from the atmosphere is via reaction
with the hydroxyl radical (OH) {k (298 K) = 8.5×
10−12 cm3 molecule−1 s−1 (Sander et al., 2006)}, the
nitrate radical (NO3) {[(k (298 K) = 5.8× 10−16 cm3
molecule−1 s−1 (Sander et al., 2006)} and photolysis
reactions (1) and (2):
HCHO+ hv −−→ H2 + CO (1)
HCHO+ hv −−→ H+ HCO (2)
Once formed, HCO reacts rapidly with O2 to give
the hydroperoxyl radical (HO2) and CO, and H reacts
with O2 to give HO2. Reactions of HO2 with O3 and
NO result in the formation of OH. Formaldehyde is
thought to be a major source of OH at high-solar zenith
angles because of the longer wavelength threshold
of the radical channel relative to the photolysis of
O3. Formaldehyde is also of interest as a source of
HO2 and OH (usually collectively referred to as odd
OHs or HOx ) in the upper troposphere (UT) (Logan
et al., 1981). Over the past decade it has emerged
that measured HOx in the UT exceeds that predicted
by models and that a range of HOx precursors may
contribute to this discrepancy. Such precursors include
formaldehyde, alkyl peroxides, acetone and isoprene
derivatives that are thought to be transported from
lower altitudes (Brune, 1998; Collins et al., 1999;
Crawford et al., 1999; Colomb et al., 2006; Fried
et al., 2008; Jaegle´ et al., 2001). These studies have
indicated the importance of deep tropical convection
and convection from the marine boundary layer on the
UT HOx budget.
Formaldehyde is a key precursor, being formed from
the degradation of most hydrocarbons and any sub-
stantial change in photolysis parameters may have
a non-negligible impact on UT HOx. The current
recommended formaldehyde absorption cross-sections
[σ (λ)] by the NASA-JPL panel (Sander et al., 2006)
and the IUPAC panel (Atkinson et al., 2006) were
determined at a resolution of ∼0.025 nm by Meller
and Moortgat (2000). Measurements at a spectral res-
olution of 0.0032 nm have been conducted by Smith
et al. (2006) showing a greater peak of formalde-
hyde cross-section in the range 300–340 nm. Direct
detection of the HCO radical has been conducted
by Gorrotxategi Carbajo et al. (2008) allowing for
absolute quantum yield [#(λ)] determination for the
radical channel of photolysis. These studies thus pro-
vide improved quantification of the parameters, which
determine the atmospheric photolysis rate of formalde-
hyde, and which are likely to influence the NASA-JPL
and IUPAC recommendations. In the current study
a global chemistry transport model (CTM) is used
to investigate the impact of these new photolysis
measurements.
2. Model and simulations
The model used is an updated version of the UK Mete-
orological Office tropospheric CTM (STOCHEM)
Copyright  2010 Royal Meteorological Society

34 M. C. Cooke et al.
described by Collins et al. (1997), with updates
reported in detail in the recent paper of Utembe et al.
(2009a). STOCHEM is a global three-dimensional
CTM, which uses a Lagrangian approach to advect
50 000 air parcels using a fourth-order Runge-Kutta
scheme with advection time steps of 3 h. The transport
and radiation models are driven by archived meteo-
rological data, generated by the Met office numerical
weather prediction models as analysis fields with a res-
olution of 1.25◦ longitude and 0.83◦ latitude and on
12 vertical levels extending to 100 hPa. The model of
Derwent et al. (2008) has been employed with updates
to the photochemical mechanism and surface emis-
sions as described below.
The chemistry employed is the most reduced version
of the common representative intermediates mecha-
nism (CRIv2-R5) (Jenkin et al., 2008; Watson et al.,
2008; Utembe et al., 2009b), which represents the
chemistry of methane and 22 emitted non-methane
hydrocarbons. Each parcel contains the concentrations
of 219 species involved in 618 photolytic, gas phase
and heterogeneous chemical reactions, with a 5-min
chemistry time step. There are also 14 species rep-
resenting the formation of secondary organic aerosol
(SOA), which are derived from the oxidation of
aromatic hydrocarbons, monoterpenes and isoprene
(Utembe et al., 2009b).
The surface emissions (man-made, biomass burn-
ing, vegetation, oceans, soil and ‘other’ surface emis-
sions) are distributed using two-dimensional source
maps. Emission totals for CO, NOx and non-methane
hydrocarbons are taken from the Precursor of Ozone
and their Effects in the Troposphere (POET) inven-
tory (Granier et al., 2005). The emissions of aromatic
species ortho-xylene, benzene and toluene were taken
from Henze et al. (2008). Biomass burning emissions
of ethyne, formaldehyde and acetic acid were produced
using scaling factors from Andreae and Merlet (2001)
per mole of CO emitted. NASA inventories are used
for aircraft NOx emissions for 1992 taken from Penner
et al. (1999). The lightning and aircraft NOx emis-
sions are monthly averages and are three-dimensional
in distribution.
Three runs were performed to investigate the effect
of formaldehyde photolysis on atmospheric trace
gases. The only differences between the three runs
were the choice of source data for the photolysis cross-
sections and quantum yields for formaldehyde. The
first two runs used data from NASA-JPL recommen-
dations in 1992 (DeMore, 1992) and 2006 (Sander
et al., 2006). The third-run used data from Sander
et al. (2006), but substituted with the data of Gor-
rotxategi Carbajo et al. (2008) and Smith et al. (2006)
over the wavelength range from 300 to 330 nm. These
runs will subsequently be referred to as JPL92, JPL06
and CarJPL06, respectively. Each model run was per-
formed for 24 months with the first 12 months being
a spin-up.
Table I. Percentage removal of HCHO from the troposphere.
Reaction JPL92 (%) JPL06 (%) CarJPL06 (%)
hν (radical) 23.2 33.5 38.1 (+4.6)
hν (molecular) 37.7 28.5 25.3 (−3.2)
OH 39.1 37.9 36.6 (−1.3)
NO3 <0.1 <0.1 <0.1
Values in brackets indicate the changes between JPL06 and CarJPL06.
3. Results
Table I shows the tropospheric removal routes of
formaldehyde and the proportion of each route for the
three global simulations. The experimental data have
developed considerably between 1992 and 2006, JPL
reports because of higher resolution cross-section mea-
surements and more accurate techniques for evaluating
the quantum yield. This development has resulted in
the radical channel of photolysis being recognised as a
more important removal route (23.2–33.5%) with the
overall removal by photolysis also being more signif-
icant (60.9–62.0%).
The remainder of the discussion will look at the dif-
ferences between the current recommended values by
Sander et al. (2006) (run JPL06) and the incorporation
of the data of Gorrotxategi Carbajo et al. (2008) (run
CarJPL06). A high-resolution UV radiation model was
used by Gorrotxategi Carbajo et al. (2008) to calcu-
late J (HCO) increases of 25–30 (±15)% between 300
and 330 nm. Table I shows that the radical channel
has become a more significant removal route (increase
of 4.6%), and overall photolysis is a more dominant
removal process (62.0–63.4%). The increased impor-
tance of the radical channel results in an increase
in the concentration of both HO2 and OH, with the
later increasing by 1.5% on a globally averaged basis
(Table II). The increase in OH causes the emitted
VOCs and CO to be removed more rapidly from the
troposphere thus the concentration of these species has
reduced on a global scale, as presented in Table II.
Table II. The mass and percentage changes for some of the
main species.
Compound
GB (JPL06) – GB (CarJPL06)
(Gg)
GB %
change
CO −2.57× 103 −0.74
O3 8.14× 101 0.03
H2 −3.52× 103 −2.50
OH 3.88× 10−3 1.41
HO2 6.69× 10−1 2.51
HCHO −4.42× 101 −4.49
NOx −1.97× 10−3 −3.29
CH4 −8.0× 103 −0.21
C2H6 −1.73× 101 −1.28
C6H6 −1.77× 100 −2.02
C5H8 −2.48× 100 −1.61
β-Pinene −1.19× 10−1 −1.04
SOA 5.00× 100 2.74
Note: global burden (GB) and the mass values quoted are a yearly mean
in gigagrams.
Copyright  2010 Royal Meteorological Society Atmos. Sci. Let. 11: 33–38 (2010)

36 M. C. Cooke et al.
0
2
4
6
8
10
12
14
0 0.1 0.2 0.3 0.4 0.5
Al
tit
ud
e
(km
)
OH (pptv)
0
2
4
6
8
10
12
14
0 5 10 15 20 25 30
HO2 (pptv)
Trace−P China (24/02−10/04)
Measured
JPL06
CARJPL06
0
2
4
6
8
10
12
14
0 500 1000 1500 2000
HCHO (pptv)
Figure 3. Comparison of OH, HO2 and HCHO with airborne measurements taken from the data collection by Emmons et al.
(2000). Red circles represent the mean of the measurements with error bars being a standard deviation in either direction. Green
star and line represent the modelled value for run JPL06. Blue star and line represent the modelled value for run CarJPL06.
zenith angles relative to summer. HO2 is predomi-
nantly produced when OH is oxidised by VOCs and
via the photolysis of formaldehyde. At higher solar
zenith angles, formaldehyde plays a more important
role in HO2 production relative to OH oxidation. The
increased importance of the radical channel (run CAR-
JPL06) of photolysis results greater in HO2 produc-
tion. Ozone is produced because of an increase in HO2
via reactions (3–5).
HO2 + NO −−→ NO2 + OH (3)
NO2 + hv −−→ NO+ O(3P) (4)
O(3P) + O2 +M −−→ O3 +M (5)
The increase in ozone ranging from 0.8 to 1.2 ppb is
not observed in the summer months because the HO2
production is dominated by OH oxidation. Therefore,
the effect of formaldehyde on ozone production is
greatest in the winter months with its significance
gradually declining as summer approaches.
Figure 2(b) shows the zonal profile for the change
in HOx concentration. HOx levels are increased most
towards the top of the model. The largest increase in
HOx at the ground level is in the northern mid to
high latitudes between 40–90◦N and 75–90 ◦S in the
southern hemisphere, with a change between 3 and
4%. The northern hemisphere has a greater change in
HOx because the emissions of HCHO precursors from
anthropogenic sources are greatest between 30 and
60◦N. The largest increase at the top of the troposphere
is towards the poles, because HOx production from
HCHO is more significant at high-solar zenith angles.
Overall, in the UT, there is an increase of 6% for
HO2 and 5% for OH. These are modest changes, but
are in keeping with the idea that several precursors
contribute to the HOx discrepancies between models
and measurements. Of the HOx produced by acetone,
alkyl peroxides and formaldehyde in the top level
of the model approximately 85% is produced via
formaldehyde photolysis. However, it must be made
clear that acetone and some of the alkyl peroxides are
oxidised to formaldehyde in the photolytic mechanism
and contribute to this 85%. The remaining 15% of
HOx from these sources is via direct production
from ROOH photolysis and from acetone prior to
formaldehyde formation.
As a result of the changes in the formaldehyde
J -values, a significant decrease was noted in the pho-
tochemical source strength for H2 and subsequently in
its global burden. Apart from direct emissions, photo-
chemical production is the largest source term for this
trace gas Simmonds et al. (2000).
4. Measurement comparison
Figure 3 shows the model and measurement data for
the vertical profiles of the OH, hydroperoxy radical
and formaldehyde off the coast of China during
Trace-P. These measurements are for one aircraft
campaign but give an indication of the significance
of the updated photolysis parameters. The model is
capable of recreating the vertical profile observed
in these measurements and most values fall within
the measurement error bars. The OH and HO2 have
increased most significantly towards the top of the
model, 12 km and higher, moving the model closer
to the mean of the measurements. The reductions
in formaldehyde also move the model closer to the
measured values. Figure 3 clearly shows that the
formaldehyde photolysis data are not the major cause
of discrepancies between model and measurements
with regards to UT HOx , however, it is a worthy
update to current CTMs.
5. Conclusions
A global tropospheric CTM has been used to explore
the changes in tropospheric trace gases resulting
from recently reported photochemical parameters for
Copyright  2010 Royal Meteorological Society Atmos. Sci. Let. 11: 33–38 (2010)

Impact of formaldehyde photolysis 37
formaldehyde photolysis. We performed three simu-
lations using formaldehyde photochemical data from
DeMore (1992), Sander et al. (2006) and Gorrotxategi
Carbajo et al. (2008). The percentages of formalde-
hyde removal via photolysis are calculated to be 60.9,
62.0 and 63.4%, respectively. The branching to the
radical channel is more significant, with 23.2, 33.5
and 38.1% of formaldehyde global removal for the
runs JPL92, JPL06 and CarJPL06, respectively. Global
levels of OH and HO2 are simulated to be 1.41 and
2.51% higher for the CarJPL06 than for the JPL06
runs. HOx levels are increased most towards the top
of the troposphere and the poles, making formaldehyde
a more significant source of UT HOx than previously
thought. The increased HOx leads to a faster destruc-
tion of VOCs, with decreases of 0.21, 2.02 and 1.61%
in global mixing ratios of methane, benzene and iso-
prene, respectively. Globally, the computed ozone lev-
els increase by 0.03%. Ozone over polluted areas of
the northern hemisphere is found to be relatively unaf-
fected in July and August. However, during February,
as shown in Figure 2(a), when there are longer periods
of higher solar zenith angles, there is a 0.8- to 1.2-ppb
increase in ozone mixing ratios over the most polluted
areas.
Acknowledgements
This work was supported by EPSRC with studentship num-
ber CHEM.SB1729.6525 for MCC; UK Natural Environmen-
tal Research Council (NERC) support for SRU is gratefully
acknowledged through Grant NE/D001846/1, as part of the
QUEST Deglaciation project; ATA thanks the Met. Office and
GWR for funding and PGC acknowledges financial support
from the Marie Curie EU project BREATHE (MEST-CT-2004-
514499). AJOE thanks the Royal Society and Wolfson Foun-
dation for a Research Merit Award. The formaldehyde photo-
chemical data were obtained with support from NERC Grants
NER/T/S/2000/00294 and NE/D001498/1. The development of
STOCHEM was supported by UK Defra under their SSNIP
Contract AQ0902 to RGD.
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