A novel approach to emission modelling of biogenic volatile organic compounds in Europe: improved seasonality and land-cover

Abstract

Biogenic volatile organic compounds (BVOC) emitted from vegetation areimportant for the formation of secondary pollutants such as ozone andsecondary organic aerosols (SOA) in the atmosphere. Therefore, BVOCemission are an important input for air quality models. To model theseemissions with high spatial resolution, the accuracy of the underlyingvegetation inventory is crucial. We present a BVOC emission model thataccommodates different vegetation inventories and uses satellite-basedmeasurements of greenness instead of pre-defined vegetation periods.This approach to seasonality implicitly treats effects caused by wateror nutrient availability, altitude and latitude on a plant stand.Additionally, we test the influence of proposed seasonal variability inenzyme activity on BVOC emissions. In its present setup, the emissionmodel calculates hourly emissions of isoprene, monoterpenes,sesquiterpenes and the oxygenated volatile organic compounds (OVOC)methanol, formaldehyde, formic acid, ethanol, acetaldehyde, acetone andacetic acid. In this study, emissions based on three differentvegetation inventories are compared with each other and diurnal andseasonal variations in Europe are investigated for the year 2006. Two ofthese vegetation inventories require information on tree-cover as aninput. We compare three different land-cover inventories (USGS GLCC,GLC2000 and Globcover 2.2) with respect to tree-cover. The often-usedUSGS GLCC land-cover inventory leads to a severe reduction of BVOCemissions due to a potential miss-attribution of broad-leaved trees andreduced tree-cover compared to the two other land-cover inventories. Toaccount for uncertainties in the land-cover classification, we introduceland-cover correction factors for each relevant land-use category toadjust the tree-cover. The results are very sensitive to these factorswithin the plausible range. For June 2006, total monthly BVOC emissionsdecreased up to -27% with minimal and increased up to +71% withmaximal factors, while in January 2006, the changes in monthly BVOCemissions were -54 and +56% with minimal and maximal factors,respectively. The new seasonality approach leads to a reduction in theannual emissions compared with non-adjusted data. The strongestreduction occurs in OVOC (up to -32%), the weakest in isoprene (aslittle as -19%). If also enzyme seasonality is taken into account,however, isoprene reacts with the steepest decrease of annual emissions,which are reduced by -44% to -49%, annual emissions of monoterpenesreduce between -30 and -35%. The sensitivity of the model to changes intemperature depends on the climatic zone but not on the vegetationinventory. The sensitivity is higher for temperature increases of 3K(+31% to +64%) than decreases by the same amount (-20 to -35%). Theclimatic zones ``Cold except summer{''} and ``arid{''} are mostsensitive to temperature changes in January for isoprene andmonoterpenes, respectively, while in June, ``polar{''} is most sensitiveto temperature for both isoprene and monoterpenes. Our model predictsthe oxygenated volatile organic compounds to be the most abundantfraction of the annual European emissions (3571-5328 Gg yr(-1)),followed by monoterpenes (2964-4124 Gg yr(-1)), isoprene (1450-2650 Ggyr(-1)) and sesquiterpenes (150-257 Gg yr(-1)).We find regions with high isoprene emissions (most notably the IberianPeninsula), but overall, oxygenated VOC dominate with 43-45% (dependingon the vegetation inventory) contribution to the total annual BVOCemissions in Europe. Isoprene contributes between 18-21 %, monoterpenes33-36% and sesquiterpenes contribute 1-2%. We compare theconcentrations of biogenic species simulated by an air quality modelwith measurements of isoprene and monoterpenes in Hohenpeissenberg(Germany) for both summer and winter. The agreement between observed andmodelled concentrations is better in summer than in winter. This canpartly be explained with the difficulty to model weather conditions inwinter accurately, but also with the increased anthropogenic influenceon the concentrations of BVOC compounds in winter. Our results suggestthat land-cover inventories used to derive tree-cover must be chosenwith care. Also, uncertainties in the classification of land-coverpixels must be taken into account and remain high. This problem must beaddressed together with the remote sensing community. Our new approachusing a greenness index for addressing seasonality of vegetation can beimplemented easily in existing models. The importance of OVOC for airquality should be more deeply addressed by future studies, especially insmog chambers. Also, the fate of BVOC from the dominant region of theIberian Peninsula should be studied more in detail.