A comprehensive emission inventory of biogenic volatile organic compounds in Europe: improved seasonality and land-cover

摘要

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, BVOC emissionare an important input for air quality models. To model these emissions withhigh spatial resolution, the accuracy of the underlying vegetation inventoryis crucial. We present a BVOC emission model that accommodates differentvegetation inventories and uses satellite-based measurements of greennessinstead of pre-defined vegetation periods. This approach to seasonalityimplicitly treats effects caused by water or nutrient availability, altitudeand latitude on a plant stand. Additionally, we test the influence ofproposed seasonal variability in enzyme activity on BVOC emissions. In itspresent setup, the emission model calculates hourly emissions of isoprene,monoterpenes, sesquiterpenes and the oxygenated volatile organic compounds(OVOC) methanol, formaldehyde, formic acid, ethanol, acetaldehyde, acetoneand acetic acid. In this study, emissions based on three different vegetationinventories are compared with each other and diurnal and seasonal variationsin Europe are investigated for the year 2006. Two of these vegetationinventories require information on tree-cover as an input. We compare threedifferent land-cover inventories (USGS GLCC, GLC2000 and Globcover 2.2) withrespect to tree-cover. The often-used USGS GLCC land-cover inventory leads toa severe reduction of BVOC emissions due to a potential miss-attribution ofbroad-leaved trees and reduced tree-cover compared to the two otherland-cover inventories. To account for uncertainties in the land-coverclassification, we introduce land-cover correction factors for each relevantland-use category to adjust the tree-cover. The results are very sensitive tothese factors within the plausible range. For June 2006, total monthly BVOCemissions decreased up to −27% with minimal and increased up to +71%with maximal 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 the annualemissions compared with non-adjusted data. The strongest reduction occurs inOVOC (up to −32%), the weakest in isoprene (as little as −19%). Ifalso enzyme seasonality is taken into account, however, isoprene reacts withthe steepest decrease of annual emissions, which are reduced by −44% to−49%, annual emissions of monoterpenes reduce between −30 and−35%. The sensitivity of the model to changes in temperature depends onthe climatic zone but not on the vegetation inventory. The sensitivity ishigher for temperature increases of 3 K (+31% to +64%) than decreasesby the same amount (−20 to −35%). The climatic zones "Cold exceptsummer" and "arid" are most sensitive to temperature changes in Januaryfor isoprene and monoterpenes, respectively, while in June, "polar" is mostsensitive to temperature for both isoprene and monoterpenes. Our modelpredicts the oxygenated volatile organic compounds to be the most abundantfraction of the annual European emissions (3571–5328 Gg yr),followed by monoterpenes (2964–4124 Gg yr), isoprene(1450–2650 Gg yr) and sesquiterpenes (150–257 Gg yr). Wefind 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 BVOC emissionsin Europe. Isoprene contributes between 18–21%, monoterpenes 33–36%and sesquiterpenes contribute 1–2%. We compare the concentrations ofbiogenic species simulated by an air quality model with measurements ofisoprene and monoterpenes in Hohenpeissenberg (Germany) for both summer andwinter. The agreement between observed and modelled concentrations is betterin summer than in winter. This can partly be explained with the difficulty tomodel weather conditions in winter accurately, but also with the increasedanthropogenic influence on the concentrations of BVOC compounds in winter.Our results suggest that land-cover inventories used to derive tree-covermust be chosen with care. Also, uncertainties in the classification ofland-cover pixels must be taken into account and remain high. This problemmust be addressed together with the remote sensing community. Our newapproach using a greenness index for addressing seasonality of vegetation canbe implemented easily in existing models. The importance of OVOC for airquality should be more deeply mbox{addressed} by future studies, especiallyin smog chambers. Also, the fate of BVOC from the dominant region of theIberian Peninsula should be studied more in detail.