Abstract
Abstract. The role of isoprene as a precursor to secondary organic aerosol (SOA) over Europe is studied with the two-way nested global chemistry transport model TM5. The inclusion of the formation of SOA from isoprene oxidation in our model almost doubles the atmospheric burden of SOA over Europe compared to SOA formation from terpenes and aromatics. The reference simulation, which considers SOA formation from isoprene, terpenes and aromatics, predicts a yearly European production rate of 1.0 Tg SOA yr−1 and an annual averaged atmospheric burden of about 50 Gg SOA over Europe. A fraction of 35% of the SOA produced in the boundary layer over Europe is transported to higher altitudes or to other world regions. Summertime measurements of organic matter (OM) during the extensive EMEP OC/EC campaign 2002/2003 are better reproduced when SOA formation from isoprene is taken into account, reflecting also the strong seasonality of isoprene and other biogenic volatile organic compounds (BVOC) emissions from vegetation. However, during winter, our model strongly underestimates OM, likely caused by missing wood burning in the emission inventories. Uncertainties in the parameterisation of isoprene SOA formation have been investigated. Maximum SOA production is found for irreversible sticking (non-equilibrium partitioning) of condensable vapours on particles, with tropospheric SOA production over Europe increased by a factor of 4 in summer compared to the reference case. Completely neglecting SOA formation from isoprene results in the lowest estimate (0.51 Tg SOA yr−1). The amount and the nature of the absorbing matter are shown to be another key uncertainty when predicting SOA levels. Consequently, smog chamber experiments on SOA formation should be performed with different types of seed aerosols and without seed aerosols in order to derive an improved treatment of the absorption of SOA in the models. Consideration of a number of recent insights in isoprene SOA formation mechanisms reduces the tropospheric production of isoprene derived SOA over Europe from 0.4 Tg yr−1 in our reference simulation to 0.1 Tg yr−1.
Highlights
Global isoprene emissions are estimated to amount to approximately 250–750 Tg C yr−1, accounting for the majority of non-methane volatile organic compounds (NMVOC) emissions (Guenther et al, 1995, 2006)
We focus on secondary organic aerosol (SOA) production in terms of “traditional” SOA, originating from the photo-oxidation of individual VOC like isoprene, terpenes, and aromatics
As we focus on the uncertainties of isoprene SOA production, the NOx-dependence for compounds other than isoprene is not considered in our simulations, but they are expected to be of importance as well (Tsigaridis et al, 2006)
Summary
Global isoprene emissions are estimated to amount to approximately 250–750 Tg C yr−1, accounting for the majority of non-methane volatile organic compounds (NMVOC) emissions (Guenther et al, 1995, 2006). Identified major uncertainties that influence the yields of SOA from isoprene and monoterpenes are the levels of ambient NOx concentrations or VOC/NOx ratios (Presto et al, 2005b; Presto and Donahue, 2006; Kroll et al, 2006), the temperature dependence of the partitioning coefficient (Offenberg et al, 2006), relative humidity (Jang and Kamens, 1998; Cocker et al, 2001), the role of acid-catalyzed heterogeneous reactions of carbonyl products (Czoschke et al, 2003; Iinuma et al, 2004; Czoschke and Jang, 2006), the possible formation of high molecular weight products (oligomers, humic like substances) (Kalberer et al, 2006; Surratt et al, 2006; Heaton et al, 2007), the role of sulphate aerosol concentration in the condensation process (Kleindienst et al, 2006; Kroll et al, 2007), the effect of UV-light (Presto et al, 2005a), and the absorption properties of pre-existing carbonaceous particulate matter We assess to what extend the new results can be reconciled with particulate organic matter observations from the EMEP OC/EC campaign in 2002–2003 (Yttri et al, 2007)
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