Isoprene Secondary Organic Aerosol in a Global Chemistry Climate Model

Abstract

Atmospheric aerosol influences the climate system, modifying atmospheric radiation directly and indirectly. Up to 90% to the total organic aerosol is formed through the oxidation of hydrocarbons and subsequent nucleation, condensation or chemical uptake onto pre-existing aerosols, defined as secondary organic aerosol (SOA). Due to thousands of individual compounds involved in SOA formation, modeling SOA related processes on a global scale is challenging. Understanding the formation of SOA is crucial to estimate its impact on the climate system, thus global models try to simulate SOA formation with different approaches. Usually, a detailed chemistry and evolution of single compounds is disregarded, due to computational limitations. Within the framework of the global chemistry climate model ECHAM-HAMMOZ, a novel explicit coupling between the sectional aerosol model HAM-SALSA and the chemistry model MOZ was established to form isoprene derived secondary organic aerosol (iSOA). Isoprene oxidation in the chemistry model MOZ is described by a semi-explicit scheme consisting of 147 reactions,embedded in a detailed atmospheric chemical mechanism with a total of 779 reactions. Low- and semi-volatile compounds produced during isoprene photo-oxidation are identified and explicitly partitioned by HAM-SALSA. Furthermore, reactive uptake of isoprene epoxidiols (IEPOX) and isoprene derived glyoxal were included as iSOA sources. With this method, every single precursor is tracked in terms of condensation, evaporation and reactive uptake in each aerosolsize bin. This approach allows the investigation of iSOA composition and its dependence on chemical regimes, aerosol acidity, choice of saturation concentration and evaporation enthalpyof each single compound. Isoprene dihydroxy dihydroperoxide (ISOP(OOH)2) and IEPOX were identified as main contributors to iSOA formation. Further study of IEPOX reactive uptake on aerosols with different pH values showed the competition between IEPOX uptake enhancementby acidic aerosol and NOx-suppression of IEPOX formation in polluted areas. Moreover, new aerosol sinks were introduced as iSOA photolysis and thermal decomposition of ISOP(OOH)2.This model framework, connecting semi-explicit isoprene oxidation with explicit treatment of aerosol tracers, leads to a global, annual isoprene SOA yield of 15 %, relative to the primary oxidation of isoprene by OH, NO3 and O3. In the modeled year 2012, 445.1 Tg (392.1 TgC) isoprene are emitted and an iSOA source of 138.5 Tg (56.7 TgC) is simulated. IEPOX contributes 42.4 Tg (21.0 TgC) and ISOP(OOH)2 78.0 Tg (27.9 TgC) to iSOA in ECHAM-HAMMOZ. The main sink process is particle wet deposition which removes 133.6 Tg (54.7 TgC). The iSOA burden reaches 1.4 Tg (0.6 TgC) . The model iSOA concentrations compare well to observed organic aerosol concentrations in regions where isoprene emissions are high.