Isoprene-derived secondary organic aerosol in the global aerosol-chemistry-climate model ECHAM6.3.0-HAM2.3-MOZ1.0

Abstract

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. Semi-volatile and low-volatile compounds produced during isoprene photooxidation are identified and explicitly partitioned by HAM-SALSA. A group contribution method was used to estimate their evaporation enthalpies and corresponding saturation vapor pressures, which are used by HAM-SALSA to calculate the saturation concentration of each iSOA precursor. With this method, every single precursor is tracked in terms of condensation and evaporation in each aerosol size bin. This approach led to the identification of dihydroxy dihydroperoxide (ISOP(OOH)2) as a main contributor to iSOA formation. Further, the reactive uptake of isoprene epoxydiols (IEPOXs) and isoprene-derived glyoxal were included as iSOA sources. The parameterization of IEPOX reactive uptake includes a dependency on aerosol pH value. This model framework connecting semi-explicit isoprene oxidation with explicit treatment of aerosol tracers leads to a global annual average isoprene SOA yield of 15% relative to the primary oxidation of isoprene by OH, NO3 and ozone. With 445.1Tg (392.1TgC) isoprene emitted, an iSOA source of 138.5Tg (56.7TgC) is simulated. The major part of iSOA in ECHAM–HAMMOZ is produced by IEPOX at 42.4Tg (21.0TgC) and ISOP(OOH)2 at 78.0Tg (27.9TgC). The main sink process is particle wet deposition, which removes 133.6 (54.7TgC). The average iSOA burden reaches 1.4Tg (0.6TgC) in the year 2012.