Observation-Constrained Kinetic Modelling of Isoprene SOA Formation in the Atmosphere

Abstract

Abstract. Isoprene has the largest global non-methane hydrocarbon emission, and the oxidation of isoprene plays a crucial role in the formation of secondary organic aerosols (SOA). Two primary processes are known to contribute to SOA formation from isoprene oxidation: (1) the reactive uptake of isoprene-derived epoxides on acidic or aqueous particle surfaces and (2) the absorptive gas-particle partitioning of low-volatility oxidation products. In this study, we developed a new multiphase condensed isoprene oxidation mechanism that include these processes with key molecular intermediates and products. The new mechanism was applied to simulate isoprene gas-phase oxidation products and SOA formation from previously published chamber experiments under a variety of conditions and atmospheric observations during the Southern Oxidant and Aerosol Studies (SOAS) field campaign. Our results show that SOA formation from most of the chamber experiments is reasonably reproduced using our mechanism except when the concentration ratios of initial nitric oxide to isoprene exceeds ~2. The SOAS simulations also reasonably agree with the measurements regarding the diurnal pattern and concentrations of different product categories. The molecular compositions of the modelled SOA indicate that multifunctional low-volatility products contribute to isoprene SOA more significantly than previously thought, with a median mass contribution of ~57 % to the total modelled isoprene SOA. This contribution, however, may vary greatly, mainly dependent on the volatility estimation and treatment of particle-phase processes (i.e., photolysis and hydrolysis). Our findings emphasize that the various pathways to produce these low-volatility species should be considered in models to more accurately predict isoprene SOA formation. The new condensed isoprene chemical mechanism can be further incorporated into regional-scale air quality models, such as the Community Multiscale Air Quality Modelling System (CMAQ), to assess isoprene SOA formation on a larger scale.

Thermodynamics and Kinetics of Atmospherically Relevant Acetalization Reactions

“…4 While the complete molecular characterization of SOA remains a challenge, it is now clear that these processes lead to large proportions of low volatility species, 5 particularly for SOA derived from isoprene. 6 However, the particular mechanisms responsible for low volatility species formation remain largely unknown.…”

“…Secondary organic aerosol (SOA), a dominant component in the global aerosol burden, is often formed via multistep atmospheric processing of gaseous biogenic and anthropogenic precursor species . While the complete molecular characterization of SOA remains a challenge, it is now clear that these processes lead to large proportions of low volatility species, particularly for SOA derived from isoprene . However, the particular mechanisms responsible for low volatility species formation remain largely unknown.…”