Abstract

Abstract. The interactions between biogenic volatile organic compounds (BVOCs), like isoprene and monoterpenes, and anthropogenic emissions of nitrogen and sulfur oxides lead to high concentrations of secondary organic aerosol (SOA) in the southeastern United States. To improve our understanding of SOA formation, we study the diurnal evolution of SOA in a land–atmosphere coupling context based on comprehensive surface and upper air observations from a characteristic day during the 2013 Southern Oxidant and Aerosol Study (SOAS) campaign. We use a mixed layer model (MXLCH-SOA) that is updated with new chemical pathways and an interactive land surface scheme that describes both biogeochemical and biogeophysical couplings between the land surface and the atmospheric boundary layer (ABL) to gain insight into the drivers of the daytime evolution of biogenic SOA. MXLCH-SOA reproduces observed BVOC and surface heat fluxes, gas-phase chemistry, and ABL dynamics well, with the exception of isoprene and monoterpene mixing ratios measured close to the land surface. This is likely due to the fact that these species do not have uniform profiles throughout the atmospheric surface layer due to their fast reaction with OH and incomplete mixing near the surface. The flat daytime evolution of the SOA concentration is caused by the dampening of the increase due to locally formed SOA by entrainment of SOA-depleted air from the residual layer. SOA formation from isoprene through the intermediate species isoprene epoxydiols (IEPOXs) and isoprene hydroxyhydroperoxides (ISOPOOHs) is in good agreement with the observations, with a mean isoprene SOA yield of 1.8 %. However, SOA from monoterpenes, oxidised by OH and O3, dominates the locally produced SOA (69 %), with a mean monoterpene SOA yield of 10.7 %. Isoprene SOA is produced primarily through OH oxidation via ISOPOOH and IEPOX (31 %). Entrainment of aged SOA from the residual layer likely contributes to the observed more oxidised oxygenated organic aerosol (MO-OOA) factor. A sensitivity analysis of the coupled land surface–boundary layer–SOA formation system to changing temperatures reveals that SOA concentrations are buffered under increasing temperatures: a rise in BVOC emissions is offset by decreases in OH concentrations and the efficiency with which SVOCs partition into the aerosol phase.

Highlights

  • Secondary organic aerosol (SOA) produced from the oxidation of volatile organic compounds (VOCs) forms an important contribution to aerosol loading (Jimenez et al, 2009; Zhang et al, 2007)

  • We find that a uniform SOA concentration in the atmospheric boundary layer (ABL) and residual layer (RL) no longer leads to a drop in SOA due to growth of the ABL, but leads to overestimated values compared to the observations during the end of the afternoon

  • We studied the diurnal evolution of biogenic secondary organic aerosol formed from daytime sources in the southeastern US by combining the MXLCH-SOA model with observations from the Southern Oxidant and Aerosol Study (SOAS) campaign

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Summary

Introduction

Secondary organic aerosol (SOA) produced from the oxidation of volatile organic compounds (VOCs) forms an important contribution to aerosol loading (Jimenez et al, 2009; Zhang et al, 2007). A large fraction of SOA is formed by biogenic volatile organic compounds (BVOCs), which are emitted in large quantities from forested areas, especially during summer (Guenther et al, 1995; Goldstein et al, 2009). SOA mass in this region has a high biogenic contribution (Ahmadov et al, 2012; Kim et al, 2015)

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