Modeling SOA formation from the oxidation of intermediate volatility &amp;lt;i&amp;gt;n&amp;lt;/i&amp;gt;-alkanes

B Aumont,J Lee-Taylor,C Mouchel-Vallon,R Valorso,S Madronich,M Camredon

doi:10.5194/acp-12-7577-2012

Abstract

Abstract. The chemical mechanism leading to SOA formation and ageing is expected to be a multigenerational process, i.e. a successive formation of organic compounds with higher oxidation degree and lower vapor pressure. This process is here investigated with the explicit oxidation model GECKO-A (Generator of Explicit Chemistry and Kinetics of Organics in the Atmosphere). Gas phase oxidation schemes are generated for the C8–C24 series of n-alkanes. Simulations are conducted to explore the time evolution of organic compounds and the behavior of secondary organic aerosol (SOA) formation for various preexisting organic aerosol concentration (COA). As expected, simulation results show that (i) SOA yield increases with the carbon chain length of the parent hydrocarbon, (ii) SOA yield decreases with decreasing COA, (iii) SOA production rates increase with increasing COA and (iv) the number of oxidation steps (i.e. generations) needed to describe SOA formation and evolution grows when COA decreases. The simulated oxidative trajectories are examined in a two dimensional space defined by the mean carbon oxidation state and the volatility. Most SOA contributors are not oxidized enough to be categorized as highly oxygenated organic aerosols (OOA) but reduced enough to be categorized as hydrocarbon like organic aerosols (HOA), suggesting that OOA may underestimate SOA. Results show that the model is unable to produce highly oxygenated aerosols (OOA) with large yields. The limitations of the model are discussed.

Highlights

Fossil fuel and biomass combustion leads to the emission of long carbon chain hydrocarbons (C>10) in the atmosphere
After 10 days of atmospheric oxidation, the carbon initially present as octane mainly ends up in the form of CO and CO2, the remaining fraction being mostly gas phase organic carbon
Simulation results show that (i) secondary organic aerosols (SOA) yield increases with the carbon chain length of the parent hydrocarbon, (ii) SOA yield decreases with decreasing COA, (iii) SOA production rates are faster with increasing COA, and (iv) the number of oxidation steps needed to describe SOA formation and evolution grows when COA decreases

Summary

Introduction

Fossil fuel and biomass combustion leads to the emission of long carbon chain hydrocarbons (C>10) in the atmosphere. A large fraction of these Intermediate Volatility Organic Compounds (IVOC) is expected to be first emitted in the condensed phase and rapidly volatilized by atmospheric dilution (Robinson et al, 2007). The SOA formation potential was recently examined in laboratory studies for few IVOC precursors, mostly alkanes (Lim and Ziemann, 2009; Presto et al, 2010), alkenes (Matsunaga et al, 2009), aromatics (Chan et al, 2009) and oxygenated IVOC (ChaconMadrid and Donahue, 2011). These experimental studies confirmed the high SOA formation potential for such precursors. Recent numerical simulations have shown that IVOC are likely a substantial source of SOA in the plume of megacities (e.g. Dzepina et al, 2011; Tsimpidi et al, 2010; Hodzic et al, 2010; Li et al, 2011; Lee-Taylor et al, 2011) and at continental scales (Jathar et al, 2011)

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