Biogenic SOA formation through gas-phase oxidation and gas-to-particle partitioning – a comparison between process models of varying complexity

E Hermansson,A Rusanen,E Swietlicki,P Roldin,D Mogensen,M Boy,R Väänänen,N Kivekäs

doi:10.5194/acp-14-11853-2014

Abstract

Abstract. Biogenic volatile organic compounds (BVOCs) emitted by vegetation play an important role for aerosol mass loadings since the oxidation products of these compounds can take part in the formation and growth of secondary organic aerosols (SOA). The concentrations and properties of BVOCs and their oxidation products in the atmosphere are poorly characterized, which leads to high uncertainties in modeled SOA mass and properties. In this study, the formation of SOA has been modeled along an air-mass trajectory over northern European boreal forest using two aerosol dynamics box models where the prediction of the condensable organics from the gas-phase oxidation of BVOC is handled with schemes of varying complexity. The use of box model simulations along an air-mass trajectory allows us to compare, under atmospheric relevant conditions, different model parameterizations and their effect on SOA formation. The result of the study shows that the modeled mass concentration of SOA is highly dependent on the organic oxidation scheme used to predict oxidation products. A near-explicit treatment of organic gas-phase oxidation (Master Chemical Mechanism version 3.2) was compared to oxidation schemes that use the volatility basis set (VBS) approach. The resulting SOA mass modeled with different VBS schemes varies by a factor of about 7 depending on how the first-generation oxidation products are parameterized and how they subsequently age (e.g., how fast the gas-phase oxidation products react with the OH radical, how they respond to temperature changes, and if they are allowed to fragment during the aging process). Since the VBS approach is frequently used in regional and global climate models due to its relatively simple treatment of the oxidation products compared to near-explicit oxidation schemes, a better understanding of the above-mentioned processes is needed. Based on the results of this study, fragmentation should be included in order to obtain a realistic SOA formation. Furthermore, compared to the most commonly used VBS schemes, the near-explicit method produces less – but more oxidized – SOA.

Highlights

Secondary organic aerosols (SOA), formed through gas-toparticle partitioning in the atmosphere, constitute a large part of the global organic aerosol load (Crippa et al, 2014; Hallquist et al, 2009; Jimenez et al, 2009) and affect the climate by absorbing or scattering radiation and acting as cloud condensation nuclei (CCN) (Scott et al, 2014)
The formation of secondary organic aerosols (SOA) has been modeled along an air-mass trajectory over northern European boreal forest using two aerosol dynamics box models where the prediction of the condensable organics from the gas-phase oxidation of Biogenic volatile organic compounds (BVOCs) is handled with schemes of varying complexity
The resulting SOA mass modeled with different volatility basis set (VBS) schemes varies by a factor of about 7 depending on how the first-generation oxidation products are parameterized and how they subsequently age

Summary

Introduction

Secondary organic aerosols (SOA), formed through gas-toparticle partitioning in the atmosphere, constitute a large part of the global organic aerosol load (Crippa et al, 2014; Hallquist et al, 2009; Jimenez et al, 2009) and affect the climate by absorbing or scattering radiation and acting as cloud condensation nuclei (CCN) (Scott et al, 2014). It has been common practice in global model studies to simplify the gas-phase oxidation using only two hypothetical oxidation products with laboratory-constrained equilibrium partitioning coefficients to estimate the formation of SOA (Kroll and Seinfeld, 2008), based on the two-product model developed by Odum et al (1996) This method does not account for continuous oxidation in the gas- or particle-phase, and atmospheric models that use this method generally underestimate organic aerosol mass (e.g., Heald et al, 2005; Spracklen et al, 2011; Volkamer et al, 2006). The above-mentioned approaches are empirical or semi-empirical and rely on experimental studies conducted under environmental conditions that resemble those in the atmosphere; conditions which might change during the SOA formation process and during future climate warming

Methods

Results

Conclusion