Abstract
Abstract. Multi-generational oxidation of volatile organic compound (VOC) oxidation products can significantly alter the mass, chemical composition and properties of secondary organic aerosol (SOA) compared to calculations that consider only the first few generations of oxidation reactions. However, the most commonly used state-of-the-science schemes in 3-D regional or global models that account for multi-generational oxidation (1) consider only functionalization reactions but do not consider fragmentation reactions, (2) have not been constrained to experimental data and (3) are added on top of existing parameterizations. The incomplete description of multi-generational oxidation in these models has the potential to bias source apportionment and control calculations for SOA. In this work, we used the statistical oxidation model (SOM) of Cappa and Wilson (2012), constrained by experimental laboratory chamber data, to evaluate the regional implications of multi-generational oxidation considering both functionalization and fragmentation reactions. SOM was implemented into the regional University of California at Davis / California Institute of Technology (UCD/CIT) air quality model and applied to air quality episodes in California and the eastern USA. The mass, composition and properties of SOA predicted using SOM were compared to SOA predictions generated by a traditional two-product model to fully investigate the impact of explicit and self-consistent accounting of multi-generational oxidation.Results show that SOA mass concentrations predicted by the UCD/CIT-SOM model are very similar to those predicted by a two-product model when both models use parameters that are derived from the same chamber data. Since the two-product model does not explicitly resolve multi-generational oxidation reactions, this finding suggests that the chamber data used to parameterize the models captures the majority of the SOA mass formation from multi-generational oxidation under the conditions tested. Consequently, the use of low and high NOx yields perturbs SOA concentrations by a factor of two and are probably a much stronger determinant in 3-D models than multi-generational oxidation. While total predicted SOA mass is similar for the SOM and two-product models, the SOM model predicts increased SOA contributions from anthropogenic (alkane, aromatic) and sesquiterpenes and decreased SOA contributions from isoprene and monoterpene relative to the two-product model calculations. The SOA predicted by SOM has a much lower volatility than that predicted by the traditional model, resulting in better qualitative agreement with volatility measurements of ambient OA. On account of its lower-volatility, the SOA mass produced by SOM does not appear to be as strongly influenced by the inclusion of oligomerization reactions, whereas the two-product model relies heavily on oligomerization to form low-volatility SOA products. Finally, an unconstrained contemporary hybrid scheme to model multi-generational oxidation within the framework of a two-product model in which ageing reactions are added on top of the existing two-product parameterization is considered. This hybrid scheme formed at least 3 times more SOA than the SOM during regional simulations as a result of excessive transformation of semi-volatile vapors into lower volatility material that strongly partitions to the particle phase. This finding suggests that these hybrid multi-generational schemes should be used with great caution in regional models.
Highlights
Organic aerosol (OA) is generally the dominant component of submicrometer-sized atmospheric particulate matter (Jimenez et al, 2009), which plays an important role in the energy budget of the earth (Pachauri et al, 2014) and the health effects of air pollution (Bernstein et al, 2004)
Results show that secondary organic aerosol (SOA) mass concentrations predicted by the UCD/CIT-statistical oxidation model (SOM) model are very similar to those predicted by a two-product model when both models use parameters that are derived from the same chamber data
OA can be directly emitted to the atmosphere in particulate form or it can be formed in situ by the oxidation of volatile organic compounds (VOCs) to yield lower volatility products that condense into the aerosol phase, so-called secondary organic aerosol (SOA)
Summary
Organic aerosol (OA) is generally the dominant component of submicrometer-sized atmospheric particulate matter (Jimenez et al, 2009), which plays an important role in the energy budget of the earth (Pachauri et al, 2014) and the health effects of air pollution (Bernstein et al, 2004). Some models have used simple chemical schemes to mimic the effects of multi-generational oxidation While these schemes differ in their details, in essence, they assume that the vapors and the products of each surrogate traditional VOC species react with the hydroxyl radical (OH) to form lower volatility products (Robinson et al, 2007; Pye and Seinfeld, 2010; Baek et al, 2011). Note that oxidation of IVOCs and POA vapors is assumed to proceed only through these ageing-type reactions, whereas oxidation of the semi-volatile products of traditional VOC precursors is an augmentation to the existing two-product or VBS parameterization Models that include these ageing schemes predict SOA mass concentrations that close the gap with measured ambient concentrations of OA mass. We use the UCD/CIT-SOM model to investigate the influence of constrained multi-generational oxidation on the mass concentrations and properties of SOA and contrast those results against predictions from a traditional two-product model and an unconstrained multi-generational oxidation model
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