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Multi-generational oxidation of volatile organic compound (VOC) oxidationproducts can significantly alter the mass, chemical composition andproperties of secondary organic aerosol (SOA) compared to calculations thatconsider only the first few generations of oxidation reactions. However, themost commonly used state-of-the-science schemes in 3-D regional or globalmodels that account for multi-generational oxidation (1) consider onlyfunctionalization reactions but do not consider fragmentation reactions,(2) have not been constrained to experimental data and (3) are added on top ofexisting parameterizations. The incomplete description of multi-generationaloxidation in these models has the potential to bias source apportionment andcontrol calculations for SOA. In this work, we used the statistical oxidation model (SOM) of Cappa and Wilson (2012), constrained byexperimental laboratory chamber data, to evaluate the regional implicationsof multi-generational oxidation considering both functionalization andfragmentation 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 theeastern USA. The mass, composition and properties of SOA predicted using SOMwere compared to SOA predictions generated by a traditional two-productmodel to fully investigate the impact of explicit and self-consistentaccounting of multi-generational oxidation. Results show that SOA mass concentrations predicted by the UCD/CIT-SOM modelare very similar to those predicted by a two-product model when both modelsuse parameters that are derived from the same chamber data. Since thetwo-product model does not explicitly resolve multi-generational oxidationreactions, this finding suggests that the chamber data used to parameterizethe models captures the majority of the SOA mass formation frommulti-generational oxidation under the conditions tested. Consequently, theuse of low and high NOx yields perturbs SOA concentrations by a factorof two and are probably a much stronger determinant in 3-D models thanmulti-generational oxidation. While total predicted SOA mass is similar forthe SOM and two-product models, the SOM model predicts increased SOAcontributions from anthropogenic (alkane, aromatic) and sesquiterpenes anddecreased SOA contributions from isoprene and monoterpene relative to thetwo-product model calculations. The SOA predicted by SOM has a much lowervolatility than that predicted by the traditional model, resulting in betterqualitative agreement with volatility measurements of ambient OA. On accountof its lower-volatility, the SOA mass produced by SOM does not appear to beas 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 hybridscheme to model multi-generational oxidation within the framework of atwo-product model in which ageing reactions are added on top of theexisting two-product parameterization is considered. This hybrid schemeformed at least 3 times more SOA than the SOM during regionalsimulations as a result of excessive transformation of semi-volatile vaporsinto lower volatility material that strongly partitions to the particlephase. This finding suggests that these hybrid multi-generationalschemes should be used with great caution in regional models

atmospheric chemistry and physics

Simulating secondary organic aerosol in a regional air quality model using the statistical oxidation model – Part 1: Assessing the influence of constrained multi-generational ageing