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. The representation of volatile organic compound (VOC) deposition andoxidation mechanisms in the context of secondary organic aerosol (SOA)formation are developed in the United Kingdom Chemistry and Aerosol (UKCA)chemistry–climate model. Impacts of these developments on both the global SOAbudget and model agreement with observations are quantified. Firstly, globalmodel simulations were performed with varying VOC dry deposition and wetdeposition fluxes. Including VOC dry deposition reduces the globalannual-total SOA production rate by 2 %–32 %, with the rangereflecting uncertainties in surface resistances. Including VOC wet depositionreduces the global annual-total SOA production rate by 15 % and isrelatively insensitive to changes in effective Henry's law coefficients. Without precursor deposition, simulated SOA concentrations are lower than observedwith a normalised mean bias (NMB) of −51  %. Hence, including SOAprecursor deposition worsens model agreement with observations even further(NMB  = - 66  %). Secondly, for the anthropogenic and biomass burning VOCprecursors of SOA (VOC ANT∕BB ), model simulations were performedby (a) varying the parent hydrocarbon reactivity, (b) varying the number of reactionintermediates, and (c) accounting for differences in volatility betweenoxidation products from various pathways. These changes were compared to ascheme where VOC ANT∕BB adopts the reactivity of a monoterpene( α -pinene), and is oxidised in a single-step mechanism with a fixedSOA yield. By using the chemical reactivity of either benzene, toluene, ornaphthalene for VOC ANT∕BB , the global annual-totalVOC ANT∕BB oxidation rate changes by −3  %, −31  %, or −66  %, respectively, compared to when using α -pinene. Increasingthe number of reaction intermediates, by introducing a peroxy radical( RO2 ), slightly slows the rate of SOA formation, but has no impact onthe global annual-total SOA production rate. However, RO2 undergoescompetitive oxidation reactions, forming products with substantiallydifferent volatilities. Accounting for the differences in product volatilitybetween RO2 oxidation pathways increases the global SOA productionrate by 153 % compared to using a single SOA yield. Overall, forrelatively reactive compounds such as toluene and naphthalene, the reductionin reactivity for VOC ANT∕BB oxidation is outweighed by accountingfor the difference in volatility of RO2 products, leading to a netincrease in the global annual-total SOA production rate of 85 % and145 %, respectively, and improvements in model agreement (NMB of −46  % and 56 %, respectively). However, for benzene, the reductionin VOC ANT∕BB oxidation is not outweighed by accounting for thedifference in SOA yield pathways, leading to a small change in the globalannual-total SOA production rate of −3  %, and a slight worsening ofmodel agreement with observations (NMB  = - 77  %). These resultshighlight that variations in both VOC deposition and oxidation mechanismscontribute to substantial uncertainties in the global SOA budget and modelagreement with observations.

geoscientific model development

The roles of volatile organic compound deposition and oxidation mechanisms in determining secondary organic aerosol production: a global perspective using the UKCA chemistry–climate model (vn8.4)