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)
Authors: Kelly Jamie M. , Doherty Ruth M. , O'Connor Fiona M. , Mann Graham W. , Coe Hugh , Liu Dantong 
Published: June 2019

Abstract. The representation of volatile organic compound (VOC) deposition and
oxidation 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 SOA
budget and model agreement with observations are quantified. Firstly, global
model simulations were performed with varying VOC dry deposition and wet
deposition fluxes. Including VOC dry deposition reduces the global
annual-total SOA production rate by 2 %–32 %, with the range
reflecting uncertainties in surface resistances. Including VOC wet deposition
reduces the global annual-total SOA production rate by 15 % and is
relatively insensitive to changes in effective Henry's law coefficients. Without precursor deposition, simulated SOA concentrations are lower than observed
with a normalised mean bias (NMB) of −51 %. Hence, including SOA
precursor deposition worsens model agreement with observations even further
(NMB =-66 %). Secondly, for the anthropogenic and biomass burning VOC
precursors of SOA (VOCANT∕BB), model simulations were performed
by (a) varying the parent hydrocarbon reactivity, (b) varying the number of reaction
intermediates, and (c) accounting for differences in volatility between
oxidation products from various pathways. These changes were compared to a
scheme where VOCANT∕BB adopts the reactivity of a monoterpene
(α-pinene), and is oxidised in a single-step mechanism with a fixed
SOA yield. By using the chemical reactivity of either benzene, toluene, or
naphthalene for VOCANT∕BB, the global annual-total
VOCANT∕BB oxidation rate changes by −3 %, −31 %, or
−66 %, respectively, compared to when using α-pinene. Increasing
the number of reaction intermediates, by introducing a peroxy radical
(RO2), slightly slows the rate of SOA formation, but has no impact on
the global annual-total SOA production rate. However, RO2 undergoes
competitive oxidation reactions, forming products with substantially
different volatilities. Accounting for the differences in product volatility
between RO2 oxidation pathways increases the global SOA production
rate by 153 % compared to using a single SOA yield. Overall, for
relatively reactive compounds such as toluene and naphthalene, the reduction
in reactivity for VOCANT∕BB oxidation is outweighed by accounting
for the difference in volatility of RO2 products, leading to a net
increase in the global annual-total SOA production rate of 85 % and
145 %, respectively, and improvements in model agreement (NMB of
−46 % and 56 %, respectively). However, for benzene, the reduction
in VOCANT∕BB oxidation is not outweighed by accounting for the
difference in SOA yield pathways, leading to a small change in the global
annual-total SOA production rate of −3 %, and a slight worsening of
model agreement with observations (NMB =-77 %). These results
highlight that variations in both VOC deposition and oxidation mechanisms
contribute to substantial uncertainties in the global SOA budget and model
agreement with observations.