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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 (VOC ANT/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 VOC ANT/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 VOC ANT/BB , the global annual-total VOC ANT/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 VOC ANT/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 VOC ANT/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. [ABSTRACT FROM AUTHOR]

In this study we examine the meteorological drivers resulting in concurrent high levels of ozone (O 3) and particulate matter smaller than 2.5 μm in diameter (PM 2.5) during two five-day air pollution episodes in 2006 (1st - 5th July and 18th – 22nd July) using an air quality model (AQUM) at 12 km horizontal resolution to simulate air pollutant concentrations. The resultant UK health burden associated with short-term exposure to simulated maximum daily 8-h O 3 (MDA8 O 3) and daily mean PM 2.5 is estimated at the national and regional level. Both episodes were found to be driven by anticyclonic conditions with light easterly and south easterly winds and high temperatures that aided pollution build up in the UK. The estimated total mortality burden associated with short-term exposure to MDA8 O 3 is similar during the chosen episodes with about 70 daily deaths brought forward (summed across the UK) during the first and second episode, respectively. The estimated health burden associated with short-term exposure to daily mean PM 2.5 concentrations differs between the first and second episode resulting in about 43 and 36 daily deaths brought forward, respectively. The corresponding percentage of all-cause mortality due to short-term exposure to MDA8 O 3 and daily mean PM 2.5 during these two episodes and across the UK regions, ranges from 3.4% to 5.2% and from 1.6% to 3.9%, respectively. The attributable percentage of all-cause mortality differs between the regions depending on the pollution levels in each episode, but the overall estimated health burdens are highest in regions with higher population totals. We estimate that during these episodes the short-term exposure to MDA8 O 3 and daily mean PM 2.5 is between 36-38% and 39–56% higher, respectively, than if the pollution levels represented typical seasonal-mean concentrations. This highlights the potential of air pollution episodes to have substantial short-term impacts on public health. • Both air pollution episodes are driven by anticyclonic conditions, light E/SE winds and high temperatures. • The estimated mortality burden associated with short-term exposure to MDA8 O 3 is up to 70 daily deaths brought forward. • The estimated mortality burden associated with short-term exposure to PM 2.5 is up to 45 daily deaths brought forward. • The corresponding percentage of all-cause mortality due to short-term exposure to MDA8 O 3 ranges from 3.4% to 5.2%. • The corresponding percentage of all-cause mortality due to short-term exposure to daily mean PM 2.5 ranges from 1.6% to 3.9% • Short-term exposure to O 3 and PM 2.5 is between 36-56% higher during these episodes compared to summer air pollution levels [ABSTRACT FROM AUTHOR]