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Discussion papers | Copyright
https://doi.org/10.5194/gmd-2018-142
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Development and technical paper 20 Jul 2018

Development and technical paper | 20 Jul 2018

Review status
This discussion paper is a preprint. It is a manuscript under review for the journal Geoscientific Model Development (GMD).

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)

Jamie M. Kelly1, Ruth M. Doherty1, Fiona M. O'Connor2, Graham W. Mann3, Hugh Coe4, and Dantong Liu4 Jamie M. Kelly et al.
  • 1School of GeoSciences, The University of Edinburgh, UK
  • 2Met Office Hadley Centre, Exeter, UK
  • 3National Centre for Atmospheric Science, School of Earth and Environment, University of Leeds, Leeds, UK
  • 4Centre for Atmospheric Sciences, School of Earth and Environmental Sciences, University of Manchester, Manchester, UK

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 is quantified. Firstly, global model simulations were performed with varying VOC dry deposition and wet deposition. 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. With 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 varying: a) the parent hydrocarbon reactivity, b) 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 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 monoterpene. 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 improvemtns 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 observatiobs (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.

Jamie M. Kelly et al.
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Jamie M. Kelly et al.
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Short summary
This study develops the representation of secondary organic aerosol (SOA) within a global chemistry-climate model (UKCA). Both dry and wet deposition within the UKCA model are extended to consider precursors of SOA. The oxidation mechanism describing SOA formation is also extended by implementing a multigenerational oxidation mechanism, with SOA yields which are dependent on oxidant concentrations.
This study develops the representation of secondary organic aerosol (SOA) within a global...
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