Impact of HO2 aerosol uptake on radical levels and O3 production during summertime in Beijing
Authors: Dyson Joanna E. , Whalley Lisa K. , Slater Eloise J. , Woodward-Massey Robert , Ye Chunxiang , Lee James D. , Squires Freya , Hopkins James R. , Dunmore Rachel E. , Shaw Marvin , Hamilton Jacqueline F. , Lewis Alastair C. , Worrall Stephen D. , Bacak Asan , Mehra Archit , Bannan Thomas J. , Coe Hugh , Percival Carl J. , Ouyang Bin , Hewitt C. Nicholas , Jones Roderic L. , Crilley Leigh R. , Kramer Louisa J. , Acton W. Joe F. , Bloss William J. , Saksakulkrai Supattarachai , Xu Jingsha , Shi Zongbo , Harrison Roy M. , Kotthaus Simone , Grimmond Sue , Sun Yele , Xu Weiqi , Yue Siyao , Wei Lianfang , Fu Pingqing , Wang Xinming , Arnold Stephen R. , Heard Dwayne E. 
Published: May 2023

Abstract. The impact of heterogeneous uptake of HO2 on aerosol surfaces on radical concentrations and the O3 production regime in Beijing in summertime was investigated. The uptake coefficient of HO2 onto aerosol surfaces, γHO2, was calculated for the AIRPRO
campaign in Beijing, in summer 2017, as a function of measured aerosol soluble copper concentration, [Cu2+]eff, aerosol liquid water content, [ALWC], and particulate matter concentration, [PM]. An average γHO2 across the entire campaign of 0.070±0.035 was calculated, with values ranging from 0.002 to 0.15, and found to be significantly lower than the value of γHO2=0.2, commonly used in modelling studies. Using the calculated γHO2 values for the summer AIRPRO campaign, OH, HO2 and RO2 radical concentrations were modelled using a box model incorporating the Master Chemical Mechanism (v3.3.1), with and without the addition of γHO2, and compared to the measured radical concentrations. The rate of destruction analysis showed the dominant HO2 loss pathway to be HO2 + NO for all NO concentrations across the summer Beijing campaign, with HO2 uptake contributing <0.3 % to the total loss of HO2 on average. This result for Beijing summertime would suggest that under most conditions encountered, HO2 uptake onto aerosol surfaces is not important to consider when investigating increasing O3 production with decreasing [PM] across the North China Plain. At low [NO], however, i.e. <0.1 ppb, which was often encountered in the afternoons, up to 29 % of modelled HO2 loss was due to HO2 uptake on aerosols when calculated γHO2 was included, even with the much lower
γHO2 values compared to γHO2= 0.2, a result which agrees with the aerosol-inhibited O3 regime recently
proposed by Ivatt et al. (2022). As such it can be concluded that in cleaner environments, away from polluted urban centres where HO2 loss chemistry is not dominated by NO but where aerosol surface area is high still, changes in PM concentration and hence aerosol surface area could
still have a significant effect on both overall HO2 concentration and
the O3 production regime. Using modelled radical concentrations, the absolute O3 sensitivity to
NOx and volatile organic compounds (VOCs) showed that, on average across the summer AIRPRO campaign,
the O3 production regime remained VOC-limited, with the exception of a
few days in the afternoon when the NO mixing ratio dropped low enough for
the O3 regime to shift towards being NOx-limited. The O3
sensitivity to VOCs, the dominant regime during the summer AIRPRO campaign,
was observed to decrease and shift towards a NOx-sensitive regime both
when NO mixing ratio decreased and with the addition of aerosol uptake. This
suggests that if [NOx] continues to decrease in the future, ozone
reduction policies focussing solely on NOx reductions may not be as
efficient as expected if [PM] and, hence, HO2 uptake to aerosol
surfaces continue to decrease. The addition of aerosol uptake into the
model, for both the γHO2 calculated from measured data and when using a fixed value of γHO2=0.2, did not have a significant effect on the overall O3 production regime across the
campaign. While not important for this campaign, aerosol uptake could be
important for areas of lower NO concentration that are already in a NOx-sensitive regime.