Impact of HO2 aerosol uptake on radical levels and O-3 production during summertime in Beijing

The impact of heterogeneous uptake of HO2 on aerosol surfaces on radical concentrations and the O-3 production regime in Beijing in summertime was investigated. The uptake coefficient of HO2 onto aerosol surfaces, gamma HO2, 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 gamma HO2 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 gamma HO2 = 0:2, commonly used in modelling studies. Using the calculated gamma HO2 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 gamma HO2, 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 O-3 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 gamma HO2 was included, even with the much lower gamma HO2 values compared to gamma HO2 D 0.2, a result which agrees with the aerosol-inhibited O-3 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 O-3 production regime. Using modelled radical concentrations, the absolute O-3 sensitivity to NOx and volatile organic compounds (VOCs) showed that, on average across the summer AIRPRO campaign, the O-3 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 O-3 regime to shift towards being NOx -limited. The O-3 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 gamma HO2 calculated from measured data and when using a fixed value of gamma HO2 D 0:2, did not have a significant effect on the overall O-3 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.