Impact of HO ₂ aerosol uptake on radical levels and O ₃ production during summertime in Beijing

The impact of heterogeneous uptake of HO 2 on aerosol surfaces on radical concentrations and the O 3 production regime in Beijing in summertime was investigated. The uptake coefficient of HO 2 onto aerosol surfaces, γHO2, was calculated for the AIRPRO campaign in Beijing, in summer 2017, as a function of measured aerosol soluble copper concentration, [Cu 2+ ] 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, HO 2 and RO 2 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 HO 2 loss pathway to be HO 2 + NO for all NO concentrations across the summer Beijing campaign, with HO 2 uptake contributing <0.3 % to the total loss of HO 2 on average. This result for Beijing summertime would suggest that under most conditions encountered, HO 2 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 HO 2 loss was due to HO 2 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 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 HO 2 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 HO 2 concentration and the O 3 production regime. Using modelled radical concentrations, the absolute O 3 sensitivity to NO x 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 NO x -limited. The O 3 sensitivity to VOCs, the dominant regime during the summer AIRPRO campaign, was observed to decrease and shift towards a NO x -sensitive regime both when NO mixing ratio decreased and with the addition of aerosol uptake. This suggests that if [NO x ] continues to decrease in the future, ozone reduction policies focussing solely on NO x reductions may not be as efficient as expected if [PM] and, hence, HO 2 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 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 NO x -sensitive regime.

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