Impact of HO₂ aerosol uptake on radical levels and O₃ production during summertime in Beijing

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. <0.1ppb, 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 NO_x 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 NO_x-limited. The O₃ 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, 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 NO_x-sensitive regime.