During the EXPLORE-YRD campaign (EXPeriment on the eLucidation of the atmospheric Oxidation capacity and aerosol foRmation, and their Effects in Yangtze River Delta) in May June 2018, we measured N2O5, NO2, O-3 and relevant parameters at a regional site in Taizhou, Jiangsu Province. The nocturnal average NO3 production rate was 1.01 +/- 0.47 ppbvh(-1), but the mixing ratio of N2O5 was low, with a maximum of 220 pptv in 1 min, suggesting rapid loss of NO3 and N2O5. The nocturnal steady-state lifetime of N2O5 was 43 + 52 s on average, which may be attributed to the elevated monoterpene and fast N2O5 uptake. VOCs (mainly monoterpenes) dominated daily NO3 loss with the percentage of 36.4% and N2O5 uptake accounted for 14.4%, when taking NO + NO3 and NO3 photolysis into consideration. We demonstrated that the nonnegligible daytime NO3 oxidation of monoterpene in YRD region, which contributes to the daytime formation of organic nitrate and secondary organic aerosol. The daily average NOx consumption rate via rapid NO3 reaction reached 0.63 ppbvh(-1), corresponding to 57.3% NOx loss in comparison with the OH oxidation pathway at this site, highlighting the key role of NO3 and N2O5 in NOx removal and subsequent photochemistry in the YRD region.

The oxidation of nitric oxide to nitrogen dioxide by hydroperoxy (HO2) and organic peroxy radicals (RO2) is responsible for the chemical net ozone production in the troposphere and for the regeneration of hydroxyl radicals, the most important oxidant in the atmosphere. In Summer 2014, a field campaign was conducted in the North China Plain, where increasingly severe ozone pollution has been experienced in the last years. Chemical conditions in the campaign were representative for this area. Radical and trace gas concentrations were measured, allowing for calculating the turnover rates of gas-phase radical reactions. Therefor; the importance of heterogeneous HO(2 )uptake on aerosol could be experimentally determined. HO2 uptake could have suppressed ozone formation at that time because of the competition with gas-phase reactions that produce ozone. The successful reduction of the aerosol load in the North China Plain in the last years could have led to a significant decrease of HO2 loss on particles, so that ozone-forming reactions could have gained importance in the last years. However, the analysis of the measured radical budget in this campaign shows that HO2 aerosol uptake did not impact radical chemistry for chemical conditions in 2014. Therefore, reduced HO2 uptake on aerosol since then is likely not the reason for the increasing number of ozone pollution events in the North China Plain, contradicting conclusions made from model calculations reported in the literature.