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. The first wintertime in situmeasurements of hydroxyl (OH), hydroperoxy (HO2) and organic peroxy(RO2) radicals(ROx=OH+HO2+RO2) in combinationwith observations of total reactivity of OH radicals, kOH inBeijing are presented. The field campaign “Beijing winter finE particleSTudy – Oxidation, Nucleation and light Extinctions” (BEST-ONE) wasconducted at the suburban site Huairou near Beijing from January toMarch 2016. It aimed to understand oxidative capacity during wintertime andto elucidate the secondary pollutants' formation mechanism in the North ChinaPlain (NCP). OH radical concentrations at noontime ranged from 2.4×106cm-3 in severely polluted air (kOH∼27s-1) to 3.6×106cm-3 in relatively cleanair (kOH∼5s-1). These values are nearly 2-foldlarger than OH concentrations observed in previous winter campaigns inBirmingham, Tokyo, and New York City. During this campaign, the total primaryproduction rate of ROx radicals was dominated by thephotolysis of nitrous acid accounting for 46 % of the identified primaryproduction pathways for ROx radicals. Other importantradical sources were alkene ozonolysis (28 %) and photolysis ofoxygenated organic compounds (24 %). A box model was used to simulate theOH, HO2 and RO2 concentrations based on the observationsof their long-lived precursors. The model was capable of reproducing theobserved diurnal variation of the OH and peroxy radicals during clean dayswith a factor of 1.5. However, it largely underestimated HO2 andRO2 concentrations by factors up to 5 during pollution episodes.The HO2 and RO2 observed-to-modeled ratios increased withincreasing NO concentrations, indicating a deficit in our understanding ofthe gas-phase chemistry in the high NOx regime. The OHconcentrations observed in the presence of large OH reactivities indicatethat atmospheric trace gas oxidation by photochemical processes can be highlyeffective even during wintertime, thereby facilitating the vigorous formationof secondary pollutants.

atmospheric chemistry and physics

Wintertime photochemistry in Beijing: observations of RO&amp;lt;sub&amp;gt;&amp;lt;i&amp;gt;x&amp;lt;/i&amp;gt;&amp;lt;/sub&amp;gt; radical concentrations in the North China Plain during the BEST-ONE campaign