Experimental budgets of OH, HO 2 and RO 2 radicals and implications for ozone formation in the Pearl River Delta in China 2014

Abstract. Hydroxyl (OH) and peroxy radicals (HO 2 , RO 2 ) were measured in the Pearl River Delta which is one of the most polluted areas in China, in autumn 2014. The radical observations were complemented by measurements of OH reactivity (inverse OH lifetime) and a comprehensive set of trace gases including CO, NO x and VOCs. OH reactivity was in the range between 15 s −1 and 80 s −1 , of which about 50 % was unexplained by the measured OH reactants. In the three weeks of the campaign, maximum median radical concentrations were 4.5 × 10 6 cm −3 for OH at noon, and 3 × 10 8 cm −3 and 2.0 × 10 8 cm −3 for HO 2 and RO 2 , respectively, in the early afternoon. The completeness of the daytime radical measurements made it possible to carry out experimental budget analyses for all radicals (OH, HO 2 , and RO 2 ) and their sum (RO x ). The maximum loss rates for OH, HO 2 , and RO 2 reached values between 10 ppbv/h and 15 ppbv/h during daytime. The largest fraction of this can be attributed to radical interconveresio reactions while the real loss rate of RO x remained below 3 ppbv/h. Within experimental uncertainties, the destruction rates of HO 2 and the sum of OH, HO 2 , and RO 2 are balanced by their respective production rates. In case of RO 2 , the budget can only be closed when the missing OH reactivity is attributed to unmeasured VOCs. Thus, the existence of unmeasured VOCs is directly confirmed by RO 2 measurements. Although the closure of the RO 2 budget is greatly improved by the additional unmeasured VOCs, a significant imbalance in the afternoon remains indicating a missing RO 2 sink. In case of OH, the destruction in the morning is compensated by the quantified OH sources from photolysis (HONO, O 3 ), ozonolysis of alkenes and OH recycling (HO 2 + NO). In the afternoon, however, the OH budget indicates a missing OH source of (4–6) ppbv/h. The diurnal variation of the missing OH source shows a similar pattern as that of the missing RO 2 sink so that both largely compensate each other in the RO x budget. These observations suggest the existence of a chemical mechanism that converts RO 2 to OH without the involvement of NO. The photochemical net ozone production rate calculated from the reaction of HO 2 and RO 2 with NO yields a daily integrated amount of 102 ppbv ozone with daily integrated RO x primary sources being 22 ppbv in this campaign. This value can be attributed to the oxidation of measured (18 %) and unmeasured (60 %) hydrocarbons, formaldehyde (14 %) and CO (8 %). An even larger integrated net ozone production of 140 ppbv would be calculated from the oxidation rate of VOCs with OH, if HO 2 and all RO 2 radicals would react with NO. However, the unknown RO 2 loss (evident in the RO 2 budget) causes 30 % less ozone production than would be expected from the VOC oxidation rate.