Experimental budgets of OH, HOsub2/sub, and ROsub2/sub radicals and implications for ozone formation in the Pearl River Delta in China 2014

摘要

Hydroxyl (OH) and peroxy radicals ( HOsub2/sub and ROsub2/sub ) 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 carbon monoxide (CO), nitrogen oxides ( NOsubx/sub=NO , NOsub2/sub ) and volatile organic compounds (VOCs). OH reactivity was in the range from 15 to 80 ssup?1/sup , of which about 50?% was unexplained by the measured OH reactants. In the 3?weeks of the campaign, maximum median radical concentrations were 4.5×10sup6/sup cmsup?3/sup for OH at noon and 3×10sup8/sup and 2.0×10sup8/sup cmsup?3/sup for HOsub2/sub and ROsub2/sub , 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, HOsub2/sub , and ROsub2/sub ) and their sum ( ROsubx/sub ). The maximum loss rates for OH, HOsub2/sub , and ROsub2/sub reached values between 10 and 15 ppbv?hsup?1/sup during the daytime. The largest fraction of this can be attributed to radical interconversion reactions while the real loss rate of ROsubx/sub remained below 3 ppbv?hsup?1/sup . Within experimental uncertainties, the destruction rates of HOsub2/sub and the sum of OH, HOsub2/sub , and ROsub2/sub are balanced by their respective production rates. In case of ROsub2/sub , the budget could be closed by attributing the missing OH reactivity to unmeasured VOCs. Thus, the presumption of the existence of unmeasured VOCs is supported by ROsub2/sub measurements. Although the closure of the ROsub2/sub budget is greatly improved by the additional unmeasured VOCs, a significant imbalance in the afternoon remains, indicating a missing ROsub2/sub sink. In case of OH, the destruction in the morning is compensated by the quantified OH sources from photolysis (HONO and Osub3/sub ), ozonolysis of alkenes, and OH recycling ( HOsub2/sub+NO ). In the afternoon, however, the OH budget indicates a missing OH source of 4 to 6 ppbv?hsup?1/sup . The diurnal variation of the missing OH source shows a similar pattern to that of the missing ROsub2/sub sink so that both largely compensate each other in the ROsubx/sub budget. These observations suggest the existence of a chemical mechanism that converts ROsub2/sub to OH without the involvement of NO, increasing the ROsub2/sub loss rate during the daytime from 5.3 to 7.4 ppbv?hsup?1/sup on average. The photochemical net ozone production rate calculated from the reaction of HOsub2/sub and ROsub2/sub with NO yields a daily integrated amount of 102 ppbv ozone, with daily integrated ROsubx/sub primary sources being 22 ppbv in this campaign. The produced ozone 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 HOsub2/sub and all ROsub2/sub radicals react with NO. However, the unknown ROsub2/sub loss (evident in the ROsub2/sub budget) causes 30 ppbv less ozone production than would be expected from the VOC oxidation rate.