Observation-based analysis of ozone production sensitivity for two persistent ozone episodes in Guangdong, China

An observation-based method (OBM) is developed to investigate the sensitivity of ozone formation to precursors during two persistent elevated ozone episodes observed at 77 stations in Guangdong. Average OH concentrations derived at the 77 stations between 08:00 and 13:00 local time stay within a narrow range of 2.5 x 10(6) to 5.5 x 10(6) cm(-3) with a weak dependence on the NOx. These values are in good agreement with OH values observed at a rural station in the Pearl River Delta (PRD). They also agree well with a box model constrained by the ambient conditions observed during the two episodes. The OBM has been used to evaluate the ozone production efficiency, epsilon(NOx or volatile organic compound, VOC), defined as the number of O-3 molecules produced per molecule of NOx (or VOC) oxidized. Average values of epsilon(NOx) and epsilon(VOC) determined by the OBM are 3.0 and 2.1 ppb ppb(-1), respectively, and both compared well with values in previous studies. Approximately 67% of the station days exhibit ozone formation sensitivity to NOx, and approximately 20% of the station days are in the transitional regime sensitive to both NOx and VOC, and only approximately 13% of the station days are sensitive to VOC. These results are in semi-quantitative agreement with the ozone formation sensitivity calculated by the box model constrained by ambient conditions observed during the two episodes. However, our OBM results differ from those of most previous investigations, which suggested that limiting the emission of VOC rather than NOx would be more effective in reducing ozone reduction in Guangdong.