Decoupled direct sensitivity analysis of regional ozone pollution over the Pearl River Delta during the PRIDE-PRD2004 campaign

High-order decoupled direct method is applied to investigate O3-precursor sensitivity during the Program of Regional Integrated Experiments of Air Quality over the Pearl River Delta (PRD) Region, China in October 2004 (PRIDE-PRD2004). First-order sensitivity coefficients of O3 show a NOx-inhibited chemistry along the polluted plumes in central and southern PRD areas, and a NOx-limited condition over larger areas of the western, eastern and northern PRD. A nonlinearity ratio derived from O3 sensitivity coefficients clearly describes the nonlinear feature of O3 chemistry over PRD. The distributions of NOx- and VOC-sensitive regimes are determined by near-ground flow conditions, and O3 sensitivity is found to shift from VOC-sensitive conditions to NOx-sensitive conditions from morning to afternoon in central and western areas of the PRD. Elevated surface O3 around mid-afternoon in October mainly occurred in VOC-sensitive regions or those NOx-limited areas in close vicinity of the VOC-sensitive regions. Ratios of H2O2/HNO3 and peroxides/HNO3 for indicating O3-precursor sensitivity are evaluated and the transition values of 0.45 for H2O2/HNO3 and 0.80 for peroxides/HNO3 were found, with an overall accuracy of about 92% for sensitivity identification at elevated O3 hours in afternoons during the campaign.