Modeling explicit tropospheric oxidation through identifying volatile organic compound (VOC) sources, their impact on air quality and their signatures in South China

Abstract

Photochemical smog, characterized by high concentrations of ozone (O₃) and fine particles, is of great concern in the urban areas, in particular megacities and city clusters like the Pearl River Delta (PRD). Ambient O₃ and its precursors were simultaneously measured for the first time at a site within the inland PRD region (WQS) and a site in Hong Kong (TC) from 22 October to 01 December 2007, in order to improve our understanding of the interplay of O₃ pollution between Hong Kong and the inland PRD region, to explore the relationships between O₃ and its precursors, and to identify the key volatile organic compound (VOC) species and emission source categories contributing to the O₃ formation. Ratio analyses for trace gases and VOCs and back trajectory calculation revealed that the air masses arriving at WQS were more aged due to regional influence, whereas the air masses at TC were mainly affected by local emissions and/or regional transport. An observation-Based Model (OBM) was employed to determine the O₃-precursor relationship. At both sites, O₃ production was found to be VOC-limited. Anthropogenic hydrocarbons played a key role in O₃ production, while reducing NO emissions aided the build up of O₃ concentrations. The contribution of carbonyls to O₃ formation was firstly input in the OBM by using measured data, the results showed that the net O₃ production derived from the OBM agreed better with the observed O₃ increment after hourly carbonyl concentrations were included. When adding carbonyls to the OBM, the derived P₀₃-NO peak increased by 64% and 47%, and the HO₂ peak increased by 43% and 39%, at WQS and TC, respectively. These results highlight the importance of carbonyls in the photochemistry in this region. To further identify which VOC species and emission source categories were likely to contribute most to regional O₃ formation in the PRD region, a photochemical trajectory model, was developed and used for the first time to simulate the formation of photochemical pollutants at WQS, Guangzhou during photochemical pollution episodes between 12 and 17 November, 2007. The simulated diurnal variations and mixing ratios of O₃ were in good agreement with observed data, indicating that the photochemical trajectory model provides a reasonable description of O₃ formation in the PRD region. Calculated photochemical ozone creation potential (POCP) indices for the region indicated that alkanes and oxygenated organic compounds had relatively low reactivity, while alkenes and aromatics presented high reactivity. Analysis of the emission inventory found that the sum of 60 of the 139 VOC species accounted for 91% of the total POCP-weighted emission. A further investigation of the relative contribution of the main emission source categories to O₃ formation suggested that mobile source was the largest contributor to regional O₃ formation (40%), followed by biogenic sources (29%), VOC products-related (23%), industry (6%), biomass burning (1%), and power plants (1%). The findings obtained in this study would advance our knowledge of air quality in the PRD region, and provide useful information to local government on effective control of photochemical smog in the region.