ALBERT

Description: In this study, the Community Multiscale Air Quality (CMAQ) modeling system is used to simulate the ozone (O3) episodes during the Program of Regional Integrated Experiments of Air Quality over the Pearl River Delta, China, in October 2004 (PRIDE-PRD2004). The simulation suggests that O3 pollution is a regional phenomenon in the PRD. Elevated O3 levels often occurred in the southwestern inland PRD, Pearl River estuary (PRE), and southern coastal areas during the 1-month field campaign. Three evolution patterns of simulated surface O3 are summarized based on different near-ground flow conditions. More than 75% of days featured interaction between weak synoptic forcing and local sea-land circulations. Integrated process rate (IPR) analysis shows that photochemical production is the dominant contributor to O3 enhancement from 09:00 to 15:00 LST (local standard time) in the atmospheric boundary layer over most areas with elevated O3 occurrence in the mid-afternoon. The simulated ozone production efficiency is 2–8 O3 molecules per NOx molecule oxidized in areas with high O3 chemical production. Precursors of O3 originating from different source regions in the central PRD are mixed during transport to downwind rural areas during nighttime and early morning, where they then contribute to the daytime O3 photochemical production. Such close interactions among precursor emissions, transports, and O3 photochemical production result in the regional O3 pollution over the PRD. Sensitivity studies suggest that O3 formation is volatile organic compound-limited in the central inland PRD, PRE, and surrounding coastal areas with less chemical aging (NOx/NOy〉0.6), but is NOx-limited in the rural southwestern PRD with photochemically aged air (NOx/NOy