Temporal characterization and regional contribution to O3 and NOx at an urban and a suburban site in Nanjing, China.

Abstract To improve our understanding of the interplay among local and regional photochemical pollutants in the typical city of the Yangtze River Delta (YRD) region, the concurrent observation of O 3 and NO x concentrations at an urban and a suburban site in Nanjing during 2008 is presented. In general, the annual mean O 3 concentration is 2.35 ppbv lower in the downtown than at suburban due to higher NO x pollution levels correlated with heavy traffic. At both sites, O 3 shows a distinct seasonality with the spring maximum and the winter minimum, while the minimum concentration of NO x appears in summertime. Besides the chemical processes of O 3 sensitivity in the daytime and the NO x titration at night, meteorological conditions also play an essential role in these monthly and diurnal variations. The ozone weekend effect that can be attributed to the weekly routine of human activities is observed in the urban atmosphere of Nanjing as well, with O 3 concentrations 2.09 ppbv higher and NO x concentrations 6.20 ppbv lower on weekends than on weekdays. The chemical coupling of NO, NO 2 and O 3 is investigated to show that the OX-component (O 3 and NO 2 ) partitioning point occurs at about 35 ppbv for NO x , with O 3 being the dominant form at lower levels and NO 2 dominating at higher levels. And it is also discovered that the level of OX is made up of two contributions, including the regional contribution affected by regional background O 3 level and the local contribution correlated with the level of primary pollution. The diurnal peak of regional contribution appears 2–5 h after the peak of local contribution, implying that OX in Nanjing might prominently affected by the pollutants from a short distance. The highest regional contribution and the second highest local contribution lead to the spring peak of O 3 observed in Nanjing, whereas the highest local contribution and the moderate regional contribution make the O 3 concentrations in summer higher than those in autumn and winter. Our results reveal the important environment impacts from meteorological conditions and human activities in the YRD region, and can help to understand O 3 pollution in these polluted areas by just using the conventional observations.