Observational study of ozone pollution at a rural site in the Yangtze Delta of China

Ozone and related trace gases (CO, NO_x, and SO₂) were measured from June 1999 to July 2000 at a rural site in the Yangtze Delta of China, a region of intensive anthropogenic activity. Elevated ozone levels were frequently observed during the study period, with the highest frequency in late spring and early summer. Over a 1 yr period, 21 d were found to have ozone concentrations exceeding the new US 8-h 80 ppb health standard. Calculation of the “SUM06” exposure index also shows relatively high (>15 ppm h) values for each season except winter. At these levels ozone may have adverse effects on human health as well as agricultural crops. Analysis of meteorological data shows that the high ozone days were associated with large-scale stagnation, intense solar radiation, and minimum rainfall. Large-scale back trajectories indicate a slow-moving/re-circulating airmass during the episodic days. Examination of chemical data shows that the observed daytime high ozone concentrations were due to downward mixing of ozone-rich air, in situ photochemical formation, and in some cases, advection to the site of aged plumes. The very high CO levels (and high CO to NO_x ratios) were found to coincide with many of the ozone episodes, suggesting a contribution from sources of emission involving incomplete combustion. It is suggested that the burning of biomass (e.g., biofeuls and crop residues) may be an important source for the observed high CO and O₃ values.