Influence of ozone and humidity on the formation of sulfate and nitrate in airborne fine particles.

Ambient concentrations of HNO3 and SO2, and particulate NO3- and SO4(2-) were simultaneously measured in daytime and nighttime in southern Taiwan, to investigate the conversion effect of these inorganic species into airborne particulate. During the episode days, the average particulate nitrate mass of accumulation mode (0.18-1.8 microm) measured over daytime and nighttime were about 3.9 and 7.6 times higher than those measured during non-episode days, respectively. The mean value of gaseous nitric acid was always higher during episode daytime than that during non-episode daytime. In addition, the SO4(2-) mass of accumulation mode during episode days was about 2.6 and 2.0 times higher than those during the daytime and nighttime of the non-episode days, respectively. Both of (1) the extent of SO2 oxidation to sulfate and NO2 oxidation to nitrate and (2) conversion ratios for sulfur (Fs) and nitrogen (Fn) were defined and calculated using field measurements. The nighttime Fn and Fs during the episode days were about 4 and 1.6 times higher, respectively, than those during the non-episode days. Furthermore, the Fs and Fn increased with the increase of relative humidity during both of the episode daytime and nighttime. A positive correlation coefficient that the Fn and Fs increases with increasing ozone concentration was found during the non-episode daytime. These results might be attributed to high NO2, SO2 and ozone concentrations in a humid atmosphere, and also the fact that the gas-to-particle conversion plays an important role during episode days.