Environ. Sci. Technol. 1986, 2 0 , 1249-1253
On the Nature of Atmospheric Oxidation Processes of SO2 to Sulfate and of NO2 to Nitrate on the Basis of Diurnal Variations of Sulfate, Nitrate, and Other Pollutants in an Urban Area Satoshl Kadowaki Aichi Environmental Research Center, 7-6 Nagare, Tsuji-machi, Kita-ku, Nagoya 462, Japan
Measurements of sulfate and nitrate (particulate plus gaseous) were made in Nagoya. Air samples were collected over a 2- or 3-h interval, and particulates were fractionated into the fine and coarse fractions. In addition, other pollutants and meteorological parameters were simultaneously monitored. Sulfur and nitrogen conversion ratios (F, and F,) defined in the text were calculated from the data. F, and F, in the summer increased by about 2 and 4 times those in the winter, respectively. F, was always higher in the daytime than in the nighttime and correlated closely with oxidant concentration. On the other hand, there was hardly any difference in F, of the daytime and the nighttime. Good correlation was found between F, and relative humidity when the oxidant concentrations were more than 20 ppb. These results indicated that dropletphase reactions are important for the SOz oxidation to sulfate, while gas-phase reactions are predominant for the NOz oxidation to nitrate.
Introduction The conversion mechanisms and rates for sulfate and nitrate formations are of considerable interest because of concern over apparent widespread environmental effects associated with acid deposition, visibility reduction, and weather and climate changes. These secondary aerosols result primarily from the oxidation of sulfur dioxide and nitrogen oxides. In order to clarify the oxidation processes and rates, numerous extensive studies have been conducted, including laboratory studies (e.g., I-3), theoretical and kinetic computer modeling studies (e.g., 4 , 5 ) , and field studies in ambient air (e.g., 6-8) and in plumes (e.g., 9, IO). It is generally recognized that the two most important oxidation processes are homogeneous gas-phase reactions involving free radicals such as OH and heterogeneous reactions in droplet phase and on aerosol particle surface. Davis et al. (9) showed that SO2 and NO2 oxidations in plume could largely be explained by the gas-phase reaction with the OH radical under midday summertime and low relative humidity ( 6 0 % ) conditions. On the other hand, McMurry et al. ( I I , 1 2 ) reported from the results of field observations and smog chamber experiments that droplet-phase reactions were important at high relative humidity (>75%), while gas-phase reactions were the predominant mechanism at low relative humidity (-35%). More data on the effect of weather conditions, such as relative humidity and sunlight, on sulfate and nitrate formations are needed to reveal the oxidation processes in ambient air. In almost all regions of Japan, there are two different summertime conditions, in which sulfate and nitrate formations are enhanced and photochemical smog episodes are often observed. The first is low relative humidity and high sunlight before the rainy season called “Baiu” has set in. The second is high relative humidity and high sunlight after the rainy season is over. Therefore, it is useful for the clarification of the oxidation processes to compare sulfate 0013-936X/86/0920-1249$01.50/0
and nitrate behavior under the two different summertime conditions. In this study, field measurements were made of diurnal variations for sulfate, nitrate, selected gaseous pollutants, and meteorological conditions in Nagoya during the summer of 1983 and the winter of 1983 and 1984. From the results, sulfur and nitrogen conversion ratios were calcuated, as defined in the text, and the oxidation processes in urban air were discussed.
Experimental Section Air samples were collected at the Aichi Environmental Research Center in a mixed residental/light industry area of Nagoya by use of a Model 21-000 Andersen sampler manufactured by 2000 INC. The sampler usually consists of eight stages followed by a backup filter. In this study to segregate atmosphericparticulates into two size fractions of coarse and fine, 1 2 . 1 pm and
