THE NATURE AND ORIGINS OF ACID SUMMER HAZE AIR-POLLUTION IN METROPOLITAN TORONTO, ONTARIO

During July and August of 1986, 1987, and 1988, a field study was cond ucted of ambient acidic aerosol levels in Toronto, Ontario. Fine parti cle mass (d(a) < 2.5 mu m) samples were collected twice daily at a cen tral-city site for the determination of particulate-phase strong acidi ty (H+) and sulfate (SO4=). Two additional H+-monitoring sites were co ncurrently operated during the summers of 1986 and 1987 to examine the spatial variability of H+ within the metropolitan area. During the su mmer of 1986, a quasicontinuous total sulfate/sulfuric acid analyzer w as also deployed to allow a determination of the chemical form of H+. Results indicate that acid aerosol episodes (H+ greater than or simila r to 100 nmole/m(3)) did occur in this city during the summer months, and that H+ peaks were well correlated with sulfate peaks. Virtually a ll of the H+ was found to be present as ammonium bisulfate (NH4HSO4). While H+ concentrations were highly correlated among the three monitor ing sites (r = 0.9), the highest H+/SO4= ratios prevailed during SO4= episode periods and at the least urbanized site. This latter trend was apparently due to greater neutralization of H+ by local ammonia at th e more urbanized sites. Comparisons of day vs night H+/SO4= ratios, an examination of air mass back-trajectories, and contemporaneous H+ mea surements at surrounding sites collectively indicated that transported regional haze air pollution from the United States is a major contrib utor to the H+ events recorded within Toronto. (C) 1994 Academic Press , Inc.