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 conducted of ambient acidic aerosol levels in Toronto, Ontario. Fine particle mass (da < 2.5 microns) samples were collected twice daily at a central-city site for the determination of particulate-phase strong acidity (H+) and sulfate (SO4 =). Two additional H(+)-monitoring sites were concurrently operated during the summers of 1986 and 1987 to examine the spatial variability of H+ within the metropolitan area. During the summer of 1986, a quasi-continuous total sulfate/sulfuric acid analyzer was also deployed to allow a determination of the chemical form of H+. Results indicate that acid aerosol episodes (H+ > or = 100 nmole/m3) did occur in this city during the summer months, and that H+ peaks were well correlated with sulfate peaks. Virtually all of the H+ was found to be present as ammonium bisulfate (NH4HSO4). While H+ concentrations were highly correlated among the three monitoring 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 the more urbanized sites. Comparisons of day vs night H+/SO4 = ratios, an examination of air mass back-trajectories, and contemporaneous H+ measurements at surrounding sites collectively indicated that transported regional haze air pollution from the United States is a major contributor to the H+ events recorded within Toronto.