Atmospheric Sulfates from Biological Sources

Bacteriogenic production of H₂S occurs in fine-grained anoxic muds, is promoted by organic and nutrient pollution of water, peaks in the warm months of the year, and is the source of most of the estimated 100 to 200 million tons of biogenic sulfur annually contributed to the global atmosphere. We tested the hypothesis that biogenic sulfur contributes to the atmospheric load of sulfate in urban and nonurban sites by statistical analyses of the 24 hour sulfate levels measured in 4 coastal and 3 Inland nonurban sites where pollutant sulfur dioxide emissions are absent or negligible, and in 8 coastal and 10 inland urban sites, all located in New England or Middle Atlantic states.

Comparisons of annual and seasonal mean sulfate levels show that in nonurban groups summertime sulfate levels significantly exceed wintertime levels, and in summer, sulfate levels in urban sites are nearly the same as in nonurban sites. Comparisons of group sulfate means in 4 New York cities near extended bodies of polluted water with those in 10 inland upstate New York cities show significantly higher levels in the cities near polluted water in spring, summer, and fall and for the year as a whole, but not in winter, when the levels were similar. When the nonurban and urban sites are grouped for proximity to coasts (where bacterial sulfate reduction is active in sediments) paired groups of coastal and inland urban and nonurban sites show no significant differences in sulfate levels in summer and fall.

Studies of the summertime sulfate means in New York state show no evidence of an elevated anthropogenic background which could explain the high summertime sulfate level observed in one nonurban site in that state, while analyses of the day to day fluctuations in urban and nonurban sites support the conclusion that nonurban sites have large local (biogenic) sulfate sources in summer and fall, and that local sulfate sources also exist in spring and may exist in winter.

We conclude that biogenic sulfate sources contribute most of the sulfate observed in the cities studied during summer and fall, and in some cities also contribute in other seasons. These biogenic contributions vary with local conditions and are estimated to contribute up to 6 µg/m³ (50%) or more to the annual geometric mean sulfate levels observed in some cities located near extensive bodies of polluted water.