A Summary of Airborne Concentrations of Sulfur- and Nitrogen-Containing Pollutants in the Northeastern United States

ABSTRACT

Airborne concentrations of SO₂, SO₄ ^2-, HNO₃, NO₃ ^-, NH₄+, and O₃ were monitored over the six-year period from September 1, 1989, through August 31, 1995, at 10 largely rural Clean Air Status and Trends Network (CASTNet) sites in the northeastern United States. Each of the sulfur- and nitrogen-containing air pollutants monitored by CASTNet displays regular, seasonal cyclical behavior and also exhibits a relatively strong high-to-low spatial concentration gradient from southwest to northeast. On average, more than 70% of the measured airborne sulfur is present as SO₂, except during the summer, when the figure drops to about 50%. During the summer, the SO₂ concentration is the lowest, SO₄ ^2- is the highest, and the fraction of airborne sulfur present as SO₄ ^2- varies considerably with location, ranging from an average of 42% at five sites in Pennsylvania to 70% at two sites in New England. Studywide, more than 70% of the measured, oxidized, airborne nitrogen (N) is present as HNO₃, except during the winter, when the figure drops to about 60%. The concentrations of gaseous SO₂ and HNO₃ are usually comparable but not always larger than the corresponding concentrations of measured sulfur and nitrogen aerosols. Nevertheless, the relatively faster deposition velocities for gases are sufficient to ensure that SO₂ and HNO₃ are usually the dominant contributors to dry sulfur and nitrogen deposition. Observed changes of 1990–1995 annual average airborne sulfur and N concentrations at 10 CASTNet sites in the Northeast are generally consistent with changes in emissions estimated to have occurred in the Northeast over the same period.