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Airborne concentrations of SO 2 , SO 4
2- , HNO 3 , NO 3
- , NH 4
+ ,
and O 3 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 cycli-cal
behavior and also exhibits a relatively strong high-to-
low spatial concentration gradient from southwest
to northeast. On average, more than 70% of the mea-sured
airborne sulfur is present as SO 2 , except during
the summer, when the figure drops to about 50%. Dur-ing
the summer, the SO 2 concentration is the lowest,
SO 4
2- is the highest, and the fraction of airborne sulfur
present as SO 4
2- varies considerably with location, rang-ing
from an average of 42% at five sites in Pennsylva-nia
to 70% at two sites in New England. Studywide, more
than 70% of the measured, oxidized, airborne nitrogen
(N) is present as HNO 3 , except during the winter, when
the figure drops to about 60%. The concentrations of
gaseous SO 2 and HNO 3 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 suffi-cient
to ensure that SO 2 and HNO 3 are usually the domi-nant
contributors to dry sulfur and nitrogen deposition.
Observed changes of 1990-1995 annual average air-borne
sulfur and N concentrations at 10 CASTNet sites
in the Northeast are generally consistent with changes
in emissions estimated to have occurred in the North-east
over the same period.
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Publication Type:
Journal
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Publication Date:
August 1999
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