The Clean Air Status and Trends Network (CASTNet) was
implemented by the U.S. Environmental Protection
Agency (EPA) in 1991 in response to Title IX of the Clean
Air Amendments of 1990, which mandated the deploy-ment
of a national ambient air monitoring network to
track progress of the implementation of emission reduc-tion
programs in terms of deposition, air quality, and
changes to affected ecosystems. CASTNet evolved from
the National Dry Deposition Network (NDDN).
CASTNet currently consists of 45 sites in the eastern
United States and 28 sites in the West. Each site measures
sulfur dioxide (SO 2 ), nitric acid (HNO 3 ), particle sulfate
(SO 4
= ), particle nitrate (NO 3
- ), and ozone. Nineteen sites
collect precipitation samples. NDDN/CASTNet uses a uni-form
set of site-selection criteria which provides the data
user with consistent measures to compare each site. These
criteria also ensure that, to the extent possible, CASTNet
sites are located away from local emission sources.
This paper presents an analysis of SO 2 and SO 4
= con-centration
data collected from 1987 through 1996 at ru-ral
NDDN/CASTNet sites. Annual and seasonal variability
is examined. Gradients of SO 2 and SO 4
= are discussed. The
variability of the atmospheric mix of SO 2 and SO 4
= is ex-plored
spatially and seasonally. Data from CASTNet are
also compared to SO 2 and SO 4
= data from concurrent moni-toring
studies in rural areas.
The Midwest and the Northeast have the highest an-nual
averages of SO 2 , greater than 10 mg/m 3 . SO 2 concen-trations
vary by a factor of 6 across the six eastern
subregions. The annual SO 2 concentrations for western
sites average less than 1 mg/m 3 . A gradient in SO 2 concen-trations
in the Ohio River Valley was observed, decreas-ing
in northeasterly, easterly, and southeasterly directions.
Also, a decreasing concentration gradient was observed
for SO 4
= from the Midwest to the East and Southeast, al-though
the gradient was not as pronounced as for SO 2 .
Both SO 2 and SO 4
= exhibited strong seasonal cycles.
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Publication Type:
Journal
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Publication Date:
November 1999
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