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The EPA Clean Air Status and Trends Network (CASTNET) utilizes a filter pack system to
measure concentrations of atmospheric sulfur and nitrogen species. The filter pack contains
four filters in sequence (Teflon®, nylon, and two cellulose filters impregnated with potassium
carbonate). The Teflon® filter samples particulate sulfate (SO4
2-), nitrate (NO3
-), chloride (Cl-),
ammonium (NH4
+), and other cations. The nylon filter collects nitric acid (HNO3) and some
sulfur dioxide (SO2). The cellulose filters collect the remaining SO2. Recent studies have
suggested that typically a fraction of the ammonium nitrate (NH4NO3) collected on the Teflon®
filter dissociates into HNO3 and ammonia. Also, NH4NO3 can react with acid sulfate particles
on the Teflon® filter to produce HNO3. These reactions could result in an overestimate of
HNO3 and an underestimate of particulate NO3
- and NH4
+. Data from CASTNET and three
other measurement programs are being used to estimate uncertainties in CASTNET filter pack
measurements of HNO3, NO3
-, and NH4
+ on a seasonal, annual, and geographic basis.
Measurements of nitrogen species from the Maryland Aerosol Research and Characterization
(MARCH) monitoring site at Ford Meade, MD were compared with CASTNET nitrogen
species measurements from the sites at Beltsville, MD; Blackwater, MD; and Arendtsville, PA.
Results indicate that CASTNET measures higher particulate NO3
- and lower gaseous HNO3
concentrations than the other measurement systems. These results are somewhat
counterintuitive. An explanation is that the CASTNET filter pack collects larger NO3
- particles
because the instrument does not include a size selection inlet (e.g., cyclone) and may not collect
HNO3 as efficiently as other measurement systems. Future work will investigate the effect of
collecting nitrate particles without a size selection inlet by measuring weekly nitrate
concentrations with a 2.5 micrometer cut-off and without cyclones.
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CNumber:
A&WMA 99th Annual Conference and Exhibition: 06/20/2006
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Publication Type:
Proceedings
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Publication Date:
June 2006
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