Intercomparison of Clean Air Status and Trends Network Nitrate and Nitric Acid Measurements with Data from other Monitoring

ABSTRACT:

The U.S. Environmental Protection Agency Clean Air Status and Trends Network (CASTNET) utilizes an open-face filter pack system to measure concentrations of atmospheric sulfur and nitrogen species. Concentration data for nitrogen species measured with filter pack systems sometimes deviate from data collected by other measurement systems used to measure the same species. The nature of these differences suggests that more than one sampling mechanism or atmospheric process is involved. The study presented here examines these differences by intercomparing CASTNET data with data from other studies, examining the results from earlier intercomparison studies, and conducting a field test to investigate the effect of particle size on filter pack measurement systems. Measurements of nitrogen species from the Maryland Aerosol Research and Characterization (MARCH) monitoring site were compared with nitrogen concentrations at three nearby CASTNET sites. Results indicate that CASTNET measured higher particulate nitrate (NO3 -) and lower gaseous nitric acid (HNO3) concentrations.

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