Intercomparison of Clean Air Status and Trends Network Nitrate and Nitric Acid Measurements with Data from Other...

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. Comparisons of NO₃^- from 34 collocated CASTNET and Inter-agency Monitoring of Protected Visual Environments (IMPROVE) sites show that CASTNET NO₃^- measurements were typically higher than the corresponding IM PROVE values. Also, results from the Lake Michigan Air Director's Consortium Midwest Ammonia Monitoring Project demonstrated NO₃^- dissociation on Teflon filters. To investigate the effect of particle size, filter pack measurement systems were operated at three CASTNET sites with and without cyclones during six 7-day measurement periods from March to August 2006. Results indicate the size-selection cyclones had a significant effect on both NO₃^- and HNO₃ concentrations, but little effect on sulfate (SO₄^2-) and ammonium (NH₄⁺) levels. NO₃^- concentrations sampled with the open-face filters were significantly higher than concentrations measured with a 2.5-μm cut point, as were HNO₃ concentrations. Although limited in spatial and temporal coverage, the field study showed that the use of an open-face filter pack may allow for the collection of coarse NO₃^- particles and for the reaction of HNO₃ with metals/salts on the Teflon filter.