The US EPA has measured hourly (semi-continuous) ambient concentrations of soluble gases (SO₂, HNO₃, NH₃) and aerosols (SO₄^-2, NO₃^-, NH₄⁺) using duplicate Monitor for AeRosols and GAses (MARGA) systems at the Beltsville, MD (BEL116) site during extended sampling periods over the past year. The effort has supplemented long-standing Clean Air Status and Trends Network (CASTNET) filter pack measurements of weekly integrated atmospheric concentrations of those pollutants with short-term concentration variations.

MARGA performance has been previously verified but it is a resource-intensive instrument that requires significant attention for field operation. Performance for all species will be assessed using time integrated sampling methods historically present at CASTNET sites (e.g. filterpack, AMoN samplers) and will be correlated with a pulsed fluorescence SO₂ analyzer to determine the efficacy of verifying overall instrument performance with a single species check.

In addition, MARGA measurements of speciated and total nitrate will be compared with that determined by the filterpack and compared with co-located hourly NOy concentrations to investigate an artifact of higher than expected total nitrate filterpack concentrations during summertime.

Measured ambient NH₃ concentration data will also be compared with historical CMAQ predicted concentration values as a function of season and time as a tool to investigate the efficacy of the model and potentially identify any artifacts or biases that may exist.