FIELD METHOD COMPARISON FOR THE CHARACTERIZATION OF ACID AEROSOLS AND GASES

This paper presents findings from two intercomparison studies of acid aerosol measurement systems, which were conducted in Uniontown and State College, PA, during the summers of 1990 and 1991, respectively. As part of these studies, acid aerosol and gas concentrations (NH3, HNO3, HNO3, SO2, H+, NK4+, NO3-, SO42-) were measured using five systems: the Harvard/EPA Annular Denuder Systems (HEADS), the Personal Annular Denuder System (PADS), the Continuous Sulfate/Thermal Speciation system (CSTS), the Micro-Orifice Impactor (MOI), and the Harvard Marple Impactor (HI). Concentrations were measured over 3, 12, and 24 h periods, with resultant acid aerosol and gas measurements compared for each system. Results from these studies show excellent agreement among the particulate measurement systems. The sulfate (SO42-), aerosol strong acidity (H+), and ammonium (NH4+) concentrations measured by all systems were highly correlated. In addition, 3 and 12 h particulate measurements were comparable for HEADS, MOI, and PADS samplers. Although significantly different, mean relative differences between HEADS measurements and those obtained using the HI and CSTS (SO42- only) systems were small. For the gases nitric acid (HNO3) and sulfur dioxide (SO2), the performance of the HEADS sampler was found to be independent of sample duration, as the 3 and 12 h HEADS measurements were comparable. The PADS sampler, however, was found to measure concentrations of both HNO3 and SO2 poorly. PADS measurements, of HNO3 in particular, were significantly lower than corresponding HEADS measurements. These lower values probably resulted from their deposition on the inlet surfaces of the PADS sampler. The performance of the measurement systems for ammonia (NH3), nitrous acid (HONO), and nitrate (NO3-) could not be determined, since their outdoor levels generally were near or below the limit of detection (LOD).