FIELD METHOD COMPARISON FOR THE CHARACTERIZATION OF ACID AEROSOLS ANDGASES

This paper presents findings from two intercomparison studies of acid aerosol measurement systems, which were conducted in Uniontown and Sta te 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: t he Harvard/EPA Annular Denuder Systems (HEADS), the Personal Annular D enuder System (PADS), the Continuous Sulfate/Thermal Speciation system (CSTS), the Micro-Orifice Impactor (MOI), and the Harvard Marple Impa ctor (HI). Concentrations were measured over 3, 12, and 24 h periods, with resultant acid aerosol and gas measurements compared for each sys tem. Results from these studies show excellent agreement among the par ticulate measurement systems. The sulfate (SO42-), aerosol strong acid ity (H+), and ammonium (NH4+) concentrations measured by all systems w ere highly correlated. In addition, 3 and 12 h particulate measurement s were comparable for HEADS, MOI, and PADS samplers. Although signific antly different, mean relative differences between HEADS measurements and those obtained using the HI and CSTS (SO42- only) systems were sma ll. For the gases nitric acid (HNO3) and sulfur dioxide (SO2), the per formance of the HEADS sampler was found to be independent of sample du ration, as the 3 and 12 h HEADS measurements were comparable. The PADS sampler, however, was found to measure concentrations of both HNO3 an d SO2 poorly. PADS measurements, of HNO3 in particular, were significa ntly lower than corresponding HEADS measurements. These lower values p robably resulted from their deposition on the inlet surfaces of the PA DS sampler. The performance of the measurement systems for ammonia (NH 3), nitrous acid (HONO), and nitrate (NO3-) could not be determined, s ince their outdoor levels generally were near or below the limit of de tection (LOD).