Ammonia and its role in midwestern haze

Ammonia is the primary basic gas in the atmosphere, and plays a critical role in the formation of fine particles through its reactions with nitric acid and sulfuric acid. Despite its importance in atmospheric chemistry, measurements of gas phase ammonia have not been made routinely by any national network. To fill this gap, the Midwest Regional Planning Organization (MRPO) and the Central Regional Air Planning Association (CenRAP _[di]) undertook a one-year sampling study of ambient ammonia at 10 sites, beginning in October 2003. The sites were chosen to represent regional background concentrations, with the exception of one urban site in Detroit. Other species measured as part of the same study are nitric acid, sulfur dioxide, and particle sulfate, nitrate, and ammonium. The sites are collocated with IMPROVE monitors and sample on a 1/6 day schedule so that each ammonia sampling day coincides with an IMPROVE sample. The samplers consist of a Teflon-coated cyclone to remove particles greater than 2.5 um, two denuders (one for acid gases, one for ammonia) in series, followed by a Teflon filter and a nylon filter (to capture nitric acid lost from particulate nitrate on the Teflon filter). Two sites are using a Rupprecht & Patashnick Model 2300 automated speciation sampler to make the same measurements. Additional measurements of ammonia and nitric acid are being made at one site (Bondville, IL) by a semi-continuous ion chromatograph, and continuous measurements of ammonia at the same site are being made by photoacoustic spectroscopy. This paper presents preliminary data from this ammonia network. Spatial and seasonal variability in ambient ammonia is compared, including urban and rural differences. The continuous and semicontinuous measurements are compared to each other and examined for diurnal variations.