Field intercomparison of measurement techniques for total NH4+ and total NO3− in ambient air

Nitrate and nitric acid concentrations at two Rocky Mountain baseline monitoring sites

A two-tiered, Teflon/nylon filterpack system was used to characterize spatial and temporal patterns of particulate nitrates and nitric acid vapors at two monitoring sites in the Rocky Mountains. Geometric means for particulate nitrates were 38.9 and 52.8 ng/m(3) for the upper and lower sites, respectively. For nitric acid, geometric means of 70.4 ng/m(3) for the upper site and 295 ng/m(3) for the lower site were observed. The relatively low concentrations found at these two sites are comparable to published values for these materials at other remote sites. Atmospheric concentrations of nitrates and nitric acid were correlated significantly at each site, and the total nitrate concentrations (NO(3)(-) plus HNO(3)) were correlated between sites. Comparisons between the two sites indicate that nitric acid concentrations were statistically greater at the lower elevation site, whereas nitrate concentrations were not significantly different. No general seasonal or annual pattern of nitrate or nitric acid concentrations were evident when comparable sampling periods were examined.