Development of a semi-continuous method for the measurement of nitric acid vapor and particulate nitrate and sulfate

An automated method for the measurement of nitric acid vapor, particulate nitrate, and particulate sulfate in the atmosphere has been developed. The method is based on a semi-continuous wet effluent denuder (WED) and wet effluent frit (WEF) sampling system, followed by ion chromatography with conductivity detection. The method provides separate measures of nitric acid and particulate nitrate, and is characterized by a sample frequency of 15 min and a detection limit of 10 pptv (parts per trillion by volume) for each species. A variety of laboratory tests used to characterize the system performance are described. Among them the particle passing efficiency of the denuder was evaluated using NH4NO3 particles (diameters < 1 mu m) generated by a new method. Ambient measurements using this method were compared with the Nylon and Teflon filter pack method at three field sites: a loblolly pine forest in rural Alabama; a suburban site in Boulder, CO; and a rural mountain site near Nederland, CO. The methods intercomparison was carried out at sites with varying nitric acid mixing ratios, with the whole data set ranging from < 0.1 to 4 ppbv nitric acid. The nitric acid measurements collected using the WED method compared very well with the filter pack method (linear regression results: slope = 1.00 +/- 0.05, r = 0.97). Particulate nitrate and sulfate measurements were compared only in Alabama. The particulate sulfate measured with the wet effluent frit and with the filter pack agreed within 12% on average. The particulate nitrate measured using the frit was much higher than that measured by the filter pack, and in addition, the WEF nitrate exhibited a strong diurnal variation while the filter pack nitrate did not. Principal component analysis has been performed on the data set as a means to elucidate the cause of this difference, and to examine the atmospheric variability observed during the Alabama study.