Basinwide nitric acid and related species concentrations observed during the Claremont nitrogen species comparison study

In conjunction with the Claremont Nitrogen Species Comparison Study, tandem filter units designed to collect HNO₃/total aerosol (TA) NO₃^- and NH₃ TA NH₄⁺ were operated at seven locations throughout the Los Angeles area, including Claremont. The sampling methods used were related to the comparison study via intensive short-term and long-term measurements made at Claremont by both the tandem filter method and the denuder difference method. Between methods, 4-h and 6-h duration HNO₃ samples taken by the tandem filter method were higher (~ 20%) than the HNO₃ results obtained by the denuder difference method. As sampling duration increased to 22 h, the tandem filter method HNO₃ and NH₃ concentration measurements increased, while the 22-h denuder difference method HNO₃ values remained indistinguishable from the average of simultaneous short-term 4- and 6-h average samples. Analysis of the basinwide data showed that Claremont experienced the highest measured HNO₃ concentrations in the Los Angeles area during the period of the comparison study, confirming that Claremont was a wise choice for the main site of the study. In contrast, an alternative site near Riverside experienced the lowest HNO₃ levels in the air basin, accompanied by high aerosol nitrate and very high NH₃ concentrations.