Measurement of atmospheric nitric acid by the filter method and comparisons with the tuneable diode laser and other methods

Abstract

As part of the Nitrogen Species Methods Comparison Study at Claremont, CA, two triple filter packs and two tuneable diode laser systems were used to measure atmospheric HNO 3; the filters also measured total suspended particulate compounds and gaseous NH 3. The filter packs sampled for 1- and 2-h and 4- and 6-h periods and the lasers operated on 5-min averaging periods. Filter samples from the 4- and 6-h periods yielded, on average, about 16% higher HNO 3 than those from the 1- and 2-h periods. Linear least squares regressions of the 1- and 2-h filter HNO 3 concentrations with the average of the two lasers were highly significant, yielding slopes of 1.22 for daytime and 0.76 for night-time periods. Similar regressions of the 4- and 6-h filter HNO 3 with the laser, annular denuder and denuder difference methods yielded slopes in the range 1.14–1.81. For measurement of atmospheric NH 3, the 4- and 6-h filter results were about 16 % lower than those from the Fourier transform infrared method.