Measurements of tropospheric nitric acid over the western United States and northeastern Pacific Ocean

Abstract

During the August–September 1986 GTE/CITE 2 aircraft mission, more than 240 measurements of nitric acid (HNO3) were made in the free troposphere as well as in the boundary layer over the northeastern Pacific Ocean and western continental United States. Marine HNO3 measurement results were strikingly similar to results from GAMETAG and other past atmospheric field experiments. The marine boundary layer HNO3 average, 62 parts per trillion by volume (pptv), was one third lower than the marine free tropospheric average, 108 pptv, suggesting that the boundary layer is a sink for tropospheric nitric acid, probably by dry deposition. Nitric acid measurements on a nighttime continental flight gave a free tropospheric average of 218 pptv, substantially greater than the daytime continental free tropospheric five‐flight average of 61 pptv. However, the nighttime results may have been influenced by highly convective conditions that existed from thunderstorms in the vicinity during that night flight. Our continental boundary layer HNO3 average of 767 pptv is an order of magnitude greater than the free tropospheric average, indicating that the boundary layer is a source of free tropospheric HNO3. The distribution of continental boundary layer HNO3 data, from averages of 123 pptv over rural Nevada and Utah to 1057 pptv in the polluted San Joaquin Valley of California suggests a close tie between boundary layer HNO3 and anthropogenic activity.