Nitric acid (HNO3) in the upper troposphere and lower stratosphere at midlatitudes: New results from aircraft‐based mass spectrometric measurements

Extensive measurements of gaseous nitric acid (HNO 3 ) have been performed in the upper troposphere and lower stratosphere using aircraft-based ion-molecule reaction mass spectrometry (IMRMS). The measurements, which took place in summer and winter between November 1994 and July 1996, cover latitudes between 29°N and 57°N and altitudes between 5.5 and 13 km. According to meteorological analyses, potential vorticity values up to 9.5 PVU (1 PVU = 10 -6 m 2 s -1 K kg -1 ) were reached. Stratospheric HNO 3 mixing ratios ranged between 300 and 2200 parts per trillion by volume (pptv). In the upper troposphere, nitric acid mixing ratios ranged between 100 and 2000 pptv, with the largest values influenced by fast vertical transport from the planetary boundary layer. These values exceed previous measurements and model calculations. The relatively high upper tropospheric nitric acid mixing ratio indicates a large rate for NO recycling from gaseous nitric acid, and possibly also an increased efficiency of aerosol activation, which may lead to an increased albedo of cirrus clouds.