First global distributions of nitric acid in the troposphere and the stratosphere derived from infrared satellite measurements

Global distributions of nitric acid (HNO 3 ) partial columns in the troposphere and the stratosphere are shown for the first time. HNO 3 vertical profiles are retrieved from a set of high‐resolution infrared spectra, measured by the nadir‐viewing Interferometric Monitor for Greenhouse Gases (IMG) instrument onboard the ADEOS satellite. Ten successive days of IMG operation in April 1997 are analyzed, yielding quasi‐global distributions. We show that the IMG measurements contain between 0.7 and 1.8 independent pieces of information on the HNO 3 vertical distribution; the extent of which depends on the observed scene. In the tropics and the midlatitudes, tropospheric and stratospheric columns can be separated, whereas, in the colder polar regions, the total column is the most relevant quantity. A detailed error budget reveals that the accuracy on the profile measurements ranges from 5–15% in the stratosphere to about 20–30% in the middle troposphere, and as much as 60% in the lowermost troposphere where the measurements are less sensitive. The global distributions of tropospheric and stratospheric partial columns are presented and discussed. In the stratosphere we measure higher columns at both poles than in the intertropical belt, as expected for the period in April analyzed here. In the troposphere, the zonal distributions show elevated values nearby NO x source regions, such as central Europe, the Eastern coast of the United States, and the North‐West of India. Typical profiles with mixing ratios up to 2 ppbv in the boundary layer and 1 ppbv in the free troposphere are identified. These novel results indicate a possible role of HNO 3 in the long‐range transport of active nitrogen and open promising perspectives for future space missions dedicated to atmospheric chemistry.