HETEROGENEOUS CONVERSION OF N2O5 TO HNO3 IN THE POST-MOUNT-PINATUBO ERUPTION STRATOSPHERE

Simultaneous stratospheric volume mixing ratio (VMR) profiles of dinitrogen pentoxide (N2O5) and nitric acid (HNO3) at sunrise between 25-degrees-N and 15-degrees-S latitude and profiles of HNO3 at sunset between 42-degrees-S and 53-degrees-S latitude have been derived from 0.01 cm-1 resolution infrared solar occultation spectra recorded 9 1/2 months after the massive eruption of the Mount Pinatubo volcano in the Philippine Islands. The measurements were obtained by the atmospheric trace molecule spectroscopy (ATMOS) Fourier transform spectrometer during the ATLAS 1 shuttle mission (March 24 to April 2, 1992). The measured HNO3 VMRs are higher at all altitudes and latitudes than corresponding values measured by the limb infrared monitor of the stratosphere (LIMS) instrument during the same season in 1979, when the aerosol loading was near background levels. The largest relative increase in the HNO3 VMR occurred near the equator at 30-km altitude, where the ATMOS/ATLAS 1 values are about a factor of 2 higher than the LIMS measurements. Two-dimensional model calculations show that the increase in HNO3 and the ATMOS/ATLAS 1 measurement of a steep decrease in the N2O5 VMR below 30 km can be explained by the enhanced conversion of N2O5 to HNO3 on the surfaces of the Mount Pinatubo sulfate aerosols. Our profile results demonstrate the global impact of the N2O5 + H2O --> 2HNO3 heterogeneous reaction in altering the partitioning of stratospheric odd nitrogen after a major volcanic eruption.