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• #### Nitrogen oxides in the arctic stratosphere: Implications for ozone abundances

In the high latitude winter stratosphere, NO$\sb2$ sequesters chlorine compounds which are extremely efficient at destroying ozone. During the nighttime, NO$\sb2$ reacts with ozone to form $\rm N\sb2 O\sb5$ which acts as a reservoir of NO$\sb2$. Under heavy aerosol loading, $\rm N\sb2O\sb5$ may react with water on aerosol surfaces to form HNO$\sb3$, a reservoir more resistant to photolysis. This heterogeneous reaction results in reduced NO$\sb2$ concentration when the sun returns at the end of the winter. A spectrograph system has been developed to measure scattered zenith skylight and thereby determine stratospheric NO$\sb2$ slant column abundance. Conversion of the measured slant column abundance to vertical column abundance requires dividing by the air mass. The air mass is the enhancement in the optical path for the scattered twilight as compared to a vertical path. Air mass values determined using a multiple scattering radiative transfer code have been compared to those derived using a Monte Carlo code and were found to agree to within 6% at a 90$\sp\circ$ solar zenith angle for a stratospheric absorber. Six months of NO$\sb2$ vertical column abundance measured over Fairbanks during the winter 1992-93 exhibited the daylight diminished and increased as the sunlight hours lengthened. The overall seasonal behavior was similar to high-latitude measurements made in the Southern Hemisphere. The ratios of morning to evening column abundance were consistent with predictions based on gas-phase chemistry. The possible heterogeneous reaction of $\rm N\sb2O\sb5$ on sulfate aerosols was investigated using Fourier Transform Infrared Spectrometer measurements of $\rm HNO\sb3$ column abundance and lidar determinations of the aerosol profile. Using an estimated $\rm N\sb2O\sb5$ column abundance and aerosol profile as input to a simple model, significant $\rm HNO\sb3$ production was expected. No increase in $\rm HNO\sb3$ column abundance was measured. From this set of data, it was not possible to determine whether significant amounts of $\rm N\sb2O\sb5$ were converted to $\rm HNO\sb3$ by this heterogeneous reaction. Better estimates of the $\rm N\sb2O\sb5$ and aerosol profile, and a more continuous set of $\rm HNO\sb3$ measurements, are needed to determine if $\rm HNO\sb3$ was actually produced.