Nitrogen Oxides in the Arctic Stratosphere: Implications for Ozone Abundances.
In the high latitude winter stratosphere, NO _2 sequesters chlorine compounds which are extremely efficient at destroying ozone. During the nighttime, NO_2 reacts with ozone to form rm N_2 O_5 which acts as a reservoir of NO_2. Under heavy aerosol loading, rm N_2O_5 may react with water on aerosol surfaces to form HNO_3, a reservoir more resistant to photolysis. This heterogeneous reaction results in reduced NO_2 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_2 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 ^circ solar zenith angle for a stratospheric absorber. Six months of NO_2 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_2O_5 on sulfate aerosols was investigated using Fourier Transform Infrared Spectrometer measurements of rm HNO_3 column abundance and lidar determinations of the aerosol profile. Using an estimated rm N _2O_5 column abundance and aerosol profile as input to a simple model, significant rm HNO_3 production was expected. No increase in rm HNO_3 column abundance was measured. From this set of data, it was not possible to determine whether significant amounts of rm N_2O_5 were converted to rm HNO_3 by this heterogeneous reaction. Better estimates of the rm N_2O _5 and aerosol profile, and a more continuous set of rm HNO_3 measurements, are needed to determine if rm HNO_3 was actually produced.
- Pub Date:
- Physics: Atmospheric Science