CIMS measurements of HNO3 and SO2 at the South Pole during ISCAT 2000

HNO3 and SO2 were measured by chemical ionization mass spectrometry at the South Pole (SP) during ISCAT 2000 (December, 2000). HNO3 mixing ratios averaged 22 pptv and ranged from less than 5 to 68 pptv. A simple steady state photochemical analysis indicates that most of the time HNO3 is deposited to the snow with a lifetime of the order of a few hours. Periods of relatively high levels of HNO3 and low levels of NO were observed when air from aloft was mixed downward, but the source of this HNO3 is uncertain. One explanation for these observations is that free tropospheric air, enriched with NOx at lower latitudes, descends to the surface at SP; this process could be an important source of nitrate to the Antarctic Plateau. Another explanation is that these descending air parcels were previously in contact with the surface and enriched with snowpack emissions of NOx upwind of SP. The measured SO2 mixing ratio was found to be less than 20 pptv on average. However, a simple steady state analysis of OH and H2SO4 observations indicates that average SO2 levels are most likely less than a few pptv.