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 22pptv and ranged from less than 5 to 68pptv. 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 NO_x 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 NO_x upwind of SP. The measured SO₂ mixing ratio was found to be less than 20pptv 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.