Soluble Acidic Gases at South Pole During ISCAT 2000

We measured HNO₃, HONO, HCOOH and CH₃COOH in the atmosphere and the firn pore air at South Pole with the mist chamber/ion chromatography (MC/IC) technique 13-27 December. Two MC samplers were operated simultaneously, one was always 85 cm above the snow surface, the other was used to sample at various heights between 30 cm below the surface up to 85 cm above. On 8 of the MC/IC sampling days we measured HNO₃ and HO₂NO₂ 10 m above the snow with a CIMS technique. Average concentrations of the soluble acids 85 cm above the snow were high for such a remote site (38, 32, 159 and 310 ppt of HNO₃, HONO, HCOOH and CH₃COOH), presumably reflecting release from the snow. Mixing ratios in the firn air (10 to 30 cm deep) were measured on 7 days and consistently revealed large enhancements of HONO, HCOOH, and CH₃COOH (2 to 10, 2 to 50, and 1.5 to 20 times higher than ambient, respectively). In contrast, HNO₃ mixing ratios in firn air ranged from 10 to 90% of ambient values at the same time. Attempts to measure gradients in the bottom 85 cm of the atmosphere (to confirm snow to air fluxes) were inconclusive. However, on five of the days when we have HNO₃ measurements at both 85 cm and 10 m above the snow the mixing ratios were elevated (1.5 to 3 fold) at the lower elevation. To ensure that these results were not due to a bias between the two instruments, we sampled with one MC inlet adjacent to the CIMS inlet 10 m above the snow for 13 hours one day. Mixing ratios by MC/IC ranged 12 to 17 ppt during this period, on average the difference between MC and CIMS values was just less than 1 ppt (range -4 to 5 ppt) and the ratio of the measurements averaged 1.17. This level of agreement suggests that the persistent gradients measured between 0.85 and 10.0 m on other days were real. Lower HNO₃ mixing ratios in firn air than ambient argue against flux of HNO₃ out of the snow, thus we suggest that HNO₃ is higher just above the snow because it is forming there from abundant NO₂ and OH.