South Pole HOx Chemistry: New Insights Based on ISCAT 2000 Observations

During the ISCAT 1998 field study a major finding was the observation that highly elevated levels of OH were present at South Pole (SP). These levels were found to be closely linked to unexpectedly high observed levels of NO released from the snowpack. The modeling interpretative analysis of these data indicated that at high levels of NO the reaction HO2 + NO was the dominant SP OH source but with CH4 oxidation being the major HOx source. Model to observation comparisons revealed that for about 80% of the data the agreement with model predictions was well within the combined uncertainties. However, for approximately 10% of the data, which coincided with the highest values of observed NO, model predicted OH values were found to be significantly lower than those observed. The argument put forward to explain this apparent discrepancy was that for the highest NO levels additional HOx sources had become significant. In particular, snow emissions of CH2O, H2O2, and/or HONO were considered. To a first approximation the concentration levels of these species might be expected to scale with those for NO. Although invoking reasonable concentration levels for these potential HOx sources did indeed demonstrate that model predictions and observations could be reconciled, the absence of hard measurements for these sources left our understanding of HOx photochemistry at South Pole incomplete. The recent ISCAT 2000 field program has provided direct measurements of all three of the proposed HOx sources. In addition, measurements of the very important HO2 species were recorded for the first time at SP. Thus, these new measurements will be used as model constraints in a more detailed SP HOx analysis to be presented. Also to be reported is a new HOx budget analysis that shows the relative importance of HOx source species, and the possible importance of additional HOx sinks.