South Pole Antarctica observations and modeling results: New insights on HOx radical and sulfur chemistry

Abstract Measurements of OH, H 2 SO 4 , and MSA at South Pole (SP) Antarctica were recorded as a part of the 2003 Antarctic Chemistry Investigation (ANTCI 2003). The time period 22 November, 2003–2 January, 2004 provided a unique opportunity to observe atmospheric chemistry at SP under both natural conditions as well as those uniquely defined by a solar eclipse event. Results under natural solar conditions generally confirmed those reported previously in the year 2000. In both years the major chemical driver leading to large scale fluctuations in OH was shifts in the concentration levels of NO. Like in 2000, however, the 2003 observational data were systematically lower than model predictions. This can be interpreted as indicating that the model mechanism is still missing a significant HO x sink reaction(s); or, alternatively, that the OH calibration source may have problems. Still a final possibility could involve the integrity of the OH sampling scheme which involved a fixed building site. As expected, during the peak in the solar eclipse both NO and OH showed large decreases in their respective concentrations. Interestingly, the observational OH profile could only be approximated by the model mechanism upon adding an additional HO x radical source in the form of snow emissions of CH 2 O and/or H 2 O 2 . This would lead one to think that either CH 2 O and/or H 2 O 2 snow emissions represent a significant HO x radical source under summertime conditions at SP. Observations of H 2 SO 4 and MSA revealed both species to be present at very low concentrations (e.g., 5 × 10 5 and 1 × 10 5  molec cm −3 , respectively), but similar to those reported in 2000. The first measurements of SO 2 at SP demonstrated a close coupling with the oxidation product H 2 SO 4 . The observed low concentrations of MSA appear to be counter to the most recent thinking by glacio-chemists who have suggested that the plateau's lower atmosphere should have elevated levels of MSA. We speculate here that the absence of MSA may reflect efficient atmospheric removal mechanisms for this species involving either dynamical and/or chemical processes.