Measurements of OH, HO ₂+RO ₂, H ₂SO ₄, and MSA at the South Pole during ISCAT 2000

Measurements of hydroxyl radical (OH), sulfuric acid (H ₂SO ₄) and methane sulfonic acid (MSA) together with the first HO ₂+RO ₂ observations at the South Pole are presented. These results were recorded as part of the Investigation of Sulfur Chemistry in the Antarctic Troposphere 2000 (ISCAT 2000) study. OH concentrations were found to be highly elevated, having a mean value over the time period of 15 November-30 December 2000 of 2.5×10 ⁶ molecule cm ^-3, thus confirming the results from ISCAT 1998. Although data were more limited for the sum of HO ₂+RO ₂, a mean value of ∼7×10 ⁷ molecule cm ^-3 was estimated. Typically, OH and HO ₂+RO ₂ both showed large day-to-day variability. Box model simulations suggest that most of this variability was a direct result of elevated and highly variable levels of nitric oxide. Comparisons of OH with overhead O ₃ column density measurements revealed that for certain time periods as much as a 30% enhancement occurred in OH as a result of decreases in column O ₃ levels. Like ISCAT 1998, the observational data for H ₂SO ₄ and MSA generally showed very low concentrations with mean values of 2.7×10 ⁵ and 8×10 ⁴ molecule cm ^-3, respectively. When compared against measured levels of particulate sulfate and methane sulfonate, these low gas phase concentrations indicate, as suggested by the more limited data from the ISCAT 1998 study, that local production of gas phase sulfur species contributes little to particle composition.