OH, HO2+RO2, H2SO4, and MSA F. Eisele, R. Mauldin, and E. Kosciuch National Center for Atmospheric Research

The NCAR chemical measurements team will provide direct observations of OH, HO2/RO2, H2SO4 and MSA at the South Pole in 2003 and these same compounds plus DMSO, and DMS on the C-130 in 2005. These measurements are important to all three major science objectives. In the 2003 study it will be critical that NOx and HOx sources and their relationship to HOx concentrations be well understood if recently observed Antarctic photochemistry is to be considered fully credible. Also of central importance will be measurements of sulfur source species as well as their oxidation products. These will ultimately define the MS and sulfate levels that are deposited on snow surfaces in Antarctica to become part of the sulfur ice record. As part of the first year (2003) ground-based study, OH, HO2/RO2, H2SO4 and MSA concentrations will be measured using an improved version of the selected ion chemical ionization mass spectrometric (SICIMS) instrument. This is the same basic instrument used previously at the South Pole. The major modification to the original system will be to the HO2/RO2 measurements. Previously, these could only be measured by switching the system over manually which required an hour or greater cycling time. In the new system this operation will be automated and take place within 5 min. The addition of HO2 measurements will place greater constraints on the models and provide important new insight as to how the HOx budget is being enhanced through direct production of HO2 by surfaced released CH2O versus direct OH production from the two other released species, H2O2 and HONO. A 4-channel version of this same instrument [Mauldin et al., 2001b] will simultaneously provide measurements of OH, HO2/RO2, gas phase H2SO4 and MSA, DMSO and DMS aboard the NCAR C-130 in the 2005 field study.

The ground-based instrument and the expanded 4-channel mass spectrometer version have been used extensively to measure OH, H2SO4 and MSA with a 30 second time resolution, both on the ground and in NASA and NSF aircraft campaigns from Greenland to the South Pole. The OH measurement technique has been validated in several informal intercomparisons. The new HO2/RO2 measurements will be calibrated utilizing the H2O photolysis at 184.9 nm in ambient air. No attempt will be made to separate HO2 and RO2 concentrations, but RO2 concentrations in Antarctica should be quite low compared to HO2. Measurements of OH, HO2/RO2, and gas phase H2SO4 and MSA will be accomplished using NO3^-as the reactant ion. The demonstrated detection limits of this technique for OH, H2SO4 and MSA are all in the range of 1 x 10⁵ molecules cm^-3 or less for a 5 minute integration time and for a one hour integration the LOD drops by ~ a factor of 5. The ambient concentration of HO2/RO2 is typically 5-100 times higher than OH, and since HO2 and RO2 must be converted into OH before being measured, their detection limit will be ~5 times higher. All of these detection limits are sufficient to measure the compounds of interest in Antarctica, and essentially define the detection limits that currently exist for these compounds.