HNO3, HONO, HCOOH, and CH3COOH

J. Dibb

University of New Hampshire

The gas phase and firn air measurements by UNH investigators are critically important to the major science objectives of ANTCI. During the 2003 ground based study, measurements of HONO, HCOOH, and CH3OOH, as HOx sources, will provide insight into why OH is elevated at the South Pole and help assess measurement/model discrepancies resulting from the ISCAT 2000 data. As a gas phase sink for both OH and NOx and a potential source of NO in the snow, HNO3 measurements will also play an important role in understanding the photochemistry and nitrogen budget at SP. These measurements will be conducted just above and below the firn surface during the first year (2003) of ANTCI.
Mist chamber sampling, using ultrapure water as the trapping solution, with ion chromatographic analysis (IC) immediately after sample collection, will be used to quantify several soluble acidic gases in the lower atmosphere and in the firn air at SP. This technique has been used for a series of airborne sampling campaigns, for related studies at Summit, Greenland, and during the 2000 ISCAT experiment at SP. The mist chamber sampler operates by generating a dense cloud of fine water droplets that quantitatively strip soluble acids from the sampled air stream. Extensive laboratory and field testing has established the reliability of the technique for quantitation of HNO3 and the carboxylic acids, and established that interference by other atmospheric gases are minimal for these acids. During ISCAT 2000 a day-long intercomparison at 10 m above the snow showed very good agreement with CIMS measurements of HNO3 at mixing ratios that averaged < 15 ppt. A similar opportunity will be available in the 2003 study.
The technique has not been fully characterized for quantitation of HONO. Laboratory testing has established that this acid is quantitatively collected, but possible interferences exist. Nitrous acid is analyzed as NO2^- in the stripping solution, hence any soluble N-oxide that dissociates to yield this ion could contribute to a positive HONO artifact. Based on our evolving understanding of N oxide photochemistry in and near the snow at Summit, it is tentatively assumed that HONO makes the dominant contribution to measured NO2^- in this environment. Inferred mixing ratios of HONO during ISCAT 2000 were problematically high, suggesting that one or more interferants may be significant at SP. Direct intercomparison of the mist chamber and the spectroscopically specific LIF technique for HONO measurement is one of our objectives during ANTCI, which should help unravel previous discrepancies.
NIST traceable standard solutions are used to calibrate the IC. Based on extensive previous testing, it is assumed that the samplers collect the target gases quantitatively (i.e., gas phase standards are not generated for calibration due to logistic difficulties operating outside in polar regions). Detection limits are a direct function of sample integration time. 30-minute sample detection limits are < 1 ppt for all four gases. Because samples are analyzed immediately, it is possible to respond to very clean air by simply sampling longer, hence very few samples are reported as below detection limit.
Two mist chamber samplers will be operated simultaneously during the ANTCI experiment. One can be fitted with a 2-m long heated inlet that allows sampling at any height within 1 m (above or below) the air/snow interface. Operating two samplers allows comparisons between the firn air and lower atmosphere, and also gradient measurements just above the snow in a manner similar to previous studies. Measurements of firn air also provide much insight into firn source strengths and variability. In addition, the UNH will collect surface and snowpit samples for quantitation of soluble ions. These samples will be returned frozen (on the ship) for analysis in the Climate Change Research Center IC laboratory.