Sources and sinks of H2SO4 in the remote Antarctic marine boundary layer
A. Jefferson,1 D. J. Tanner,1 F. L. Eisele,1,2 and H. Berresheim3
Abstract. A steady state analysis of H2SO4 sources and sinks in the Antarctic marine boundary layer was performed using measurements from project SCATE, Sulfur Chemistry in the Antarctic Troposphere Experiment. The calculations show that the SO2 levels needed to account for the observed gas phase H2SO4 ranged from about 17 to 300 parts per trillion by volume (pptv) with an average SO2 concentration of 100 pptv, far more than previous measurements of SO2 in this region which range between 7 and 17 pptv [Berresheim, 1987; Pszenny et al., 1989; P. Quinn, personal communication, 1996]. Boundary layer oxidation of DMS via an SO2 intermediate was found to be an insufficient source of H2SO4 in this region. Likely alternative sources of H2SO4 include oxidation of boundary layer DMDS and vertical entrainment of air from higher altitudes.
Selected Ion Chemical Ionization Mass Spectrometric Measurement of OH
D. Tanner, A. Jefferson and F. Eisele*
Atmospheric Chemistry Division, National Center for Atmospheric Research, Boulder, CO 80307
Abstract
Ion assisted OH measurements are presented from two different standpoints. First, instrument operation and calibration are discussed along with recent improvements in measurement techniques. A chemical model of inlet titration chemistry in both clean and polluted air is also included in this section. The results of OH measurements using the present technique are presented for the entire campaign effort. Hydroxyl radical concentrations of 2-4 x 106 molecules cm-3 were typical at noon for clean, clear days. In polluted air masses OH concentrations up to 8 x 106 molecules cm-3 were observed. Variation of OH with changing solar flux and differing air masses is clearly shown and discussed.
OH photochemistry and methane sulfonic acid formation
in the coastal Antarctic boundary layer
A. Jefferson,1 D. J. Tanner,1 F. L. Eisele,1,2 D. D. Davis,2 G. Chen,2
J. Crawford,2 J. W. Huey,3 A. L. Torres,4 and H. Berresheim5
Abstract. Studies of dimethylsulfide (DMS) oxidation chemistry were conducted at Palmer Station on Anvers Island, Antarctica, during the austral summer of 1993/1994. Part of the study involved gas phase measurements of OH, methane sulfonic acid (MSA), and H2SO4 using a chemical ionization mass spectrometer, as well as measurements of the NO, CO, and O3 concentrations. Mean 24 hour concentrations from February 16-23 of OH, MSA, and H2SO4 were 1.1 ´ 105, 9.5 ´ 105, and 1.61 ´ 106 molecules cm-3, respectively. Model calculations of OH compared well with observed levels (e.g., within 30%). The modeling results suggest that the dominant source of OH is from the reaction of O(1D) with H2O, where O(1D) is the product of O3 photolysis. Because of the clean atmospheric environment and predicted low nonmethyl hydrocarbon levels in Antarctica, the dominant OH sink was found to be reaction with CO and CH4. Particulate levels of MSA were higher than could be attributed to condensation of boundary layer (BL) gas phase MSA on to the aerosol surface. Alternate mechanisms for generating MSA in the particle phase were speculated to involve either in-cloud oxidation of dimethylsulfoxide or OH oxidation of DMS in the atmospheric buffer layer above the boundary layer followed by condensation of gas phase MSA on aerosols and transport back to the BL [Davis et al., this issue].
Measurements of the H2SO4 mass accommodation coefficient onto polydisperse aerosol
Abstract. The loss rate of H2SO4 vapor onto submicron particles was measured for three different particle substrates. The experimental technique involved direct flow tube measurements of H2SO4 decay rates onto a polydisperse aerosol using chemical ionization mass spectroscopic detection. The aerosols of this study were partially hydrated crystalline salts with diameters in the size range of 20 to 400 nm. The mass accommodation coefficients, a, were calculated from the first - order rate constants for H2SO4 loss to be 0.73 + 0.21 and 0.79 + 0.23 for loss onto (NH4)2SO4 and NaCl, respectively. Measurements of the loss rate of H2SO4 onto a NaCl aerosol coated with stearic acid resulted in lower mass accommodation coefficients with values of 0.31 and 0.19 for aerosol with high and low stearic acid coverage, respectively. The observed decrease in a on an aerosol with a hydrocarbon coating suggests that aerosol composition is a key factor in H2SO4 adsorption on to a particle surface.