JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 92, NO. D8, PAGES 9779-9787, AUGUST 20, 1987 GASEOUS DEPOSITION TO SNOW 1. EXPERIMENTAL STUDY OF SO 2 AND NO 2 DEPOSITION Marc P. Valdez Department of Atmospheric Sciences, University of Arizona, Tucson Roger C. Bales and Deena A. Stanley Department of Hydrology and Water Resources, University of Arizona, Tucson George A. Dawson Department of Atmospheric Sciences, University of Arizona, Tucson Abstract. A series of 48 laboratory experi- ments has determined depth profiles of S(IV) and S(VI) accumulation in snow exposed to 20-140 ppb SO 2 for 6-12 hours. Surface deposition velocity, calculated from the amount of sulfur taken up, divided by the gas-phase concentration, averaged 0.06 cms -1. Snow held near -2øC had an average deposition velocity of 0.04 cm s -1, with newsnow having a value about double that for older, meta- morphosed snow (0.05 versus 0.02 cms-l). Snow held near 0øC had an average deposition velocity of 0.07 cms -1, and snow held below zero, but with surface melting due to sunlight, had a value of 0.06 cms -1. When sunlight and temperature allowed draining of meltwater, dep?sition veloc- ities were much higher (0.14 cm s ). Deposition velocities for snow at -2øC were as much as 60% higher for measurements over 6 hours versus 10-12 hours. Penetration of sulfur into the snow was generally about 7 cm for colder experiments and only 4 cm for snow with more liquid water pres- ent. Uptake of SO 2 by snow is apparently deter- mined largely by the liquid-water-to-air ratio in the snowpack. SO 2 and ozone concentrations had only small effects on deposition velocity. Most sulfur in the snow was found as S(VI), even in the absence of ozone, indicating that another oxidant is readily available, especially in new snow. Sunlight had no effect on uptake, other than increasing the liquid-water-to-air ratio by surface melting of the snow. Four measurements of NO 2 deposition to snow were also made; uptake was small, with deposition velocities averaging 0.005 cm s-1 in the dark and 0,012 cm s-1 in the sunlight. 1. Introduction Dry deposition of SO 2 to seasonal snow covers is an area of surprisingly little previous work [Whelpdale and Shaw, 1974; Barrie and Walmsley, 1978; Dovland and Eliassen, 1976; Granat and Johansson, 1983; Cadle et al., 1985], despite its importance in winter accumulation of acidity in mountain snowpacks. The subsequent "acid shock" phenomenon resulting from preferential release of ions during the early part of snowpack melting, is well-documented [e.g., Haapala et al., 1975; Skartveit, 1981; Skartveit and Gjessing, 1979]. Copyright 1987 by the American Geophysical Union. Paper number 7D0421. 0148-0227/87/007D-0421505.00 The concentration of ions in early meltwater has been observed to be 2-5 times the average [Johan- nessen et al., 1975; Cadle et al., 1984; Tsiouris et al., 1985] and varies depending on the original distribution in the snow [Colbeck, 1981]. Related studies to date on the accumulation and loss of ions have emphasized the wet and dry deposition of particles [e.g., McMahon and Denison, 1979; Slinn, 1977; Scott, 1981; Garland, 1978; Ibrahim et al., 1983], the wet deposition of gases [e.g., Brimblecombe and Dawson, 1984; Summers, 1977], and the movement of ions in melt- waters of seasonal snow covers [e.g., Johannessen and Henriksen, 1978; Tranter et al., 1986]. In none of these other studies are the nature of the snowpack and chemical interactions at the snow grain surfaces explicitly considered. By con- trast, dry deposition of SO 2 is a multistep proc- ess involving adsorption/absorption of gas trans- ported to the snow-air interface, followed by oxidation to produce sulfate. Both the chemical and physical steps will influence sulfate accumu- lation. The extent of accumulation should there- fore depend on the nature of the snow (e.g., wet versus dry, metamorphosed or freshly fallen) and the presence of oxidants. Little is known of the factors affecting the uptake of NO 2. The purpose of the work described in this paper was to elucidate these physical and chem- ical interactions affecting the uptake of SO 2 by snow. Specifically, the influences of ozone, illumination, temperature, and exposure time on SO 2 deposition velocity were investigated experi- mentally (this paper) and theoretically in a com- panion paper [Bales et al., this issue]. A few measurements of NO 2 uptake were also made. 2. Methods 2.1. Study Site The study was undertaken at the Fort Valley research station of the U.S. Forest Service •ear Flagstaff, Arizona, at an elevation of 2250 m. The weather during the study period (December 1985 through March 1986) was mild. About half of the experiments used snow from an early winter storm in November 1985. This early snow was very well metamorphosed, with rounded grains of 0.5- to 1-mm diameter. Several smaller storms in February and March deposited fresh snow. Night- time temperatures throughout the period were generally -3 ø to -12øC; daytime temperatures were nearly always above 0øC, and highs were often in 9779