Gaseous deposition to snow: 1. Experimental study of SO2 and NO2 deposition

Abstract

A series of 48 laboratory experiments has determined depth profiles of S(IV) and S(VI) accumulation in snow exposed to 20–140 ppb SO2 for 6–12 hours. Surface deposition velocity, calculated from the amount of sulfur taken up, divided by the gas‐phase concentration, averaged 0.06 cm s−1. Snow held near −2°C had an average deposition velocity of 0.04 cm s−1, with new snow having a value about double that for older, metamorphosed snow (0.05 versus 0.02 cm s−1). Snow held near 0°C had an average deposition velocity of 0.07 cm s−1, and snow held below zero, but with surface melting due to sunlight, had a value of 0.06 cm s−1. When sunlight and temperature allowed draining of meltwater, deposition velocities were much higher (0.14 cm s−1). Deposition velocities for snow at −2°C 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 present. Uptake of SO2 by snow is apparently determined largely by the liquid‐water‐to‐air ratio in the snowpack. SO2 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 NO2 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.