Chemistry of Wintertime Wet Deposition

Abstract

Winter precipitation differs from summer precipitation in the northeastern United States and eastern Canada, as evidenced by a higher SO/sub 4//sup - -//NO/sub 3//sup -/ ratio in summer than winter. Part of this difference can be attributed to differences between rain and snow since snow has a lower SO/sub 4//sup - -//NO/sub 3//sup -/ ratio than summer rains, or even winter rains. Previous researchers found higher concentrations of all species in snow than rain, but NO/sub 3//sup -/, in particular, was far higher in snow. They could not attribute these differences to air temperatures, synoptic patterns, precipitation rate, wind direction or wind speed. They suggested that since both winter rain and snow originate from sub-freezing clouds, differences must be due to more efficient below-cloud scavenging by snowflakes than raindrops. Higher NO/sub 3//sup -/ levels were found in snow than winter rain which was attributed to higher HNO/sub 3/ scavenging by snow. This is in qualitative agreement with HNO/sub 3/ scavenging studies. The authors examined four years of winter precipitation data from southeastern Michigan. Precipitation concentrations of SO/sub 4//sup - -/ and NO/sub 3//sup -/ are considered in terms of precipitation amount, ambient concentrations, wind direction and cloud temperatures.