Chemistry of atmospheric precipitation in the north-central united states: Influence of sulfate, nitrate, ammonia and calcareous soil particulates

Abstract

The supply of alkaline soil dust and gaseous NH 3 available to neutralize anthropogenic acids in the atmosphere controls the acidity of precipitation in the north-central United States. Major ions and trace metals were determined in precipitation-event and snow-core samples from sites along a 600 km transect from the North Dakota prairie to the forests of northeastern Minnesota, collected during the period April 1978–June 1979. Acidity increased 4-fold from west to east as the effect of alkaline dust and NH 3 decreased with increasing distance from the cultivated prairie; calcium and Mg 2+ decreased 2 to 3-fold across the transect. However, minimum concentrations of NH 4 + and SO 4 2− were observed at Itasca, the central site. Natural emissions of these elements were important in the west, while anthropogenic emissions were responsible for the higher concentrations in the east. Wet deposition of H + decreased 8-fold and deposition of NO 3 − and SO 4 2− decreased 1.5 to 2-fold from Hovland in the east to Tewaukon in the west. Wet deposition of the metal cations increased from Hovland to Tewaukon. Dry deposition followed a similar trend. Winter snow cover and freezing temperatures, which decreased airborne soil dust and the evolution of NH 3 from the prairie soils, led to an increase in precipitation acidity at all sites. The acid increase was accompanied by a decrease in alkaline metal cations, especially Ca 2+, and in NH 4 +. At Hovland SO 4 2− and NO 3 − also increased during the winter. The occurrence of snow events at Tewaukon that were appreciably more acid than the snowpack accumulated there indicates that snow was neutralized after it fell by alkaline dust entrained in resuspended snow, or deposited separately. Winter inputs of acid are especially important because they are released during a short period in the spring. Over half of the acid input at Hovland occurred during the winter. Precipitation inputs of P and N probably benefit nutrient-poor ecosystems in the north-central United States. However, acid precipitation at Hovland is approaching levels known to cause adverse effects in sensitive regions. The geology of northeastern Minnesota, adjoining Ontario, and northern Wisconsin is similar to that in areas where lake acidification and losses of fish populations have occurred, so that such problems may eventually occur in central North America.