Chemical patterns of bulk atmospheric deposition in the state of Colorado

Samples of bulk deposition from the atmosphere were taken over a 1‐year interval between May 1982 and May 1983 at 42 sites distributed over the state of Colorado. Collections were complete for the entire year at 36 of these sites. The collections were made biweekly and were analyzed for major cations, major anions, total particulate material, and the hydrogen ion ( p H). A p H below 5.6 was characteristic of the annual precipitation over two thirds of the state, including all of the high‐elevation areas in central Colorado as well as substantial portions of the plains. The lowest p H values (average below 5.0) were found near the Continental Divide in the northern third and southern third of the state. Emissions of NO x and SO 2 from power plants within or near the borders of the state are of a magnitude similar to that of the total deposition of SO 4 = and NO 3 − N within the state. Gradients of acidity within the state are to a large degree explained by three factors: (1) location of power plants and urban areas with respect to prevailing winds, (2) differential transport of strong acids and alkalis, and (3) amount of wet precipitation. At lower elevations, carbonates neutralize strong mineral acids, as shown by the distributions of strong acid anions and divalent cations. Because of their association with terrestrial particulate materials, alkalis are transported over shorter distances than the gaseous precursors of mineral acids. Thus air masses moving over the mountains, which lack large sources of either alkalis or acids, show an increasing bias toward acidifying substances at higher elevations. This causes a statewide negative relation between p H and elevation. Superimposed on the elevation effect is a moisture effect caused by a strong relationship between strong acid deposition and amount of wet precipitation.