Climate-change signals in national atmospheric deposition program precipitation data

National Atmospheric Deposition Program (NADP)/National Trends Network precipitation type, snow-season duration, and annual timing of selected chemical wet-deposition maxima vary with latitude and longitude within a 35-year (1979–2013) data record for the contiguous United States and Alaska. From the NADP data collected within the region bounded by 35.6645°–48.782° north latitude and 124°–68° west longitude, similarities in latitudinal and longitudinal patterns of changing snow-season duration, fraction of annual precipitation recorded as snow, and the timing of chemical wet-deposition maxima, suggest that the chemical climate of the atmosphere is linked to physical changes in climate. Total annual precipitation depth has increased 4–6 % while snow season duration has decreased from approximately 7 to 21 days across most of the USA, except in higher elevation regions where it has increased by as much as 21 days. Snow-season precipitation is increasingly comprised of snow, but annually total precipitation is increasingly comprised of liquid precipitation. Meanwhile, maximum ammonium deposition occurs as much as 27 days earlier, and the maximum nitrate: sulfate concentration ratio in wet-deposition occurs approximately 10–21 days earlier in the year. The maximum crustal (calcium + magnesium + potassium) cation deposition occurs 2–35 days earlier in the year. The data suggest that these shifts in the timing of atmospheric wet deposition are linked to a warming climate, but the ecological consequences are uncertain.