Trends in cation, nitrogen, sulfate and hydrogen ion concentrations in precipitation in the United States and Europe from 1978 to 2010: a new look at an old problem

Industrial emissions of SO2 and NOx, resulting in the formation and deposition of sulfuric and nitric acids, affect the health of both terrestrial and aquatic ecosystems. Since the mid-late 20th century, legislation to control acid rain precursors in both Europe and the US has led to significant declines in both SO4–S and H ? in precipitation and streams. However, several authors noted that declines in streamwater SO4–S did not result in stoichiometric reductions in stream H, and suggested that observed reductions in base cation inputs in precipitation could lessen the effect of air pollution control on improving stream pH. We examined long-term precipitation chemistry (1978–2010) from nearly 30 sites in the US and Europe that are variably affected by acid deposition and that have a variety of industrial and land-use histories to (1) quantify trends in SO4–S, H , NH4–N, Ca, and NO3–N, (2) assess stoichiometry between H and SO4–S before and after 1990, and (3) examine regional synchrony of trends. We expected that although the overall efforts of developed countries to reduce air pollution and acid rain by the mid-late 20th century would tend to synchronize precipitation chemistry among regions, geographically varied patterns of fossil fuel use and pollution control measures would produce important asynchronies among European countries and the United States. We also expected that control of particulate versus gaseous emission, along with trends in NH3 emissions, would be the two most significant factors affecting the stoichiometry between SO4–S and H . Relationships among H, SO4–S, NH4–N, and cations differed markedly between the US and Europe. Controlling for SO4–S levels, H ? in precipitation was significantly lower in Europe than in the US, because (1) alkaline dust loading from the Sahara/Sahel was greater in Europe than the US, and (2) emission of NH3, which neutralizes acidity upon conversion to NH4 , is generally significantly higher in Europe than in the US. Trends in SO4–S and H ? in precipitation were close to stoichometric in the US throughout the period of record, but not in Europe, especially eastern Europe. Ca in precipitation declined significantly before, but not after 1990 in most of the US, but Ca declined in eastern Europe even after 1990. SO4–S in precipitation was only weakly related to fossil fuel consumption. The stoichiometry of SO4–S and H ? may be explained in part by emission controls, which varied over time and among regions. Control of particulate emissions reduces alkaline particles that neutralize acid precursors as well as S-containing Responsible Editor: Dale Johnson K. Lajtha (&) Department of Crop and Soil Sciences, Oregon State University, Corvallis, OR 97331, USA e-mail: [email protected] J. Jones College of Earth, Ocean, and Atmospheric Sciences, Oregon State University, Corvallis, OR 97331, USA e-mail: [email protected] 123 Biogeochemistry DOI 10.1007/s10533-013-9860-2