Industrial emissions of SO
2 and NO
x, 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 SO
4–S and H
+ in precipitation and streams. However, several authors noted that declines in streamwater SO
4–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 SO
4–S, H
+, NH
4–N, Ca, and NO
3–N, (2) assess stoichiometry between H
+ and SO
4–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 NH
3 emissions, would be the two most significant factors affecting the stoichiometry between SO
4–S and H
+. Relationships among H
+, SO
4–S, NH
4–N, and cations differed markedly between the US and Europe. Controlling for SO
4–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 NH
3, which neutralizes acidity upon conversion to NH
4 +, is generally significantly higher in Europe than in the US. Trends in SO
4–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. SO
4–S in precipitation was only weakly related to fossil fuel consumption. The stoichiometry of SO
4–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 particulates, reducing SO
4–S and Ca more steeply than H
+, consistent with trends in the northeastern US and Europe before 1990. In contrast, control of gaseous SO
2 emissions results in a stoichiometric relationship between SO
4–S and H
+, consistent with trends in the US and many western European countries, especially after 1991. However, in many European countries, declining NH
3 emissions contributed to the lack of stoichiometry between SO
4–S and H
+.Recent reductions in NO
x emissions have also contributed to declines in H
+ in precipitation. Future changes in precipitation acidity are likely to depend on multiple factors including trends in NO
x and NH
3 emission controls, naturally occurring dust, and fossil fuel use, with significant implications for the health of both terrestrial and aquatic ecosystems.
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