Trends of Sulfur Dioxide in Air and Sulfate in Air and Precipitation

摘要

Time trends of ambient air and precipitation concentrations from 1989 to 2001 are presented for 43 sites in the United States,with 37 of the sites being east of the Mississippi River.The air quality data are from the Clean Air Status and Trends Network(CASTNet)and the precipitation data are from the National Atmospheric Deposition Program/National Trends Network(NADP/NTN).Both networks have as an objective to determine regional patterns and trends and thus the sites are located in rural areas as opposed to urban/suburban areas.Linear trend analysis was done on the annual median concentrations of the three sulfur species(aerosol and precipitation sulfate and gaseous sulfur dioxide)for the 43 sites to calculate the percent change over the 13-year period.Scatter plots were examined to determine that a linear trend model is reasonable.Almost all percent change values determined in this way were negative,that is,concentrations decreased over time.The median percent changes over the 13 years,by species,for 24 midwestern and northeastern sites are:-29% for aerosol sulfate,-45% for sulfur dioxide,and -29% for precipitation sulfate.It is particularly interesting that the sulfur dioxide decrease is greater than the aerosol sulfate decrease,and that the precipitation sulfate trend agrees better with the aerosol sulfate trend than with the ambient sulfur dioxide trend.The magnitude of the trends and the trend differences in space and between the three species are important features to be captured by regional sulfur models.Emissions of sulfur are compared to the air and precipitation trends and comments are made on the issue of linearity(proportionality)between emission reductions and trends in sulfur species concentrations.