Progress toward clean cloud water at Whiteface Mountain New York

Fossil fuel combustion is the primary source of anthropogenic acidity in cloud water. Since 1994 there is a measurable decrease in hydrogen, sulfate, and nitrate ion concentrations in cloud water collected at Whiteface Mountain. In this paper we assess these changes from the point of view of progress toward clean air conditions.The cleanest clouds crossing Whiteface Mountain, those with the lowest total ion concentrations, are found to have pH values in the range 5.0–5.25. Furthermore, there is a linear relationship between pH, sulfate and nitrate ion concentrations in cloud water. With this in mind, we define clean cloud water values of sulfate, nitrate and hydrogen as mean concentrations observed in cloud water samples with pH in the range 5.0–5.25.We then compare mean annual values to clean air values to determine annual ratios. In 1994, ratios for SO42−, NO3− and H+ were respectively 26.9, 13.1 and 29.9 times above the clean air value. In 2009, the SO42−, NO3− and H+ corresponding ratios were 4.2, 2.7, and 4.8 times above the clean air value. In other words, comparison of the 1994 and 2009 results suggest reductions in anthropogenic concentrations of SO42−, NO3− and H+, by 84%, 79% and 84% respectively.To verify our approach, we have calculated corresponding changes in the aerosol SO42− ratio, with clean air concentrations equal to the natural background aerosol SO42− value used for the Regional Haze Rule. These results compared favorably to our cloud water SO42− ratio.

► Identification of cleanest clouds passing over Whiteface Mountain New York.
► We examine cloud water values since 1994 and compare to emission reductions.
► SO4 analyses compares favorably to the background value used for Regional Haze Rule.
► We show acidity improvements from policy decisions but there is more work to do.