We appreciate the value of the recent article by Brown and Froemke (2012) in providing an updated assessment of nonpoint-source threats to water quality in the United States. However, we highlight some concerns with the measure chosen to quantify atmospheric deposition in the United States. Our primary concern is that the selective use of only atmospheric wet deposition of nitrate plus sulfate as the chosen index of the level of atmospheric deposition perpetuates a longstanding impression that atmospheric deposition is low and of minimal environmental and ecological importance throughout the western United States. Brown and Froemke's figure 2i shows varying levels of elevated deposition in the eastern half of the country, with no areas showing elevated deposition in the West.
The source of the deposition data in Brown and Froemke's article is the National Atmospheric Deposition Program National Trends Network (NADP/NTN), which measures atmospheric wet deposition. The long-term data from the NADP have proven invaluable in advancing our understanding of deposition effects and temporal deposition trends in the United States. However, as in the Brown and Froemke article, NADP data have been frequently used as the sole measure of atmospheric deposition without acknowledging the limitations of measuring only wet deposition. Actual air pollution inputs can be dramatically underestimated when dry deposition is ignored, particularly in areas with semiarid or arid climates.
Dry deposition and deposition in fog or cloudwater—both of which can be major atmospheric deposition inputs—are not measured by the NADP/NTN. The Clean Air Status and Trends Network (CASTNet) is a dry deposition network, with monitoring sites collocated at a subset of the NADP/ NTN sites. However, there are insufficient data points in the CASTNet network to create dry deposition maps. Dry deposition fluxes of nitrogen (N) and other pollutants are important in all regions of the United States but can constitute as much as 85 percent or more of the N deposition inputs in arid regions.
Brown and Froemke considered both the acidification and nutrient effects of atmospheric deposition on water quality. The authors mentioned the impressive reductions in sulfate deposition in the eastern United States in past decades. The deposition of nitrate is also decreasing in some areas. However, in the United States, ammonium is becoming an increasingly larger fraction of total atmospheric N deposition, with increasing ammonium deposition trends in many areas (Lehman et al. 2005). However, Brown and Froemke did not include ammonium in their atmospheric deposition metric.
Wet plus dry deposition of both reduced (i.e., ammonia, ammonium) and oxidized (i.e., nitric acid vapor, various nitrogen oxides, nitrate) N forms should be included in any assessment of N deposition effects on water quality, either as a nutrient effect or when considering acidification effects. However, it does not seem appropriate to include sulfate deposition when evaluating nutrient effects on water quality.
We argue that N deposition is not well represented unless dry deposition of N in its various gaseous and particulate forms, including reduced forms of N (e.g., ammonium and ammonia), and, in some cases, cloudwater deposition are also included. Otherwise, a skewed N deposition map will result. An example of a more appropriate portrayal of N deposition in the United States is the total (wet + dry) N deposition map recently published in BioScience (Baron et al. 2011, figure 3c).
Finally, the use of the sum of nitrate and sulfate in wet deposition expressed on a mass basis (kilograms per hectare) as a measure of deposition inputs (Brown and Froemke 2012) is also highly unconventional. Each of these ions has a unique molecular weight and on a chemical basis are not appropriately summed unless they are first converted to moles or equivalents (the latter is more appropriate when considering acidification effects; e.g., equivalents per hectare per year).
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