The U.S. Geological Survey’s National Water Quality Assessment Program is analyzing long-term changes in nitrogen deposition for surface and ground water studies on a national basis. Atmospheric deposition of nitrogen is one of the key components needed for an inclusive nitrogen trend analysis. The National Atmospheric Deposition Program’s (NADP) National Trends Network provides estimates of nitrogen in atmospheric wet deposition beginning in 1978, but there was a need for data predating this time period. The National historical data sources that were coalesced for the nitrogen deposition estimates were: 1) Air Force Cambridge Research Center’s study from 1955 – 1956; 2) Public Health Service and the National Center for Atmospheric Research from 1960 - 1966; and 3) National Atmospheric Deposition Program from 1981 – 1984. A fourth study conducted during 1972 - 1982, conducted by the National Oceanic and Atmospheric Administration, U.S. Environmental Agency, and World Meteorological Organization was not included in the analysis, but the data were digitized.

Historical data sets were reconstructed by: digitizing the data; estimating and interpolating latitude and longitude based on the most probable National Weather Service stations for that time; associating the precipitation collection station with the closest National Climatic Data Center precipitation depth; and making appropriate data substitutions for missing precipitation depths or concentration values. Once the nitrogen loads were calculated, inorganic nitrogen wet deposition maps were generated for years 1955-1956, 1962-1965, and 1981-1984. The nitrogen deposition estimates, in kilograms per hectare, are presented as 2,338.383-meter by 2,338.383-meter resolution raster datasets. The units and resolution used are the same as those used in the National Atmospheric Deposition Program National Trends Network raster datasets from 1985 to 2012, for easier comparison between these sources.

As a result of this investigation, all data from the four national studies, as well as a regional USGS study (1966 – 1967), are now available online in tabular datasets. These data sets include precipitation chemistry results (calcium, magnesium, potassium, sodium, ammonium, nitrate, nitrogen, chloride, sulfate, pH, and specific conductance when available), precipitation depth, calculated site-specific precipitation-weighted concentra­tions, and raster datasets of nitrogen from wet deposition. The USGS has made these data available in appendixes  of: Estimates of inorganic nitrogen wet deposition from precipitation for the conterminous United States, 1955–84, by Gronberg, J.M., Ludtke, A.S., and Knifong, D.L, 2014, available online at: http://pubs.usgs.gov/sir/2014/5067 or http://dx.doi.org/10.3133/sir20145067