About the NADP Concentration and Deposition Maps

Mapping Uncertainty

NADP annual concentration and deposition maps represent a modeled, spatial interpolation of quality-controlled point observation data from NTN and MDN sites. Uncertainty in the measurements and the spatial interpolation is an unavoidable part of the process. The intended purpose of these maps is to represent regional trends in concentration and deposition data. As such, we do not have measurement sites near local sources of air pollutants, such as urban areas and industrial sites. Uncertainty within these maps varies geographically and has not been quantified. Higher levels of uncertainty can be expected when looking at regions with large topographic variability, near urban and industrial areas, and in regions isolated from nearby NADP sites. Uncertainty also increases when viewing the maps at smaller scales and evaluating small-scale features.

Users of the maps are warned to use caution when making decisions based upon localized estimates within the map where measurements were not taken.

Interpolation Methods

The concentration gradient maps are generated using the Inverse Distance Weighting interpolation method.

The deposition surfaces are not directly interpolated. Instead, they are generated as a product of two interpolated surfaces – the corresponding concentration surface, and a precipitation surface that combines NADP-measure precipitation values with modeled estimates

The annual composite precipitation surfaces are derived from an adapted version of a high resolution precipitation model developed by the PRISM Climate Group, and supplemented with NADP precipitation observations. PRISM stands for “Parameter-elevation Regression on Independent Slopes Model“. The PRISM modeled precipitation estimates incorporate point observation data, a reliable digital elevation model (DEM), and expert knowledge of complex climatic variables that result in high resolution, continuous, digital grid estimates of total annual precipitation.

NADP modified the original PRISM surfaces by adding in the additional NADP precipitation observations. The annual precipitation surfaces were created by using an inverse distance weighting (IDW) method to calculate a weighted value for every grid cell within a 30 km radius of each NADP precipitation site. The weighted values are calculated using a combination of the PRISM modeled precipitation data, and the NADP observed precipitation values. The weighting function was established so that as you approached the edge of the 30 km radius the values of the weighted grid cells approached that of PRISM. Outside of the 30 km radius the annual precipitation grid cells were populated using only PRISM data.

Compared to generating a precipitation surface using only NADP station data, this process should result in annual precipitation, and subsequently deposition maps with improvements in the estimation of interpolated values, especially in regions of highly complex terrain such as the Rocky Mountains.

Acceptable Use Policy

If you intend on reprinting any of these maps for use in a publication, please read our Use Conditions.