National Trends Network

The NTN is the only network providing a long-term record of precipitation chemistry across the United States.

Sites predominantly are located away from urban areas and point sources of pollution. Each site has a precipitation chemistry collector and gauge. The automated collector ensures that the sample is exposed only during precipitation (wet-only-sampling).

Site operators follow standard operational procedures to help ensure NTN data comparability and representativeness. They collect samples weekly on Tuesday morning, using only containers cleaned at the NADP Analytical Laboratory (NAL) at the Wisconsin State Laboratory of Hygiene (WSLH). They weigh the collection bucket to determine sample volume and transfer the sample from the collection bucket to a shipping bottle. All samples are sent to the NAL for analysis, data entry, verification, and screening.

The NAL measures free acidity (H+ as pH), conductance, calcium (Ca2+), magnesium (Mg2+), sodium (Na+), potassium (K+), sulfate (SO42-), nitrate (NO3), chloride (Cl), and ammonium (NH4+). The NAL also measures orthophosphate, but only for quality assurance as an indicator of sample contamination.

The NAL reviews field and laboratory data for completeness and accuracy, and flags samples that were mishandled, compromised by precipitation collector failures, or grossly contaminated. The NAL delivers all data and information to the NADP Program Office, which applies a final set of checks and resolves remaining discrepancies. Data then are made available on the NADP website.

Information on joining the network is available here.

Data Notices
Maps / GIS

Annual Maps

Annual gradient maps of precipitation-weighted mean concentrations and deposition are available for NTN. These maps are available in PDF and grid formats (up to 2018).

Additional Maps

NADP’s Total Deposition Science Committee (TDEP) has developed total deposition (wet + dry) maps using a hybrid approach that combines measured air concentration, wet deposition data from NTN, and modeled deposition velocity and dry deposition data.

The animated maps display the National Trends Network (NTN) concentration and deposition maps from 1985 to 2016 for sulfate, nitrate, and ammonium and the deposition maps for nitrogen from nitrate and ammonium. PDF and PowerPoint versions are available.

About the maps

In 2011, the NADP Technical Committee instituted changes in the way NADP annual concentration and deposition maps are produced. It was decided to modify the original map series from a discrete contour map style to a continuous color gradient map style while incorporating an external, highly resolved precipitation dataset.  More information about the process used to create these maps is available below.

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.

Field Methods

Every Tuesday morning at 9:00 a.m., NTN site operators across the country head to their monitoring stations to retrieve their samples of the previous week’s precipitation, whether rain, snow, or sleet.

Samples are collected continuously in a wet deposition collector. The collector opens automatically during wet weather, allowing the precipitation to fall into a collection bucket lined with a sample bag, and then closes as soon as the precipitation stops.

At the site, the operator replaces the collection bucket and bag with one a new each week. The operator then takes the sample to a field laboratory, where it is weighed. Next, the sample is transferred to a shipping bottle and sent to the NAL in Madison, Wisconsin.

Each site is also equipped with a weighing-bucket rain gauge that provides a continuous record of rainfall amounts. Rainfall is recorded to the nearest 0.01 in. The rain gauge also monitors the wet deposition collector, recording whether the collector was properly open during wet periods and closed during dry periods.

Operator Support Information

Lab Methods

At the NADP Analytical Laboratory (NAL), the concentrations of the important inorganic chemicals in precipitation are analyzed. These chemicals or analytes include acidity (measured as pH), sulfate, nitrate, ammonium, chloride, and base cations.

The data are checked to ensure their quality, and special codes are applied to help in interpreting the data. These sample validity codes indicate the quality of the sample, for instance, whether it was collected according to standard protocols and whether there is any sign of contamination.

After this initial data processing, the data are sent electronically to the NADP Program Office, which performs further checks of accuracy and completeness. The entire data record is confirmed as being consistent with standard NTN procedures and documentation. Sophisticated data processing programs are run to detect discrepancies in the coding of the sample validity and completeness of each sites data records. Any such discrepancies are carefully screened and resolved.

Only after data verification is complete is the sample record entered into the NADP database, ready for distribution to the public and inclusion in NADP publications.

The current listing of laboratory SOPs are available. Copies are available upon request.