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AIRMoN Wet Deposition Program

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The longest U.S. network record of precipitation chemistry in the modern era is that which started as the Department of Energy's Multistate Atmospheric Power Production Pollution Study (MAP3S) in 1976. With the end of the acid deposition decade, and with the cessation of a formal research program under the National Acid Precipitation Assessment Program, the MAP3S network was terminated by DOE and was eventually transferred to NOAA where it remains a program of ARL, constituting one tier of the AIRMoN-Wet program.
Total inorganic total nitrogen at State College, PA 1977-1996


The major wet deposition network -- the National Atmospheric Deposition Program (NADP) -- has a large array of about 200 stations. Weekly samples of precipitation are collected at these stations and then sent to a single central laboratory for chemical analysis. The weekly sampling network has three severe drawbacks:

  • sample chemistry can be affected by biological activity in precipitation that remains at ambient temperatures for several days before analysis;
  • week-long averaging means that several rainfall events frequently are combined into single samples;
  • while strict weekly sampling protocols ensure high quality at a reasonable price, research to improve sample quality is often not possible.

These drawbacks are important considerations for NOAA. The first means that accurate quantification of ammonium and (rarely) nitrate ion deposition is not possible; the AIRMoN-Wet approach is to collect daily samples and to keep them chilled until analysis so as to minimize biological activity. The second complicates any interpretation or analysis that involves a coupling with meteorology, and the third tends to stifle innovation required to adapt emerging technology to improve sampling methodologies, to speed communication to keep sites on-line, or to reduce labor expenses. None of these drawbacks should be construed as complaints against the NADP weekly program. NADP provides the routine monitoring required for long-term understanding of trends and deposition across the entire country; prime users of these data include ecologists, agriculturists, foresters, and power companies affected by Clean Air Act legislation. AIRMoN-Wet is a value added research program designed to enhance the quality and utility of NADP data while meeting the additional requirements required to meet the NOAA mission.

AIRMoN-Wet Field and Laboratory Procedures

Routine aspects of the AIRMoN-Wet program are coordinated carefully with NADP. Siting protocols, field equipment, some sampling protocols, and most laboratory analysis protocols are the same for both programs. Protocols in AIRMoN-Wet differ from NADP National Trends Network (NTN) only to the extent required to ensure temporally adequate (daily) sampling collection and enhanced sample quality. AIRMoN protocols that differ from NADP NTN include:

  • daily collections
  • chilling of samples until analysis
  • reliance on a National Weather Service "stick" gage instead of a Belfort 5-780 weighing gage for official precipitation amount measurement
  • no filtration of samples prior to analysis
  • a sampling order for trace samples that emphasizes analysis of anthropogenic substances over soil cations.

As with NADP NTN, AIRMoN-Wet samples are analyzed by the Illinois State Water Survey in Champaign, Illinois, and network coordination is handled through the National Resource Ecology Laboratory at Colorado State University in Fort Collins, Colorado.

New equipment currently under evaluation for use with the AIRMoN program includes the adaptation of several strain gages as potential replacements for the aging Belfort 5-780. Strain gages have fewer moving parts, no linkages to stick or corrode, and are readily adapted to generate digital signals that can be transmitted to a datalogger. Pending the outcome of our current field study, AIRMoN will replace the current recording gages with rain gages equipped with dataloggers and cell phones. The new equipment will be queried remotely, avoiding the time and agony of deciphering information currently analyzed by hand from Belfort paper charts. The new gages will be accompanied by software allowing remote submission of field information by site operators. The current system requires hardcopy submission of field data followed by double punching of data by laboratory personnel to ensure high levels of quality. After the initial expenditures to upgrade equipment, it is anticipated that AIRMoN-wet will produce more accurate data at a lower cost.

In 1996 all of the AIRMoN-Wet stations were equipped with Canadian precipitation bulk gages to begin to address the major discrepancies in chemical loadings that currently exist at the border. Deposition biases differ for different ions between the two countries; however, many of the biases are statistically significant, amounting to about 14% for sulfate deposition on an annual basis. Resolution of this issue will allow refinement of chemical loadings from precipitation for North America, issues of critical importance to the Commission for Environmental Cooperation and the International Joint Commission. The Canadian Air and Precipitation Monitoring Network (CAPMoN)-AIRMoN comparison is an outgrowth of the longstanding CAPMoN-NADP comparison begun in the mid-1980s (Vet et al., 1989; Sirois et al., 1997).

As a first step to compare data from different networks, NADP weekly data have been compared to AIRMoN-wet data collected at collocated stations. For some ions such as ammonium, differences between the weekly and daily networks are systematic and amount to approximately 15% on an annual basis. (For the case of ammonium, the differences are driven by biological decay of some samples. Measured differences are in keeping with results found between NADP and MAP3S during the early 1980s (Lamb and Comrie, 1993). Other differences are more difficult to quantify and are probably driven by contamination problems. AIRMoN data clearly show that NADP bucket lid seals occasionally contaminate samples with sodium; NADP no longer ships samples in plastic buckets, largely because of complaints from the AIRMoN/MAP3S community.

Research Plans

Monitoring Station

A number of projects will be attempted in the next year or two, depending on available funding. We will continue to upgrade electronics in motor box assemblies that drive precipitation collectors, and we will attempt to complete strain gage research so that we can move the equipment to a permanent field position. A four-way field intercomparison between NADP, AIRMoN-Wet, CAPMoN, and the wet-side program under the EPA-led National Dry Deposition Program is under consideration, with the likely establishment of a proper comparison at Pennsylvania State University. Further field studies with various biocide treatments are still contemplated, but not planned for the next year or two due to budget constraints. At some point, chilling of samples should be replaced by introduction of a simple biocide such as chloroform or thymol to cut shipping costs and better stabilize samples.

Perhaps more importantly, we will attempt to move forward with the early detection aspects of AIRMoN-Wet to detect cleaner air due to mandated emissions reductions. Computation of back trajectories in combination with cluster analysis and gridded tabulations of trajectory has continued at a low level for several years; some of this work should come to fruition during the next year. The main purpose of these efforts will be to reduce the uncertainty in our analyses by removing variability introduced by meteorology.

Linkages with Other Parts of NOAA

The AIRMoN-Wet program is an integral part of the overall AIRMoN program. AIRMoN-Wet is managed primarily from ARL Silver Spring; the AIRMoN-Dry part of the program is managed byARL/ATDD in Oak Ridge, Tennessee. The ARL modeling community in Silver Spring and ASMD in Research Triangle Park routinely apply these data to the issue of nutrient deposition in coastal areas.

Linkages with Other Organizations

AIRMoN-Wet data are fully integrated into international air quality studies as promoted by the International Joint Commission between the U.S. and Canada, the U.S.-Canada Air Accord, the Committee for Environmental Cooperation (established as a result of the North American Free Trade Agreement), and the World Meteorological Organization Global Atmospheric Watch program based in Geneva, Switzerland. U.S. agencies involved in these international agreements include the State Department, EPA, and the National Park Service. The Atmospheric Environment Service and the Ontario Ministry of the Environment are our Canadian counterpart agencies. Through involvement with the National Atmospheric Deposition Program, AIRMoN scientists are part of a consortium of eight federal agencies which require deposition data for a variety of purposes.

Recent Publications

Hicks, B., and R. Artz, Estimating Background Precipitation Quality from Network Data, Environmental Pollution, 75, 137-143, 1992.

Lamb, D., and L. Comrie, Comparability and Precision of MAP3S and NADP/NTN Precipitation Chemistry Data at an Acidic Site in Eastern North America, Atmospheric Environment, 27A, 1993-2008, 1993.

NADP/AIRMoN. The NADP/Atmospheric Integrated Research Monitoring Network-wet Site Operator's Manual: Illinois State Water Survey, Champaign, IL.

NADP/AIRMoN. The NADP/AIRMoN-wet Quality Assurance Manual." Illinois State Water Survey, Champaign, IL.

Sirois, Alain, Robert Vet, and Dennis Lamb, A Comparison of Precipitation Chemistry Measurements Obtained by the CAPMoN and NADP/NTN Networks. Advances in Environmental Research, In Press, 1997.

Vet, Robert J., A. Sirois, D. Lamb, and R. Artz. Intercomparison of Precipitation Chemistry Data Obtained Using CAPMoN and NADP/NTN Protocols. NOAA Technical Memorandum ERL ARL-174, Air Resources Laboratory, Silver Spring, Maryland, 1989.

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