SCIENTISTS AT STATE AGRICULTURAL EXPERIMENT STATIONS (SAES) RECOGNIZED THE NEED FOR A NATIONAL NETWORK TO ASSESS THE MAGNITUDE, GEOGRAPHIC EXTENT, AND TEMPORAL VARIATIONS IN THE DEPOSITION OF ACIDIC SUBSTANCES AND NUTRIENTS IN PRECIPITATION. THE NATIONAL ATMOSPHERIC DEPOSITION PROGRAM (NADP) IS THE CURRENT VERSION OF THIS SAES INITIATED PROJECT (NATIONAL RESEARCH SUPPORT PROJECT-3). THE NADP OPERATES THREE PRECIPITATION CHEMISTRY NETWORKS (NATIONAL TRENDS NETWORK [NTN], ATMOSPHERIC INTEGRATED RESEARCH MONITORING NETWORK [AIRMOM, TO BE DISCONTINUED IN SEPTEMBER 2019], AND MERCURY DEPOSITION NETWORK [MDN]), TWO ATMOSPHERIC CONCENTRATION NETWORKS (ATMOSPHERIC MERCURY NETWORK [AMNET], AMMONIA MONITORING NETWORK [AMON]), TWO ANALYTICAL LABORATORIES (CENTRAL ANALYTICAL LABORATORY [CAL] AND MERCURY (HG) ANALYTICAL LABORATORY [HAL] ), AND THE NETWORK EQUIPMENT DEPOT. EACH NETWORK HAS ITS OWN OBJECTIVES AND CORRESPONDING ANALYTES AND SAMPLING PERIOD. SULFUR AND NITROGEN OXIDES EMITTED FROM INDUSTRIAL AND TRANSPORTATION SOURCES, UTILITIES, AND METROPOLITAN AREAS ENTER THE ATMOSPHERE AND ARE TRANSFORMED INTO ACIDIFYING COMPOUNDS. THESE POLLUTANTS ARE TRANSPORTED IN THE ATMOSPHERE AND ARE REMOVED, IN PART, AS ACIDIC WET AND DRY DEPOSITION. ECOLOGICAL IMPACTS FROM THIS DEPOSITION INCLUDE CHANGES TO LAKE AND STREAM CHEMISTRY, REDUCED FOREST GROWTH, REDUCED SOIL FERTILITY, AND INCREASED WEATHERING AND CORROSION OF EXPOSED STRUCTURES. AEROSOLS RESULTING FROM THE EMISSIONS REDUCE VISIBILITY AND ALTER THE RADIATIVE BALANCE OF THE CLIMATE SYSTEM. EPIDEMIOLOGICAL STUDIES LINK ADVERSE HUMAN HEALTH WITH FINE PARTICLES CONTAINING SULFATE AND NITRATE. DEPOSITION OF ATMOSPHERIC MERCURY HAS BEEN IDENTIFIED AS AN IMPORTANT NEGATIVE INPUT TO MANY ECOSYSTEMS. THE GOAL OF THE NADP IS TO MONITOR THE NATION'S PRECIPITATION AND ATMOSPHERE FOR A RANGE OF CHEMICAL CONSTITUENTS, TO DETERMINE WHETHER SPATIAL AND TEMPORAL TRENDS IN CONCENTRATION AND WET/DRY DEPOSITION CAN BE IDENTIFIED. THE RESULTS FROM THIS MONITORING ALLOW DATA USERS, SUCH AS POLICY MAKERS AND SCIENTIST, TO MAKE INFORMED DECISIONS AND ADVANCE SCIENTIFIC UNDERSTANDING. NADP ACCOMPLISHES THIS USING FIVE MONITORING NETWORKS OPERATED ACROSS NORTH AMERICA. THE SITE OPERATIONS AND NETWORKS ARE MANAGED TO ENSURE CONTINUITY ACROSS ALL SITES AND NETWORKS, RESULTING IN A CONSISTENT, QUALITY ASSURED, DATA SET. THE PROGRAM HAS GROWN TO A MONITORING SYSTEM OF OVER 300 DIFFERENT SITE LOCATIONS. THIS PROJECT HAS FOUR OBJECTIVES: 1) COORDINATION OF THE NTN, MDN, AIRMON, AMON, AND AMNET SAMPLING NETWORKS, AND ASSOCIATED SPECIAL PROJECTS; 2) CHEMICAL ANALYSIS, SITE SUPPORT, AND DATA VALIDATION FOR ALL NETWORK SITES INCLUDING THOSE SITES FUNDED IN THIS AGREEMENT; 3) QUALITY ASSURANCE OF ALL MEASUREMENTS TO ENSURE THAT NETWORK OPERATIONS ARE CONSISTENT WITH DOCUMENTED NETWORK QUALITY ASSURANCE PLANS AND STANDARD OPERATIONAL PROCEDURES; AND 4) MAINTAIN DATA AVAILABILITY, THAT IS IN AN EASILY ACCESSIBLE FORMAT FOR ALL RESEARCH SCIENTISTS, POLICY PROFESSIONAL, AGRICULTURAL AND INDUSTRY LEADERS, AND THE GENERAL PUBLIC, TO SUPPORT RESEARCH OF ANY AND ALL TYPES. THE NADP OPERATES AS A COOPERATIVE PROGRAM, WITH INPUT FROM ALL STAKEHOLDERS. SPECIFICALLY, THE NADP HAS A DECISION-MAKING EXECUTIVE COMMITTEE AND SEVERAL CONSTITUENT SUBCOMMITTEES. OPERATIONALLY, PRECIPITATION SAMPLES ARE COLLECTED USING STANDARD WET-DRY PRECIPITATION COLLECTORS AND STANDARD FIELD AND ANALYTICAL METHODS. THE PROGRAMS RESULTS IN MEASUREMENTS OF FREE ACIDITY (AS PH), SPECIFIC CONDUCTANCE, CALCIUM, MAGNESIUM, SODIUM, POTASSIUM, SULFATE, NITRATE, CHLORIDE, AMMONIUM, MERCURY, AND GASEOUS AMMONIA AND MERCURY ON A REGULAR BASIS. MOST SAMPLES ARE COLLECTED ON A WEEKLY BASIS, SPECIFICALLY EVERY TUESDAY MORNING AND SHIPPED TO NADP LABORATORIES FOR ANALYSIS UNDER STRICT QUALITY ASSURANCE AND QUALITY CONTROLLED PROCEDURES. THIS DATA IS FREELY AVAILABLE TO ALL, AND IS USED BY RESEARCHERS, POLICY PROFESSIONALS, AND EDUCATORS. THE NADP PROGRAMS EXPECTED OUTCOMES INCLUDE: FREELY AVAILABLE AND QUALITY ASSURED DATA; IMPLEMENTATION OF TWO ANNUAL PUBLIC MEETINGS WITH REPORTS TO SITE OPERATORS, SUPERVISORS, AND SPONSORS; AND SCIENTIFICALLY SOUND AND AUDITED SITE OPERATION. THE ANTICIPATED BENEFITS FROM THE PROGRAM INCLUDE MONITORING THE DEPOSITION OF SPECIFIC CHEMICAL CONSTITUENTS TO THE NATION'S NATURAL AND MANAGED ECOSYSTEMS, DETECTION OF TRENDS IN THE DEPOSITION OF THESE CONSTITUENTS, AND DISBURSEMENT OF SCIENTIFICALLY VALID ENVIRONMENTAL DATA, MADE AVAILABLE FOR ALL RESEARCH AND POLICY ENDEAVORS.