The National Atmospheric Deposition Program/National Trends Network (NADP/NTN) was initiated in 1978 by the Association of State Agricultural Experiment Stations to monitor long-term atmospheric chemistry and the effects pollutants have on aquatic and terrestrial systems. As of fall 2023, precipitation was being collected at approximately 260 NTN sites in the United States, including Puerto Rico and the U.S. Virgin Islands, and additionally Bermuda and Canada. Beginning in 1996, the NADP/Mercury Deposition Network (MDN) has monitored mercury (Hg) in precipitation, and includes approximately 80 sites in the United States including Puerto Rico, plus Canada and Taiwan. The U.S. Geological Survey started the Precipitation Chemistry Quality Assurance Project (PCQA) for NADP/NTN in 1978 and for NADP/MDN in 2004. The quality-assurance programs assess and document the quality of wet-deposition data for NADP/NTN and NADP/MDN. During 2021-2022, the NADP Central Analytical Laboratory (CAL) and the Mercury Analytical Laboratory (HAL) were located at the Wisconsin State Laboratory of Hygiene at the University of Wisconsin, Madison (UW). The CAL analyzed all weekly precipitation samples collected by NADP/NTN, and the HAL analyzed all weekly total mercury samples from the MDN. In 2023, these two laboratories were combined and rebranded as the National Atmospheric Laboratory (NAL). The PCQA verifies the quality of NADP data-collection processes for both the National Trends Network (NTN) and Mercury Deposition Network (MDN). PCQA data are available in separate tables for each quality-assurance program: (1) NTN Interlaboratory-Comparison, (2) MDN Interlaboratory-comparison program, (3) NTN Field-Audit program, (4) MDN System-Blank program, and (5) Co-located Sampler program. The Interlaboratory-Comparison programs (1 & 2) compare the variability and bias of the NADP National Atmospheric Laboratory’s chemical results to other labs that measure low-ionic strength samples, including labs that serve international precipitation monitoring networks. The Field Audit and System Blank programs (3 & 4) quantify the contamination and stability of NADP NTN and MDN samples, respectively by analysis and comparison of equipment rinse samples to unaltered parent solutions. The Co-located Sampler program uses identical, NTN collectors located within 5 meters of each other to obtain replicate samples that are used to calculate the overall variability of NTN sample data. Measured parameters include precipitation depth, pH, specific conductance, and ion concentrations for calcium, magnesium, sodium, potassium, ammonium, chloride, nitrate, sulfate, phosphate, and total mercury. Periodically, the PCQA prepares an interpretive report of the QA data. These data support the 2021-2022 USGS External QA Report for the NADP NTN and MDN. Data users should consult the metadata file for explanations of the data fields. All negative values for chemical constituents are the negatives of the analytical detection limits. For example, a concentration value reported as less than 0.005 milligrams per liter is reported in the data files as -0.005. Data users may also consult the NADP website for data descriptions as well as USGS external quality assurance reports, which are availble from the online USGS Publications Warehouse.