Precision of atmospheric dry deposition data from the Clean Air Status and Trends Network

A collocated, dry deposition sampling program was begun in January 1987 by the US Environmental Protection Agency to provide ongoing estimates of the overall precision of dry deposition and supporting data entering the Clean Air Status and Trends Network (CASTNet) archive. Duplicate sets of dry deposition sampling instruments were installed adjacent to existing instruments and have been operated for various periods at 11 collocated field sites. All sampling and operations were performed using standard CASTNet procedures. The current study documents the bias-corrected precision of CASTNet data based on collocated measurements made at paired sampling sites representative of sites across the network. These precision estimates include the variability for all operations from sampling to data storage in the archive. Precision estimates are provided for hourly, instrumental ozone (O3) concentration and meteorological measurements, hourly model estimates of deposition velocity (Vd) from collocated measurements of model inputs, hourly O3 deposition estimates, weekly filter pack determinations of selected atmospheric chemical species, and weekly estimates of Vd and deposition for each monitored filter pack chemical species and O3. Estimates of variability of weekly pollutant concentrations, expressed as coefficients of variation, depend on chemical species: NO3− 8.1%; HNO3 6.4%; SO2 4.3%; NH4+ 3.7%; SO42− 2.3%; and O3 1.3%. Precision of estimates of weekly Vd from collocated measurements of model inputs also depends on the chemical species: aerosols 2.8%; HNO3 2.6%; SO2 3.0%; and O3 2.0%. Corresponding precision of weekly deposition estimates are: NO3− 8.6%; HNO3− 5.2%; SO2 5.6%; NH4+ 3.9%; SO42− 3.5%; and O3 3.3%. Precision of weekly concentration, Vd estimates, and deposition estimates are comparable in magnitude and slightly smaller than the corresponding hourly values. Annual precision estimates, although uncertain due to their small sample size in the current study, are consistent with the corresponding weekly values.