A comparison of the precipitation chemistry measurements obtained by the CAPMoN and NADP/NTN networks

Precipitation chemistry measurements obtained by the Canadian Air and Precipitation Monitoring Network (CAPMoN) and the U.S. National Atmospheric Deposition Program/National Trends Network (NADP/NTN) have been examined using more than 7 yr of collocated data from two sites, namely, Sutton, Quebec, Canada and State College, Pennsylvania, U.S.A. In the case of the CAPMoN data, weekly precipitation-weighted mean concentrations, total sample depths and total standard gauge depths were computed from daily data and compared to the corresponding weekly sampling data of the NADP/NTN network. Seasonal and annual precipitation-weighted mean concentrations and deposition values were also computed for both networks and compared. Statistically significant between-network biases were found to exist in the weekly results for most of the measured variables, particularly standard gauge depth, sample depth, pH, H+, NO3-, NH4+, Na+; the NADP/NTN values were consistently lower than those of CAPMoN with the exception of pH and Na+. The magnitude of the biases was less than 35% of the median CAPMoN weekly value for the 7 yr. For most of the measured parameters, the variability of the between-network differences represented less than 20% of the median CAPMoN weekly value. Both the between-network biases and variabilities were functions of several physical parameters, the most dominant being the sample depth and the ionic concentration. For seasonal and annual deposition values, statistically significant between-network biases were found for H+, SO42-, NO3-, Ca2+, NH4+ for both periods; for Mg2+ and K+ for seasonal data; and Cl- for yearly data, with the NADP/NTN deposition values being lower than those of CAPMoN. The relative biases ranged from 7 to 37%. Part of the between-network bias in the deposition estimates was directly attributable to a strong bias in the standard gauge depths of the two networks.