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

Precipitation chemistry measurements obtained by the Canadian Air and Preci pitation Monitoring Network (CAPMoN) and the U.S. National Atmospheric Depo sition Program/National Trends Network (NADP/NTN) have been examined using more than 7 yr of collocated data from two sites, namely, Sutton, Quebec, C anada and State College, Pennsylvania, U.S.A. In the case of the CAPMoN dat a, weekly precipitation-weighted mean concentrations, total sample depths a nd total standard gauge depths were computed from daily data and compared t o 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 significa nt between-network biases were found to exist in the weekly results for mos t of the measured variables, particularly standard gauge depth, sample dept h, pH, H+, NO3-, NH4+, Na+; the NADP/NTN values were consistently lower tha n those of CAPMoN with the exception of pH and Na+. The magnitude of the bi ases 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 dif ferences represented less than 20% of the median CAPMoN weekly value. Both the between-network biases and variabilities were functions of several phys ical parameters, the most dominant being the sample depth and the ionic con centration. For seasonal and annual deposition values, statistically signif icant 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 r elative biases ranged from 7 to 37%. Part of the between-network bias in th e deposition estimates was directly attributable to a strong bias in the st andard gauge depths of the two networks.