Comparability and precision of MAP3S and NADP/NTN precipitation chemistry data at an acidic site in eastern North America

Precipitation chemistry data derived from more than six years of concurrent sampling under the MAP3S and NADP/NTN protocols at the Penn State site in rural Pennsylvania are compared. Preprocessing of the data (sample and gage depths, concentrations of H3O+, SO42−, NO3−, NH4+) included aggregating the event and daily MAP3S data into weekly averages that coincide with the weekly data of the NADP/NTN program. A simple statistical model was applied to the differences of the paired data to look for systematic biases between the networks. The most notable bias occurred for ammonium concentration (NADP/NTN low), a finding consistent with other studies. A minor, but statistically significant, bias was also found for sulfate coccentration (NADP/NTN high). Both the ammonium and sulfate biases may be related to the relatively long time that the NADP/NTN samples remain in the field, thus strengthening arguments in favor of daily sampling protocols. The hydronium and nitrate concentrations exhibited remarkably high levels of comparability between the daily and weekly protocols. Overall, the data from the MAP3S and NADP/NTN networks are comparable, with the possible exception of ammonium, for some applications. The field results from about two years of duplicate sampling under the MAP3S protocol demonstrate that measurement precisions of only a few percent are possible when the protocol is consistently followed and the equipment is well maintained.