Weekly and daily precipitation chemistry network comparisons in the eastern U.S.: NADP/NTN vs MAP3S/AIRMoN

This study is a comparison of the weekly aggregated MAP3S (1981–1989) and AIRMoN (1992–1995) daily precipitation chemistry record, to the NADP/NTN weekly data at four collocated sites in the eastern U.S.A. Data for conductivity, H+, NO-3, and SO2-4 all strongly support the hypothesis that the weekly and daily network concentrations of these ions are comparable for both time periods. Data for ammonium show a statistically significant bias for both time periods with the daily record having concentrations approximately 14% higher than the weekly values. This bias also has been observed in other studies of weekly and daily networks, and may be a function of residence time samples remain in the field and/or in storage before analysis. Significantly higher Cl- and K+ concentrations in the MAP3S data set from 1986 to 1989 suggest a contamination problem and use of these data is not recommended for this time period. Potassium concentrations during the other years of record compare poorly between the daily and weekly networks on a weekly basis. Annual data compare well. The other base cation weekly concentrations (Ca2+, Mg2+, Na+) also do not compare well, with average weekly differences ranging from 23 to 51%. However, both the daily and weekly networks show a very similar temporal decline in the sum of base cation concentrations of about 30% from the periods 1981–1985 to 1992–1995. Network Ca2+ and Mg2+ concentration trends also compare well for the entire period of record. Intranetwork variability, assessed in a separate study at several collocated NADP sites in the eastern U.S.A., could account for 1/5 to 1/2 of the internetwork bias for most ions. Intranetwork relative error may account for 1/4 to 2/3 of the internetwork relative error. This study supports the concept that the MAP3S and AIRMoN networks represent a continuous event/daily network data base of high quality for conductivity, H+, NO-3, and SO2-4 concentrations. Comparisons in trends of NH+4, Ca2+ and Mg2+ also suggest that these ions represent a quality long-term database. Daily network NH+4 (and possibly Na+) concentrations may be better estimates of actual precipitation concentrations than the weekly network data.