Network comparability and the reconstruction of a precipitation chemistry record at a monitoring site in the eastern United States

Abstract

Prior comparability studies of precipitation chemistry networks in eastern North America have reported that data derived from daily sampling are preferred over those obtained from weekly integrated sampling. However, the record of daily wet-only precipitation chemistry in the United States was interrupted for more than 3 years (from 5 January 1990 to 26 December 1993), precluding the generation of a single, continuous record since the start of operations in September 1976. A first attempt was made to reconstruct the record during the intervening (“gap”) years at a site in central Pennsylvania, where the Canadian Air and Precipitation Monitoring Network (CAPMoN) collocated daily sampling operations beginning in 1987. First, data from the original daily wet deposition monitoring program (the Multistate Atmospheric Power Production Pollution Study (MAP3S)) and from the most recent program (the Atmospheric Integrated Research Monitoring Network (AIRMoN)) were separately compared with the CAPMoN data. The internetwork biases so obtained were then merged and used to adjust the uninterrupted, overlapping CAPMoN data, thus enabling us to combine the different data sets and reconstruct the whole record for the major ions (SO42−, NO3−, NH4+, and H3O+). The between-network differences derived from the paired daily data (US datum–CAPMoN datum) show statistically significant negative biases (MAP3S, AIRMoN<CAPMoN) of about 0.5 mm for rain gage depth and 4–5 μeq l−1 for the concentrations of SO42− and NO3−. The relative precision obtained for [SO42−] and [NO3−] ranged from 9% to 13%, which is similar to the findings of previous weekly based intercomparison studies. The reconstructed record (1988–2000) reveals the highest daily concentrations for all major ions for 1991, followed by a consistent decrease over the following 6 years.