D. L. Sisterson LeRoy J. Schroder J. David S. Bigelow 1989 <p>Differences between field and laboratory pH values reported by the National Atmospheric Deposition Program/National Trends Network (NADP/NTN) monitoring program from 1984 through 1986 are investigated. Median differences in hydrogen ion concentration between laboratory and field pH determinations at sites averaged -4.6 <i><span>μ</span></i>equiv/L in natural precipitation samples on an annual basis. The median difference found in external quality assurance samples analyzed during the same time period was -11 <i><span>μ</span></i>equiv/L. The results suggest a systematic bias in pH values reported by the NADP/NTN network. The bias appears to have a fixed component of approximately -7 <i><span>μ</span></i>equiv/L, which can be attributed to the sampling bucket and lid, and a seasonal and regional component that ranges from +4 to -22 <i><span>μ</span></i>equiv/L at the 10th and 90th percentiles. Differences were found to be independent of sample pH and sample volume. The magnitude of the bias has implications for the interpretation of previously published pH and hydrogen ion concentration and deposition values in the western United States.</p> application/pdf 10.1021/es00065a018 en American Chemical Society An interpretation of differences between field and laboratory pH values reported by the National Atmospheric Deposition Program/National Trends Network monitoring program article