An interpretation of differences between field and laboratory pH values reported by the National Atmospheric Deposition Program/National Trends Network monitoring program

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 μequiv/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 μequiv/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 μequiv/L, which can be attributed to the sampling bucket and lid, and a seasonal and regional component that ranges from +4 to -22 μequiv/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.