Monthly and annual bias in weekly (NADP/NTN) versus daily (AIRMoN) precipitation chemistry data in the Eastern USA

Previous comparisons of the data from the National Atmospheric Deposition Program, National Trends Network (NTN) against collocated event sampled data and daily sampled data suggest a substantial bias in the concentration of ammonium [NH4+] and concentrations of several base cations, while the comparability of other ion concentrations ranges among the studies. Eight years of collocated data from five NTN and Atmospheric Integrated Research and Monitoring Network (AIRMoN) sites are compared here. Unlike previous analyses, the data from these two data sets were analyzed in the same laboratory using the same analytical methods; therefore, factors that influence concentration differences can be isolated to sampling frequency and sample preservation techniques. For comparison, the relative biases for these data have been calculated using both median value and volume-weighted mean concentrations, following two different approaches in the literature. The results suggest a relative bias of about 10% in [NH4+] (NTN less than AIRMoN), which is smaller than previous estimates that included the influence of inter-laboratory comparisons. The annual relative bias of [H+] increases over the analysis period, which results in a larger total relative bias for [H+] than found in a previous analysis of AIRMoN and NTN data. When comparing NTN and AIRMoN data on monthly time scales, strong seasonal variations are evident in the relative bias for [H+], [NH4+], and [SO42−]. Large biases in [SO42−] (NTN greater than AIRMoN) on monthly times scales have not been detected in previous analyses where data for all seasons were considered together.