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BIASES IN CASTNET FILTER PACK RESULTS ASSOCIATED WITH SAMPLING PROTOCOL
Citation:
Sickles II, J E. AND D. S. Shadwick. BIASES IN CASTNET FILTER PACK RESULTS ASSOCIATED WITH SAMPLING PROTOCOL. ATMOSPHERIC ENVIRONMENT 36(29):4687-4698, (2003).
Impact/Purpose:
Our main objective is to assess the exposure of selected ecosystems to specific atmospheric stressors. More precisely, we will analyze and interpret environmental quality (primarily atmospheric) data to document observable changes in environmental stressors that may be associated with legislatively-mandated emissions reductions.
Description:
In the current study, single filter weekly (w) results are compared with weekly results aggregated from day and night (dn) weekly samples. Comparisons of the two sampling protocols for all major constituents (SO42-, NO3-, NH4+, HNO3, and SO2) show median bias (MB) of < 5 nmol m-3 (0.1 ppb) and median relative bias (MRB) of < 10%. Examination of seasonal results reveal larger biases in some cases, especially during summer. Systematic discrepancies are observed for all constituents except for SO42-. Composite dn results exceed the w results (in all cases except for the summer nylon HNO3), and the magnitude depends on the constituent and on the season. To use CASTNet results for trends analyses, it may be useful to put all of the data on the same basis. Algorithms derived from linear regression analyses are offered to convert the recent results to a uniform basis. To improve accuracy, adjustments are indicated for summer Teflon NO3 and nylon HNO3, and in each season for Teflon NH4 and Total SO2. Nylon filters are also shown to have variable collection characteristics for SO2 that are especially sensitive to humidity. A network-wide change in the SO2 collection and/or retention characteristics for the nylon filters is found in April 1997.