Evaluation of the filter pack for long-duration sampling of ambient air

Abstract

A 14-week filter pack (FP) sampler evaluation field study was conducted at a site near Bondville, IL to investigate the impact of weekly sampling duration. Simultaneous samples were collected using collocated filter packs (FP) from two independent air quality monitoring networks (CASTNet and Acid-MODES) and using duplicate annular denuder systems (ADS). Precision estimates for most of the measured species are similar for weekly ADS and composited FPs. There is generally good agreement between the weekly CASTNet FP results aggregated from weekly daytime and weekly nighttime samples and those aggregated from daily 24 h Acid-MODES samples; although SO 2 is the exception, and CASTNet concentrations are higher than Acid-MODES. Comparison of weekly ADS results with composited weekly FP results from CASTNet shows good agreement for SO 2- 4. With the exception of the two weeks where the FP exceeded the ADS, both HNO 3 and the sum of particulate and gaseous NO - 3 show good agreement. The FP often provides good estimates of HNO 3, but when used to sample atmospheres that have experienced substantial photochemical reactivity, FP HNO 3 determinations using nylon filters may be biased high. It is suggested that HNO 2 or some other oxidized nitrogen compound can accumulate on a regional scale and may interfere with the FP determination of HNO 3. FP particulate NO - 3 results are in fair agreement with the ADS. Since FP SO 2 results are biased low by 12–20%, SO 2 concentration in the CASTNet data archive should be adjusted upward. Nylon presents problems as a sampling medium in terms of SO 2 recovery and specificity for HNO 3. Additional comparative sampler evaluation studies are recommended at several sites over each season to permit comprehensive assessment of the concentrations of atmospheric trace constituents archived by CASTNet.