Significant Loss of Ion Concentrations in 1998 Due to Clogging of Module B Nylon Filters
During 1998, numerous nylon filters in the IMPROVE module B sampler clogged reducing the flow rate to unacceptable levels and the filters were invalidated. The problem was primarily at eastern sites, but did include several events at western sites (Table 1). The loss is biased toward samples with high mass and high reconstructed extinction.
In 1998, the manufacturer of the nylon material (MSI) inadvertently changed the specifications, increasing the pressure drop. The pressure drop of material from Whatman/Gelman had an even higher pressure drop, so it would not have helped to change manufacturer. With the higher pressure drop, the 25mm filter was just at the edge of acceptable flow rates. At high ambient concentrations, the new filters had a pressure drop that significantly reduced the flow rate and valid concentrations could not be obtained.
Long Term Solution
The loss has been minimized in the IMPROVE Version II samplers by increasing the nylon filter diameter from 25mm to 37mm.
Recommendations for Data Analysis
The sulfate concentrations can be replaced by scaling the sulfur concentrations by a factor of three. Care must be taken at Cape Romain, SC, Okefenokee, GA and Chassahowitzka, FL, where the summer time sulfur concentration can be underestimated (see the underestimation of sulfur concentration issue).
No substitution is available for the missing nitrate concentrations.
Sulfur-Sulfate History by B. Eldred - Detailed information on the potential causes of the sulfur underestimation.
Table 1. Loss of nylon filters when the flow dropped more than 25% (major clogs).