Vapor adsorption artifact in the sampling of organic aerosol: Face velocity effects

The concentration (μgC m^-3) of aerosol organic carbon as determined by collection with quartz fiber filters was found to exhibit a significant decrease with increasing filter face velocity. A similar face velocity dependence was found for organic carbon on back-up quartz fiber filters behind either quartz fiber front filters (QQ combination) or Teflon membrane front filters (TQ combination). Additionally, the concentration of organic carbon on the back-up filter in the TQ combination was always greater than on the back-up filter in the QQ combination. The most likely explanation for these observations is the adsorption or organic vapors onto the filter medium itself. The amount of organic carbon adsorbed onto the quartz fiber front filter can be approximated by measuring the amount of organic carbon on a quartz fiber back-up filter behind a Teflon front filter with both filter combinations sampling at the same face velocity. Making the correction in such a manner removes most of the face velocity dependence inthe resultant concentrations. For 24 h sampling in Portland, Oregon, at a face velocity of 40 cm s^-1, the correction exceeded 50% for uncorrected organic carbon concentrations of 2 μgC m^-3 but decreased to 15% at 16 μgC m^-3.