The goal of this study is to better understand the magnitude and nature of measured OC artifacts on field blanks (bQF) and quartz-fiber filters behind quartz-fiber front filters (QBQ) and to evaluate strategies to compensate for these artifacts. Specific objectives are: Compile existing databases of mass, elements, and ions, non-processed data for OC, EC, and thermal fractions, as well as laboratory, field, and trip blanks for collocated IMPROVE, STN/CSN, and/or SEARCH sites. Describe how blanks have been handled and used in each network. Compare the magnitudes of adsorbed organic vapors estimated by different methods and evaluate how well the applied methods compensate for these artifacts in the reported OC concentrations. Examine the influence of water-soluble organics as powders and in solution on the blank/backup filters and the quantification of OC and EC with the IMPROVE and STN/CSN thermal/optical protocols. Acquire information that can better relate carbon measurements from the IMPROVE and STN/CSN network.