When particles are collected onto quartz-fiber filters, some of the organic vapors in the air are adsorbed onto the fibers. This adsorption also occurs passively when filters are exposed to the atmosphere with no air drawn through them. These vapors leave the sample during thermal analyses (Watson et al., 2005) and are interpreted as part of the measured organic carbon (OC). This positive OC artifact yields higher values for OC in PM2.5 and PM10 samples than are actually in ambient air (Kukreja and Bove, 1976). The adsorbed VOCs also char within the filter, thereby causing differences in elemental carbon (EC) levels determined by transmittance and reflectance corrections (Chen et al., 2004; Chow et al., 2004). Semi-volatile organic compounds (SVOCs) can be collected as particles, but portions of them can evaporate and leave the particle in the gas stream owing to increases in temperature or decreases of their gas phase concentrations in the sampled air (Galasyn et al., 1984). Evaporated SVOCs yield a negative OC artifact because they should have been reported as part of the PM2.5 or PM10 mass (Obeidi and Eatough, 2002). Several U.S. networks, including the Interagency Monitoring of Protected Visual Environments (IMPROVE) network (Figure 1-1; Watson, 2002), the Maryland Aerosol Research and Characterization (MARCH-Atlantic) study (Chen et al., 2001; 2002; 2003), and the Southeastern Aerosol Research and Characterization (SEARCH) network (Figure 1-2; Hansen et al., 2003; 2006), use quartz-fiber front filters (QF) to collect OC and EC and use backup filters and/or field blanks to correct for the OC artifact.