Chemical mass balance analysis was performed using a large dataset of molecular marker concentrations to estimate the contribution of biomass smoke to ambient organic carbon (OC) and fine particle mass in Pittsburgh, Pennsylvania. Source profiles were selected based on detailed comparisons between the ambient data and a large number of published profiles. The fall and winter data were analyzed with fireplace and woodstove source profiles, and open burning profiles were used to analyze the spring and summer data. At the upper limit, biomass smoke is estimated to contribute on average 520+/-140 ng-C m(-3) or 14.5% of the ambient OC in the fall, 210+/-85 ng-C m(-3) or 10% of the ambient OC in the winter, and 60 + 21 ng-C/m(-3) or 2% of the ambient OC in the spring and summer. In the fall and winter, there is large day-to-day variability in the amount of OC apportioned to biomass smoke. The levels of biomass smoke in Pittsburgh are much lower than in some other areas of the United States, indicating significant regional variability in the importance of biomass combustion as a source of fine particulate matter. The calculations face two major sources of uncertainty. First, the ambient ratios of levoglucosan, resin acids, and syringhaldehyde concentrations are highly variable implying that numerous sources with distinct source profiles contribute to ambient marker concentrations. Therefore, in contrast to previous CMB analyses, we find that at least three distinct biomass smoke source profiles must be included in the CMB model to explain this variability. Second, the marker-to-OC ratios of available biomass smoke profiles are highly variable. This variability introduces uncertainty of more than a factor of 2 in the amount of ambient OC apportioned to biomass smoke by different statistically acceptable CMB solutions. The marker-to-OC ratios of source profiles are critical parameters to consider when evaluating CMB solutions.