Estimates of indirect aerosol radiative forcing have focused on increased sulfate aerosol mass concentrations caused by anthropogenic emissions of gas-phase sulfur dioxide, implicitly neglecting the impact of direct particulate emissions. Emissions of primary particles and gas-phase precursors have different effects on cloud condensation nuclei (CCN) concentrations as they impact CCN concentrations via different microphysical pathways. We present a theoretical analysis and evidence from a three-dimensional global model of aerosol microphysics to show that particulate emissions are more efficient per unit mass than gas-phase emissions at increasing CCN concentrations. Both analyses show that the few percent of anthropogenic sulfur emitted as particulate sulfate results in an increase in CCN concentrations comparable to that resulting from much larger emissions of gas-phase sulfur dioxide. Therefore, models should explicitly distinguish between the microphysical impacts of particulate and gas-phase emissions to accurately estimate the magnitude of the indirect effect of aerosols on climate.