Measurements during recent field campaigns downwind of the Indian subcontinent, Asia, and the northeastern United States reveal a substantial decrease in the relative humidity dependence of light scattering, fσsp(RH), with increasing mass fraction of particulate organic matter (POM) for submicrometer aerosol. Using data from INDOEX (INDian Ocean EXperiment), ACE Asia (Aerosol Characterization Experiment -- Asia), and ICARTT (International Consortium for Atmospheric Research on Transport and Transformation), we have identified, within measurement limitations, the impact of POM on the fσsp(RH) of accumulation mode sulfate-POM mixtures. The result is a parameterization that quantifies the POM mass fraction - fσsp(RH) relationship for use in radiative transfer and air quality models either as input or as validation. The parameterization is valid where the aerosol consists of an internally mixed sulfate-carbonaceous accumulation mode and other externally mixed components (e.g. sea salt, dust) and is applicable on both global and regional scales.