The aerosol monitoring network of the NOAA Earth System Research Laboratory routinely provides data on the aerosol absorption and spectral scattering and backscattering coefficients. An algorithm is introduced that retrieves from the primary measurements the aerosol asymmetry parameter, the optical property needed in addition to those mentioned above to completely characterize aerosol with respect to its direct radiative effect on climate. The algorithm is tested on synthetic data and shown to retrieve the spectral aerosol asymmetry parameter for aerodynamic particle diameters Dp,aer < 1.0 ¿m with ~0.8% systematic uncertainty and ~1.1% systematic uncertainty for Dp,aer < 10.0 ¿m. The algorithm is applied to data from six ESRL stations representing aerosol in the dried state of continental, marine, arctic, and free tropospheric origin. The retrieved asymmetry parameters show a significant dependence on wavelength. Both wavelength dependence and absolute value of the asymmetry parameter depend on location or air mass type. The median values range from 0.57 for free tropospheric aerosol to 0.65 for marine aerosol at 550 nm wavelength and Dp,aer < 10.0 ¿m. It is shown that the accuracy of the asymmetry parameters retrieved with the algorithm is significantly larger than that of asymmetry parameters derived from backscatter fractions assuming a Henyey-Greenstein phase function.