Measurements of the sensitivity of aerosol hygroscopicity and the κ parameter to the O/C ratio.

We report measurements of the subsaturated hygroscopic growth of aerosol particles composed of single organic components of varying oxygen-to-carbon ratio up to relative humidities approaching saturation using the techniques of aerosol optical tweezers and an electrodynamic balance. The variation in the hygroscopicity parameter κ between compounds of even the same O/C ratio is found to be significant with, for example, a range in κ values from 0.12 to 0.38 for compounds with an O/C of 1. The measurements are compared with a review of all of the available literature data for which both the κ value and O/C ratio are reported, and a new parametrization is determined. Critical supersaturations predicted using this parametrization yield values that have associated uncertainties that are comparable to typical uncertainties in experimental measurements of critical supersaturations. However, the systematic variability between κ parametrizations determined from different studies remains large, consistent with the O/C ratio providing only an approximate guide to aerosol hygroscopicity and reflecting significant variations for aerosols of different chemical functionality, composition, and oxidation history.