Direct Comparison of the Submicron Aerosol Hygroscopicity of Water-Soluble Sugars

Water-soluble organic compounds (WSOCs) readily uptake water and form atmospheric droplets. Understanding the water-uptake ability of these WSOCs can improve our understanding of their radiative effects and thus can improve current climate models. In this study, we measure the subsaturated and supersaturated droplet growth of four WSOCs: levoglucosan, sucrose, raffinose, and trehalose. Specifically, we use three distinct nanoscale droplet growth methods: cavity ring-down spectroscopy (CRDS), hygroscopic tandem differential mobility analysis (H-TDMA), and cloud condensation nuclei (CCN) counting and report optical growth factor (fRH), growth factor (Gf), refractive indices, and critical activation diameters (dp50) for each aerosol system. Köhler theory (assuming a droplet surface tension equal to that of pure water) is applied to all droplet growth measurements, and the single-parameter hygroscopicity, κ, is reported for the four WSOCs measured on the different platforms. The three experimental methods measure decreased hygroscopicity with an increase in WSOC molecular weight. When assuming the droplets are dilute, κ values from measurements are consistent with known values. The three experimental methods have comparable precision with systematic deviations in the average κ for each method. Few studies have employed fRH data to obtain κ values. The current two methods to estimate κ from fRH data produce similar results, are dry aerosol size-dependent, and agree with κ values that assume dilute solution behavior. Thus, the presented measured data set and analysis contribute to the comparison of subsaturated and supersaturated data from optical and geometric methods for organic aerosol.