Apportionment of light scattering and hygroscopic growth to aerosol composition

During a recent campaign at the NOAA CMDL monitoring station on Sable Island, Canada (43.93° N, 60.01° W) a dual-nephelometer humidigraph measured the hygroscopic growth factor of aerosol scattering, fRH(σsp), one of the key parameters necessary for estimating short-wave aerosol radiative forcing. Measurements revealed less growth for anthropogenically influenced aerosols than for marine, fRH(σsp) of 1.7 ± 0.1 vs. 2.7 ± 0.4, where fRH(σsp) = σsp(85%)/σ(sp40%). A combined measurement-modeling approach was used to estimate σsp and its RH-dependence, based on the measured particle size distribution and composition. The model suggested that differences in the particle size distribution, assuming the same aerosol composition, could not explain the observed differences in fRH(σsp) We have confirmed with individual particle analysis, that aerosol composition was indeed responsible for the difference in fRH(σsp). As well, the scattering contribution of organic carbon for the influenced case is at least as much as sulfate aerosol.