Coupling aerosol size distributions and size‐resolved hygroscopicity to predict humidity‐dependent optical properties and cloud condensation nuclei spectra

Abstract

Aerosol size distributions and size‐resolved hygroscopicity were measured with a Differential Mobility Analyzer/Tandem Differential Mobility Analyzer (DMA/TDMA) system during the May 2003 Atmospheric Radiation Measurement Program (ARM) Aerosol Intensive Operational Period (IOP) at a rural site in northern Oklahoma. Size and hygroscopicity information were combined in order to model the aerosol as a population of multicomponent particles, each consisting of organic carbon, elemental carbon, mineral dust, ammonium sulfate, and water, or some subset of these components. Comparisons of both derived optical properties and derived cloud condensation nuclei (CCN) spectra with direct measurements are used to provide independent validation of this multicomponent aerosol model. The aerosol hygroscopic growth factor, f(RH), relating total submicron scattering at high relative humidity to that at low relative humidity, is predicted with this aerosol model. The f(RH) values computed using measurements made with the Aerosol Observing System are closer to those predicted when the hygroscopic fraction of the aerosol is assumed to be in the metastable state than to those values predicted assuming the aerosol is crystalline. Overpredictions of f(RH) are generally limited to the measured f(RH) plus 0.15 over the RH range from 40 to 80% RH. Underpredictions are enhanced at RH greater than ∼65%, suggesting underestimation of the increase in scattering enhancement with increasing RH. The multicomponent aerosol model is also used to predict the cumulative and differential critical supersaturation distributions of the aerosol population. The resulting derived CCN spectra are compared to spectra measured directly with two Desert Research Institute (DRI) CCN spectrometers. Among the 1490 pairs of DMA/TDMA‐predicted and DRI‐measured CCN concentrations at various critical supersaturations from 0.02–1.05%, the sample number‐weighted mean R2 value for the linear regressions is 0.74. CCN concentrations are slightly overpredicted at both the lowest (0.02–0.04%) and highest (0.80–1.05%) supersaturations measured. Overall, the multicomponent aerosol model based on the measurements of size distributions and size‐resolved hygroscopicity yields reasonable predictions of the humidity‐dependent optical properties and of the CCN spectra of the aerosol.