Approximated Expression of the Hygroscopic Growth Factor for Polydispersed Aerosols

Aerosol hygroscopic growth plays an important role in characterization of the atmospheric aerosol. Aerosol physico-chemical and optical properties are dependent on relative humidity as well as on their size and composition. Usually, atmospheric particles take up water and thereby change their optical properties. These altered optical properties impact aerosol radiative forcing. For example, the Interagency Monitoring of Protected Visual Environments (IMPROVE) network measures light scattering coefficients using a nephelometer. Here, hygroscopic growth factor is important in interpretation of aerosol characteristics and related phenomena such as visibility and extinction properties. One of the widely used expression for extinction coefficient as a function of aerosol mass concentration and hygroscopic growth factor (fRH) is the reconstructured equation proposed by Malm et al. (1994). Malm’s reconstructured equation assume the single f(RH), which is independent on particle size. For given water, different aerosol size distribution should yield different growth rate. Therefore, size distribution parameters should be explained as a function of geometric mean diameter and geometric standard deviation. In this study, hygroscopic growth factor (fRH) growth curves for polydispersed aerosol are approximated for different aerosol size distribution parameters (i.e. geometric mean diameter and geometric standard deviation). Water contents for ammonium sulfate and ammonium nitrate are calculated based on the Aerosol Inorganics Model (AIM) thermodynamic equilibrium model with the “no solids” option of pure ammonium sulfate (Clegg et al., 1998).

Aerosol light extinction coefficient(bext) at given RH was calculated using Mie theory for polydispersed aerosol, for the change in particle volume and refractive index from the addition of water. Finally, we approximated fRH for polydispersed aerosol as quadratic function of RH. The parameterized coefficients are based on thermodynamic model results and MLR (Multilinear regression). The approximated coefficients from this study are changed with particle size.

Subsequently, the obtained approximated fRH compared and the comparison results showed reasonable agreement with model calculated results.