Retrieval of Broadband Optical Properties from Ambient Aerosols Measurements Using Inverse Mie Calculations

The role of atmospheric aerosols in earth’s radiative balance is crucial. A thorough knowledge about the spectral optical properties of various types of aerosols is necessary to quantify the net radiative forcing produced by aerosol–light interactions. In this study, we exploited an open-source inverse algorithm based on the Python—PyMieScatt survey iteration method, to retrieve the wavelength dependent Mie-equivalent complex refractive indices of ambient aerosols. This method was verified by obtaining the broadband complex refractive indices of monodisperse polystyrene latex spheres and polydisperse common salt aerosols, using laboratory data collected with a supercontinuum broadband cavity enhanced extinction spectrometer operating in the 420–540 nm wavelength range. Field measurements of ambient aerosol were conducted using a similar cavity enhanced extinction spectrometer (IBBCEES) operating in the wavelength range of 400–550 nm, a multi-wavelength aethalometer, and a scanning mobility particle sizer, in Changzhou city, People’s Republic of China. The absorption coefficients for the entire wavelength range were retrieved using the absorption Ångström exponents calculated from a pair of measured absorption coefficients at known wavelengths. The survey iteration method takes scattering and absorption coefficients, wavelength, and size distributions as inputs; and it calculates the Mie-equivalent wavelength dependent complex refractive index (RI = n ± ik) and estimated errors. The retrieved field RI values ranged from 1.66 ≤ n ≤ 1.80 to 1.65 ≤ n ≤ 1.86 and from 0.036 ≤ k ≤ 0.038 to 0.062 ≤ k ≤ 0.067 in the wavelength range (400–550 nm), for low and high aerosol loading conditions, respectively. Additionally, we derived the spectral dependencies of scattering and absorption coefficients along with the n and k Ångström exponents (AE). The nAE and kAE estimated values suggest a stronger wavelength dependence for aerosol light scattering compared to absorption, and a decreasing trend for the spectrally dependent single scattering albedo during both loading conditions. The extremum of errors in the retrieved n and k values were quantified by considering (a) uncertainties in input parameters in the broad spectral region (400–550 nm), (b) using CAPS extinction values at 530 nm and (c) an estimated size distribution incorporating the coarse particles (at 530 nm)