Title: Optical properties of atmospherically relevant aerosols: Instrumentation, measurements and retrieval

Abstract

The structure and composition of the atmosphere plays a vital role in balancing the incoming and outgoing solar radiations of our planet. Major pollutants such as trace gases and minute particles suspended in atmosphere, called aerosols, contribute much to this phenomenon affecting the global climate system. The optical properties of aerosols, like extinction coefficient (scattering plus absorption), single scattering albedo (SSA, ratio of scattering to extinction), aerosol optical depth, and Ångström exponent, describe the direct aerosol-radiation interactions. It is challenging to retrieve the broadband optical properties of aerosols due to the spatial and temporal variability of their concentration, their variable size distribution, composition, and mixing, and the limited detection limits of the instrumentation making such measurements. The accurate measurement of complex refractive index (RI = n ± ik), an intensive property of aerosol, is essential for modelling their radiative effects. Along with the knowledge about size distribution and wavelength, RI can be used to perform the forward Mie calculations which results in the accurate retrievals of optical parameters. Conversely, many studies have been conducted to perform the inverse calculations of Mie theory, to retrieve the aerosol complex RI from the optical parameters measured using several instruments, provided the size distribution of the aerosols are known. In this research work, the optical properties of aerosols were retrieved using both experimental and theoretical approaches. In the first part of this research work, we developed a new instrument exploiting the advantages of a visible-band supercontinuum (SC) light source with those of a dual-cavity cavity enhanced absorption spectrometer (CEAS) setup, which make use of the theory of incoherent broadband cavity-enhanced absorption spectroscopy (IBBCEAS). This instrument is capable of measuring light extinction by atmospheric aerosol over a broad wavelength range of 420-540 nm, which is important for carbonaceous aerosols. The newly developed supercontinuum broadband dual-cavity cavity enhanced extinction spectrometer (SC-BD-CEES) was calibrated and tested in the laboratory using absorption of NO2, as well as polydisperse (sodium chloride and kerosene soot) and monodisperse (polystyrene latex spheres) aerosol types. A comparison of both theoretical predictions and experimental measurements with the data obtained from our instrument resulted in good agreements. This study demonstrated that the SC-BD-CEES is a sensitive instrument with sufficient dynamic range and accuracy for laboratory and field applications. In the second part of this study, we exploited PyMieScatt’s survey iteration technique to retrieve the wavelength dependent complex refractive indices of ambient aerosols for the first time. As a primary validation of this method the broadband (420 – 540 nm) refractive indices of common calibration aerosols polystyrene latex (PSL) spheres and sodium chloride (NaCl) were retrieved, from the data collected using a supercontinuum dual-cavity cavity enhanced extinction spectrometer (SC-BD-CEES). Extinction and absorption of light due to ambient aerosols were measured using an incoherent broadband cavity enhanced extinction spectrometer (IBBCEES) and an aethalometer. The refractive indices in a wavelength range from 410 – 550 nm, at two different aerosol loading conditions were retrieved. The data measured on June 23rd 2019 from a location in the Yangtze river delta in the southwest of Changzhou city, P R China, was used for analysis. In addition to the refractive index, the Ångström exponents (AE) and dependence of broadband refractive indices on wavelength dependent single scattering albedo (SSA) for the atmospheric aerosols were also calculated. The results indicated the presence of absorbing aerosols in the ambient air which was validated by comparing the results obtained from AERONET data and HYSPLIT model. PyMieScatt survey iteration is a reliable inverse algorithmic method, used for the retrieval of complex RIs and its spectral dependencies, which, along with other optical parameters, can be used for source apportionment and for quantifying the effective radiative forcing of aerosols.