Characterization of the Optical Properties of Nitroaromatic Brown Carbon Aerosols Using Single Particle Techniques
MALSHA AMUGODA, Pedro De Allende, Bilal Shokoor, James F. Davies, University of California, Riverside
Abstract Number: 234
Working Group: Aerosol Processes and Properties in Changing Environments in the Anthropocene
Abstract
The scattering and absorption of light by aerosol particles depend on both the real and imaginary part of the complex refractive index (RI). Multifunctional nitroaromatic brown carbon (BrC) chromophores show significant light absorption in the short wave visible range of the solar spectrum. Predicting the contribution of aerosol particles containing BrC to the radiative budget is challenging due to the dependence of the optical properties on the exact chemical composition, with factors such as pH playing an important role.
In this study, we describe a method to retrieve the wavelength-dependent complex RI of aqueous BrC microdroplets levitated in a linear quadrupole electrodynamic balance (LQ-EDB) under controlled relative humidity (RH). A sample BrC droplet is levitated alongside a non-light-absorbing probe droplet and both are illuminated with broadband LED light (425 to 780 nm). Backscattered light is collected consecutively from each droplet and the spectra are processed together to obtain an intensity-calibrated spectrum for the BrC droplet.
We use this method to characterize the optical properties of a series of nitroaromatic chromophores (NAC) and their internal mixtures with inorganic salts. We identify that the optical properties of the NAC also depend on their volatility and hygroscopicity, and we characterize the evolution of optical properties with composition. Furthermore, based on the initial mixing ratio, RH, and volatility, we observe that some samples undergo liquid-liquid phase separation, thereby changing their optical properties. Moreover, increasing the bulk pH to form sodium salts of NAC produces a marked change in the particle properties, including scattering, absorption, volatility, and phase state. Overall, this study aims to connect the optical properties of BrC aerosol with their chemical composition, phase state, and evolution over time.