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The impact of water droplets on soils has recently been found to drive emissions of airborne soil organic particles (ASOP). The chemical composition of ASOP include macromolecules such as polysaccharides, tannins, and lignin (derived from degradation of plants and biological organisms), which determine light absorbing (brown carbon) particle properties. Optical properties of ASOP were inferred from the quantitative analysis of the electron energy-loss spectra acquired over individual particles using transmission electron microscopy. The optical constants of ASOP are compared with those measured for laboratory generated particles composed of Suwanee River Fulvic Acid (SRFA) reference material, which is used as a laboratory surrogate of ASOP. The chemical composition of the particles was analyzed using energy dispersive X-ray spectroscopy, electron energy-loss spectroscopy, and synchrotron-based scanning transmission X-ray microscopy with near edge X-ray absorption fine structure spectroscopy. ASOP and SRFA exhibit similar carbon composition, with minor differences in other elements present. When ASOP are heated to 350 degrees C their absorption increases as a result of pyrolysis and partial volatilization of semivolatile organic constituents. The retrieved refractive index (RI) at 532 nm of SRFA particles, ASOP, and heated ASOP were 1.22-0.07i, 1.29-0.07i, and 1.90-0.38i, respectively. Retrieved imaginary part of the refractive index of SRFA particles derived from EELS measurements was higher and the real part was lower compared to data from more common optical methods. Therefore, corrections to the EELS data are needed for incorporation into models. These measurements of ASOP optical constants confirm that they have properties characteristic of atmospheric brown carbon and therefore their potential effects on the radiative forcing of climate need to be assessed in atmospheric models.

来源平台：ACS EARTH AND SPACE CHEMISTRY