Humidity driven nanoscale chemical separation in complex organic matter

Environmental context The absorption of water by mixtures of organic matter in aerosols influences various atmospheric processes, such as scattering of solar radiation and cloud formation. We use X-ray techniques to elucidate the swelling behaviour of representative organic matter under humid, sub-saturated conditions, and show chemical separation according to the functional group composition in the organic mixtures upon water uptake. The results will further our understanding of the complex role of aerosol organic matter in atmospheric processes. Abstract Humic-like substances (HULIS) represent an important fraction of particulate organic matter in the atmosphere. Understanding the water uptake by HULIS and the associated morphology evolution will improve the assessment of their ability to act as cloud condensation nuclei as well as their light scattering properties. The water uptake properties of Suwannee River Fulvic Acid and of tannic acid used as proxy for atmospheric HULIS, were investigated using X-ray absorption spectroscopy in combination with a scanning transmission X-ray microscope. For both compounds, continuous water uptake was observed, whereby in fulvic acid phase separation occurred, resulting in an inhomogeneous organic matrix. Within the inhomogeneous mixture, different regions with different amounts of water uptake could be differentiated based on their spectral signatures in near-edge X-ray absorption fine structure (NEXAFS) spectra, thus based on carbon functional group signatures, indicating that carboxyl-poor compounds separated from carboxyl-rich compounds upon water uptake. The differentiation into fractions with high/low water uptake ability is further refined by considering phenols, aromatic groups, and O-alkylic groups.