Exploring water-soluble organic aerosols structures in urban atmosphere using advanced solid-state 13C NMR spectroscopy

Abstract

Abstract Water-soluble organic matter (WSOM) in air particles has profound effects on climate and human health. At the heart of this environmental significance of WSOM lies a complex set of compounds, of which a major fraction still often remains undeciphered. Yet, not all environmental problems require delving into the molecular-level identification of WSOM constituents. Understanding the contribution of different functional groups to whole aerosol WSOM composition offers a highly important structural dataset that enables a better representation of WSOM in climate studies. For the first time, advanced solid-state 13C nuclear magnetic resonance (NMR) techniques, including nearly quantitative 13C multiple cross polarization/magic angle spinning (multiCP/MAS), multiCP/MAS with dipolar dephasing, multiCP/MAS with 13C chemical shift anisotropy filter, and two-dimensional 1H–13C heteronuclear correlation (2D HETCOR), are applied to acquire an accurate quantitative structural description of whole aerosol WSOM collected in an urban atmosphere. Two urban aerosol WSOM samples collected in two short periods of time, under different wintry weather conditions, were investigated. NMR data successfully pinpointed the variability of whole aerosol WSOM composition, allowing to suggest source-specific structural characteristics for each sample in two short periods of time. A new structural model of urban aerosol WSOM was build based on this compositional data, showing the presence of three independent classes of compounds that vary both in content and molecular diversity within short periods of time: heteroatom-rich aliphatic (either chain or branched), carbohydrate-like moieties, and highly substituted aromatic units. These findings establish advanced solid-state NMR as a promising tool for probing the chemical structures of inhomogeneous aerosol WSOM in rapidly changing atmospheric conditions, allowing to resolve discrepancies between modeled and measured aerosol WSOM.