Investigating the water-soluble organic functionality of urban aerosols using two-dimensional correlation of solid-state 13 C NMR and FTIR spectral data Regina M.B.O. Duarte * , Sandra M.S.C. Freire, Armando C. Duarte Department of Chemistry & CESAM, University of Aveiro, 3810-193 Aveiro, Portugal highlights graphical abstract  Two-dimensional (2D) correlation spectroscopy was used to study aerosol WSOM.  The analytical approach is based on data from 13 C NMR and FTIR analysis.  Seasonal differences in urban WSOM composition have been described.  Urban WSOM samples consist of two major components that vary across the seasons.  Possible sources of identified WSOM components have been suggested. article info Article history: Received 3 March 2015 Received in revised form 22 June 2015 Accepted 25 June 2015 Available online 30 June 2015 Keywords: Urban organic aerosols Water-soluble organic matter CP-MAS 13 C NMR spectroscopy FTIR-ATR spectroscopy Two-dimensional correlation spectroscopy abstract Two-dimensional (2D) correlation spectroscopy, applied to one-dimensional solid-state cross polariza- tion magic angle spinning (CP-MAS) 13 C nuclear magnetic resonance (NMR), Fourier transform infrared coupled to attenuated total reflectance (FTIR-ATR), and the combination of CP-MAS 13 C NMR and FTIR- ATR data, has been used to glean new structural information on the most hydrophobic water-soluble organic matter (WSOM) from fine urban air particles collected during different seasons. CP-MAS 13 C NMR analysis confirmed that unsubstituted saturated aliphatics, aromatics, oxygenated aliphatics, and carboxylic acids accounted for most (84e92%) of the characterized carbon functional groups. Applying 2D correlation spectroscopy to CP-MAS 13 C NMR and FTIR-ATR spectra indicates an increase in the contribution of hydroxyl and carboxylic groups to the aliphatic component with increasing air tem- perature, suggesting the photo-chemical origin of these WSOM structures. Urban samples from colder periods are clearly impacted by biomass combustion sources, although the presence of aged and pro- cessed organic aerosols cannot be ruled out. Using 2D FTIR- 13 C NMR hetero-spectral correlation spec- troscopy, it has been demonstrated that the studied WSOM samples consists of at least two different classes of compounds: (i) an aliphatic component rich in both carboxylic and hydroxyl functional groups, and (ii) lignin-derived structures. © 2015 Elsevier Ltd. All rights reserved. 1. Introduction Organic aerosols (OA) comprise a multitude of molecular structures, physical properties, and sources (primary and secondary formation) (P€ oschl, 2005; Duarte and Duarte, 2013). This inherent * Corresponding author. E-mail address: regina.duarte@ua.pt (R.M.B.O. Duarte). Contents lists available at ScienceDirect Atmospheric Environment journal homepage: www.elsevier.com/locate/atmosenv http://dx.doi.org/10.1016/j.atmosenv.2015.06.043 1352-2310/© 2015 Elsevier Ltd. All rights reserved. Atmospheric Environment 116 (2015) 245e252