Examining the structural properties of hydrophilic and hydrophobic organic aerosols using 1H NMR: Diurnal variations and source apportionment

Abstract

The behaviour of the carbonaceous aerosols during the rainy season and the diurnal variations in their structural groups have not been thoroughly examined. The present study aims to understand the structural composition of hydrophilic and hydrophobic organic aerosols (OA) at an urban background location in Mumbai, India. The carbonaceous fractions, i.e., Elemental (EC) and Organic (OC) Carbon, accounted for 14–34% of the total PM₁₀ (Particulate matter with aerodynamic diameter ≤10 μm). The PM₁₀ and EC were maximum in the morning, while OC was the highest in the evening. The aliphatic structural groups were more concentrated in the total fraction, contributing 53–62% of the total resonances. The total concentrations of the structural groups in both hydrophilic (29.2 ± 9.8 μmol/m³) and total (197.8 ± 154.3 μmol/m³) fractions were highest in the morning. Traffic emissions impacted the morning and evening aerosols, as suggested by the broad aliphatic and sharp aromatic resonances observed in the total fraction. This is further corroborated by the variability in EC and OC, their significant correlations with Volatile OC and Nitrogen oxides, and their contribution to regression models and principal components. The afternoon aerosols demonstrated characteristics of Secondary OA. Our work extends the present understanding of the diurnal variability and the heterogeneity of the hydrophilic and hydrophobic structural groups in organic aerosols.