Chromophores and chemical composition of brown carbon characterized at an urban kerbside by excitation-emission spectroscopy and mass spectrometry

Abstract. The optical properties, chemical composition, and potential chromophores of brown carbon (BrC) aerosol particles were studied during typical summer and winter time at a kerbside in downtown Karlsruhe, a city in central Europe. The average absorption coefficient and mass absorption efficiency at 365 nm (Abs₃₆₅ and MAE₃₆₅) of BrC were lower in the summer period (1.6 ± 0.5 Mm^-1, 0.5 ± 0.2 m² g^-1) than in the winter period (2.8 ± 1.9 Mm^-1, 1.1 ± 0.3 m² g^-1). Using a Parallel factor (PARAFAC) analysis to identify chromophores, two different groups of highly oxygenated humic-like substances (HO-HULIS) dominated in summer and contributed 96 ± 6 % of total fluorescence intensity. In contrast, less oxygenated-HULIS (LO-HULIS) dominated the total fluorescence intensity in winter with 57 ± 12 %, followed by HO-HULIS with 31 ± 18 %. The statistical analysis of AMS data (positive matrix factorization) and Aqualog excitation-emission spectra (parallel factor analysis) showed that the LO-HULIS chromophores are most likely emitted from biomass burning in winter. Less volatile oxygenated organic aerosol shows good correlations (r > 0.7; p < 0.01, respectively) with HO-HULIS components in summer. The LO-HULIS have a negative correlation (r = -0.6, p < 0.01) with O₃, which indicates that the LO-HULIS may be depleted by reaction with ozone. In contrast, the HO-HULIS had a positive correlation (r = 0.7, p < 0.01) with O₃, indicating that they may result from oxidation reactions.

Five nitro-aromatic compounds (NACs) were identified by CIMS (C₇H₇O₃N, C₇H₇O₄N, C₆H₅O₅N, C₆H₅O₄N, and C₆H₅O₃N) which contributed 0.03 ± 0.01 % to the total organic mass, but can explain 0.3 ± 0.1 % of the total absorption of methanol-extracted BrC at 365 nm in winter. Furthermore, we identified 316 potential brown carbon molecules which accounted for 2.5 ± 0.6 % of the organic aerosol mass. Using an average mass absorption efficiency (MAE₃₆₅) of 9.5 m² g^-1 for these compounds, we can estimate their mean light absorption to be 1.2 ± 0.2 Mm^-1, accounting for 32 ± 15 % of the total absorption of methanol-extracted BrC at 365 nm. The potential BrC molecules assigned to the LO-HULIS component had a higher average molecular weight (265 ± 2 Da) and more nitrogen-containing molecules (62 ± 1%) than the molecules assigned to the HO-HULIS components. Our analysis shows that the LO-HULIS, with a high contribution of nitrogen-containing molecules originating from biomass burning, dominate aerosol fluorescence in winter and HO-HULIS, with less nitrogen-containing molecules from less volatile oxygenated organic aerosol, dominate in summer.