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

Abstract

Abstract. The optical properties, chemical composition, and potential chromophores of brown carbon (BrC) aerosol particles were studied during typical summertime and wintertime 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 methanol-soluble BrC (MS-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 the 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 %. Positive matrix factorization (PMF) analysis of organic compounds detected in real time by an online aerosol mass spectrometer (AMS) led to five characteristic organic compound classes. The statistical analysis of PARAFAC components and PMF factors showed that LO-HULIS chromophores were most likely emitted from biomass burning in winter. HO-HULIS chromophores could be low-volatility oxygenated organic aerosol from regional transport and oxidation of biogenic volatile organic compounds (VOCs) in summer. Five nitro-aromatic compounds (NACs) were identified by a chemical ionization mass spectrometer (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 MS-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 m2g−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 MS-BrC at 365 nm. This indicates that a small fraction of brown carbon molecules dominates the overall absorption. 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, dominates aerosol fluorescence in winter, and HO-HULIS, with fewer nitrogen-containing molecules as low-volatility oxygenated organic aerosol from regional transport and oxidation of biogenic volatile organic compounds (VOC), dominates in summer.

Molecular Analysis of Secondary Organic Aerosol and Brown Carbon from the Oxidation of Indole

“…Notably, Absind-SOA at both times exceeds the total absorption (0.84 ± 0.24 Mm −1 ) of ambient BrC measured during the summer at a rural background location in Melpitz, Germany (Moschos et al, 2018). Additionally, these calculated absorption values fall within the same range of total light absorption at 365 nm (1.6 ± 0.5 Mm −1 in summer 300 and 2.8 ± 1.9 Mm −1 in winter) observed for BrC aerosol in an urban area, the city of Karlsruhe (Jiang et al, 2022).…”

“…In addition, particle number concentrations were measured by CPC (3022a, TSI Inc.). SMPS and CPC data analysis was also shown in a previous study (Gao et al, 2022). The chemical composition and aerodynamic size of ind-SOA were characterized by a high-resolution time-of-flight aerosol mass spectrometer (HR-ToF-AMS, Aerodyne Inc. hereafter AMS).…”

“…S1. The detailed description of the AIDA chamber, its operation modes, and the associated measurement techniques were published elsewhere (Gao et al, 2022;Saathoff et al, 2009). Briefly, ind-SOA was produced from ozonolysis of indole in the presence/absence of NO2 and (NH4)2SO4 seed particles, with experiment-specific conditions listed in Table 1.…”

“…The Teflon filters analyzed by FIGAERO-CIMS provide the organic molecular composition of the ind-SOA. The instrument and its modes of operation have been described in detail elsewhere (Jiang et al, 2022). Briefly, components A second quartz filter in each experiment was extracted with 8 mL acetonitrile (Optima LC/MS grade, Fisher Chemical) under sonication.…”

“…CC BY 4.0 License. (Jiang et al, 2022). The MAC of the BrC fractions in the extracts were calculated as a function of wavelength according to Hecobian et al (2010):…”