Chromophores and chemical composition of brown carbon characterized at an urban kerbside by excitation–emission spectroscopy and mass spectrometry
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Published:2022-11-24
Issue:22
Volume:22
Page:14971-14986
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ISSN:1680-7324
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Container-title:Atmospheric Chemistry and Physics
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language:en
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Short-container-title:Atmos. Chem. Phys.
Author:
Jiang Feng, Song JunweiORCID, Bauer Jonas, Gao LinyuORCID, Vallon Magdalena, Gebhardt Reiner, Leisner Thomas, Norra Stefan, Saathoff Harald
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.
Publisher
Copernicus GmbH
Subject
Atmospheric Science
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