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Excitation-emission matrix (EEM) fluorescence spectroscopy has been widely used to characterize chemical components of brown carbon (BrC), yet the molecular basics and formation mechanisms of chromophores decomposed by parallel factor (PARAFAC) analysis are not fully understood. Here, water-soluble organic carbon (WSOC) in aerosols from Karachi, Pakistan, were characterized with EEM spectroscopy and Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS). Three PARAFAC components were identified, including two humic-like (C1 and C2), and one protein-like (C3) species. Among them, the C2 shows the longest emission maxima (~494 nm), and correlates tightly with the mass absorption efficiency at 365 nm (MAE365), the character of BrC. Molecular families associated with each of the three components were determined by Spearman correlation analysis between FT-ICR MS peaks and PARAFAC component intensities. The C1 and C2 components are associated with nitrogen-enriched compounds, despite that C2 more with higher aromaticity, higher N content, and highly oxygenated compounds. The formulas associated with C3 include fewer nitrogen-containing species, with a lower unsaturated degree and oxidation state. A dominant oxidation pathway for the formation of C1 and C2 components was suggested, notwithstanding their different precursor types. A large number of formulas associated with C2 were found to be located in the "potential BrC" region, overlapped with BrC-associated formulas, and readily correlated tightly with MAE365. This suggests that the compounds illuminating C2 may have also contributed substantially to the BrC light absorption. These findings were important for future studies using the EEM-PARAFAC method to explore the compositions, processes, and sources of atmospheric BrC. [ABSTRACT FROM AUTHOR]

Water‐soluble organic carbon (WSOC) is a substantial component of carbonaceous aerosols in South/East Asia. However, the poorly constrained sources, molecular compositions, and light absorption properties of WSOC over South/East Asian marine regions lead to large uncertainty in climate effects. Herein, we collected marine aerosol samples from South China Sea to the eastern Indian Ocean to investigate the sources, molecular compositions, and light absorption properties of WSOC. The δ13C of Marine‐influenced WSOC (−20.6 ± 1.4‰) was higher than those in Continent‐influenced (South Asia‐influenced: −24.4 ± 1.1‰ and Southeast Asia‐influenced: −22.7 ± 1.0‰, p < 0.01) regions. Additionally, combining with 5‐day back trajectories and the relationships of δ13C‐WSOC with chemical tracers (Cl−/Na+ and nonsea salt K+/K+), we found that the Marine‐influenced WSOC was mostly related to oceanic emissions, which was more enriched in saturated primary marine biological compounds with lower oxidation level. In contrast, the Continent‐influenced WSOC was significantly affected by C3 biomass burning and secondary organic aerosols formation, and it has a higher fraction of aromatic and highly oxidized compounds. Moreover, the high aromatic level of Continental‐influenced WSOC may result in high mass absorption efficiency at 365 nm (MAE365) but low absorption Ångström exponent (AAE) values. Taken together with previous WSOC light absorption properties studies, we found that the MAE365 of WSOC in urban, rural and background, and marine regions correlated well with AAE. However, the response and correlation coefficients of MAE365 to AAE were highest in urban regions, followed by rural and background and marine regions, which is likely associated with different aromatic compound contents. Therefore, in the future, more attention should be paid to the sources and atmospheric processes of condensed aromatic compounds. Key Points: The Continent‐influenced and Marine‐influenced WSOC were significantly affected by anthropogenic and oceanic emissions, respectivelyThe Continental‐influenced WSOC was more enriched in aromatic compounds, with stronger light absorption capacity than Marine‐influenced WSOCDifference in response and correlation coefficients of MAE365 to AAE in different regions may be related to varied aromatic compound contents [ABSTRACT FROM AUTHOR]