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Abstract. Brown carbon (BrC) plays an essential impact on radiative forcingdue to its ability to absorb sunlight. In this study, the optical propertiesand molecular characteristics of water-soluble and methanol-soluble organiccarbon (OC; MSOC) emitted from the simulated combustion of biomass and coalfuels and vehicle emissions were investigated using ultraviolet–visible (UV–vis)spectroscopy, excitation–emission matrix (EEM) spectroscopy, andFourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS)coupled with electrospray ionization (ESI). The results showed that thesesmoke aerosol samples from biomass burning (BB) and coal combustion (CC) hada higher mass absorption efficiency at 365 nm (MAE 365 ) than vehicleemission samples. A stronger MAE 365 value was also found in MSOC thanwater-soluble organic carbon (WSOC), indicating low polar compounds wouldpossess a higher light absorption capacity. Parallel factor (PARAFAC) analysisidentified six types of fluorophores (P1–6) in WSOC including twohumic-like substances (HULIS-1) (P1 and P6), three protein-like substances(PLOM) (P2, P3, and P5), and one undefined substance (P4). HULIS-1 was mainly fromaging vehicle exhaust particles; P2 was only abundant in BB aerosols; P3 wasubiquitous in all tested aerosols; P4 was abundant in fossil burningaerosols; and P5 was more intense in fresh vehicle exhaust particles. TheMSOC chromophores (six components; C1–6) exhibited consistentcharacteristics with WSOC, suggesting the method could be used to indicatethe origins of chromophores. FT-ICR mass spectra showed that CHO and CHONwere the most abundant components of WSOC, but S-containing compoundsappeared in a higher abundance in CC aerosols and vehicle emissions than BBaerosols, while considerably fewer S-containing compounds largely with CHOand CHON were detected in MSOC. The unique formulas of different sources in the van Krevelen (VK) diagram presented different molecular distributions.To be specific, BB aerosols with largely CHO and CHON had a medium H ∕ C andlow O ∕ C ratio, while CC aerosols and vehicle emissions largely withS-containing compounds had an opposite H ∕ C and O ∕ C ratio. Moreover, thelight absorption capacity of WSOC and MSOC was positively associated withthe unsaturation degree and molecular weight in the source aerosols. Theabove results are potentially applicable to further studies on the EEM-based ormolecular-characteristic-based source apportionment of chromophores inatmospheric aerosols.

atmospheric chemistry and physics

Molecular compositions and optical properties of dissolved brown carbon in biomass burning, coal combustion, and vehicle emission aerosols illuminated by excitation–emission matrix spectroscopy and Fourier transform ion cyclotron resonance mass spectrometry analysis