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

• Published in: Atmospheric chemistry and physics Vol. 20; no. 4; pp. 2513 - 2532
• Main Authors: Tang, Jiao, Li, Jun, Su, Tao, Han, Yong, Mo, Yangzhi, Jiang, Hongxing, Cui, Min, Jiang, Bin, Chen, Yingjun, Tang, Jianhui, Song, Jianzhong, Peng, Ping'an, Zhang, Gan
• Format: Journal Article
• Language: English
• Published: Katlenburg-Lindau Copernicus GmbH 02-03-2020; Copernicus Publications
• Subjects: Absorption; Aerosols; Ageing; Aging; Air pollution; Analysis; Analytical methods; Atmospheric aerosols; Atmospheric physics; Automotive emissions; Biomass; Biomass burning; Burning; Carbon; Chemical composition; Chemical compounds; Chromophores; Coal; Coal combustion; Combustion; Components; Cyclotron resonance; Digestive absorption; Electromagnetic absorption; Emission analysis; Emission spectroscopy; Excitation spectra; Fluorescence; Fluorophores; Fossils; Fourier analysis; Fourier transforms; Fuel consumption; Ionization; Ions; Light absorption; Mass spectra; Mass spectrometry; Mass spectroscopy; Methanol; Molecular weight; Optical properties; Organic carbon; Radiative forcing; Resonance; Scientific imaging; Smoke; Spectroscopy; Vehicle emissions; Water chemistry
• ISSN: 1680-7324; 1680-7316; 1680-7324
• DOI: 10.5194/acp-20-2513-2020

Brown carbon (BrC) plays an essential impact on radiative forcing
due to its ability to absorb sunlight. In this study, the optical properties
and molecular characteristics of water-soluble and methanol-soluble organic
carbon (OC; MSOC) emitted from the simulated combustion of biomass and coal
fuels and vehicle emissions were investigated using ultraviolet–visible (UV–vis)
spectroscopy, excitation–emission matrix (EEM) spectroscopy, and
Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS)
coupled with electrospray ionization (ESI). The results showed that these
smoke aerosol samples from biomass burning (BB) and coal combustion (CC) had
a higher mass absorption efficiency at 365 nm (MAE365) than vehicle
emission samples. A stronger MAE365 value was also found in MSOC than
water-soluble organic carbon (WSOC), indicating low polar compounds would
possess a higher light absorption capacity. Parallel factor (PARAFAC) analysis
identified six types of fluorophores (P1–6) in WSOC including two
humic-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 from
aging vehicle exhaust particles; P2 was only abundant in BB aerosols; P3 was
ubiquitous in all tested aerosols; P4 was abundant in fossil burning
aerosols; and P5 was more intense in fresh vehicle exhaust particles. The
MSOC chromophores (six components; C1–6) exhibited consistent
characteristics with WSOC, suggesting the method could be used to indicate
the origins of chromophores. FT-ICR mass spectra showed that CHO and CHON
were the most abundant components of WSOC, but S-containing compounds
appeared in a higher abundance in CC aerosols and vehicle emissions than BB
aerosols, while considerably fewer S-containing compounds largely with CHO
and 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 and
low O ∕ C ratio, while CC aerosols and vehicle emissions largely with
S-containing compounds had an opposite H ∕ C and O ∕ C ratio. Moreover, the
light absorption capacity of WSOC and MSOC was positively associated with
the unsaturation degree and molecular weight in the source aerosols. The
above results are potentially applicable to further studies on the EEM-based or
molecular-characteristic-based source apportionment of chromophores in
atmospheric aerosols.