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

Abstract

Abstract. 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.

Measurement report: Long emission-wavelength chromophores dominate the light absorption of brown carbon in Aerosols over Bangkok: impact from biomass burning

“…The MAE at nm (MAE365) of the WSOC was 0.83±0.25 m 2 g −1 C, which was higher than that of the MSOC (0.26±0.12 m 2 g −1 C), indicating that more water-soluble BrC with stronger light absorption capability could be extracted with ultrapure deionized water, whereas water-insoluble BrC is characterized by lower light absorption capability over Bangkok. These results were consistent with those from vehicular exhaust samples in our previous study, where MAE365 values of the WSOC (0.71±0.30 m 2 g −1 C) were higher than those of the MSOC (0.26±0.09 m 2 g −1 C) (Tang et al, 2020b).…”

“…S7, high TCC values were found as expected between the 85-model components and the 145model components, indicating that the two models identified similar chromophores. Although one exceptional component was detected each for the WSOC and MSOC fractions by the new 145-model, these fluorescent components were only highly characterized by source samples, as reported in our previous study (Tang et al, 2020b).…”

“…The much higher HIX values in the WSOC compared to the MSOC suggest that WSOC may have a higher degree of aromaticity or a more condensed chemical structure. Our previous study revealed that MSOC has a higher molecular weight but lower aromaticity index than the corresponding WSOC in combustion experiment aerosol samples, indicating a more aliphatic structure in the MSOC (Tang et al, 2020b). The HIX values of WSOC were highest in the hot season (3.9±1.1), followed by the pre-hot season (3.3±1.1), cool season (2.9±0.36), and monsoon (2.5±0.22), whereas those of the MSOC tended to be higher in the hot and cool seasons than in the monsoon and pre-hot seasons.…”

“…as well as source emission samples including biomass burning, coal combustion and vehicle emission which were encircled by a violet, yellow, and blue region, respectively. Note that the fluorescence characteristic of source samples was described elsewhere (Tang et al, 2020b), but the fluorescence indices was first reported in this study. Pre-hot season is from January 18 to February 29, 2016; hot season is from March 2 to May 31, 2016; monsoon is from June 2 to October 30, 2016; cool season is from November 1, 2016 to January 28, 2017.…”

“…To further explore the potential sources of the EEM-PARAFAC components, we added 60 source samples to the matrices. The source sample EEM data were described in our previous study (Tang et al, 2020b), including those of 33 biomass-burning samples (IDs: 1-33), 17 coal-combustion samples of two fluorescence spectra between the two models (refer to Text S3 of Supplement). Note that a higher TCC value would indicate a higher degree of similarity of the spectra.…”