Carbonaceous aerosols play an important role in the Earth's radiative balance. One such aerosol, black carbon (BC), absorbs significant amounts of light and exerts a warming effect, while organic carbon (OC) was initially thought to only scatter solar radiation. However, recent studies show that there are certain types of OC that absorb radiation efficiently in the near-ultraviolet (UV) (300–400 nm) and visible ranges, which are called brown carbon (BrC). According to previous studies, BrC in atmospheric aerosols mainly originates from emissions from biomass burning (BB) and coal combustion (CC), vehicle exhaust, and the formation of secondary organic aerosol (SOA). Recently, many studies have investigated the optical properties and molecular characteristics of BrC in laboratory-simulated combustion and their light absorption in controlled vehicle emissions. However, there were no available studies on the comprehensive characteristics of BrC in various sources and their variations in optical and chemical information impacted by these sources. Therefore, investigating BrC in different sources would improve our understanding of the evolution of BrC absorption.
Researchers from Chinese Academy of Sciences and Fudan University investigated 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 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.
The study is published in Atmospheric Chemistry and Physics on March 2, Phd student TANG Jiao at University of Chinese Academy of Sciences (institute: Guangzhou Institute of Geochemistry, Chinese Academy of Sciences) is the first author. Prof. LI Jun and ZHANG Gan at University of Chinese Academy of Sciences and Guangzhou Institute of Geochemistry of Chinese Academy of Sciences are the corresponding authors.
