Atmospheric brown carbon (BrC) is a light-absorbing component that affects radiative forcing; however, this effect requires further clarification, particularly with respect to BrC emission sources, chromophores, and optical properties. In the present study, the concentrations, optical properties, and emission factors of organic carbon (OC), water-soluble OC (WSOC), and humic-like substances (HULIS) in fine particulate matter (PM2.5) emitted from vehicles in three road tunnels ( the Wucun, Xianyue, and Wenxing tunnels in Xiamen, China) were investigated. The mass concentrations and light absorption of OC, WSOC, and HULLS were higher at the exits of each tunnel than at entrances, demonstrating that vehicle emissions were a BrC source. At each tunnel's exit, the average light absorption contributed by HULIS-BrC to water-soluble BrC (WS-BrC) and total BrC at 365 nm was higher than the corresponding carbon mass concentration contributed by HULIS (HULIS-C) to WSOC and OC, indicating that the chromophores of HULIS emitted from vehicles had a disproportionately high effect on the light absorption characteristics of BrC. The emission factors (ER) of HUUS-C and WSOC mass concentrations were highest at the Xianyue tunnel; however, the EFs of HULIS-BrC and WS-BrC light absorption were highest at the Wenxing tunnel, indicating that the chromophore composition of BrC was different among the tunnels and that the mass concentration EFs did not correspond directly to the light absorption EFs. (C) 2021 Elsevier B.V. All rights reserved.

Carbonaceous aerosols over the Tibetan Plateau originate primarily from biomass burning and vehicle emissions (BB and VEs, respectively). The light absorption characteristics of these carbonaceous aerosols are closely correlated with the burning conditions and represent key factors that influence climate forcing. In this study, the light absorption characteristics of elemental carbon (EC) and water-soluble organic carbon (WSOC) in PM2.5 (fine particulate matter smaller than 2.5 mu m) generated from BB and VEs were investigated over the Tibetan Plateau (TP). The results showed that the organic carbon (OC)/EC ratios from BB- and VE-sourced PM2.5 were 17.62 +/- 10.19 and 1.19 +/- 0.36, respectively. These values were higher than the ratios in other regions, which was primarily because of the diminished amount of oxygen over the TP. The mass absorption cross of EC (MAC(EC)) at 632 nm for the BB-sourced PM2.5 (6.10 +/- 1.21 m(2).g(-1)) was lower than that of the VE-sourced PM2.5 (8.10 +/- 0.98 m(2).g(-1)), indicating that the EC content of the BB-sourced PM2.5 was overestimated because of the high OC/EC ratio. The respective absorption per mass (alpha/rho) values at 365 nm for the VE- and BB-sourced PM2.5 were 0.71 +/- 0.17 m(2).g(-1) and 0.91 +/- 0.18 m(2).g(-1). The alpha/rho value of the VEs was loaded between that of gasoline and diesel emissions, indicating that the VE-sourced PM2.5 originated from both types of emissions. Because OC and WSOC accounts for most of the carbonaceous aerosols at remote area of the TP, the radiative forcing contributed by the WSOC should be high, and requires further investigation.