The light absorbing and molecule characteristic of PM2.5 brown carbon observed in urban Shanghai.

Both brown carbon (BrC) and the non-absorbing components coated on black carbon (BC) aerosols can enhance the light absorption of BC aerosols. BrC is a complicated mixture of organic compounds and not well characterized, which hinders exploring the links between BrC and optical properties. We conducted an in-depth field study on optical properties of ambient aerosols at a monitoring site in Shanghai, China via real-time monitoring and offline analysis. Results showed that BrC caused light absorption coefficients were 3.3 ± 3.3 Mm⁻¹, 2.2 ± 5.0 Mm⁻¹, 1.2 ± 1.2 Mm⁻¹ at λ = 370, 470 and 520 nm, respectively, accounting for 11%, 10%, 6% of the total aerosol absorption for the corresponding wavelengths. A larger proportion of long-chain aliphatic organosulfates (OSs, C n H 2n+2 O 4 S, (CH 2) n O 5 S, (CH 2) n O 6 S) with double bond equivalent (DBE) values of 0 or 1 accounted for 5–20% of the light absorption (λ = 365 nm) for soluble brown carbon (BrC), which were dominating for the days with less N-containing aromatic compounds appearing. Furthermore, the structure of C n H 2n+2 O 4 S, (CH 2) n O 5 S, (CH 2) n O 6 S were explored using target MS/MS of HPLC-Q-ToF-MS: (CH 2) n O 5 S series, the most abundant family of OSs, were constructed by functionalizing a saturated hydrocarbon with one sulfate and one carbonyl group. C n H 2n+2 O 4 S series were oxidized with only one sulfate group in the aliphatic chain R. (CH 2) n O 6 S series were proposed as aliphatic OSs with one ester group. We speculated aliphatic OSs were formed via acid catalyzed perhydrolysis of hydroperoxides derived from long-chain alkanes releasing from diesel fueled vehicles, followed by the reaction with sulfate anion radicals. Therefore, relevant technologies should be further explored to reduce the impacts from vehicle emissions. [Display omitted] • BrC generally contributed to 10% of the total aerosol absorption at λ = 370–470 nm. • Long-chain aliphatic OSs are strong chromophores on less biomass burning days. • C n H 2n+2 O 4 S, (CH 2) n O 5 S, (CH 2) n O 6 S are aliphatic OSs with one O 5 S and O 6 S. • The OSs may be formed via acid catalyzed perhydrolysis of hydroperoxides. [ABSTRACT FROM AUTHOR]