Water-insoluble organic carbon in PM2.5 over China: light-absorbing properties, potential sources, radiative forcing effects and possible light-absorbing continuum.

Water-insoluble carbon (WIOC) constitutes a substantial portion of organic carbon (OC) and contributes significantly to light absorption by brown carbon (BrC), playing pivotal roles in climate forcing and human health. China as hotspots regions with high level of OC and BrC, information regarding the sources and light-absorbing properties of WIOC on national scale remains scarce. Here, we investigated the light-absorbing properties and sources of WIOC in ten representative urban cities across China. On average, WIOC accounted for 33.4 ± 7.66 % and 40.5 ± 9.73 % of the concentrations and light-absorbing efficiency at 365 nm (Abs₃₆₅) of extractable OC (EX-OC, comprising relatively hydrophobic OC [WIOC and humic-like substances: HULIS-C], and hydrophilic OC [non-humic-like substances: non-HULIS-C]). The mass absorption efficiency of WIOC at 365 nm (MAE₃₆₅) was (1.59 ± 0.55 m²/gC) comparable to that of HULIS (1.54 ± 0.57 m²/gC) but significantly higher than non-HULIS (0.71 ± 0.28 m²/gC), indicating that hydrophobic OC possesses a stronger light-absorbing capacity than hydrophilic OC. Biomass burning (31.0 %) and coal combustion (31.1 %) were the dominant sources of WIOC, with coal combustion sources exhibited the strongest light-absorbing capacity. Moreover, employing the simple forcing efficiency (SFE_300–700nm) method, we observed that WIOC exhibited the highest SFE_300–700nm (6.57 ± 5.37 W/g) among the EX-OC fractions. The radiative forcing of EX-OC was predominantly contributed by hydrophobic OC (WIOC: 39.4 ± 15.5 % and HULIS: 39.5 ± 12.1 %). Considering the aromaticity, sources, and atmospheric processes of different carbonaceous components, we propose a light-absorbing carbonaceous continuum, revealing that components enriched with fossil sources tend to possess stronger light-absorbing capacity, higher aromatic levels, increased molecular weights, and greater recalcitrance in the atmosphere. Reducing fossil fuel emissions emerges as an effective means of mitigating both gaseous (CO₂) and particulate light-absorbing carbonaceous warming components. [ABSTRACT FROM AUTHOR]