Measurement report: Brown carbon aerosol in polluted urban air of the North China Plain – day–night differences in the chromophores and optical properties

Brown carbon (BrC) aerosol is light-absorbing organic carbon that affects radiative forcing and atmospheric photochemistry. The BrC chromophoric composition and its linkage to optical properties at the molecular level, however, are still not well characterized. In this study, we investigate the day–night differences in the chromophoric composition (38 species) and optical properties of water-soluble and water-insoluble BrC fractions (WS-BrC and WIS-BrC) in aerosol samples collected in Shijiazhuang, one of the most polluted cities in China. We found that the light absorption contribution of WS-BrC to total BrC at 365 nm was higher during the day (62±8 %) than during the night (47±26 %), which is in line with the difference in chromophoric polarity between daytime (more polar nitrated aromatics) and nighttime (more less-polar polycyclic aromatic hydrocarbons, PAHs). The high polarity and water solubility of BrC in the daytime suggests the enhanced contribution of secondary formation to BrC during the day. There was a decrease in the mass absorption efficiency of BrC from nighttime to daytime (2.88±0.24 vs. 2.58±0.14 for WS-BrC and 1.43±0.83 vs. 1.02±0.49 m2 g C−1 for WIS-BrC, respectively). Large polycyclic aromatic hydrocarbons (PAHs) with four- to six-ring PAHs and nitrophenols contributed to 76.7 % of the total light absorption between 300–420 nm at nighttime, while nitrocatechols and two- to three-ring oxygenated PAHs accounted for 52.6 % of the total light absorption during the day. The total mass concentrations of the identified chromophores showed larger day–night difference during the low-pollution period (day-to-night ratio of 4.3) than during the high-pollution period (day-to-night ratio of 1.8). The large day–night difference in BrC composition and absorption, therefore, should be considered when estimating the sources, atmospheric processes, and impacts of BrC.