Measurement report: Diurnal variations of brown carbon during two distinct seasons in a megacity in Northeast China

Abstract. Brown carbon (BrC) represents an important target for the “win-win” strategy of mitigating climate change and improving air quality. However, estimating co-benefits of BrC control remains difficult for China, partially because current measurement results are insufficient to represent the highly variable emission sources and meteorological conditions across different regions. In this study, we investigated, for the first time, the diurnal variations of BrC during two distinct seasons in a largely unexplored megacity in Northeast China. The winter campaign conducted in January of 2021 was characterized by low temperatures rarely seen in other Chinese megacities (down to about −20 °C). The mass absorption efficiencies of BrC at 365 nm (MAE₃₆₅) were found to be ~10 % higher at night. The variations of MAE₃₆₅ could not be explained by the influence of residential biomass burning emissions or secondary aerosol formation, but were strongly associated with the changes of a diagnostic ratio for the relative importance of coal combustion and vehicle emissions (R_S/N). Given that most coal combustion activities were uninterruptible, the higher nighttime MAE₃₆₅ in winter were attributed primarily to increased emissions from heavy-duty diesel trucks. The spring campaign conducted in April of 2021 was characterized by frequent occurrences of agricultural fires, as supported by the intensive fire hotspots detected around Harbin and the more-than-doubled levoglucosan to organic carbon ratios (LG/OC) compared to winter campaign. In spring, MAE₃₆₅ depended little on R_S/N but exhibited a strong positive correlation with LG/OC, suggesting open burning emissions as the dominant influencing factor for BrC’s light absorption capacity. MAE₃₆₅ were ~70 % higher at night for the spring campaign, pointing to the prevalence of nighttime agricultural fires, which were presumably in response to local bans on open burning. It is noteworthy that the agricultural fire emissions resulted in distinct peak at ~365 nm for the light absorption spectra of BrC, and a candidate for the compounds at play was inferred to be C₇H₇NO₄. Due to the presence of the ~365 nm peak, the absorption Ångström exponents could not be properly determined for the agricultural fire-impacted samples. In addition, the ~365 nm peak became much less significant during the day, likely due to photo-bleaching of the relevant chromophores.