16 Feb 2023
 | 16 Feb 2023
Status: this preprint is currently under review for the journal ACP.

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

Yuan Cheng, Xu-bing Cao, Jiu-meng Liu, Ying-jie Zhong, Qin-qin Yu, Qiang Zhang, and Ke-bin He

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 (RS/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 RS/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.

Yuan Cheng et al.

Status: open (until 30 Mar 2023)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on acp-2023-51', Anonymous Referee #1, 21 Feb 2023 reply
  • RC2: 'Comment on acp-2023-51', Anonymous Referee #2, 21 Mar 2023 reply

Yuan Cheng et al.

Data sets

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

Yuan Cheng et al.


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Short summary
BrC aerosols were explored in the northernmost megacity in China during a frigid winter and an agricultural-fire-impacted spring. BrC were more light-absorbing at night for both seasons, with more pronounced diurnal variations in spring, and the dominant drivers were identified as regulations on heavy-duty diesel trucks and open burning. Agricultural fires resulted in unique absorption spectra of BrC, which were characterized by distinct peak at ~365 nm.