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 (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)
- 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 for Measurement report: Diurnal variations of brown carbon during two distinct seasons in a megacity in Northeast China https://doi.org/10.5281/zenodo.7590785
Yuan Cheng et al.
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This manuscript investigated diurnal variations of BrC in a megacity in Northeast China. The studied region is distinct, because it has quite different meteorological conditions and emission sources compared to well-known hotspots such as Beijing and surrounding regions. So far, however, aerosols in this region remained poorly understood with limited studies, e.g., regarding their chemical compositions, physical properties, sources and impacts. In this context, the authors conducted field measurements during two distinct seasons in a “largely unexplored” megacity in Northeast China, and traced the diurnal variations of BrC back to the changes of aerosol sources. The unique light absorption spectra of BrC observed during open burning episodes are especially interesting. Therefore, I think this manuscript represents a valuable contribution to better understanding of haze pollution in Northeast China, and my overall assessment is that it could be considered for publication in ACP after addressing the comments below.
Major point. MAE, which can be converted to the imaginary part of the complex refractive index of BrC, is an important parameter for climate models. In addition to a summary of the observational results, implications of this study should also be involved in the Conclusions section. To my understanding, although the winter is much colder in Northeast China compared to Beijing, MAE did not show apparent difference between these two regions. This is a potentially important point for the spatial distribution of MAE, but was completely ignored by the authors. In addition, the authors should make clear recommendations regarding whether the diurnal variations of MAE need to be considered in climate models.