Concurrent photochemical whitening and darkening of ambient brown carbon
Abstract. The light-absorbing organic aerosol (OA), known as brown carbon (BrC), has important radiative impacts, however its sources and evolution after emission remain to be elucidated. In this study, the light absorption at multiple wavelengths, mass spectra of OA and microphysical properties of black carbon (BC) were characterized at a typical sub-urban environment in Beijing. The absorption of BC is constrained by its size distribution and mixing state, being subtracted from total absorption to obtain the absorption of BrC, then by applying the least-correlation of BC absorption with secondary BrC, the absorption contributed by BC, primary BrC and secondary BrC was apportioned. The multi-linear regression analysis on the factorized OA mass spectra indicated the OA from traffic and biomass burning emission contributed to primary BrC. Importantly, the moderately oxygenated OA (O/C=0.62) was revealed to highly correlate with secondary BrC. These OA had higher nitrogen content, in line with the nitrogen-containing functional groups detected by the Fourier transform infrared spectrometer. The photooxidation was found to result in reduced contribution of primary BrC about 20 % but enhanced contribution of secondary BrC by 30 %, implying the concurrent whitening and darkening of BrC. This provides field evidence that the photochemically produced secondary nitrogen-containing OA can considerably compensate some bleaching effect on the primary BrC, hereby causing radiative impacts.
Qian Li et al.
Status: final response (author comments only)
RC1: 'Comment on acp-2022-483', Anonymous Referee #1, 28 Aug 2022
- AC1: 'Reply on RC1', Dantong Liu, 20 Oct 2022
RC2: 'Comment on acp-2022-483', Anonymous Referee #2, 31 Aug 2022
- AC2: 'Reply on RC2', Dantong Liu, 20 Oct 2022
Qian Li et al.
Qian Li et al.
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General Comments: BrC aerosols are short-lived climate forcers and contribute substantially to anthropogenic radiative forcing. Their sources and evolution pathways need to be elucidated. This manuscript titled "Concurrent photochemical whitening and darkening of ambient brown carbon" explores these research questions using diurnal measurements of microphysical (SP2), light absorption (Aethalometer) and chemical characteristics (HR-Tof-AMS and FTIR) at a sub-urban site in Beijing. The manuscript assessed diurnal variation of AMS based PMS source factors, apportioned absorption coefficient at 375 nm for BC, primary BrC and secondary BrC and multiple linear regression between absorption and PMF factors. Overall, the study has some interesting findings about bleaching and darkening of BrC during night time, daytime (photochemical oxidation) and role of nitration in governing these BrC behaviours. However, the manuscript has many shortcomings in its current version. It needs through language editing and clarifications at many places throughout the manuscript. The study has relevance to the atmospheric research community and can be accepted for publication in the journal after major revision. The detailed comments are given below:
R Satish, N Rastogi On the use of brown carbon spectra as a tool to understand their broader composition and characteristics: a case study from crop-residue burning samples. - ACS omega, 2019. https://doi.org/10.1021/acsomega.8b02637
R Satish, P Shamjad, N Thamban, S Tripathi, N Rastogi Temporal characteristics of brown carbon over the central Indo-Gangetic Plain. - Environmental science & technology, 2017. https://doi.org/10.1021/acs.est.7b00734
Further, BrC and BC emissions from different sources are very different. For e.g., vehicular emissions are highly rich in BC, but not in BrC. For biomass burning, its vice-versa. How these scenarios will impact the [σabs/[rBC]pri ratio and σabs-SOA estimation. The cluster analysis (Fig S1) and AMS results indicate that scenario is likely (Fig. 1) at the sampling site. How this will impact the overall findings of this study.
If these are based on σabs values, then don’t use words such as “mean contribution of absorption for BC, primary BrC and secondary BrC” as σabs values were not weighted with corresponding solar flux values. Instead, you can use words such as “mean contribution of absorption coefficient for BC, primary BrC and secondary BrC”. Please keep this point in mind throughout the manuscript.
Emerging investigator series: heterogeneous OH oxidation of primary brown carbon aerosol: effects of relative humidity and volatility, 10.1039/D0EM00311E, Environ. Sci.: Processes Impacts, 2020, 22, 2162-2171
Please modify the sentence and cite them properly
“The FTIR peaks of 1630cm-1 and 860cm-1 are integrated the absorption areas above the baseline.”