Contrasting Nighttime Heterogeneous and Daytime Photochemical Aging Drive the Optical Evolution of Black Carbon

Zhang, Yin; Zhai, Jinghao; Zeng, Yaling; Shi, Shao; Cai, Baohua; Yang, Ke; Yan, Yu; Yuan, Xin; Hu, Tianlong; Wang, Chen; Fu, Tzung-May; Zhu, Lei; Shen, Huizhong; Ye, Jianhuai; Yang, Xin

doi:10.5194/egusphere-2026-1850

Preprints

https://doi.org/10.5194/egusphere-2026-1850

Preprints

27 Apr 2026

| 27 Apr 2026

Status: this preprint is open for discussion and under review for Atmospheric Chemistry and Physics (ACP).

Contrasting Nighttime Heterogeneous and Daytime Photochemical Aging Drive the Optical Evolution of Black Carbon

Yin Zhang, Jinghao Zhai, Yaling Zeng, Shao Shi, Baohua Cai, Ke Yang, Yu Yan, Xin Yuan, Tianlong Hu, Chen Wang, Tzung-May Fu, Lei Zhu, Huizhong Shen, Jianhuai Ye, and Xin Yang

Abstract. Black carbon (BC) play a critical role in the climate system, yet their atmospheric aging processes and consequent optical impacts in real-world atmospheres remain insufficiently understood. In this study, we present integrated single-particle measurements using a single particle soot photometer (SP2) and a single-particle aerosol mass spectrometer (SPAMS) during a field campaign in urban Shenzhen, China. The mean refractory BC (rBC) concentration was 1.2 μg m⁻³, with core mass median diameters (MMD) of 155–170 nm. The diurnal variation in the coating-to-core mass ratio (MR) indicated that BC underwent continuous aging. Nighttime aging was primarily driven by heterogeneous nitrate uptake, whereas daytime photochemical aging was characterized by condensation of nitrate, sulfate, and oxidized organics. Despite their distinct mechanisms, both aging pathways elevated the MR and produced similar net enhancements in the mass absorption cross section (MAC) with an overnight increase of ~0.8 m² g⁻¹ and a daytime increase of ~1.0 m² g⁻¹. These comparable net increments were due to the offsetting effect of intensive fresh emissions during the day. Specifically, the MAC enhancement rates driven by nighttime heterogeneous reactions and daytime photochemical aging were determined to be 0.34 and 0.59 m² g⁻¹ h⁻¹, respectively. This study provides direct observational evidence of diurnally contrasting BC aging pathways and quantitatively constrains the optical enhancement rates under real urban conditions.

Received: 01 Apr 2026 – Discussion started: 27 Apr 2026

Publisher's note: Copernicus Publications remains neutral with regard to jurisdictional claims made in the text, published maps, institutional affiliations, or any other geographical representation in this paper. While Copernicus Publications makes every effort to include appropriate place names, the final responsibility lies with the authors. Views expressed in the text are those of the authors and do not necessarily reflect the views of the publisher.

Download & links

Preprint (PDF, 751 KB)

Supplement (598 KB)

Download & links

Yin Zhang, Jinghao Zhai, Yaling Zeng, Shao Shi, Baohua Cai, Ke Yang, Yu Yan, Xin Yuan, Tianlong Hu, Chen Wang, Tzung-May Fu, Lei Zhu, Huizhong Shen, Jianhuai Ye, and Xin Yang

Status: open (until 19 Jun 2026)

Post a comment Subscribe to comment alert

RC1:
'Comment on egusphere-2026-1850', Anonymous Referee #1, 22 May 2026 reply
Review of “Contrasting Nighttime Heterogeneous and Daytime Photochemical Aging Drive the Optical Evolution of Black Carbon”
This manuscript presents an integrated analysis of refractory black carbon (rBC) measurements from an SP2, single-particle chemical information from SPAMS, and bulk aerosol optical measurements from a PAX during an urban field campaign in Shenzhen. The main contribution of the study is the attempt to connect diurnal changes in rBC coating state and chemical composition with corresponding changes in light absorption, and to distinguish between nighttime heterogeneous aging and daytime photochemical aging pathways. The dataset is valuable, and the combined SP2–SPAMS–PAX perspective has the potential to provide useful observational constraints on the evolution of BC mixing state and optical properties in an urban environment.
However, I have several concerns about the strength of the mechanistic interpretation. In particular, some of the proposed aging pathways are plausible but not fully demonstrated by the available measurements. I recommend that the authors revise the manuscript to more clearly distinguish between direct observations, inferred relationships, and mechanistic attribution. Additional sensitivity analyses and more cautious wording would substantially strengthen the paper.
Major comments
1 The explanation for the daytime secondary coating-thickness mode needs stronger support.The manuscript reports the emergence of a daytime secondary mode in coating thickness, around 60 nm, and attributes this to accelerated photochemical aging of a subset of BC particles and the effect of PBL expansion, which dilutes freshly emitted BC and thereby increases the relative contribution of aged particles. This explanation as written it is not fully convincing.

The explanation invoking PBL dilution is not physically sufficient as written. Uniform dilution would reduce the concentrations of all coating-thickness classes proportionally and would not change the normalized coating-thickness distribution or generate a secondary mode. The observed daytime secondary maximum would require a change in the relative contributions of distinct BC populations, for example entrainment/mixing of more aged particles from aloft, preferential dispersion of fresh near-surface emissions, or rapid aging of a subset of particles. The authors should clarify which mechanism they intend and provide supporting evidence. Alternatively, the interpretation should be softened and framed as one possible explanation rather than a demonstrated mechanism.
2 The nighttime nitrate aging mechanism is over-attributed to N2O5 hydrolysis. The manuscript argues that nighttime aging is driven by heterogeneous nitrate uptake, likely through NO3/N2O5 chemistry and subsequent N2O5 hydrolysis on BC surfaces. While this pathway is certainly possible under nighttime urban conditions, the presented observations do not seem sufficient to uniquely identify it as the dominant mechanism.
An alternative explanation is thermodynamic partitioning of nitrate during cooler nighttime conditions, especially involving HNO3/NH4NO3. If temperature decreases and aerosol liquid water or ammonium availability favors particulate nitrate, then increased nitrate on BC-containing particles could occur without requiring N2O5 hydrolysis as the dominant pathway. The authors should discuss this possibility explicitly. If they wish to maintain the N2O5 interpretation, they should provide additional evidence, such as constraints from O3, NO2, RH, temperature, aerosol liquid water, nitrate partitioning calculations, or other indicators that distinguish heterogeneous N2O5 chemistry from nighttime partitioning.
A more cautious framing would be that nighttime nitrate accumulation is consistent with heterogeneous nitrogen chemistry, potentially including N2O5 hydrolysis, but that thermodynamic partitioning of nitrate may also contribute.
3 The claim that BC surfaces specifically promote sulfate formation should be softened or better supported.
The manuscript states that enhanced sulfate signals in BC-containing particles indicate that the BC surface specifically promotes sulfate formation. This is an interesting possibility, but the current evidence appears indirect. SPAMS relative peak areas provide valuable particle-level chemical information, but they are not direct formation-rate measurements. Differences between BC-containing and non-BC particles could also reflect particle size, source covariance, condensation/coagulation history, detection efficiency, classification biases, or differences in mixing state rather than surface-catalyzed sulfate production specifically.
The authors should either provide stronger support for this claim or rephrase it more cautiously. For example, they could state that the observations are “consistent with preferential sulfate accumulation on BC-containing particles” rather than concluding that BC surfaces specifically promote sulfate formation.
4 Role of coagulation in BC mixing-state evolution.

The discussion focuses primarily on condensation and heterogeneous uptake as mechanisms that increase rBC coating thickness and MR. However, coagulation between BC-containing and non-BC particles can also transfer secondary material to the BC-containing population and contribute to internal mixing. This distinction is important because an increase in nitrate, sulfate, ammonium, or organic material associated with BC-containing particles does not necessarily imply that these species formed or condensed directly on BC surfaces. It may also reflect coagulation with pre-existing non-BC particles that already contain these species. The authors should discuss whether coagulation is expected to be important under the observed number concentrations and timescales, and how it could affect their interpretation of daytime and nighttime aging mechanisms. At minimum, the manuscript should acknowledge that SP2/SPAMS observations of increased coating or enhanced secondary ion signals on BC-containing particles cannot unambiguously distinguish condensation/heterogeneous uptake from coagulation-driven mixing.

5 The derived MR and coating thickness are central to the manuscript, but the associated uncertainties need more discussion.
The authors appropriately note that MR is not directly measured but is estimated from SP2-derived quantities using a core–shell Mie model, assumed refractive indices, and assumed densities. However, MR and coating thickness are then used heavily throughout the interpretation, including the discussion of diurnal aging, the MR > 3 threshold, and the calculation of MAC for thickly coated BC.
The authors should provide a clearer uncertainty discussion for the inferred coating thickness and MR. In particular, the assumed rBC core refractive index m = 2.26 + 1.26i should be accompanied by the wavelength. Since this value is high compared with commonly used visible-wavelength BC refractive indices, the authors should clarify that it is used for the SP2 scattering/LEO retrieval, presumably at 1064 nm, and not for interpreting the 532 nm optical measurements. A sensitivity test or discussion of the effect of alternative core refractive indices, coating refractive index, coating density, core density, and core–shell morphology would make the paper more robust.
6 The fresh-BC MAC baseline of 7.13 m^2 g^{-1} is important and should be treated more cautiously.
The manuscript estimates a fresh-BC MAC by extrapolating the relationship between MAC and the fraction of thickly coated particles to zero thickly coated fraction. This value is then used as the baseline in Eq. 4 to estimate excess absorption due to aging. Because this baseline directly affects the inferred absorption enhancement and the subsequent MAC calculations for thickly coated BC, the uncertainty in this value should be discussed more explicitly.
The intercept-based fresh-BC MAC is an empirical estimate, not a direct measurement of freshly emitted BC. The authors should provide the uncertainty in the intercept and discuss how sensitive the derived b_abs^E, MAC_thickly, and enhancement rates are to the choice of baseline MAC. This is especially important because the reported diurnal MAC changes are relatively modest, on the order of 0.8–1.0 m^2 g^{-1}.
7 The optical enhancement “rates” should be framed as apparent rates rather than intrinsic process-specific rates.
The manuscript reports nighttime and daytime MAC enhancement rates and attributes them to nighttime heterogeneous aging and daytime photochemical aging, respectively. However, the observed bulk optical evolution is also influenced by emissions, dilution, PBL evolution, and removal processes. The authors acknowledge that removal cannot be fully isolated, but the language in the conclusions still implies a stronger process-level attribution than the measurements can support.
I suggest referring to these as “apparent” or “campaign-specific” enhancement rates associated with the observed nighttime and daytime periods. The authors should avoid implying that these are intrinsic kinetic rates of nitrate-driven or photochemical aging unless additional process-level constraints are provided.
8 The BrC correction at 532 nm should be summarized more clearly in the main text.
The MAC is calculated from PAX absorption at 532 nm divided by SP2-derived rBC mass. The authors correctly note that brown carbon may also absorb at 532 nm and that a correction was applied, with details in the Supporting Information. Because the paper’s conclusions depend on relatively subtle MAC changes, the main text should include enough information for the reader to evaluate whether residual BrC absorption could influence the reported diurnal trends.
At minimum, the authors should briefly summarize the correction method, its magnitude, and whether the correction varies systematically between daytime and nighttime. This is particularly relevant because daytime photochemical aging and organic aerosol formation could plausibly affect BrC absorption.
9 The “natural chamber” framing should be used with caution.
The selected period appears to be relatively dominated by local emissions and minimally affected by long-range transport, which makes it useful for studying diurnal aging. However, the term “natural chamber” may give the impression of a more closed system than is warranted. The period still includes strong diurnal variations in emissions, boundary-layer dynamics, non-zero wind speed, dilution, and removal. I suggest either softening this language or explicitly acknowledging the limitations of the analogy.
Minor comments and technical corrections
In Section 2.2, where the rBC core refractive index m = 2.26 + 1.26i is introduced, please specify the wavelength. It should also be clear that this refractive index is used in the SP2 coating retrieval and not in the 532 nm PAX-based MAC analysis.

In Eq. 4, the asterisk should be replaced by a proper multiplication sign or centered dot.

Figure 1: Please explain C1, C2, and C3 in the figure caption. The reader should not have to infer from the text that these refer to trajectory clusters.

Figure 2: It would be useful to know whether the diurnal variations shown are robust when the data are split by the trajectory clusters C1, C2, and C3, or at least whether the selected local-emissions period behaves similarly to the full-period diurnal average. If the main conclusions are based on the C2/local period, this should be made visually and methodologically clear.

Figure 6: Please define the blue symbols in the caption and/or legend.

The reported MAC values and MAC enhancement rates should be put into context with previous studies. For example, how do the inferred fresh-BC MAC, observed MAC range, and enhancement rates compare with prior urban SP2/PAX or SP2/photoacoustic studies?

The manuscript should be careful with wording such as “direct observational evidence” when the conclusion depends on multiple inferred quantities, including coating thickness from LEO fitting, MR from assumed densities and core–shell geometry, and BrC-corrected MAC from bulk absorption.

Some statements would benefit from being changed from causal to observational language. For example, “BC enhances sulfate formation” could be revised to “BC-containing particles showed enhanced sulfate signals” unless the causal mechanism is directly demonstrated.

Please clarify whether the MR > 3 threshold from Liu et al. (2017) is expected to apply quantitatively to this dataset, source region, and SP2 retrieval approach, or whether it is used only as a qualitative indicator of thick coating.

The manuscript would benefit from a concise uncertainty paragraph summarizing the main uncertainties in the derived quantities: rBC mass calibration, coating thickness, MR, BrC correction, fresh-BC MAC baseline, and inferred enhancement rates. I do not necessarily expect a comprehensive uncertainty propagation, but the manuscript should include a clearer qualitative or semi-quantitative discussion of the dominant uncertainties affecting the SP2-derived coating thickness/MR and the inferred MAC enhancement rates.

Reply
Citation: https://doi.org/10.5194/egusphere-2026-1850-RC1

Yin Zhang, Jinghao Zhai, Yaling Zeng, Shao Shi, Baohua Cai, Ke Yang, Yu Yan, Xin Yuan, Tianlong Hu, Chen Wang, Tzung-May Fu, Lei Zhu, Huizhong Shen, Jianhuai Ye, and Xin Yang

Supplement

https://doi.org/10.5194/egusphere-2026-1850-supplement

Yin Zhang, Jinghao Zhai, Yaling Zeng, Shao Shi, Baohua Cai, Ke Yang, Yu Yan, Xin Yuan, Tianlong Hu, Chen Wang, Tzung-May Fu, Lei Zhu, Huizhong Shen, Jianhuai Ye, and Xin Yang

Viewed

Total article views: 395 (including HTML, PDF, and XML)

HTML	PDF	XML	Total	Supplement	BibTeX	EndNote
303	68	24	395	31	19	17

HTML: 303
PDF: 68
XML: 24
Total: 395
Supplement: 31
BibTeX: 19
EndNote: 17

Views and downloads (calculated since 27 Apr 2026)

Month	HTML	PDF	XML	Total
Apr 2026	97	34	5	136
May 2026	192	29	17	238
Jun 2026	14	5	2	21

Cumulative views and downloads (calculated since 27 Apr 2026)

Month	HTML	PDF	XML	Total
Apr 2026	97	34	5	136
May 2026	192	29	17	238
Jun 2026	14	5	2	21

Viewed (geographical distribution)

Total article views: 390 (including HTML, PDF, and XML) Thereof 390 with geography defined and 0 with unknown origin.

Country	#	Views	%

Latest update: 13 Jun 2026

Short summary

Black carbon warms our climate, but its atmospheric changes remain unclear. We measured individual urban particles with advanced instruments to track this process. We found that black carbon ages through distinct chemical reactions at night versus sunlight-driven processes during the day. Both pathways coat the particles, significantly increasing their ability to absorb light. This helps accurately predict their warming effects.


Total:	0
HTML:	0
PDF:	0
XML:	0