the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 10 Sep 2026
Observation of dark brown carbon in urban aerosols and its contribution to surface dimming
Abstract. Dark brown carbon (d-BrC) aerosols are commonly associated with biomass burning and wildfire emissions, yet their occurrence and climatic impacts in urban environments remain elusive. Southeast Texas – a hub for petrochemical industries at the intersection of continental and marine air masses – is strongly influenced by aerosol-induced surface dimming and extreme meteorological events. Here, the in-situ photoacoustic measurements reveal that over 80 % of aerosol light absorption at blue and near-infrared wavelengths is due to non-black carbon (BC) aerosols. The computer-controlled scanning electron microscopy and particle-scale electron energy loss spectroscopy demonstrates the presence of refractory d-BrC in the coastal urban atmosphere as an absorption contributor. The diurnal pattern of BC and non-BC absorption suggests association of the non-BC aerosols such as d-BrC particles with local non-biomass burning combustion sources, potentially including traffic, flares, and industrial activities. Observationally constrained radiative-transfer calculations show that d-BrC contributes 40 % of daytime-mean surface dimming and 50 % of the total top-of-atmosphere radiative forcing attributable to light-absorbing carbon. Such radiative changes that influence local meteorology may also impact ~25 % of the global population residing in coastal urban regions like Houston. Our findings demonstrate the presence and climatic importance of d-BrC in a coastal urban environment in addition to wildfire plumes, highlighting the need for incorporation of d-BrC in aerosol-climate models.
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Status: open (until 22 Oct 2026)
- RC1: 'Comment on egusphere-2026-4745', Anonymous Referee #1, 30 Sep 2026 reply
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General comment
This manuscript combines ensemble optical measurements and particle-resolved characterization to investigate dark brown carbon (d-BrC) in a coastal urban environment. The identification of particles with optical properties consistent with d-BrC is potentially important. However, the evidence for their presence is stronger than the quantitative constraints on their contribution to ambient absorption and radiative effects. In particular, the inferred non-BC absorption depends on the assumed BC optical properties, whereas the radiative estimates additionally rely on using OC4 as a proxy for d-BrC abundance. These two inference pathways are not sufficiently connected or validated. I recommend major revision, focusing on the robustness of the BC subtraction, the representativeness of the particle measurements, and the uncertainty in the reported radiative contributions.
Specific comments
1. In Section 2.3, please clarify whether the adopted MAC of 11.25 m2/g represents bare BC or already includes absorption enhancement, and therefore whether multiplying it by an additional factor of 1.5 is justified. The cited Beeler and Chakrabarty (2021) reference also appears unrelated to BC optical properties and should be corrected. Beyond the specified ranges of enhancement factor and AAE, the uncertainty analysis should include MAC uncertainty, SP2 calibration and undetected mass, IPN measurement uncertainty, and the effects of detection-limit screening. Please provide uncertainty intervals for the residual absorption at both wavelengths.
2. In sections 2.3 and 3.1, a wavelength-independent enhancement factor and a single power-law AAE may not adequately represent the absorption spectrum of aged, coated BC. Please test whether physically plausible BC size distributions and mixing states can reproduce the observed absorption at both 405 and 1047 nm simultaneously, preferably using available campaign constraints. A useful diagnostic is the MAC required for SP2-measured rBC to explain all measured absorption. Importantly, the reported residuals are large, particularly at 1047 nm; their explanation by BC variability should be quantitatively tested rather than either assumed or dismissed.
3. In Section 3.2, the STEM-EELS results support the presence of d-BrC-like particles, but do not by themselves establish their contribution to ensemble absorption or exclude substantial bias in the BC subtraction. Please report the number of particles optically characterized, their selection criteria, and the variability and uncertainty of their retrieved optical properties. The manuscript should clearly distinguish this population from the much larger CCSEM dataset. Please also explain how collection, heating, and electron-beam exposure may affect the representativeness of the characterized particles.
4. In sections 2.7 and 3.4, the radiative estimates rely on assigning OC4 to d-BrC, although low volatility does not uniquely imply strong absorption, as illustrated by Particle 3. Please evaluate how the results change when only a fraction of OC4 is assigned to d-BrC. In addition, compare filter-based EC with contemporaneous SP2 rBC, since different BC mass measurements are used in the subtraction and radiative calculations. Most importantly, please forward-calculate absorption from the adopted BC and d-BrC populations and compare it with IPN observations. This absorption-closure test is needed to connect the particle-based radiative calculations with the ensemble residual.
5. In Section 3.4, abstract and conclusions, the reported 40% contribution to surface dimming and 50% contribution to TOA radiative effects should be presented as conditional estimates for the modeled BC+d-BrC system, with uncertainties reflecting the abundance proxy and optical assumptions. Please also revisit the claim that setting surface albedo to zero isolates absorption-driven dimming, because aerosol scattering remains active under this condition. Finally, clarify which scenario and averaging period produce the reported 1.46 W/m2 boundary-layer heating and demonstrate consistency with the stated surface and TOA energy budgets. The main conclusions should distinguish robust evidence of d-BrC occurrence from less-constrained estimates of its climatic importance.