Black carbon, the sooty residue of incomplete combustion from diesel engines, coal-fired power plants and biomass burning, has long been recognized as one of the most consequential short-lived climate pollutants on Earth. Second only to carbon dioxide in its contribution to global warming, this dark aerosol absorbs sunlight with extraordinary efficiency, heating the atmosphere from within. Yet despite decades of study, the climate effects of black carbon remain stubbornly uncertain, and a new study published in Atmospheric Chemistry and Physics suggests that a surprisingly mundane process—the speed at which fresh soot particles age in the atmosphere—may be a far more powerful lever on regional climate than previously appreciated, particularly over the densely populated and monsoon-dominated landscapes of East Asia.
The research, led by Peng Gao and Bingliang Zhuang of Nanjing University together with colleagues, tackles a persistent weakness in climate modeling. When black carbon is freshly emitted, its particles are hydrophobic, repelling water and resisting removal from the air. Over time, through condensation of soluble vapors and coagulation with other particles, the soot becomes coated, hydrophilic, and internally mixed with other aerosol species. This transformation, known as aging, fundamentally changes everything that matters for climate: how the particles absorb and scatter light, how readily they act as cloud condensation nuclei, and how quickly they are rained out of the atmosphere. Many climate models, however, sidestep this complexity by assigning black carbon a fixed e-folding aging timescale, often around one day, regardless of where or when the particles find themselves.
That fixed assumption is demonstrably wrong in the real atmosphere. Chamber experiments in Beijing have measured aging timescales as short as 4.6 hours, while studies in Los Angeles and Mexico City found soot largely aged within about three hours. In clean, remote regions such as the Qinghai–Tibet Plateau, aging can stretch beyond a day. To capture this variability, the team incorporated a dynamic aging parameterization, derived from particle-resolved simulations of condensation and coagulation, into the regional climate and chemistry coupled model RegCM-Chem. Rather than a constant, the aging timescale in their scheme emerges from local conditions: it depends on the rate of condensable sulfate production and the total aerosol number concentration, meaning that soot ages fastest precisely where pollution is heaviest.
The simulations, spanning summers from 2008 to 2021 across a domain covering East, South and Southeast Asia at 60-kilometer resolution, revealed striking spatial contrasts. Over the North China Plain and the Sichuan Basin—two of the most heavily polluted regions on the planet—the black carbon aging timescale dropped below ten hours, with a minimum monthly mean of just 2.84 hours. Across most of eastern China and the Indian landmass, aging was consistently faster than the model’s default 27.6 hours. In remote areas such as the Tibetan Plateau and over the oceans, where condensable material is scarce, the process could take several days. The spatial pattern of aging, in effect, mirrors the map of anthropogenic emissions.
Accelerated aging reshaped the atmospheric budget of black carbon in counterintuitive ways. Because aged, hydrophilic particles are efficiently scavenged by rain, faster aging promoted wet deposition and reduced the regional mean black carbon column burden over East Asia by 0.12 milligrams per square meter. Yet at the surface, concentrations actually increased over eastern China, including the North China Plain and the southeastern coast, by more than 0.2 micrograms per cubic meter in places. The explanation lies in a compensating mechanism: hydrophilic particles, with their larger characteristic diameters and higher solubility, are less susceptible to dry deposition than their hydrophobic counterparts. Where summer precipitation is limited, this suppressed dry deposition dominates, leaving more soot lingering near the ground even as the total atmospheric burden shrinks.
These changes in loading and composition fed directly into the aerosol’s optical behavior. Black carbon aerosol optical depth increased significantly over eastern China, driven both by higher near-surface concentrations and by the greater extinction efficiency of aged particles, whose shortwave extinction coefficient is roughly 1.3 times that of fresh soot. The strengthened absorption amplified the instantaneous direct radiative forcing at the top of the atmosphere and deepened the dimming at the surface, where the regional mean forcing reached −3.00 watts per square meter. The additional atmospheric heating, exceeding 0.12 kelvin per day in localized layers, then set off a cascade of dynamical adjustments that rippled through the East Asian summer monsoon system.
The circulation response was pronounced. Anomalous southerly winds along the southeastern Chinese coast, coupled with a cyclonic circulation near 850 hectopascals over southern China, transported warm, moist air from the South China Sea inland, enhancing moisture convergence and raising cloud fraction there by one to two percent. These dynamics favor the development of the East Asian summer monsoon itself. Over northern China, the story inverted: compensating subsidence flanking the black carbon heating center near 35 degrees north, together with the semi-direct suppression of cloud formation, reduced cloud cover by two to three percent and dried the lower troposphere. The net result was a characteristic dipole—wetter, cloudier conditions in the south, drier and warmer conditions in the north.
Clouds responded to the aged soot through a second, microphysical pathway as well. With its enhanced hygroscopicity—assigned a value of 0.22 based on observational constraints—hydrophilic black carbon activates more readily as cloud condensation nuclei. Under the dynamic aging scheme, the mean cloud droplet number concentration in the lower troposphere over East Asia increased by 9.37 per cubic centimeter on top of the 36.98 per cubic centimeter induced by black carbon overall, shrinking droplet effective radii and thickening cloud optical depth. Sensitivity tests varying the assumed hygroscopicity between 0.11 and 0.34 showed that while droplet numbers were only modestly affected, the resulting cloud radiative and dynamical adjustments amplified the surface forcing response considerably, underscoring how small microphysical uncertainties can balloon into large climatic ones.
When all pathways were combined, the effective radiative forcing at the top of the atmosphere over East Asia reached +3.60 watts per square meter from black carbon–radiation interactions, −0.58 from black carbon–cloud interactions, and +0.90 for the total. Crucially, the total was not the simple sum of its parts. The team explicitly quantified a nonlinear interaction term and found that coupled circulation, moisture and cloud adjustments produced residuals comparable to, or larger than, the isolated pathway responses—for example, a −2.12 watts per square meter residual at the top of the atmosphere over northeastern China. In the total response, black carbon warmed northern China by about 0.044 kelvin while cooling the south by 0.042 kelvin, and produced a south-wet, north-dry precipitation pattern with regional anomalies of +0.13 and −0.09 millimeters per day.
The implications extend well beyond East Asia. Black carbon’s global effective radiative forcing spans −0.28 to +0.41 watts per square meter in the latest IPCC assessment, and its indirect forcing in liquid clouds ranges from −0.64 to +0.23 watts per square meter across models—uncertainties that this work traces, in part, to how aging is represented. The authors caution that their dynamic scheme did not fully resolve the persistent underestimation of black carbon concentrations in models, and that observational constraints on particle size distributions, mixing states and absorption enhancement remain scarce. Still, the message is clear: a process operating on timescales of mere hours, long treated as a fixed constant in climate models, exerts a first-order influence on regional radiative forcing, monsoon circulation and the distribution of floods and droughts. For a pollutant whose mitigation is among the most tractable climate interventions available, getting its life cycle right may be essential to predicting—and managing—the climate of the world’s most populous region.
Subject of Research: The sensitivity of black carbon aerosol climate effects in East Asia to dynamic aerosol aging processes in a regional climate model
Article Title: Sensitivity of dynamic aging on the climate effects of black carbon aerosols over East Asia in summer
Article References: Gao, P., Zhuang, B., Hu, Y., Zhou, Y., Zhao, R., Wang, Q., Li, S., Wang, T., Li, M., & Xie, M. (2026). Sensitivity of dynamic aging on the climate effects of black carbon aerosols over East Asia in summer. Atmospheric Chemistry and Physics, 26(19), 13861-13884. https://doi.org/10.5194/acp-26-13861-2026
Image Credits: AI Generated
DOI: 10.5194/acp-26-13861-2026
Keywords: black carbon, aerosol aging, East Asia, East Asian summer monsoon, radiative forcing, RegCM-Chem, aerosol-cloud interactions, wet deposition, cloud condensation nuclei, regional climate modeling, North China Plain, air pollution
