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

The physical story begins at a scale almost too small to imagine. The exchange of carbon dioxide between air and water is controlled by a concentration gradient across the mass boundary layer, a film of water only microns thick at the very top of the sea. Temperature and salinity variations within this layer and in the upper metres beneath it act on the gas in two opposing but reinforcing ways. At the interface itself, cooler, fresher water holds more dissolved carbon dioxide, raising solubility and pulling gas out of the air. At the base of the boundary layer, the same variations reshuffle the carbonate system—the chemical equilibrium among dissolved carbon dioxide, bicarbonate and carbonate ions—in a direction that further steepens the gradient. Scientists have already incorporated the temperature side of this story, the so-called cool-skin and diurnal warm-layer corrections, into global carbon budgets. Salinity, however, has remained the neglected sibling, because the haline skin is normally far too thin to drive meaningful carbonate repartitioning. Rain is the exception that proves the rule.

Rain alters air–sea carbon exchange through three distinct mechanisms. First, raindrops hammering the surface generate turbulence that enhances gas transfer, pushing carbon dioxide in whichever direction the local gradient points. Second, raindrops themselves carry dissolved carbon dioxide, delivering a small but measurable direct input known as wet deposition. Third, and most elusive, rain freshens and cools the upper ocean, diluting the salt content of a buoyant layer one to three metres deep that can persist for hours after the storm has passed. That freshening lowers the fugacity of carbon dioxide at the surface and repartitions the carbonate system below, strengthening the ocean’s pull on atmospheric carbon. While the first two mechanisms have been satisfactorily parameterized, the third has been crippled by a paucity of observations: nearly all routine oceanographic measurements come from ships, Argo floats and moorings sampling at five metres or deeper, entirely below the zone where rain’s influence is strongest.

The scale mismatch is the crux of the problem. Satellite instruments can detect the salinity anomalies left by rain, but rain is extraordinarily patchy on sub-kilometre scales, so even the finest satellite pixels—eight kilometres across—smear the signal. Earlier global analyses, notably a 2024 study by Parc and colleagues, attempted to capture the dilution effect using a one-dimensional prognostic model and an instantaneous empirical correction for satellite salinity, arriving at a combined rain enhancement of the ocean carbon sink of 5.3 to 7.7 percent, with wet deposition contributing more than half. The new study takes a different route. The authors selected the Rain Impact Model 3, or RIM-3, a physically grounded scheme that tracks the time evolution of freshwater lenses over the preceding twenty-four hours of precipitation history, and which has been validated against salinity retrievals from the Aquarius and SMAP satellites. Crucially, RIM-3 acknowledges what instantaneous models cannot: fresh layers linger long after the rain stops.

The results are striking. Running RIM-3 against high-resolution satellite rainfall data for the year 2000, the team found that dilution alone roughly doubles the effect estimated by the earlier work, making it the dominant term in the rain-driven carbon budget. In total, rain boosts global ocean carbon uptake by 13.6 percent relative to a wind-only calculation, against the wind-driven flux of 1.468 petagrams of carbon per year. Wet deposition contributes about 3.8 percent, consistent with both the previous study and an earlier independent estimate, while rain-induced turbulence adds a smaller, interacting share—the turbulence and dilution effects reinforce one another because they occur in the same places at the same times. When the authors ran the same analysis with the older instantaneous parameterization, the dilution effect came out at less than half the RIM-3 value, underscoring how sensitive the answer is to how faithfully a model represents the persistence of freshwater at the sea surface.

Regionally, the story is even more dramatic. Rain matters most where precipitation is heavy, winds are light and the carbon dioxide gradient between air and sea is weak—conditions that converge spectacularly in the Intertropical Convergence Zone of the tropical Pacific and, above all, in the tropical Indian Ocean. In the western equatorial Pacific, previous work has shown that accounting for rain can flip a region from a weak source of carbon dioxide to a strong sink over the course of a year. The new maps reveal zonal peaks in rain-driven flux centred on the tropical convergence belts and midlatitude storm tracks, with the southern midlatitude peak amplified by the breadth of the Indian Ocean. Outside a small wind-dominated patch of the eastern equatorial Pacific, rain always pushes the net flux toward greater uptake—either strengthening absorption where the ocean already takes up carbon or suppressing outgassing where it releases it. The authors even suggest that regions where rain effects are strongest could be promising candidates for ocean alkalinity enhancement, a proposed carbon removal technology.

Rain also cools the surface, a mechanism the main analysis does not explicitly include. Combining known temperature sensitivities with an observational relationship between rain rate and skin temperature yields a rough additional uptake of 0.027 petagrams of carbon per year, about 1.9 percent of the wind-driven flux. The authors judge this a likely upper bound, since the rain–temperature relationship probably saturates at high rain rates, though persistence of cooler temperatures after rainfall could push the other way. Either way, cooling appears to be a real but secondary player, less important than dilution and deposition, and the total rain effect on the ocean sink may ultimately reach around fifteen percent.

The implications for global carbon accounting are uncomfortable. Current Global Carbon Budget estimates do not explicitly account for rain effects, and the surface ocean measurements that feed them—about ninety-five percent of which are taken at five metres depth—capture only part of the story. Using the physics of RIM-3, the authors calculate that a hypothetical global network of continuous five-metre carbon dioxide measurements would implicitly capture about sixty-two percent of the rain-induced interfacial flux. But real observations are sparse in space and time relative to the episodic nature of rainfall, and an analysis of the SOCATv2025 database shows that today’s observing system captures only about thirty percent of the interfacial rain influence. Worryingly, the enormous growth in observational coverage in recent decades has not translated into better coverage of rain-affected conditions, and five-metre measurements capture especially little of the effect in the equatorial oceans where rain matters most.

Taken together, the authors conclude that current global carbon budget estimates are probably underestimating the ocean carbon sink by 10 ± 5 percent. That figure echoes earlier work showing that correcting for cool-skin and warm-layer effects brought flux estimates into line with independent measurements of ocean carbon inventory, and it suggests rain deserves the same rigorous treatment. The team is careful to note that the point is not to crown RIM-3 as the definitive model but to expose how poorly constrained rain dilution remains, and what that gap costs our understanding of the planet’s largest carbon sink. What is needed, they argue, is a coordinated observational campaign: widespread, systematic measurements of very near-surface salinity profiles and rain rates, with particular emphasis on the equatorial oceans, alongside process studies that directly measure carbon dioxide fluxes, salinity and carbon dioxide anomalies within the boundary layer during and after rain. Until then, a substantial slice of the ocean’s climate service may be hiding, quite literally, beneath a layer of fresh water thinner than a swimming pool is deep.

Subject of Research: The effect of rain-induced salinity dilution on global air–sea carbon dioxide flux and the ocean carbon sink

Article Title: Global carbon dioxide flux into the surface ocean intensified by rain-induced dilution

Article References: Global carbon dioxide flux into the surface ocean intensified by rain-induced dilution. (n.d.). https://doi.org/10.1038/s41561-026-02112-z

Image Credits: AI Generated

DOI: 10.1038/s41561-026-02112-z

Keywords: ocean carbon sink, air-sea CO2 flux, rain-induced dilution, sea surface salinity, carbonate chemistry, wet deposition, gas transfer velocity, satellite remote sensing, global carbon budget, Intertropical Convergence Zone, mass boundary layer, Nature Geoscience

The study focused on the coast of Newfoundland and Labrador, where waves from the Atlantic Ocean crash against an extensive shoreline and strong winds whip spray far inland. According to the researchers, this is close to a worst-case scenario for sea-spray-driven PFAS transport: abundant wind, crashing surf and a long coastline all combine to maximise the generation of marine aerosols. If the ocean were truly pumping significant quantities of forever chemicals into coastal environments through spray, the team reasoned, Newfoundland would be one of the places where the signal would be unmistakable. Senior author Cora Young, an atmospheric chemist and Guy Warwick Rogers Chair in York University’s Faculty of Science, said the location was chosen precisely because it offered the ideal conditions to detect such an effect.

We designed this study specifically to see if sea spray was contributing to a mobilization of PFAS in coastal regions and Newfoundland is a great place to do that, Young explained, noting that it is a marine location and very windy, so the team expected quite a lot of PFAS from sea spray. Instead, she said, the researchers were quite surprised that they did not see anything indicating sea spray was contributing to PFAS in the area. The finding directly contradicts the expectation, rooted in earlier modelling and laboratory work, that the ocean-to-land transfer of these chemicals through aerosols represents a significant and direct exposure pathway for people and ecosystems near the coast.

To test the hypothesis rigorously, the team moved beyond simulations and carried out on-the-ground field work. Lead author Daniel Persaud, a PhD student at York, together with Young and their collaborators, collected both wet and dry deposition samples at several locations stretching up to 30 kilometres from the eastern coast. Wet deposition refers to PFAS delivered by precipitation such as rain, while dry deposition captures the dust and particles, along with gases, that settle out of the atmosphere onto the ground. Measuring both pathways matters, because many studies record only one or the other, and the balance between them shapes how scientists estimate where forever chemicals ultimately land.

The sampling sites were forested watersheds within the Newfoundland and Labrador Boreal Ecosystem Latitudinal Transect, a network of study locations chosen because they are all influenced by marine aerosols and lie close to the ocean. Because the sites are windy and near the water, the researchers expected a strong marine-aerosol signal in their chemistry. To track it, they used sodium as a tracer of sea spray, since sodium is abundant in ocean-derived particles. Persaud said the measured PFAS showed little association with sodium, indicating that the forever chemicals found at the sites were not arriving with the salt spray from the Atlantic.

That does not mean the landscape was clean. The researchers did detect PFAS at every study site, but the pattern of compounds pointed to a different origin. They mostly found perfluoroalkyl acids, a subclass of PFAS that includes perfluoroalkyl carboxylic acids and perfluoroalkanesulfonic acids. These substances are not produced from local sources in Newfoundland. Instead, the evidence suggested they were the atmospheric breakdown products of PFAS precursor compounds that had been emitted elsewhere, transformed in the air during long-range transport, and then deposited on the island. In other words, the chemicals arrived from distant industrial and commercial centres, not from the waves at the shore.

The likely sources of those precursors lie elsewhere in Canada and the United States, and potentially even further afield. PFAS and their precursors are typically found in firefighting foams, non-stick materials and performance chemicals, and they are notorious for their ability to travel long distances through the atmosphere, reaching even the Arctic. Because perfluoroalkyl acids are extremely persistent and can bioaccumulate in living organisms, their continued deposition in remote regions raises ongoing environmental concerns. The new results do not make those concerns disappear; they simply redirect attention away from the ocean surface and back toward atmospheric transport of emissions from faraway sources.

The implications may extend well beyond eastern Canada. Young noted that Newfoundland offers something close to a perfect storm for sea-spray generation, with driving wind, crashing waves and an extensive coastline that together maximise the amount of aerosol produced. If sea spray were a major route for moving PFAS from ocean to land, it should have shown up there. According to Young, this sort of sea spray is really not a major source of global pollution according to the study. That conclusion, she suggested, amounts to good news: researchers had worried that PFAS could move from ocean to land and back again indefinitely, but the field evidence indicates the problem is not as large as feared.

One of the study’s most instructive findings concerned the role of dry deposition. The team measured both wet and dry deposition, but that combined approach is not universally practised, and their results underline how important it is to do so in order to understand the full impact of PFAS in the environment. Young said she still finds it surprising that dry deposition remains so important even in a place like Newfoundland, where rainfall is frequent. The observation suggests that in many other landscapes, from remote deserts to the Arctic, the settling of particle-bound and gaseous PFAS may be a larger contributor than precipitation alone would indicate, and that studies relying only on rain and snow could substantially underestimate total deposition.

Persaud said the distinction between wet and dry pathways helps researchers better predict where forever chemicals will be found as they move around the globe and where they will ultimately end up, benefiting not only his team but other researchers going forward. The work was a collaboration with scientists at Memorial University in St. John’s, where Young previously worked and collected samples, the Canadian Forest Service and Natural Resources Canada in Corner Brook, and the Aquatic Contaminants Research Division of Environment and Climate Change Canada in Burlington, Ontario. The paper, titled The Occurrence of Legacy Per- and Polyfluoroalkyl Substances (PFAS) Deposited along a Latitudinal Transect in Newfoundland and Labrador, Canada, was published in the Royal Society of Chemistry journal Environmental Science: Processes & Impacts under DOI 10.1039/D6EM00257A. For coastal residents wondering whether summer sea spray carries a hidden dose of forever chemicals, the answer from this windswept corner of Canada appears to be reassuring: the ocean, at least, is not the culprit it was made out to be.

Subject of Research: Atmospheric deposition of PFAS forever chemicals in coastal Newfoundland and the role of sea spray as a transport pathway

Article Title: Exposure to forever chemicals through sea spray may be overblown

Article References: Exposure to forever chemicals through sea spray may be overblown. (n.d.). Original publication

Image Credits: AI Generated

DOI: Not provided

Keywords: PFAS, forever chemicals, sea spray, marine aerosols, Newfoundland and Labrador, atmospheric deposition, wet deposition, dry deposition, perfluoroalkyl acids, long-range transport, York University, environmental chemistry