Forests Feed the Stratosphere: Monsoons Lift Tree Emissions into Earth’s Aerosol Layer

Deep above the tropical Pacific, at altitudes where airliners cruise and the air is a hundred times drier than at sea level, a persistent haze of tiny particles has puzzled atmospheric scientists for decades. This background stratospheric aerosol layer, the quiet residue left between volcanic eruptions and wildfire injections, shapes Earth’s radiation balance and provides surfaces for chemical reactions that influence ozone. Yet its composition and origins have remained stubbornly uncertain. A new study published in Nature Geoscience now argues that a surprisingly large share of this high-altitude haze comes not from volcanoes, industry, or fires, but from the world’s forests, and that the delivery mechanism is none other than the planet’s great monsoon circulations.

The research team, led by Kai Qie and Pengfei Yu of Jinan University together with colleagues at the Chinese Academy of Sciences, Lanzhou University, the NSF National Center for Atmospheric Research, NOAA, and the University of Colorado Boulder, assembled an unusually broad evidence base. They combined in situ aerosol measurements from nine airborne field campaigns, including ACCENT, CR-AVE, Pre-AVE, POSIDON, ACCLIP, ATom, StratoClim, ACRIDICON-CHUVA, and OP3, with simulations from a state-of-the-art sectional aerosol climate model, CESM1-CARMA. This pairing of direct observations with a microphysically detailed model allowed the researchers to quantify both what is floating in the lower stratosphere and how it got there.

The first headline finding is striking: biogenic secondary organic aerosol, particles formed when reactive gases emitted by vegetation are oxidized in the atmosphere, accounts for roughly forty percent of the global mean annual background stratospheric aerosol burden. In other words, nearly half of the quiet-time particle load above the weather is made of material that began its journey as volatile organic compounds wafting from leaves. The particles are transported into the upper troposphere and lower stratosphere through convective pathways that the authors describe as previously underappreciated, operating over the monsoon systems of South America, central Africa, Indonesia-Australia, and southern Asia.

The second discovery concerns the sheer scale of this vertical flux. The team estimates that between 0.42 and 0.72 teragrams of biogenic organic aerosol cross the tropopause each year. Set against the average rate of volcanic aerosol injection into the stratosphere over the past two decades, a period of relative volcanic quiescence, this biogenic flux is five to ninety percent larger. That comparison reframes the stratospheric aerosol budget: during years without major eruptions, forests may deliver as much particulate matter aloft as the volcanoes that have traditionally dominated thinking about the layer.

The physical mechanism hinges on the peculiar power of monsoon convection. Monsoon systems host some of the deepest and most vigorous thunderstorms on Earth, with updrafts capable of lifting air from the boundary layer to the tropical tropopause layer within hours. Satellite measurements of outgoing longwave radiation, used by the team as a proxy for deep convection, show that aerosol extinction in the lower stratosphere peaks seasonally in step with monsoon activity over each region. The Asian summer monsoon, long recognized as a pathway for pollution and short-lived chlorine compounds into the stratosphere, emerges here as one node in a global network that also includes the tropical monsoons of the Amazon, the Congo basin, and the maritime continent.

The chemistry behind the particles is equally consequential. Plants emit enormous quantities of isoprene and other biogenic volatile organic compounds, and recent work has shown that under the cold, low-pressure conditions of the upper troposphere these compounds can oxidize and nucleate into new particles, with isoprene-derived products such as epoxydiols and isoprene nitrates playing key roles. The new simulations reproduce observed isoprene column densities from the CrIS satellite and match vertical profiles measured during the CAFE Brazil and POSIDON campaigns, lending credibility to the modeled chain that runs from forest canopy to stratospheric particle.

Not everything that rises is delivered. Deep convection is a double-edged sword for aerosols, because the same updrafts that loft particles also drive precipitation that scavenges them. Earlier modeling work by some of the same authors quantified how efficiently in-cloud removal strips aerosols from convective outflow, which is precisely why the survival of a substantial biogenic fraction in the upper troposphere and lower stratosphere is notable. The new analysis suggests that gas-phase precursors carried upward in monsoon outflow continue to form particles after reaching the upper troposphere, effectively manufacturing aerosol at altitudes where wet removal can no longer touch it.

The observational fingerprints come from an impressive suite of instruments. Aircraft-based particle spectrometers and aerosol mass spectrometers measured size distributions and composition from the boundary layer to around nineteen kilometers, while the SAGE II and SAGE III-ISS satellite instruments provided two decades of stratospheric extinction profiles that the team compared against model output during the volcanically quiet 1999 to 2004 window. The agreement between cloud-removed satellite extinction statistics and CESM1-CARMA simulations across the tropical eastern Pacific, southern Atlantic, and southern Indian Ocean supports the conclusion that the model captures the real background aerosol budget rather than an artifact of parameterization.

Why does this matter beyond atmospheric chemistry? Stratospheric aerosols scatter sunlight and alter stratospheric heating rates, and they host heterogeneous reactions central to ozone depletion chemistry. Any accurate accounting of climate sensitivity, geoengineering proposals that would deliberately inject sulfur into the stratosphere, or projections of ozone recovery must therefore include a realistic background, and that background now appears to be substantially organic and biologically sourced. The findings also connect land use to the upper atmosphere: deforestation reduces forest volatile emissions, and earlier work has suggested such changes can cool climate by cutting organic aerosol production, a linkage this study sharpens considerably.

The study, published on 25 September 2026 in Nature Geoscience, carries implications that reach from the Amazon canopy to the edge of space. It demonstrates that the boundary between the living surface of the planet and the chemically stratospheric realm is far more porous than textbook pictures suggest, and that the seasonal breathing of tropical forests, funneled upward by monsoon circulations, leaves a measurable signature in the stratospheric aerosol layer year after year. As the authors emphasize, global monsoon convections play an important role in the background stratospheric aerosol budget, a conclusion that will now need to be woven into climate models, ozone assessments, and any serious discussion of deliberate climate intervention.

Subject of Research: Transport of biogenic secondary organic aerosols into the lower stratosphere via global monsoon convection

Article Title: Persistent contribution of biogenic emissions to lower stratospheric aerosols via global monsoon convections

Article References: Qie, K., Shao, M., Peng, Y., Wang, X., Chen, W., Pan, L. L., Schill, G., Rosenlof, K. H., Toon, O. B., Bian, J., & Yu, P. (2026). Persistent contribution of biogenic emissions to lower stratospheric aerosols via global monsoon convections. Nature Geoscience. https://doi.org/10.1038/s41561-026-02115-w

Image Credits: AI Generated

DOI: 10.1038/s41561-026-02115-w

Keywords: stratospheric aerosols, biogenic emissions, secondary organic aerosol, monsoon convection, isoprene, upper troposphere, CESM1-CARMA, SAGE II, tropopause, atmospheric chemistry, climate, Persistent