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1. New particle formation from isoprene under upper-tropospheric conditions

   https://doi.org/10.1038/s41586-024-08196-0  

   Shen, Jiali; Russell, Douglas M; DeVivo, Jenna; Kunkler, Felix; Baalbaki, Rima; Mentler, Bernhard; Scholz, Wiebke; Yu, Wenjuan; Caudillo-Plath, Lucía; Sommer, Eva; et al (December 2024, Nature)

   Abstract Aircraft observations have revealed ubiquitous new particle formation in the tropical upper troposphere over the Amazon^1,2and the Atlantic and Pacific oceans^3,4. Although the vapours involved remain unknown, recent satellite observations have revealed surprisingly high night-time isoprene mixing ratios of up to 1 part per billion by volume (ppbv) in the tropical upper troposphere⁵. Here, in experiments performed with the CERN CLOUD (Cosmics Leaving Outdoor Droplets) chamber, we report new particle formation initiated by the reaction of hydroxyl radicals with isoprene at upper-tropospheric temperatures of −30 °C and −50 °C. We find that isoprene-oxygenated organic molecules (IP-OOM) nucleate at concentrations found in the upper troposphere, without requiring any more vapours. Moreover, the nucleation rates are enhanced 100-fold by extremely low concentrations of sulfuric acid or iodine oxoacids above 10⁵ cm⁻³, reaching rates around 30 cm⁻³ s⁻¹at acid concentrations of 10⁶ cm⁻³. Our measurements show that nucleation involves sequential addition of IP-OOM, together with zero or one acid molecule in the embryonic molecular clusters. IP-OOM also drive rapid particle growth at 3–60 nm h⁻¹. We find that rapid nucleation and growth rates persist in the presence of NO_xat upper-tropospheric concentrations from lightning. Our laboratory measurements show that isoprene emitted by rainforests may drive rapid new particle formation in extensive regions of the tropical upper troposphere^1,2, resulting in tens of thousands of particles per cubic centimetre. 

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2. Enhanced growth rate of atmospheric particles from sulfuric acid

   https://doi.org/10.5194/acp-20-7359-2020  

   Stolzenburg, Dominik; Simon, Mario; Ranjithkumar, Ananth; Kürten, Andreas; Lehtipalo, Katrianne; Gordon, Hamish; Ehrhart, Sebastian; Finkenzeller, Henning; Pichelstorfer, Lukas; Nieminen, Tuomo; et al (January 2020, Atmospheric Chemistry and Physics)

   Abstract. In the present-day atmosphere, sulfuric acid is the mostimportant vapour for aerosol particle formation and initial growth. However,the growth rates of nanoparticles (<10 nm) from sulfuric acidremain poorly measured. Therefore, the effect of stabilizing bases, thecontribution of ions and the impact of attractive forces on molecularcollisions are under debate. Here, we present precise growth ratemeasurements of uncharged sulfuric acid particles from 1.8 to 10 nm, performedunder atmospheric conditions in the CERN (EuropeanOrganization for Nuclear Research) CLOUD chamber. Our results showthat the evaporation of sulfuric acid particles above 2 nm is negligible,and growth proceeds kinetically even at low ammonia concentrations. Theexperimental growth rates exceed the hard-sphere kinetic limit for thecondensation of sulfuric acid. We demonstrate that this results fromvan der Waals forces between the vapour molecules and particles anddisentangle it from charge–dipole interactions. The magnitude of theenhancement depends on the assumed particle hydration and collisionkinetics but is increasingly important at smaller sizes, resulting in asteep rise in the observed growth rates with decreasing size. Including theexperimental results in a global model, we find that the enhanced growth rate ofsulfuric acid particles increases the predicted particle number concentrationsin the upper free troposphere by more than 50 %. 

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3. Role of methanesulfonic acid in atmospheric particle nucleation and growth

   https://doi.org/10.1038/s41586-026-10810-2  

   Baalbaki, Rima; Shen, Jiali; Simon, Mario; Klebach, Hannah; Ruhl, Samuel; DeVivo, Jenna; Wang, Mingyi; Scholz, Wiebke; Dada, Lubna; Rörup, Birte; et al (July 2026, Nature)

   Abstract Dimethyl sulfide (DMS; CH₃SCH₃) from marine phytoplankton is a notable source of atmospheric sulfur¹. Its oxidation products include sulfuric acid (SA; H₂SO₄) and methanesulfonic acid (MSA; CH₃SO₃H), which has a higher yield than SA below 10 °C (ref. ²). Although SA is known to drive the formation of new particles³, which may subsequently grow and act as cloud condensation nuclei (CCN), the role of MSA remains unclear⁴. Here, in experiments performed under atmospheric conditions at the CERN CLOUD (Cosmics Leaving OUtdoor Droplets) chamber, we show that MSA nucleates together with ammonia (NH₃) below −10 °C, at rates comparable with SA-NH₃. Moreover, MSA and SA nucleate synergistically below −10 °C, forming multi-acid molecular clusters with NH₃. Even at ultralow NH₃levels, MSA drives particle growth at or near the kinetic limit below 9 °C and above 40% relative humidity (RH). Because MSA and SA generally coexist at similar concentrations in cool marine regions, our findings indicate that nucleation rates may be accelerated up to tenfold and growth rates up to twofold compared with SA-NH3 alone. Our global model simulations indicate that MSA can enhance CCN concentrations, especially in polar regions. We propose that MSA might be an important driver of biogenic particles in cool, pristine marine regions of both the present-day and pre-industrial atmospheres and yet is unaccounted for in global climate models⁵. 

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4. Synergistic HNO3–H2SO4–NH3 upper tropospheric particle formation

   https://doi.org/10.1038/s41586-022-04605-4  

   Wang, Mingyi; Xiao, Mao; Bertozzi, Barbara; Marie, Guillaume; Rörup, Birte; Schulze, Benjamin; Bardakov, Roman; He, Xu-Cheng; Shen, Jiali; Scholz, Wiebke; et al (May 2022, Nature)

   Abstract New particle formation in the upper free troposphere is a major global source of cloud condensation nuclei (CCN) 1–4 . However, the precursor vapours that drive the process are not well understood. With experiments performed under upper tropospheric conditions in the CERN CLOUD chamber, we show that nitric acid, sulfuric acid and ammonia form particles synergistically, at rates that are orders of magnitude faster than those from any two of the three components. The importance of this mechanism depends on the availability of ammonia, which was previously thought to be efficiently scavenged by cloud droplets during convection. However, surprisingly high concentrations of ammonia and ammonium nitrate have recently been observed in the upper troposphere over the Asian monsoon region 5,6 . Once particles have formed, co-condensation of ammonia and abundant nitric acid alone is sufficient to drive rapid growth to CCN sizes with only trace sulfate. Moreover, our measurements show that these CCN are also highly efficient ice nucleating particles—comparable to desert dust. Our model simulations confirm that ammonia is efficiently convected aloft during the Asian monsoon, driving rapid, multi-acid HNO 3 –H 2 SO 4 –NH 3 nucleation in the upper troposphere and producing ice nucleating particles that spread across the mid-latitude Northern Hemisphere. 

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5. Calibrating interdependent photochemistry, nucleation, and aerosol microphysics in chamber experiments

   https://doi.org/10.5194/amt-19-3363-2026  

   Donahue, Neil M; Hofbauer, Victoria; Finkenzeller, Henning; Stolzenburg, Dominik; Bauer, Paulus S; Chiu, Randall; Dada, Lubna; Duplissy, Jonathan; He, Xu-Cheng; Heinritzi, Martin; et al (January 2026, Atmospheric Measurement Techniques)

   Abstract. Laboratory experiments addressing complex phenomena such as atmospheric new-particle formation and growth typically involve numerous instruments measuring a range of key coupled variables. In addition to independent calibration, the combined dataset provides not just constraints on the parameters of interest but also on the critical instrument calibrations. Here we find good agreement between production and loss rates of sulfuric acid (H2SO4) in an experiment performed at the CERN CLOUD chamber involving oxidation of sulfur dioxide (SO2) in the presence of ammonia (NH3) at 58 % relative humidity, driving new-particle formation and growth of particles by H2SO4+NH3 nucleation initiated by O3 photolysis via several light sources. This closure requires consistency across numerous parameters, including: the particle number and size distribution; their condensation sink for H2SO4; the particle growth rates; the concentration of H2SO4; and the nucleation coefficients for both neutral and ion-induced pathways. Our study shows that accurate agreement can be achieved between production and loss of condensable vapors in laboratory chambers under atmospheric conditions, with accuracy ultimately tied to particle number measurement (i.e. a condensation particle counter). This, in turn implies parameters such as the H2SO4 concentration and particle size distributions can be determined to a comparable precision. 

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