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1. 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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2. Temperature, humidity, and ionisation effect of iodine oxoacid nucleation

   https://doi.org/10.1039/d4ea00013g  

   Rörup, Birte; He, Xu-Cheng; Shen, Jiali; Baalbaki, Rima; Dada, Lubna; Sipilä, Mikko; Kirkby, Jasper; Kulmala, Markku; Amorim, Antonio; Baccarini, Andrea; et al (May 2024, Environmental Science: Atmospheres)

   Iodine oxoacids are recognised for their significant contribution to the formation of new particles in marine and polar atmospheres. Nevertheless, to incorporate the iodine oxoacid nucleation mechanism into global simulations, it is essential to comprehend how this mechanism varies under various atmospheric conditions. In this study, we combined measurements from the CLOUD (Cosmic Leaving OUtdoor Droplets) chamber at CERN and simulations with a kinetic model to investigate the impact of temperature, ionisation, and humidity on iodine oxoacid nucleation. Our findings reveal that ion-induced particle formation rates remain largely unaffected by changes in temperature. However, neutral particle formation rates experience a significant increase when the temperature drops from +10 ^oC to −10 ^oC. Running the kinetic model with varying ionisation rates demonstrates that the particle formation rate only increases with a higher ionisation rate when the iodic acid concentration exceeds 1.5 × 10⁷ cm^sup>−3, a concentration rarely reached in pristine marine atmospheres. Consequently, our simulations suggest that, despite higher ionisation rates, the charged cluster nucleation pathway of iodic acid is unlikely to be enhanced in the upper troposphere by higher ionisation rates. Instead, the neutral nucleation channel is likely to be the dominant channel in that region. Notably, the iodine oxoacid nucleation mechanism remains unaffected by changes in relative humidity from 2% to 80%. However, under unrealistically dry conditions (below 0.008% RH at +10 ^oC), iodine oxides (I₂O₄ and I₂O₅) significantly enhance formation rates. Therefore, we conclude that iodine oxoacid nucleation is the dominant nucleation mechanism for iodine nucleation in the marine and polar boundary layer atmosphere. 

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3. 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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4. Summertime Airborne Measurements of Ammonia Emissions From Cattle Feedlots and Dairies in Northeastern Colorado

   https://doi.org/10.1029/2023JD039043  

   Juncosa_Calahorrano, Julieta F; Pollack, Ilana B; Sullivan, Amy P; Roscioli, J Robert; Caulton, Dana R; McCabe, Megan E; Li, En; Pierce, Jeffrey R; Fischer, Emily V (December 2023, Journal of Geophysical Research: Atmospheres)

   Abstract Phase One of the Transportation and Transformation of Ammonia (TRANS²Am) field campaign took place in northeastern Colorado during the summer of 2021. One of the goals of TRANS²Am was to measure ammonia (NH₃) emissions from cattle feedlots and dairies. Most of these animal husbandry facilities are co‐located within oil and gas development, an important source of methane (CH₄) and ethane (C₂H₆) in the region. Phase One of TRANS²Am included 12 near‐source research flights. We present estimates of NH₃emissions ratios with respect to CH₄(NH₃EmR), with and without correction of CH₄from oil and gas, for 29 feedlots and dairies in the region. The data shows larger emissions ratios than previously reported in the literature with a large range of values (i.e., 0.1–2.6 ppbv ppbv⁻¹). Facilities housing cattle and dairy had a mean (std) of 1.20 (0.63) and 0.29 (0.08) ppbv ppbv⁻¹, respectively. We also found that only 15% of the total ammonia (NH_x) is in the particle phase (i.e., ) near major sources during the warm summertime months. We examined the evolution of NH₃in one plume that was sampled at different distances and altitudes up to 25 km downwind and estimated the NH₃lifetime against deposition and partitioning to the particle phase to be 87–120 min. Finally, we calculated estimates of NH₃emission rates from four optimally sampled facilities. These ranged from 4 to 29 g NH₃ · h⁻¹ · hd⁻¹. 

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5. Continuous Monitoring of Sulfur Dioxide Removal Using K-Band Molecular Rotational Resonance Spectroscopy

   https://doi.org/10.1177/00037028251317215  

   Jasti, Sai Eswar; Jamil, Md Abrar; Chandrasekaran, Chandru P; Wei, Suying; Twagirayezu, Sylvestre (July 2025, Applied Spectroscopy)

   Kazarian, S (Ed.)

   Sulfur dioxide (SO₂), an air pollutant, poses significant threats to both public health and the environment. It is one of the six air pollutants regulated by the U.S. Environmental Protection Agency (EPA) under the Clean Air Act. In efforts to determine the application of molecular rotational resonance (MRR) spectroscopy for monitoring SO₂and its removal from point sources, a K-band MRR technique was evaluated. This method was applied to measure the products of heated mixtures of SO₂and oxygen (O₂) in the presence of ammonium metavanadate (NH₄VO₃) as a catalyst. The observed MRR spectrum revealed the presence of SO₂, water vapor (H₂O), and ammonia (NH₃) due to the sensitivity of MRR to only polar species. SO₂removal was further confirmed by the disappearance of SO₂as NH₃formed. The work presented here analyzed the measurements of SO₂and validated K-band MRR for monitoring SO₂removal. It was observed that the K-band MRR maintains its linearity and other polar species in the mixture did not interfere with MRR signature of SO₂. The limit of detection, better than 1%, was determined by evaluating targeted K-band MRR signal response of SO₂removal obtained at varying partial pressures of SO₂in the mixture and using the MRR signal of pure SO₂at 3 mTorr as a reference (100%). Additionally, the results showed that the accuracy and precision of K-band MRR for measuring SO₂partial pressure were satisfactory. 

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