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1. 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 %.
2. 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 thatmore »spread across the mid-latitude Northern Hemisphere.« less
3. The gas-phase formation mechanism of iodic acid as an atmospheric aerosol source

   https://doi.org/10.1038/s41557-022-01067-z  

   Finkenzeller, Henning ; Iyer, Siddharth ; He, Xu-Cheng ; Simon, Mario ; Koenig, Theodore K. ; Lee, Christopher F. ; Valiev, Rashid ; Hofbauer, Victoria ; Amorim, Antonio ; Baalbaki, Rima ; et al ( November 2022 , Nature Chemistry)

   Abstract Iodine is a reactive trace element in atmospheric chemistry that destroys ozone and nucleates particles. Iodine emissions have tripled since 1950 and are projected to keep increasing with rising O 3 surface concentrations. Although iodic acid (HIO 3 ) is widespread and forms particles more efficiently than sulfuric acid, its gas-phase formation mechanism remains unresolved. Here, in CLOUD atmospheric simulation chamber experiments that generate iodine radicals at atmospherically relevant rates, we show that iodooxy hypoiodite, IOIO, is efficiently converted into HIO 3 via reactions (R1) IOIO + O 3  → IOIO 4 and (R2) IOIO 4  + H 2 O → HIO 3  + HOI +  (1) O 2 . The laboratory-derived reaction rate coefficients are corroborated by theory and shown to explain field observations of daytime HIO 3 in the remote lower free troposphere. The mechanism provides a missing link between iodine sources and particle formation. Because particulate iodate is readily reduced, recycling iodine back into the gas phase, our results suggest a catalytic role of iodine in aerosol formation.
4. Measurement report: Sulfuric acid nucleation and experimental conditions in a photolytic flow reactor

   https://doi.org/10.5194/acp-21-1987-2021  

   Hanson, David R. ; Menheer, Seakh ; Wentzel, Michael ; Kunz, Joan ( January 2021 , Atmospheric Chemistry and Physics)

   Abstract. Nucleation rates involving sulfuric acid and watermeasured in a photolytic flow reactor have decreased considerably over atime period of several years. Results show that the system – flow reactor,gas supplies and lines, flow meters, valves, H2SO4 photo-oxidantsources – has reached a baseline stability that yields nucleationinformation such as cluster free energies. The baseline nucleation rate ispunctuated by temporary bursts that in many instances are linked to cylinderchanges, delineating this source of potential contaminants. Diagnostics wereperformed to better understand the system, including growth studies to assessH2SO4 levels, chemiluminescent NO and NOx detection toassess the HONO source, and deployment of a second particle detector toassess the nanoparticle detection system. The growth of seed particles showstrends consistent with the sizes of nucleated particles and provides ananchor for calculated H2SO4 concentrations. The chemiluminescentdetector revealed that small amounts of NO are present in the HONO source,∼ 10 % of HONO. The second condensation-type particlecounter indicates that the nanoparticle mobility sizing system has a bias atlow sulfuric acid levels. The measured and modeled nucleation ratesrepresent upper limits to nucleation in the binary homogeneous system,H2SO4-H2O, as contaminants might act to enhance nucleationrates and ion-mediated nucleation may contribute. Nonetheless, theexperimental nucleation rates, which have decreased by an order ofmore »magnitudeor larger since our first publication, extrapolate to some of the lowest ratesreported in experiments with photolytic H2SO4. Results fromexperiments with varying water content and with ammonia addition are alsopresented and have also decreased by an order of magnitude from our previouswork; revised energetics of clusters in this three-component system arederived which differ from our previous energetics mainly in the five-acid andlarger clusters.« less
5. Nanoparticle growth by particle-phase chemistry

   https://doi.org/10.5194/acp-18-1895-2018  

   Apsokardu, Michael J. ; Johnston, Murray V. ( January 2018 , Atmospheric Chemistry and Physics)

   The ability of particle-phase chemistry to alter the molecular composition and enhance the growth rate of nanoparticles in the 2–100 nm diameter range is investigated through the use of a kinetic growth model. The molecular components included are sulfuric acid, ammonia, water, a non-volatile organic compound, and a semi-volatile organic compound. Molecular composition and growth rate are compared for particles that grow by partitioning alone vs. those that grow by a combination of partitioning and an accretion reaction in the particle phase between two organic molecules. Particle-phase chemistry causes a change in molecular composition that is particle diameter dependent, and when the reaction involves semi-volatile molecules, the particles grow faster than by partitioning alone. These effects are most pronounced for particles larger than about 20 nm in diameter. The modeling results provide a fundamental basis for understanding recent experimental measurements of the molecular composition of secondary organic aerosol showing that accretion reaction product formation increases linearly with increasing aerosol volume-to-surface-area. They also allow initial estimates of the reaction rate constants for these systems. For secondary aerosol produced by either OH oxidation of the cyclic dimethylsiloxane (D₅) or ozonolysis of β-pinene, oligomerization rate constants on the order of 10⁻³ to 10⁻¹ M⁻¹ s⁻¹ are neededmore »to explain the experimental results. These values are consistent with previously measured rate constants for reactions of hydroperoxides and/or peroxyacids in the condensed phase.« less