Researchers at the California Institute of Technology participated in the Cosmics Leaving Outdoor Droplets (CLOUD) collaboration at CERN, performing experiments that simulate the physics and chemistry of atmospheric particle formation and growth over a wide range of conditions encountered in the atmosphere.  Nucleation of new particles from the vapor phase, and the subsequent growth of those particles generate more than half of the particles that act as cloud condensation nuclei (CCN) in the global atmosphere.  Atmospheric particles, especially the CCN are a leading source of uncertainty in radiative forcing of climate.  Understanding how anthropogenic activities have already changed Earth’s atmosphere, and improving our ability to predict the CCN changes that will occur with changing emissions as the world decarbonizes is essential for informing climate change mitigation policies.   The CLOUD facility at CERN has developed unique facilities and, through the collaborating institutions throughout Europe and the US, capabilities for measurement of the fundamental processes leading to new particle and CCN formation.   

Atmospheric new particle formation occurs when molecular clusters formed by nucleation of extremely low volatility molecules that then grow as a succession of molecules of increasing volatility can activate and condense onto the nascent particles.  Measurements made with a variety of instruments, including ones developed at Caltech to probe cluster growth in the overlap between mass spectrometric measurements of small molecular clusters, and physical characterization of larger, thermodynamically stable particles.  Only those that grow fast enough escape coagulation with larger, pre-existing particles become CCN.  

Globally, sulfuric acid dominates initial cluster formation, but atmospheric sulfuric acid concentrations are usually too low to enable growth directly to CCN formation.  Highly oxidized organics concentrations are, however, high enough to grow the nuclei rapidly through the so-called “valley of death” to become CCN.  State-of-the-art mobility analyzers make it possible to quantify particle growth through this critical region of particle-size space, which extends well beyond the range of the mass spectrometric measurements.  High resolution mass spectrometers map the oxidation pathways in the CLOUD experiments, while a battery of mobility analyzers provide the detailed size distributions from about 1.7 nm to well beyond the threshold for acting as CCN that are needed to quantify the losses of vapors and growing clusters to larger particles.  To measure the effects of atmospheric ions on nucleation and growth, a new instrument was developed at Caltech that expands the size range of incipient particles measured, and probes both charged and neutral particles.

Mobility measurements were central to the discovery that, below 0 C, nitric acid can play a key role in atmospheric new particle formation and growth.  During winter in highly polluted megacities, growth of nuclei that initially grow only very slowly accelerates by orders of magnitude as they pass a threshold at of a few nanometers, carrying them to CCN sizes.  In the megacity environment, the rapid growth depletes the ammonium nitrate vapor concentrations, so these bursts only are possible in short, localized bursts.  Further experiments reveal that nucleation may occur in upper troposphere conditions where nitric and sulfuric acids and ammonia carried aloft by convective systems and evolved from detrained ice crystals, possibly dominating the number concentration in the tropical upper troposphere, particularly when influenced by the Asian monsoon.  

The CLOUD experiments probe both particle formation from both anthropogenic (e.g., aromatics), biogenic (e.g., terpenes and isoprene) volatile organic compounds.  Particle formation from iodine species and dimethyl sulfide has been previously observed, but mechanisms and contributions to atmospheric aerosols have been uncertain.  Experiments at CLOUD have probe the molecular pathways and quantify particle formation from these diverse precursors.  

The timescale of measurements in the CLOUD chamber is a matter of hours, but atmospheric reactions continue for much longer times after emissions, leading to multiple generations of oxidation products.   A flow tube system developed at Caltech has been adapted to as a feed system for the CLOUD chamber.  Caltech researchers contributed to the design of this reactor, and the interface between it and the larger chamber.  FLOTUS has recently been installed at CLOUD, and has been employed in preliminary experiments.  Large, highly-charged aerosol particles are found in some convective systems.  The Caltech team further contributed to the design and evaluation of a charged-particle generation system for the CLOUD chamber, enhancing the Caltech mobility analyzer system to measure the charge state of that aerosol and its effects on the aerosol dynamics. 

Beyond the advances in the science of atmospheric particle formation, four Caltech doctoral students formed key parts of their doctoral theses; additional four postdoctoral scholars advanced their research experience through participation in the experiments at CLOUD, and analysis of data arising from that work, and in the design and optimization of new capabilities for the CLOUD experiment.  

Last Modified: 12/12/2023
Modified by: Richard C Flagan