New Particle Formation

Why do we care new particle formation (NPF) in the atmosphere? 

Atmospheric aerosols can profoundly influence public health and climate. Around half of global cloud condensation nuclei (CCN) come from nucleation process. And CCN can further grow to form clouds, which affect Earth’s energy balance. Depending on the size, number concentration and height of droplets, clouds strongly influence the local, regional, and global radiation budget. Understanding NPF process is key to understanding climate change

What is the nucleation process? 

Nucleation is a phase transformation process from a vapor to form nanoparticles. The formation of stable molecular clusters requires growth that overcomes the Gibbs energy barrier (typically ∼1 nm critical cluster size). After overcoming the Gibbs energy barrier, the molecular cluster becomes stable and can further grow to larger aerosol particles (>1 nm size) and form CCN (>100 nm size) by condensation of vapors. Figure 1 shows the atmospheric binary homogeneous nucleation of H2SO4 and H2O nucleation and subsequent growth process (Curtius, 2009). 

Figure 1. Schematic presentation of the atmospheric binary homogeneous nucleation of sulfuric acid and water nucleation and subsequent growth process. 


What is CLOUD? 

Cosmics Leaving Outdoor Droplets (CLOUD) is a simulation chamber located at CERN, the European Council for Nuclear Research, in Geneva, Switzerland. CLOUD provides precise control over physical and chemical conditions, combined with extremely low contamination, making it a unique research tool to study nucleation of particles and ice nucleation over a wide range of atmospheric conditions, including remote marine atmosphere, urban polluted boundary layer, forested ecosystems, and the upper troposphere/ lower stratosphere.

What have we learned from CLOUD? 

Since its inception in 2009, CLOUD experiments have systematically investigated NPF from marine, anthropogenic and biogenic precursor gases over a wide range of atmospheric conditions to provide a molecular level understanding of nucleation and early particle growth. 

Figure 2 summarizes the currently known mechanisms for NPF in the atmosphere (Kirkby et al., 2023). CLOUD experiments have identified several key vapors that contribute to NPF, shown in red boxes, namely, iodic acid (HIO3), methanesulfonic acid (MSA, likely), sulfuric acids (H2SO4), nitric acid (HNO3, under some conditions), and ultra-low-volatility organic compounds (ULVOCs). Stablizers, which can reduce the evaporation of embryonic molecular clusters in the nucleation process, include amines, ammonia (NH3) and iodous acid (HIO2), shown in blue. Gaseous precursors, including sulfur dioxide (SO2), dimethyl sulfide (DMS), iodine (I2) or organic compounds, can be oxidized by atmospheric oxidants like hydroxyl radicals (OH•) and ozone (O3) to form ultra-low-volatility vapors, which may in turn nucleate to form new particles, known as nucleation, and/or condense onto the existing particles, contributing to particle growth. NPF and growth occurs essentially throughout the troposphere and, at elevated vapor concentrations, is also responsible for smog episodes in highly polluted urban environments.

Figure 2. Mechanisms for NPF in the atmosphere explored by CLOUD (Kirkby et al., 2023).


Integrating laboratory experiments, field observations, and modeling

We participate in annual campaigns at CLOUD/CERN with DOAS and CIMS to measure condensable vapors and develop instrumentation that we also deploy in the field (e.g., TI3GER project). The CLOUD campaigns bring together researchers from 3 US and 15 European research groups that form the CLOUD collaboration. Throughout the year, activities to coordinate the data analysis include discussions and data workshops, a summer school, and the annual Science Team meeting and campaign at CERN. The experimental conditions probed in each campaign are guided by field measurements and inform the parameterization of NPF in global models. 

Ongoing and future themes include:

  • Acid-base system: sulfuric acid–ammonia and sulfuric acid–amines
  • Oxygenated organic molecules: biogenic VOCs and the oxidation products, including isoprene and α-pinene, and anthropogenic VOCs and corresponding oxidation products, including toluene, trimethylbenzene, naphthalene and cresol 
  • Iodine oxoacids
  • Mixtures of chemical systems are of recent interest as well 

These mechanisms are identified and confirmed in the real world, via field, mountain, and aircraft observations. For further details see our recent review paper (Kirkby et al., 2023).

Recent publications:

Finkenzeller et al.: The gas-phase formation mechanism of iodic acid as an atmospheric aerosol source, Nature Chemistry, 15, 129–135., 2023.

He et al.: Iodine oxoacids enhance nucleation of sulfuric acid particles in the atmosphere. Science, 382, 6676, 1308-1314., 2023.

Wang et al.: Measurement of iodine species and sulfuric acid using bromide chemical ionization mass spectrometers, Atmos. Meas. Tech., 14, 4187–4202, doi:10.5194/amt-14-4187-2021, 2021.

Kirkby et al.: Atmospheric new particle formation from the CERN CLOUD experiment. Nat. Geosci. 16, 948–957., 2023

Other references:

Curtius, J.: Nucleation of atmospheric particles, EPJ Web of Conferences, 1, 199-209, 2009.