Aerosols are known to affect the climate and human health both directly and indirectly. Two types of aerosols exist in the atmosphere, primary and secondary.  Primary aerosol particles are introduced directly into the atmosphere and include sources such as soot, mineral dust, sea salt and pollen.  Secondary particles are formed from gases and grow in size over time. The formation and growth of secondary particles is typically modeled using classical nucleation theory (CNT).   One of the most studied systems for new particle formation is sulfuric acid reacting with water vapor.  Figure 1 shows pictorially how this process is modeled to occur.

In the first step, SO₂ is converted into H₂SO₄ which complexes with water molecules.  This H₂SO₄-H₂O complex acts as the nucleating seed for new particle growth.  This complex can grow with the addition of water and H₂SO₄ molecules until it can reach a diameter in excess of 1 micrometer in size. 

The number of particles in the atmosphere predicted by models is often several times smaller than measured concentrations.  To reconcile the difference between measured and modeled particle concentrations requires identification of new mechanisms for particle formation.  Formation of a nucleating seed is the first step in particle formation.  We experimentally and computationally tested the hypothesis that organic acids and radicals (molecules with unpaired electrons) can serve as nucleating seeds.  Particle formation was studied with both formic acid (HC(O)OH) and HO₂ radical as nucleating seeds.  HO₂ radical is the most abundant radical in the atmosphere, and previous work has shown that HO₂ radical can hydrogen-bond with water to form an HO₂-H₂O complex which can serve as the nucleating seed.

This hypothesis was tested using a slow-flow reactor coupled to two scanning mobility particle sizers (SMPS) (Figure 2). Particle size distributions and concentrations were measured as functions of reaction time, initial concentration of HO₂ radical, formic acid, and concentration of water vapor.  The experimental instrument and method were tested with formic acid and water vapor, a previously studied system.  The experimental work was complemented with high level ab initio studies on both the HC(O)OH-H₂O and HO₂-H₂O complexes.  The computational studies provided valuable insights into the mechanism and thermodynamics of new particle formation.  The results of this work also suggest that radicals and organic acids commonly found in the atmosphere react with NH₃, NH₂(CH₃), NH(CH₃)₂ or N(CH₃)₃ to serve as nucleating sites for generation of particles in the atmosphere. 

These new mechanisms for particle formation may help close the gap between measured and modeled concentrations of particles in the atmosphere. 

Last Modified: 10/28/2017
Modified by: Jaron C Hansen