Zhao, Jun (2007-12). Experimental and theoretical investigation of nucleation and growth of atmospheric aerosols. Doctoral Dissertation. | Thesis individual record

Aerosol particles have profound impacts on human health, atmospheric radiation,
and cloud microphysics and these impacts are strongly dependent on particle sizes.
However, formation and growth of atmospheric particles are currently not well
understood. In this work, laboratory and theoretical studies have been performed to
investigate the formation and growth of atmospheric particles. The first two parts of the
dissertation are a laboratory investigation of new particle formation and growth, and a
theoretical study of atmospheric molecular complexes and clusters. The nucleation rate
was considerably enhanced in the presence of cis-pinonic acid and ammonia. The
composition of the critical cluster was estimated from the dependence of the nucleation
rate on the precursor concentration and the time evolution of the clusters was then
simulated using molecular dynamic simulations. Results from quantum chemical
calculations and quantum theory of atoms in molecules (QTAIM) reveal that formation
of strong hydrogen bonding between an organic acid and sulfuric acid is likely
responsible for a reduction of the nucleation barrier by modifying the hydrophobic
properties of the organic acid and allowing further addition of hydrophilic species (e.g.,
H2SO4, H2O, and possibly NH3) to the hydrophilic side of the clusters. This promotes growth of the nascent cluster to overcome the nucleation barrier and thus enhances the
nucleation in the atmosphere.
The last part of this dissertation is the laboratory investigation of heterogeneous
interactions of atmospheric carbonyls with sulfuric acid. Direct measurement has been
performed to investigate the heterogeneous uptake of atmospheric carbonyls on sulfuric
acid. Important parameters have been obtained from the time-dependent or timeindependent
uptake profiles. The results indicated that the acid-catalyzed reactions of
larger aldehydes (e.g. octanal and 2, 4-hexadienal) in sulfuric acid solution were
attributed to aldol condensation in high acidity. However such reactions do not
contribute much to secondary organic aerosol (SOA) formation due to the low acidity
under tropospheric conditions. On the other hand, heterogeneous reactions of light
dicarbonyl such as methylglyoxal likely contribute to SOA formation in slightly acidic
media. The reactions of methylglyoxal in the atmospheric aerosol-phase involve
hydration and subsequent polymerization, which are dependent on the hygroscopicity,
rather than the acidity of the aerosols.

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