Atmospheric Transformations of Synthetic Nanoparticles

Overview: 

We have conducted a series of experiments on SOA condensation to nanoparticles and also developed a numerical model simulating the condensation of a range of organics with different vapor pressures to particles in the presence of a background aerosol distribution also containing semi-volatile organics.  One paper has been published in ES&T

Growth of Synthetic Nanoparticle

We performed a series of coating experiments on nanoparticles suspended in our smog chamber. This formed the core of a Masters thesis by Joohyung (Jack) Lee, resulting in the second paper above. Jack generated MeOx (TiO2 and CeO2) nanoparticles in our chamber by nebulizing an aqueous suspension of either bare or PAA-coated MeOx particles. He formed SOA from α-pinene by oxidizing with ozone and tracked growth of MeOx particles as well as a smaller mode of homogeneously nucleated SOA particles. All particles grow according to their share of the condensational sink (surface area corrected for gas-phase diffusion). This is surprising because organics are semi-volatile. Equilibration of organic composition should cause significant deviation from simple condensational growth. Jack also observed inhibition of SOA nucleation in the presence of bare MeOx particles but substantial nucleation in the presence of PAA coated particles. Our interpretation is that PAA suppresses adsorption of SOA vapors to the nanoparticles. 

Ellis Robinson is exploring the mixing via evaporation and recondensation of different pure organic substances mimicking SOA species. This physics is thought to dominate organic mixing but has never been directly tested under atmospherically relevant conditions. The results above strongly suggest that such a test is important.  He is perfecting measurements of individual 100-700 nm particles with our high-resolution mass spectrometer and using isotopic labeling to investigate mixing of various organic species via evaporation and condensation. William Burroughs has shown that we can “release and catch” MeOx particles in our chamber after a coating experiment they have been processed for 1-2 hours in our smog chamber.

Modeling Growth of Nanoparticles in the Atmosphere

We have written a dynamical model to describe the growth by condensation of NPs in the atmosphere. In this model inorganics (principally sulfuric acid vapors) and organics are treated as separate phases. Modeling growth via sulfuric acid condensation is straightforward. Growth of organics is interesting. Organic vapors are semi volatile, forming a complex mixture with a wide range of volatility. At equilibrium the volatility distribution of all particles will be the same, but the dynamics, including potential condensed-phase diffusion limitations in highly viscous mixtures, require extensive study. 

The consequences of these simple constraints have not been considered by any treatment of NP growth in the atmosphere.  Our group has pioneered the treatment of volatility based modeling of semi-volatile mixtures. However, before this project our focus has been on equilibrium modeling of the semi-volatile mass concentrations in the condensed phase. Motivated by CEINT we are now fully considering the dynamics of semi-volatile condensation to and evaporation from particles.