Vapor scavenging by atmospheric aerosol particles

Abstract

Particle growth due to vapor scavenging was studied using both experimental and computational techniques. Vapor scavenging by particles is an important physical process in the atmosphere because it can result in changes to particle properties (e.g., size, shape, composition, and activity) and, thus, influence atmospheric phenomena in which particles play a role, such as cloud formation and long range transport. In the modelling portion of this thesis, the influence of organic vapor on the evolution of a particle mass size distribution was investigated using a modified version of MAEROS (a multicomponent aerosol dynamics code) (Gelbard and Seinfeld, 1984). The modelling study attempted to identify the sources of organic aerosol observed by Novakov and Penner (1993) in a field study in Puerto Rim. Potential sources were hypothesized to be organic vapor emissions from either forest vegetation near the sampling site OT from the ocean's surfactant layer followed by gas-to-particle conversion. Comparison of model parameters with literature values suggested that the observed organic aerosol was fomed by nucleation and condensation of terpene vapor emissions onto a pre-existing typical marine aerosol size distribution. Organic vapor emissions from the ocean were found to be an unlikely source of the observed organic aerosol. Experimentally, vapor scavenging and particle growth were investigated using two techniques. The influence of the presence of organic in a particle on the particle's hydroscopicity was investigated using an electrodynamic balance. It was found that, far a Tween8O/NaCl system, having organic associated with an inorganic salt particle slowed the deliquescence rate of the particle. Additionally, for the Tween80/NaC1 system the deliquescence humidity was lowered due to the presence of organicsuggesting an increase in particle hydroscopicity. It was also shown bat the presence of an organic carbon (e.g., azelaic acid, Tween80 or dodecyl sulfate sodium salt) on a carbon black particle made the carbon black particle Rygrophilic, while a pure carbon black particle was observed to be hydrophobic. The charge on a particle (which is required for study of a particle in the eleclmdynamic balance) was investigated theoretically and experimentally. Particle charge was found to have a negligible effect on particle growth in electrodynamic balance studies. A prototype apparatusthe refhctive index thermal diffusion chamber (RITDC)was developed to study multiple particles in the same environment at the same time. Proof of concept experiments showed that it is possible to determine particle composition with rime from changes in particle refractive index measured using the RITDC. Further refinements to the apparatus were suggested.

Simulation of Aerosol Coagulation and Deposition Under Multiple Flow Regimes with Arbitrary Computational Precision

“…Although a number of other models have been introduced (Landgrebe and Pratsinis 1990;Prakash et al 2003, and summaries in Seigneur et al 1986;Zhang et al 1999), the most widely used coagulation model for confined atmospheres is the MAEROS code (Gelbard 1982) or its variants, which have been implemented in other larger computer code systems, for example, MELCOR (Summers et al 1994). MAEROS has also found numerous applications for open atmospheric modeling (Kreidenweis 1993;Andrews 1996).…”

“…This code has been incorporated in many thermal hydraulic applications to assess the accidental nuclear source term (Summers et al 1994;Powers et al 1996), and in other applications where changes in particle size distribution are of interest, such as vapor scavenging of atmospheric aerosols (Andrews 1996) and gas-to-particle conversion models (Kreidenweis 1993). In MAEROS, the particle size boundaries are selected to obey a geometric constraint limiting the size of each section to double that of the lower section, v k ≥ 2 v k-1 .…”