AAAR 35th Annual Conference October 17 - October 21, 2016 Oregon Convention Center Portland, Oregon, USA
Abstract View
Modeling the Size Dependence of Particle Composition and Growth Rates
MICHAEL APSOKARDU, Murray Johnston, University of Delaware
Abstract Number: 695 Working Group: Aerosol Chemistry
Abstract Particle composition measurements of ambient new particle formation (NPF) events suggest that sulfate, base (typically ammonia) and carbonaceous matter are the three major constituents of particles. A challenge of understanding NPF is explaining observed particle growth rates. Sulfate, from sulfuric acid, alone is incapable of explaining observed growth rates, and recent research confirms that carbonaceous matter is responsible for the fraction of growth not attributed to sulfuric acid. However, the ways in which carbonaceous matter contribute to particle growth are not fully understood. For organic compounds, molecular properties such as molar mass and condensed phase vapor pressure (which is modified by the Kelvin effect) can strongly affect mass transport from the gas phase to the particle phase. Since ambient air contains many types of organic compounds across a wide range of molecular properties, it is important to understand how molecular properties influence the particle growth process.
In this work, we have developed and used a kinetic model to study the size dependence of particle growth by carbonaceous matter. The model iteratively calculates volume and composition of a particle as a function of time when it is exposed to gas phase species. The molecular inputs (gas phase mixing ratios and molecular properties such as molar mass and vapor pressure) are similar to those encountered in our previous studies of ambient NPF. These include sulfuric acid, ammonia, water, and organic compounds distributed through a wide range of volatility bins. Using this model, size dependencies of particle composition and growth rate are elucidated. Non-continuum flow contributes significantly to the size dependence below 10nm. While the Kelvin effect is also greatest below 10nm, it can influence particle growth and composition up to 100nm and greater. Of particular interest are the relative amounts of more volatile vs. less volatile organic compounds in the particle as it grows. Where possible, simulations of particle growth and composition are compared to ambient and laboratory measurements.