AAAR 32nd Annual Conference
September 30 - October 4, 2013
Oregon Convention Center
Portland, Oregon, USA
Abstract View
Particle Evolution near Major Roadways Based on Observed Ultrafine Particle Concentration Profiles under Stable Conditions
Wonsik Choi, SUZANNE PAULSON, UCLA
Abstract Number: 107 Working Group: Urban Aerosols
Abstract Ultrafine particles (UFP, < 100 nm), emitted from vehicular exhaust the dominant source in the urban areas, have been known to cause adverse health effects by recent epidemiological and toxicological studies as well as to affect visibility and climate indirectly. Most studies conducted during daytime found smaller nanoparticles emitted from traffic sources disappear within 300 m downwind, changing size distributions rapidly near major roadways. Particle evolution near roadways has been addressed primarily using modeling studies, but gaps between model results and observations still remain. The purpose of this study is to investigate how freshly-emitted UFP evolve in their characteristics during transport periods under stable conditions in near roadways, when transport times are much longer than during daytime.
We investigate the effects of particle dynamics on UFP plume decay rates based on size-segregated UFP concentration profiles measured with Mobile Monitoring Platform equipped with Fast Mobility Particle Sizer for stable pre-sunrise hours. Total 32 size bins were regrouped into 5 size groups based on observed size distributions for data analysis efficiency. Particle loss with distance for smaller size bins was larger than that only due to dilution/dispersion, implying particle dynamics likely affects observed UFP decay rates with distance. Brownian coagulation rates were not able to explain those gaps, and hence additional processes such as evaporation and coagulation enhancement due to van der Waals forces are needed. The upper limit of Hamaker constant for fleshly emitted UFP from vehicles, which is a key parameter to estimate enhanced coagulation rates due to van der Waals forces, is also presented.