AAAR 32nd Annual Conference
September 30 - October 4, 2013
Oregon Convention Center
Portland, Oregon, USA
Abstract View
Diurnal and Seasonal Trends in the Apparent Density of Ambient Fine and Coarse Particles in Los Angeles
SINA HASHEMINASSAB, Payam Pakbin, Ralph J. Delfino, Constantinos Sioutas, University of Southern California
Abstract Number: 213 Working Group: Aerosol Physics
Abstract Particle density, an important parameter in the transport and deposition of particles, depends on both physical characteristics of the particles and also their chemical composition. In this study, the day to day and diurnal variations in apparent density of ambient fine (dp< 2.5 micro-meter) and coarse (2.5 micro-meter< dp <10 micro-meter) particulate matter (PM) were investigated for a 7-month period in a location situated near downtown Los Angeles. The number size distribution of particles was measured by a Scanning Mobility Particle Sizer (SMPS) and an Optical Particle Sizer (OPS). Mass concentration of ambient fine and coarse PM were measured by a Beta Attenuation Monitor (BAM) and a Continuous Coarse Particulate Matter (CCPM) monitor, respectively. The hourly apparent density was calculated by the ratio of the mass concentration to volume concentration, calculated by converting the number size distribution to volume size distribution, with the assumption of spherical particles. Moreover, the impact of traffic emission was assessed by continuous measurement of black carbon, an indicator of diesel exhaust, using an Aethalometer. The results revealed that the daily averaged apparent density of CPM is greater than fine PM (1.9 ± 0.4 and 1.5 ± 0.3 g.cm-3, respectively). The apparent density also showed a strong daily variation in both size ranges. The minimum apparent density of fine PM was observed in the morning and afternoon rush hours (1.4 ± 0.2 g.cm-3), indicating the impact of traffic emitted agglomerate soot particles on PM2.5 density, while highest during early afternoon (1.7 ± 0.6 g.cm-3), mainly because of higher photochemical activity. Similarly to fine PM, lowest apparent density of CPM was found during the morning rush hours (1.5 ± 0.5 g.cm-3). However, after morning rush hours, CPM apparent density started increasing, as the wind speed peaks, and remained high until midnight (2.0 ± 0.2 g.cm-3).