10th International Aerosol Conference
September 2 - September 7, 2018
America's Center Convention Complex
St. Louis, Missouri, USA

Abstract View


CCN Activity and Particle Growth of Aging Diesel Exhaust Particles

Humphrey Chukwuto, FRANK BOWMAN, University of North Dakota

     Abstract Number: 414
     Working Group: Carbonaceous Aerosol

Abstract
Diesel exhaust particles emitted into the atmosphere undergo aging that changes their physical and chemical properties, but these processes are complex and not well understood. Laboratory chamber experiments were conducted to investigate the aging of diesel exhaust in the presence of UV light, ammonium sulfate particles, and/or α-pinene vapors. Particle growth and changes in number concentration and size distribution were monitored with a SMPS. Changes in CCN activity were measured with a CCN counter. Results from dark experiments with diesel exhaust alone showed no significant secondary organic aerosol formation and diesel exhaust particles that remained CCN inactive. Aging experiments with UV radiation resulted in a significant increase in particle mass, with deposition-corrected particle mass after 4 hours that was 20-60% higher than the initial mass of freshly emitted diesel soot particles. CCN activity also increased over time indicating that photooxidation of diesel exhaust vapors produces SOA more hygroscopic than the freshly emitted diesel particles. The addition of α-pinene to diesel exhaust in UV aging experiments further enhanced both CCN activity and SOA formation. Experiments with ammonium sulfate particles showed no discernible effect on SOA formation for either diesel exhaust alone or diesel exhaust/α-pinene mixtures. CCN activity measurements were consistent with a largely external mixture of hygroscopic ammonium sulfate particles and non-hygroscopic diesel soot, coated with moderately hygroscopic SOA. The results from this study underscore the importance of aging via photooxidation of SOA precursors and mixing with water-soluble particles in transforming hydrophobic soot into more CCN-active particles.