| Soot particles are an essential atmospheric constituent because of their effects on human health, regional and global climate. The impact of soot on climate and human health depends on their ability to interact with water vapor. Freshly emitted soot particles are hydrophobic but the chemical and physical transformations (aging) they undergo while airborne can make them hydrophilic enough to retain water and act as nuclei for cloud formation. The aging of soot is a complicated process and represents a significant uncertainty in quantifying the net radiative forcing of soot particles. In this study, the cloud condensation nuclei (CCN) activity and the secondary organic aerosol formation of diesel exhaust aged in the presence of one or more of the following: UV-radiation, alpha-pinene, and ammonium sulfate seed particles were investigated. Four sets of experiments (diesel exhaust, diesel exhaust + alpha-pinene, diesel exhaust + ammonium sulfate seed particle, and diesel exhaust + alpha-pinene + ammonium sulfate seed experiments) were conducted. The changes in size distribution, number concentration and CCN activity of the aged diesel exhaust particles were monitored with Scanning Mobility Particle Sizer (SMPS) and Cloud Condensation Nuclei Counter (CCNc).;Results from dark experiments with diesel exhaust alone reveal no significant secondary organic aerosol (SOA) formation, nor CCN activation for fresh and aged diesel exhaust particles, as indicated by decreases in particle mass with time (mass decreased by a factor of 0.3), near zero kappa (kappa CCN ~ 0) and activation ratio (< 1%) values at all times, and large critical diameters. xx Aging of diesel exhaust particles in the presence of UV- radiation in the diesel exhaust experiments resulted in a significant increase in mass (increased by factor of 1.0-1.4), kappa value (ranging from 0.03--0.08), activation ratio (ranging from 0.1--0.75), and a substantial decrease in critical diameter after 200 minutes of aging.;The addition of 39 ppbv of alpha-pinene (diesel exhaust + alpha-pinene experiments) further enhanced CCN activation and SOA formation of diesel exhaust particles exposed to UV radiation. kappaCCN values (0.08--0.15) and particle mass (increased by a factor 1.4--1.7) increased further by 88% and 21%, respectively, when compared to the diesel exhaust experiments. Also, the addition of alpha-pinene reduced the particle diameter required for activation.;Experiments with a mixture of diesel exhaust and ammonium sulfate seed particles under UV and dark conditions investigated the effect on ammonium sulfate seed particles on SOA formation and CCN activation. Over the course of an experiment, particle mass in dark experiments decreased by a factor of 0.9--0.8, while particle mass in the UV experiments increased by a factor of 1.1--1.2. The result shows that SOA formation was higher for diesel exhaust experiments than when a mixture of diesel exhaust and ammonium seed particle were exposed to UV radiation. kappaCCN of the aged particles, which seems to be correlated with the ammonium sulfate fraction and the % increase in particle mass, was found to range from 0.02--0.3 for UV experiments.;The addition of 39 ppbv of alpha-pinene to the mixture of diesel exhaust and ammonium sulfate had a significant effect on particle mass (increased by 60%) but no discernible effect on kappaCCN when compared to diesel exhaust +ammonium sulfate experiments.;The results from this study underscore the importance of exposure to sunlight (UV), mixing with water-soluble particles, such as ammonium sulfate, and SOA precursors such as gas phase diesel exhaust and alpha-pinene, in transforming hydrophobic soot into more CCN-active particles. The increase in hygroscopicity and CCN activity of aged diesel particles is expected to reduce their atmospheric lifetime. |
