Investigation of Nucleation Bursts During the Pittsburgh Air Quality Study

Homogeneous nucleation is one of the major sources of atmospheric particles on a global scale. Understanding nucleation is important for quantifying its role in shaping the ambient aerosol distribution and its effects on cloud properties and the planetary energy balance. Over 100 days with nucleation events were investigated during a sampling campaign sampling continental aerosols in Pittsburgh, Pennsylvania. Over 90,000 size distributions were collected over 12 months using Scanning Mobility Particle Sizers (SMPS) at three locations, including both urban and rural sites. Particle size distributions were measured down to 3 nm at the main site and to 10 nm at the other sites. The frequency of nucleation events was surprising. Approximately 50% of the study days were characterized by nucleation events. These events appear to occur over a large area and are not directly related to the emissions from the urban area. Some nucleation events occurred near simultaneously at samplers 500 km apart. Theories under investigation for the nucleation mechanism include sulfuric acid-water, sulfuric acid-water-ammonia, and secondary organic nucleation. The chemistry of the freshly nucleated and growing particles was investigated by collecting over 20,000 single particle mass spectra using Laser Ablation Aerosol Mass Spectrometry on particles as small as 20 nm. Results of TDMA and hygroscopic growth measurements of nuclei mode particles will also be presented. A large number of high-frequency gas, particle, and meteorological measurements were taken with collocated instruments. Data will be analyzed to elucidate possible cause-effect relationships and the dataset will be compared to theoretical estimates of nucleation rates for a number of mechanisms.