Abstract: 

Atmospheric aerosols are among the least understood components of the climate system. Aerosols affect climate both directly, by scattering light, and indirectly through their interactions with clouds. Aerosols may offset up to a third of the warming due to climate change; however, the uncertainty associated with the aerosol indirect effect complicates quantification. Aerosols can be emitted directly into the atmosphere. They can also form in the atmosphere through the condensation of gases, a process known as new particle formation (NPF). NPF is believed to account for approximately half of the world’s cloud condensation nuclei (CCN). Understanding NPF is essential for accurate climate modeling. Ultrafine aerosols, which have diameters under 100 nm, are difficult to study with most techniques because their mass is so small. However, understanding the composition and formation of ultrafine aerosols is particularly important for clarifying how NPF contributes to CCN concentrations worldwide. During this defense, ultrafine aerosol composition and NPF during two separate field campaigns will be discussed. During SAIL (Surface Atmosphere Integrated Field Laboratory), an 18-month-long campaign in the Colorado Rocky Mountains, NPF was observed on 32% of days. Ultrafine aerosol composition during the campaign was measured indirectly using a Hygroscopic Tandem Differential Mobility Analyzer (HTDMA). Aerosol hygroscopicity remained low throughout the campaign and did not increase during NPF, indicating that organic compounds play a role in NPF at the site. EPCAPE-UPP (Eastern Pacific Cloud Aerosol Precipitation Experiment-Ultrafine Particle Properties) was a 2-month-long sub-campaign conducted at a coastal site in San Diego, California. Ultrafine particle hygroscopicity and chemical composition were measured using an HTDMA and a Thermal Desorption Chemical Ionization Mass Spectrometer (TDCIMS). The TDCIMS is a unique instrument capable of measuring size-resolved ultrafine particle chemical composition in real time. Ultrafine particle hygroscopicity was observed to decrease at the site during periods of heavy cloud cover. TDCIMS data shows that salt and hygroscopic organics decrease in concert with decreasing hygroscopicity. Two NPF events were observed during EPCAPE-UPP; these events were accompanied by increased sulfate and methane sulfonate in 30 nm particles, leading us to believe that sulfuric acid and methane sulfonic acid were driving the NPF events.