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Ultrafine Aerosols (UA) Group
UA Group Members:
Summary of Activities:
Regional and Global Air Quality
One of the main research thrusts of the Ultrafine Aerosols (UA) group is to understand the impacts of urban emissions on regional air quality through its modification of aerosol physico -chemical properties. The UA group specifically focuses its efforts on “ ultrafine particles,” those smaller than 100 nm in diameter whose impacts span all scales from local to regional and global. In March 2006, as part of the MILAGRO campaign, the UA group and collaborators from the University of Minnesota , the Georgia Institute of Technology, the University of Colorado , and the Lawrence Berkeley National Laboratories deployed a suite of instruments to study aerosol formation and growth at the “T1” ground-based site NE northeast of Mexico City during the MILAGRO campaign. These activities followed the first measurements of ultrafine aerosol size distributions in the Mexico City Metropolitan Area, performed by the UA group in 2003, where we found that new particle formation events occur intensely and frequently both inside and outside of the metropolitan area. The preliminary results from MILAGRO show that, once again, new particle formation and subsequent condensational growth occur frequently outside of Mexico City, and are often the dominant processes affecting number concentrations in that area. We also found that these newly formed particles consisted of a complex mixture of both organic and inorganic compounds. These observations were made using the Thermal Desorption Chemical Ionization Mass Spectrometer (TDCIMS), which, with funding support from NSF and the Department of Energy, has been modified recently modified for the study to allow the study of secondary organic compounds in ultrafine particles. The UA group is also the first to measure ultrafine aerosol size distributions in Kansas City , MO , as part of a field study to understand impacts of social disparity in US urban centers. This study, called “Geospatial Analysis of Asthma and Environmental Variables in Kansas City: Implications for Social Disparity,” took place during the Summer 2006 in collaboration with the researchers from the University of Missouri, Kansas City, and Children's Mercy Hospital of Kansas City.
Chemistry in the Climate System
Another major research objective of the UA group is to address the need for atmospheric measurements that elucidate aerosol-cloud-climate interactions. Ultrafine aerosols from a variety of sources, including those generated from urban air pollution as well as naturally from plant emissions, possess the ability to form cloud droplets through the uptake of water in supersaturated air. This ability for ultrafine aerosols to form cloud droplets depends ultimately on their chemical composition and size. In collaboration with researchers from the Georgia Institute of Technology, the UA group has built a suite of instruments to study the effects of ultrafine aerosol chemical composition on aerosol hygroscopicity and cloud condensation nuclei (CCN) activity. This effort is partially funded through a grant from NOAA's Office of Global Programs. Three essential elements to this suite are instruments that measure hygroscopicity (uptake of water onto particles that are exposed to air at 90% Relative Humidity), CCN concentration (fraction of particles that activate into cloud droplets when exposed to a supersaturation of water in air), and particle chemical composition using TDCIMS. During FY06, we performed measurements at the “T1” site northeast of Mexico City during the MILAGRO campaign. Results from this campaign will be used to parameterize models that couple the size and chemical composition of aerosols to cloud droplet concentration. Future plans for these instruments include laboratory measurements aerosol generated from the oxidation of actual plant emissions using the recently developed Biogenic Aerosol Facility. This facility was developed in collaboration with scientists and postdoctoral fellows from BAI and ASP, and has already proved to be a unique and effective means for studying aerosol generation and growth as well as the composition and climatic impacts of biogenic ultrafine aerosols.
Science Highlight
During the MILAGRO campaign, members of the Ultrafine Aerosols group, working with scientists from the University of Minnesota , the Georgia Institute of Technology, and the University of Colorado , studied the properties of particles that arise from a phenomenon known as “nucleation.” Nucleation occurs when certain key gases, such as sulfuric acid, reach a sufficiently high concentration that they cluster together to form a very small particle. We can detect these particles when their size approaches 3 nm, corresponding to a cluster of 160 sulfuric acid molecules. These events are sudden and dramatic, such as the event pictured in Figure 1d on March 16th, 2006 , at a site located about 40 km northeast of Mexico City . As the figure shows, the 6-hour long event started at 3:00 PM GMT, or 9:00 AM local time, and seems to have occurred when the wind shifted from the North (shown in Figure 1b) and the sun rose (as shown in the ultraviolet radiation, or UVB, measurements in Figure 1c). Figure 1a shows measurements we performed to study the impacts of the newly formed particles on climate using a “Cloud Condensation Nucleus,” or CCN, counter. The CCN counter counts the number of particles of a given size, such as for 100 nm diameter particles plotted in Figure 1a, that are capable of forming cloud droplets when exposed to a given “ supersaturation ” of water in the air. Supersaturation is a measure of the amount of excess water vapor above 100% Relative Humidity (RH) that is present in the air. The large red region in Figure 1a lines up with the particles formed from the nucleation event, meaning that 100 nm diameter particles created from nucleation can readily form cloud droplets. These results from these measurements will be used as input to computer models that will study the impact of nucleation events on clouds and climate.
Figure 1. Measurements of climatically relevant aerosol properties during a new particle formation event that began at 3:00 PM (GMT) on 16 March 2006 at the T1 site during MILAGRO. (a) CCN activation fraction of 100nm diameter particles; (b) wind direction; (c) wind speed, UVB, and relative humidity; (d) particle size distribution function.
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