HR: 1340h
AN: A13C-1356    [Abstracts]
TI: Interaction of nitric acid and cirrus clouds in the tropical upper troposphere during TC4
AU: * Scheuer, E
AF: University of New Hampshire, Institute for the Study of Earth, Oceans, and Space Morse Hall 358, Durham, NH 03824, United States
AU: Dibb, J E
EM: jack.dibb@unh.edu
AF: University of New Hampshire, Institute for the Study of Earth, Oceans, and Space Morse Hall 358, Durham, NH 03824, United States
AU: Twohy, C
EM: twohy@coas.oregonstate.edu
AF: Oregon State University, College of Oceanography and Atmospheric Sciences Oceanography 104, Corvallis, OR 97331, United States
AU: Rogers, D
EM: dcrogers@ucar.edu
AF: National Center for Atmospheric Research, 10802 Airport Court, Broomfield, CO 80021, United States
AB: Impacts of cirrus clouds on climate and the composition of the tropical upper troposphere were major foci of NASA's Tropical Composition, Cloud, and Climate Coupling Experiment (TC4). One chemical impact that has been postulated is removal and vertical redistribution of HNO₃ through uptake on cirrus particles. Limited previous sampling campaigns have provided somewhat differing results and are not fully compatible with theory developed from lab studies. While deployed to San Jose, Costa Rica during the TC4 field campaign (July-August, 2007) the NASA DC-8 research aircraft spent approximately 41 flight hours in the tropical upper troposphere (> 9 km pressure altitude) during 15 flights. Close to half of the sampling time in the UT consisted of "racetrack" patterns in cirrus clouds from anvil outflow. Interstitial mixing ratios of HNO₃ were depressed (mean 36 pptv) compared to clear air encountered in the UT (mean 70 pptv). Full depletion of HNO₃, however, was never observed (minimum 5 pptv). HNO₃ mixing ratios observed in-cloud varied considerably. HNO₃ mixing ratios generally increased as condensed water decreased. On several flights, the same cloud region was traversed multiple times. Condensed cloud water content data revealed persistent regions of "thicker" cloud with less HNO₃ and contrasting "thinner" cloud with relatively enhanced HNO₃. Microphysical measurements of cloud properties from in situ instruments on the DC-8 may allow factors controlling the partitioning of HNO₃ between the gas phase and cirrus particles in these relatively warm clouds (> 215K) to be determined. Also, strong enhancements of HNO₃ at the base of the anvil systems apparently reflect vertical redistribution of HNO₃ by sedimenting cirrus particles and subsequent particle sublimation and HNO₃ evaporation. The impact of released HNO₃, however, appears to be restricted to a very thin layer just below the cloud.
DE: 0320 Cloud physics and chemistry
DE: 0340 Middle atmosphere: composition and chemistry
DE: 0365 Troposphere: composition and chemistry
SC: Atmospheric Sciences [A]
MN: 2007 Fall Meeting