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Published July 1, 2007 | public
Journal Article

On the Source of Organic Acid Aerosol Layers above Clouds


During the July 2005 Marine Stratus/Stratocumulus Experiment (MASE) and the August−September 2006 Gulf of Mexico Atmospheric Composition and Climate Study (GoMACCS), the Center for Interdisciplinary Remotely-Piloted Aircraft Studies (CIRPAS) Twin Otter probed aerosols and cumulus clouds in the eastern Pacific Ocean off the coast of northern California and in southeastern Texas, respectively. An on-board particle-into-liquid sampler (PILS) quantified inorganic and organic acid species with ≤5-min time resolution. Ubiquitous organic aerosol layers above cloud with enhanced organic acid levels were observed in both locations. The data suggest that aqueous-phase reactions to produce organic acids, mainly oxalic acid, followed by droplet evaporation is a source of elevated organic acid aerosol levels above cloud. Oxalic acid is observed to be produced more efficiently relative to sulfate as the cloud liquid water content increases, corresponding to larger and less acidic droplets. As derived from large eddy simulations of stratocumulus under the conditions of MASE, both Lagrangian trajectory analysis and diurnal cloudtop evolution provide evidence that a significant fraction of the aerosol mass concentration above cloud can be accounted for by evaporated droplet residual particles. Methanesulfonate data suggest that entrainment of free tropospheric aerosol can also be a source of organic acids above boundary layer clouds.

Additional Information

© 2007 American Chemical Society. Received for review December 21, 2006. Revised manuscript received April 6, 2007. Accepted April 20, 2007. This work was supported by NOAA grant NA06OAR4310082 and Office of Naval Research grant N00014-04-1-0118. The authors gratefully acknowledge the NOAA Air Resources Laboratory (ARL) for provision of the HYSPLIT transport and dispersion model.

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