Grantee Research Project Results
Acid-catalyzed Reactions in Sulfuric Acid Aerosols: Characterization and Impact on Ice NucleationEPA Grant Number: F5B10198
Title: Acid-catalyzed Reactions in Sulfuric Acid Aerosols: Characterization and Impact on Ice Nucleation
Investigators: Beaver, Melinda R
Institution: University of Colorado at Boulder
EPA Project Officer: Hahn, Intaek
Project Period: August 1, 2005 through July 31, 2008
Project Amount: $91,624
RFA: STAR Graduate Fellowships (2005) RFA Text | Recipients Lists
Research Category: Academic Fellowships
Cirrus clouds composed of water ice are quite abundant in the upper troposphere and have a major impact on climate. There is some indication that cirrus cloud abundances have been increasing in recent decades although the reason for the increase is not clear. The radiative effect of cirrus clouds depends on particle characteristics such as size and number density. These parameters are in turn controlled by the nucleation of the ice particles. There has been a great deal of interest recently in determining which ambient particles in the upper troposphere act as ice nuclei. Most previous studies have focused on pure inorganics such as sulfates as potential nuclei. Recent studies have also included simple mixtures of organics with inorganics. One very recent development in tropospheric aerosol chemistry is the suggestion that acid-catalyzed reactions may produce very complex mixtures of organics with sulfates. The products of these reactions have not yet been well characterized, nor has the effect of such complex organics on ice nucleation been explored. I propose to generate complex organic/sulfate aerosols using acid catalyzed chemistry. I will then characterize the resulting products and determine the ice nucleating ability of the resulting aerosol. These studies should provide data on ice nucleation that is more representative of the complex aerosol likely to exist in the upper troposphere than is currently available.
Cirrus clouds form in the upper troposphere when ambient aerosol particles cool, take up water, and eventually nucleate ice. Ice nucleation by inorganic aerosol is relatively well understood. However, it has recently been established that organic compounds compose a large fraction of atmospheric aerosol mass. Acid-catalyzed reactions in sulfuric acid particles have been proposed to be important in incorporating the organics into particles. This project aims to characterize the acid-catalyzed reactions and to determine the impact of such reactions on ice nucleation.
Sub-micrometer sulfuric acid particles will be generated in the laboratory by atomization, characterized, and then reacted with organic compounds capable of acid-catalyzed reactions. The products formed in the particles will be investigated by a combination of real-time aerosol mass spectrometry (AMS) and collection techniques utilizing traditional analytical tools (NMR and GC-MS). Ice nucleation will be investigated using a low temperature aerosol flow tube apparatus coupled to an FTIR spectrometer.
Several different experimental results are possible. It may be that as long as the water content of the aerosol is known, ice nucleation conditions can be predicted using an accepted model for homogeneous ice nucleation. However, in aerosol systems where larger organics form due to acid catalysis, heterogeneous ice nucleation at a warmer temperature mechanism can be imagined. Alternatively, a depression in freezing could occur, possibly due to organic compounds acting to impede water transport.