Date of Award

Spring 2000

Project Type

Dissertation

Program or Major

Earth Sciences

Degree Name

Doctor of Philosophy

First Advisor

Patrick M Crill

Abstract

Tropospheric methyl bromide (CH3Br) is a significant source of ozone (O3) destroying bromine to the stratosphere. Due to this threat, the cessation of the production and use of fumigant CH3Br has been slated for 2005 by the Montreal Protocol nations. This decision was based on CH3Br's relatively long atmospheric lifetime which is estimated from the current understanding of its natural and anthropogenic sources and sinks. The research presented in this dissertation has focused on determining the magnitude and characteristics of the biogeochemical exchange of atmospheric CH3Br with upland and wetland soils to attain a better understanding of these portions of the tropospheric budget of CH 3Br.

An accurate and precise method to determine ambient mixing ratios of CH3Br was developed and implemented. Samples were analyzed for CH3Br by preconcentrating a large volume of air (200 to 700 mL) on a packed sample loop held at -70°C. The sample loop was then heated to 120° and flushed into a gas chromatograph equipped with an oxygen doped electron capture detector (GC-ECD).

Laboratory incubations of soils collected from around the world were completed to determine if the upland soil sink was universal. Manipulation of soil samples were completed to determine the characteristics of uptake. Field flux measurements over a growing season were completed at two upland sites in the Northeastern United States using a chamber method. Laboratory incubations and field measurements revealed that upland soils are net sinks of atmospheric CH3Br. The process of uptake appears to be bacterial. There was also some indication from field measurements that there are mechanisms for production of CH3Br in these upland systems.

Exchange of CH3Br at two temperate wetlands was studied over an entire growing season revealing an estimate of net emissions of CH 3Br to the atmosphere of approximately 2.2 Gg yr-1 from global wetlands. This is approximately 4% of the missing source term in the tropospheric budget of CH3Br. This result reveals a significant terrestrial source of atmospheric CH3Br. This research also implies that the exchange of CH3Br in terrestrial systems is a complex interaction of in situ production and consumption.

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