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Department of Physics
Portland State University

Current Research Areas

Isotopic atmospheric methane

Emissions from rice agriculture

Role of trees in the methane budget

Methyl halide emissions from estuaries

Carbon flux from cities

Method development

 

Understanding trends in atmospheric methane through analyses of a rare air archive

In 1980s and through the 1990s, direct measurements of atmospheric methane from several global monitoring networks showed that the growth rate slowed from ~1% per year (in the 1980s) to near zero between 1999 and 2007. The causes for this decline is a long-standing scientific question and a subject that remains unresolved.

One tool to better understand trends in sources and sinks of methane is through the use of stable isotopes. By comparing trends in the carbon and hydrogen isotopic composition (δ13C and δD) of atmospheric methane in time series data to the isotopic signatures of sources, we can disentangle trends in methane sources and sinks. Our recent work analyzed the isotopic composition of atmospheric methane in more than 300 archived historic air samples from the Oregon Health & Sciences University - Portland State University air archive (1978-1998). These sampels were collected as part of one of the earliest global methane long-term monitoring programs run by Dr. Reinhold Rasmussen and Dr. Aslam Khalil at the Oregon Graduate Institute (OGI). The archive has samples from collection locations including Cape Meares, Oregon (pictured above); South Pole, Antarctica; Mauna Loa and Cape Kumukahi, Hawaii; Tutuila, American Samoa; Sable Island, Nova Scotia. Resulting datasets provide historical time series data for isotopic information during a time when data are rare.
The resulting dataset can be downloaded here. Please see "data_readme" for information on datause and appropriate citation.

To interpret these new data in the context of changing methane sources during this period we employed a three dimensional global chemical transport model inversion which used long-term record of measurements of methane concentrations from the OGI and NOAA-ESRL in addition to the new and more recent historical isotopic data (from Quay et al.; Tyler et al; INSTAAR). Results of this analysis were pubished in the journal Proceedings of the National Academy of Sciences:

Andrew L. Rice, Christopher L. Butenhoff, Doaa G. Teama, Florian H. Röger, M. Aslam K. Khalil, and Reinhold A. Rasmussen Atmospheric methane isotopic record favors fossil sources flat in 1980s and 1990s with recent increase PNAS 2016 ; published ahead of print September 12, 2016, doi:10.1073/pnas.1522923113

This work was covered by the Washington Post and several other media outlets

 

Funding for this work was provided by the US National Sciences Foundation (NSF AGS grant 0952307), the MJ Murdock Charitable Trust (grant 2012183) and the Miller Foundation through the Institute for Sustainable Solutions.