A 30-Year Record of the Isotopic Composition of Atmospheric Methane

Description

<p> Methane (CH₄) is one of the most important greenhouse gases after water vapor and carbon dioxide due to its high concentration and global warming potential 25 times than that of CO₂(based on a 100 year time horizon). Its atmospheric concentration has more than doubled from the preindustrial era due to anthropogenic activities such as rice cultivation, biomass burning, and fossil fuel production. However, the rate of increase of atmospheric CH₄ (or the growth rate) slowed from 1980 until present. The main reason for this trend is a slowdown in the trend of CH₄sources. Measuring stable isotopes of atmospheric CH₄ can constrain changes of CH₄sources. The main goal of this work is to interpret the CH₄ trend from 1978-2010 in terms of its sources using measurements of CH₄ mixing ratio and its isotopes. </p><p> The current work presents the measurements and analysis of CH₄ and its isotopes (&delta;¹³C and &delta;D) of four air archive sample sets collected by the Oregon Graduate Institute (OGI). CH₄ isotope ratios (&delta;¹³C and &delta;D) were measured by a continuous flow isotope ratio mass spectrometer technique developed at PSU. The first set is for Cape Meares, Oregon which is the oldest and longest set and spans 1977-1999. The integrity of this sample set was evaluated by comparing between our measured CH₄ mixing ratio values with those measured values by OGI and was found to be stable. Resulting CH₄ seasonal cycle was evaluated from the Cape Meares data. The CH₄ seasonal cycle shows a broad maximum during October-April and a minimum between July and August. The seasonal cycles of &delta;¹³C and &delta;D have maximum values in May for &delta;¹³C and in July for &delta;D and minimum values between September-October for &delta;¹³C and in October for &delta;D. These results indicate a CH₄ source that is more enriched January-May (e.g. biomass burning) and a source that is more depleted August-October (e.g. microbial). In addition to Cape Meares, air archive sets were analyzed from: South Pole (SPO), Samoa (SMO), Mauna Loa (MLO) 1992-1996. The presented &delta;D measurements are unique measured values during these time periods at these stations. </p><p> To obtain the long-term in isotopic CH₄ from 1978-2010, other datasets of Northern Hemisphere mid-latitude sites are included with Cape Meares. These sites are Olympic Peninsula, Washington; Monta&ntilde;a de Oro, California; and Niwot Ridge, Colorado. The seasonal cycles of CH₄ and its isotopes from the composite dataset have the same phase and amplitudes as the Cape Meares site. CH₄ growth rate shows a decrease over time 1978-2010 with three main spikes in 1992, 1998, and 2003 consistent with the literature from the global trend. CH₄ lifetime is estimated to 9.7 yrs. The &delta;¹³C trend in the composite data shows a slow increase from 1978-1987, a more rapid rate of change 1987-2005, and a gradual depletion during 2005-2010. The &delta;D trend in the composite data shows a gradual increase during 1978-2001 and decrease from 2001-2005. From these results, the global CH₄ emissions are estimated and show a leveling off sources 1982-2010 with two large peak anomalies in 1998 and 2003. The global average &delta;¹³C and &delta;D of CH₄ sources are estimated from measured values. The results of these calculations indicate that there is more than one source which controls the decrease in the global CH4 trend. From 1982-2001, &delta;¹³C and &delta;D of CH₄ sources becomes more depleted due to a decrease in fossil and/or biomass burning sources relative to microbial sources. From 2005-2010, &delta;¹³C of CH₄ sources returns to its 1981 value. There are two significant peaks in &delta;¹³C and &delta;D of CH₄ sources in 1998 and 2003 due to the wildfire emissions in boreal areas and in Europe.</p>

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Identifer	oai:union.ndltd.org:PROQUEST/oai:pqdtoai.proquest.com:3557627
Date	04 May 2013
Creators	Teama, Doaa Galal
Publisher	Portland State University
Source Sets	ProQuest.com
Language	English
Detected Language	English
Type	thesis