A global study of tropospheric methane chemistry and emissions
University of Cambridge
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Heimann, I. (2017). A global study of tropospheric methane chemistry and emissions (doctoral thesis). https://doi.org/10.17863/CAM.56036
Methane is the second most important greenhouse gas after CO2 and affects atmospheric temperatures directly and indirectly. Through its chemical loss in the atmosphere, methane also influences tropospheric ozone concentrations, a major air polluter. Experiments have been performed with a chemistry-climate model investigating the effects of source and sink changes on atmospheric methane in a present day (2000) and future (2100) climate. The model setup has been altered so that methane emissions are accounted for rather than using a fixed surface concentration boundary condition. Furthermore, a new chemistry scheme has been implemented into the model in which oxidant fields are prescribed removing methane’s self-feedback. This complements the existing, interactive chemistry scheme. Interactive methane concentrations and its atmospheric lifetime were found to be slightly low biased relative to observations with a source strength of 548 Tg(CH4) yr-1, in line with recent estimates. This low bias may be linked to tropospheric ozone, NOx and CO biases in the model, all influencing the tropospheric OH radical, methane’s major sink. OH was high biased relative to observational estimates, a common feature of chemistry-climate models. Methane emissions were increased to 585 Tg(CH4) yr-1 with a latitudinal shift to larger tropical emissions. This increase resulted in excellent agreement between modelled and observed methane levels, both globally and in the latitudinal gradient. The 7% methane emission increase also increased tropospheric ozone levels. This highlighted methane’s negative impacts on air quality, particularly important because of recent and projected methane emission increases. Probing the sink effect on atmospheric methane levels with the new non-interactive chemistry scheme showed that reducing the OH sink strength and altering its latitudinal distribution improved global methane levels and led to reasonable agreement with observations. However, the latitudinal gradient of non-interactive methane was overestimated suggesting that either the source or the sink distribution require adjustment. Analysis furthermore showed that a tropical shift of methane emissions only slightly improved the latitudinal gradient of non-interactive methane. Future climate experiments were performed which show that the OH sink increase due to higher temperatures (and thus larger tropospheric water vapour content) is counteracted by projected methane emission increases. While projected CO emissions decreased to the end of the 21st century, model calculations here showed that the tropospheric CO burden increased due to chemical production during methane loss. These experiments highlighted that methane emission mitigation is not only beneficial in slowing climate warming but also in improving air quality, affecting the ozone budget.
This record's DOI: https://doi.org/10.17863/CAM.56036
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