User:Marcbremenkamp/sandbox

Update to last paragraph of the intro section of atmospheric methane page:

The known sources of methane are predominantly located near the Earth's surface. In combination with vertical atmospheric motions and methane's relatively long lifetime, methane is considered to be a well-mixed gas. In other words, the concentration of methane is taken to be constant with respect to height within the troposphere. The dominant sink of methane in the troposphere is reaction with hydroxyl radicals that are formed by reaction of singlet oxygen atoms with water vapor. Methane is also present in the stratosphere, where methane's concentration decreases with height.

Update to Methane as a Greenhouse Gas section:

Methane in the Earth's atmosphere is a strong greenhouse gas with a global warming potential (GWP) 104 times greater than CO2 in a 20-year time frame; methane is not as persistent a gas as CO2 and tails off to about GWP of 28 for a 100-year time frame. This means that a methane emission will have 28 times the impact on temperature of a carbon dioxide emission of the same mass over the following 100 years. Methane has a large effect but for a relatively brief period, having an estimated lifetime of 9.1 years in the atmosphere, whereas carbon dioxide has a small effect for a long period, having an estimated lifetime of over 100 years.

The globally averaged concentration of methane in Earth's atmosphere increased by about 150 percent from 722 ± 25 ppb in 1750 to 1803.2 ± 1.2 ppb in 2011. As of 2011, methane contributed radiative forcing of 0.48 ± 0.05 Wm-2, or about 17% of the total radiative forcing from all of the long-lived and globally mixed greenhouse gases. According to NOAA, the atmospheric methane concentration has continued to increase since 2011 to an average global concentration of 1850.5 ppb as of July 2018.

Updated chlorine reaction section:

The reaction of methane and chlorine atoms acts as a primary sink of Cl atoms and is a primary source of hydrochloric acid (HCl) in the stratosphere.

CH4 + Cl → CH3 + HCl

The HCl produced in this reaction leads to catalytic ozone destruction in the stratosphere.

Update to Oil and Natural Gas Supply Chain section:

Methane is a primary component of natural gas, and thus during the production, processing, storage, transmission, and distribution of natural gas, a significant amount of methane is lost into the atmosphere.

According to the EPA Inventory of U.S Greenhouse Gas Emissions and Sinks: 1990-2015 report, 2015 methane emissions from natural gas and petroleum systems totaled 8.1 Tg per year in the United States. Individually, the EPA estimates that the natural gas system emitted 6.5 Tg per year of methane while petroleum systems emitted 1.6 Tg per year of methane. Methane emissions occur in all sectors of the natural gas industry, from drilling and production, through gathering and processing and transmission, to distribution. These emissions occur through normal operation, routine maintenance, fugitive leaks, system upsets, and venting of equipment. In the oil industry, some underground crude contains natural gas that is entrained in the oil at high reservoir pressures. When oil is removed from the reservoir, associated gas is produced.

However, a review of methane emissions studies reveals that the EPA Inventory of Greenhouse Gas Emissions and Sinks: 1990-2015 report likely significantly underestimated 2015 methane emissions from the oil and natural gas supply chain. The review concluded that in 2015 the oil and natural gas supply chain emitted 13 Tg per year of methane, which is about 60% more than the EPA report for the same time period. The authors write that the most likely cause for the discrepancy is an under sampling by the EPA of so-called "abnormal operating conditions", during which large quantities of methane can be emitted.