The trace gas methyl chloride comes primarily from vegetation and the burning of biomass, making it a major natural source of ozone-depleting chlorine in the stratosphere. Previous studies have shown that in the troposphere, methyl chloride concentrations vary not by altitude—as in the stratosphere—but rather by latitude, peaking in the tropics because of higher emissions there. Methyl chloride has a global atmospheric lifetime of about 1 year because of its reaction with short-lived radicals, but it lasts much longer in the stratosphere, which makes it a potentially effective tracer of atmospheric dynamics.
Although scientists have suspected that these characteristics might be used to track air masses in the upper troposphere and lower stratosphere, they have previously lacked the data necessary to confirm this. Now Umezawa et al., using a data set collected by instruments placed aboard commercial airliners, have determined that these measurements can be used to establish the origin of air masses.
The team analyzed data gathered between 2008 and 2012 by the In-service Aircraft for a Global Observing System—Civil Aircraft for the Regular Investigation of the Atmosphere Based on an Instrument Container (IAGOS-CARIBIC) observatory, a set of instruments packaged in an air freight container and deployed aboard a Lufthansa Airbus. They discovered seasonal variations in both methyl chloride and nitrous oxide concentrations in the lowermost stratosphere. Although these were in phase from winter to early summer, the correlation disappeared in the late summer and fall, indicating that both gases are influenced by descending air from the high stratosphere early in the year but by inflow of tropical tropospheric air in summer.
Using this airborne data set, the team calculated the lifetime of methyl chloride in the stratosphere as lasting 35 ± 7 years, half the length of the only previous estimate. In addition, by employing a mass balance approach, the researchers determined that the average proportion of tropical air in the lowermost stratosphere is 37% in spring and 61% in autumn. The researchers attribute the difference to a “tongue” of tropical tropospheric air that ventilates the lowermost stratosphere during the summer. Since they observed this tongue only in the methyl chloride data, the researchers conclude that this gas is the most effective tracer, particularly for tropical tropospheric air masses. (Journal of Geophysical Research: Atmospheres, doi:10.1002/2015JD023729, 2015)
—Terri Cook, Freelance Writer
Citation: Cook, T. (2016), Methyl chloride can track tropical air in the lower stratosphere, Eos, 97, doi:10.1029/2016EO045097. Published on 3 February 2016.