Airborne Observations of Halogenated Trace Gases in the Upper Troposphere and Lowermost Stratosphere

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Chlorinated and brominated tracers in the atmosphere are harmful to the ozone layer and many are climate active gases with high radiative efficiencies. Production and use of substances such as chlorofluorocarbons, long-lived chlorinated solvents and halons are regulated in the Montreal Protocol and its amendments, most recently extended in the Kigali amendment in October 2016. The success of regulation is reflected in decreasing atmospheric mixing ratios of many substances. As a consequence of the phase-out of several substances, the atmospheric load of their replacements such as long-lived hydrofluorocarbons (HFC) is increasing, resulting in future global warming. Now, new short-lived HFC (so-called HFO, hydrofluoroolefins) are coming into use, with their mixing ratios in the atmosphere already having reached detectable amounts. The proposed project focuses on new observations of halogenated gases in the upper troposphere and lowermost stratosphere within the CARIBIC project employing a passenger aircraft for atmospheric measurements. Research topics to be addressed are (1) HFOs in the UT/LS, (2) interhemispheric gradients of halogenated gases in the upper troposphere and (3) the impact of the South Asian summer monsoon.With a flight altitude of 10-12 km the platform allows to assess mixing ratios of halogenated tracers in the upper troposphere as an entry point for quasi isentropic transport into the lowermost stratosphere. With atmospheric mean lifetimes of the order of days to weeks, the newly used HFOs live sufficiently long to be transported to the upper troposphere. The measurements around the tropopause will provide data to assess the potential of further transport of halogens into the lowermost stratosphere. In summer, the South Asian summer monsoon provides another pathway for the transport of pollutants to the lowermost stratosphere, and it has a strong impact on the chemical composition of the atmosphere uplifting pollutants from densely populated regions and developing economies with increasing emissions of halogenated substances. Reported halocarbon emissions and emission estimates derived from observed trends in the atmosphere do not always match. For example, atmospheric mixing ratios of CCl4 decrease at a slower rate than would be expected from its lifetime and reported emissions, and similar findings were recently reported for CFC-11. Most halocarbons are emitted from industrial processes predominantly in the northern hemisphere resulting in pronounced interhemispheric differences. Variations in the interhemispheric gradient may be related to changes either in atmospheric transport or in emission budgets. Adding to the database provided by station networks, airborne measurements have the potential to put further observational constraint to the current model framework for deriving top-down emissions. This question will be addressed using data from flights into the southern hemisphere and an existing 12-box model.