Bromine from short–lived source gases in the Northern Hemisphere UTLS

Abstract

<p><strong>Abstract.</strong> We present novel measurements of five short-lived brominated source gases (CH<sub>2</sub>Br<sub>2</sub>, CHBr<sub>3</sub>, CH<sub>2</sub>ClBr, CHCl<sub>2</sub>Br and CHClBr<sub>2</sub>) obtained using a gas chromatograph-mass spectrometer system on board the High Altitude and Long Range Research Aircraft (HALO). The instrument is extremely sensitive due to the use of chemical ionisation, allowing detection limits in the lower parts per quadrillion (10<sup>−15</sup>) range. Data from three campaigns using the HALO aircraft are presented, where the Upper Troposphere/Lower Stratosphere (UTLS) of the Northern Hemisphere mid to high latitudes were sampled during winter and during late summer to early fall. We show that an observed decrease with altitude in the stratosphere is consistent with the relative lifetimes of the different compounds. Distributions of the five source gases and total organic bromine just below the tropopause shows an increase in mixing ratio with latitude, in particular during polar winter. This increase in mixing ratio is explained by increasing lifetimes at higher latitudes during winter. As the mixing ratio at the extratropical tropopause are generally higher than those derived for the tropical tropopause, extratropical troposphere-to-stratosphere transport will result in elevated levels of organic bromine in comparison to air transported over the tropical tropopause. The observations are compared to model estimates using different emission scenarios. A scenario which has emissions most strongly concentrated to low latitudes cannot reproduce the observed latitudinal distributions and will tend to overestimate bromine input through the tropical tropopause from CH<sub>2</sub>Br<sub>2</sub> and CHBr<sub>3</sub>. Consequently, the scenario also overestimates the amount of brominated organic gases in the stratosphere. The two scenarios with the highest overall emissions of CH<sub>2</sub>Br<sub>2</sub> tend to overestimate mixing ratios at the tropical tropopause but are in much better agreement with extratropical tropopause values, showing that not only total emissions but also latitudinal distributions in the emissions are of importance. While an increase in tropopause values with latitude is reproduced with all emission scenarios during winter, the simulated extratropical tropopause values are on average lower than the observations during late summer to fall. We show that a good knowledge of the latitudinal distribution of tropopause mixing ratios and of the fractional contributions of tropical and extratropical air is needed to derive stratospheric inorganic bromine in the lowermost stratosphere from observations. Depending on the underlying emission scenario, differences of a factor 2 in reactive bromine derived from observations and model outputs are found for the lowermost stratosphere, based on source gas injection. We conclude that a good representation of the contributions of different source regions is required in models for a robust assessment of the role of short-lived halogen source gases on ozone depletion in the UTLS.</p>

A convolution of observational and model data to estimate age of air spectra in the northern hemispheric lower stratosphere

“…The focus of WISE was on isentropic exchange processes between troposphere and stratosphere around the mid-latitude tropopause. The flight tracks for both campaigns are shown in Keber et al (2019). The campaign data are binned into grids of equivalent latitudes (Allen and Nakamura, 2003) and potential temperature differences to the WMO tropopause, with a bin size of 5° × 5 K and treated as phase averages.…”

“…GhOST-MS is a dualchannel gas chromatograph coupled with an Electron Capture Detector (ECD) in an isothermal channel and a quadrupole mass spectrometer (MS) in a temperature programed channel. The set-up and relevant precision values are given in Keber et al (2019). All GhOST-MS data in this study are reported on SIO-05 scales.…”