Global seasonal distribution of CH2Br2 and CHBr3 in the upper troposphere and lower stratosphere

<div class="page" role="region" data-page-number="1" aria-label="Page 1" data-listening-for-double-click="true" data-loaded="true"> <div class="textLayer"><span dir="ltr" role="presentation">Halogens from long-lived anthropogenic substances contribute to the depletion of stratospheric ozone. Besides these&#160;</span><span dir="ltr" role="presentation">long-lived substances, chlorinated and brominated substances with lifetimes of less than 6 months are additional&#160;</span><span dir="ltr" role="presentation">sources of stratospheric halogens. These substances, also known as very short-lived substances (VSLSs), have both&#160;</span><span dir="ltr" role="presentation">natural and anthropogenic origins.</span> <span dir="ltr" role="presentation">The contribution of chlorinated VSLSs (Cl-VSLSs) to stratospheric chlorine is&#160;</span><span dir="ltr" role="presentation">a few percent.</span> <span dir="ltr" role="presentation">In comparison, brominated VSLSs (Br-VSLS) contribute to about a quarter of the stratospheric&#160;</span><span dir="ltr" role="presentation">bromine. The relative contribution of VSLSs to stratospheric halogen loading is expected to increase as the Montreal&#160;</span><span dir="ltr" role="presentation">Protocol controlled substances progressively decrease.</span> <span dir="ltr" role="presentation">Due to their short lifetimes, VSLSs rapidly release their&#160;</span><span dir="ltr" role="presentation">halogen content into the lowermost stratosphere, a region where changes in ozone have a relatively large impact on&#160;</span><span dir="ltr" role="presentation">surface climate.</span></div> <div class="textLayer"><br role="presentation" /><span dir="ltr" role="presentation">Here we present the global seasonal distribution of the two major Br-VSLSs CH</span>_{<span dir="ltr" role="presentation">2</span>}<span dir="ltr" role="presentation">Br</span>_{<span dir="ltr" role="presentation">2&#160;</span>}<span dir="ltr" role="presentation">and CHBr</span>_{<span dir="ltr" role="presentation">3</span>}<span dir="ltr" role="presentation">, which account for&#160;</span><span dir="ltr" role="presentation">about 80 % of total organic Br-VSLS. Measurements from four High Altitude and Long Range Research Aircraft&#160;</span><span dir="ltr" role="presentation">(HALO) missions, the HIAPER Pole-to-Pole Observations (HIPPO) mission, and the Atmospheric Tomography&#160;</span><span dir="ltr" role="presentation">(ATom) mission were used for this purpose.</span> <span dir="ltr" role="presentation">Observational results show a similar seasonality of CH</span>_{<span dir="ltr" role="presentation">2</span>}<span dir="ltr" role="presentation">Br</span>_{<span dir="ltr" role="presentation">2</span>}&#160;<span dir="ltr" role="presentation">in the&#160;</span><span dir="ltr" role="presentation">free and upper troposphere of both hemispheres and less clear seasonality with larger variations for CHBr</span>_{<span dir="ltr" role="presentation">3</span>}<span dir="ltr" role="presentation">.</span> <span dir="ltr" role="presentation">The&#160;</span><span dir="ltr" role="presentation">distribution of CH</span>_{<span dir="ltr" role="presentation">2</span>}<span dir="ltr" role="presentation">Br</span>_{<span dir="ltr" role="presentation">2&#160;</span>}<span dir="ltr" role="presentation">in the lowermost stratosphere suggests differences in hemispheric autumn, where the influx&#160;</span><span dir="ltr" role="presentation">of tropospheric air seen in northern hemispheric summer to autumn is not evident in the Southern Hemisphere.&#160;</span><span dir="ltr" role="presentation">However, the southern hemispheric database is insufficient to quantify this difference.</span> <span dir="ltr" role="presentation">The observed distributions&#160;</span><span dir="ltr" role="presentation">were additionally compared to distributions based on model results of TOMCAT and CAM-Chem, both using the&#160;</span><span dir="ltr" role="presentation">emission inventory of Ord&#243;&#241;</span><span dir="ltr" role="presentation">ez et al. (2012). Neither model was able to reproduce the seasonal distribution of CH</span>_{<span dir="ltr" role="presentation">2</span>}<span dir="ltr" role="presentation">Br</span><span dir="ltr" role="presentation">₂&#160;</span><span dir="ltr" role="presentation">in the Southern Hemisphere. In contrast, both models show a pronounced seasonality of CHBr</span>_{<span dir="ltr" role="presentation">3</span>}&#160;<span dir="ltr" role="presentation">in both hemispheres,&#160;</span><span dir="ltr" role="presentation">which is not confirmed by observations.</span> <span dir="ltr" role="presentation">The distributions of both substances in the lowermost stratosphere are&#160;</span>overall well captured by the models, except for southern hemispheric autumn with considerably lower mixing ratios&#160;in the observations.</div> </div>