Organic and inorganic bromine measurements around the extratropical tropopause and lowermost stratosphere: Insights into the transport pathways and total bromine

• Main Authors: Rotermund, Meike K., Bense, Vera, Chipperfield, Martyn P., Engel, Andreas, Grooß, Jens-Uwe, Hoor, Peter, Hüneke, Tilman, Keber, Timo, Kluge, Flora, Schreiner, Benjamin, Schuck, Tanja, Vogel, Bärbel, Zahn, Andreas, Pfeilsticker, Klaus
• Format: Text
• Language: English
• Published: 2021
• Subjects: Norwegian Sea; Rigby; North Atlantic
• Online Access: https://doi.org/10.5194/acp-2021-202; https://acp.copernicus.org/preprints/acp-2021-202/

We report on measurements of total bromine (Br tot ) in the upper troposphere and lower stratosphere taken during 15 flights with the German High Altitude and LOng range research aircraft (HALO). The research campaign WISE (Wave-driven ISentropic Exchange) included regions over the North Atlantic, Norwegian Sea and north-western Europe in fall 2017. Br tot is calculated from measured total organic bromine (Br org ) added to inorganic bromine (Br y inorg ), evaluated from measured BrO and photochemical modelling. Combining these data, the weighted-mean [Br tot ] is 19.2 ± 1.2 ppt in the northern hemispheric lower stratosphere (LS) in agreement with expectations for Br tot in the middle stratosphere (Engel and Rigby et al. (2018)). The data reflects the expected variability in Br tot in the LS due to variable influx of shorter-lived brominated source and product gases from different regions of entry. A closer look into Br org and Br y inorg , as well as simultaneously measured transport tracers (CO and N 2 O) and an air mass lag-time tracer (SF 6 ), suggests that bromine-rich air masses persistently protruded into the lowermost stratosphere (LMS) in boreal summer, creating a high bromine region (HBrR). A subsection, HBrR*, has a weighted average of [Br tot ] = 20.9 ± 0.8 ppt. The most probable source region is former air from the tropical upper troposphere and tropopause layer (UT/TTL) with a weighted mean [Br tot ] = 21.6 ± 0.7 ppt. CLaMS Lagrangian transport modelling shows that the HBrR air mass consists of 51.2 % from the tropical troposphere, 27.1 % from the stratospheric background, and 6.4 % from the mid-latitude troposphere (as well as contributions from other domains). The majority of the surface air reaching the HBrR is from the Asian monsoon and its adjacent tropical regions, which greatly influences trace gas transport into the LMS in boreal summer and fall. Tropical cyclones from Central America in addition to air associated with the Asian monsoon region contribute to the elevated Br tot observed in the UT/TTL. TOMCAT global 3–D model simulations of a concurrent increase of Br tot show an associated O 3 change of −2.6 ± 0.7 % in the LS and −3.1 ± 0.7 % near the tropopause. Our study of varying Br tot in the LS also emphasizes the need for more extensive monitoring of stratospheric Br tot globally and seasonally to fully understand its impact on LMS O 3 and its radiative forcing of climate, as well as in aged air in the middle stratosphere to elucidate the stratospheric trend in bromine.