Abstract
Abstract. We report on measurements of total bromine (Brtot) 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 northwestern Europe in fall 2017. Brtot is calculated from measured total organic bromine (Brorg) added to inorganic bromine (Bryinorg), evaluated from measured BrO and photochemical modeling. Combining these data, the weighted mean [Brtot] is 19.2±1.2 ppt in the northern hemispheric lower stratosphere (LS), in agreement with expectations for Brtot in the middle stratosphere (Engel and Rigby et al., 2018). The data reflect the expected variability in Brtot in the LS due to variable influx of shorter lived brominated source and product gases from different regions of entry. A closer look into Brorg and Bryinorg, as well as simultaneously measured transport tracers (CO and N2O) and an air mass lag time tracer (SF6), 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 [Brtot] = 20.9±0.8 ppt. The most probable source region is air recently transported from the tropical upper troposphere and tropopause layer (UT/TTL) with a weighted mean of [Brtot] = 21.6±0.7 ppt. CLaMS Lagrangian transport modeling 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 midlatitude 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 Brtot observed in the UT/TTL. TOMCAT global 3-D model simulations of a concurrent increase of Brtot show an associated O3 change of -2.6±0.7 % in the LS and -3.1±0.7 % near the tropopause. Our study of varying Brtot in the LS also emphasizes the need for more extensive monitoring of stratospheric Brtot globally and seasonally to fully understand its impact on LMS O3 and its radiative forcing of climate, as well as in aged air in the middle stratosphere to elucidate the stratospheric trend in bromine.
Highlights
Bromine is thought to account for approximately one-third of the global stratospheric O3 depletion, and it is effective at destroying O3 in the lowermost stratosphere (LMS) (e.g., Salawitch et al, 2005; Hossaini et al, 2015; WMO, 2018; Chipperfield et al, 2020)
There are four major contributions to total bromine in the troposphere, as suggested by previous research (e.g., Engel and Rigby et al, 2018): (1) CH3Br that has both natural and anthropogenic sources, with a contribution of 6.8 ppt to Brtot in 2016; (2) the four major halons consisting of H-1211 (CClBrF2), H-1301 (CBrF3), H1202 (CBr2F2), and H-2402 (CBrF2CBrF2), all from anthropogenic activities and contributing 7.8 ppt to Brtot in 2016; (3) bromine compounds inferred from naturally emitted socalled very short-lived substances (BrVSLS); and (4) inorganic bromine previously released from brominated VSLS or sea salt, of which a fraction of both is eventually transported into the stratosphere
We report on inferred total bromine, Brtot, in the upper troposphere and lower stratosphere (UTLS) for all available data during the WISE campaign in the fall of 2017
Summary
Bromine is thought to account for approximately one-third of the global stratospheric O3 depletion, and it is effective at destroying O3 in the lowermost stratosphere (LMS) (e.g., Salawitch et al, 2005; Hossaini et al, 2015; WMO, 2018; Chipperfield et al, 2020). The LMS is defined as the atmospheric layer extending from the local tropopause (TP) to the potential temperature = 380 K isentrope, thereby excluding the tropics (Holton et al, 1995), and we define the lower stratosphere (LS) from the local TP up to the highest measurements of this study at ∼ 410 K. Numerous observational and theoretical studies have addressed the variable contributions (3)– (4), which together are assessed to contribute 5 ± 2 ppt to stratospheric bromine (e.g., Engel and Rigby et al, 2018, and references therein). These include studies performed from the ground, ships, manned and unmanned aircraft, and highflying balloons at different locations/seasons and globally from satellites. Additional studies addressing Brtot measurements in aged air of the middle and upper stratosphere at a given location and season are crucial
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