Abstract
Abstract. Oceanic emissions of halogenated very short-lived substances (VSLS) are expected to contribute significantly to the stratospheric halogen loading and therefore to ozone depletion. The amount of VSLS transported into the stratosphere is estimated based on in-situ observations around the tropical tropopause layer (TTL) and on modeling studies which mostly use prescribed global emission scenarios to reproduce observed atmospheric concentrations. In addition to upper-air VSLS measurements, direct observations of oceanic VSLS emissions are available along ship cruise tracks. Here we use such in-situ observations of VSLS emissions from the West Pacific and tropical Atlantic together with an atmospheric Lagrangian transport model to estimate the direct contribution of bromoform (CHBr3), and dibromomethane (CH2Br2) to the stratospheric bromine loading as well as their ozone depletion potential. Our emission-based estimates of VSLS profiles are compared to upper-air observations and thus link observed oceanic emissions and in situ TTL measurements. This comparison determines how VSLS emissions and transport in the cruise track regions contribute to global upper-air VSLS estimates. The West Pacific emission-based profiles and the global upper-air observations of CHBr3 show a relatively good agreement indicating that emissions from the West Pacific provide an average contribution to the global CHBr3 budget. The tropical Atlantic, although also being a CHBr3 source region, is of less importance for global upper-air CHBr3 estimates as revealed by the small emission-based abundances in the TTL. Western Pacific CH2Br2 emission-based estimates are considerably smaller than upper-air observations as a result of the relatively low sea-to-air flux found in the West Pacific. Together, CHBr3 and CH2Br2 emissions from the West Pacific are projected to contribute to the stratospheric bromine budget with 0.4 pptv Br on average and 2.3 pptv Br for cases of maximum emissions through product and source gas injection. These relatively low estimates reveal that the tropical West Pacific, although characterized by strong convective transport, might overall contribute less VSLS to the stratospheric bromine budget than other regions as a result of only low CH2Br2 and moderate CHBr3 oceanic emissions.
Highlights
Organic brominated compounds, emitted at the surface through natural and anthropogenic processes, are the primary source of stratospheric bromine
source gas injection (SGI) estimated for CHBr3 emissions in the tropical Atlantic is relatively low with 1.1 % of all emitted SG being transported into the tropical stratosphere while in the West Pacific 3.8 % of all emitted SG are entrained. (Note that one unit of SG emissions of CHBr3 (CH2Br2) contains three times the amount of bromine compared to one unit of SGI or product gas injection (PGI) entrainment.) The difference results from stronger convective activity in the West Pacific which acts as the main mechanism for the fast vertical uplift of air masses
We present results from a modeling study which estimates for CHBr3 and CH2Br2 from the West Pacific to provide a upper-air very short-lived substances (VSLS) abundances based on observations of local- relative source gas contribution to the stratospheric bromine ized oceanic emissions
Summary
Organic brominated compounds, emitted at the surface through natural and anthropogenic processes, are the primary source of stratospheric bromine. The upper limit of SGI and PGI estimates obtained from observations and models would imply a relatively large impact of stratospheric inorganic bromine produced from VSLS (BrVy SLS) on midlatitude ozone depletion (Salawitch et al, 2005). It is of interest to determine if global modeling studies, which often rely on uniformly mixed background mixing ratios, might miss the influence of strongly localized sources This would be problematic in case of a correlation between emission strength and efficiency of transport into the TTL, resulting in systematic over- or underestimates of PGI and SGI when using averaged emission fluxes. Analyzing how emission rates and convective activity influence SGI and PGI will help to understand the relative importance of these two processes for the stratospheric Bry budget
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