Evaluating global emission inventories of biogenic bromocarbons

Abstract

Emissions of halogenated very short-lived substances (VSLS) are poorly constrained. However, their inclusion in global models is required to simulate a realistic inorganic bromine (Br_y) loading in both the troposphere, where bromine chemistry perturbs global oxidizing capacity, and in the stratosphere, where it is a major sink for ozone (O₃). We have performed simulations using a 3-D chemical transport model (CTM) including three <i>top-down</i> and a single <i>bottom-up</i> derived emission inventory of the major brominated VSLS bromoform (CHBr₃) and dibromomethane (CH₂Br₂). We perform the first concerted evaluation of these inventories, comparing both the magnitude and spatial distribution of emissions. For a quantitative evaluation of each inventory, model output is compared with independent long-term observations at National Oceanic and Atmospheric Administration (NOAA) ground-based stations and with aircraft observations made during the NSF HIAPER Pole-to-Pole Observations (HIPPO) project. For CHBr₃, the mean absolute deviation between model and surface observation ranges from 0.22 (38%) to 0.78 (115%) parts per trillion (ppt) in the tropics, depending on emission inventory. For CH₂Br₂, the range is 0.17 (24%) to 1.25 (167%) ppt. We also use aircraft observations made during the 2011 "<b>S</b>tratospheric Ozone: <b>H</b>alogen <b>I</b>mpacts in a <b>V</b>arying <b>A</b>tmosphere" (SHIVA) campaign, in the tropical West Pacific. Here, the performance of the various inventories also varies significantly, but overall the CTM is able to reproduce observed CHBr₃ well in the free troposphere using an inventory based on observed sea-to-air fluxes. Finally, we identify the range of uncertainty associated with these VSLS emission inventories on stratospheric bromine loading due to VSLS (Br_y^VSLS). Our simulations show Br_y^VSLS ranges from ~ 4.0 to 8.0 ppt depending on the inventory. We report an optimised estimate at the lower end of this range (~ 4 ppt) based on combining the CHBr₃ and CH₂Br₂ inventories which give best agreement with the compilation of observations in the tropics

Stratospheric Injection of Brominated Very Short‐Lived Substances: Aircraft Observations in the Western Pacific and Representation in Global Models

“…A similar discrepancy exists between CAM‐chem‐SD CHBr 3 and measurements obtained during CAST (not shown), which sampled altitudes below 8 km in the TWP during winter 2014 (Andrews et al, ). Previous evaluations of the Ordóñez et al () climatology, used by CAM‐chem‐SD for emissions of VSLS, have identified the potential to overestimate the actual emissions of CHBr 3 based on a comparison of model output to ground‐based and Southeast Asian aircraft observations (Hossaini et al, , ). However, CAM‐chem‐SD shows very good agreement with ATTREX measurements of CHBr 3 sampled at higher altitudes.…”

“…Eyring et al () recommended that CCMI models incorporate the WMO 2011 best estimate for Br y VSLS of 5 ppt (Montzka et al, ) in one of two manners: either by explicitly including CHBr 3 and CH 2 Br 2 (the two major VSLS) in the model or by increasing the surface mixing ratio of CH 3 Br (a traditional, non‐VSLS source of bromine) by 5 ppt relative to baseline, to act as a surrogate for the additional bromine from VSLS. Recent studies have evaluated different emission inventories for VSLS that utilize more detailed emission schemes than recommended for the first phase of CCMI (Hossaini et al, , ; Lennartz et al, ), but model output from these recent runs are not provided in the public CCMI archive and are representative only of present time conditions. Here our focus is on the analysis of archived CCMI model simulations over long historical (1960–2010) and forecast (1960–2100) time scales conducted to study the interaction between climate change and atmospheric chemistry (Morgenstern et al, ; Revell et al, ).…”

Tropospheric Halogen Chemistry: Sources, Cycling, and Impacts

“…In the recent work of Hossaini and co-workers, 173 the TOMCAT 3D CTM was combined with the existing ocean emission inventories (see also section 3.2.2) of CHBr 3 and CH 2 Br 2 239,243−245 to evaluate their tropospheric distribution and resulting stratospheric bromine injection. They reported a range of bromine injection to the stratosphere of ∼4.0–8.0 pmol/mol depending on the emission inventory, and quantitatively evaluated the different emissions by comparing the model results with available ground-based and aircraft observations from recent field campaigns.…”

Revisiting the hemispheric asymmetry in midlatitude ozone changes following the Mount Pinatubo eruption: A 3‐D model study

“…The chemistry scheme includes a detailed description of the O x , NO y , Cl y , Br y , and HO x families, as well as source gases. The model also includes the brominated VSLS CH 2 Br 2 and CHBr 3 , which yield an additional 6 pptv of stratospheric bromine [ Hossaini et al, ]. The model includes a treatment of heterogeneous reactions on sulfate aerosols and polar stratospheric clouds.…”