Intercomparison Between Surrogate, Explicit, and Full Treatments of VSL Bromine Chemistry Within the CAM‐Chem Chemistry‐Climate Model. Issue 4 (23rd February 2021)

• Record Type: Journal Article
• Authors: Fernandez, Rafael P.; Barrera, Javier A.; López‐Noreña, Ana Isabel; Kinnison, Douglas E.; Nicely, Julie; Salawitch, Ross J.; Wales, Pamela A.; Toselli, Beatriz M.; Tilmes, Simone; Lamarque, Jean‐François; Cuevas, Carlos A.; Saiz‐Lopez, Alfonso
• Is Part Of: Geophysical research letters. Volume 48:Issue 4(2021)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2021-02-23
• Subjects: CAM‐Chem -- CCMI -- lowermost stratospheric ozone -- tropospheric oxidation capacity -- very‐short lived bromine; Geophysics -- Periodicals; Planets -- Periodicals; Lunar geology -- Periodicals; 550
• DOI: 10.1029/2020GL091125
• Languages: English
• ISSNs: 0094-8276

Abstract

Abstract: Many Chemistry‐Climate Models (CCMs) include a simplified treatment of brominated very short‐lived (VSL Br ) species by assuming CH3 Br as a surrogate for VSL Br . However, neglecting a comprehensive treatment of VSL Br in CCMs may yield an unrealistic representation of the associated impacts. Here, we use the Community Atmospheric Model with Chemistry (CAM‐Chem) CCM to quantify the tropospheric and stratospheric changes between various VSL Br chemical approaches with increasing degrees of complexity (i.e., surrogate, explicit, and full). Our CAM‐Chem results highlight the improved accuracy achieved by considering a detailed treatment of VSL Br photochemistry, including sea‐salt aerosol dehalogenation and heterogeneous recycling on ice‐crystals. Differences between the full and surrogate schemes maximize in the lowermost stratosphere and midlatitude free troposphere, resulting in a latitudinally dependent reduction of ∼1–7 DU in total ozone column and a ∼5%–15% decrease of the OH/HO2 ratio. We encourage all CCMs to include a complete chemical treatment of VSL Br in the troposphere and stratosphere. Plain Language Summary: The atmospheric bromine burden is dominated by anthropogenic long‐lived bromocarbons, such as methyl bromide (CH3 Br) and halons (i.e., CBr2 F2 ). Due to their small reactivity, these species do not influence tropospheric chemistry and are transported unaltered to the stratosphere, where they contribute to ozone layer depletion. The Montreal Protocol has banned the industrial production of halons and phased out the production of CH3 Br, and consequently their atmospheric abundances are declining. Accordingly, the relative contribution of natural very short‐lived bromine (VSL Br ) species, such as bromoform (CHBr3 ) and dibromomethane (CH2 Br2 ), has increased. Given that VSL Br decompose more rapidly than long‐lived species, their impact on upper tropospheric chemistry and lowermost stratospheric ozone cannot be neglected. In addition, heterogeneous recycling of inorganic bromine on sea‐salt aerosol and ice‐crystals enhances the tropospheric bromine burden. However, many Chemistry‐Climate Models include a simplified approach by assuming CH3 Br as a surrogate for VSL Br ; while those that include an explicit VSL Br approach only consider a simplified tropospheric chemical processing. Here, we compare a surrogate, an explicit and the full chemical treatment of VSL Br source and product gases, and quantify the global impacts of these natural bromocarbons on tropospheric and stratospheric ozone, as well as on other oxidizing agents. Key Points: Using CH3 Br as surrogate for brominated very short‐lived (VSL Br ) species reproduces upper stratosphere bromine, but the impact on lowermost stratospheric ozone is underestimated An explicit approach for CHBr3 and CH2 Br2 captures the expected bromine stratospheric injection but underestimates tropospheric impacts Only the full chemical treatment of VSL Br sources results in a coherent bromine representation in the troposphere and lowermost stratosphere