Halogen chemistry reduces tropospheric O<sub>3</sub> radiative forcing

Abstract

Abstract. Tropospheric ozone (O 3 ) is a global warming gas, but the lack of a firm observational record since the preindustrial period means that estimates of its radiative forcing (RF TO 3 ) rely on model calculations. Recent observational evidence shows that halogens are pervasive in the troposphere and need to be represented in chemistry-transport models for an accurate simulation of present-day O 3 . Using the GEOSChem model we show that tropospheric halogen chemistry is likely more active in the present day than in the preindustrial. This is due to increased oceanic iodine emissions driven by increased surface O 3 , higher anthropogenic emissions of bromo-carbons, and an increased flux of bromine from the stratosphere. We calculate preindustrial to present-day increases in the tropospheric O 3 burden of 113 Tg without halogens but only 90 Tg with, leading to a reduction in RF TO 3 from 0.43 to 0.35 Wm −2 . We attribute ∼ 50 % of this reduction to increased bromine flux from the stratosphere, ∼ 35 % to the ocean-atmosphere iodine feedback, and ∼ 15 % to increased tropospheric sources of anthropogenic halogens. This reduction of tropospheric O 3 radiative forcing due to halogens (0.087 Wm −2 ) is greater than that from the radiative forcing of stratospheric O 3 (∼ 0.05 Wm −2 ). Estimates of RF TO 3 that fail to consider halogen chemistry are likely overestimates (∼ 25 %).

The novel HALO mini-DOAS instrument: inferring trace gas concentrations from airborne UV/visible limb spectroscopy under all skies using the scaling method

“…Hence, below 9.5 km altitude BrO could not be detected above the detection limit. The amount and distribution of halogen oxides such as BrO (panel e) in the troposphere is a matter of current debate (Harder et al, 1998;Fitzenberger et al, 2000;Saiz-Lopez and von Glasow, 2012;Volkamer et al, 2015;Wang et al, 2015;Schmidt et al, 2016;Sherwen et al, 2016;Werner et al, 2017) and is of significant scientific interest due to its potential influence on tropospheric ozone chemistry (von Glasow et al, 2004) and thus radiative forcing (Sherwen et al, 2017). Reported tropospheric background profiles at polar latitudes include those by Fitzenberger et al (2000), who derive tropospheric BrO profiles above Kiruna (Sweden) from balloon measurements and conclude that tropospheric [BrO] amounting to 0.4-2.3 ppt was present, assuming a uniform distribution within the troposphere.…”

Global modeling of tropospheric iodine aerosol

“…This work uses GEOS‐Chem (http://www.geos-chem.org) version v10 at 4° × 5° resolution with a coupled halogen chemistry scheme as described and evaluated for present day [ Sherwen et al , ] and for the preindustrial [ Sherwen et al , ]. This incorporates previous halogen development in GEOS‐Chem [ Bell et al , ; Eastham et al , ; Parrella et al , ; Sherwen et al , ; Schmidt et al , ], with gas‐phase chemistry based on Jet Propulsion Laboratory/International Union of Pure and Applied Chemistry recommendations [ Sander et al , ; Atkinson et al , , , ] and heterogeneous chemistry from previous work [ Abbatt et al , ; Braban et al , ; Ammann et al , ; Sherwen et al , ].…”

“…To probe the possible changes of iodine aerosol between preindustrial and the present day, the model was also run with preindustrial emissions as described previously [ Parrella et al , ; Sherwen et al , ]. Anthropogenic emissions of O 3 precursors are switched off and natural emissions maintained at their present day values.…”

Scientific assessment of background ozone over the U.S.: Implications for air quality management

“…Bottom figure from NAB O 3 simulation described in Lin et al, 2017. DOI: https://doi.org/10.1525/elementa.309.f3 chemistry (Sherwen et al, 2017) or dry deposition (e.g., Val Martin et al, 2014)). Some of the studies in Table S1 have attempted to bias-correct USB or NAB O 3 estimates by simply assuming the bias is entirely due to USB O 3 (Lin et al, 2012b) or by assuming that the relative model contributions from individual sources are accurate such that USB O 3 is adjusted proportionally to its contribution to total simulated O 3 (Dolwick et al, 2015).…”