Estimating the climate significance of halogen-driven ozone loss in the tropical marine troposphere

Abstract

Abstract.We have integrated observations of tropospheric ozone, very short-lived (VSL) halocarbons and reactive iodine and bromine species from a wide variety of tropical data sources with the global CAM-Chem chemistry-climate model and offline radiative transfer calculations to compute the contribution of halogen chemistry to ozone loss and associated radiative impact in the tropical marine troposphere. The inclusion of tropospheric halogen chemistry in CAM-Chem leads to an annually averaged depletion of around 10 % (∼2.5 Dobson units) of the tropical tropospheric ozone column, with largest effects in the middle to upper troposphere. This depletion contributes approximately −0.10 W m −2 to the radiative flux at the tropical tropopause. This negative flux is of similar magnitude to the ∼0.33 W m −2 contribution of tropospheric ozone to presentday radiative balance as recently estimated from satellite observations. We find that the implementation of oceanic halogen sources and chemistry in climate models is an important component of the natural background ozone budget and we suggest that it needs to be considered when estimating both preindustrial ozone baseline levels and long term changes in tropospheric ozone.

Detection of iodine monoxide in the tropical free troposphere

“…Tropospheric ozone is a greenhouse gas and the primary source for OH radicals, which are an important sink for methane in the tropical atmosphere (38). Atmospheric models estimate that halogen-mediated ozone loss could deplete the tropospheric ozone column by 10% per year (18). These estimates are as of yet unconstrained by halogen radical observations in the FT and predict ∼0.02 ppt IO in the upper troposphere over the tropical Atlantic (18), which is a factor of five less IO than the upper limit reported for this region (28) and the FT-IO mixing ratio that we find over the Central Pacific ocean.…”

“…Atmospheric models estimate that halogen-mediated ozone loss could deplete the tropospheric ozone column by 10% per year (18). These estimates are as of yet unconstrained by halogen radical observations in the FT and predict ∼0.02 ppt IO in the upper troposphere over the tropical Atlantic (18), which is a factor of five less IO than the upper limit reported for this region (28) and the FT-IO mixing ratio that we find over the Central Pacific ocean. The possibility of iodine recycling from aerosols could further extend the effective iodine lifetime and add an additional ozone loss pathway that is not yet considered in atmospheric models.…”

“…Distributions of chlorophyll-a (Chl-a) in the surface ocean are currently being used to scale very short-lived marine iodocarbon emission (16,18). Positive correlations between satellite IO and Aqua/Moderate Resolution Imaging Spectroradiometer (MODIS) satellite-derived Chl-a (SI Text and Figs.…”

“…Furthermore, the relevance of halogen chemistry for ozone loss rates is assessed using atmospheric models that cannot be validated because of the lack of halogen radical observations in the tropical free troposphere (FT). First model estimates suggest that the combined effect of iodine and bromine species could lead to 10% depletion of tropospheric ozone per year (18); the largest impact of the halogen-driven ozone loss is expected in the middle to upper troposphere. To date, there are no aircraft measurements of IO.…”

Injection of iodine to the stratosphere

“…The most recent estimates indicate that between 2 and 8 parts per trillion by volume (pptv) of VSL Br are injected to the stratosphere [World Meteorological Organization (WMO), 2014]. In the case of iodine, the oceans provide the main source of iodine compounds to the atmosphere [Carpenter et al, 2013;Saiz-Lopez et al, 2012a] where they reduce the global warming effect of ozone in the marine troposphere [Saiz-Lopez et al, 2012b]. Recent studies have reported values of 0.2-0.4 pptv of iodine monoxide (IO) in the free troposphere over the subtropical station of Izaña in the Atlantic Ocean (Canary Islands) [Puentedura et al, 2012] and 0.1-0.2 pptv range throughout the tropical free troposphere of the Pacific Ocean , demonstrating the ubiquitous presence of the radical in the marine free troposphere.…”