Abstract
Abstract. Naturally emitted from the oceans, iodine compounds efficiently destroy atmospheric ozone and reduce its positive radiative forcing effects in the troposphere. Emissions of inorganic iodine have been experimentally shown to depend on the deposition to the oceans of tropospheric ozone, whose concentrations have significantly increased since 1850 as a result of human activities. A chemistry–climate model is used herein to quantify the current ocean emissions of inorganic iodine and assess the impact that the anthropogenic increase in tropospheric ozone has had on the natural cycle of iodine in the marine environment since pre-industrial times. Our results indicate that the human-driven enhancement of tropospheric ozone has doubled the oceanic inorganic iodine emissions following the reaction of ozone with iodide at the sea surface. The consequent build-up of atmospheric iodine, with maximum enhancements of up to 70% with respect to pre-industrial times in continental pollution outflow regions, has in turn accelerated the ozone chemical loss over the oceans with strong spatial patterns. We suggest that this ocean–atmosphere interaction represents a negative geochemical feedback loop by which current ocean emissions of iodine act as a natural buffer for ozone pollution and its radiative forcing in the global marine environment.
Highlights
Tropospheric ozone (O3) is a short-lived greenhouse gas (GHG) with a positive radiative forcing (RF) of 0.4 W m−2 (0.2–0.6 W m−2) (Myhre et al, 2013)
Given that anthropogenic activities have led to an increase of 20–55 % in tropospheric ozone since 1850 (Myhre et al, 2013) and that the current halogen-mediated troposphericozone loss in the tropical regions accounts for −0.1 W m−2 of the radiative flux at the tropical tropopause (i.e. ∼ 1/3 of the total tropospheric O3 RF; Saiz-Lopez et al, 2012b), this study aims at (i) assessing how the anthropogenic increase in tropospheric ozone has affected the abiotic oceanic emission of iodine source gases (ISG) and (ii) describing a geochemical feedback mechanism between ozone and iodine that mitigates the positive radiative forcing of tropospheric ozone in the global marine environment
In agreement with observations (Myhre et al, 2013), our simulations indicate that anthropogenic activities since 1850 have caused a mean ozone increase of 40 % in the marine boundary layer (MBL)
Summary
Tropospheric ozone (O3) is a short-lived greenhouse gas (GHG) with a positive radiative forcing (RF) of 0.4 W m−2 (0.2–0.6 W m−2) (Myhre et al, 2013). Laboratory studies have demonstrated the potential of the ocean to emit inorganic hypoiodous acid (HOI) and, to a lesser extent, molecular iodine (I2) following the reaction of ozone with iodide at the sea surface (Carpenter et al, 2013; MacDonald et al, 2014). The oceanic emission of inorganic iodine source gases (ISG; i.e. HOI, I2) has been experimentally shown to depend on the deposition of tropospheric ozone to the oceans (estimated to be in the range of 200–350 Tg O3 yr−1; Ganzeveld et al, 2009), wind speed and sea surface temperature (SST) (Garland et al, 1980; Carpenter et al, 2013; MacDonald et al, 2014)
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