Abstract
Abstract. Despite the potential influence of iodine chemistry on the oxidizing capacity of the troposphere, reactive iodine distributions and their impact on tropospheric ozone remain almost unexplored aspects of the global atmosphere. Here we present a comprehensive global modelling experiment aimed at estimating lower and upper limits of the inorganic iodine burden and its impact on tropospheric ozone. Two sets of simulations without and with the photolysis of IxOy oxides (i.e. I2O2, I2O3 and I2O4) were conducted to define the range of inorganic iodine loading, partitioning and impact in the troposphere. Our results show that the most abundant daytime iodine species throughout the middle to upper troposphere is atomic iodine, with an annual average tropical abundance of (0.15–0.55) pptv. We propose the existence of a "tropical ring of atomic iodine" that peaks in the tropical upper troposphere (~11–14 km) at the equator and extends to the sub-tropics (30° N–30° S). Annual average daytime I / IO ratios larger than 3 are modelled within the tropics, reaching ratios up to ~20 during vigorous uplift events within strong convective regions. We calculate that the integrated contribution of catalytic iodine reactions to the total rate of tropospheric ozone loss (IOx Loss) is 2–5 times larger than the combined bromine and chlorine cycles. When IxOy photolysis is included, IOx Loss represents an upper limit of approximately 27, 14 and 27% of the tropical annual ozone loss for the marine boundary layer (MBL), free troposphere (FT) and upper troposphere (UT), respectively, while the lower limit throughout the tropical troposphere is ~9%. Our results indicate that iodine is the second strongest ozone-depleting family throughout the global marine UT and in the tropical MBL. We suggest that (i) iodine sources and its chemistry need to be included in global tropospheric chemistry models, (ii) experimental programs designed to quantify the iodine budget in the troposphere should include a strategy for the measurement of atomic I, and (iii) laboratory programs are needed to characterize the photochemistry of higher iodine oxides to determine their atmospheric fate since they can potentially dominate halogen-catalysed ozone destruction in the troposphere.
Highlights
The oceans provide the main source of iodine to the atmosphere
Three different vertical regions on consecutive non-overlapping altitude intervals were defined within the tropics (20◦ N–20◦ S) and mid-latitudes (50–20◦ N and 20–50◦ S): marine boundary layer (MBL), expanding from the ocean surface up to ∼900 m a.s.l (∼900 hPa); the free troposphere (FT), from ∼900 m (900 hPa) to ∼8.5 km (350 hPa); and the upper troposphere (UT) from ∼8.5 km (350 hPa) up to the model tropopause
We suggest that the combined release of I atoms from CH3I photolysis and the photolytic recycling of gaseous IxOy within the JIxOy scheme can account for the increase in IOx lifetime required to reconcile our current understanding of iodine chemistry to recent field measurements throughout the mid- to upper troposphere (Fig. 3a, see Sect. 3.4)
Summary
The oceans provide the main source of iodine to the atmosphere. Methyl iodide (CH3I) and other very short-lived (VSL) iodocarbons (e.g. CH2I2, C2H5I, C3H7I, CH2ICl, CH2IBr) are produced by biotic and photochemical processes, and released to the atmosphere from supersaturated ocean waters (Carpenter et al, 2012; Saiz-Lopez et al, 2012a). Laboratory studies have established the gaseous emission of molecular iodine (I2) following the reaction of aqueous iodide with atmospheric ozone at the sea surface (Garland and Curtis, 1981; Sakamoto et al, 2009; Hayase et al, 2010). Several modelling studies and analysis of experimental data have suggested that the HOI / I2 additional inorganic source must surpass the emission strength of organic VSL iodocarbons in order to reproduce observed iodine monoxide (IO) measure-. INO2 → I + NO2 IONO2 → IO + NO2 INO + INO → I2 + 2NO INO2 + INO2 → I2 + 2NO2 OIO + NO → IO + NO2 HI + NO3 → I + HNO3 IO + BrO → Br + I + O2 IO + BrO → Br + OIO
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