Abstract
Abstract. We present a global simulation of tropospheric iodine chemistry within the GEOS-Chem chemical transport model. This includes organic and inorganic iodine sources, standard gas-phase iodine chemistry, and simplified higher iodine oxide (I2OX, X = 2, 3, 4) chemistry, photolysis, deposition, and parametrized heterogeneous reactions. In comparisons with recent iodine oxide (IO) observations, the simulation shows an average bias of ∼ +90 % with available surface observations in the marine boundary layer (outside of polar regions), and of ∼ +73 % within the free troposphere (350 hPa < p < 900 hPa) over the eastern Pacific. Iodine emissions (3.8 Tg yr−1) are overwhelmingly dominated by the inorganic ocean source, with 76 % of this emission from hypoiodous acid (HOI). HOI is also found to be the dominant iodine species in terms of global tropospheric IY burden (contributing up to 70 %). The iodine chemistry leads to a significant global tropospheric O3 burden decrease (9.0 %) compared to standard GEOS-Chem (v9-2). The iodine-driven OX loss rate1 (748 Tg OX yr−1) is due to photolysis of HOI (78 %), photolysis of OIO (21 %), and reaction between IO and BrO (1 %). Increases in global mean OH concentrations (1.8 %) by increased conversion of hydroperoxy radicals exceeds the decrease in OH primary production from the reduced O3 concentration. We perform sensitivity studies on a range of parameters and conclude that the simulation is sensitive to choices in parametrization of heterogeneous uptake, ocean surface iodide, and I2OX (X = 2, 3, 4) photolysis. The new iodine chemistry combines with previously implemented bromine chemistry to yield a total bromine- and iodine-driven tropospheric O3 burden decrease of 14.4 % compared to a simulation without iodine and bromine chemistry in the model, and a small increase in OH (1.8 %). This is a significant impact and so halogen chemistry needs to be considered in both climate and air quality models. 1 Here OX is defined as O3 + NO2 + 2NO3 + PAN + PMN+PPN + HNO4 + 3N2O5 + HNO3 + BrO + HOBr + BrNO2+2BrNO3 + MPN + IO + HOI + INO2 + 2INO3 + 2OIO+2I2O2 + 3I2O3 + 4I2O4, where PAN = peroxyacetyl nitrate, PPN = peroxypropionyl nitrate, MPN = methyl peroxy nitrate, and MPN = peroxymethacryloyl nitrate.
Highlights
The chemistry of the troposphere controls the concentration of a range of climate gases including ozone (O3) and methane (CH4)
In order to explore our current understanding of the tropospheric chemistry of iodine we present a global modelling study of tropospheric iodine chemistry, using the GEOSChem chemical transport model
In line with previous studies (McFiggans et al, 2000), we consider that the uptake of hypoiodous acid (HOI), INO2, and INO3 leads to the recycling of iodine back into the gas phase as 1/2I2 on sea-salt aerosol alone, whereas irreversible loss via uptake of HI leads to the generation of aerosol phase iodine
Summary
Iodine chemistry has been evaluated by a number of box model studies (Sander et al, 1997; Mahajan et al, 2009; McFiggans et al, 2000, 2010; Read et al, 2008; Saiz-Lopez et al, 2007) and a few global model studies (Prados-Roman et al, 2015a; Saiz-Lopez et al, 2012a, 2014). The role of reactive halogens have been investigated in global chemical transport models (Parrella et al, 2012) and chemistry-climate models (Ordóñez et al, 2012; Saiz-Lopez et al, 2014). Iodine, and chlorine chemistry into a global model led to significant changes in the composition troposphere.
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