Iodine oxide in the global marine boundary layer

Prados-Román, C.; Cuevas, Carlos A.; Hay, T.; Fernández, Rafael P.; Mahajan, Anoop S.; Royer, Sarah-Jeanne; Galí, Martí; Simó, Rafel; Dachs, Jordi; Großmann, Katja; Kinnison, Douglas E.; Lamarque, J. F.; Saiz‐Lopez, Alfonso

doi:10.5194/acp-15-583-2015

Cited by 96 publications

(132 citation statements)

References 28 publications

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“…A gas-phase reaction has been reported in chamber studies (Hall, 1995;Pal and Ariya, 2004;Sumner et al, 2005;Snider et al, 2008;Rutter et al, 2012), but this is likely to have been influenced by the walls of the chamber (as seen in Pal and and presence of secondary organic aerosols (in Rutter et al, 2012). j Atmospheric concentrations of I and IO (Dix et al, 2013;Prados-Roman et al, 2015;Volkamer et al, 2015) are too low for these reactions to be significant. k Rates are too slow to be of relevance.…”

Section: Chemical Mechanismmentioning

confidence: 99%

A new mechanism for atmospheric mercury redox chemistry: implications for the global mercury budget

et al. 2017

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Abstract. Mercury (Hg) is emitted to the atmosphere mainly as volatile elemental Hg 0 . Oxidation to water-soluble Hg II plays a major role in Hg deposition to ecosystems. Here, we implement a new mechanism for atmospheric Hg 0 / Hg II redox chemistry in the GEOS-Chem global model and examine the implications for the global atmospheric Hg budget and deposition patterns. Our simulation includes a new coupling of GEOS-Chem to an ocean general circulation model (MITgcm), enabling a global 3-D representation of atmosphereocean Hg 0 / Hg II cycling. We find that atomic bromine (Br) of marine organobromine origin is the main atmospheric Hg 0 oxidant and that second-stage HgBr oxidation is mainly by the NO 2 and HO 2 radicals. The resulting chemical lifetime of tropospheric Hg 0 against oxidation is 2.7 months, shorter than in previous models. Fast Hg II atmospheric reduction must occur in order to match the ∼ 6-month lifetime of Hg against deposition implied by the observed atmospheric variability of total gaseous mercury (TGM ≡ Hg 0 + Hg II (g)). We implement this reduction in GEOS-Chem as photolysis of aqueous-phase Hg II -organic complexes in aerosols and clouds, resulting in a TGM lifetime of 5.2 months against deposition and matching both mean observed TGM and its variability. Model sensitivity analysis shows that the interhemispheric gradient of TGM, previously used to infer a longer Hg lifetime against deposition, is misleading because Southern Hemisphere Hg mainly originates from oceanic emissions rather than transport from the Northern Hemisphere.The model reproduces the observed seasonal TGM variation at northern midlatitudes (maximum in February, minimum in September) driven by chemistry and oceanic evasion, but it does not reproduce the lack of seasonality observed at southern hemispheric marine sites. Aircraft observations in the lowermost stratosphere show a strong TGM-ozone relationship indicative of fast Hg 0 oxidation, but we show that this relationship provides only a weak test of Hg chemistry because it is also influenced by mixing. The model reproduces observed Hg wet deposition fluxes over North America, Europe, and China with little bias (0-30 %). It reproduces qualitatively the observed maximum in US deposition around the Gulf of Mexico, reflecting a combination of deep convection and availability of NO 2 and HO 2 radicals for second-stage HgBr oxidation. However, the magnitude of this maximum is underestimated. The relatively low observed Hg wet deposition over rural China is attributed to fast Hg II reduction in the presence of high organic aerosol concentrations. We find that 80 % of Hg II deposition is to the global oceans, reflecting the marine origin of Br and low concentrations of organic aerosols for Hg II reduction. Most of that deposition takes place to the tropical oceans due to the availability of HO 2 and NO 2 for second-stage HgBr oxidation.

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Section: Chemical Mechanismmentioning

confidence: 99%

A new mechanism for atmospheric mercury redox chemistry: implications for the global mercury budget

et al. 2017

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“…Models that were used to calculate radiative forcing of tropospheric O 3 (RF TO 3 ) in the past do not contain this halogen chemistry (Hauglustaine et al, 1994;Levy et al, 1997;Myhre et al, 2013;Young et al, 2013), raising concerns that they may have a systematic bias in their simulation of past, present, and future tropospheric O 3 and more widely the composition of the troposphere. As increased observations of tropospheric halogens are made (Dix et al, 2013;Gómez Martín et al, 2013;Mahajan et al, 2010Mahajan et al, , 2012Prados-Roman et al, 2015;Read et al, 2008;Volkamer et al, 2015;Wang et al, 2015) and models are used to understand them (Parrella et al, 2012;Saiz-Lopez et al, 2012a, 2014Schmidt et al, 2016;Sherwen et al, 2016a, b), these concerns grow.…”

Section: Introductionmentioning

confidence: 99%

“…The tropospheric impact of halogens in polar regions during springtime has been known for some time (Barrie et al, 1988;Jacob et al, 1992), but their significance for the global troposphere has only been evident in the last decade (Read et al, 2008;Saiz-Lopez et al, 2012a;Prados-Roman et al, 2015;Wang et al, 2015). Reviews of the appropriate processes are given elsewhere (Simpson et al, 2015).…”

Section: Introductionmentioning

confidence: 99%

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

et al. 2017

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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 %).

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“…Oceanic emissions of the inorganic gases I 2 and HOI, produced from ozonolysis of surface iodide (Carpenter et al 2013;MacDonald et al 2014), are believed to contribute~2-2.5 Tg I yr. −1 , around 80 % of global iodine emissions (Sherwen et al 2015;Prados-Roman et al 2015). However, in certain regions -including at high latitudes where surface ocean iodide levels are low, and in locations where wind speeds are high and surface ozone (O 3 (g)) mixing ratios are relatively low -organic gases may contribute 50 % or more of the iodine source .…”

Section: Introductionmentioning

confidence: 99%

“…The presence of IO establishes that active atmospheric iodine chemistry is occurring, driven by photochemical breakdown of iodine precursors. Iodine is a significant ozone-depleting agent in the MBL, being responsible for 20-30 % of the total ozone loss in that region Prados-Roman et al 2015;Sherwen et al 2015). Calculations from global chemistry transport models indicate that globally iodine provides an odd oxygen (Ox) loss of 12-16 % of the total, at around~720 Tg yr. −1 Sherwen et al 2015).…”

Section: Introductionmentioning

confidence: 99%

A nocturnal atmospheric loss of CH2I2 in the remote marine boundary layer

et al. 2015

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Ocean emissions of inorganic and organic iodine compounds drive the biogeochemical cycle of iodine and produce reactive ozone-destroying iodine radicals that influence the oxidizing capacity of the atmosphere. Di-iodomethane (CH 2 I 2 ) and chloro-iodomethane (CH 2 ICl) are the two most important organic iodine precursors in the marine boundary layer. Ship-borne measurements made during the TORERO (Tropical Ocean tRoposphere Exchange of Reactive halogens and Oxygenated VOC) field campaign in the east tropical Pacific Ocean in January/February 2012 revealed strong diurnal cycles of CH 2 I 2 and CH 2 ICl in air and of CH 2 I 2 in seawater. Both compounds are known to undergo rapid photolysis during the day, but models assume no night-time atmospheric losses. Surprisingly, the diurnal cycle of CH 2 I 2 was lower in amplitude than that of CH 2 ICl, despite its faster photolysis rate. We speculate that night-time loss of CH 2 I 2 occurs due to reaction with NO 3 radicals. Indirect results from a laboratory study under ambient atmospheric boundary layer conditions indicate a k CH2I2+NO3 of ≤4 × 10 −13 cm 3 molecule −1 s −1 ; a previous kinetic study carried out at ≤100 Torr found k CH2I2+NO3 of 4 × 10 −13 cm 3 molecule −1 s −1 . Using the 1-dimensional atmospheric THAMO model driven by sea-air fluxes calculated from the seawater and air measurements (averaging 1.8 +/− 0.8 nmol m −2 d −1 for CH 2 I 2 and 3.7 +/− 0.8 nmol m −2 d −1 for CH 2 ICl), we show that J Atmos Chem (2017)

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Iodine oxide in the global marine boundary layer

Cited by 96 publications

References 28 publications

A new mechanism for atmospheric mercury redox chemistry: implications for the global mercury budget

A new mechanism for atmospheric mercury redox chemistry: implications for the global mercury budget

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

A nocturnal atmospheric loss of CH2I2 in the remote marine boundary layer

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Iodine oxide in the global marine boundary layer

Cited by 96 publications

References 28 publications

A new mechanism for atmospheric mercury redox chemistry: implications for the global mercury budget

A new mechanism for atmospheric mercury redox chemistry: implications for the global mercury budget

Halogen chemistry reduces tropospheric O&lt;sub&gt;3&lt;/sub&gt; radiative forcing

A nocturnal atmospheric loss of CH2I2 in the remote marine boundary layer

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