Iodine chemistry in the troposphere and its effect on ozone

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 I x O y oxides (i.e. I 2 O 2 , I 2 O 3 and I 2 O 4 ) 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 (IO x Loss ) is 2-5 times larger than the combined bromine and chlorine cycles. When I x O y photolysis is included, IO x 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.

Global impacts of tropospheric halogens (Cl, Br, I) on oxidants and composition in GEOS-Chem

“…The major I y sink is HOI deposition, which represents 59 % of the deposi- from industrial areas flow over high predicted oceanic iodide concentrations and lead to increased oceanic inorganic iodine emissions. Within the vertical there is an average of ∼ 0.5-1 pmol mol −1 of I y , consistent with previous model studies ( Saiz-Lopez et al, 2014;Sherwen et al, 2016a). The lowest concentrations of I y are seen just above the marine boundary layer, where I y loss via wet deposition is most favourable due to partitioning towards water-soluble HOI.…”

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

“…For example, the partitioning between ICl and IBr emissions following uptake of condensable iodine compounds to sea-salt aerosol is not well known. Changing the ICl to IBr ratio from 0.85 : 0.15 (as used here and in Sherwen et al, 2016b) to a the IBr yield (0.5 : 0.5), as used in other studies (McFiggans et al, 2000;Saiz-Lopez et al, 2014), increases the reduction in the O 3 radiative from the 25 % found here to 34 %. There is also a question as to whether models have some aspects of this halogen chemistry "tuned" into them through enhanced O 3 deposition to the ocean surface, or other mechanisms for the present day.…”

A negative feedback between anthropogenic ozone pollution and enhanced ocean emissions of iodine

“…A summary of the model set-up and simulations used in this study are provided below, whereas a detailed description of the model as well as the iodine chemical scheme and reaction rates is described elsewhere Ordóñez et al, 2012;Saiz-Lopez et al, 2014).…”

“…odd oxygen, hydrogen, nitrogen, iodine, bromine, chlorine) (Brasseur and Solomon, 2005; see also Saiz-Lopez et al, 2014), we calculate that the industrialisation process has on average increased the rate of the total ozone chemical loss in the global MBL from 1.89 nmol mol −1 d −1 to 3.19 nmol mol −1 d −1 , mainly driven by changes in the abundance of odd oxygen, hydrogen and iodine. On a global annual average, 25 % of this enhanced ozone loss rate results from the human-driven boosting of inorganic iodine emissions that has accelerated iodine-mediated ozone destruction from 0.54 nmol mol −1 d −1 in pre-industrial times to 0.89 nmol mol −1 d −1 in the present day.…”

“…Note, however, that these values should be regarded as lower limits. Based on the recent study by Saiz-Lopez et al (2014), we investigated the effect that the photolysis of the higher oxides I 2 O x (x = 2, 3 or 4) -formed after the reactions of IO with itself (x = 2), of IO with OIO (x = 3) or of OIO with itself (x = 4) (Bloss et al, 2001;Spietz et al, 2005;Gómez Martín et al, 2005, 2007 -could have on the RF of tropospheric ozone. Our results indicate that if the photolysis of I 2 O x is considered, the current negative effect of the enhanced iodine-mediated ozone loss in the marine troposphere would mitigate the warming long-wave radiative effect of tropospheric ozone by up 20 % globally and by up to 40 % in the NH.…”