Iodine's impact on tropospheric oxidants: a global model study in GEOS-Chem

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 parametrised heterogeneous reactions. In comparisons with recent Iodine Oxide (IO) observations the iodine simulation shows an average bias of ~+66 % available surface observations in the marine boundary layer (outside of polar regions), and of ~+73 % within the free troposphere (350 < hPa < 900) 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 rate (748 Tg OX yr−1) is by photolysis of HOI (78 %), photolysis of OIO (21 %), and reaction of 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 parameters and conclude that the simulation is sensitive to choices in parameterisation 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. This is a significant impact and so halogen chemistry needs to be considered in climate and air quality models.

Modeling the observed tropospheric BrO background: Importance of multiphase chemistry and implications for ozone, OH, and mercury

“…Iodine drove a 750 Tg a −1 increase in the O x loss rate and had the greatest proportional effect in the MBL and upper troposphere. Sherwen et al [] used the bromine simulation detailed in Parrella et al [] which results in smaller decrease in tropospheric ozone burden (6.5%) than the 14% from chlorine and bromine presented here. The increased BrO concentrations calculated in this work may impact the Sherwen et al [] simulation through the coupling reaction BrO + IO.…”

Atmospheric ethanol in London and the potential impacts of future fuel formulations

“…To investigate the wider impacts of using ethanol blended gasoline, a regional version of the GEOS-Chem chemical transport model 31 (v10, www.geos-chem.org) was run at at 0. period discarded as 'spin up'.…”

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

“…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 .…”

“…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).…”