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We present a global simulation of tropospheric iodine chemistry within the GEOSChem 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 Tgyr-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 Iγ 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 TgOX 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. [ABSTRACT FROM AUTHOR]

Measurements of the radical species OH and HO2 were made using the Fluorescence Assay by Gas Expansion (FAGE) technique during a series of nighttime and daytime flights over the UK in summer 2010 and winter 2011. OH was not detected above the instrument's 1 o limit of detection during any of the nighttime flights or during the winter daytime flights, placing upper limits on [OH] of 1.8 x 106 molecule cm-3 and 6.4 x 105 molecule cm-3 for the summer and winter flights, respectively. HO2 reached a maximum concentration of 3.2 x 108 molecule cm-3 (13.6 pptv) during a nighttime flight on 20 July 2010, when the highest concentrations of NO3 and O3 were also recorded. Analysis of the rates of reaction of OH, O3, and the NO3 radical with measured alkenes indicates that the summer nighttime troposphere can be as important for the processing of VOCs as the winter daytime troposphere. Analysis of the instantaneous rate of production of HO2 from the reactions of O3 and NO3 with alkenes has shown that, on average, reactions of NO3 dominated nighttime production of HO2 during summer, and reactions of O3 dominated nighttime HO2 production during winter. [ABSTRACT FROM AUTHOR]

Reaction with the hydroxyl radical (OH) is the dominant removal mechanism for virtually all volatile organic compounds (VOCs) in the atmosphere, however it can be difficult to reconcile measured OH reactivity with known sinks. Unresolved higher molecular weight VOCs contribute to OH sinks, of which monoaromatics are potentially an important sub-class. A method based on comprehensive two-dimensional gas chromatography coupled to time of flight mass spectrometry (GCxGC-TOFMS) has been developed that extends the degree with which larger VOCs can be individually speciated from whole air samples (WAS). The technique showed excellent sensitivity, resolution and good agreement with an established GC-FID method, for compounds amenable to analysis on both instruments. Measurements have been made of VOCs within the UK east coast marine boundary layer and free troposphere, using samples collected from five aircraft flights in winter 2011. Ten monoaromatic compounds with an array of different alkyl ring substituents have been quantified, in addition to the simple aromatics, benzene, toluene, ethyl benzene and ∑m- and p-xylene. These additional compounds were then included into constrained box model simulations of atmospheric chemistry occurring at two UK rural and suburban field sites in order to assess the potential impact of these larger monoaromatics species on OH reactivity; they have been calculated to contribute an additional 2-6% to the overall modelled OH loss rate, providing a maximum additional OH sink of ~ 0.9 s-1. [ABSTRACT FROM AUTHOR]