Coupling of HO<sub>x</sub>, NO<sub>x</sub> and halogen chemistry in the antarctic boundary layer

Abstract

A modelling study of radical chemistry in the coastal Antarctic boundary layer, based upon observations performed in the course of the CHABLIS (Chemistry of the Antarctic Boundary Layer and the Interface with Snow) campaign at Halley Research Station in coastal Antarctica during the austral summer 2004/2005, is described: a detailed zero-dimensional photochemical box model was used, employing inorganic and organic reaction schemes drawn from the Master Chemical Mechanism, with additional halogen (iodine and bromine) reactions added. The model was constrained to observations of long-lived chemical species, measured photolysis frequencies and meteorological parameters, and the simulated levels of HO_x, NO_x and XO compared with those observed. The model was able to replicate the mean levels and diurnal variation in the halogen oxides IO and BrO, and to reproduce NO_x levels and speciation very well. The NO_x source term implemented compared well with that directly measured in the course of the CHABLIS experiments. The model systematically overestimated OH and HO₂ levels, likely a consequence of the combined effects of (a) estimated physical parameters and (b) uncertainties within the halogen, particularly iodine, chemical scheme. The principal sources of HO_x radicals were the photolysis and bromine-initiated oxidation of HCHO, together with O(¹D) + H₂O. The main sinks for HO_x were peroxy radical self- and cross-reactions, with the sum of all halogen-mediated HO_x loss processes accounting for 40% of the total sink. Reactions with the halogen monoxides dominated CH₃O₂-HO₂-OH interconversion, with associated local chemical ozone destruction in place of the ozone production which is associated with radical cycling driven by the analogous NO reactions. The analysis highlights the need for observations of physical parameters such as aerosol surface area and boundary layer structure to constrain such calculations, and the dependence of simulated radical levels and ozone loss rates upon a number of uncertain kinetic and photochemical parameters for iodine species

Tropospheric ozone and its precursors from the urban to the global scale from air quality to short-lived climate forcer

“…Many questions remain on the mechanism of release of chlorine/bromine species from the snowpack (see Abbatt et al, 2012, for a detailed review). An important issue is whether it can explain observed HO x and NO x levels (SaizLopez et al, 2008;Bloss et al, 2010), as well as observed halogens levels. A modelling study by Thomas et al (2011Thomas et al ( , 2012 indicated that up to 10 ppt of tropospheric BrO can be explained by a mechanism involving nitrate formation in the interstitial snow; if this is the case, the resulting formation of NO x may compensate for the depletion of O 3 due to reactive Br, possibly leading to net ozone formation.…”

Impacts of bromine and iodine chemistry on tropospheric OH and HO<sub>2</sub>: comparing observations with box and global model perspectives

“…In this work we use a chemistry scheme based on a subsection of the hydrocarbons (ethane, propane, iso-butane, n-butane, iso-pentane, n-pentane, hexane, ethene, propene, 1-butene, acetylene, isoprene, toluene, benzene, methanol, acetone, acetaldehyde and DMS) available from the Master Chemical Mechanism version 3.2 (MCM v3.2 http://mcm.leeds.ac.uk/MCM/home.htt) Saunders et al, 2003), with a halogen chemistry scheme described by Saiz-Lopez et al (2006), Whalley et al (2010) and Edwards et al (2011). We also include the reaction between OH and CH 3 O 2 (Bossolasco et al, 2014;Fittschen et al, 2014;Assaf et al, 2016;Yan et al, 2016), with a rate coefficient of 1.6 × 10 −10 cm 3 s −1 (Assaf et al, 2016) and products HO 2 + CH 3 O (Assaf et al, 2017), the impact of which on the HO 2 : OH ratio and CH 3 O 2 budget is described in the Supplement. The total number of species in the model is ∼ 1200, with ∼ 5000 reactions.…”

“…Emissions for CH 3 I follow Bell et al (2002), while those of other organic iodine species use parameterisations based on chlorophyll a in the tropics and constant oceanic and coastal fluxes in extratropical regions (Ordonez et al, 2012). Emissions of inorganic iodine species (HOI and I 2 ) use the results of Carpenter et al (2013), with oceanic iodide concentrations parameterised by MacDonald et al (2014). The iodine chemistry scheme includes 26 unimolecular and bimolecular reactions, 3 three-body reactions, 21 photolysis reactions and 7 heterogeneous reactions, using recommendations by Atkinson et al (2007) and Sander et al (2011) where available.…”

“…Observations of BrO and IO radicals within the MBL have demonstrated widespread impacts on atmospheric composition and chemistry (Alicke et al, 1999;Sander et al, 2003;Leser et al, 2003;Saiz-Lopez and Plane, 2004;SaizLopez et al, 2004SaizLopez et al, , 2006Peters et al, 2005;Whalley et al, 2007;Mahajan et al, 2010a;Commane et al, 2011;Dix et al, 2013;Gomez Martin et al, 2013;Le Breton et al, 2017), including significant effects on HO x concentrations and on the HO 2 : OH ratio in coastal and marine locations (Bloss et al, 2005a(Bloss et al, , 2010Sommariva et al, 2006;Kanaya et al, 2007;Whalley et al, 2010).…”

Observations of hydroxyl and peroxy radicals and the impact of BrO at Summit, Greenland in 2007 and 2008

“…suggested that halogen may be present at Summit, perturbing the HO x cycling and enhancing OH levels. In contrast, a later study at Halley Bay, Antarctica (75 • 35 S, 26 • 19 W) found average OH levels of 3.9 × 10 5 molec cm −3 in February with typical maximum (local noontime) levels of 7.9 × 10 5 molec cm −3 (Bloss et al, 2007(Bloss et al, , 2010. The OH levels at Halley Bay were slightly higher than measured at Palmer station in the same season of the year but significantly lower than observed at South Pole.…”