The chemistry of OH and HO<sub>2</sub> radicals in the boundary layer over the tropical Atlantic Ocean

Abstract

Abstract. Fluorescence Assay by Gas Expansion (FAGE) has been used to detect ambient levels of OH and HO2 radicals at the Cape Verde Atmospheric Observatory, located in the tropical Atlantic marine boundary layer, during May and June 2007. Midday radical concentrations were high, with maximum concentrations of 9 ×106 molecule cm−3 and 6×108 molecule cm−3 observed for OH and HO2, respectively. A box model incorporating the detailed Master Chemical Mechanism, extended to include halogen chemistry, heterogeneous loss processes and constrained by all available measurements including halogen and nitrogen oxides, has been used to assess the chemical and physical parameters controlling the radical chemistry. The model was able to reproduce the daytime radical concentrations to within the 1 σ measurement uncertainty of 20% during the latter half of the measurement period but significantly under-predicted [HO2] by 39% during the first half of the project. Sensitivity analyses demonstrate that elevated [HCHO] (~2 ppbv) on specific days during the early part of the project, which were much greater than the mean [HCHO] (328 pptv) used to constrain the model, could account for a large portion of the discrepancy between modelled and measured [HO2] at this time. IO and BrO, although present only at a few pptv, constituted ~19% of the instantaneous sinks for HO2, whilst aerosol uptake and surface deposition to the ocean accounted for a further 23% of the HO2 loss at noon. Photolysis of HOI and HOBr accounted for ~13% of the instantaneous OH formation. Taking into account that halogen oxides increase the oxidation of NOx (NO → NO2), and in turn reduce the rate of formation of OH from the reaction of HO2 with NO, OH concentrations were estimated to be 9% higher overall due to the presence of halogens. The increase in modelled OH from halogen chemistry gives an estimated 9% shorter lifetime for methane in this region, and the inclusion of halogen chemistry is necessary to model the observed daily cycle of O3 destruction that is observed at the surface. Due to surface losses, we hypothesise that HO2 concentrations increase with height and therefore contribute a larger fraction of the O3 destruction than at the surface.

Understanding in situ ozone production in the summertime through radical observations and modelling studies during the Clean air for London project (ClearfLo)

“…Further details on the instrument for OH and HO2 detection can be found in Whalley et al (2010) with only an outline of the specific set-up and running conditions during ClearfLo described here. The radical 20 measurements were made from a 20 ft air-conditioned shipping container which had been converted into a mobile laboratory.…”

“…The primary source of OH from the photolysis of ozone and subsequent reaction of the excited state oxygen atom 15 with H2O, which is often considered the dominant radical source in many other environments (e.g in the remote marine atmosphere (Whalley et al, 2010)) tends only to play a minor role in urban centres, with this source accounting for < 6% of the total radical sources during MCMA-2006(Dusanter et al, 2009) which took place in Mexico City.…”

Impacts of bromine and iodine chemistry on tropospheric OH and HO&lt;sub&gt;2&lt;/sub&gt;: 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.…”

“…In general, observationally constrained box model simulations suggest that halogens in the troposphere will increase OH concentrations, primarily because of a change in the HO 2 to OH ratio occurring as a result of reactions of halogen oxides (XO) with HO 2 to produce a hypohalous acid (HOX) which photolyses to give an OH radical and a halogen atom (Kanaya et al, 2002(Kanaya et al, , 2007Bloss et al, 2005a;Sommariva et al, 2006Sommariva et al, , 2007Whalley et al, 2010). Other impacts on the HO x photochemical system are observed (impacts from changes to NO x chemistry etc.…”

“…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).…”