Tropospheric OH and HO2 radicals: field measurements and model comparisons

Abstract

The hydroxyl radical, OH, initiates the removal of the majority of trace gases in the atmosphere, and together with the closely coupled species, the hydroperoxy radical, HO(2), is intimately involved in the oxidation chemistry of the atmosphere. This critical review discusses field measurements of local concentrations of OH and HO(2) radicals in the troposphere, and in particular the comparisons that have been made with numerical model calculations containing a detailed chemical mechanism. The level of agreement between field measurements of OH and HO(2) concentrations and model calculations for a given location provides an indication of the degree of understanding of the underlying oxidation chemistry. We review the measurement-model comparisons for a range of different environments sampled from the ground and from aircraft, including the marine boundary layer, continental low-NO(x) regions influenced by biogenic emissions, the polluted urban boundary layer, and polar regions. Although good agreement is found for some environments, there are significant discrepancies which remain unexplained, a notable example being unpolluted, forested regions. OH and HO(2) radicals are difficult species to measure in the troposphere, and we also review changes in detection methodology, quality assurance procedures such as instrument intercomparisons, and potential interferences.

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

“…This box modelling study is consistent with previous studies (Kanaya et al, 2002(Kanaya et al, , 2007Bloss et al, 2005a;Sommariva et al, 2007;Whalley et al, 2010;Mahajan et al, 2010a;Stone et al, 2012) in that it implies that halogen chemistry is likely to increase the OH concentration of the marine boundary layer (and potentially other regions of the troposphere) as it enhances the HO 2 to OH conversion through the production of HOBr and HOI. We now look at the impact of halogen chemistry on the concentrations of OH and HO 2 at the Cape Verde Atmospheric Observatory within the framework of a global atmospheric chemistry model.…”

Potential Role of Stabilized Criegee Radicals in Sulfuric Acid Production in a High Biogenic VOC Environment

“…In general, the observed OH levels for this study are comparable with previously reported OH levels from other moderately polluted environments. 39 The observed daytime H 2 SO 4 maximum was ∼7 × 10 6 molecules cm −3 . This is much lower than the reported values (∼2 × 10 7 molecules cm −3 ) for airborne observations over the boundary layer of Northeastern U.S. and the Ohio Valley region during NEAQS-2004.…”

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

“…For example, Bigi and Harrison (2010) report a steady increase in ozone between 1996-2008 in North Kensington; an urban background site in London. 20 To implement efficient reduction strategies for ozone, a detailed understanding of the factors controlling free radicals is critical since the reaction of HO2 and RO2 radicals with NO, forming NO2, followed by the subsequent photolysis of NO2 represents the only net formation pathway to tropospheric ozone: Measurements of radicals have been made at various urban and sub-urban locations worldwide, both during the summer and winter (Stone et al (2012) and references therein). Observations of OH and HO2 in the urban atmosphere have primarily been 5 made using fluorescence assay by gas expansion (FAGE), and comparisons with predicted radical concentrations using chemistry box models constrained with co-located radical precursor measurements have revealed varying levels of success in replicating observations.…”