Parameterisation and impact of aerosol uptake of HO<sub>2</sub> on a global tropospheric model

Abstract

Abstract. HO 2 is an important atmospheric trace gas, whose sink to aerosol is poorly understood yet significant. Previous parameterisations of the rate of uptake have been limited by the lack of laboratory studies. This paper creates a parameterisation for γ HO 2 based on the available laboratory studies. The calculated global mean γ HO 2 is 0.028, significantly lower than previous work (0.2). Modelled concentrations of HO 2 show significant regional sensitivity to the value of γ HO 2 (up to +106 % at the surface with the parameterisation of γ HO 2 in this work as compared with a value of 0.2), but global sensitivity is small (+3.2 %). The modelled response in O 3 is also highly regional, being up to +27 % at the surface over China, and only +0.3 % globally (with the parameterisation of γ HO 2 in this work as compared with a value of 0.2). The impact of γ HO 2 on sulfate is more complex, with up to +16 % over China and −5 % over high latitudes, resulting in a global change of +1.2 % (with the parameterisation of γ HO 2 in this work as compared with a value of 0.2). Uncertainty in the reaction mechanism and hence products (previously assumed to be H 2 O 2 ) impacts the processing of sulfur and hence aerosol loads. Further laboratory studies are desirable to constrain the rate of reaction and to elucidate the reaction mechanism and products.

Heterogeneous reaction of HO<sub>2</sub> with airborne TiO<sub>2</sub> particles and its implication for climate change mitigation strategies

“…At RH relevant to the lower stratosphere (<40%) the measurements showed that γ (HO 2 ) is in the range 0.020-0.028 at 295 K. An inverse temperature dependence of γ (HO 2 ) onto dry seasalt aerosols has previously been observed (Remorov et al, 2002); although there have been no systematic experimental studies of the temperature dependence of γ (HO 2 ), parameterisations have developed (Thornton et al, 2008;Macintyre and Evans, 2011). At stratospherically relevant temperatures (T = 200-220 K), γ (HO 2 ) is likely to be considerably larger than observed at 295 K; however it is not possible to cool the aerosol flow tube/SMPS system to verify this experimentally.…”

The influence of clouds on radical concentrations: observations and modelling studies of HO&lt;sub&gt;x&lt;/sub&gt; during the Hill Cap Cloud Thuringia (HCCT) campaign in 2010

“…To investigate both the impact of the uptake and whether H 2 O 2 is produced, three simulations are run, (i) with no cloud uptake of HO 2 , (ii) with cloud uptake (an assumed pH of 5) of HO 2 using the Thornton mechanism to produce H 2 O 2 , and (iii) with cloud uptake (assumed pH of 5) of HO 2 to produce H 2 O. All simulations include HO 2 uptake onto aerosol with γ HO 2 of 0.2, which is the standard value used in GEOS-Chem (Martin et al, 2003;Macintyre and Evans, 2011). Figure 10 shows the annual fractional change in surface HO 2 , OH, H 2 O 2 and O 3 concentrations with cloud uptake switched on, and with H 2 O 2 being either produced or not.…”

“…The general consensus, until recently, was that these reactions would ultimately produce H 2 O 2 (Jacob, 1996), but the significance of the reactions depends critically on whether this is the case or whether, instead, H 2 O is produced (Macintyre and Evans, 2011). This is significant, as H 2 O 2 can photolyse to return odd hydrogen (HO x = OH + HO 2 ) to the gas phase, whilst cloud uptake of HO 2 to form H 2 O provides a terminal sink for HO x .…”

Radical chemistry at night: comparisons between observed and modelled HO&lt;sub&gt;x&lt;/sub&gt;, NO&lt;sub&gt;3&lt;/sub&gt; and N&lt;sub&gt;2&lt;/sub&gt;O&lt;sub&gt;5&lt;/sub&gt; during the RONOCO project

“…For HO 2 , γ HO 2 = 0.028 is used based on the mean value reported by the parameterisation by Macintyre and Evans (2011). For NO 3 , a value of γ NO 3 = 0.001 is used.…”