Mass accommodation coefficient for HO₂ radicals on aqueous particles

Abstract

The rate of reaction of gas phase HO2 radicals with a monodisperse, submicrometer aerosol was measured in a flow tube reactor at atmospheric pressure. At the relative humidity of the experiments (75%), the aerosol consisted of concentrated solution droplets of either LiNO3 or NH4HSO4. When the aerosol contained a sufficient amount of Cu(II) ions, reaction of HO2 with the aerosol was observed. The mass accommodation coefficient for HO2 on aqueous particles was determined to be greater than 0.2. This implies that in clouds HO2 mass transport will be limited by gas-phase diffusion and HO2 will be in equilibrium at the gas-liquid interface. Reactions of HO2 with Cu(I) and Cu(II) ions in submicrometer aerosols may have a significant role in converting atmospheric odd hydrogen radicals into H202.

Measurements of uptake coefficients for heterogeneous loss of HO2 onto submicron inorganic salt aerosols

“…Mozurkewich et al 23 observed highly efficient uptake of HO 2 onto Cu(II)-doped aerosols with  HO2 >0.2 (the value depending on the Cu(II) molality), which has been confirmed in recent studies 25,26 .…”

“…However, HO 2 uptake coefficients as determined by previous laboratory studies at room temperature are inconsistent and span a wide range of values. Measured HO 2 reactive uptake coefficients reported in the literature are summarized in Table 1 23 conducted the first laboratory measurements of HO 2 uptake onto aqueous aerosol particles doped with Cu(II) ions to catalyze the oxidation of HO 2 , and hence its removal in the bulk phase. These studies enabled the determination of the mass accommodation coefficient (), or the probability that a collision of a gas molecule with the aerosol surface leads to its incorporation into the aerosol.…”

Uptake of HO<sub>2</sub> radicals onto Arizona test dust particles using an aerosol flow tube

“…where Kn is the Knudsen number defined by where D g is the gas phase diffusion constant of HO 2 (0.25 cm 2 s −1 ) at room temperature (Mozurkewich et al, 1987). The Brown correction (Brown, 1978) was then applied to the uptake coefficients, in order to take into account the laminar flow in the flow tube.…”

“…The impact on HO 2 concentrations due to uptake onto these particles is unknown due to a lack of laboratory data. Sea-salt aerosols could also contribute significantly to the difference between the models and observations, especially if they contain small amounts of transition metal ions (Fitzsimmons et al, 2013), such as copper, which has been shown in laboratory studies to increase the HO 2 uptake coefficient significantly (George et al, 2013;Mozurkewich et al, 1987;Taketani et al, 2008;Thornton and Abbatt, 2005). Uptake coefficients of N 2 O 5 to illite aerosols (γ = 0.04-0.09, dependent upon RH), which is one of the most abundant clay minerals in dust, were measured to be considerably larger than for uptake to ATD (γ ∼ 0.006, weak if any dependence upon RH) (Tang et al, 2014).…”

Simulation of trace gas redistribution by convective clouds - Liquid phase processes

“…In the measurements of the sticking coefficients of SO 2 (Gardner et al, 1987) and HO 2 (Mozurkewich et al, 1987) onto aqueous solutions, it has been found that the corresponding values of α i are slightly larger than 0.01. Wang and Crutzen (1995) indicated that under typical natural atmospheric conditions the uptake of trace gases into cloud or rain droplets depends only weakly on the values of sticking coefficient because of the limitations on diffusive transfer.…”