Measurements of HO₂ uptake to aqueous aerosol: Mass accommodation coefficients and net reactive loss

Abstract

[1] We present room temperature measurements of HO 2 uptake to aqueous submicron aerosol obtained with an entrained aerosol flow tube coupled to a chemical ionization mass spectrometer. Two aqueous aerosol types were examined, H 2 SO 4 at 35-40% relative humidity (RH) and (NH 4 ) 2 SO 4 at 40-45% RH. By doping the aerosol with Cu(II) ($0.1 M in aerosol) to create an efficient aerosol sink, we determine lower limits to the mass accommodation coefficient, a HO2 , to be 0.8 ± 0.3 for H 2 SO 4 and 0.5 ± 0.1 for (NH 4 ) 2 SO 4 particles. In the absence of Cu(II), net reactive loss of HO 2 on H 2 SO 4 aerosol was slow, and we measured a reaction probability, g HO2 , less than 0.01. In contrast, loss of HO 2 to (NH 4 ) 2 SO 4 aerosol, buffered to pH = 5.1 but without Cu(II), was efficient in our experiment. For this system, aerosol-induced loss of gas-phase HO 2 was observed to follow second-order kinetics, and we infer from our measurements a second-order aqueous phase reaction rate coefficient of 1 ± 0.25 Â 10 7 M À1 s À1 in good agreement with an estimate of 2 ± 1 Â 10 7 M À1 s À1 based on literature values of HO 2 aqueous-phase chemical parameters. These results imply that heterogeneous loss of HO 2 to submicron aqueous sulfate aerosol will be strongly temperature dependent with negligible contribution to odd hydrogen radical (HO x ) removal rates at temperatures warmer than $270 K, but potentially large contributions to HO x loss (e.g., >50% of total HO x loss) in colder regions of the troposphere depending on the available aqueous aerosol volume.Citation: Thornton, J., and J. P. D. Abbatt (2005), Measurements of HO 2 uptake to aqueous aerosol: Mass accommodation coefficients and net reactive loss,

An experimental technique for the direct measurement of N<sub>2</sub>O<sub>5</sub> reactivity on ambient particles

“…Future experiments using the entrained aerosol flow reactor can and will likely include: i) measurement of the gas-phase reactivity of NO 3 to fully investigate nocturnal nitrogen processing, ii) determination of the ClNO 2 yield following uptake of N 2 O 5 on chloride containing particles (McNeill, et al, 2006;Thornton and Abbatt, 2005b), and iii) investigation of the reactivity of other trace gases of tropospheric significance (e.g., HO 2 , HNO 3 , HOBr) on ambient particles (Thornton and Abbatt, 2005a).…”

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

“…These values are not consistent and range from > 0.1 to < 0.01 (Cooper and Abbatt, 1996;Thornton and Abbatt, 2005;Hanson et al, 1992;Gershenzon et al, 1995). The most recent measurement of γ (HO 2 ) on aqueous-phase sulfuric acid aerosols was conducted at 35 % RH (Thornton and Abbatt, 2005).…”

“…O − 2 can form on surfaces via direct surface-oxygen electron transfer, photo-induced electron transfer, surface intermolecular electron transfer or decomposition of hydrogen peroxide (H 2 O 2 ) (Anpo et al, 1999). The dissociation of HO 2 to O − 2 occurs in bulk liquid (pK a = 4.7; Thornton and Abbatt, 2005); however, water surrounding the particles only begins to acquire liquidlike properties once the coverage of water is greater than ∼ 2 monolayers. If significant formation of O − 2 does occur above ∼ 2 monolayers of H 2 O, then, owing to its much higher reactivity, some deviation from linearity of γ (HO 2 ) versus monolayers of water (Fig.…”

Chemical and physical characteristics of aerosol particles at a remote coastal location, Mace Head, Ireland, during NAMBLEX

“…There are few available measurements of uptake coefficient of HO 2 to salt solutions typical of the marine boundary layer. Thornton and Abbatt (2005) measured reactive uptake coefficients of HO 2 to deliquesced sulphuric acid and ammonium sulphate particles and showed that the loss was not first order in HO 2 . They also estimate that at typical tropospheric concentrations of HO 2 overall reaction probabilities with the aerosol types at room temperature would be much less than 0.01.…”