Investigation of HO2 uptake onto Cu(ii)- and Fe(ii)-doped aqueous inorganic aerosols and seawater aerosols using laser spectroscopic techniques

Abstract

Heterogeneous HO₂ uptake onto aerosols, which affects tropospheric chemistry, is known as a potential sink of HO_x (OH + HO₂+RO₂); however, the associated reaction kinetics are still unclear. Also, the measurement of the HO₂ uptake coefficient (γ) remains challenging and the γ values reported so far based on laboratory studies vary by orders of magnitude depending on the aerosol properties, initial HO₂ concentration, and the experimental conditions. Here, we established a novel approach for detecting the loss rate of HO₂ uptake by inorganic aerosols derived from NaCl, (NH₄)₂SO₄, Na₂SO₄, or diluted seawater using laser-pump and laser-induced fluorescence techniques and clarified the effects of transition metal ions (TMIs), Cu(II) and Fe(II) on the process. Our results confirmed the enhancement effects of Cu(II) and Fe(II) on the uptake process. Furthermore, soluble Cu(II) enhanced the uptake with a kinetic reaction rate coefficient two orders of magnitude higher than that of Fe(II) ions. The second order rate constants of bulk phase reactions for Cu(II)- and Fe(II)-doped (NH₄)₂SO₄ are 1.5 × 10⁷ M⁻¹ s⁻¹ and 1.3 × 10⁵ M⁻¹ s⁻¹, respectively. However, uncertainties regarding the pH of the aerosols and TMI participation in the aerosol phase still exist. Notwithstanding, the uptake coefficients here reported will complement current IUPAC recommendations. Additionally, future studies can benefit from investigations of the effects of aerosol pH on the HO₂ uptake process, the determination of uptake onto other TMI-doped aerosol types, and testing with ambient aerosols. Our findings can also be incorporated into multiphase kinetic models for a better understanding of the bulk and surface processes and for the evaluation of spatiotemporal oxidation products under ambient atmospheric conditions.