Ionic Strength Effect Alters the Heterogeneous Ozone Oxidation of Methoxyphenols in Going from Cloud Droplets to Aerosol Deliquescent Particles

Abstract

Methoxyphenols are one of the most abundant classes of biomarker tracers for atmospheric wood smoke pollution. The reactions of atmospheric oxidants (ozone, OH) with methoxyphenols can contribute to the formation of secondary organic aerosols (SOA). Here, for the first time, we use the wellestablished vertical wetted wall flow tube (VWWFT) reactor to assess the effect of ionic strength (I), pH, temperature, and ozone concentration on the reaction kinetics of ozone with acetosyringone (ACS), as a representative methoxyphenol compound. At fixed pH 3, typical for acidic atmospheric deliquescent particles, and at I = 0.9 M adjusted by Na 2 SO 4 , the uptake coefficient (γ) of O 3 increases by 2 orders of magnitude from γ = (5.0 ± 0.8) × 10 −8 on neat salt solution (Na 2 SO 4 ) to γ = (6.0 ± 0.01) × 10 −6 on a mixture of ACS and Na 2 SO 4 . The comparison of the uptake coefficients of O 3 at different pH values indicates that the reaction kinetics strongly depends on the acidity of the phenolic group of ACS. The observed different reactivity of gas-phase ozone with ACS has implications for ozone uptake by the dilute aqueous phase of cloud droplets and by aerosol deliquescent particles loaded with inorganic salts, and it can affect the formation of SOA in the atmosphere.

Ionic Strength Effect Triggers Brown Carbon Formation through Heterogeneous Ozone Processing of Ortho-Vanillin

“…42 When Na 2 SO 4 and NaNO 3 are added to the aqueous solution, the Henry's law coefficient of ozone ought to be adjusted by the Setchenow equation that takes into account the ionic strength effect. 24,28,49 For details about the estimation of the Henry's law coefficients of O 3 at different salt concentrations, the readers are referred to our previous studies. 24,28 The estimated Henry's law coefficients of O 3 at different Na 2 SO 4 and NaNO 3 concentrations are reported in Table S4.…”

“…24,28,49 For details about the estimation of the Henry's law coefficients of O 3 at different salt concentrations, the readers are referred to our previous studies. 24,28 The estimated Henry's law coefficients of O 3 at different Na 2 SO 4 and NaNO 3 concentrations are reported in Table S4.…”

“…The ionic strength can alter the kinetics and products distribution of reactions that occur within the aerosol deliquescent particles, by which it can affect the aerosol composition and optical properties. 22,24,28,32,[50][51][52][53] The presence of inorganic ions modifies the pKa of o-VL, which in turn impacts its protonation/deprotonation degree and, as a consequence, its absorption spectrum. 32,33 Figure S1 shows the absorbance of 1×10 −5 mol L −1 o-VL at different ionic strengths, adjusted by Na 2 SO 4 .…”

“…23 At pH 3, which is relevant for acidic aerosol particles, a sharp increase has been observed for the uptake coefficients of O 3 on aqueous acetosyringone, from γ = (1.39 ± 0.38)×10 −7 in the absence of salt to γ = (1.17 ± 0.01)×10 −6 at I = 0.9 mol L −1 , adjusted with Na 2 SO 4 . 24 In this study, we use a vertical wetted wall flow tube (VWWFT) reactor to investigate the influence of sulfate and nitrate ions on the heterogeneous reactions of O 3 with o-VL, in the dark and under light irradiation (320 nm<λ<400 nm), at pH 5.6 that corresponds to moderately acidic aerosol particles. Indeed, there are lately more and more studies showing that the pH values of aerosol particles under hazy and foggy conditions in China are higher compared to pH values of aerosols in Europe and the USA 21,[25][26][27] .…”

“…Cheng et al (2016) 21 up have been provided in our previous papers. 24,28 The flow tube is 80-cm long, with an internal diameter (d) = 0.9 cm. Gaseous ozone was generated by a commercial ozone generator (UVP, LLC Upland).…”