Organosulfates from Dark Aqueous Reactions of Isoprene-Derived Epoxydiols Under Cloud and Fog Conditions: Kinetics, Mechanism, and Effect of Reaction Environment on Regioselectivity of Sulfate Addition

Abstract

Atmospheric oxidation of isoprene yields large quantities of highly water-soluble isoprene epoxydiols (IEPOX) that partition into fogs, clouds, and wet aerosols. In aqueous aerosols, acid-catalyzed ring-opening of IEPOX followed by nucleophilic addition of inorganic sulfate or water forms organosulfates and 2-methyltetrols, respectively, contributing substantially to secondary organic aerosol (SOA). However, the fate of IEPOX in clouds, fogs and evaporating hydrometeors is not well understood. Here we investigate the rates, product branching ratios, and stereochemistry of organosulfates from reactions of dilute IEPOX (5 to 10 mM) under a range of sulfate concentrations (0.3 to 50 mM) and pH values (1.83-3.38) in order to better understand the fate of IEPOX in clouds and fogs. From these aqueous dark reactions of β-IEPOX isomers (trans- and cis-2-methyl-2,3-epoxybutane-1,4-diols), which are the predominant IEPOX isomers, products were identified and quantified using hydrophilic interaction liquid chromatography coupled to an electrospray ionization high-resolution quadrupole time-of-flight mass spectrometer operated in negative ion mode (HILIC/(-)ESI-HR-QTOFMS). We found that regiochemistry and stereochemistry were affected by pH and the tertiary methyltetrol sulfate (C₅H₁₂O₇S) was promoted by increasing solution acidity. Furthermore, the rate constants for the reaction of IEPOX under cloud-relevant conditions are up to one order of magnitude lower than reported in the literature for aerosol-relevant conditions due to markedly different solution activity. Nevertheless, the contribution of cloud and fog water reactions to IEPOX SOA may be significant in cases of lower aqueous-phase pH (model estimate) or during droplet evaporation (not studied).

Keywords: 2-methyl-2,3-epoxybutane-1,4-diols; 2-methyltetrols; Biogenic-anthropogenic interaction; IEPOX; Molecular tracers; Multiphase chemistry; Stereochemistry; aqSOA.

Figure 1.

HILIC/(–)ESI-HR-QTOFMS extracted ion chromatograms (EICs) of authentic standards of 2-MTs (deprotonated ion, EIC m/z 135.066, black) and MTSs (deprotonated ion EIC, m/z 215.023, blue). MTS isomers 1,3,4-trihydroxy-3-methylbutan-2-yl sulfates (peaks A and B, RT 3.0 and 3.5 min, respectively) and 1,3,4-trihydroxy-2-methylbutan-2-yl sulfates (peaks C and D, RT 5.7 and 6.6 min, respectively). The HILIC conditions in this study do not resolve the 2-MT diastereomers 2-methylthreitol and 2-methylerythritol.

Figure 2.

Growth in 2-methyltetrols (2-MTs) (panel A) and methyltetrol sulfates (MTSs) (panel B) during experiments with 5 mM trans-β-IEPOX varying sulfate (experiments #9–12 in Table 1).

Figure 3.

(panel A) Comparison of k_H+ to the regression model of Cole-Filipiak et al. (k_H+,mod). Grey circles: published observations (letters denote sources: M, C, E, A, L, P, m, and N). Solid black line: 1:1 line (equal to original regression model, which incorporated observations of M, C, L and P). Dotted black line: fit to all published observations including those of E, A, m, and N, added here as an extension of the original. Solid blue line: fit to all studies except M, C, and this work, which ignore sulfate in calculation of k_H+. Orange diamonds: k_H+ estimated from structure-activity relationships by Eddingsaas and DFT simulations of Piletic and coworkers. Red circles: observed k_H+, this work (trans-β-IEPOX). Red numbers: experiment number in Table 3. (panel B) Observed of k_H+, this work, as a function of sulfate ion activity (mol L⁻¹ basis) (trans-β-IEPOX; derived via equation 4). Numbers: experiment number in Table 3.

Figure 4.

Branching ratio, β, for experiments #9–12 varying sulfate ion activity (mmol L⁻¹ basis) (red circles). Diamonds: Eddingsaas et al. for cis-2,3-epoxybutane-1,4-diol; Line: Piletic et al. Blue square: branching ratio calculated using relative nucleophilic strengths of sulfate and H₂O.

Figure 5.

HILIC/(–)ESI-HR-QTOFMS EICs of the four MTS isomers detected from trans-β-IEPOX (pink EIC, peaks A and D) and cis-β-IEPOX (orange EIC, peaks B and C). Hypothesized products, product properties and reaction mechanisms are noted above peaks.

Figure 6.

Transition state proposed for nucleophilic attack on protonated trans-β-IEPOX leading to the major observed isomer.

Figure 7.

Regioselectivity of tertiary sulfate under varying SO₄²⁻ activity (panel A) and varying H₃O⁺ activity (panel B), following equation 6 (Section 3.4). The linear best fit and 95% confidence interval of the slope are shown. Activities correspond to mol L⁻¹.

Scheme 1.

Reaction of trans-β-IEPOX with acidic sulfate by A⁻¹ (top), A-2 (middle) or general acid (bottom) mechanisms. Products 1–4 are shown to the right; product 3 is only expected from the reaction of cis-β-IEPOX (not shown).