Heterogeneous OH oxidation of isoprene-epoxydiol-derived organosulfates: kinetics, chemistry and formation of inorganic sulfate

Abstract

Abstract. Acid-catalyzed multiphase chemistry of epoxydiols formed from isoprene oxidation yields the most abundant organosulfates (i.e., methyltetrol sulfates) detected in atmospheric fine aerosols in the boundary layer. This potentially determines the physicochemical properties of fine aerosols in isoprene-rich regions. However, chemical stability of these organosulfates remains unclear. As a result, we investigate the heterogeneous oxidation of aerosols consisting of potassium 3-methyltetrol sulfate ester (C5H11SO7K) by gas-phase hydroxyl (OH) radicals at a relative humidity (RH) of 70.8 %. Real-time molecular composition of the aerosols is obtained by using a Direct Analysis in Real Time (DART) ionization source coupled to a high-resolution mass spectrometer. Aerosol mass spectra reveal that 3-methyltetrol sulfate ester can be detected as its anionic form (C5H11SO7-) via direct ionization in the negative ionization mode. Kinetic measurements reveal that the effective heterogeneous OH rate constant is measured to be 4.74±0.2×10-13 cm3 molecule−1 s−1 with a chemical lifetime against OH oxidation of 16.2±0.3 days, assuming an OH radical concentration of 1.5×106 molecules cm−3. Comparison of this lifetime with those against other aerosol removal processes, such as dry and wet deposition, suggests that 3-methyltetrol sulfate ester is likely to be chemically stable over atmospheric timescales. Aerosol mass spectra only show an increase in the intensity of bisulfate ion (HSO4-) after oxidation, suggesting the importance of fragmentation processes. Overall, potassium 3-methyltetrol sulfate ester likely decomposes to form volatile fragmentation products and aqueous-phase sulfate radial anion (SO4⚫-). SO4⚫- subsequently undergoes intermolecular hydrogen abstraction to form HSO4-. These processes appear to explain the compositional evolution of 3-methyltetrol sulfate ester during heterogeneous OH oxidation.

Effect of inorganic-to-organic mass ratio on the heterogeneous OH reaction rates of erythritol: implications for atmospheric chemical stability of 2-methyltetrols

“…Further study that measures the γ eff for size-selected monodisperse and polydisperse particles is desired to better investigate the effect of particle size distribution on γ eff calculation. These results agree with the literature that the addition of AS decreases the overall rate of heterogeneous OH oxidation with organic compounds (Mungall et al, 2017;Kwong et al, 2018b;Lam et al, 2019b). We carried out molecular dynamics (MD) simulations to gain a better insight into the effect of dissolved ions on the heterogeneous OH reactivity of erythritol.…”

“…The heterogeneous OH oxidation of erythritol particles and erythritol-AS particles was investigated using an aerosol flow tube reactor at 20 • C and 85 % RH. Experimental details have been given elsewhere (Cheng et al, 2016;Chim et al, 2017a, b;Lam et al, 2019b). Briefly, aqueous droplets were atomized using an atomizer and were directly mixed with ozone, oxygen (O 2 ), dry nitrogen (N 2 ), humidified N 2 and hexane before being introduced into the reactor.…”

“…There remains large uncertainty in how inorganic salts alter the heterogeneous kinetics and chemistry of organic compounds (McNeill et al, 2007(McNeill et al, , 2008Dennis-Smither et al, 2012). A few recent laboratory studies have revealed that the presence of dissolved inorganic ions (e.g., ammonium sulfate, AS) can reduce the heterogeneous OH reactivity of organic compounds in aqueous organic-inorganic particles (e.g., methanesulfonic acid and 3-methylglutaric acid) but does not significantly alter the reaction mechanisms (Mungall et al, 2017;Kwong et al, 2018b;Lam et al, 2019b). In these studies, only a single inorganic-to-organic mass ratio was chosen to examine the impacts of the salts on the heterogeneous reactivity of organic compounds.…”

Aerosol characteristics at the Southern Great Plains site during the HI-SCALE campaign

“…The C 5 H 6 O + ion has been recognized as a unique marker for IEPOX SOA (Robinson et al, 2011;Lin et al, 2012Lin et al, , 2013Hu et al, 2015). During both iSOA events, the time series of C 5 H 6 O + ion track that of IEPOX SOA and SO 4 (Fig.…”

“…During both events, generally speaking, the sum of isoprene and its first-generation oxidation products did not track the evolution of IEPOX SOA well, but at times an enhancement in IEPOX SOA followed after a peak in the sum of m/z 69 and 71. Given the good correlation between SO 4 and IEPOX SOA, we speculate that the variation of IEPOX SOA at the SGP site was mostly driven by variations in the concentration and the acidity of particles, as IEPOX SOA precursors appear to be abundant during most of the campaign (Lin et al, 2012). Back trajectories provide some insights into the transport pathways associated with the iSOA events (Fig.…”