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The 2-methyltetrols have been widely chosen as chemical tracers for isoprene-derived secondary organic aerosols. While they are often assumed to be relatively unreactive, a laboratory study reported that pure erythritol particles (an analog of 2-methyltetrols) can be heterogeneously oxidized by gas-phase OH radicals at a significant rate. This might question the efficacy of these compounds as tracers in aerosol source-apportionment studies. Additional uncertainty could arise as organic compounds and inorganic salts often coexist in atmospheric particles. To gain more insights into the chemical stability of 2-methyltetrols in atmospheric particles, this study investigates the heterogeneous OH oxidation of pure erythritol particles and particles containing erythritol and ammonium sulfate (AS) at different dry inorganic-to-organic mass ratios (IOR) in an aerosol flow tube reactor at a high relative humidity of 85 %. The same reaction products are formed upon heterogenous OH oxidation of erythritol and erythritol-AS particles, suggesting that the reaction pathways are not strongly affected by the presence and amount of AS. On the other hand, the effective OH uptake coefficient, eff, is found to decrease by about a factor of 20 from 0:450:025 to 0:020:001 when the relative abundance of AS increases and the IOR increases from 0.0 to 5.0. One likely explanation is the presence of dissolved ions slows down the reaction rates by decreasing the surface concentration of erythritol and reducing the frequency of collision between erythritol and gas-phase OH radicals at the particle surface. Hence, the heterogeneous OH reactivity of erythritol and likely 2-methyltetrols in atmospheric particles would be slower than previously thought when the salts are present. Given 2-methyltetrols often coexist with a significant amount of AS in many environments, where ambient IOR can vary from 1:89 to 250, our kinetic data would suggest that 2-methyltetrols in atmospheric particles are likely chemically stable against heterogeneous OH oxidation under humid conditions.

Abstract. Information on liquid–liquid phase separation (LLPS) and viscosity (ordiffusion) within secondary organic aerosol (SOA) is needed to improvepredictions of particle size, mass, reactivity, and cloud nucleatingproperties in the atmosphere. Here we report on LLPS and viscosities withinSOA generated by the photooxidation of diesel fuel vapors. Diesel fuelcontains a wide range of volatile organic compounds, and SOA generated bythe photooxidation of diesel fuel vapors may be a good proxy for SOA fromanthropogenic emissions. In our experiments, LLPS occurred over the relativehumidity (RH) range of ∼70 % to ∼100 %,resulting in an organic-rich outer phase and a water-rich inner phase. Theseresults may have implications for predicting the cloud nucleating propertiesof anthropogenic SOA since the presence of an organic-rich outer phase athigh-RH values can lower the supersaturation with respect to water requiredfor cloud droplet formation. At ≤10 % RH, the viscosity was ≥1×108 Pa s, which corresponds to roughly the viscosity of tarpitch. At 38 %–50 % RH, the viscosity was in the range of 1×108 to 3×105 Pa s. These measured viscosities areconsistent with predictions based on oxygen to carbon elemental ratio (O:C)and molar mass as well as predictions based on the number of carbon,hydrogen, and oxygen atoms. Based on the measured viscosities and theStokes–Einstein relation, at ≤10 % RH diffusion coefficients oforganics within diesel fuel SOA is ≤5.4×10-17 cm2 s−1 and the mixing time of organics within 200 nm diesel fuel SOAparticles (τmixing) is 50 h. These small diffusion coefficientsand large mixing times may be important in laboratory experiments, where SOAis often generated and studied using low-RH conditions and on timescales ofminutes to hours. At 38 %–50 % RH, the calculated organic diffusioncoefficients are in the range of 5.4×10-17 to 1.8×10-13 cm2 s−1 and calculated τmixing values arein the range of ∼0.01 h to ∼50 h. These valuesprovide important constraints for the physicochemical properties ofanthropogenic SOA.

Abstract. Atmospheric particles, consisting of inorganic salts, organic compounds anda varying amount of water, can continuously undergo heterogeneous oxidationinitiated by gas-phase oxidants at the particle surface, changing thecomposition and properties of particles over time. To date, most studiesfocus on the chemical evolution of pure organic particles upon oxidation. Togain more fundamental insights into the effects of inorganic salts on theheterogeneous kinetics and chemistry of organic compounds, we investigatethe heterogeneous OH oxidation of 3-methylglutaric acid (3-MGA) particlesand particles containing both 3-MGA and ammonium sulfate (AS) in anorganic-to-inorganic mass ratio of 2 in an aerosol flow tube reactor at ahigh relative humidity of 85.0 %. The molecular information of theparticles before and after OH oxidation is obtained using the directanalysis in real time (DART), a soft atmospheric pressure ionization sourcecoupled to a high-resolution mass spectrometer. Optical microscopymeasurements reveal that 3-MGA–AS particles are in a single liquid phaseprior to oxidation at high relative humidity. Particle mass spectra showthat C6 hydroxyl and C6 ketone functionalization products arethe major products formed upon OH oxidation in the absence and presence ofAS, suggesting that the dissolved salt does not significantly affectreaction pathways. The dominance of C6 hydroxyl products over C6 ketone products could be explained by the intermolecular hydrogenabstraction by tertiary alkoxy radicals formed at the methyl-substitutedtertiary carbon site. On the other hand, kinetic measurements show that theeffective OH uptake coefficient, γeff, for 3-MGA–AS particles(0.99±0.05) is smaller than that for 3-MGA particles (2.41±0.13) by about a factor of ∼2.4. A smaller reactivity observed in 3-MGA–AS particles might be attributed to a higher surfaceconcentration of water molecules and the presence of ammonium and sulfateions, which are chemically inert to OH radicals, at the particle surface.This could lower the collision probability between the 3-MGA and OHradicals, resulting in a smaller overall reaction rate. Our results suggestthat inorganic salts likely alter the overall heterogeneous reactivity oforganic compounds with gas-phase OH radicals rather than reaction mechanismsin well-mixed aqueous organic–inorganic droplets at a high humidity, i.e.,85 % relative humidity (RH). It also acknowledges that the effects of inorganic salts on theheterogeneous reactivity could vary greatly, depending on the particlecomposition and environmental conditions (e.g., RH and temperature). Forinstance, at lower relative humidities, aqueous 3-MGA–AS droplets likelybecome more concentrated and more viscous before efflorescence, possiblygiving rise to diffusion limitation during oxidation under relatively dry orcold conditions. Further studies on the effects of inorganic salts on thediffusivity of the species under different relative humidities within theorganic–inorganic particles are also desirable to better understand therole of inorganic salts in the heterogeneous reactivity of organiccompounds.

Abstract. Atmospheric particles, consisting of inorganic salts, organic compounds anda varying amount of water, can continuously undergo heterogeneous oxidationinitiated by gas-phase oxidants at the particle surface, changing thecomposition and properties of particles over time. To date, most studiesfocus on the chemical evolution of pure organic particles upon oxidation. Togain more fundamental insights into the effects of inorganic salts on theheterogeneous kinetics and chemistry of organic compounds, we investigatethe heterogeneous OH oxidation of 3-methylglutaric acid (3-MGA) particlesand particles containing both 3-MGA and ammonium sulfate (AS) in anorganic-to-inorganic mass ratio of 2 in an aerosol flow tube reactor at ahigh relative humidity of 85.0 %. The molecular information of theparticles before and after OH oxidation is obtained using the directanalysis in real time (DART), a soft atmospheric pressure ionization sourcecoupled to a high-resolution mass spectrometer. Optical microscopymeasurements reveal that 3-MGA–AS particles are in a single liquid phaseprior to oxidation at high relative humidity. Particle mass spectra showthat C6 hydroxyl and C6 ketone functionalization products arethe major products formed upon OH oxidation in the absence and presence ofAS, suggesting that the dissolved salt does not significantly affectreaction pathways. The dominance of C6 hydroxyl products over C6 ketone products could be explained by the intermolecular hydrogenabstraction by tertiary alkoxy radicals formed at the methyl-substitutedtertiary carbon site. On the other hand, kinetic measurements show that theeffective OH uptake coefficient, γeff, for 3-MGA–AS particles(0.99±0.05) is smaller than that for 3-MGA particles (2.41±0.13) by about a factor of ∼2.4. A smaller reactivity observed in 3-MGA–AS particles might be attributed to a higher surfaceconcentration of water molecules and the presence of ammonium and sulfateions, which are chemically inert to OH radicals, at the particle surface.This could lower the collision probability between the 3-MGA and OHradicals, resulting in a smaller overall reaction rate. Our results suggestthat inorganic salts likely alter the overall heterogeneous reactivity oforganic compounds with gas-phase OH radicals rather than reaction mechanismsin well-mixed aqueous organic–inorganic droplets at a high humidity, i.e.,85 % relative humidity (RH). It also acknowledges that the effects of inorganic salts on theheterogeneous reactivity could vary greatly, depending on the particlecomposition and environmental conditions (e.g., RH and temperature). Forinstance, at lower relative humidities, aqueous 3-MGA–AS droplets likelybecome more concentrated and more viscous before efflorescence, possiblygiving rise to diffusion limitation during oxidation under relatively dry orcold conditions. Further studies on the effects of inorganic salts on thediffusivity of the species under different relative humidities within theorganic–inorganic particles are also desirable to better understand therole of inorganic salts in the heterogeneous reactivity of organiccompounds.

Information on liquid-liquid phase separation (LLPS) and viscosity (or diffusion) within secondary organic aerosol (SOA) is needed to improve predictions of particle size, mass, reactivity, and cloud nucleating properties in the atmosphere. Here we report on LLPS and viscosities within SOA generated by the photooxidation of diesel fuel vapors. Diesel fuel contains a wide range of volatile organic compounds, and SOA generated by the photooxidation of diesel fuel vapors may be a good proxy for SOA from anthropogenic emissions. In our experiments, LLPS occurred over the relative humidity (RH) range of ∼ 70% to ∼ 100 %, resulting in an organic-rich outer phase and a water-rich inner phase. These results may have implications for predicting the cloud nucleating properties of anthropogenic SOA since the presence of an organic-rich outer phase at high-RH values can lower the supersaturation with respect to water required for cloud droplet formation. At ≤ 10%RH, the viscosity was ≥ 1×108 Pa s, which corresponds to roughly the viscosity of tar pitch. At 38 %-50%RH, the viscosity was in the range of 1×108 to 3×105 Pa s. These measured viscosities are consistent with predictions based on oxygen to carbon elemental ratio (O V C) and molar mass as well as predictions based on the number of carbon, hydrogen, and oxygen atoms. Based on the measured viscosities and the Stokes-Einstein relation, at ≤ 10%RH diffusion coefficients of organics within diesel fuel SOA is ≤ 5:4×10-17 cm2 s-1 and the mixing time of organics within 200 nm diesel fuel SOA particles (τmixing) is 50 h. These small diffusion coefficients and large mixing times may be important in laboratory experiments, where SOA is often generated and studied using low-RH conditions and on timescales of minutes to hours. At 38 %-50%RH, the calculated organic diffusion coefficients are in the range of 5:4×10-17 to 1:8×10-13 cm2 s-1 and calculated τmixing values are in the range of ∼ 0:01 h to ∼ 50 h. These values provide important constraints for the physicochemical properties of anthropogenic SOA.