scholarly journals Effects of Inorganic Salts on the Heterogeneous OH Oxidation of Organic Compounds: Insights from Methylglutaric Acid-Ammonium Sulfate

2019 ◽  
Author(s):  
Hoi Ki Lam ◽  
Sze Man Shum ◽  
James F. Davies ◽  
Mijung Song ◽  
Andreas Zuend ◽  
...  
Keyword(s):  
Relative Humidity ◽  
Gas Phase ◽  
Ammonium Sulfate ◽  
Organic Compounds ◽  
Particle Surface ◽  
High Relative Humidity ◽  
Inorganic Salts ◽  
Oh Radicals ◽  
Kinetic Measurements ◽  
Inorganic Particles

Abstract. Atmospheric particles, consisting of inorganic salts, organic compounds and a varying amount of water, can continuously undergo heterogeneous oxidation initiated by gas-phase oxidants at the particle surface, changing the composition and properties of particles over time. To date, most studies focus on the chemical evolution of pure organic particles upon oxidation. To gain more fundamental insights into the effects of inorganic salts on the heterogeneous kinetics and chemistry of organic compounds, we investigate the heterogeneous OH oxidation of 3-methylglutaric acid (3-MGA) particles and particles containing both 3-MGA and ammonium sulfate (AS) in an organic-to-inorganic mass ratio of 2 in an aerosol flow tube reactor at a high relative humidity of 85.0 %. The molecular information of the particles before and after OH oxidation is obtained using the Direct Analysis in Real Time (DART), a soft atmospheric pressure ionization source, coupled to a high-resolution mass spectrometer. Optical microscopy measurements reveal that 3-MGA-AS particles are in a single liquid phase prior to oxidation at high relative humidity. Particle mass spectra show that C6 hydroxyl and C6 ketone functionalization products are the major products formed upon OH oxidation in the absence and presence of AS, suggesting that the dissolved salt does not significantly affect reaction pathways. The dominance of C6 hydroxyl products over C6 ketone products could be explained by the intermolecular hydrogen abstraction by tertiary alkoxy radicals formed at the methyl-substituted tertiary carbon site. On the other hand, kinetic measurements show that the effective 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 surface concentration of water molecules, and the presence of ammonium and sulfate ions, which are chemically inert to OH radicals, at the particle surface. This could lower the collision probability between the 3-MGA and OH radicals, resulting in a smaller overall reaction rate. Our results suggest that inorganic salts likely alter the overall heterogeneous reactivity of organic compounds with gas-phase OH radicals rather than reaction mechanisms in well-mixed aqueous organic-inorganic particles.

scholarly journals Effects of inorganic salts on the heterogeneous OH oxidation of organic compounds: insights from methylglutaric acid–ammonium sulfate

2019 ◽  
Vol 19 (14) ◽  
pp. 9581-9593 ◽  
Author(s):  
Hoi Ki Lam ◽  
Sze Man Shum ◽  
James F. Davies ◽  
Mijung Song ◽  
Andreas Zuend ◽  
...  
Keyword(s):  
Relative Humidity ◽  
Gas Phase ◽  
Ammonium Sulfate ◽  
Organic Compounds ◽  
Particle Surface ◽  
High Relative Humidity ◽  
Inorganic Salts ◽  
Oh Radicals ◽  
Inorganic Particles ◽  
Heterogeneous Reactivity

Abstract. Atmospheric particles, consisting of inorganic salts, organic compounds and a varying amount of water, can continuously undergo heterogeneous oxidation initiated by gas-phase oxidants at the particle surface, changing the composition and properties of particles over time. To date, most studies focus on the chemical evolution of pure organic particles upon oxidation. To gain more fundamental insights into the effects of inorganic salts on the heterogeneous kinetics and chemistry of organic compounds, we investigate the heterogeneous OH oxidation of 3-methylglutaric acid (3-MGA) particles and particles containing both 3-MGA and ammonium sulfate (AS) in an organic-to-inorganic mass ratio of 2 in an aerosol flow tube reactor at a high relative humidity of 85.0 %. The molecular information of the particles before and after OH oxidation is obtained using the direct analysis in real time (DART), a soft atmospheric pressure ionization source coupled to a high-resolution mass spectrometer. Optical microscopy measurements reveal that 3-MGA–AS particles are in a single liquid phase prior to oxidation at high relative humidity. Particle mass spectra show that C6 hydroxyl and C6 ketone functionalization products are the major products formed upon OH oxidation in the absence and presence of AS, suggesting that the dissolved salt does not significantly affect reaction pathways. The dominance of C6 hydroxyl products over C6 ketone products could be explained by the intermolecular hydrogen abstraction by tertiary alkoxy radicals formed at the methyl-substituted tertiary carbon site. On the other hand, kinetic measurements show that the effective 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 surface concentration of water molecules and the presence of ammonium and sulfate ions, which are chemically inert to OH radicals, at the particle surface. This could lower the collision probability between the 3-MGA and OH radicals, resulting in a smaller overall reaction rate. Our results suggest that inorganic salts likely alter the overall heterogeneous reactivity of organic compounds with gas-phase OH radicals rather than reaction mechanisms in 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 the heterogeneous reactivity could vary greatly, depending on the particle composition and environmental conditions (e.g., RH and temperature). For instance, at lower relative humidities, aqueous 3-MGA–AS droplets likely become more concentrated and more viscous before efflorescence, possibly giving rise to diffusion limitation during oxidation under relatively dry or cold conditions. Further studies on the effects of inorganic salts on the diffusivity of the species under different relative humidities within the organic–inorganic particles are also desirable to better understand the role of inorganic salts in the heterogeneous reactivity of organic compounds.

scholarly journals Liquid–liquid phase separation in particles containing organics mixed with ammonium sulfate, ammonium bisulfate, ammonium nitrate or sodium chloride

2013 ◽  
Vol 13 (23) ◽  
pp. 11723-11734 ◽  
Author(s):  
Y. You ◽  
L. Renbaum-Wolff ◽  
A. K. Bertram
Keyword(s):  
Phase Separation ◽  
Sodium Chloride ◽  
Liquid Phase ◽  
Relative Humidity ◽  
Ammonium Sulfate ◽  
Inorganic Salt ◽  
Liquid Phase Separation ◽  
Inorganic Salts ◽  
Organic Species ◽  
Liquid Liquid Phase Separation

Abstract. As the relative humidity varies from high to low values in the atmosphere, particles containing organic species and inorganic salts may undergo liquid–liquid phase separation. The majority of the laboratory work on this subject has used ammonium sulfate as the inorganic salt. In the following we studied liquid–liquid phase separation in particles containing organics mixed with the following salts: ammonium sulfate, ammonium bisulfate, ammonium nitrate and sodium chloride. In each experiment one organic was mixed with one inorganic salt and the liquid–liquid phase separation relative humidity (SRH) was determined. Since we studied 23 different organics mixed with four different salts, a total of 92 different particle types were investigated. Out of the 92 types, 49 underwent liquid–liquid phase separation. For all the inorganic salts, liquid–liquid phase separation was never observed when the oxygen-to-carbon elemental ratio (O : C) &amp;geq; 0.8 and was always observed for O : C < 0.5. For 0.5 &amp;leq; O : C < 0.8, the results depended on the salt type. Out of the 23 organic species investigated, the SRH of 20 organics followed the trend: (NH4)2SO4 &amp;geq; NH4HSO4 &amp;geq; NaCl &amp;geq; NH4NO3. This trend is consistent with previous salting out studies and the Hofmeister series. Based on the range of O : C values found in the atmosphere and the current results, liquid–liquid phase separation is likely a frequent occurrence in both marine and non-marine environments.

scholarly journals Condensational uptake of semivolatile organic compounds in gasoline engine exhaust onto pre-existing inorganic particles

2011 ◽  
Vol 11 (1) ◽  
pp. 3461-3492
Author(s):  
S.-M. Li ◽  
J. Liggio ◽  
L. Graham ◽  
G. Lu ◽  
J. Brook ◽  
...  
Keyword(s):  
Air Quality ◽  
Gas Phase ◽  
Organic Compounds ◽  
Gasoline Engine ◽  
Particle Mass ◽  
Seed Particle ◽  
Dilution Ratio ◽  
Ambient Conditions ◽  
Engine Exhaust ◽  
Inorganic Particles

Abstract. This paper presents the results of laboratory studies on the condensational uptake of gaseous organic compounds in the exhaust of a light-duty gasoline engine onto preexisting sulfate and nitrate seed particles. Significant condensation of the gaseous organic compounds in the exhaust occurs onto pre-existing inorganic particles on a time scale of 2–5 min. The amount of condensed organic mass (COM) is proportional to the seed particle mass, suggesting that the uptake is due to dissolution, not adsorption. The solubility decreases as a power function with increased dilution of the exhaust, ranging from 0.23 g/g at a dilution ratio of 81, to 0.025 g/g at a dilution ratio of 2230. The solubility increases nonlinearly with increasing concentration of the total hydrocarbons in the gas phase (THC), rising from 0.12 g/g to 0.26 g/g for a CTHC increase of 1 to 18 μg m−3, suggesting that more organics are partitioned into the particles at higher gas phase concentrations. In terms of gas-particle partitioning, the condensational uptake of THC gases in gasoline engine exhaust can account for up to 30% of the total gas+particle THC. By incorporating the present findings, regional air quality modelling results suggest that the condensational uptake of THC onto sulfate particles alone can be comparable to the primary particle mass under moderately polluted ambient conditions. These findings are important for modelling and regulating the air quality impacts of gasoline vehicular emissions.

scholarly journals Deliquescence, efflorescence, and phase miscibility of mixed particles of ammonium sulfate and isoprene-derived secondary organic material

2012 ◽  
Vol 12 (4) ◽  
pp. 9903-9943 ◽  
Author(s):  
M. L. Smith ◽  
A. K. Bertram ◽  
S. T. Martin
Keyword(s):  
Liquid Phase ◽  
Relative Humidity ◽  
Ammonium Sulfate ◽  
Organic Material ◽  
Important Variable ◽  
Oxidation Products ◽  
Liquid Separation ◽  
Inorganic Particles ◽  
Photo Oxidation ◽  
Hygroscopic Properties

Abstract. The hygroscopic phase transitions of ammonium sulfate mixed with isoprene-derived secondary organic material were investigated in aerosol experiments. The organic material was produced by isoprene photo-oxidation at 40% relative humidity. The low volatility fraction of the photo-oxidation products condensed onto ammonium sulfate particles. The particle-phase organic material had oxygen-to-carbon ratios of 0.67 to 0.74 for mass concentrations of 20 to 30 μg m−3. The deliquescence, efflorescence, and phase miscibility of the mixed particles were investigated using a dual arm tandem differential mobility analyzer. The isoprene photo-oxidation products induced deviations in behavior relative to pure ammonium sulfate. Compared to an efflorescence relative humidity (ERH) of 30 to 35% for pure ammonium sulfate, efflorescence was eliminated for mixed aqueous particles having organic volume fractions ε of approximately 0.6 and greater. Compared to a deliquescence relative humidity (DRH) of 80% for pure ammonium sulfate, the DRH steadily decreased for increasing ε, approaching a DRH of 40% for ε of 0.9. Parameterizations of the DRH(ε) and ERH(ε) curves were as follows: DRH(ε)= Σ i ci,d xi valid for 0 ≤ ε ≤ 0.86 and ERH(ε)= Σ i ci,e xi valid for 0 ≤ ε ≤ 0.55 for the coefficients c0,d= 80.67, c0,e = 28.35, c1,d= −11.45, c1,e = −13.66, c2,d = 0, c2,e = 0, c3,d = 57.99, c3,e = −83.80, c4,d = −106.80, and c4,d = 0. The molecular description that is thermodynamically implied by these strongly sloped DRH(ε) and ERH(ε) curves is that the organic isoprene photo-oxidation products, the inorganic ammonium sulfate, and water form a miscible liquid phase even at low relative humidity. This phase miscibility is in contrast to the liquid-liquid separation that occurs for some other types of secondary organic material. These differences in liquid-liquid separation are consistent with a prediction recently presented in the literature that the bifurcation between liquid-liquid phase separation versus mixing depends on the oxygen-to-carbon ratio of the organic material. The conclusions are that the influence of secondary organic material on the hygroscopic properties of ammonium sulfate varies with organic composition and that the degree of oxygenation of the organic material, which is a measurable characteristic of complex organic materials, is an important variable influencing the hygroscopic properties of mixed organic-inorganic particles.

scholarly journals Condensational uptake of semivolatile organic compounds in gasoline engine exhaust onto pre-existing inorganic particles

2011 ◽  
Vol 11 (19) ◽  
pp. 10157-10171 ◽  
Author(s):  
S.-M. Li ◽  
J. Liggio ◽  
L. Graham ◽  
G. Lu ◽  
J. Brook ◽  
...  
Keyword(s):  
Air Quality ◽  
Gas Phase ◽  
Organic Compounds ◽  
Gasoline Engine ◽  
Particle Mass ◽  
Seed Particle ◽  
Organic Mass ◽  
Dilution Ratio ◽  
Engine Exhaust ◽  
Inorganic Particles

Abstract. This paper presents the results of laboratory studies on the condensational uptake of gaseous organic compounds in the exhaust of a light-duty gasoline engine onto preexisting sulfate and nitrate seed particles. Significant condensation of the gaseous organic compounds in the exhaust occurs onto these inorganic particles on a time scale of 2–5 min. The amount of condensed organic mass (COM) is proportional to the seed particle mass, suggesting that the uptake is due to dissolution determined by the equilibrium partitioning between gas phase and particles, not adsorption. The amount of dissolution in unit seed mass, S, decreases as a power function with increased dilution of the exhaust, ranging from 0.23 g g−1 at a dilution ratio of 81, to 0.025 g g−1 at a dilution ratio of 2230. It increases nonlinearly with increasing concentration of the total hydrocarbons in the gas phase (THC), rising from 0.12 g g−1 to 0.26 g g−1 for a CTHC increase of 1 to 18 μg m−3, suggesting that more organics are partitioned into the particles at higher gas phase concentrations. In terms of gas-particle partitioning, the condensational uptake of THC gases in gasoline engine exhaust can account for up to 30% of the total gas + particle THC. The organic mass spectrum of COM has the largest fragment at m/z 44, with mass ratios of mass fragments 43/44 and 57/44 at 0.59 and 2.91, much lower than those reported for gasoline engine primary organic aerosols. The mass fragment 44/total organic mass ratio of 0.097 indicates that COM contains large oxygenated components. By incorporating the present findings, regional air quality modelling results suggest that the condensational uptake of THC onto sulfate particles alone can be comparable to the primary particle mass under moderately polluted ambient conditions. These findings are important for modelling and regulating the air quality impacts of gasoline vehicular emissions.

scholarly journals Glyoxal's impact on dry ammonium salts: fast and reversible surface aerosol browning

2020 ◽  
Author(s):  
David O. De Haan ◽  
Lelia N. Hawkins ◽  
Kevin Jansen ◽  
Hannah G. Welsh ◽  
Raunak Pednekar ◽  
...  
Keyword(s):  
Relative Humidity ◽  
Gas Phase ◽  
Ammonium Sulfate ◽  
Aerosol Particles ◽  
Ammonium Salts ◽  
Carbon Formation ◽  
Cloud Droplets ◽  
Brown Carbon ◽  
Dicarbonyl Compounds ◽  
Dry Conditions

Abstract. Alpha-dicarbonyl compounds are believed to form brown carbon in the atmosphere via reactions with ammonium sulfate (AS) in cloud droplets and aqueous aerosol particles. In this work, brown carbon formation in AS and other aerosol particles was quantified as a function of relative humidity (RH) during exposure to gas-phase glyoxal (GX) in chamber experiments. Under dry conditions (RH 

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

2020 ◽  
Vol 20 (6) ◽  
pp. 3879-3893 ◽  
Author(s):  
Rongshuang Xu ◽  
Hoi Ki Lam ◽  
Kevin R. Wilson ◽  
James F. Davies ◽  
Mijung Song ◽  
...  
Keyword(s):  
Gas Phase ◽  
Chemical Stability ◽  
Particle Surface ◽  
Surface Concentration ◽  
Reaction Rates ◽  
Atmospheric Particles ◽  
Oh Radicals ◽  
Reaction Products ◽  
Organic Mass ◽  
Chemical Tracers

Abstract. 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, γeff, is found to decrease by about a factor of ∼20 from 0.45±0.025 to 0.02±0.001 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.

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