14:30 〜 14:45
[AAS11-21] Competitive multiphase reactions of deliquesced aerosol particles in the presence of SO2 and NO2 regulated by aerosol pH
キーワード：multiphase reaction, pH, sulfate, nitrate
Sulfur dioxide (SO2) and nitrogen dioxide (NO2) are ubiquitous in the atmosphere and precursors for sulfate and nitrate. Multiphase oxidation of SO2 by NO2 in deliquesced aerosol particles is key pathway for sulfate production.
2NO2 + SO32-/HSO3- + H2O -> 2HONO + SO42- (+ H+) (R1)
The pathway has been proposed to drive sulfate formation during haze events.1 Heterogeneous NO2 hydrolysis is also an important pathway to produce nitrate.
2NO2 + H2O -> NO3- + HONO + H+ (R2)
Recent work has shown rapid NO2 hydrolysis in deliquesced aerosol particles.2 However, the relative importance of the two pathways to the production of inorganics remains to be understood. Multiphase oxidation of SO2 by NO2 depends on aerosol pH,3 whereas heterogeneous NO2 hydrolysis is insensitive to pH.2 In this study, we study multiphase reactions in deliquesced aerosol particles in the presence of SO2 and NO2 at various pH. We find the competitive sulfate and nitrate productions regulated by aerosol pH. The experiments of multiphase reactions in the presence of SO2 and NO2 were performed using a custom-made aerosol flow cell coupled with a Raman spectroscope. Four types of particles were studied at various pH and relative humidities (RHs) of 50-86%: (1) sodium chloride (NaCl); (2) ammonium chloride (NH4Cl); (3) sodium nitrate (NaNO3); and (4) ammonium nitrate (NH4NO3). RH in the flow cell was controlled using a mixed flow of dry and wet gas streams of N2. Standard gases of NO2, SO2, and NH3 were also introduced to achieve desirable mixing ratios in the flow cell. Particularly, NH3 gas was used to control the particle pH. Particle pH was measured by the Extended Aerosol Inorganics Model.
Raman spectra of particles were acquired using a Raman spectrometer with a 30 mW 532nm laser. A 50x objective lens with a numerical aperture of 0.55 was used to focus the laser onto a sample particle. Sulfate and nitrate concentrations were quantified using the peak area ratio of the corresponding solute to water. Raman spectra of NaCl particles at 1 ppm NO2, 1 ppm SO2, and >3 ppm NH3 revealed not only sulfate production but also S2O62-. The S2O62- production is evident for SO3- formation from its self-reaction. SO3- is formed as an intermediate from the reaction of dissolved SO2 and NO2, and then its reaction with another NO2 and self-reaction lead to the formation of SO42- and S2O62-, respectively. On the other hand, the reaction of SO3- by O2 reaction initiates the well-known chain reactions to form sulfate. We find the sulfate formation in air is approximately 38 times faster than that in N2 at given gas concentrations. Therefore, the SO3- + O2 reaction may outweigh reaction R1 in air, implying that the reaction experiments with air, comparable to many other studies, potentially overestimate the reaction kinetics. Figure 1 presents sulfate and nitrate formation rates as a function of pH for sodium chloride particles at RH 80% at 1 ppm NO2 and 1 ppm SO2 in N2. As pH increases from 5.8 to 6.3, sulfate formation rates exponentially increase by ~2 orders of magnitude, which is consistent with earlier work.3 Nitrate is observable below pH = 6. Nitrate formation rates increase exponentially with decreased pH by ~2 orders of magnitude. The results suggest that sulfate production dominates over nitrate production at high pH, whereas nitrate production is dominant at low pH (<6), highlighting that reactions R1 and R2 are competitive and highly dependent on pH.
References
(1) Cheng et al., Sci. Adv. 2016, 2 (12), e1601530.
(2) Gen et al., Environ. Sci. Technol. 2024, 58 (18), 7904–7915.
(3) Liu, T.; Abbatt, J. P. D. Nat. Chem. 2021, 13 (12), 1173–1177.
2NO2 + SO32-/HSO3- + H2O -> 2HONO + SO42- (+ H+) (R1)
The pathway has been proposed to drive sulfate formation during haze events.1 Heterogeneous NO2 hydrolysis is also an important pathway to produce nitrate.
2NO2 + H2O -> NO3- + HONO + H+ (R2)
Recent work has shown rapid NO2 hydrolysis in deliquesced aerosol particles.2 However, the relative importance of the two pathways to the production of inorganics remains to be understood. Multiphase oxidation of SO2 by NO2 depends on aerosol pH,3 whereas heterogeneous NO2 hydrolysis is insensitive to pH.2 In this study, we study multiphase reactions in deliquesced aerosol particles in the presence of SO2 and NO2 at various pH. We find the competitive sulfate and nitrate productions regulated by aerosol pH. The experiments of multiphase reactions in the presence of SO2 and NO2 were performed using a custom-made aerosol flow cell coupled with a Raman spectroscope. Four types of particles were studied at various pH and relative humidities (RHs) of 50-86%: (1) sodium chloride (NaCl); (2) ammonium chloride (NH4Cl); (3) sodium nitrate (NaNO3); and (4) ammonium nitrate (NH4NO3). RH in the flow cell was controlled using a mixed flow of dry and wet gas streams of N2. Standard gases of NO2, SO2, and NH3 were also introduced to achieve desirable mixing ratios in the flow cell. Particularly, NH3 gas was used to control the particle pH. Particle pH was measured by the Extended Aerosol Inorganics Model.
Raman spectra of particles were acquired using a Raman spectrometer with a 30 mW 532nm laser. A 50x objective lens with a numerical aperture of 0.55 was used to focus the laser onto a sample particle. Sulfate and nitrate concentrations were quantified using the peak area ratio of the corresponding solute to water. Raman spectra of NaCl particles at 1 ppm NO2, 1 ppm SO2, and >3 ppm NH3 revealed not only sulfate production but also S2O62-. The S2O62- production is evident for SO3- formation from its self-reaction. SO3- is formed as an intermediate from the reaction of dissolved SO2 and NO2, and then its reaction with another NO2 and self-reaction lead to the formation of SO42- and S2O62-, respectively. On the other hand, the reaction of SO3- by O2 reaction initiates the well-known chain reactions to form sulfate. We find the sulfate formation in air is approximately 38 times faster than that in N2 at given gas concentrations. Therefore, the SO3- + O2 reaction may outweigh reaction R1 in air, implying that the reaction experiments with air, comparable to many other studies, potentially overestimate the reaction kinetics. Figure 1 presents sulfate and nitrate formation rates as a function of pH for sodium chloride particles at RH 80% at 1 ppm NO2 and 1 ppm SO2 in N2. As pH increases from 5.8 to 6.3, sulfate formation rates exponentially increase by ~2 orders of magnitude, which is consistent with earlier work.3 Nitrate is observable below pH = 6. Nitrate formation rates increase exponentially with decreased pH by ~2 orders of magnitude. The results suggest that sulfate production dominates over nitrate production at high pH, whereas nitrate production is dominant at low pH (<6), highlighting that reactions R1 and R2 are competitive and highly dependent on pH.
References
(1) Cheng et al., Sci. Adv. 2016, 2 (12), e1601530.
(2) Gen et al., Environ. Sci. Technol. 2024, 58 (18), 7904–7915.
(3) Liu, T.; Abbatt, J. P. D. Nat. Chem. 2021, 13 (12), 1173–1177.