17:15 〜 18:45
[PPS04-P03] Laboratory Study of SO2 Uptake by Sulfuric Acid Droplets in the Venusian Atmospheric Condition
キーワード：金星、エアロゾル
Sulfur dioxide (SO2) is strongly coupled with atmospheric processes on Venus, including chemical reactions, radiative forcing, and cloud formation. Cloud droplets are mainly constituted of sulfuric acid, an oxidation product of SO2. Observations have shown that the volume mixing ratio of SO2 decreases by more than two orders of magnitude from the lower cloud to the cloud-top region (Marcq et al., 2018). However, the SO2 depletion cannot be explained by the gas-phase chemistry alone, suggesting the presence of missing sinks of SO2 in the cloud layer (Bierson & Zhang, 2020). The mechanisms of SO2 depletion remain to be understood.
The reactive uptake of SO2 into aerosol droplets occurs with gas phase oxidants and its mechanisms have been extensively studied in atmospheric chemistry of the Earth. For instance, the gas phase oxidants including nitrogen dioxide (NO2), ozone (O3), and hydrogen peroxide (H2O2) can oxidize dissolved SO2 and hence enhance SO2 uptake (e.g., Gen et al., 2019; Rattigan et al., 2000). However, the reactive uptake of SO2 has not been investigated in the Venusian atmospheric condition.
Here, we perform laboratory experiments to examine the uptake of SO2 by sulfuric acid droplets using an electrodynamic balance (EDB). A sulfuric acid droplet of ~8 µm in radius was levitated in the EDB chamber under the ambient temperature (~298 K) and pressure (1 atmosphere). This condition corresponds to an altitude of 50-55 km on Venus. The mixture of carbon dioxide (CO2: ~100 %) and SO2 (100 ppm) gasses were continuously introduced into the chamber to represent the Venusian atmospheric condition. In addition, 10 ppm NO2 was also used as a potential gas phase oxidant (e.g., Liu & Abbatt, 2021) to explore its effect on SO2 uptake. Droplet size was measured with the Mie scattering method. The size change during SO2 uptake was measured to estimate the SO2 uptake rate. We assume that dissolved SO2 is oxidized by dissolved NO2 to form sulfate (SO42-).
Uptake experiments show that sulfuric acid droplets do not grow in size in the presence of CO2 and SO2 alone. This result is not surprising because SO2 solubility in water is extremely low under acidic conditions such as in sulfuric acid solution. In contrast, we find the droplet growth at a rate of ~8 nm/hour in radius with an addition of NO2. The next step is to ascertain the dependence of radial growth on NO2 concentration by varying the concentration of NO2. Then, through extrapolation of the concentration of NO2 to the value on Venus, we estimate whether the growth rate can explain the depletion of SO2 on Venus using a numerical model. Our results highlight that the uptake of SO2 by the sulfuric acid droplets can be facilitated by gaseous oxidants, which warrants future observations of oxidant species in the Venusian atmosphere.
References
Bierson, C. J., & Zhang, X. (2020). Chemical cycling in the Venusian atmosphere: a full photochemical model from the surface to 110 km. Journal of Geophysical Research: Planets, 125(7), e2019JE006159.
Gen, M., Zhang, R., Huang, D. D., Li, Y., & Chan, C. K. (2019). Heterogeneous SO2 oxidation in sulfate formation by photolysis of particulate nitrate. Environmental Science & Technology Letters, 6(2), 86-91.
Liu, T., & Abbatt, J. P. (2021). Oxidation of sulfur dioxide by nitrogen dioxide accelerated at the interface of deliquesced aerosol particles. Nature Chemistry, 13(12), 1173-1177.
Marcq, E., Mills, F. P., Parkinson, C. D., & Vandaele, A. C. (2018). Composition and chemistry of the neutral atmosphere of Venus. Space Science Reviews, 214, 1-55.
Rattigan, O. V., Boniface, J., Swartz, E., Davidovits, P., Jayne, J. T., Kolb, C. E., & Worsnop, D. R. (2000). Uptake of gas-phase SO2 in aqueous sulfuric acid: Oxidation by H2O2, O3, and HONO. Journal of Geophysical Research: Atmospheres, 105(D23), 29065-29078.
The reactive uptake of SO2 into aerosol droplets occurs with gas phase oxidants and its mechanisms have been extensively studied in atmospheric chemistry of the Earth. For instance, the gas phase oxidants including nitrogen dioxide (NO2), ozone (O3), and hydrogen peroxide (H2O2) can oxidize dissolved SO2 and hence enhance SO2 uptake (e.g., Gen et al., 2019; Rattigan et al., 2000). However, the reactive uptake of SO2 has not been investigated in the Venusian atmospheric condition.
Here, we perform laboratory experiments to examine the uptake of SO2 by sulfuric acid droplets using an electrodynamic balance (EDB). A sulfuric acid droplet of ~8 µm in radius was levitated in the EDB chamber under the ambient temperature (~298 K) and pressure (1 atmosphere). This condition corresponds to an altitude of 50-55 km on Venus. The mixture of carbon dioxide (CO2: ~100 %) and SO2 (100 ppm) gasses were continuously introduced into the chamber to represent the Venusian atmospheric condition. In addition, 10 ppm NO2 was also used as a potential gas phase oxidant (e.g., Liu & Abbatt, 2021) to explore its effect on SO2 uptake. Droplet size was measured with the Mie scattering method. The size change during SO2 uptake was measured to estimate the SO2 uptake rate. We assume that dissolved SO2 is oxidized by dissolved NO2 to form sulfate (SO42-).
Uptake experiments show that sulfuric acid droplets do not grow in size in the presence of CO2 and SO2 alone. This result is not surprising because SO2 solubility in water is extremely low under acidic conditions such as in sulfuric acid solution. In contrast, we find the droplet growth at a rate of ~8 nm/hour in radius with an addition of NO2. The next step is to ascertain the dependence of radial growth on NO2 concentration by varying the concentration of NO2. Then, through extrapolation of the concentration of NO2 to the value on Venus, we estimate whether the growth rate can explain the depletion of SO2 on Venus using a numerical model. Our results highlight that the uptake of SO2 by the sulfuric acid droplets can be facilitated by gaseous oxidants, which warrants future observations of oxidant species in the Venusian atmosphere.
References
Bierson, C. J., & Zhang, X. (2020). Chemical cycling in the Venusian atmosphere: a full photochemical model from the surface to 110 km. Journal of Geophysical Research: Planets, 125(7), e2019JE006159.
Gen, M., Zhang, R., Huang, D. D., Li, Y., & Chan, C. K. (2019). Heterogeneous SO2 oxidation in sulfate formation by photolysis of particulate nitrate. Environmental Science & Technology Letters, 6(2), 86-91.
Liu, T., & Abbatt, J. P. (2021). Oxidation of sulfur dioxide by nitrogen dioxide accelerated at the interface of deliquesced aerosol particles. Nature Chemistry, 13(12), 1173-1177.
Marcq, E., Mills, F. P., Parkinson, C. D., & Vandaele, A. C. (2018). Composition and chemistry of the neutral atmosphere of Venus. Space Science Reviews, 214, 1-55.
Rattigan, O. V., Boniface, J., Swartz, E., Davidovits, P., Jayne, J. T., Kolb, C. E., & Worsnop, D. R. (2000). Uptake of gas-phase SO2 in aqueous sulfuric acid: Oxidation by H2O2, O3, and HONO. Journal of Geophysical Research: Atmospheres, 105(D23), 29065-29078.