Reactive uptake of SO2 in H2SO4 droplets under Venus-analogous conditions: Laboratory study using a single particle levitation method

Heterogeneous reactions involving cloud particles are known to influence the chemical balance of planetary atmospheres. Understanding these interactions is especially important for Venus, where cloud particle chemistry likely plays a significant role in shaping the observed vertical distribution of sulfur dioxide (SO₂). Observations show that the concentration of SO₂ decreases by three orders of magnitude from the bottom to the top of the cloud layers. However, this SO₂ depletion cannot be explained by gas-phase chemistry alone, suggesting a missing SO₂ sink within the cloud layers. A potential mechanism for SO₂ depletion is the reactive uptake of SO₂ by cloud droplets, which is a well-documented process in Earth’s atmosphere, particularly in the presence of oxidants. However, it is highly uncertain whether the reactive uptake mechanism can contribute significantly to SO₂ depletion in the cloud layers of Venus because the solubility of SO₂ in sulfuric acid (H₂SO₄) is extremely low. This unaccounted-for pathway necessitates experimental validation under Venus-analogous conditions.

Here, we performed laboratory experiments to examine the uptake of SO₂ by a single H₂SO₄ droplet of ~10 µm in the presence of nitrogen dioxide (NO₂) as an oxidant for SO₂ oxidation. A single sulfuric acid droplet was levitated using an electrodynamic balance (EDB), a device that uses electric fields to levitate a charged particle in mid-air. The droplet was levitated at ambient temperature (~298 K) and pressure (1 atm), conditions approximately corresponding to an altitude of 50-55 km on Venus. The radius of the droplet was determined by analyzing the Mie scattering spectrum of white light scattered by the droplet, allowing precise quantification of size growth due to reactive uptake.

We find that the size growth of the H₂SO₄ droplet occurs only when both SO₂ and NO₂ are present, indicating SO₂ oxidation by NO₂ within the droplet. The growth rate increases with NO₂ concentration, and the reactive uptake coefficient of SO₂, γ, is parameterized by the number density of NO₂ (cm^-3), n_NO2, as log₁₀ γ = 0.572 × log₁₀ n_NO2 - 15.03 . Numerical simulations suggest that γ = 10^-7 is required to reproduce the observed SO₂ concentration at the top of the cloud layer. Our results underscore that the reactive uptake of SO₂ by droplets may play an important role in SO₂ depletion in the Venusian cloud layers, warranting future observations of oxidants in the Venusian atmosphere.