Uptake of SO2 into Sulfuric Acid Droplets through the Oxidation by NO2 under Venus-Analogous Conditions

Abstract

Sulfur dioxide (SO₂) is the primary sulfur-bearing gas on Venus and plays a pivotal role in its atmospheric chemistry. Observations show that SO₂ concentration decreases by 3 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. Here, we show for the first time that SO₂ uptake and subsequent oxidation within droplets could serve as an additional sink in the Venusian cloud layers. We performed laboratory experiments to examine the uptake of SO₂ by sulfuric acid (H₂SO₄) droplets of ∼10 μm in the presence of nitrogen dioxide (NO₂) as an oxidant. We find that the size growth of H₂SO₄ droplets occurs only when both SO₂ and NO₂ are present, indicating the SO₂ oxidation by NO₂ in H₂SO₄ droplets. The growth rate increases with NO₂ concentration, and the reactive uptake coefficient of SO₂, γ_SO2, is parameterized by the number density of NO₂ (cm^–3), n_NO2, as log₁₀γ_SO2 = 0.572 × log₁₀n_NO2 – 15.03. Numerical simulations suggest that γ_SO2= 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 H₂SO₄ droplets may play an important role in SO₂ depletion in the cloud layers, warranting future observations of oxidants in the Venusian atmosphere.