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

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, γSO2, is parameterized by the number density of NO2 (cm–3), nNO2, as log10γSO2 = 0.572 × log10nNO2 – 15.03. Numerical simulations suggest that γSO2= 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.