Abstract: The two major components of acid rain are sulfuric acid (H2SO4) and nitric acid (HNO3). Sulfur dioxide (SO2) is the main precursor of H2SO4. Atmospheric sulfur dioxide is oxidized homogeneously by reaction of SO2 with OH and O2 leading to SO3, which then reacts with water to form sulfuric acid. This is the now accepted acid rain mechanism for generation of atmospheric sulfuric acid. In this talk we will review the traditional acid rain mechanism and we will introduce a new acid rain mechanism that relies on the photochemistry of SO2 and show how this new chemistry can be an important ingredient in the overall mechanism of acid rain formation not yet considered by current atmospheric models.

Sulfur dioxide has been proposed in solar geoengineering as a precursor of H2SO4 aerosol, a cooling agent active in the stratosphere to contrast climate change due to the anthropogenic emissions of greenhouse carbon dioxide. Considering the introduction of SO2 in the stratosphere, the photochemistry of HOSO is critical to understanding the role of SO2 mitigating climate change. The spectroscopy and photochemistry this new species provide important insights that help to better understand SO2 chemistry in earth's upper atmosphere.