The photochemistry of SO2 at the air-water interface of water droplets leads to the formation of HOSO radicals. Using first-principles simulations, we show that HOSO displays an unforeseen strong acidity (pK(a) = -1) comparable with that of nitric acid and is fully dissociated at the air-water interface. Accordingly, this radical might play an important role in acid rain formation. Potential implications are discussed.