Photochemical Aging of Atmospheric Fine Particles as a Potential Source for Gas-Phase Hydrogen Peroxide.

Atmospheric hydrogen peroxide (HO), as an important oxidant, plays a key role in atmospheric sulfate formation, affecting the global radiation budget and causing acid rain deposition. The disproportionation reactions of hydroperoxyl radicals (HO) in both gas and aqueous phases have long been considered as dominant sources for atmospheric HO. However, these known sources cannot explain the significant formation of HO in polluted areas under the conditions of high NO levels and low ambient relative humidity (RH). Here, we show that under relatively dry conditions during daytime, atmospheric fine particles directly produce abundant gas-phase HO. The formation of HO is verified to be by a reaction between the particle surface -OH group and HO radicals formed by photooxidation of chromophoric dissolved organic matters (CDOMs), which is slightly influenced by the presence of high NO levels but remarkably accelerated by water vapor and O. In contrast to aqueous-phase chemistry, transition metal ions (TMIs) are found to significantly suppress HO formation from the atmospheric fine particles. The HO formed from relatively dry particles can be directly involved in in situ SO oxidation, leading to sulfate formation. As CDOMs are ubiquitous in atmospheric fine particles, their daytime photochemistry is expected to play important roles in formation of HO and sulfate worldwide.