Hydrogen Peroxide in Gas, Liquid and Aerosol Phases over Beijing

Abstract:

Multiphase partitioning is an important way for cycling of atmospheric oxidants and the growth of particulate matters. Peroxides are an important oxidant in the gas, liquid and particle phases and a highly active free radical reservoir in the atmosphere. They are widely involved in atmospheric chemical reactions and have an important impact on atmospheric oxidation power Thus, they are considered as an indicator of atmospheric oxidation capacity. The atmospheric peroxides are very unstable and prone to decomposition reactions, making accurate measurements difficult. We developed the peroxide measuring instrument, which can quantitatively determine atmospheric hydrogen peroxide (H₂O₂) and certain organic peroxides. We carried out multiple simultaneous observations of atmospheric peroxides in gas-liquid and gas-particle phases in the Beijing area. It is found that the observed H₂O₂ level in Beijing gas, rain and PM_2.5 particles was similar to that in the United States urban areas. Taking into account that the Beijing atmosphere can quickly consume H₂O₂ by high sulfur dioxide (SO₂) and other reducing substances and transition metals (such as iron ion), such a observed H₂O₂ level indicates that the Beijing atmospheric oxidation capacity is significantly stronger than that in cities of the United States. Moreover, unlike the observed H₂O₂ gas-liquid partitioning, which in general obeyed the Henry's law, the observed effective gas-particle partitioning coefficient of H₂O₂ was far greater than that expected by the existing theory. It was found that the particulate phase organic peroxide concentration was 1 order of magnitude higher than the particulate phase H₂O₂ concentration. The decomposition /hydrolysis of organic peroxides may be the main "missing source" of H₂O₂ in PM_2.5 particles.