Distribution and regeneration of hydroxyl free radicals in gaseous and particulate phases of pollutants in near-ground ambient air

Abstract

Hydroxyl free radicals play the main role in atmospheric oxidative capacity. The formation of secondary fine particulate matter and the degradation of gaseous pollutants in the troposphere are dominated by hydroxyl free radical reactions. The harmful effect of particulate matters to human health is closely related to the free radicals distributed in the particulate matter phase. In this paper, the distribution of hydroxyl free radicals in near-ground ambient air was studied. The hydroxyl free radicals exist not only in the gaseous phase but also in the particulate phase, especially during heavy haze pollution. It is noteworthy that the concentration of hydroxyl free radicals in the particulate phase is about two times higher than that in the gaseous phase. The possible regeneration mechanisms are proposed. The excited electrons (e−) and holes (h+) formed e− - h+ pairs in particulate matters are identified to play a key role in promoting the regeneration of hydroxyl free radicals in particulate phase. Environmentally persistent free radicals in particulate phase could also further promote the chain reaction of free radicals and ultimately result in explosive regeneration in an environment that has been excited. The correlation between radical concentrations in gas species, particulate phases and particulate number concentration has been summary based on the actual monitoring site including Thermal power plant, Yumen street and Chemical street sites.