Seasonal variation characteristics of hydroxyl radical pollution and its potential formation mechanism during the daytime in Lanzhou

Hydroxyl free radicals(OH radicals) play the main role in atmospheric chemistry and their involving reactions are the dominant rate determining step in the formation of secondary fine particulate matter and in the removal of air pollutants from the atmosphere.In this paper,we studied the seasonal variation characteristics of OH radicals during the daytime in Lanzhou and explored the potential formation mechanism of high concentration OH radicals.We found that the OH radicals in four seasons was 2.7×106,2.6×106,3.1×106, and2.2×106 cm-3,respectively.Since the rainfall was concentrated in summer,the wet deposition had a significant effect on removing OH radicals.Among the four pollutants(including ozone(O3),volatile organic compounds(VOCs),nitrogen dioxide(NO2) and fine particulate matter(PM2.5)),the variation of OH radicals were closely related to ozone concentration especially in spring and summer.In autumn,the correlation between PM2.5 and OH radicals were the closest among the observing pollutants and its formation mechanism was different conventional regeneration pathway.In Event 1,high concentration of ozone was the main source of OH radicals;under the high humidity condition,except for ozone,the multiple factors including VOCs,NO2 and PM2.5 interplayed and leaded to the Event 2.

Potential factors and mechanism of particulate matters explosive increase induced by free radicals oxidation

Atmospheric particulate pollution in China has attracted much public attention.Occasionally, the particle number concentration increases sharply in a short time period,which is defined as a "particulate matter explosive increase". Heavy particulate matter pollution not only reduces visibility but also has an adverse effect on human health. Hence,there is an urgent need to discover the causes of particulate matter explosive increase.During this campaign, the particle number concentration and free radicals were measured at a tall building on the campus of Lanzhou University of Technology. Additionally, we examined a series of chemicals to reproduce the observed particulate matter explosive increase in a smog chamber to determine its potential factors. Then, we analyzed the mechanism of particulate matter explosive increase in the presence of free radicals. We found that, among the potential inorganic and organic sources analyzed, a mixture of organic and SO_2 in the research region had a major effect on particulate matter explosive increase. Moreover, free radical oxidation has a large effect, especially in the formation of organic particulates.