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 × 10, 2.6 × 10, 3.1 × 10, and 2.2 × 10 cm, 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 (O), volatile organic compounds (VOCs), nitrogen dioxide (NO) and fine particulate matter (PM)), the variation of OH radicals were closely related to ozone concentration especially in spring and summer. In autumn, the correlation between PM 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, NO and PM interplayed and leaded to the Event 2.