Urban Atmospheric Chemistry During the PUMA Campaign 2: Radical Budgets for OH, HO_2 and RO_2

摘要

A detailed photochemical box model was used to investigate the key reaction pathways between OH, HO_2 and RO_2 radicals during the summer and winter PUMA field campaigns in the urban city-centre of Birmingham in the UK. The model employed the most recent version of the Master Chemical Mechanism and was constrained to 15-minute average measurements of long-lived species determined in situ at the site. The results showed that in the summer, OH initiation was dominated by the reactions of ozone with alkenes, nitrous acid (HONO) photolysis and the reaction of excited oxygen atoms atoms with water. In the winter, ozone+alkene reactions were the primary initiation route, with a minor contribution from HONO photolysis. Photolysis of aldehydes was the main initiation route for HO_2, in both summer and winter. RO_2 initiation was dominated by the photolysis of aldehydes in the summer with a smaller contribution from ozone+alkenes, a situation that was reversed in the winter. At night, ozone+alkene reactions were the main radical source. Termination, under all conditions, primarily involved reactions with NO (OH) and NO_2 (OH and RCO_3). These results demonstrate the importance of ozone+alkene reactions in urban atmospheres, particularly when photolysis reactions were less important during winter and at nighttime. The implications for urban atmospheric chemistry are discussed.