Process analysis of typical summertime ozone episodes over the Beijing area

The 2008 Summer Olympic Games will be held in Beijing; however, summer is the season in which Beijing is marked by frequent high-O 3 episodes. Observation analysis reveals that the urban plume is transported by a southerly wind and undergoes chemical reaction during propagation, resulting in the common occurrence of high O 3 concentrations in the urban downwind area; this is the typical scenario of the formation of high summertime O 3 levels over the Beijing area. The typical high-O 3 episode is well reproduced by the 3-dimensional air-quality model CMAQ-MADRID (Community Multi-scale Air-Quality Model of Aerosol Dynamics, Reaction, Ionization, and Dissolution). Radical cycle analysis reveals that the Beijing urban area was in a VOC-limited regime for O 3 formation, while the urban downwind area changed gradually to a NO x -limited condition. The urban downwind area, Dingling, is readily affected by intrusion of the urban plume. Detailed process analysis indicated that although the O 3 peaks reached the same level on 26 and 27 June 2000, the dominant process contributing to the O 3 increase was different on each day. On 26 June, the major contributor to the O 3 peak at Dingling was the arrival of high O 3 levels with propagation of the urban plume; in contrast, on 27 June O 3 precursors were transported rapidly to the urban downwind area by strong transportation processes that occurred earlier than those on the previous day, resulting in O 3 production by local photochemistry becoming the major contributor to the high-O 3 episode.