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-O3 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 O3 concentrations in the urban downwind area; this is the typical scenario of the formation of high summertime O3 levels over the Beijing area. The typical high-O3 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 O3 formation, while the urban downwind area changed gradually to a NOx-limited condition. The urban downwind area, Dingling, is readily affected by intrusion of the urban plume. Detailed process analysis indicated that although the O3 peaks reached the same level on 26 and 27 June 2000, the dominant process contributing to the O3 increase was different on each day. On 26 June, the major contributor to the O3 peak at Dingling was the arrival of high O3 levels with propagation of the urban plume; in contrast, on 27 June O3 precursors were transported rapidly to the urban downwind area by strong transportation processes that occurred earlier than those on the previous day, resulting in O3 production by local photochemistry becoming the major contributor to the high-O3 episode.