Photochemical production of ozone in Beijing during the 2008 Olympic Games

Abstract

As a part of the CAREBeijing-2008 campaign, observations of O₃, oxides of nitrogen (NO_x and NO_y), CO, and hydrocarbons (NMHCs) were carried out at the air quality observatory of the Peking University in Beijing, China during August 2008, including the period of the 29th Summer Olympic Games. The measurements were compared with those of the CAREBeijing-2006 campaign to evaluate the effectiveness of the air pollution control measures, which were conducted for improving the air quality in Beijing during the Olympics. The results indicate that significant reduction in the emissions of primary air pollutants had been achieved; the monthly averaged mixing ratios of NO_x, NO_y, CO, and NMHCs decreased by 42.2, 56.5, 27.8, and 49.7 %, respectively. In contrast to the primary pollutants, the averaged mixing ratio of O₃ increased by 42.2 %. Nevertheless, it was revealed that the ambient levels of total oxidant (O_x = O₃+NO₂+1.5 NO_z) and NO_z were reduced by 21.3 and 77.4 %, respectively. The contradictions between O₃ and O_x were further examined in two case studies. Ozone production rates of 30–70 ppbv h⁻¹ and OPEx of ~8 mole mole⁻¹ were observed on a clear-sky day in spite of the reduced levels of precursors. In that case, it was found that the mixing ratio of O₃ increased with the increasing NO₂/NO ratio, whereas the NO_z mixing ratio leveled off when NO₂/NO>8. Consequently, the ratio of O₃ to NO_z increased to above 10, indicating the shift from VOC-sensitive regime to NO_x-sensitive regime. However, in the other case, it was found that the O₃ production was inhibited significantly due to substantial reduction in the NMHCs. According to the observations, it was suggested that the O₃ and/or O_x production rates in Beijing should have been reduced as a result of the reduction in the emissions of precursors during the Olympic period. However, the nighttime O₃ levels increased due to a decline in the NO-O₃ titration, and the midday O₃ peak levels were elevated because of the shift in the photochemical regime and the inhibition of NO_z formation.