Elevated ground-level ozone (O 3 ), reflecting atmospheric oxidative capacity, are of increasing concern. High levels of total oxidants (O x = O 3 + NO 2 ) have been persistently observed as a feature of Beijing's air pollution. Beijing is a well-known megacity requiring the enforcement of stringent air quality controls as rapid economic growth continues. To evaluate the effect of air quality controls in recent years, ground-based on-line measurements at an urban site were conducted in summer and the variations in O 3 with simultaneous changes in NO x and volatile organic compounds (VOCs) between 2005 and 2011 were analyzed. Both NO x and total VOCs in Beijing decreased over the study period, 1.4 ppbv yr −1 and 1.6 ppbv yr −1 , respectively. However, VOCs reactivity, in terms of OH loss rate, showed an indistinct statistical trend due to unsteady variations from naturally emitted isoprene, though some anthropogenic species showed decreasing trends, such as pentane, benzene and toluene. Meanwhile, daytime average O 3 increased rapidly at an annual rate of 2.6 ppbv yr −1 , around 5 % yr −1 between 2005 and 2011. Considering the influence of NO titration effect and elevated regional ozone background in the North China Plain (NCP), the main reason for such an increase in oxidants was subject to "local" photochemistry. A simplified model was used to evaluate the effect of changes in the levels of ozone precursors on ozone production. We found that between 2001 and 2006, the production rate of total oxidants, P (O x ) increased rapidly due to increased VOC levels and decreasing NO 2 , while from 2006 to 2011 P (O x ) re-mained high, though decreased slightly as a consequence of the decrease in both VOC reactivity (−5 % yr −1 ) and NO x (−4 % yr −1 ). Observations have shown that Beijing's efforts to control air pollution were somehow effective in cutting ozone precursors, but still left higher ground-level ozone. We surmised that it resulted from potential contributions from OVOCs and regional transport near Beijing. Therefore, Beijing needs deeper cooperation with adjacent provinces to control ozone pollution together. To impel this kind of joint prevention and control program, ground-level ozone should become a mandatory index for air quality management, and a faster reduction of VOCs, especially reactive VOCs, in urban areas, should coordinate with national NO x emission control programs.

Abstract. China implemented systematic air pollution control measures during the 2008 Beijing Summer Olympics and Paralympics to improve air quality. This study used a versatile mobile laboratory to conduct in situ monitoring of on-road air pollutants along Beijing's Fourth Ring Road on 31 selected days before, during, and after the Olympics air pollution control period. A suite of instruments with response times of less than 30 s was used to measure temporal and spatial variations in traffic-related air pollutants, including NO x , CO, PM 1.0 surface area (S(PM 1 )), black carbon (BC), and benzene, toluene, the sum of ethylbenzene, and m-, p-, and o-xylene (BTEX). During the Olympics (8-23 August, 2008), on-road air pollutant concentrations decreased significantly, by up to 54% for CO, 41% for NO x , 70% for SO 2 , 66% for BTEX, 12% for BC, and 18% for S PM 1 , compared with the pre-control period (before 20 July). Concentrations increased again after the control period ended (after 20 September), with average increases of 33% for CO, 42% for NO x , 60% for SO 2 , 40% for BTEX, 26% for BC, and 37% for S(PM 1 ), relative to the control period. Variations in pollutants concentrations were correlated with changes in traffic speed and the number and types of vehicles on the road. Throughout the measurement periods, the concentrations of NO x , CO, and BTEX varied markedly with the numbers of light-and medium-duty vehicles (LDVs and MDVs, respectively) on the road. Only after 8 August was a noticeable relationship found between BC and S(PM 1 ) and the number of heavy-duty vehicles (HDVs). Additionally, Correspondence to: T. Zhu (tzhu@pku.edu.cn) BC and S(PM 1 ) showed a strong correlation with SO 2 before the Olympics, indicating possible industrial sources from local emissions as well as regional transport activities in the Beijing area. Such factors were identified in measurements conducted on 6 August in an area southwest of Beijing. The ratio of benzene to toluene, a good indicator of traffic emissions, shifted suddenly from about 0.26 before the Olympics to approximately 0.48 after the Olympics began. This finding suggests that regulations on traffic volume and restrictions on the use of painting solvents were effective after the Olympics began. This study demonstrated the effectiveness of air pollution control measures and identified local and regional pollution sources within and surrounding the city of Beijing. The findings will be invaluable for emission inventory evaluations and model verifications.

Abstract. In conjunction with hosting the 2008 Beijing Olympics, the municipal government implemented a series of stringent air quality control measures. To assess the impacts on variation of ambient non-methane hydrocarbons (NMHCs), the whole air was sampled by canisters at one urban site and two suburban sites in Beijing, and 55 NMHC species were quantified by gas chromatography equipped with a quadrupole mass spectrometer and a flame ionization detector (GC/MSD/FID) as parts of the field Campaign for the Beijing Olympic Games Air Quality program (CareBeijing). According to the control measures, the data were presented according to four periods: 18-30 June, 8-19 July, 15-24 August (during the Olympic Games), and 6-15 September (during the Paralympic Games). Compared with the levels in June, the mixing ratios of NMHCs obtained in the Olympic and Paralympic Games periods were reduced by 35% and 25%, respectively. Source contributions were calculated using a chemical mass balance model (CMB 8.2). After implementing the control measures, emissions from target sources were obviously reduced, and reductions in vehicle exhaust could explain 48-82% of the reductions of ambient NMHCs. Reductions in emissions from gasoline evaporation, paint and solvent use, and the chemical industry contributed 9-40%, 3-24%, and 1-5%, respectively, to reductions of ambient NMHCs. Sources of liquefied petroleum gas (LPG) and biogenic emissions were not controlled, and contributions from these sources from July to September were stable or even higher than in June. Ozone formation potentials (OFPs) were calculated for the measured NMHCs. The total OFPs during the Olympic and Paralympic Games were reduced by 48% and 32%, respectively, compared with values in June. Reductions in the OFPs of alkenes and aromatics explained 77-92% of total OFP reductions. The alkenes andCorrespondence to: M. Shao (mshao@pku.edu.cn) aromatics were mainly from vehicle exhausts, and reductions of vehicle exhaust gases explained 67-87% of reductions in alkenes and 38-80% of reductions in aromatics. These findings demonstrate the effectiveness of the air quality control measures enacted for the 2008 Olympics and indicate that controlling vehicular emissions could be the most important measure to improve air quality in Beijing.