A comprehensive study on ozone pollution in a megacity in North China Plain during summertime: Observations, source...

Tropospheric ozone (O₃) pollution has been becoming prominent in North China Plain (NCP) in China since last decade. In order to clarify the source contribution and formation mechanism of O₃, the critical precursors of volatile organic compounds (VOCs) were measured with both on-line and off-line methods in Luoyang City in summer of 2019. The concentrations of nitrogen oxides (NO_x, sum of NO and NO₂) and O₃ were simultaneously monitored. Fifty-seven VOCs measured in U.S. Photochemical Assessment Monitoring Station (PAMS) showed daily concentrations in a range of 14.5 ± 5.33 to 29.2 ± 11.2 ppbv in Luoyang, which were comparable with those in other Chinese megacities. The mass compositions of VOCs were determined, with comparatively low proportions of alkanes (<50%) but high fractions of photoreactive alkenes and alkyne. Source apportionment of VOCs was conducted by Hybrid Environmental Receptor Model (HERM). The results indicated that industrial (38.5%) and traffic (32.0%) were the two dominated pollution sources of VOCs in the urban, while the biogenic and residential sources had contributions of 15.8% and 13.8%, respectively. To further measure the O₃ formation sensitivity and its source attribution, the WRF-CHEM model was adopted in this study. The variation of O₃ between the observation and the stimulation using the local emission inventory showed an index of agreement (IOA) of 0.85. The simulation conducted by WRF-CHEM indicated an average of 43.5% of the O₃ was associated with the regional transportation, revealing the importance of inter-regional prevention and control policy. Traffic and biogenic emissions were the two major pollution sources to an O₃ episode occurred from July 21 to July 27, 2019 (when O₃ concentration over 150 μg m^-3) in Luoyang, with average contributions of 22.9% and 18.3%, respectively. The O₃ isopleths proved that its formation in the atmosphere of Luoyang was in transitional regime and collectively controlled by both VOCs and NO_x. This was different from the observations in main cities of NCP before implantations of strict emission controls. The isopleths additionally designated that the O₃ formation regime would move forward or shift to NO_x regime after a reduction of over 45% during the episode. Similar patterns were also reported in other Chinese megacities such as Beijing and Shanghai, due to the tightening of the NO_x control policies. Our results do support that the simultaneous controls of NO_x and VOCs were effective in reductions of tropospheric O₃ in Luoyang. Meanwhile, joint regional control policies on the emissions of NO_x and VOCs can potentially overwhelm the current O₃ pollutions in China.