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

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 (NOx, 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 NOx. 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 NOx 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 NOx control policies. Our results do support that the simultaneous controls of NOx and VOCs were effective in reductions of tropospheric O3 in Luoyang. Meanwhile, joint regional control policies on the emissions of NOx and VOCs can potentially overwhelm the current O3 pollutions in China.