关键词: 挥发性有机物, 经验动力学模拟方法, 后向轨迹聚类, 气象因素, 臭氧

Abstract: Objective　Volatile organic compounds (VOCs) are important precursors of tropospheric ozone (O3) and secondary organic aerosols, playing a critical role in regional atmospheric photochemical pollution. In recent years, with the rapid urbanization and industrial development in Xi'an, China, air pollution problems characterized by increasing O3 concentrations have become increasingly prominent in summer. Therefore, understanding the temporal variation characteristics of VOCs and their relationship with O3 formation is essential for developing effective air pollution control strategies. In this study, continuous monitoring of VOCs was conducted in the urban area of Xi'an during the summer of 2023. The main objectives were to analyze the concentration levels and temporal variation characteristics of VOCs, investigate the influence of meteorological conditions and anthropogenic emissions on VOCs, identify the regional transport pathways of air masses affecting VOCs concentrations, and evaluate the contribution of VOCs to ozone formation. Methods　Continuous monitoring data of VOC concentrations in Xi'an from June to August 2023 were collected and analyzed to investigate their temporal variation characteristics. Regression analysis was applied to examine the relationships between VOC concentrations and meteorological factors, including temperature, relative humidity, wind speed, and precipitation. In addition, traffic flow data were introduced to evaluate the influence of motor vehicle emissions on VOC variations in the urban area. To further investigate the regional transport characteristics of air masses affecting VOC concentrations, the HYSPLIT backward trajectory model was used to calculate and cluster the trajectories of air masses arriving in Xi'an during the observation period. Meanwhile, the OZIPR photochemical model was applied to analyze the sensitivity relationship between VOCs and nitrogen oxides (NOx) during O3 formation through EKMA curve analysis. By integrating monitoring data, meteorological information, and model simulation results, the study comprehensively assessed the influencing factors of VOC concentrations and their potential impact on O3 formation in Xi'an. Results and Discussion　The results showed that the average concentration of VOCs in Xi'an during the summer of 2023 was (3.001 ± 0.634) μg/m³, indicating a relatively high level of VOC pollution in the urban atmosphere. The monthly variation characteristics showed that the VOC concentrations in Xi'an in July and August were significantly higher than those in June, which may be related to higher temperatures and stronger photochemical activity in midsummer. The diurnal variation of VOCs exhibited a pronounced "V"-shaped pattern and showed a negative correlation with ozone concentrations. Specifically, the concentrations of VOCs reached their peaks at approximately 08:00 in the morning and around 22:00 at night. The morning peak was mainly related to the increase of traffic emissions during rush hours and the relatively stable atmospheric conditions that limited the diffusion of pollutants. The nighttime peak occurred after the traffic rush hours, indicating a certain lag effect caused by atmospheric accumulation and reduced boundary layer height during nighttime. The lowest VOC concentrations generally appeared between 16:00 and 18:00, which may be attributed to enhanced atmospheric mixing and stronger photochemical reactions that promote the consumption of VOCs. Meteorological factors exhibited different degrees of influence on VOC concentrations. Among them, temperature showed the strongest correlation with VOC variations, with a significant negative correlation (−0.617 < R < −0.456), indicating that higher temperatures tend to enhance photochemical reactions and accelerate the consumption of VOCs in the atmosphere. Relative humidity also showed a notable correlation with VOC concentrations, generally presenting a positive relationship ( −0.535 < R < −0.486), which suggests that humid conditions may favor the accumulation of VOCs or the reduction of VOC dispersion. In contrast, wind speed and precipitation showed relatively weak correlations with VOC concentrations during the observation period. The analysis of traffic data further indicated that motor vehicle emissions were an important source contributing to VOC variations in Xi'an. The concentrations of VOCs on weekdays were generally higher than those on non-working days, reflecting the significant influence of urban traffic activities on atmospheric VOC levels. Furthermore, the backward trajectory clustering analysis based on the HYSPLIT model indicated that the air masses affecting Xi'an during the summer of 2023 were mainly transported over short and medium distances. Among the identified transport pathways, the air masses originating from the southeast had the greatest impact on urban VOC concentrations in Xi'an, suggesting that regional transportation from surrounding areas may contribute to the accumulation of VOCs in the city. In addition, the EKMA curve analysis based on the OZIPR model showed that O3 formation in Xi'an during summer was in a VOCs-sensitive regime, which indicating that reducing VOC emissions is more effective than reducing NOx emissions in controlling O3 pollution under current atmospheric conditions. The results also highlight the close relationship between VOC concentrations and O3 formation, as an increase in VOC levels is often accompanied by an increase in O3 concentrations. Conclusion　This study systematically analyzed the temporal variation characteristics, influencing factors, and regional transport patterns of VOCs in the summer of 2023 in Xi'an City. The results demonstrated that the VOC concentrations exhibited clear diurnal and monthly variation patterns, and were strongly influenced by meteorological conditions and motor vehicle emissions. Regional air mass transport also played an important role in shaping VOC distribution in the urban atmosphere. Moreover, O3 formation in summer 2023 in Xi'an during was primarily controlled by VOCs-sensitive photochemical conditions, indicating that reducing VOCs emissions will be an effective strategy for mitigating O3 pollution. These findings provide important scientific evidence for understanding the characteristics of VOC pollution and valuable support for the formulation of targeted air pollution prevention and control measures in Xi'an.

Key words: volatile organic compounds, empirical kinetic modeling approach (EKMA), backward trajectory clustering; meteorological factor, ozone