Long-term trends and variations in haze-related weather conditions in north China during 1980–2018 based on emission-weighted stagnation intensity

Abstract Recently, climatological and environmental researchers have paid significant attention to the long-term trends and variations in haze-related weather conditions in North China (NC). This study investigates this topic using a quantified air stagnation index (ASIE) that combines stagnation intensity with fixed emission information, given that haze occurrence depends strongly on the rate of emission. ASIE has a close spatial and temporal relationship with observed PM2.5 concentrations, and a strong sensitivity to haze occurrence in NC. The annual ASIE increased by 18.2% over the period 1980–2018 due to significant decreases in planetary boundary layer height and ventilation. However, there was an apparent drop during 2013–2018, which suggests that lower stagnation intensity may take effect on the improved air quality in NC reported in recent years. Such low-frequency oscillation occurred twice during 1980–2018. Hence, if the current trend of decreasing stagnation intensity reverses, haze events may become more common in the future. In addition, the interannual variations in stagnation intensity were very significant. The percentage change of ASIE was as high as 50%–70% in some years. Finally, using the year-to-year growth ratio of ASIE, we highlight the difficulty of the “clean air challenge” due to the variations in stagnation in NC. The results suggest that the enforcement of the emission reduction plan should be tailored according to the stagnation conditions in the region and period of interest.