Winter haze over North China Plain from 2009 to 2016: Influence of emission and meteorology.

Abstract Analysis of observed PM 2.5 in Beijing since 2009 reveals that winter haze over North China Plain (NCP) peaked in 2012 and 2013 and there was an improvement in air quality until 2016. The variation of wintertime PM 2.5 from 2009 to 2016 is influenced by both emission changes and meteorology conditions, and we quantified the relative contributions from these two aspects. Sensitivity simulation by GEOS-Chem suggested that emission reductions over NCP in 2013–2017 caused 10% decrease of regional mean PM 2.5 concentration in 2016 winter compared to 2012 winter level. We removed emission influence on PM 2.5 to get PM 2.5 that influenced by meteorology (met-influenced PM 2.5). For met-influenced PM 2.5 , compared to original-observed PM 2.5 (the US Embassy data), percentage of clean days (daily PM 2.5 ≤ 75 μg m⁻³) decreases while that of polluted (75 μg m⁻³ < daily PM 2.5 ≤150 μg m⁻³) and heavily polluted (150 μg m⁻³ < daily PM 2.5 ≤ 250 μg m⁻³) days increases. However, proportion of extremely polluted (daily PM 2.5 > 250 μg m⁻³) days stays unchanged, even if emission reduction is doubled, indicating that the extremely polluted situation over NCP is dominated by meteorological conditions, and emission control from 2013 to 2017 has little effects on the extremely polluted days. We developed an effective haze day index (HDI) to represent the weather conditions conducive to haze days. HDI is constructed based on the normalized near surface meridional wind (V850), temperature difference (δT) between near surface (850 hPa) and upper atmosphere (250 hPa), and the relative humidity at 1000 hPa (RH1000). HDI is skillful to detect 72% of the severe haze days (daily PM 2.5 > 150 μg m⁻³). On average, the anomalously high V850 is the main cause of severe haze, while in 2012 winter, RH1000 favorable for secondary aerosols' formation is the largest contributor to haze. Graphical abstract Proportion of the four pollution levels for (a) original observation, (b) met-influenced observation and (c) new met-influenced observation when doubled emission influence is removed. Changes begin to distinct since 2014. Percentage of clean days decreases and that of polluted and heavily polluted days increases from (a) to (b) and (c), indicating that the meteorology conditions from 2013 to 2016 winter would make the air quality worse than it was if the emission control strategies had not been implemented. Notably, the percentage of extremely polluted days stays stable, demonstrating that the emission control since 2013 is of no help to reduce the extremely polluted days in NCP. Image 1 Highlights • We quantified the contribution from emission and meteorology to wintertime PM 2.5. • Emission control over NCP has little benefit on extremely polluted days. • Haze day index (HDI) is skillful to detect 72% of the severe haze days. • The anomalously high meridional winds is the main cause of severe haze these years. • The RH favorable for aerosol formation is the largest contributor to haze in 2012. Emission control from 2013 to 2017 has little benefits on the reduction of extremely polluted days; on average, the anomalously high V850 is the main cause of severe haze. [ABSTRACT FROM AUTHOR]