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

Analysis of observed PM2.5 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 PM2.5 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 PM2.5 concentration in 2016 winter compared to 2012 winter level. We removed emission influence on PM2.5 to get PM2.5 that influenced by meteorology (met-influenced PM2.5). For met-influenced PM2.5, compared to original-observed PM2.5(the US Embassy data), percentage of clean days (daily PM2.5 ≤ 75 μg m−3) decreases while that of polluted (75 μg m−3 150 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. 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 o