Study on the synoptic flow patterns and boundary layer process of the severe haze events over the North China Plain in January 2013

Air quality is significantly influenced by the synoptic, regional and local meteorological conditions. This study aims at elucidating the relation between synoptic flow patterns and low visibility events of haze and fog over the North China Plain (NCP), and the contribution of synoptic flow patterns and boundary layer structure to the severe haze events over the NCP in January 2013. Nine synoptic flow types are statistically identified over the Northern China for autumn and winter of 2004-2014. The flow types with high pressure to the northeast of the NCP (NEH, type 8), weak low pressure band (L-, type 5), high pressure to the southeast (SHE, type 4), and high pressure to the north (NH, type 6) are associated with high occurrence frequencies of low visibility events (48.3%, 42.0%, 37.2%, and 36.7%). The meteorological conditions of these flow patterns reveal synergistic contribution of weak wind and high relative humidity (RH) to low visibility. Quantitative measures for dispersion conditions (recirculation, ventilation, and stagnation) suggest undesirable ventilation and frequent stagnation of the flow types 9 (EH, high pressure to the east), 4, 5, and 8. In January 2013 three regional haze episodes are identified from the distribution of visibility over the NCP, i.e., 10-16 January (EP 1), 22-24 January (EP 2), and 28 January-1 February (EP 3), which were largely associated with the flow types 5, 8, 4, and 9. Coverage of the hazy area exhibited northward expansion in the EP 2 and EP 3 when the RH increased. The abnormally high RH could be attributed to the flow type 6 (NH), which has the highest frequency of precipitation (13.7%) and RH among the nine flow types, and occurred more frequently in that month than in January 2004-2014. The simulation results indicate the evolution of the planetary boundary layer and southerly advection, which was responsible for the high RH and persistent temperature inversion that contributed to the long-lasting haze events.