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Great difference exists in the aerosol optical depth (AOD) between summer and winter over the North China Plain (NCP). Monthly mean AOD at 550 nm derived from the MODIS (MODerate Resolution Imaging Spectroradiometer) products during 2000–2014 over the area of 30–40° N and 110–125° E exhibits an annual maximum in June (0.855 ± 0.130) and a minimum in December (0.381 ± 0.032). This seasonality of AOD is in the opposite phase with the surface particulate matter (PM) concentration (higher in winter and lower in summer). The possible causes for the higher AOD in June (compared with December) include (a) a higher boundary layer height (BLH) that results in more efficient transport and mixing of aerosol particles to a higher altitude (corresponding to a lower particle concentration near surface) as revealed by the Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations profile, (b) a higher relative humidity (RH) due to the inshore monsoon circulation that leads to enhancement of aerosol extinction, (c) emission from the regional open stalk burning in the summer harvest season (as seen from MODIS fire products), and (d) the typical eastward open topographical basin over NCP. Under the assumption that the aerosol and water vapor are well mixed within the boundary layer, analysis on multi-year average shows that the differences in BLH, RH and surface PM concentration can explain up to 81% of the variance of monthly averaged AOD over NCP. A preliminarily hypothesis is also suggested to interpret the shift of AOD pattern from winter to summer with an abrupt increase of AOD from May to June, as well as an increase of surface PM 2.5 concentration over NCP during the early phase of northward progress of the East Asia summer monsoon front. [ABSTRACT FROM AUTHOR]

Regional haze-fog events over the North China Plain (NCP) have attracted much attention in recent years. Their increase has been attributed to anthropogenic emissions of air pollutants and synoptic weather conditions. We investigated the influence of local meteorological conditions and large-scale circulation on the haze-fog events over the NCP during 2001–2012, and found a significant interannual increase in the number of summer regional haze-fog days. Analysis indicated that local meteorological conditions could partly explain the increase but failed to explain the spatial variation; meanwhile, regional circulation change induced by large-scale circulation and orographic forcing unveiled a possible spatiotemporal variation mechanism. In summer, the prevalent southerly winds over the NCP were obstructed by the Taihang and Yanshan mountains, steadying the outflow direction to the southeast, while different inflow direction controlled by large-scale circulation had different effects on regional circulation. In weak (strong) East Asian summer monsoon years, an intensified eastward (westward) zonal inflow wind component reinforced (weakened) the negative vorticity and formed an anomalous anticyclone (cyclone), which strengthened (weakened) the downward motion, so the dissipation capability was weakened (strengthened) and the wind speed decreased (increased), ultimately resulting in the increased (decreased) occurrence of haze-fog. We also found that the circulation anomaly had a good relationship with strong El Niño and La Niña events. There was more haze-fog over the NCP in the summers that followed a La Niña event, and less in summers that followed an El Niño event. This suggested the possibility that summer haze-fog phenomena could be predicted based on the phase of ENSO. [ABSTRACT FROM AUTHOR]