Long-Term Variations and Influencing Factors of Low-Visibility Events over the Coast of China

The frequency of low visibility pollution events in China have changed greatly during recent decades. In addition, the coexistence of fog and haze (fog-haze day) has become more and more common occurrence. In order to understand the long-term variation characteristics and influencing factors of low visibility events over coastal of China, 37 consecutive years (from 1980 to 2017) data of 67 meteorological stations, which located less than 5 km from the coastline, are statistically analyzed. The effects of the meteorological and pollution conditions are investigated based on reanalysis data and satellite observation. In general, the pollution over the northern coast is more serious than that over southern coast, with the most polluted in the Bohai Bay area. The frequency of low visibility in port areas has significantly increased due to the impact of ship emissions. The number of fog days and haze days exhibit the opposite temporal variation pattern, i.e., fog days show a decreasing trend from 1980 to 2004 followed by a rise after 2004, while haze days increases at first and then decreases. The fog-haze events at high relative humidity (90 %-95 %) are more frequent than that at low relative humidity (80 %-85 %). Atmospheric thermodynamic structure, air-sea temperature difference, monsoon and pollutant emission are critical factors affecting the variations of low visibility events. The key meteorological factors affecting low visibility events are temperature, wind and water vapor pressure. Some climate indices are also strongly correlated with low visibility events, especially sea surface temperature (SST) index including West Wind Drift Current SST Index, Atlantic Multi-decadal Oscillation Index, NINO A SSTA Index, etc.