Impact of water vapor content on visibility: Fog-haze conversion and its implications to pollution control

Atmospheric water vapor content, which reflects the total change of water vapor in the whole atmosphere, has an important effect on the atmospheric visibility (VIS). To investigate the variation of the VIS in Beijing during the past decades and to address the key factors affecting the VIS, the relationships of the VIS with the PM2.5 concentrations, the relative humidity (RH) and the total column water vapor (TCWV) have been explored in this study. The evolution characteristics of these factors in the fog-haze conversion process during the low-visibility (LVIS, VIS <10 km) events are also analyzed. Based on the relations between the TCWV and VIS, a novel method to distinguish haze and fog has been developed and employed in this study. The results show that the frequency of LVIS increases with the increasing of RH, TCWV and PM2.5 concentration. Under dry conditions (RH <= 40%), the VIS variation is mainly affected by the PM2.5 concentrations. Under moderate RH conditions (86% > RH > 40%), the VIS is more influenced by TCWV and RH. Under high RH and extremely low visibility conditions (RH > 86% and VIS <1 km), the water vapor is absorbed by aerosols, resulting in the conversion of haze to fog. This process leads to sharp decreases in TCWV and VIS, but an increase in PM2.5 concentrations simultaneously. This study provides a meaningful approach to identify the VIS evolution under the variations of RH, TCWV and PM2.5, which is helpful to the forecasting and early warning of LVIS events.