Impacts of relative humidity and PM2.5 concentration on atmospheric visibility: A comparative study of hourly observations of multiple stations

To comprehensively investigate fundamental characteristics and associated influencing mechanisms in different regions of Zhejiang province, based on hourly visibility data during 2013-2014 at Hangzhou station, Ningbo station and Wenzhou station, comparative analysis is conducted to study visibility variability at different time-scales for the three cities. It is found that the occurrence frequency of different visibility levels for the three cities are broadly consistent; with the rise of visibility level, the occurrence frequency gradually decreases. Besides, the visibility displays significant diurnal cycle. There are two low-visibility periods during the year, i. e. December to February and May to June. Generally speaking, the visibility in Ningbo is better while those in Hangzhou and Wenzhou are similar. Power spectrum analysis indicates that visibility in the three cities exhibits significant diurnal periodicity; many distinct peaks occur in the high-frequency range, and some peaks are also prominent in the low-frequency band. Further studies have been carried out to investigate the mechanism responsible for the change in visibility. Relative humidity and PM2.5 concentration are critically important and can effectively influence visibility. Increases in relative humidity and PM2.5 concentration can lead to the decline of visibility. At the same relative humidity level, the visibility drops rapidly in the beginning but decreases much slower after the "point of inflection". At the same level of PM2.5 concentration, the visibility gradually falls with the elevation of the relative humidity, which proves the importance of water vapor. Using the relative humidity and PM2.5 concentration as the impact factors, a quantitative statistical-model is constructed with nonlinear fitting scheme. It is demonstrated that the fitting result is pretty good. Finally, some existing problems and valuable scientific issues are discussed.