Analysis of variation characteristics and influencing factors of atmospheric visibility in Tianjin urban area from 2013 to 2016

Using the MODLE6000 type forward-scattering visibility meter produced by Belfort, the atmospheric visibility was measured over 2013-2016 in Tianjin urban area, and the variation characteristics and influencing factors of atmospheric visibility were analyzed, combined with the simultaneous PM_2.5 concentration and related meteorological data. The results showed that the average atmospheric visibility was 11.23 km during the measurement period. The annually averaged visibility increased year by year in the 3 years, which should be related to the decrease of PM_2.5 concentration in Tianjin area. The seasonal variation of atmospheric visibility in Tianjin urban area is obvious, with higher visibility in spring, lower in summer and lowest in autumn. There are significant morning and evening peaks in diurnal change of visibility, and the decrease of visibility is mainly due to joint impacts of PM_2.5 concentration and diurnal variation of RH. The continuous low visibility (<10 km) weather events lasting more than 72 hours have decreased in recent years. However, the continuous low visibility (<5 km) weather events lasting more than 72 hours still occurred frequently in recent years. The low visibility weather events occurred mostly in autumn and winter, which were mainly related to the adverse diffusion conditions.