Spatial and Temporal trend in horizontal visibility and extinction coefficient in Northwest Iran

Horizontal visibility variations represent changes in air quality. This study is carried out to investigate trend in horizontal visibility in northwest of Iran during 1985-2020. In this regard hourly observational visibility data from 27 meteorological stations was obtained from IRIMO. After screening and selecting the reliable data series, extinction coefficient was calculated by Kashmeider's approach. Ridit analysis and Mann-Kendall statistical tests were used to detect trends in visibility and extinction coefficient. Also, 5 visibility categories were determined and the percentages of each category got calculated. Results showed that the annual regional-average of horizontal visibility (extinction coefficient) was calculated as ~11.4 (0.173) km in the studied period. Ridit output variations showed that visibility improvement / weakening periods was observed in different times according to the environmental conditions of each station. In most stations, the increasing trend of horizontal visibility often formed through the reduction of average visibility percentages; increase in good and very good visibilities and finally stability in bad and very bad visibilities. The highest (lowest) seasonal extinction coefficient obtained for winter (summer) with 0.179 (0.168) km. A perfect inverse correspondence was observed between ridit outputs and extinction coefficient in terms of fluctuation periods and trend direction in all stations. Spatial distribution of parameters showed that the highest (lowest) values of ridit (extinction coefficient) were obtained in several stations located in the north of the studied area. In 18 stations out of 27 studied stations, the increasing (decrease) trend of ridit (extinction coefficient) was detected, which both indicate the increasing trend of the horizontal visibility in the northwest of Iran.