Atmospheric visibility and its affecting factors for the Geophysics Station, Tehran, for years 2001-2011

Atmospheric visibility is an important parameter that can affect human activities as well as being a good indicator of the air pollution condition in urban areas. Hence its monitoring is important for urban such areas as Tehran which often suffer from acute air pollution episodes. Atmospheric visibility is closely related to the meteorological conditions determined by local and regional factors such as topography, latitude, closeness to water body, etc.
    In this study, the atmospheric visibility for the years 2001-2011 at the Institute of Geophysics, Tehran, including the effects of meteorological conditions such as rain, fog and suspended dust on its variations were considered. The visibility and meteorological data were obtained from Meteorology Organization of Iran and the air pollution data are acquired from the air quality company of the municipalily of Tehran. In this ten-year period, the mean annual visibility of this city was about 8.7 km while the seasonal visibilities for spring, summer, autumn, and winter were 9.2, 9.7, 9.1, and 6.7 km, respectively. The largest visibility occured in August (9.9 km) while the least one occured in December (5.7 km). On the whole, the visibility was affected by rain, fog and dust from which dust was more effective in reducing the visibility especially in winters in which  its frequent value was less than 3 km, and is less than the other periods. Also in the cold season, the average visibility for relative humidities less than 60 percent increased from 1 to 2 km. The average annual PM10 concentration for the years 2007-2009, for which the whole data were available, was 68.9 μg/m3, with its maximum for August and December.
    The relationship between the visibilities and some other air pollution and meteorological factors were also considered. The largest negative correlation between the PM10 concentration and the visibility occured in spring and summer. This was due to the fact that the moisture content of air in this period was more than that of the rest of the year in this area. Moisture usually contributes to the formation of very fine urban aerosols and also makes larger aerosols known as haze. 
    City of Tehran is in the vicinity of a high mountain range that induces a very marked local circulation. This circulation is particularly marked in this area which is often away from active mid-latitude storm tracks leading to calm conditions. This study showed that as the daily wind turns from northerly katabatic to southerly anabatic during the morning transition, the visibility deteriorates due to the movement of the more polluted air over the station.
    This study also showed that poor visibilities less than 3 km often occured with high concentrations of PM10 in calm conditions over the city in which urban aerosols dominate especially in winter, or when strong dusty winds prevail.