Abstract
The characteristics of meteorological conditions and air pollution were investigated in a valley city (Yangsan) on bad visibility days (from 05:00 to 09:00 LST) of the cold half year (November 2008 to April 2009). This analysis was performed using the hourly observed data of meteorological variables (temperature, wind speed and direction, relative humidity, and 2 m and 10 m temperature) and air pollutants (<TEX>$NO_2$</TEX>, <TEX>$SO_2$</TEX>, <TEX>$PM_{10}$</TEX>, and <TEX>$O_3$</TEX>). In addition, visibility data based on visual measurements and a visibility meter were used. The bad visibility days were classified into four types: fog, mist, haze, and the mixture (mist+haze). The results showed that the bad visibility days of the four types in the valley city were observed to be more frequently (about 50% of the total study period (99 days except for missing data)) than (27%) those near coastal metropolitan city (Busan). The misty days (39%) in the valley city were the most dominant followed by the hazy (37%), mixture (14%), and foggy days (10%). The visibility degradation on the misty days in Yangsan was closely related to the combined effect of high-level relative humidity due to the accumulation of water vapor from various sources (e.g. river, stream, and vegetation) and strong inversion due to the development of surface radiative cooling within the valley. On the hazy days, the visibility was mainly reduced by the increase in air pollutant (except for <TEX>$O_3$</TEX>) concentrations from the dense emission sources under local conditions of weaker winds from the day before and stronger inversion than the misty days. The concentrations of <TEX>$NO_2$</TEX>, <TEX>$PM_{10}$</TEX>, and <TEX>$SO_2$</TEX> (up to +36 ppb, <TEX>$+25{\mu}g/m^3$</TEX>, and +7 ppb) on the hazy days were a factor of 1.4-2.3 higher than those (+25 ppb, <TEX>$+14{\mu}g/m^3$</TEX>, and +3 ppb) on the misty days.
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