Seasonal characteristics of haze observed by continuous visibility monitoring in the urban atmosphere of Kwangju, Korea.

Continuous visibility monitoring has been carried out in Kwangju, Korea since May 1999. The total light extinction coefficient bext measured by a transmissometer and reveals seasonal trends in urban visual air quality, especially under hazy conditions with a visual range of less than 15 km. Seasonal atmospheric visibility under low relative humidity during the winter was observed to be better than during any other seasons. Summertime visibility was severely degraded due to highly increased light scattering by hygroscopic particles under high humidity atmospheric conditions. Visibility during spring and fall was also moderate. However, yellow sand in spring caused the lowest visibility conditions over the measurement area for a few days. With continuous monitoring using the transmissometer, the daily average seasonal visual range was measured to be 13.1, 9.2, 11.0, and 13.9 km in spring, summer, fall and winter, respectively. Under the atmospheric humidity condition less than 60%, visual range was observed to be 16.1, 13.9, 15.1, and 16.6 km in spring, summer, fall, and winter, respectively. The mean light extinction budget by sulfate and nitrate aerosols was determined to be the highest value of 63.71% during the summer and the lowest value of 27.08% during spring. During the 'yellow sand dust' period, a mean light extinction budget by soil particles was estimated to be at an unusually high value of 44.22%.