Traffic and seasonal dependence of the light absorption coefficient in santiago de chile.

We have designed and built a compact, low-cost automated system to measure the optical absorption coefficient of air. Because most of the light absorption is due to black carbon, this method is a direct measure of the amount of black carbon in the atmosphere. The equipment was used to measure absorption over a period of one year in a central area of Santiago. Our results show a strong correlation with the daily traffic pattern. The highest value of the absorption coefficient during most of the year occurs during the morning rush hour (0700-0800), and the lowest value either early in the morning (0300-0500) or in the afternoon (1400-1700). The absorption coefficient also shows a strong dependence with the season of the year, with values 10-20 times higher in winter than in summer. The data show that, during most of the year, the amount of black carbon present in the atmosphere is due to traffic. At night, during winter, the high concentration of black carbon is due to the temperature inversion effect.