**2.1 Site**

Beijing, the capital of the People's Republic of China, is located at 39°56' N latitude and 116°20' E longitude. East, North and West of Beijing are surrounded by mountains. The climate of Beijing is an East Asia monsoon type, with cold and dry winters, and hot and humid summers. During winter, the Siberian air masses that move southward across the Mongolian Plateau are accompanied by cold and dry air. In summer, the air mass is hot owing to warm and humid monsoon winds from the southeast, bringing most of the annual precipitation in Beijing area.

The East Asian Monsoon season is the dominating climate of Beijing. In spring (March, April, May), the content of the water vapor in the atmosphere is low and there is little rain in this region. The rainy season begins in June, ends in August and then comes in the dry season. The main rainfall is in July. In autumn (September, October, November), the sky condition is always clear; for Beijing, it is often controlled by the anticyclone. In winter (December, January, February), Siberian anticyclones frequently take place in the Beijing area. Rainless and cloudless conditions are the dominant sky conditions in the region. In this paper, spring and winter are called the dry season because there are few rainfall events in these months. . Summer and autumn are called the humid season, for most of the rainfall occur in these two seasons.

The monsoon starts in July, and ends in October when the dry season begins. There are many active synoptic systems (depressions) in the humid period,. while stabilization system control prevails in the dry season and most days are clear in this period.

In the humid season, the southern wind from the ocean prevails bringing abundant vapor in to the atmosphere, thus the vapor content of the atmosphere is high in this season. Water vapor markedly affects the long wavelength radiation by absorbing them, leaving the UV spectral portion and the short wavelength spectral radiation for possible scattering and reflection. . Consequently, the general decrease in the global radiation could cause the ratio of UV to global radiation to increase as the water vapor increases.

In the dry season, the northern wind from the continent is the prevailing wind bringing dry and cold air. So the vapor content is low in the dry season and the ratio of UV to global radiation decreases as the water vapor decreases.

The seasonal variation pattern of the ratio is that the highest and lowest values appear in summer and winter respectively, and the ratio in autumn is smaller than that in spring. This variation trend is controlled by the water vapor content.

The experimental site is located in down town Beijing area, between the Fourth and the Third Ring Roads. This area is part of downtown Beijing. There are domestic dwellings to the north and south of this site,and a freeway near to it's east. The measurements in this area can represent the average conditions for Beijing. The sampling instruments are installed on a flat platform at the top of the chemical laboratories' building, of the Institute of Atmospheric Physics, Chinese Academy of Sciences.
