Tầng trung lưu: từ khoảng 50 km đến 80-85 km, nhiệt độ giảm theo độ cao đạt đến -75 °C. Phần đỉnh tầng có một ít hơi nước, thỉnh thoảng có một vài vệt mây bạc gọi là mây dạ quang. | Factors That Affect Air Pollution 329 jZ T .T y fỊị 1 J- Cubatao Brazil just may be the most polluted city in the world. Located south of Sao Paulo this heavily industrialized area of 100 000 people lies in a coastal valley-known by local residents as the valley of death. Temperature inversions and stagnant air combine to trap the many pollutants that spew daily into the environment. Recently nearly one-third of the downtown residents suffered from respiratory disease and more babies are born deformed there than anywhere else in South America. pollutants trapped within the cool marine air are occasionally swept eastward by a sea breeze. This action carries smog from the coastal regions into the interior valleys see Fig. . THE ROLE OF TOPOGRAPHY The shape of the landscape topography plays an important part in trapping pollutants. We know from Chapter 3 that at night cold air tends to drain downhill where it settles into low-lying basins and valleys. The cold air can have several effects It can strengthen a preexisting surface inversion and it can carry pollutants downhill from the surrounding hillsides see Fig. . Valleys prone to pollution are those completely encased by mountains and hills. The surrounding mountains tend to block the prevailing wind. With light winds and a shallow mixing layer the poorly ventilated cold valley air can only slosh back and forth like a murky bowl of soup. Air pollution concentrations in mountain valleys tend to be greatest during the colder months. During the warmer months daytime heating can warm the sides of the valley to the point that upslope valley winds vent the pollutants upward like a chimney. Valleys susceptible to stagnant air exist in just about all mountainous regions. The pollution problem in several large cities is at least partly due to topography. For example the city of Los Angeles is surrounded on three sides by hills and mountains. Cool marine air from off the ocean moves inland and pushes against the .
