Research contents of air pollution meteorology

Meteorology of air pollution studies the diffusion, migration, transformation and removal of pollutants after entering the atmosphere by meteorological methods, providing scientific research basis for preventing and controlling air pollution and improving the atmospheric environment. The main contents of the book include atmospheric turbulence, atmospheric diffusion, pollution deposition theory and micrometeorology principle; The calculation of air pollution concentration under the conditions of plain, city and complex terrain focuses on atmospheric diffusion and air pollution on local and urban scales.

Air pollution and meteorology in plain areas: mainly study the transport and diffusion law of air pollutants over flat terrain, and estimate the impact of pollutants discharged from chimneys and leaked from ground workshops on the surrounding environment and downwind areas. It is an important basis for selecting the position and height of chimneys and rationally arranging factories and residential areas. Because the wind direction and wind speed in the plain area are basically the same on a certain horizontal plane, the transportation law of pollutants is relatively simple. The pollution frequency of pollution sources to surrounding areas is determined by the local wind direction frequency. Obviously, the leeward area with the highest frequency and wind direction has the most pollution opportunities. When the airflow bypasses the buildings in the factory area, a downward airflow will be generated on the downwind side. If the chimney mouth is in this airflow, pollutants will be carried to the leeward area. In order to avoid the impact on the nearby area, at least the chimney is one and a half times higher than the nearby buildings.

Atmospheric diffusivity is closely related to atmospheric stability (see atmospheric static stability). Under sunny and windy conditions: at night, radiation inversion occurs at a height of several hundred meters near the ground, the atmosphere is stable, turbulence is suppressed and diffusion is slow; At noon, when the vertical temperature drop rate is greater than1c/100m, the atmosphere is in an unstable state, with the development of thermal turbulence and strong diffusion ability. When the vertical drop rate of air temperature is equal to1c/100m, the atmosphere is in a neutral state, turbulence is maintained, and the diffusion capacity is between stable and unstable States. Under the condition of strong wind or cloud, although the increase of wind speed can increase the turbulence intensity, the intensity of inversion is weakened and the development of thermal convection is also affected because of the existence of strong wind or cloud. F pasquale classifies atmospheric stability into six categories according to solar radiation intensity (strong, medium and weak according to solar altitude angle, cloud shape and cloud amount), cloud amount and wind speed (see table), in which A is the most unstable, B is unstable, C is slightly unstable, D is neutral (cloudy day or night), E is stable and F is the most stable. When calculating the concentration of pollutants, the idealized model summarized by diffusion experiments is adopted. The formula for calculating the ground pollutant concentration χ in the downwind direction of continuous point source discharged from high chimney is: where q is the source intensity (the mass discharged per unit time); Is the average wind speed; Taking the emission source as the origin, the Y axis on the horizontal plane is perpendicular to the average wind direction, and the vertical direction of the Z axis is the positive direction; σy and σz are the atmospheric diffusion parameters in Y direction and Z direction, respectively, and H is the sum of chimney height and flue gas rising height (called chimney effective height). Under various stability conditions, the change of atmospheric diffusion parameters relative to the downwind distance x of the emission source can be found in the figure or calculated according to σ=ax. The values of a and b are related to atmospheric stability and determined by experiments. The gas leaked from ground equipment and workshop can be calculated as non-point source or line source, and the calculation formula can be derived from point source formula.

Air pollution and meteorology in the land-water boundary area: in coastal or lakeside areas, the local circulation caused by the temperature difference between land and water is called sea (lake) land wind (see sea-land wind). The low-level airflow transports the discharged pollutants to a certain distance, and then returns to the original place from the high altitude, which increases the pollution concentration in the original place. Sometimes the pollutants carried away by the land breeze are brought back by the sea breeze, which also increases the concentration of pollutants in the air. The photochemical smog in Los Angeles, USA, is produced under this circulation condition. In spring and summer, the water temperature is much lower than the surface temperature. The air on the water surface is heated when it flows over the land surface, which destroys the inversion layer formed above the water surface. At this time, the pollutants accumulated in the upper part of the inversion layer are brought to the ground by thermal convection, which increases the concentration of pollutants there. This is the so-called "smoking" phenomenon. This phenomenon can last for a long time because the lake surface inversion can last for several hours. Similarly, in autumn and winter, when the stable air from the land surface flows through the unfrozen water surface, there will also be a phenomenon of "smoking". Because the roughness of land surface is generally greater than that of water surface, the atmospheric turbulence diffusion on land surface is usually stronger than that on water surface. Air Pollution and Meteorology in Mountainous Areas: Terrain fluctuation makes the daily radiation intensity and radiation cooling of the sun uneven, and the resulting thermal circulation is called topographic wind. There is an uphill wind on the hillside during the day and a downhill wind at night; There is a valley wind during the day and a mountain wind at night in the valley (see valley wind). There will also be a closed circulation of mountain valley wind in the deep valley, and the reverse airflow above it is called reverse mountain valley wind. Between the mountain and the plain, there is also a kind of wind with large gradient (also called mountain-plain wind). In different positions of the valley, the air flow at different heights is very different, so the pollutant transport paths at different discharge points are also different. If the height of the chimney on the hillside is lower than the thickness of the slope wind (tens of meters), pollutants will be carried to the plain or valley by the downhill wind, which is not conducive to increasing the emission height of the chimney by using the hillside topography. If the discharge port is in a closed cycle, it is not conducive to the discharge of pollutants, which will increase the concentration of local pollutants. Smoke from tall chimneys caused serious pollution on windward slopes. The dynamic action of airflow across mountains produces downward airflow or vortex on leeward slope; Under unstable conditions, the valley wind will also sink when the wind speed is high. This will make the smoke from the chimney incline downward or sink to the ground. In the winding section of the valley, there is a small wind because of the terrain obstruction, which will aggravate the pollution in this area. The duration of inversion in mountainous areas is longer than that in plain areas, and multi-layer inversion may occur. The inversion layer and the valley form a "pipeline", which limits the diffusion of pollutants and aggravates the pollution in the downwind area. Pollutants accumulate between the two inversion layers, and when the inversion layer is destroyed, the phenomenon of "smoking" appears. Although the airflow disturbance caused by topography increases the turbulence intensity, the atmospheric diffusion parameters in mountainous areas are several times larger than those in plain areas. Because the horizontal transport is not as good as that in plain areas, the air pollution in mountainous areas (especially mountain basins and valleys) is usually more serious than that in plain areas. Urban air pollution and meteorology: Urban heat island effect makes the low-altitude temperature not appear at night (see urban climate), but it is still covered by a stable layer more than several hundred meters, and forms an urban mixed layer below the stable layer. Due to mixing, the vertical distribution of concentration in this layer tends to be uniform. At the same time, the heat island effect makes the cold air from the countryside converge and rise to the city, forming a heat island circulation. The horizontal and radial confluence of the circulation makes the pollutants close to the ground gather in the city, which intensifies the pollution of the city; On the other hand, its concentrated updraft makes the smoke from high chimneys rise and be transported to far places, which can also reduce the pollution to the city. In addition, the buildings in the city increase the roughness of the ground, weaken the vertical variation of wind speed, and form a complex local circulation due to the "passage" between buildings. All of the above shows that the transport process of pollutants in cities is quite complicated. But generally speaking, due to the disturbance of air flow and the thermal convection of urban buildings, the turbulence in cities is stronger than that in plain areas, so the atmospheric diffusion parameters are much larger than that in plain areas.