Dynamic Simulation Of Photochemical Smog Formation In City And Its Effect Factor

Photochemical Smog is a kind of air pollution with high harm to human health and environment. Generally photochemical smog is composed of ozone,Peroxyacetyl nitrates (PAN) and nitric acid (HNO3) pollutants which are formed by photochemical reaction of nitrogen oxides and hydrocarbons under ultraviolet ray radiation. With the development of city economy in our country, the amounts of vehicles increases dramatically in cities, the photochemical smog problems have become more and more serious in some cities. Photochemical smog, which is mainly caused by vehicle exhausts in some cities has received a considerable research interest.In order to understand the impact of vehicle exhausts on photochemical smog in city and the relationships among the pollutant concentrations during photochemical fog formation reaction, the photochemical smog formation mechanism is explored firstly, and the current pollution levels and developing trends of exhaust pollution in big city are discussed. Then the simplified set of photochemistry dynamic model is established, and a numerical simulation of photochemical smog formation at different initial conditions is carried out. The validation of the simplified model is examined, by comparing the simulation results with the experimental data published. The impact of temperature and atmospheric pressure on the creation and destruction of ozone and PAN are studied. Finally the Empirical Kinetics Modeling Approach graphs of ozone and PAN are plotted, and the mitigation measures of photochemical smog pollution in different kinds of cities are proposed. The conclusions are summarized as follows:1. The index of photochemical smog pollution grade in city is established. Using the ratio of NO_x to SO₂, it is conclude that the urban air pollution characteristic in this country is being transformed to the exhaust type of motor vehicle (photochemical smog type) from the soot type. The transport of ozone in city is analyzed, and it is pointed out that at the same time when one pays attention to the pollution in city one ought to think much of the photochemical smog pollution at some distance downwind.2. The formation process of photochemical smog is numerically simulated using the explicit Runge-Kutta method and the MATLAB tool, the numerical simulation of photochemical smog formation process is carried out based on the simplified photochemistry dynamic model. So the present model can be thought as a usable method, in which simulate the accumulation and consumption of ozone in a loop system (no sources). The simulation results show that the initial hydrocarbon (RH) is steadily consumed, and that the aldehydes (RCHO) actually increases initially due to conversion of some of the RH into RCHO. The conversion of NO to NO₂ is evident. The concentrations of ozone and PAN are accumulated initially, and then they are consumed in some times. It can also be seen from the simulation results that—under the condition of low concentration ratio of NO_x/SO₂, when the initial concentration of NO_x is fixed, increasing NMHC leads to increasing O₃ and PAN. While initial concentration of NMHC is fixed, increasing NO_x results in increasing both O₃ and PAN. However the time of achieving the maximum of ozone value is retarded. When the initial concentration of NMHC and NO_x is doubled, the yields of O₃ and PAN are increased rapidly, the time of achieving the maximum of ozone concentration is shortened.3. The impact of vehicle exhausts on photochemical smog in city is studied emphatically. When the concentration of NMHC is high in city, an increase in NO_x concentration caused by vehicle exhausts aggravates photochemical smog pollution in city.4. The relationships between the photochemical smog formation and evolution and temperature and atmospheric pressure are studied. The simulated results show that increasing the temperature of environment leads to decreasing the maximum concentration of O₃ and the increase of PAN concentration; while increasing the atmospheric pressure leads to decreasing the maximum concentration of O₃, and the change of PAN concentration is slight, the times of O₃ achieving the maximum concentration and PAN occurring reduce and formaldehyde HCHO,RH and RCHO exhausting are prolonged.5. The contour of ozone and PAN concentrations is plotted by using the Empirical Kinetics Modeling Approach. The results reveal that the relationships between the maximum and formed rate of O₃ and PAN and the ratio of NMHC to NO_x are not a linear. Using the ridged line of EKMA, the type of photochemical smog pollution in city can be classified into A and B. Finally the mitigation measures of photochemical smog pollution in different kinds of city are proposed.