| Seen from the chemical,the form of photochemical smog is actual a process that hydrocarbons(HC)and nitrogen oxides(NOx)conduct oxidation at a slow pace under the sunlight and at the same time producing ozone(O3). In this process,some of radicals especially hydroxyl radicals play a very important role. Volatile organic compounds(VOCs),as a lively hydrocarbons, mainly make for the radical transformation and value-added role.VOCs mainly from fuel combustion and vehicle exhaust and oil refining and organic chemical production,and so on.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 issue research interest.The nature of Photochemical smog is ozone pollution. In order to understand the impact of vehicle exhausts on photochemical ozone in city, VOCs in particularly and the relationships among the pollutant concentrations during photochemical smog formation reaction. In this paper 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. Finally the Empirical Kinetics Modeling Approach (EKMA) graphs of ozone are plotted.As an example of Shenzheng City, Verified the reliability of the model, The conclusions are summarized as follows:1. VOCs by an average of ours motor vehicle emission factors.2. VOCs by photochemical Ozone formation in the role.3. Studide and simulated photochemistry dynamic of vehicle exhausts based on photochemistry dynamic model.4. Based on the photochemical reaction and the chemical reaction dynamics of VOCs,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 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 NMHC/NOx, when the initial concentration of NOx is fixed, increasing NMHC leads to increasing O3. While initial concentration of NMHC is fixed, increasingNOx results in increasing both O3. However the time of achieving the maximum of ozone value is retarded. When the initial concentration of NMHC and NOx is doubled, the yields of O3 are increased rapidly, the time of achieving the maximum of ozone concentration is shortened.5.The impact of VOCs on photochemical ozone formation by using ozone isopleth plotting package (OZIPP). Study found that when the initial concentration of nitrogen oxidesφ(NOx) and other initial conditions remain unchangeable andφ(NOx) remain high,an increase ofφ(VOCs) will increasec growth rate ofφ(O3) and the moment that ozone Max coming. whenφ(NOx) remain low, the increase ofφ(VOCs) may inhibit the formation of photochemical ozone;the higher the proportion of the VOCs component of olefin and aromatic polymer the higher the concentration ratio of the formation of photochemical。ozone. Therefore,on the purpose of controlling emission of VOCs, we should put an emphases on controlling more sources of pollution,such as ethylene,xylene and toluene.
|
PDF Full Text Request