عنوان مقاله [English]
The concentration of ground-level ozone is the result of thousands of complex chemical reactions. Basically, an increase of ozone concentration occurs in the presence of NOx, VOCs, and the sun’s radiation. This study deals with analyzing the ground-level ozone in Isfahan city from November 22 to December 21 (a full solar month) in 2009. According to the observations made by the Isfahan Meteorological Organization, photochemical smog was visible over the city during this month. The data used in this study include NO2, NO and O3 concentrations and the meteorological variables of temperature, relative humidity and wind speed which have been measured in Isfahan in 2009. The analyses were carried out for the sunshine hours in two time periods of 9 to 12 am and 6 am to 15 pm whose main characteristics are:
A) During 6–15 period: the sun rises from 6 am and by becoming closer to the dusk, i.e. about 15 pm, both the radiation intensity, and temperature decrease;
B) During 9–12 period: the higher temperature, radiation intensity, and traffic are the effective factors in the emission of pollutants when compared to the other hours of the day.
The days under study are classified based on maximum, minimum and average ozone concentration. In order to analyze the tropospheric ozone and smog creation, in this study, the photochemical and semi-empirical models were used. The kinetic and mechanism of a number of photochemical reactions effective in ozone formation were taken into account in order to analyze the changes in ozone concentration. Calculations were carried out by using the Excel and Matlab software programs. Making use of the steady-state approximation method and considering oxygen atom in the steady state, the reaction rates have been computed. The differential relation obtained (d[O3]/dt = k2k1[NO2]- k3[O3][NO]) is a function of three variables of NO, NO2 and O3 concentrations. The amounts of reaction rate (d[O3]/dt) and also the rate constants k2k1 and k3 were also calculated. Analysis of the experimental relation between the activation energy and the results obtained from calculations indicate that the reactions that take place in the troposphere can be considered rank 3 reactions. In the troposphere, the quantum of energy (hν), which is released in some reactions, is very strong. The activation energies obtained for all days of this study include negative values, and this confirms the fact that the energy of the photons of the sun is needed to change NO2 and O2 to O3 in the troposphere. Based on the negative activation energies obtained, we can consider the reaction NO2+O2+ hν→NO+O3 as the mechanism for the tropospheric ozone production in Isfahan.
The creation of photochemical smog, SP(t) with t denoting the time, during the mentioned days was studied based on the Jonson’s semi-empirical model. The relations obtained based on the changes in NO and O3 concentrations with respect to time show that smog creation follows a quadratic nonlinear relation.
In general, the increase of the concentration of pollutants on the ground as a result of photolysis reactions has led to the production of ozone concentrations. The results achieved from the analysis of reaction rate, smog creation, and the resulting curves indicate that ozone concentration has not been uniformly increasing or decreasing during the studied days, but there were both the increasing and decreasing trends. In general, the photochemical reactions taken place in the atmosphere of Isfahan city have caused both production and loss of ozone. Consequently, the investigation showed that the changes of the ozone concentrations under the effects of the solar radiation followed the same pattern in autumn 2009. On the other hand, the changes in the ozone concentration on the ground level caused changes in smog creation during the studied time. With regard to the above-mentioned arguments and on the basis of the effect of nearly the same conditions, a constant process prevailed. It could be predicted that this pattern will be the same in the future years.