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The assessment of the atmospheric air quality in Tomsk with the use of a mesoscale photochemical model

The paper presents the results of studying the influence of the development of meteorological situation in clear weather with a weak wind on the level of atmospheric surface air pollution in the city of Tomsk (Russia). Mesoscale meteorological and photochemical models with a new kinetic scheme were applied to the analysis. The main local weather conditions were identified, in which numerical calculations show increased values of the air pollution index. The numerical predictions have confirmed an increase in the ground-level concentration of pollutants in the presence of temperature inversion or isothermy.


Introduction
Processes occurring in the lower part of the atmosphere, i.e. in the atmospheric boundary layer (ABL) significantly influence the human life and activity. Special attention must be paid to the deterioration of the surface air quality due to an increase in the volume of pollutants as a result of natural disasters, anthropogenic emissions (the results of intensive human economic activity in large urban areas, accidental environmental pollution). A part of the incoming pollutants enter into chemical and photochemical reactions can lead to the generation of the urban smog. Many components thereof are characterized by a high toxicity and reduce visibility. Due to the requirements to ensure the environmental safety of major population centers, research into the causes and rate of the air pollution is becoming of particular importance. A significant effect that should be taken into account when studying urbanized territories is an anthropogenic atmospheric phenomenon urban "heat island" (UHI) which means a temperature difference of several degrees for a city and a suburb. It is such temperature differences due to which typical of the city circulation of air masses is formed under conditions of a weak transfer in the ABL. This may lead to adverse local environmental conditions within large cities or their suburbs. Therefore, one of the current most urgent fundamental problems of the Earth Sciences is the problem of the research of the basic laws of the atmospheric processes with the use of mathematical modeling methods in order to provide monitoring and forecasting of meteorological and chemical parameters of the surface layer state over populated areas with a specific exchange of mass, momentum and energy between the air and land [1]. Currently, there is a large number of studies on mathematical modeling of air quality in cities. Based on the review of these works, we can identify the following main methods of studying the meteorological regime and atmospheric pollution [2]: empirical-statistical, statistical, analog modeling, mathematical modeling. For example, empirical-statistical and statistical methods associate various meteorological parameters and properties of the "underlying" terrain. Mathematical models used to assess air quality can be divided into two categories: energy (statistical) and hydrodynamic models. Energy models are designed to study meteorological regime in the surface air over the city. They are based on the heat balance equation. The hydrodynamic modeling method is characterized by some authors as the most fruitful and promising. It is based on the solution of a system of equations describing the meteorological regime of air flow formation in the urban environment depending on the horizontal and vertical components of wind speed, temperature, specific humidity, etc. [3]. Both Russian and foreign scientists work within the framework of the given classification. For example, in [4], the authors study an advective-diffusion model to study the distribution of pollutants in the atmosphere in order to determine whether pollutants are mostly concentrated at the ground level or near the source of emission. This model takes into account wind, diffusion coefficient, and temperature. Numerical solution of this model is carried out by the explicit finite difference method. The calculations carried out by the authors show that the atmospheric conditions affecting the concentration of pollutants are not final since model parameters also affect the atmospheric state. The article [5] is devoted to the study of atmospheric pollution from the mathematical point of view. The authors describe spatial and temporal distribution of pollutants released into the atmosphere and perform mathematical modeling in the FlexPDE software package. This mathematical model can be a useful tool to estimate air pollution in industrial areas. In [6], the authors used the weather research and forecasting model coupled with chemistry (WRF-CHEM) which includes anthropogenic and biogenic emissions in Southeast Asia. The article presents a comparison of the results of the model-based computations and ground-based measurements which shows that the WRF-CHEM model simulates meteorological parameters quite well and predicts O3, NO2 and CO. However, the value of the simulated NO2 concentration was underestimated in comparison with satellite observations. This could affect O3 modeling on the surface. The authors suggest that if we correct the underestimated NO2 values the model will probably simulate the values of O3 concentrations more accurately. Based on the above review of research by modern authors, we can conclude that there is a large variety of mathematical models, but they all have their own areas of applicability and methods of solution. Moreover, the choice of the model and its solution method depends on many factors: available input data, geographical location of the city (environmental conditions, terrain, etc.), weather conditions, etc. The purpose of this work is to numerically study, using mathematical models, the influence of the evolvement of the meteorological situation in clear weather with a weak wind on the level of atmospheric surface air pollution with primary and secondary pollutants as a result of emissions from industrial enterprises and vehicles of the city. Tomsk (Russia) with population of more than 500 000 people is taken as an example for the calculation.

Mesoscale photochemical model
The Eulerian model of turbulent diffusion is used to calculate the concentration of pollutant components taking into account the chemical interactions between them. The model includes transport equations describing advection, turbulent diffusion, and chemical reactions [7]: Here 3 is the number of chemical components of the pollutant whose concentration is to be determined; x, y are horizontal coordinates, the Ox axis is directed Eastward, Oy is Northward, z is the vertical coordinate, t is time, T is the period of modeling. The computational domain is a parallelepiped , Lx, Ly are horizontal dimensions of the area, H is the height of the research area, hz(x, y) is the terrain height above sea level; - Equations (1) are not closed-form ones since in addition to the Ci concentration which is required to be determined there are additional unknown correlations ciu, civ, ciw which model the turbulent diffusion of the pollutant. The closing relations are obtained within thr equilibrium approximation for differential equations of transfer of turbulent momentum, heat and mass fluxes under conditions of local homogeneity of the atmospheric boundary layer [8]. Currently, emissions from vehicles make a major contribution to the formation of secondary air pollutants in cities including generation of aerosol particles of several microns in size which jeopardize human health. Two approaches are mainly used in mathematical modeling of secondary aerosol particle formation. The first [9] approach describes the processes of nucleation, condensation and coagulation that lead to the formation of aerosol particles in detail. The second one [10] represents aerosol formation as overall reactions whose rates are determined empirically in laboratory facilities. In this paper, the second approach is chosen due to its lower computational complexity. For this purpose, the semi-empirical kinetic mechanism GRS (General Reactions Set) [10] developed in CSIRO was added to the 3D photochemical mesoscale model [6,7,11] available to the team of authors instead of the shortcut kinetic scheme of the formation of secondary urban air pollutants with photochemical reaction products developed at the Danish Meteorological Institute [12]. Along with the photochemical reactions of ozone formation in the city atmosphere, the GRS also considers generalized reactions of atmospheric aerosol formation (inert non-gaseous carbon compounds SNGOC, nitrogen SNGN and sulfur SNGS). In the developed modification of the kinetic mechanism, the reactions of ground-level ozone formation are presented in more detail: instead of two reactions of the basic GRS mechanism, six reactions are considered where nitrogen dioxide and monoxide, various states of atomic oxygen (excited and ground) are involved apart from ozone. The advantage of the proposed modified semi-empirical kinetic mechanism is that, along with a more detailed description of gaseous chemical reactions, the overall reactions of formation of aerosol particles of 10 μm and 2.5 μm in the urban atmosphere are also quite compactly described.
Smokestacks of industrial enterprises (about 300) located in Tomsk and the Tomsk region, as well as city roads were considered as sources of air pollutants. The diurnal traffic emission rate was modeled according to the following law: max (0.05+0.95 sin (π(t[h]-6)/18),0.0). Table 1. Chemical reactions of the modified kinetic mechanism of aerosol formation.

Reactions
Reaction rate Source 1.

Some computation results
To test the developed set of models for the study of local meteorological conditions and atmospheric air quality over large localities, computations were compared with measurements made with the use of instruments of the Joint Use Center (JUC) "Atmosphere" of V.E. Zuev Institute of Atmospheric Optics of the Siberian Branch of the RAS (http://juc.iao.ru), as well as instruments of the meteorological station of Tomsk. Figure 1 shows graphs of calculated and measured values of surface meteorological parameters, such as temperature and relative humidity at a height of 2 meters above the earth's surface, as well as wind speed and direction at a height of 10 meters. The calculations are compared with the measurements made by instruments of the TOR-station of the CUC "Atmosphere" and the Tomsk meteorological station. Figure 1 shows that on this day (July 3, 2013), a weak east wind was observed in Tomsk with changes in surface temperature and humidity during the day which is typical for low-cloud weather. There was no precipitation on that that (http://rp5.ru). Comparing the obtained values, it can be noted that the mesoscale meteorological model predicts surface wind characteristics numerically quite well. Some underestimation of the decrease in air temperature in the morning indicates the need to pay attention to the modeling of the interaction of the atmospheric boundary layer with the terrain. During the daytime and evening hours, the model reproduces changes in relative humidity and temperature quite well.     Figure 3 shows graphs of changes in the maximum API values for the territory under consideration 50x50km 2 for each calculation hour during the day for the cold (October-April) and warm (May-September) periods of the year ( Table 2). The figure shows that the maximum API values are observed from 10 to 17 o'clock with a low surface wind speed in cold months (Figure 3a, b, Table 2). And not one but two local extremes are possible in the period from 10 to 17 o'clock. In warm weather (Figure 3c, d; Table 2), there are usually two peaks: morning from 5 to 10 o'clock and eveningfrom 16 to 22. The closer the date under consideration (Table 2)  concentration of pollutants is explained by the presence of an elevated temperature inversion at 00 and 04 o'clock, with the lower boundary at the altitudes of about 100 and 50 m, respectively. As is known, inversion is a stable layer with no turbulent movements. Pollutants from high-rise sources (pipes or smokestacks of industrial enterprises) located within the city accumulate under the inversion layer. Note that at a height of 10m, the API values are relatively small. Starting at sunrise (8 o'clock), the surface layer is turbulized due to convection, which in low wind conditions leads to the alignment of the vertical API profile and an increase in its values at a height of 10 m, i.e. to surface air quality deterioration. At a later time, according to the computations, the pollutant is well mixed due to a high level of turbulence in the atmospheric boundary layer and is deposited in the urban area; and the value of the air pollution index at an altitude of 10 m decreases increasing again only in isothermy conditions (or slight temperature inversion) at 20 o'clock.    inversion. But due to the weak ingress of the pollutant from ground sources, its accumulation at an altitude of 10m is not numerically predicted. During the daytime, due to the intense flow of solar radiation, calculations show a well-developed convective boundary layer with turbulent mixing which, even with a weak wind and an increased level of air pollution emissions during daytime hours, provides safe conditions for air quality in the city.

Conclusion
The paper presents the mesoscale meteorological and photochemical models developed at Tomsk State University to study and forecast the development of the meteorological situation and assess air quality over the city. For inclusion in the photochemical model, a new kinetic scheme is proposed which is based on the kinetic schemes developed in CSIRO [10] and DMI [12]. Its feature is the use of a reduced semi-empirical description of chemical and photochemical reactions that lead to the formation of secondary pollutants as a result of anthropogenic emissions. The models were validate on the data of the measurements of meteorological parameters and groundlevel concentrations of pollutants obtained by the Joint Use Center "Atmosphere" of the V.E. Zuev Institute of Atmospheric Optics of the Siberian Branch of the RAS. Using the models, the influence of meteorological parameters on air quality in the city of Tomsk at the weak wind and partly cloudy weather was studied. Meteorological conditions specific for the dates when the computations showed increased API values were identified. Numerical computations confirmed an increase in the concentration of pollutants in the presence of temperature inversion (or isothermy): the largest amount of pollutants is registered under the inversion layer. The lower the base of the inversion is, the higher the level of pollutants at the earth's surface is. Lowering the inversion leads to an increase in the level of pollution at the Earth's surface.