**1. Introduction**

Air pollution is caused by the emission of gaseous, liquid, and solid substances in amounts that cause environmental damage, adversely affecting flora and fauna, water, soil, and human health [1]. The main air pollutants include nitrogen compounds (NO, NO2), carbon compounds (CO, CO2), sulfur dioxide (SO2), heavy metals (mercury, nickel, lead, arsenic, cadmium), hydrocarbons, and their derivatives, as well as particulate matter pollutants PM10, PM2.5, and PM1.0. Particulate matter pollutants have a negative impact on human health, both directly, by penetrating the body causing allergies and lung diseases, and indirectly, by acting as a carrier for heavy metals, microorganisms, and bacteria [2–4]. Therefore, it is important both to monitor the concentrations of pollutants in the air and to effectively control the amounts of pollutants emitted. Unfortunately, the regulations of the European Union set permissible dust concentrations only for the PM10 and PM2.5 fractions [5]. The permissible level of PM10 is 50 μg/m<sup>3</sup> for the daily average and 40 μg/m<sup>3</sup> for the annual average. For PM2.5, the maximum limit is 25 μg/m3 (annual average). According to WHO recommendations from 2005 [6], the average annual concentration of PM10 should not exceed 20 μg/m3, with a daily average of 50 μg/m3, whereas for PM2.5, the annual average concentration should not exceed 10 μg/m3 with a daily average of 25 μg/m3. No limits have been set for the PM1.0 fraction, although it is increasingly considered the most dangerous type of PM.

Particulate matter is not the only dangerous type of air pollution, however. Gaseous pollutants such as SO2, which is highly toxic with a suffocating odor, also pose a problem. Sulfur dioxide has a high specific gravity and relative density, which causes it to slowly spread through the atmosphere. It arises mainly as a result of burning solid and liquid

**Citation:** Cichowicz, R.; Dobrza ´nski, M. Modeling Pollutant Emissions: Influence of Two Heat and Power Plants on Urban Air Quality. *Energies* **2021**, *14*, 5218. https://doi.org/ 10.3390/en14175218

Academic Editor: Angel A. Juan

Received: 5 August 2021 Accepted: 17 August 2021 Published: 24 August 2021

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**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

fuels contaminated with sulfur (e.g., hard coal, crude oil) in combustion engines, power plants, and combined heat and power plants [7,8]. The amount of SO2 introduced into the environment largely depends on the quality of the fuel used. Sulfur compounds contribute to acidification of the environment, which leads to the formation of acid rain, lower soil fertility, inhibition of plant growth, and plant death [9]. Sulphur dioxide pollution is "seasonal", in the sense that higher concentrations are observed during the winter/heating seasons, while in summer/vegetation seasons, there are lower concentrations of SO2. According to a European Union Directive 2008/50/EC [5], the permissible average daily concentration of SO2 is 125 μg/m3, and the permissible average hourly concentration is 350 μg/m3. These levels are the acceptable values for the protection of human health. The WHO [6] sets a much lower limit of 20 μg/m3 for the daily average. Unfortunately, the WHO guidelines do not provide a limit value for the annual average of SO2.

Nitrogen compounds NOx (NO, NO2) are another significant threat. These compounds are formed during the combustion of fuels at high temperatures, which leads to the oxidation of nitrogen contained in the fuels and in the atmosphere. The main sources of NO2 are road transport (so-called "linear emissions") [10–12] and energy and heating systems ("point emissions") [13]. The most dangerous nitrogen compounds are odorless and colorless nitrogen oxides and brown-colored suffocating NO2. Nitrogen oxides could contribute to photochemical smog and high ozone levels. However, more and more scientific works indicate to the contrary that nitrogen oxides can lead to ozone depletion in the air [14]. Nitrogen dioxide emissions are mainly caused by heavy traffic (linear emissions), as well as by heating systems and the energy sector (point emissions) [12]. The environmental damage caused by NOx includes eutrophication, which is associated with the degradation of terrestrial and aquatic ecosystems [15]. Nitrous oxides also contribute to the formation of tropospheric ozone [16] and acidification of the environment [17]. According to European standards, the daily average NO2 limit is 130 μg/m<sup>3</sup> (these levels are the limit values for the protection of human health) [5]. According to WHO guidelines [6], the permissible average annual NO2 concentration is 40 μg/m3, and the hourly average is 200 μg/m3.

The basic method for determining the state of air quality is to measure pollutant concentrations. Stationary ground stations monitor pollutant concentrations in manual daily and automatic continuous systems [18–20]. However, the small number of such stations and the distance between them mean that the data they collect can only be used to evaluate the state of air quality on a global or national scale. It is not possible to assess the impact of individual emitters on the state of local air quality [21,22]. Local analyses are influenced by a number of important factors, such as wind direction and strength, meteorological conditions, topography, and roughness of the terrain [23–25]. To take into account all of these variables in the analysis, it would be necessary to have a complex network of numerous measuring stations located around the area. This is a very time-consuming and costly solution, so computer programs are used to simulate the concentrations and spread of pollutants based on detailed emitter data. Examples of such software include Aero 2010, Emitor, OPA03 [26], AERMOD [27], ENVI-met, and Austal 2000 [28].

Local analysis should also take into account the division of pollutant emissions into so-called "low" and "high" emissions. Low emissions are from pollution sources up to a height of about 40 m from ground level, i.e., from "line emitters" such as communication routes [29], "point emitters" such as the flue gas systems used in small industrial plants and individual households, and "surface emitters" such as densely built-up and inhabited residential quarters with individual heating systems [30]. "High emissions" are mainly produced by large industrial plants, power plants, and combined heat and power plants [31–33].

Here, we analyze an area of the city of Lodz (in the center of Poland, in Central Europe). The main sources of "high" and "point" emissions in Lodz are two coal-fired CHP plants. Despite technological progress and the introduction of substitutes in the form of biomass, hard coal is still the main raw material used to generate energy, with several hundred megagrams being burned each year. As a result, facilities such as combined heat and power plants are popularly considered to be the main emitters of air pollutants. In this study, we set out to determine whether the CHP plants are in fact the largest emitter of pollutants and the extent of their impact on the local environment.
