*2.2. Methodology of Analysis*

We used both actual measurements and numerical calculations of the dispersion of emissions from selected pollution sources. Based on analysis of the research area, three probable sources of air pollution were selected: EC3 heat and power plants fired with hard coal, light, and heavy fuel oil; road transport, and individual heating systems. The actual measurements were performed in the first quarter of the year, in the period from January to March 2021 (this is the winter heating season in central and eastern Europe), and in the third quarter of the year, from June to August 2021 (the summer period in Poland). Mobile measuring equipment was used, consisting of measuring and sampling devices installed on an unmanned aerial vehicle (UAV) and on a transport platform (TP). The use of the UAV allowed for measurements at heights from 5 m to 50 m above the ground. The TP was used for measurements at a height of about 2 m above the ground. The measuring apparatus was equipped with a laser-scattered (LS) sensor, which was used to measure PM10, PM2.5, and PM1.0 (10,000 particles per second). It was also equipped with ElectroChemical (EC) sensors to measure H2S (3 ppb–1 ppm), O2 (0.20–100%), VOCs (Ethanol, Iso-Butane, 0–500 ppm, sensor type: MOS) and SO2 (0.5–2000 ppm). Validation of the measurement data of particulate matter was performed on the basis of data from an accredited measuring station VIEP (the method equivalent to the reference method), while the gaseous pollutants were validated in relation to the VEGA-GC microchromatograph (equipped with a thermal conductivity detector TCD, minimum concentration of 500 ppb (0.005 ppm)). Numerical analyses of pollutant dispersion were carried out using the ArcGis [23] program, which was used to produce a graphical presentation of the actual measurement data, and the OPA03 program by Eko-Soft [24], which was used to simulate the concentrations of pollutants from selected pollution sources. Interpolation in ArcGis was carried out using the Empirical Bayesian Kriging 3D method. Both software programs are described in detail in [14,25]. The calculations performed in the OPA03 program are

based on the legal acts in force in Poland [26] and the European Union [4]. The OPA03 software is based on the proprietary algorithm of the EKO-Soft company, in accordance with the methodology described in the Polish law [26]. Data on the wind rose in the analyzed period, emitter parameters (such as the number of chimneys and their height, speed of exhaust gases, mass concentration of pollutants emitted, average hourly number of vehicles, and type of fuel) were added to the program. Details for individual pollutant emitters are presented further in the article.

**Figure 2.** Map of the main areas affecting air quality: 1—analyzed area from the west intersection "I" to the east intersection "II"; 2—area of large-format stores; 3—green areas; 4—single-family houses; 5—multi-family houses; 6—industrial areas: warehouse, offices, small handicraft industries, and EC3 heat and power plant (photo background source: Google Earth Pro).

The input data for the calculation of pollutant emissions from the EC-3 CHP plant were provided for scientific purposes by Veolia Energia Łód´z and are the actual measurements of emissions from the CHP plant taken during the analyzed period. According to annual data, the maximum recorded emissions of PM10, PM2.5, and SO2 were 2.667 kg/h, 1.143 kg/h, and 128.81 kg/h, respectively. In accordance with the methodology presented in [27], the average volume of traffic measured during field measurements was adopted for the analysis. Different vehicle types and fuels were considered. According to [28], gasoline-powered passenger cars account for 55% of all vehicles using the analyzed communication artery, diesel-powered cars accounted for 30%, and LPG gas-powered cars for 15%. Vans were divided between those with diesel engines (75%) and those with gasoline engines (25%). Tractors and buses were 100% diesel, and 100% of motorcycles had gasoline engines (detailed data on pollutant emissions are provided later in this paper). Finally, we considered individual heating systems in single-family houses located in the immediate and close vicinity of the studied area. Individual heating systems are used for domestic hot water in summer and for heating in winter. It was assumed in the calculations that 70% of the buildings used hard coal as fuels, and 30% used natural gas.

In the numerical analysis in the OPA03 software, the simulation can be performed with or without the background level of pollution in the air. The background level of pollutions is understood as the concentration of pollutants in the air without the analyzed pollutant emitter. Background levels of pollution were not included in the numerical analyses, to illustrate the individual impact of pollution sources on the dispersion of air pollutants. A common level of 2 m above ground level was adopted for the analysis. Particulate matter pollutants PM10, PM2.5, and PM1.0 were included in the field measurements, as well as gaseous SO2 and VOCs. For the purposes of comparison, the numerical analysis was based on PM10 and SO2 emissions.

#### *2.3. Meteorological Conditions*

Characteristic data for the winter (1st quarter of the year) and summer (3rd quarter of the year) periods in central Poland were selected for the analysis. The winter period was from January to March, which is the so-called the heating period because the outside air temperatures oscillate predominantly around 0 ◦C. For this reason, it was decided to choose two representative measurement series, A and B, for which the average air temperature was about 6 ◦C with a relative humidity of about 76% (Table 1). In the summer period, from June to August, average air temperatures above 18 ◦C predominate. Therefore, it was decided to choose two series, C and D, in which the average temperature was higher than 20 ◦C.

**Table 1.** Meteorological data for representative measurement series during the winter and summer periods (source: [29]).


In the analysis of the dispersion of pollutants, another important parameter is the speed and direction of the wind. In the city of Łód ´z in 2021, winds from the west W (11%) and west–north WSW, and SW (9%) directions were prevailing, with wind speeds ranging from 0 m/s to 7.5 m/s (Figure 3). In the winter period from January to March, the average wind speed was 2.92 m/s (11% W). In the summer period from June to August, the average wind speed was 15% lower, amounting to 2.47 m/s (9% WSW).

**Figure 3.** Wind rose for the city of Łód ´z in 2021 (own study based on data from source [29]).
