*2.1. Research Object*

The choice of Wroclaw and Cracow as the study object was mainly based on their excessive emissions of air pollutants, but not only that. Cracow was chosen because it is the most polluted city in Poland, which is mainly influenced by its geographical location. The unfavorable location of the city in the Vistula valley, surrounded (on three sides) by terrain elevations, means the polluted air hangs over the city instead of being removed. In addition, the buildings of air corridors, which so far allowed to remove some pollutants from the city and introduce fresh air into it, and low emissions significantly affect the pollution distribution of this city. Wroclaw, on the other hand, due to its characteristic

dense buildings (mainly old tenement houses and estates of old single-family houses), also struggles with high air pollution, especially in the autumn and winter period. In this case, the first step (identification of sources) is understood as the determination of all factors that create the possibility of an increase in the concentration of particles in the air. The distribution of emission sources should be adopted according to the modeling needs. For this reason, the emission can be divided into:


The research area was chosen based on data from documents such as the National Air Quality Program or Annual Air Quality Assessments for the Poland Voivodeships, performed by the Inspectorate of Environmental Protection in Poland. Based on the analysis of the maps prepared in those documents, the areas of increased emission were indicated, and its source was identified. The maps contained in the above-mentioned documents concern the distribution of the average annual concentration of PM10 in Wroclaw and Cracow [21,22]. In Wroclaw, the highest average annual concentration of PM10 occurs in the south of the agglomeration and in the eastern part of the city. In case of Cracow, the highest average annual concentration is found in the central part of the agglomeration. The air quality is worst in the following districts: Kazimierz, Old Town, Pr ˛adnik Biały, Pr ˛adnik Czerwony.

The second step concerns air quality measurements. Air quality measurements were taken in two popular tourist and academic cities in Poland—Wroclaw and Cracow respectively. Both cities are known to their poor air quality. In 2018, Cracow appeared in 8th place on the list of European cities with the worst air quality. The poor air quality in Cracow is mainly due to the city's location. According to [23], the inflow of air from the surrounding areas results in a deterioration of the air quality in Cracow. Unfortunately, the city is surrounded by communities where low-quality coal stoves are still the dominant method of heating houses [23]. Measurement locations were selected in the first step and they represent each type of air pollution source.

Measurements were carried out in the following locations of Wroclaw:


Measurements were carried out in the following locations of Cracow:


It is worth noting that the locations of the measurement points do not coincide with those used by Environmental Protection Inspectorate. Measurement points have been selected to reflect the real problems of residents suffering from poor air quality. The measurements took place in two sessions: morning and afternoon. Each measurement lasted 30 min. During this time the measuring device collected 186 samples (one sample/10 s). With those 12 measurements (six in Wroclaw and six in Cracow) a total of 2232 samples were collected. Measurements were taken in early autumn, when the heating season was slowly beginning. The ambient temperature was in the 14–20 ◦C range. The weather conditions during the measurements were not the same. Rainfall accompanied all morning measurements only in Wroclaw. Moreover, it intensified with each subsequent measurement.

#### *2.2. Measuring Equipment*

The study used an outdoor environmental air quality monitoring device in selected cities (Wroclaw and Cracow). The number and mass distribution, and immissions of particulate matter were used as criteria for urban air quality. A TSI Optical Particle Sizer (OPS) 3330 was used to measure the number and mass of particulate matter (PM) in Wroclaw and Cracow. At this point, it is important to emphasize the difference between emissions and immissions. Emission is the mass of substances released directly into the environment, both from natural and anthropogenic sources. The amount of these substances is determined in units of weight over time, e.g., g/h, kg/year. Immission, also called concentration of pollutants, is the amount of a dust or gas pollutants in a given volume of air unit. Immission (the object of our interest) is determined in units of weight of the substance per unit volume, e.g., μ/m3, g/m3 [24].

The analyzer used in our research enables the measurement of particles in the range from 0.3 to 10 μm for concentrations from 0 to 3000/cm3. TSI's 3330 Optical Particle Sizer (OPS) is a lightweight, portable device that provides fast and accurate measurement of particle concentration and size distribution using single particle counting technology. Sixteen measurement channels were used during the tests [25]. The quality of the measurement is ensured by the manufacturer's calibration standards.
