## 2 Experiments  
### Study Area  
The area of interest is situated in the Moravian-Silesian Region in the northeast of Czechia, bordering Poland. It consists of the Trimec Trough lying along the Olza River, surrounded by the Silesian Beskids from the northeast (up to 700 m above sea level) and the Moravian-Silesian Beskids from the southwest (up to 1000 m above sea level). In the centre of the region, the town of Trimec is situated (WGS84: 49.677892N, 18.670754E) with a population of about 35,000 inhabitants (Czech Statistical Office, Czechia, 2020). See Figure 1.  
According to the Koppen climate classification [47; 48], most of the area belongs to the climate group _Cfb--Temperate oceanic climate_ with the coldest month averaging temperature above 0 \({}^{\circ}\)C, all months with average temperatures below 22 \({}^{\circ}\)C, and at least four months averaging above 10 \({}^{\circ}\)C. There is no significant precipitation difference between seasons. A small part of the area covering the Moravian-Silesian Beskids comes under the groups _Dfb--Warm-summer humid continental climate_ (with similar climate characteristics as _Cfb_) and _Dfc--Subarctic climate_. According to the Czech Hydrometeorological Institute (CHMI), the normal annual sum of precipitation (1981-2010) in the area ranges between 800 and 1000 mm, exceeding the Czech average significantly.  
The modern history of Trimec city is inseparably connected with iron and steelmaking. The Trimec Iron and Steel Works (WGS84: 49.6903064N, 18.6484139E) have operated since 1839, and, nowadays, it is the second-largest factory of its kind in Czechia, producing about 2000 kt of pig iron and 2500 kt of crude steel per year [49]. Though PM emissions associated with the production are in a long-term decline, the factory remains an important source of pollution (see Figure 2).  
Figure 1: Situation map of the area of interest (AQM—air quality monitoring).  
The Trinec Iron and Steel Works and the surrounding area were chosen for the study because they were found to correspond well with the main objectives. The factory is situated in a narrow valley with clear prevailing wind directions, which enables the study of the spatial distribution of pollution in a relatively closed area. The PM emissions of the Trinec Iron and Steel Works dropped to 15% of the original values in the last decade (Figure 2), which should entail a considerably lower pollution load of the surrounding environment. Moreover, within the distance of 5 km, the border of the protected area of Beskids is located, representing a vulnerable environment that should be taken under consideration, too.  
The pollution in the area is measured by two air quality monitoring stations (AQM): Trinec-Kanada station (WGS84: 49.672402N, 18.643058E), monitoring NO, NO\({}_{2}\), NO\({}_{x}\), PM\({}_{2.5}\) and PM\({}_{10}\) concentrations, and Trinec-Kosmos station (WGS84: 49.668123N, 18.677791E), monitoring O\({}_{3}\), PM\({}_{2.5}\) and PM\({}_{10}\) concentrations [50]. However, information on heavy metal concentration is absent, which presents one of the motivations for performing this survey. In 2017 (when the biomonitoring was performed), the daily PM\({}_{10}\) concentrations surpassed the tolerated number of exceedance (TE), and the annual average PM\({}_{2.5}\) concentrations surpassed the legal limit values of 20 \(\upmu\)g.m\({}^{-3}\)[3, 51]. When the PM concentrations are displayed in relation to wind direction (see Figure 3), it suggests that the most important source of the pollutant is the nearby factory. Though the last published data from 2019 revealed that LVs for observed pollutants were met, according to the CHMI, it was mainly due to exceptionally favourable meteorological conditions and an overall decrease of emissions in the region [50].  
Figure 2: Emissions and production of the Trinec Iron and Steel Works.