Detailed Assessment of the Effects of Meteorological Conditions on PM10 Concentrations in the Northeastern Part of the Czech Republic

This article assessed the links between PM10 pollution and meteorological conditions over the Czech-Polish border area at the Třinec-Kosmos and Věřňovice sites often burdened with high air pollution covering the years 2016–2019. For this purpose, the results of the measurements of special systems (ceilometers) that monitor the atmospheric boundary layer were used in the analysis. Meteorological conditions, including the mixing layer height (MLH), undoubtedly influence the air pollution level. Combinations of meteorological conditions and their influence on PM10 concentrations also vary, depending on the pollution sources of a certain area and the geographical conditions of the monitoring site. Gen1erally, the worst dispersion conditions for the PM10 air pollution level occur at low air temperatures, low wind speed, and low height of the mixing layer along with a wind direction from areas with a higher accumulation of pollution sources. The average PM10 concentrations at temperatures below 1 ◦C reach the highest values on the occurrence of a mixing layer height of up to 400 m at both sites. The influence of a rising height of the mixing layer at temperatures below 1 ◦C on the average PM10 concentrations at Třinec-Kosmos site is not as significant as in the case of Věřňovice, where a difference of several tens of μg·m−3 in the average PM10 concentrations was observed between levels of up to 200 m and levels of 200–300 m. The average PM10 hourly concentrations at Třinec-Kosmos were the highest at wind speeds of up to 0.5 m·s−1, at MLH levels of up to almost 600 m; at Věřňovice, the influence of wind speeds of up to 2 m·s−1 was detected. Despite the fact that the most frequent PM10 contributions come to the Třinec-Kosmos site from the SE direction, the average maximum concentration contributions come from the W–N sectors at low wind speeds and MLHs of up to 400 m. In Věřňovice, regardless of the prevailing SW wind direction, sources in the NE–E sector from the site have a crucial influence on the air pollution level caused by PM10.


Introduction
PM 10 is a problematic pollutant with a wide spectrum of effects on human health, mostly on the respiratory and cardiovascular systems of the human body. The hazards of this pollutant lie not only in its quantity, thus in high measured concentrations, but also in the morphology of the particles and their qualitative composition. The suspended particulate matter is capable of creating bonds, which may, of course, result in transferring a range of other elements, e.g., heavy metals and polycyclic aromatic hydrocarbons, into the human body. The effects on the human body have clinically proven to be negative, especially in the form of carcinogenesis [1][2][3][4][5], for some of these transported substances.

Experiments
For the assessment, sites of the Czech Hydrometeorological Institute [40] have been chosen-Třinec-Kosmos [41] and Věřňovice [42], located in the northeastern part of the Moravian-Silesian Region of the Czech Republic (Figure 1). The monitored sites are about 34 km apart in a straight line. Both sites belong to the National Air Quality Monitoring Network of the Czech Republic [43]. At the Třinec-Kosmos and Věřňovice sites, PM 10 concentrations are measured by means of a radiometric method based on the absorption of beta radiation in a sample collected on filter material by means of an Automatic suspended particulate monitor MP101M [44]. The wind direction and speed are measured at a standard height of 10 m by means of a WindSonic machine, based on ultrasound technology. The air temperature is scanned at a height of 2 m above the ground, by means of a Cormet company machine. At both sites, special ceilometer devices are placed that are traditionally used to measure the height of the cloud base and the cloud amount in individual layers, the vertical visibility, and the aerosol concentration in the ground atmospheric layer. These ceilometers contain a presentation module that enables the measurement and display of the structure of the atmospheric boundary layer based on an algorithm that determines the thickness of the mixing layer depending on the aerosol concentration in the atmosphere. In both cases, these are Vaisala CL31 Ceilometers [45].
Atmosphere 2020, 11, 497 4 of 21 It is necessary to point out that measurement by means of ceilometers is limited by height. Although the upper measurement limit reaches the height of a few kilometers, it is impossible to regard the Earth's surface level as the bottom limit. In the vicinity of the Earth's surface, the measurement is limited by higher noise that can reach up to a height of 50 m [46]. For the purposes of the analysis, the average hourly values of the mixing layer height were used, calculated from the data measured at 16-second intervals from the ceilometers at the Třinec-Kosmos and Věřňovice sites. The output hourly values of PM10, temperature, wind speed, and direction were also used in the analysis. Due to large diurnal mixing layer height (MLH) variation, the analysis also included the daily averages of the parameters. Correlation coefficients in the analysis are calculated from the hourly and daily data in relation to the MLH, which was divided into 20 intervals and, subsequently, from their arithmetic means and medians. In the case of daily MLH averages, intervals from a height of 150 m are available. The assessed period is from 2016 to 2019. The data utilization rate used for the monitored period was high. In the case of hourly data on PM10 and meteorological variables (air temperature, wind direction, and speed) the average at both sites is 98%; it is 87% from the ceilometers' data for the mixing layer height. In order to assess higher PM10 concentrations and exceedance numbers, a limit value of 50 µg.m −3 was purposefully chosen, which corresponds to the value of the daily pollution limit for PM10 [13]. The term "cold period of the year" is used for the months of January through March and October through December; the term "warm period of the year" is used for the months of April through September.
In the analysis, concentration roses were also used for illustrative purposes, which show average PM10 concentrations for the given wind direction and speed, as well as weighted concentration roses [47]. The difference between a concentration rose and a weighted concentration rose is that the latter provides information about how often a given wind direction and speed combination occurs and states to what extent the concentrations detected for a given wind speed and direction affect the overall average concentration for a given period. The comparison of the two roses may show a significant pollution source located, however, in a sector from which the wind only rarely blows and thus does not contribute significantly to the overall average concentration. The concentration rose reveals what the pollution situation was at maximum concentrations of a given contaminant at a particular site; the weighted concentration rose shows from what wind direction and at what speed the pollution came to the largest extent for the whole period. For the above-described reasons, both rose types for the same site and period may vary significantly [48]. In the presented roses, there is information about calm air, which is a situation with a wind speed of 0-0.2 m.s −1 .
The Třinec-Kosmos site is situated in the center of the city of Třinec, in a typical residential development. In the immediate vicinity of the site there are a parking lot as well as a residential For the purposes of the analysis, the average hourly values of the mixing layer height were used, calculated from the data measured at 16-second intervals from the ceilometers at the Třinec-Kosmos and Věřňovice sites. The output hourly values of PM 10 , temperature, wind speed, and direction were also used in the analysis. Due to large diurnal mixing layer height (MLH) variation, the analysis also included the daily averages of the parameters. Correlation coefficients in the analysis are calculated from the hourly and daily data in relation to the MLH, which was divided into 20 intervals and, subsequently, from their arithmetic means and medians. In the case of daily MLH averages, intervals from a height of 150 m are available. The assessed period is from 2016 to 2019. The data utilization rate used for the monitored period was high. In the case of hourly data on PM 10 and meteorological variables (air temperature, wind direction, and speed) the average at both sites is 98%; it is 87% from the ceilometers' data for the mixing layer height. In order to assess higher PM 10 concentrations and exceedance numbers, a limit value of 50 µg·m −3 was purposefully chosen, which corresponds to the value of the daily pollution limit for PM 10 [13]. The term "cold period of the year" is used for the months of January through March and October through December; the term "warm period of the year" is used for the months of April through September.
In the analysis, concentration roses were also used for illustrative purposes, which show average PM 10 concentrations for the given wind direction and speed, as well as weighted concentration roses [47]. The difference between a concentration rose and a weighted concentration rose is that the latter provides information about how often a given wind direction and speed combination occurs and states to what extent the concentrations detected for a given wind speed and direction affect the overall average concentration for a given period. The comparison of the two roses may show a significant pollution source located, however, in a sector from which the wind only rarely blows and thus does not contribute significantly to the overall average concentration. The concentration rose reveals what the pollution situation was at maximum concentrations of a given contaminant at a particular site; the weighted concentration rose shows from what wind direction and at what speed the pollution came to the largest extent for the whole period. For the above-described reasons, both rose types for the same site and period may vary significantly [48]. In the presented roses, there is information about calm air, which is a situation with a wind speed of 0-0.2 m·s −1 .
The Třinec-Kosmos site is situated in the center of the city of Třinec, in a typical residential development. In the immediate vicinity of the site there are a parking lot as well as a residential development type of road. In the northeastern (NE) direction from the site, the closest pollution line source is a frequented road with an average number of vehicles of more than 9000 every 24 h [49]. The Třinecké železárny a.s. industrial plant premises are situated in the northwestern (NW) sector about 1.5 km from the site. The closest housing development with individual heating is situated in the N-NE direction about 500 m from the site, another is in the S-SW direction about 1 km from the site. The shortest route towards the Polish border is in the NE direction about 3 km from the site. The Třinec-Kosmos site is situated at the end of the Jablunkov Furrow, which starts at Jablunkov Pass and separates the Moravian-Silesian and the Silesian Beskydy mountain ranges. The shortest distance to the foothills of the Beskydy mountains is just 4.5 km southwest of the station. In the furrow is the Olše River basin that drains off the surrounding undulating relief [50,51]. The direction of the furrow also determines the prevailing wind direction along the southeast/northwest axis (Figure 2a) [52]. In the vicinity of the site, at a distance of 40 m in the NE direction, there is a high-rise, a roughly 50-meter-high building that may, with regard to the prevailing wind direction, slightly muffle the wind influence from the northeast.

Results
The suspended particulate matter concentrations at both sites are assessed depending on the mixing layer height and other meteorological variables-air temperature, wind speed, and direction.

Dependence of Suspended Particulate Matter Concentrations on the Mixing Layer Height
Based on the average daily courses of hourly concentrations of PM10 and MLH in the period between 2016 and 2019 for both stations, it is obvious that the lowest MLH is measured in the evening and night hours and the early morning hours (Figure 3). During this period, the highest PM10 concentrations were observed at the Věřňovice station. In both cases, higher PM10 concentrations were observed in the cold part of the year, when the daily variability was also greater than in the warm part of the year. According to the existing pollution assessment analyses of the Třinec-Kosmos site, apart from the significant contribution of secondary particles, a combination of sources from abroad and local heating and industry, as well as road traffic, contribute significantly to the PM 10 concentrations [52]. At lower wind speeds of up to 2 m·s −1 , with a prevailing southeastern (SE) wind direction, a significant influence of dense housing development with individual heating is very likely to be seen at the site. Conversely, in situations with a NW wind direction, a significant influence of the industrial sources of Třinecké železárny is assumed, which includes both high-as well as low-emitting sources. At higher wind speeds, the influence of sources situated north of the site at a greater distance is also apparent here in the cold period of the year, which points at foreign sources from Poland.
The Věřňovice site is situated in the cadastral area of the Dolní Lutyně municipality, about 300 m east of the Věřňovice village and about 1 km south of the Czech-Polish border. In the immediate vicinity of the site, there is an infrequently used and partly paved road flanked by deciduous trees and Atmosphere 2020, 11, 497 6 of 21 fields. Roughly 1.5 km in the NW direction, there is a highway with roughly 11,000 vehicles a day [49]. The closest significant industrial source, Elektrárna Dětmarovice a.s., is situated about 3 km southeast of the site. The Věřňovice site is situated in open terrain in the vicinity of the River Olše. At the site, the prevailing wind direction is along the southwest/northeast axis where the more frequent flow is southwest (Figure 2b). Věřňovice is situated in the area of the Ostrava Basin where the "Moravian Gate" ends; this divides the Podbeskydská hilly area and Nízký Jeseník [50,51]. The shape of the gate along the southwest/northeast axis significantly determines the prevailing wind direction of the region where Věřňovice is situated. It is apparent from the existing analyses that local fireplaces have the greatest contribution to the exceedance of PM 10 pollution limits here among the primary sources, mostly those used on the territory of Poland [52].

Results
The suspended particulate matter concentrations at both sites are assessed depending on the mixing layer height and other meteorological variables-air temperature, wind speed, and direction.

Dependence of Suspended Particulate Matter Concentrations on the Mixing Layer Height
Based on the average daily courses of hourly concentrations of PM 10 and MLH in the period between 2016 and 2019 for both stations, it is obvious that the lowest MLH is measured in the evening and night hours and the early morning hours ( Figure 3). During this period, the highest PM 10 concentrations were observed at the Věřňovice station. In both cases, higher PM 10 concentrations were observed in the cold part of the year, when the daily variability was also greater than in the warm part of the year.

Results
The suspended particulate matter concentrations at both sites are assessed depending on the mixing layer height and other meteorological variables-air temperature, wind speed, and direction.

Dependence of Suspended Particulate Matter Concentrations on the Mixing Layer Height
Based on the average daily courses of hourly concentrations of PM10 and MLH in the period between 2016 and 2019 for both stations, it is obvious that the lowest MLH is measured in the evening and night hours and the early morning hours ( Figure 3). During this period, the highest PM10 concentrations were observed at the Věřňovice station. In both cases, higher PM10 concentrations were observed in the cold part of the year, when the daily variability was also greater than in the warm part of the year. The distribution of the PM10 and MLH hourly values is depicted in Figure 4. The distribution of the PM 10 and MLH hourly values is depicted in Figure 4.
In order to work out the dependence of PM 10    The correlation coefficients (hourly and daily values) of PM10 dependences on MLH differ over the period from 2016 to 2019 at both sites; it is, however, possible to state that they are statistically significant dependences in all the cases (Table 1). When comparing the MLH arithmetic and median averages in the individual years, the highest values were reached in 2019 at both sites; they were higher at Věřňovice than at Třinec-Kosmos (Table 2). The correlation coefficients (hourly and daily values) of PM 10 dependences on MLH differ over the period from 2016 to 2019 at both sites; it is, however, possible to state that they are statistically significant dependences in all the cases (Table 1). When comparing the MLH arithmetic and median averages in the individual years, the highest values were reached in 2019 at both sites; they were higher at Věřňovice than at Třinec-Kosmos (Table 2).

Dependence of the Suspended Particulate Matter Concentrations on the Mixing Layer Height and the Air Temperature
The distribution of temperature and MLH hourly values are depicted in Figure 6.

Dependence of the Suspended Particulate Matter Concentrations on the Mixing Layer Height and the Air Temperature
The distribution of temperature and MLH hourly values are depicted in Figure 6.   The correlation dependences between the temperature and the mixing layer height changed over the years of 2016 to 2019 at both sites. The correlation dependence is significant every year of the monitored period (Table 3). concentrations are significantly dependent on the mixing layer height and temperature in the case of both sites over the entire monitored period from 2016 to 2019. The average PM10 concentrations reach their highest values at temperatures below 1 °C. At the Třinec-Kosmos location, the highest average daily and hourly PM10 concentrations were observed at temperatures below 1 °C and a mixing layer height lower than 200 to 300 m. At temperatures above 5 °C, the average daily PM10 concentrations were highest at an MLH between 200 and 300 m. At a higher MLH the PM10 concentration values were almost identical. The average daily and hourly PM10 concentrations at the Věřňovice station reached the highest values during the entire period of analysis in combination with the relationship between the temperature and height of the mixing layer, at temperatures below 1 °C and a mixing layer height below 200 m. At the same temperature, but at an MLH of 200-300 m, the average daily concentrations were lower, but still double the limit value of 50 µg.m −3 . A more significant PM10 concentration dependence on temperature as well as MLH was observed at Věřňovice. At Třinec, a lower temperature had a substantial influence on PM10 concentrations; the mixing layer height influence was shown mostly up to an MLH of 400 m. The combination of a low temperature and low MLH level corresponds to the general assumption about the occurrence of poorer dispersion conditions in the cold period of the year as opposed to the assumed occurrence of better dispersion conditions in the warm period of the year (Figure 8a, 9a). When observing the correlations among the numbers of exceedances of PM10 daily concentration values of 50 µg.m −3 and the MLH and temperature (Figure 8b, 9b), a decreasing count of exceedances of this value is apparent with a rising temperature and increasing MLH. The highest number of 50 µg.m −3 value exceedances The correlation dependences between the temperature and the mixing layer height changed over the years of 2016 to 2019 at both sites. The correlation dependence is significant every year of the monitored period (Table 3). The PM 10 concentrations are significantly dependent on the mixing layer height and temperature in the case of both sites over the entire monitored period from 2016 to 2019. The average PM 10 concentrations reach their highest values at temperatures below 1 • C. At the Třinec-Kosmos location, the highest average daily and hourly PM 10 concentrations were observed at temperatures below 1 • C and a mixing layer height lower than 200 to 300 m. At temperatures above 5 • C, the average daily PM 10 concentrations were highest at an MLH between 200 and 300 m. At a higher MLH the PM 10 concentration values were almost identical. The average daily and hourly PM 10 concentrations at the Věřňovice station reached the highest values during the entire period of analysis in combination with the relationship between the temperature and height of the mixing layer, at temperatures below 1 • C and a mixing layer height below 200 m. At the same temperature, but at an MLH of 200-300 m, the average daily concentrations were lower, but still double the limit value of 50 µg·m −3 . A more significant PM 10 concentration dependence on temperature as well as MLH was observed at Věřňovice. At Třinec, a lower temperature had a substantial influence on PM 10 concentrations; the mixing layer height influence was shown mostly up to an MLH of 400 m. The combination of a low temperature and low MLH level corresponds to the general assumption about the occurrence of poorer dispersion conditions in the cold period of the year as opposed to the assumed occurrence of better dispersion conditions in the warm period of the year (Figures 8a and 9a). When observing the correlations among the numbers of exceedances of PM 10 daily concentration values of 50 µg·m −3 and the MLH and temperature (Figures 8b and 9b), a decreasing count of exceedances of this value is apparent with a rising temperature and increasing MLH. The highest number of 50 µg·m −3 value exceedances for PM 10 daily concentrations is observed at temperatures below 1 • C and MLHs below 300 m. For both sites, a slight increase in the number of 1-hour PM 10 concentrations above 50 µg·m −3 is apparent at an MLH level above 1000 m as opposed to 800 to 1000 m.
Atmosphere 2020, 11, x FOR PEER REVIEW 11 of 23 for PM10 daily concentrations is observed at temperatures below 1 °C and MLHs below 300 m. For both sites, a slight increase in the number of 1-hour PM10 concentrations above 50 µg.m −3 is apparent at an MLH level above 1000 m as opposed to 800 to 1000 m.

Dependence of Suspended Particulate Matter Concentrations on the Mixing Layer Height, Wind Speed, and Direction
The distribution of wind speed and MLH hourly values are depicted in Figure 10.

Dependence of Suspended Particulate Matter Concentrations on the Mixing Layer Height, Wind Speed, and Direction
The distribution of wind speed and MLH hourly values are depicted in Figure 10.
(a) (b) The distribution of wind speed and MLH hourly values are depicted in Figure 10. At the Třinec-Kosmos site, a wind speed measured at 10 m does not significantly change with a rising MLH. At the Věřňovice site, the wind speed change is more apparent with a rising MLH. Here, the difference of the average values of the wind speed on the occurrence of MLH levels of up to 200 m and above 400 m makes up 1 and more m·s −1 (Figure 11). The correlation dependences of the wind speed on MLH are low at the Třinec-Kosmos site; the Věřňovice site shows a statistically more significant dependence than the Třinec-Kosmos site. Higher wind speeds are observed at the Veřňovice station, compared to the Třinec-Kosmos station. The differences are due to the more complex orography of the Třinec region compared to the relatively flat and open region surrounding Věřňovice. The station in Třinec is located in a housing development, at the foothills of the Beskydy Mountains (Chapter 2). This means that the correlation between MLH and wind speed at the Třinec-Kosmos station is much less significant or even insignificant, compared to the station in Věřňovice.
The correlation dependences of the wind speed and MHL also differ from site to site throughout the years of the assessed period. A statistically more significant dependence of the wind speed on the MLH is apparent at the Věřňovice site (Table 4).

Dependence of Suspended Particulate Matter Concentrations on the Mixing Layer Height, Wind Speed, and Direction
The distribution of wind speed and MLH hourly values are depicted in Figure 10. At the Třinec-Kosmos site, a wind speed measured at 10 m does not significantly change with a rising MLH. At the Věřňovice site, the wind speed change is more apparent with a rising MLH. Here, the difference of the average values of the wind speed on the occurrence of MLH levels of up to 200 m and above 400 m makes up 1 and more m.s −1 (Figure 11). The correlation dependences of the wind speed on MLH are low at the Třinec-Kosmos site; the Věřňovice site shows a statistically more significant dependence than the Třinec-Kosmos site. Higher wind speeds are observed at the Veřňovice station, compared to the Třinec-Kosmos station. The differences are due to the more complex orography of the Třinec region compared to the relatively flat and open region surrounding Věřňovice. The station in Třinec is located in a housing development, at the foothills of the Beskydy Mountains (Chapter 2). This means that the correlation between MLH and wind speed at the Třinec-Kosmos station is much less significant or even insignificant, compared to the station in Věřňovice. The correlation dependences of the wind speed and MHL also differ from site to site throughout the years of the assessed period. A statistically more significant dependence of the wind speed on the MLH is apparent at the Věřňovice site (Table 4).    PM 10 concentrations are significantly dependent on the MLH and wind speed. At the Třinec-Kosmos site, situations with calm air and wind speeds of up to 0.5 m·s −1 had an apparent influence on higher hourly PM 10 concentrations. At wind speeds between 0.5-2 m·s −1 , the highest average hourly concentrations were observed at MLHs of 200-300 m. The average daily PM 10 concentrations were always the highest at MLHs below 200 m. Average daily wind speeds below 0.5 m·s −1 were not available for the two stations of interest (Třinec-Kosmos and Věřňovice). The daily average wind speed values were 0.5 m·s −1 or more. The influence of the wind speed on PM 10 concentrations decreased with the mixing layer height at the Třinec-Kosmos site. At wind speeds above 2 m·s −1 , the PM 10 concentrations were lowest at the site. During the observation of the occurrence of the hourly PM 10 concentrations above 50 µg·m −3 , the highest count of these higher values was undoubtedly at wind speeds of 1-2 m·s −1 and 0.5-1 m·s −1 and MLHs of up to 300 m. The lowest number of daily PM 10 concentrations above 50 µg·m −3 was observed at wind speeds above 2 m·s −1 and MLHs above 400 m ( Figure 12). wind speeds of 1-2 m.s and 0.5-1 m.s and MLHs of up to 300 m. The lowest number of daily PM10 concentrations above 50 µg.m −3 was observed at wind speeds above 2 m.s −1 and MLHs above 400 m ( Figure 12).
The situation at the Věřňovice site was slightly different. The highest average hourly PM10 concentrations were observed at MLHs below 200 m and wind speeds between 0.5-2 m.s −1 , or under calm conditions and wind speeds below 0.5 m.s −1 . The highest average daily PM10 concentrations were observed at MLHs below 200 m and wind speeds below 2 m.s −1 . The lowest average PM10 concentrations were reached at all levels with the highest wind speeds above 2 m.s−1. The highest number of 50 µg.m −3 value exceedances for PM10 daily concentrations was observed at wind speeds between 1 and 2 m.s −1 and MLHs of 200 to 300 m, at lower wind speeds below 1 m.s −1 , and MLHs below 200 m ( Figure 13). The prevailing wind directions vary noticeably over the whole period from 2016 to 2019 at both sites (Figure 14). At the Třinec-Kosmos site, the prevailing wind direction is along the southeast/northwest axis, where the SE wind direction prevails. The average relative frequencies of counts of wind directions changed in the individual months throughout the whole period; the  The prevailing wind directions vary noticeably over the whole period from 2016 to 2019 at both sites (Figure 14). At the Třinec-Kosmos site, the prevailing wind direction is along the southeast/northwest axis, where the SE wind direction prevails. The average relative frequencies of counts of wind directions changed in the individual months throughout the whole period; the The prevailing wind directions vary noticeably over the whole period from 2016 to 2019 at both sites (Figure 14). At the Třinec-Kosmos site, the prevailing wind direction is along the southeast/northwest axis, where the SE wind direction prevails. The average relative frequencies of counts of wind directions changed in the individual months throughout the whole period; the prevailing SE wind direction, however, remained. The least frequent wind direction was NE. At the Věřňovice site, the highest frequency of counts of the wind direction is along the southwest/northeast axis; the frequencies of counts of the prevailing wind direction, however, differed throughout the year. On average, the southwestern Atmosphere 2020, 11, 497 13 of 21 (SW) wind direction prevailed in the cold months (October to March). In the warm part of the year, the NE and NW wind directions prevailed. The SE wind direction at the Věřňovice site was the least frequent throughout the whole monitored period.
Atmosphere 2020, 11, x FOR PEER REVIEW 14 of 23 prevailing SE wind direction, however, remained. The least frequent wind direction was NE. At the Věřňovice site, the highest frequency of counts of the wind direction is along the southwest/northeast axis; the frequencies of counts of the prevailing wind direction, however, differed throughout the year. On average, the southwestern (SW) wind direction prevailed in the cold months (October to March). In the warm part of the year, the NE and NW wind directions prevailed. The SE wind direction at the Věřňovice site was the least frequent throughout the whole monitored period. The average monthly PM10 concentrations reached the highest values at the lowest average MLH in the cold months of the year at all wind directions at Třinec-Kosmos. The lowest average monthly PM10 concentrations were reached during the SE wind direction. During the SE and E wind directions, the MLHs also reached the lowest values of the average monthly heights of the mixing layer. In comparing the years from 2016 to 2019, it is apparent that on average, 2019 showed higher values of the mixing layer height than the preceding years, which is also reflected in the lower average PM10 concentrations ( Figure 15). The weighted concentration roses for PM10 divided according to the mixing layer height (Figure 16) show that the most frequent contribution to the average PM10 concentrations for the period from 2016 to 2019 comes to the Třinec-Kosmos site from a southeasterly direction, which basically corresponds The average monthly PM 10 concentrations reached the highest values at the lowest average MLH in the cold months of the year at all wind directions at Třinec-Kosmos. The lowest average monthly PM 10 concentrations were reached during the SE wind direction. During the SE and E wind directions, the MLHs also reached the lowest values of the average monthly heights of the mixing layer. In comparing the years from 2016 to 2019, it is apparent that on average, 2019 showed higher values of the mixing layer height than the preceding years, which is also reflected in the lower average PM 10 concentrations (Figure 15).
Atmosphere 2020, 11, x FOR PEER REVIEW 14 of 23 prevailing SE wind direction, however, remained. The least frequent wind direction was NE. At the Věřňovice site, the highest frequency of counts of the wind direction is along the southwest/northeast axis; the frequencies of counts of the prevailing wind direction, however, differed throughout the year. On average, the southwestern (SW) wind direction prevailed in the cold months (October to March). In the warm part of the year, the NE and NW wind directions prevailed. The SE wind direction at the Věřňovice site was the least frequent throughout the whole monitored period. The average monthly PM10 concentrations reached the highest values at the lowest average MLH in the cold months of the year at all wind directions at Třinec-Kosmos. The lowest average monthly PM10 concentrations were reached during the SE wind direction. During the SE and E wind directions, the MLHs also reached the lowest values of the average monthly heights of the mixing layer. In comparing the years from 2016 to 2019, it is apparent that on average, 2019 showed higher values of the mixing layer height than the preceding years, which is also reflected in the lower average PM10 concentrations ( Figure 15). The weighted concentration roses for PM10 divided according to the mixing layer height (Figure 16) show that the most frequent contribution to the average PM10 concentrations for the period from 2016 to 2019 comes to the Třinec-Kosmos site from a southeasterly direction, which basically corresponds The weighted concentration roses for PM 10 divided according to the mixing layer height (Figure 16) show that the most frequent contribution to the average PM 10 concentrations for the period from 2016 to 2019 comes to the Třinec-Kosmos site from a southeasterly direction, which basically corresponds to the described prevailing wind directions at this location. This information, however, does not necessarily mean that the maximum PM 10 concentration contributions also come from the same direction; winds from other directions can carry higher concentrations of PM 10 , which may lead to the exceedance of the daily limit of the pollution value for PM 10 of 50 µg·m −3 . The division of the weighted concentration roses according to the mixing layer height describes the PM 10 concentration at the height of the ground measurement at the site when the mixing layer height reached various levels.
Atmosphere 2020, 11, x FOR PEER REVIEW 15 of 23 to the described prevailing wind directions at this location. This information, however, does not necessarily mean that the maximum PM10 concentration contributions also come from the same direction; winds from other directions can carry higher concentrations of PM10, which may lead to the exceedance of the daily limit of the pollution value for PM10 of 50 µg.m −3 . The division of the weighted concentration roses according to the mixing layer height describes the PM10 concentration at the height of the ground measurement at the site when the mixing layer height reached various levels.  The weighted concentration roses for PM10, divided according to the mixing layer height (Figure 18), show that the most frequent contribution to the average PM10 concentrations for the period from 2016 to 2019 come to the Věřňovice site from the NE sector at wind speeds of up to 2 m.s −1 , despite the fact that, on the whole, the SW wind direction significantly prevails at the site. The division of the The weighted concentration roses for PM 10 , divided according to the mixing layer height (Figure 18), show that the most frequent contribution to the average PM 10 concentrations for the period from 2016 to 2019 come to the Věřňovice site from the NE sector at wind speeds of up to 2 m·s −1 , despite the fact that, on the whole, the SW wind direction significantly prevails at the site. The division of the weighted concentration roses according to the mixing layer height does not describe the PM 10 concentrations and the wind direction at a certain MLH, but a situation at a ground measurement height at the site and during the time when the mixing layer height reached the described height.  When comparing the PM10 concentration contributions at the sites after the division according to the wind directions and according to the MLH levels (Figures 15-20), it is apparent that at higher MLHs the contributions generally reach lower values for all wind directions. When comparing Figures 14, 17 and 18, a difference in the layout of the frequency of counts of wind directions between both sites becomes apparent. While at the Třinec-Kosmos site, a boundary of the prevailing wind direction along the southeast/northwest axis is more sharply visible in the pictures, at Věřňovice, despite the fact that the prevailing wind direction is along the southwest/northeast axis, the frequencies of counts of directions are spread more evenly to the other sectors, as well. This situation corresponds to the information described in Chapters 1 and 2.
The concentration roses ( Figure 19) for 2016-2019 show that the maximum PM 10 concentration contributions come to the Třinec-Kosmos site at a low MLH level of up to 200 m and low wind speeds of up to 1 m·s −1 from the northern (N) to the western (W) sector. For MLHs of 200-400 m, an influence of the wind direction from the E sector is also significantly shown, as well as the influence from the N and W sectors. The average maximum PM 10 concentration contributions at the site occur at MLHs of up to 400 m above the ground and at wind speeds of up to 2 m·s −1 . On the occurrence of a higher MLH level of up to 600 m, a higher wind speed of above 4 m·s −1 from the N and NE directions contributes to the resulting situation more significantly. From the point of view of the PM 10 concentration roses divided according to the mixing layer height for the Třinec-Kosmos site, it is possible to regard the SW wind directions as those directions with the lowest PM 10 contribution; at higher wind speeds of above 4 m·s −1 , the S-SE directions also contribute the lowest amount.      When comparing the PM 10 concentration contributions at the sites after the division according to the wind directions and according to the MLH levels ( Figures 15-20), it is apparent that at higher MLHs the contributions generally reach lower values for all wind directions.

Discussion
The detection of dependences of PM 10 concentrations on the mixing layer height may point out at possible pollution sources in the area; it would, however, be necessary to check the exact determination of the source type by other methods used in assessing the field of air quality, e.g., by more detailed air pollutant measurements and by methods for pollution source identification, e.g., Positive Matrix Factorization (PMF) [53] and back trajectories [54]. The high PM 10 concentrations in combination with the low mixing layer height provide information about the high influence of low-emitting sources of air pollution (individual heating) on the air quality at a given site. On the other hand, higher PM 10 concentrations reached at higher MLH levels show the influence of industrial sources, sources with emission release at greater heights. High concentrations combined with a low wind speed, perhaps even with no wind, show the influence of sources in the vicinity of the site. Conversely, higher wind speeds combined with high PM 10 concentrations correspond to the premise of transport from more remote places to the site [16,17,[24][25][26].
The findings of this work correspond to the conclusions of Air Quality Improvement Program [52] elaborated for both sites and areas of interest in which the sites are located and also to the "Air Pollution Sources Contribution to PM 2.5 Concentration in the Northeastern Part of the Czech Republic" analysis [55].
As mentioned before in Section 2, the meteorological variables data used the analysis from the ground measurements of temperature at a height of 2 m above the ground; the wind direction and speed were measured at 10 m above the ground. The values of these variables at various mixing layer heights were not included in the analysis. A vertical wind direction and speed and temperature profile would be a significant addition to the assessment that would, among other things, provide information about the occurrence of temperature inversions. This information could be completed with distant measurement data or from model calculations in a subsequent assessment. It is necessary to point out again that the ceilometer measurement is biased by noise at the lowest level above the ground [46], which interferes up to roughly 50 m according to the existing available sources.
Although the PM 10 concentrations are significantly dependent both on the wind speed and the mixing layer height, the dependence of the wind speed and MLH does not show a significant statistical dependence. The differences are also apparent between both assessed sites with a statistically higher dependence detected at the Věřňovice site. Orography is assumed to play an important role in this aspect (Section 2). While the Věřňovice site is situated in open terrain, the Třinec-Kosmos site is situated in an urban development at the end of Jablunkov Furrow at the foothills of the Moravian-Silesian Beskydy mountains.
Correlation dependences for the wind direction and MLH have not been elaborated. The authors do not consider this aspect important. The prevailing wind direction is different at both sites, the difference lying mostly in the area geomorphology where the sites are situated. The authors find the wind direction and speed assessment sufficiently useful, divided according to the MLHs mentioned in this article. These indicators serve primarily for detection of the origin and location of possible pollution sources that influence the given area.
Another opportunity for analysis is the assessment of other pollutants' dependence on the mixing layer height. Among the examples are ground-level ozone and processing for the warm part of the year [56], as well as a more detailed assessment of the behavior of pollutants such as sulfur dioxide, nitrogen dioxide, and suspended particles during winter smog situations when there is a low mixing layer height [32].
The disadvantage of an assessment of the relationships between meteorology and ambient air quality with regard to the mixing layer height lies in the fact that there are no available data from a longer sequence of ceilometer measurement. The assessment of the mixing layer height trends from a long-term point of view would surely bring additional information about the development or changes of the dispersion conditions, especially in the areas with poor air quality. The observed information may serve as a basis for the evaluation of current situations with high PM 10 concentrations, for announcing and invalidating smog situations, and model solutions of pollution situation forecasts.

Conclusions
The presented analysis provides evidence of the influence of the mixing layer height, combined with other meteorological variables, on PM 10 concentrations in the area of the Czech-Polish border, which faces problems with a complex representation of a substantial number of various pollution source types. The influence on the pollution situation in combination with meteorological variables at a given site also differs depending on the varied geographical conditions. The influence of a temperature below 1 • C on the pollution situation is noticeable at both sites. The highest numbers of exceedances of PM 10 daily concentrations of 50 µg·m −3 occur at low temperatures combined with a mixing layer height of up to 300 m. In the case of Třinec-Kosmos, the average PM 10 concentrations at temperatures below 1 • C reach the highest values on the occurrence of a mixing layer height of up to 300 to 400 m. The decrease in PM 10 concentrations with increasing MLHs at temperatures below 1 • C is not very significant. In this case, the influence of a rising height of the mixing layer at temperatures below 1 • C on the average PM 10 concentrations is not as significant as in the case of Věřňovice, where a difference of several tens of µg·m −3 in the average PM 10 concentrations is observed between levels of up to 200 m, and levels of 200-300 m.
The influence of the wind speed on the occurrence of various MLHs on PM 10 concentrations is also noticeable. The average PM 10 hourly concentrations at Třinec-Kosmos are the highest at wind speeds of up to 0.2 m·s −1 , or 0.5 m·s −1 , at MLH levels of up to almost 600 m, at Věřňovice the influence of wind speeds of up to 2 m·s −1 is detected. In both cases, throughout the assessment of the number of PM 10 daily concentrations above 50 µg·m −3 , it is then clear that higher values occur in the case of Třinec at wind speeds of 0.5 up to 2 m·s −1 and MLHs of 200-300 m. At the Věřňovice station, the highest number of limit value exceedances was observed at wind speeds between 1-2 m·s −1 and at wind speeds above 2 m·s −1 and MLHs of 200-300 m.
The influence of the wind direction combined with the MLH is different at both sites; however, it is crucial as well. Despite the fact that the most frequent PM 10 contributions come to the Třinec-Kosmos site from the SE direction, the average maximum concentration contributions come from the W-N sectors at low wind speeds and MLHs of up to 400 m. In Věřňovice, regardless of the prevailing SW wind direction, sources in the NE-E sector from the site have a crucial influence on the air pollution level caused by PM 10