3.1 PM2.5 concentrations at the AQMS in Western Macedonia
Figure 2 and
Figure 3 illustrate the box plots of the annual mean PM
2.5 and PM
10 concentrations, as registered by the AQMS in the region of Western Macedonia Lignite Center for the years 2019, 2020, 2021 and 2022. The concentrations of PM
2.5 and PM
10 varied among the AQMS. Notably, AQMS S7 and S8 registering the highest PM
2.5 values. As for the years 2020 and 2021 when the lockdown restrictions were implemented, there is a general decrease in the mean, median and interquartile range of PM
2.5 concentrations at all the AQMS expect for AQMS S6 and S10 where a slight increase is detected in 2020. Similarly, mean, median and interquartile range of PM
10 concentrations generally decreased in 2020 at all the AQMS expect for AQMS S6.
In
Figure 2 the box-plots of PM
2.5 concentrations for the years 2019, 2020, 2021 and 2022 for the AQMS S1, S2, S4, S5, S6, S7, S8, S9 and S10 are shown. In 2019, the mean annual PM
2.5 concentrations were 13.7 μg/m
3, 12.8 μg/m
3, 17.0 μg/m
3, 12.7 μg/m
3, 11.9 μg/m
3, 22.9 μg/m
3, 30.5 μg/m
3, 11.9 μg/m
3 and 12.5 μg/m
3 for the AQMS S1, S2, S4, S5, S6, S7, S8, S9, S10, respectively. In 2020, the mean PM
2.5 concentrations were 12.9 μg/m
3, 11.9 μg/m
3, 15.2 μg/m
3, 11.6 μg/m
3, 12.8 μg/m
3, 18.9 μg/m
3, 28.0 μg/m
3, 10.2 μg/m
3 and 12.9 μg/m
3 for the AQMS S1, S2, S4, S5, S6, S7, S8, S9, S10, respectively. A minor decreasing trend followed in the year 2021. In 2021, the mean PM
2.5 concentrations were 12.8 μg/m
3, 10.6 μg/m
3, 13.7 μg/m
3, 10.3 μg/m
3, 12.2 μg/m
3, 20.5 μg/m
3, 23.3 μg/m
3, 10.8 μg/m
3, 11.0 μg/m
3 for the AQMS S1, S2, S4, S5, S6, S7, S8, S9, S10, respectively. In 2022, a slight increase in PM
2.5 concentrations was detected which is not apparent at all AQMS. Specifically, the mean PM
2.5 concentrations in 2022 were 12.0 μg/m
3, 11.7 μg/m
3, 14.9 μg/m
3, 11.6 μg/m
3, 12.0 μg/m
3, 18.2 μg/m
3, 26.6 μg/m
3 , 11.4 μg/m
3 and 12.0 μg/m
3 for the AQMS S1, S2, S4, S5, S6, S7, S8, S9, S10, respectively.
As we mentioned above the AQMS S7 and S8 were constantly recording the highest values. The AQMS S8 was located in Meliti for the years under study after its relocation from Vevi in 2018, while the AQMS S7 was located in Florina. In both of these areas, district heating networks are currently not available. This means that traditional heating systems such as fireplaces and wood burning in stoves are prevalent, which of course are major source for air pollution in these areas [
12]. It is well-known that combustion appliances that burn fossil fuels, wood or biomass emit a large number of air pollutants (CO, PM, NOx, Volatile Organic Compounds (VOCs) and Polycyclic Aromatic Hydrocarbons (PAHs)) as well as CO
2 emissions [
32]Bari et al., 2011; [
33]Fameli et al., 2021.
Given that the amounts of energy needed for heating applications depends on the prevailing weather conditions and that due to its geographic location, the cold period in Western Macedonia is longer in comparison to the rest of the country, the region is the most energy-consuming region in Greece, in terms of Heating Degree-Days [
30]. According to Zoras et al. (2007) the climatological monthly mean temperature in Kozani and Florina ranged from values below 3 °C in January to almost 24 °C in July, while the precipitation amounts are higher in November and December [
16].
In contrast to Florina, Kozani, Ptolemaida, Amyntaio and Filotas are equipped with district heating networks that utilize the thermal load of the neighbouring lignite-fired power stations [
37] Pitoska et al., 2021]. The district energy systems have the advantage to produce heat and/or power with limited environmental impacts and reduced CO
2 and other GHG (Greenhouse Gas) emissions. As well as they have economic benefits given that they are highly efficient systems and minimize energy wastes [
36] Margaritis et al., 2015]. Pitoska et al. (2021) conducted a questionnaire survey in the city of Ptolemaida and found that the participants recognize the high efficiency of district heating in reducing pollutants and protecting the environment [
37].
Figure 3 displays the box-plots of PM
10 concentrations for the years 2019, 2020, 2021 and 2022 for the AQMS S1, S2, S4, S5, S6, S7, S8, S9 and S10. In 2019, the mean annual PM
10 concentrations were 22.9 μg/m
3, 21.1 μg/m
3, 26.4 μg/m
3, 21.2 μg/m
3, 18.2 μg/m
3, 29.9 μg/m
3, 40.1 μg/m
3, 22.2 μg/m
3 and 27.0 μg/m
3 for the AQMS S1, S2, S4, S5, S6, S7, S8, S9, S10, respectively. In 2020, the mean PM
10 concentrations were 19.5 μg/m
3, 19.8 μg/m
3, 22.4 μg/m
3, 20.0 μg/m
3, 19.3 μg/m
3, 25.9 μg/m
3, 36.5 μg/m
3, 17.4 μg/m
3 and 23.1 μg/m
3 for the AQMS S1, S2, S4, S5, S6, S7, S8, S9, S10, respectively. In most of the AQMS, there was a decrease in annual mean and median PM
10 concentrations in 2021 followed by elevated values in 2022. Specifically, the mean PM
10 concentrations in 2021 were 19.5 μg/m
3, 18.3 μg/m
3, 20.0 μg/m
3, 18.6 μg/m
3, 20.0 μg/m
3, 28.9 μg/m
3, 33.8 μg/m
3, 19.0 μg/m
3 and 20.7 μg/m
3 for the AQMS S1, S2, S4, S5, S6, S7, S8, S9, S10, respectively. The mean PM
10 concentrations in 2022 were 19.3 μg/m
3, 20.9 μg/m
3, 22.4 μg/m
3, 19.5 μg/m
3, 18.4 μg/m
3, 26.9 μg/m
3, 32.6 μg/m
3, 22.4 μg/m
3 and 18.5 μg/m
3 for the AQMS S1, S2, S4, S5, S6, S7, S8, S9, S10, respectively.
In general, the air pollutants in the region of Western Macedonia exhibit clear seasonality patterns with their highest values during the winter months and their lowest values during the summer period. The great variations in the air pollutant values were largely affected by the emissions sources in the region and the meteorological conditions. As previously mentioned, Western Macedonia is largely dominated by the mining activities and the electricity generation in the lignite-fired power plants. The activities in the open-pit lignite mines such as mining, transportation of soil and coal, movement of trucks on unpaved roads are major sources of PM
10 in the region [
13,
14,
15,
17,
19,
20]. Prior studies have identified the role of these activities in the overall air quality in the region [
13,
14,
15,
16,
17,
18,
19,
20].
Figure 4a and 4b show the mean monthly variation in PM
2.5 and PM
10 concentrations and the 95% confidence interval in the mean for the AQMS S1, S2, S4, S5, S6, S7, S8, S9 and S10 for the years 2019, 2020, 2021 and 2022.
The higher mean monthly PM
2.5 concentrations were detected during the cold period (October to April) while the warm period (May to September) the lowest mean monthly concentrations PM
2.5 concentrations were observed (
Figure 4a). As a general trend for the period 2019-2022, the mean monthly PM
2.5 values among the AQMS S1, S2, S4, S5, S6, S7, S8, S9 and S10 varied greatly. The highest mean monthly PM
2.5 concentrations observed at AQMS S7 and S8. In March, April and May of 2020, the mean monthly PM
2.5 were 19.8 ± 7.1 μg/m
3, 16.5 ± 7.3 μg/m
3 and 9.5 ± 4.3 μg/m
3 for the AQMS S7 and 34.5 ± 11.0 μg/m
3, 28.0 ± 9.5 μg/m
3 and 11.2 ± 6.0 μg/m
3 for the AQMS S8. For the corresponding period of 2019 the mean monthly PM
2.5 concentrations are generally higher. Specifically, the corresponding values in March, April and May 2019 for the AQMS S7 were 22.6 ± 8.0 μg/m
3, 18.1 ± 7.6 μg/m
3 and 9.2 ± 3.6 μg/m
3, and for the AQMS S8 were 42.7 ± 11.4 μg/m
3, 28.4 ± 9.1 μg/m
3 and 15.5 ± 6.1 μg/m
3.
As expected, there is a significant decrease in PM
2.5 concentrations in 2020 compared to 2019 (
Figure 4a). As for the AQMS S1, S2, S4, S5, S6, S9 and S10, the mean monthly PM
2.5 concentrations were also decreased in March and April of 2020 compared to March and April of 2019. However, for the majority of the AQMS, in May of 2020 there was an increase in PM
2.5 concentrations in terms of monthly values, compared to the previous year. Specifically, the mean monthly PM
2.5 concentrations in May 2020 were 10.9 ± 5.8 μg/m
3, 9.0 ± 4.5μg/m
3, 10.2 ± 4.1μg/m
3, 9.0 ± 4.5 μg/m
3, 10.0 ± 5.6 μg/m
3, 9.5 ± 4.3 μg/m
3, 11.2 ± 6.0 μg/m
3, 7.3 ± 4.3μg/m
3 and 9.1 ± 4.7 μg/m
3 for the AQMS S1, S2, S4, S5, S6, S7, S8, S9 and S10, respectively. In contrast, in May 2019 the mean monthly values were 8.1 ± 3.1 μg/m
3, 6.9 ± 2.4 μg/m
3, 8.3 ± 2.9 μg/m
3, 7.1 ± 2.9 μg/m
3, 7.0 ± 2.5 μg/m
3, 9.2 ± 3.6 μg/m
3, 15.5 ± 6.1 μg/m
3, 7.9 ± 3.2 μg/m
3 and 7.5 ± 2.7 μg/m
3, for the AQMS S1, S2, S4, S5, S6, S7, S8, S9 and S10, respectively. The slight increase in concentrations could be attributed to the fact that from 4 May 2020, after the lockdown period of March and April 2020, the restrictions on movement and business activity began to be gradually lift.
As for PM10 concentrations, the mean monthly values exhibit spatial heterogeneity and no seasonality patterns can be detected at all AQMS. Solely, AQMS S7 and S8 exhibit clear seasonality patterns with the highest PM concentrations in winter period and the lowest concentrations in summer period. As mentioned above PM concentrations in Meliti and Florina are highly dependent on heating systems. Also, high PM10 concentrations during the winter period are detected at AQMS S4.
In general, researches on air quality in Western Macedonia prior to COVID-19 pandemic have also found that the highest PM
10 concentrations registered in the warm period of the year [
13,
14,
15,
19,
20]. For instance, Triantafyllou (2000) studied the PM
10 concentrations in the southern part of the Eordea Basin from January 1991 to December 1994 and found that PM
10 concentrations are higher during summer and early autumn and lower during spring [
13]. This pattern in PM
10 concentrations could be attributed to multiple causes including seasonal changes in the atmospheric dispersion characteristics and the absence of scavenging by precipitation [
13,
14,
15,
19,
20]. Also, the wind-induced resuspension is a secondary source of PM
10 in the open-pit coal mines during the warm period [
19,
20].
As we can see from
Figure 4(b) at all the AQMS the mean monthly PM
10 concentrations were decreased in March and April of 2020 compared to March and April of 2019. In detail, the mean monthly PM
10 concentrations in March 2020 were 19.2 ± 10.6 μg/m
3, 18.1 ± 11.3 μg/m
3, 22.3 ± 8.10 μg/m
3, 17.5 ± 9.25 μg/m
3, 18.6 ± 11.8 μg/m
3, 24.6 ± 10.2 μg/m
3, 38.6 ± 11.6 μg/m
3, 15.5 ± 9.0 μg/m
3, 17.3 ± 9.8 μg/m
3 for the AQMS S1, S2, S4, S5, S6, S7, S8, S9 and S10, respectively. In April 2020, the mean monthly PM
10 concentrations were 18.70 ± 9.9 μg/m
3, 16.20 ± 8.3 μg/m
3, 19.30 ± 5.7 μg/m
3, 14.90 ± 7.1 μg/m
3, 17.70 ± 7.9 μg/m
3, 21.30 ± 7.9 μg/m
3, 33.00 ± 10.6 μg/m
3, 14.33 ± 6.8 μg/m
3 and 17.07 ± 8.2 μg/m
3 for the AQMS S1, S2, S4, S5, S6, S7, S8, S9 and S10, respectively. As for March and April of 2019, the mean monthly PM
10 concentrations were considerably higher. The mean monthly PM
10 in March 2019
, were 26.2 ± 8.20 μg/m
3, 22.5 ± 8.6 μg/m
3, 28.2 ± 9.9 μg/m
3, 20.7 ± 8.1 μg/m
3, 21.4 ± 7.5 μg/m
3, 34.1 ± 11.1 μg/m
3, 55.0 ± 13.7 μg/m
3, 25.7 ± 10.7 μg/m
3 and 23.3 ± 7.84 μg/m
3, and in April 2019 were 24.70 ± 16.5 μg/m
3, 23.34±14.0 μg/m
3, 27.90 ± 12.8 μg/m
3, 20.30 ± 14.2 μg/m
3, 20.43 ± 14.6 , 28.50 ± 15.7 μg/m
3, 38.60 ± 15.5 μg/m
3, 22.40 ± 14.1 μg/m
3, and 24.00 ± 16.7 μg/m
3 for the AQMS S1, S2, S4, S5, S6, S7, S8, S9 and S10, respectively.
On the other hand, at all the AQMS the mean monthly PM10 concentrations in May of 2020 were increased compared to May of 2019. In May 2019, the mean monthly PM10 concentrations were 15.3 ± 6.0 μg/m3 , 11.7 ± 4.0 μg/m3, 15.0 ± 3.9 μg/m3 , 12.0 ± 3.6 μg/m3 , 10.9 ± 3.7 μg/m3 , 13.7 ± 3.9 μg/m3 , 23.4 ± 7.1 μg/m3 , 18.1 ± 8.2 μg/m3 and 17.0 ± 7.7 μg/m3, while in May 2020, the mean monthly PM10 concentrations were 24.2 ± 19.5 μg/m3, 22.7 ± 16.8 μg/m3, 23.1 ± 13.3 μg/m3, 24.4 ± 17.5 μg/m3, 23.5 ± 20.8 μg/m3, 22.7 ± 18.2 μg/m3, 19.9 ± 13.4 μg/m3, 20.4 ± 15.3 μg/m3 and 24.5 ± 19.6 μg/m3 for the AQMS S1, S2, S4, S5, S6, S7, S8, S9 and S10, respectively.
As for the years 2021 and 2022, it seems that the concentrations of PM remained at the levels of 2020 (Figures 2, 3, 4a and 4b). This could not be attributed merely to the COVID-19 pandemic, but also on other factors. In the period between the end of 2019 and mid-2020, the units I and II of the Power Station Kardia and the units I and II of the Power Station Amyntaio seized to operate. The closure of these units had an obvious effect on the air quality improvement in the region in the last decade, as reported in prior studies [
19,
20]. Sachanidis et al. (2022) reported that the improvement in ambient air quality in the Western Macedonia Lignite Center is correlated with the reduction on the excavated rock volumes and the lignite amount produced [
25]. Importantly, they highlighted that the overall better air quality is measured higher in terms of the number of exceedances of PM limit values. The PM exceedances are mainly correlated with air pollution episodes attributed to the lignite mining activities, prevailing meteorological conditions and the long-range dust transport seasonal phenomena [
25]. The emissions from the combustion of lignite in the power stations, the mining activities of the lignite coal and the transport of fugitive dust sources and fly ash cause local air pollution phenomena [
14].
As for the air quality improvement during the lockdown in Greece, previous studies have also reported a decrease in air pollutants concentrations, but additional factors also have an effect on air pollutants concentrations. Kotsiou et al. (2021) examined air quality during the pandemic in Volos, a coastal port city in Greece with almost 86,000 inhabitants, in accordance with the national census data of 2021 [
31]. They found that the lockdown resulted in 37.4% reduction in mean daily PM
2.5 in 2020 compared to 2019 levels but during the strictest lockdown (from 23 March to 4 May), the occurrence of high levels of PM
2.5 were not avoided, even there were restrictions in human activity patterns [
31].
In our study, the period of the first national lockdown that began on the 23
rd of March 2020 and ended in the first days of May 2020 is shown in
Figure 5 and
Figure 6. More specifically,
Figure 5a shows the daily mean PM
2.5 concentrations for the AQMS S1, S2, S4, S5, S6, S7, S8, S9 and S10 for the period from the 1
st of February 2019 to the 1
st of June 2019 and
Figure 5b shows the corresponding period in 2020. Similarly,
Figure 6a and 6b show the daily mean PM
10 concentrations for the AQMS S1, S2, S4, S5, S6, S7, S8, S9 and S10 for the above mentioned periods.
During the first lockdown period (from 23 March to 4 May 2020) the daily PM
2.5 concentrations reduced. Reductions in daily mean PM
2.5 concentrations were observed at all AQMS in the weeks before the lockdown and during the 6-week lockdown (from 23 March to 4 May 2020) (
Figure 5b). During the 6-week lockdown the mean PM
2.5 concentrations were 13.1 μg/m
3, 11.6 μg/m
3, 15.0 μg/m
3, 10.7 μg/m
3, 12.9 μg/m
3, 17.1 μg/m
3, 29.8 μg/m
3, 8.6 μg/m
3, 10.0 μg/m
3 for the AQMS S1, S2, S4, S5, S6, S7, S8, S9 and S10, respectively. In this period there was a decrease of -16%, -27%, -21%, -25, -9%, -1%, -3%, -36% and -19% in the mean PM
2.5 concentrations for the AQMS S1, S2, S4, S5, S6, S7, S8, S9 and S10, respectively. During the corresponding period in 2019 the mean PM
2.5 concentrations were 15.6 μg/m
3,15.8 μg/m
3, 19.1 μg/m
3, 14.3 μg/m
3, 14.2 μg/m
3, 17.2 μg/m
3, 30.9 μg/m
3, 13.5 μg/m
3 and 12.4 μg/m
3 for the AQMS S1, S2, S4, S5, S6, S7, S8, S9 and S10, respectively.
Specifically, during the period from the 23rd of March 2019 to the 4th of May 2019, the maximum daily PM2.5 concentrations were above 25 μg/m3, while the daily PM2.5 concentrations reached up to maximum daily values of 55 μg/m3 at the AQMS S8. In 2020, high levels of PM2.5 concentrations were also observed, with the daily maximum PM2.5 reaching up to 44.1 μg/m3 and 52.2 μg/m3 at the AQMS S7 and S8.
As we can see in Figures 6a and 6b, the daily mean PM10 concentrations during the first lockdown period (from 23 March to 4 May 2020) also reduced. During the 6-week lockdown the average PM10 concentrations were 18.7 μg/m3, 16.9 μg/m3, 20 μg/m3, 15.6 μg/m3, 18.2 μg/m3, 21.9 μg/m3, 34.4 μg/m3, 14.5 μg/m3 and 16.8 μg/m3 for the AQMS S1, S2, S4, S5, S6, S7, S8, S9 and S10, respectively. In this 6-week period there was a decrease of -25%, -28%, -30%, -26, -13%, -18%, -17%, -40% and 30% in the average PM
10 concentrations for the AQMS S1, S2, S4, S5, S6, S7, S8, S9 and S10, respectively. During the corresponding period in 2019 the mean PM
10 concentrations were 24.9 μg/m
3, 23.4 μg/m
3, 28.4 μg/m
3, 21.2 μg/m
3, 20.9 μg/m
3, 26.6 μg/m
3, 41.6 μg/m
3, 24.3 μg/m
3, 24.0 μg/m
3 for the AQMS S1, S2, S4, S5, S6, S7, S8, S9 and S10, respectively. However, it is worth mentioned that in April 2019 there was a 5-day period of extremely high PM
10 values with spatial homogeneity (
Figure 6a). From the 24
th of April to the 27
th of April 2019 the PM
10 concentrations gradually increased at all the AQMS under study. This event that occurred in the second half of April 2019 is attributed to a large-scale Saharan dust episode over Greece and Europe [
34] Peshev et al. (2022)]. In Greece dust episode are generally common phenomena during spring and autumn that mainly affect the southern part of Greece. Similarly, a long-lasting Saharan dust episode occurred in Greece from the 14
th of May to the 20
th of May 2020 with high values of PM
10 concentrations at all the AQMS under study (
Figure 6b) [
35] Kokkalis et al, 2021]. As has been mentioned above long-range transport is an important mechanism for particle pollution episodes [
8].
3.2 PM concentrations in correlation with meteorological parameters and air pollutants
Figure 7a and 7b show the mean monthly variations in daily PM
2.5 , PM
10 , SO
2, NO
2, NO and NOx concentrations (μg/m
3) along with the mean air temperature (°C) for the AQMS S6 and S7 for the year of 2019 and 2020. AQMS S6 is located in Amyntaio which is a town with 4,348 inhabitants and AQMS S7 is located in Florina, which is a town with a population 17,188 inhabitants [
38].
As we previously discussed, the AQMS S7 registered high PM
2.5 and PM
10 concentrations because it is highly affected from the local sources of air pollution from the traditional heating systems. The highest air pollutants concentrations were observed during the winter months when the lowest air temperatures were observed. For example, in January of 2019 the mean air temperature was below 0°C (T
mean=-1.8° C ± 4.2°C), while PM
10 and PM
2.5 reached up to 82.6 μg/m
3 and 80 μg/m
3, respectively. The PM
2.5/PM
10 ratio reached the value of 1.0 indicating the major contribution of fine particles attributable to anthropogenic air pollution sources. Based on previous studies the high ratios which have been found in Florina indicate the great contribution of the individual household heating systems (e.g., fireplaces and woodstoves) and biomass burning [
20]. In March and April of 2020, the air temperature was lower compared to of the same months in 2019. The monthly mean air temperature was 8.4°C±3.4°C and 11.4°C±5.0°C in March and April of 2020, 10.4°C±3.4°C and 12.1°C±2.3°C, for the same months in 2019. However, the PM concentrations were lower in 2020 compared to 2019 indicating that lockdown restrictions had an effect on air pollutant concentrations. As we have previously mentioned, PM
10 concentrations in March and April of 2020 were considerably lower compared to the corresponding months in 2019. The mean monthly SO
2, NO
2, NO, NOx concentrations were 19.6 μg/m
3, 4.80 μg/m
3, 13.9 μg/m
3 and 18.7 μg/m
3 in March of 2019, and 7.59 μg/m
3, 12.6 μg/m
3, 9.80 μg/m
3 and 22.4 μg/m
3in April of 2019, respectively. In 2020 the air pollutants concentrations are lower. In detail, in March 2020 the mean monthly SO
2, NO
2, NO, NOx concentrations were 8.28 μg/m
3, 3.54 μg/m
3, 5.46 μg/m
3 and 9.00 μg/m
3, while in April 2020 8.55 μg/m
3, 3.19 μg/m
3, 3.88 μg/m
3 and 7.07 μg/m
3, respectively.
At the AQMS S6 the mean monthly SO2, NO2, NO, NOx concentrations were 7.23 μg/m3, 1.61 μg/m3, 51.4 μg/m3 and 53.0 μg/m3 in March 2019, and 3.24 μg/m3, 1.40 μg/m3, 29.7 μg/m3 and 31.1 μg/m3 in April 2019. Specifically, the mean monthly concentrations decreased sharply from May 2019 onwards to average monthly SO2, NO2, NO, NOx concentrations of 3.35 µg/m3, 1.62 µg/m3, 3.98 µg/m3 and 5.60 µg/m3, respectively.
In March of 2020 the concentrations of SO2, NO2, NO, NOx were 5.33 µg/m3, 1.87 µg/m3, 1.81 µg/m3 and 3.38 µg/m3, while in April of 2020 the concentrations of SO2, NO2, NO, NOx were 5.65 µg/m3, 2.05 μg/m3, 1.97 µg/m3 and 3.85 µg/m3, respectively. Both PM2.5 and PM10 concentrations were relative lower in 2020 compared to 2019 but no major differences were detected.
As for the meteorological conditions at the AQMS S6, the air temperature in relative low but at higher levels compared to AQMS S7. The monthly mean air temperature in January of 2019 was also below 0 °C, while in March and April of 2019 was at 10.2°C ± 2.95°C and 11.8°C ± 2.25°C, respectively. During the lockdown period in 2020, the monthly mean air temperature in March and April was 8.23°C ± 3.53°C and 11.2°C ± 4.94°C, respectively.
Figure 8(a) and 8(b) show the correlation coefficients between the daily air pollutants concentrations and meteorological parameters for the AQMS S6 and S7 for the year 2020. At AQMS S6 high positive correlations were found for PM
2.5 and PM
10 (r=0.8), while moderate or even negative correlations were found for both PM
2.5 and PM
10 with NOx, NO
2, NO and SO2 (
Figure 7(a)). As for the meteorological parameters, PM
10 concentrations were positive correlated with air temperature (r=0.11) and negative with wind speed (r=-0.16). This also explains the fact that PM
10 concentrations are higher during the warm period when the temperature are higher. In contrast, PM
2.5 concentrations were negative correlated with air temperature (r=-0.3).
At AQMS S7 the PM concentrations have positive correlation with the air pollutants (PM, NOx, NO
2 and NO) and relative humidity but negative correlation with SO
2, wind speed, wind direction, and temperature (
Figure 7 (b)). The highest values of correlation coefficient were found for PM
2.5 and PM
10 (r=0.91), PM
2.5 and NOx (r=0.82), PM
2.5 and NO
2 (r=0.66), PM
2.5 and NO (r=0.73), while moderate correlation coefficient were found for PM
2.5 and RH (r=0.38). The highest negative correlation were found for PM
2.5 and temperature (r=-0.61), and PM
2.5 and wind speed (r=-0.36). Similarly, PM
10 were highly correlated with NOx (r=0.70), NO
2 (r=0.66) and NO (r=0.63), while negative correlations were found with SO
2 (r=-0.14), wind speed (r=-0.36), wind direction (r=-0.22), and temperature (r=-0.35). The correlation coefficients at the AQMS S7 also explain the high dependence of PM with meteorological conditions.