A graphical presentation (maps of transects) of all in-situ mobile dataset (depicted on Fig. 3) were plotted
with the software R software (ggmap package). Krakow administrative boundaries were obtained from GAMD database.
Each point depicted on maps represents the average value of PM10 concentration recorded by the dust meter in 20 seconds with a frequency of up to 3Hz (approx. 40–60 readings). In addition, samples collected in the exact same location (for example during stops next to the reference stations) were averaged to one reading.
All transects were obtained during evening and night hours (8:00 p.m. − 2:30 a.m.), when the atmospheric conditions were considered as stable (transect presented on Fig. 3.b and Fig. 3.c) or slightly stable (remaining transects), according to the Pasquill Stability Classes.
All transects except for the first one (Fig. 3.a) were considered as statistically significant, since the Pearson and Spearman coefficients were found to be above 0.5. The first transect was then rejected due to an insufficient number of reference stations necessary for the calculation of statistical coefficients.
In the analysis we have also considered the changes in meteorological parameters, well known to influence the PM10 concentration in ambient air, since the main functionality of our mobile monitoring system was to determine the hot spot occurrence in the villages and cities surrounding Krakow. Danek et al20 for instance indicate that the probability of „low” emission (explained earlier as emission up to 40 m high, derived mainly from domestic households and car traffic) rises when the ambient air temperature is close or less than 0°C; air humidity is above 70%; and the wind speed is low. Moreover, during long term research carried out by KAS, it was concluded that the „low” emissions is directly correlated with the hours of human activity in the place of residence, that is in the morning (approx. 6:00–9:00 a.m.) and in the evening (5:00 p.m. − 1:00 a.m.). All above mentioned factors were considered during our mobile monitoring campaigns.
When analyzing all individual monitoring campaigns it was found that the lowest concentrations of PM10 concentration ranging from 7 µg/m3 to 31 µg /m3 were recorded in January 13th on Thursday evening, when the relative humidity was approximately 84.7+/- 5.5%, SW wind direction and ambient temperature of about 3.1 ± 0.2℃. Transect depicted on Fig. 3.b was recorded in the appropriate time interval (between 20:00–23:00). However, since the wind speed on January 13th was much more intensive when compared to other campaigns weather conditions, this has probably contributed to the reduction of PM10 concentration. The average PM10 dust concentration that night was close to regional background value [31]. Surprisingly, when considering other meteorological parameters such as ambient temperature it was found that the highest 20 second averaged PM10 concentration (approximately 225 µg/m3 ) were recorded in February 13th campaign when the outside air temperature was the highest from all 6 sampling campaigns, being above 5°C.
Transect presented on Fig. 3.f, was conducted on March 3rd 2022 in slightly stable weather conditions. The beginning of the transect was mainly marked as yellow, which refers to an elevated PM10 concentrations above 50 µg/m3 with a single hot spot found in the village of Libertów and on the route to Wieliczka (both places located on the south side from the center of Krakow), where PM10 concentration was recorded as high as 174 µg/m3. Towards the end of the measurement the north side of Krakow in the city of Niepołomice gradual purification of ambient air was observed, and the PM10 concentration decreased approximately from 70 µg/m3 to 40 µg/m3 (presented as green scale).This phenomena is also visible in SEM dataset, which shows over 80 µg/m3 in the city center at 22:00 and below 40 µg/m3 at 02:00.
Much more diverse PM10 concentrations were recorded during campaign conducted on January 19th, February 13th and February 15th 2022 (results are depicted on Fig. 3.c - Fig. 3.e.) The transects revealed big difference in PM10 concentration in the Krakow city center and its borders. The rural areas (villages) North and West of the city center deserve special attention, as there is a sharp shift from low PM10 dust concentration 23–30 µg/m3 to as high concentration as 176–225 µg/m3.
All transects were then combined to one single map. Figure 4.a depicts all the readings (Fig. 3.b - Fig. 3.f) while Fig. 4.b summarizes only 3 selected campaigns’ transects previously presented on Fig. 3.c - Fig. 3.e. Values of PM10 concentration in Fig. 4 represents the PM10 averaged value for a given point obtained within a radius of 150 m. Rapid changes in dust concentrations have been marked and numbered. Administrative borders of Krakow city also marked on the maps
The increased concentration of PM10 was recurrently recorded in the west boarder of Kraków, in places marked as 1 (the city of Zabierzów), 3 (the Vistula riverbed, villages of Liszki and Piekary), and 5 (the city of Skawina). A repeated episodes of high PM10 dust concentration being recorded in the very same locations proves them to be a local „low” emission hot spots in the vicinity of Krakow. Point marked as 3 on Fig. 2 requires an additional study, as it is located next to the Vistula riverbed - a place where local pollutants can accumulate. The highest concentrations of PM10 reaching up to 250 µg/m3 were recorded repeatedly during the entire campaign, in all sampling location 1 (Zabierzów), 2 Zielonki, 3 (Liszki), 4 (Zabierzów and Wieliczka) as well as 5 (Skawina). Above mentioned „low” emissions hot spots coincides with the largest number of solid fossil fuel furnaces, which have not yet been replaced in these municipalities as part of Voivodeship air protection programs. According to data provided by Stowarzyszenie Metropolia Krakowska28 by the end of 2021 there were still 19315 furnaces to be replaced in rural municipalities in proximity to Krakow borders. Without proper and centralized measures these municipalities will still remain being responsible for secondary air pollution load for the Kraków city.