Figure 2a is drawn using publicly available data on wind speed and its direction recorded at the FMF (black arrows) and ARSO 2 (red arrows) stations. The data are presented from 21:00 on Dec 31, 2016, to 04:00 Jan 1, 2017. The speed did not exceed 0.6 m/s at any of the measurement stations, and after midnight, the wind blew in the northeast (from southwest to northeast) direction until 02:00 at the FMF station and in the southwest direction at the ARSO 2 station. After 02:00, the wind direction at the FMF station changed to southeast.
The average hourly temperature (T) and relative humidity (Rh) are shown in Fig. 2b from Dec 28, 2016, to Jan 3, 2017. As both meteorological stations (ARSO 2, FMF) recorded similar time modulations of T and Rh values, only the data from ARSO 2 station are shown. The periodical modulation of the average temperature is out of phase with the Rh. When the temperature increases, Rh decreases because the partial pressure of water increases with temperature. As evidenced in Table 5, their correlation is indeed strong, negative, and statistically significant (r = − 0.85, p < 0.001). On Jan 1, at 00:00, the temperature was − 5.6 °C and Rh was 85% at the ARSO 2 station. The lowest recorded temperature of − 8.4 °C during our campaign was on Jan 1, 2017, at 08:00.
Air pollution by PM
—data from public database (ARSO)
Based on public data from ARSO (ARSO), mass concentrations of PM10 measured in 1-h intervals from Dec 28, 2016, to Jan 1, 2017, are shown in Fig. 3a. The vertical dashed line marks midnight (Jan 1, 2017, at 00:00), when the intensity of the fireworks was highest. The peak in the PM10 mass concentration was on Jan 1 at 02:00 with the concentration reaching 234 µg/m3 at ARSO 1 station and 299 µg/m3 at ARSO 2 station. The average daily mass concentrations of PM10 for ARSO 1, ARSO 2, and ARSO 3 were 153, 137, and 126 µg/m3, respectively. These values were the highest average daily mass concentrations in the years 2016 and 2017. On Jan 1, 2017, the average daily mass concentration of PM2.5 measured at the ARSO 3 station was 114 µg/m3 (ARSO). This value turned out to be the highest among all the values recorded in 2016 and 2017.
The daily average mass concentrations of PM2.5 and PM10 at ARSO 3 station from Dec 1, 2016, to Jan 31, 2017, are shown in Fig. 3b. The dashed vertical line marks Jan 1, 2017, while the horizontal dash-dot line marks the maximum average daily mass concentration value of air pollution by PM10 (50 µg/m3) that should not be exceeded more than 35 times in a year as it is determined by the Slovenian government (Republic of Slovenia). The annual average mass concentration for PM2.5 set by the National directive and by the EU Air Quality Directive (2008/50/EC) (European Parliament) is 25 μg/m3 and was not exceeded neither in 2016 (22.50 µg/m3) nor in 2017 (20.24 µg/m3). It is worth noting that the WHO guideline limits the daily mass concentration of PM2.5 to 25 μg/m3 (WHO) and this value was exceeded multiple times, especially in the heating season (from November to February).
Air pollution by nanoparticles
Mass concentrations and the cumulative mass concentration of the particles collected by the DLPI are presented in Fig. 4. The largest mass (920 µg) was found on stage 6 with D50 of 400 nm. Particles captured on stages 5 (260 nm) to 8 (1 µm) add to more than 75% of all the mass. The total mass of all collected particles was 3.909 mg, which corresponds to a mass concentration of 6515 µg/m3.
Collected particles on the stages 3 and 5 with D50 of 108 nm and 260 nm, respectively, were analysed with ICP-MS. These two stages were selected because SMPS measurements have shown the largest number concentration of particles with the size from 70 to 300 nm (Fig. 5a). Mass concentrations of detected elements are presented in Table 2 in decreasing order. Because aluminium foil was used as a substrate in the DLPI, the data for aluminium are not shown. Among all the detected elements, iron (Fe), manganese (Mn), zinc (Zn), and magnesium (Mg) were detected in relatively large (μg/m3 and hundreds of ng/m3) quantities. Elements such as copper (Cu), chromium (Cr), barium (Ba), lead (Pb), nickel (Ni), vanadium (V), and strontium (Sr) were detected in smaller quantities (10–100 ng/m3), while rubidium (Rb), tin (Sn), cobalt (Co), and arsenic (As) were detected in traces (below 10 ng/m3). The amount of cobalt and arsenic is just above the detection threshold and can be neglected.
Elevated mass concentrations of different heavy metals were also measured at the ARSO 3 monitoring station (ARSO). They were determined from PM10 samples with the ICP-MS method after chemical decomposition according to the SIST EN 14,902:2005 standard. Data on daily concentrations of heavy metals (Table 3) shows that on Jan 1, 2017, mass concentrations of Al, V, Fe, Cu, Zn, Ga, Sr, Cd, Pb, Ba, and Rb strongly exceeded the average daily concentration from Jan 3 to Jan 31. In fact, the mass concentrations of Al, Ba, Sr, and Cu were more than 10 times, and Sr more than 140 times, higher than their average daily mass concentrations from Jan 3 to Jan 31, highlighting the negative side of New Year’s celebrations.
Figure 5 presents the normalized concentration of NPs given as the diameter in log scale vs time (a) and the total concentration of all NPs in the size range from 14.6 to 685.4 nm (b) for the period from Dec 28, 2016, to Jan 3, 2017. During the New Year night, the maximum value of the total concentration was reached 45 min after midnight. When the total concentration of NPs increased, the majority of the detected NPs were in the size range between 50 and 300 nm (Fig. 5a).
The Aethalometer data (Fig. 5b) shows a diurnal modulation of black carbon concentrations with higher values in the evenings. This is due to the activity of sources, such as holiday traffic and an increased use of wood and fossil fuels for heating, and due to the reduced mixing of the atmosphere and a very low mixing layer height. From Dec 28, 2016, to Jan 3, 2017, the black carbon concentrations resemble the SMPS total concentration data shown in Fig. 5b. The Pearson correlation coefficient between black carbon and SMPS measurements (taken every 3 min) is 0.88 (p < 0.001).
Correlations of heavy metals and PM
Intercorrelations of daily heavy metal concentrations for January 2017 are given in Table 4 and visually represented in Fig. 6. In Table 4, star symbols above the diagonal show the significance of the corresponding Pearson correlation coefficients reported under the diagonal. The heavy metals can be roughly divided into two groups (clusters) with high correlation. The first group contains Al, V, Cu, Zn, Ga, Sr, Cd, Pb, Ba, and Rb while the second one contains Cr, Mn, Fe, Ni, Co, and Mo. The correlations between the heavy metals and PM10 are shown in Table 4, last row and column. All except As are significantly positively correlated with PM10. Moreover, most of the heavy metals are strongly positively correlated with PM10, and only three (Cr, Ni, and Sb) are moderately positively correlated with PM10.
In Table 5, correlations of T, Rh, NPs, BC, and PM10 are shown. Rh, NP, BC, and PM10 are significantly positively correlated, while temperature is significantly negatively correlated with all parameters.
Additional trend tests for NPs, PM
(ARSO 1 and ARSO 2) data
The results show that in the period in question (from Dec 28, 2016, 00:00 to Jan 2, 2017, 23:00), all three variables (NPs, PM10 – ARSO 1, and PM10 – ARSO 2) exhibit a significant trend: NPs (BB Mann–Kendall test: τ = 0.25, p < 0.001), PM10 – ARSO 2 (BB Mann–Kendall test: τ = 0.47, p < 0.001), PM10 – ARSO 1 (Mann–Kendall test: τ = 0.47, p < 0.001). Furthermore, all these trends were positive, as suggested by a positive Sen’s slope (NP: 42.64, PM10 – ARSO 2: 0.71, PM10 – ARSO 1: 0.80), indicating that the closer the New Year was, the more polluted the air was. In our case, Sen’s slope of NP is 42.64 which means that the value of NP (i.e. number concentration of measured particles) increases, on average, each hour by 42.64 units (Sen’s slope values of PM10 are interpreted in the similar manner).