3.1. NO2
The panels of
Figure 3, where the monthly mean concentrations for 2020 are plotted against the corresponding values for 2014–2019, clearly show the typical seasonality of NO
2 concentration levels and the impact of COVID-19 lockdown on air quality in the Po Valley. Note that during the cold season, the regional mean and maximum values are typically about twice as high as during the warm season. In January, the two datasets displayed rather similar features, with statistically non-significant differences for both the overall means (42.5 vs. 42.1 µg m
−3) and for the whole data distribution (
Table 2). Relative variations of the monthly means were almost equally split between positive (increase) and negative (decrease) values, but with 50% of the stations being in the ±10% range. Conversely, from February on, together with the usual seasonal pattern of the concentration levels, we can see the progressive shift of the 2020 datasets towards lower values, with both significantly lower overall means and a different data distribution (
Figure 4). The reductions of the overall mean ranged between −4.6 µg m
−3 (February, −12.1%) and −11.8 µg m
−3 (March, −37.4%), but in relative terms were even higher in April (−9.3 µg m
−3, −40.9%) in the full lockdown period. In February, relative reductions of the monthly mean were observed at about 80% of the stations—but mostly (87%) as little as −30% (−5.1 µg m
−3 on average); in March and April, reductions were observed at almost all the stations (
Figure S2) and to a larger extent. In March, reductions were observed at 98.6% of the stations, with 28.4% being down to −30% (−5 µg m
−3) and 71.6% between −50% and −30% (−14.6 µg m
−3). Figures for April were quite similar, with reductions observed at 94% of the stations but even stronger in their relative magnitude: 25.6% down to −30% (−2.7 µg m
−3); and 74.1% were between −50% and −30% (−12.3 µg m
−3). In spite of the progressive loosening of the lockdown measures, reductions were still largely present in May (94%) and June (91.2%), but to a decreasing relative extent: in May, reductions by 30% (−2.8 µg m
−3) were 38.9% and those in the 30% to 50% range (−10.6 µg m
−3) were 61.1%, whereas, conversely, in June they were 61.1% (−3.2 µg m
−3) and 38.9% (−8.6 µg m
−3).
The analysis of the stratified dataset by zones shows that the different areas of the Po Valley were rather homogeneously affected by the emission reductions due to lockdown. Both the time pattern of the relative reductions, U-shaped with the minimum values in April, and their magnitude, in particular as mean values, were fairly common to the different areas (
Figure 5). However, a slightly higher average relative reduction (−42%, −14.6 µg m
−3) and larger maximum relative reductions (−72%, −35.1 µg m
−3) were observed for the stations of the urban agglomerates in the March–May period compared with the other locations—all about −36% (−7.4 µg m
−3) and with maximum values around −67% (−27.3 µg m
−3), respectively. The further stratification of the urban agglomerates subset by station type showed higher relative reductions at traffic-exposed sites (−47% on the average, ranging between −75% and −25%) than at urban background sites (−39% on the average, ranging between −72% and −3%); in absolute terms, at traffic stations the reduction (−20.3 µg m
−3) almost doubled that of the background stations (−10.8 µg m
−3). More generally, the stratification by station type highlighted that traffic sites experienced larger reductions (−41% on the average, −15.5 µg m
−3) than background sites (−32%, −6.6 µg m
−3), particularly rural background sites (−19%, −2.6 µg m
−3). Interestingly, data from one traffic station located near to the Milano-Torino motorway reported relative reductions of around −70% (about −30 µg m
−3) during the lockdown period and only around −30% (−10 µg m
−3) from May on. The extent of the concentration reduction observed during the lockdown was consistent with the emission reduction on motorways, estimated to be in the order of −70% for passenger cars and light duty vehicles and −50% for heavy duty vehicles [
32].
The time patterns of the overall daily mean and median concentrations in 2020 calendar days are compared with the corresponding values of 2014–2019 period in
Figure 6, where concentration ranges with respect to the quartiles of the 2014–2019 distributions are also plotted. The progressive reduction of the concentration values over the whole Po Valley is clearly evident and well summarized by
Table 3. In January and February, the overall mean concentrations were in line with the values observed in the 2014–2019 period, almost always falling within the interquartile range (IQR
14–19 = q3
14–19–q1
14–19, roughly about 30–50 µg m
−3) and the maximum values, in spite of some peaks (e.g., 2nd week of January, February 8th–10th) and sinks (e.g., first week of February) driven by the peculiar meteorological conditions in those days of 2020. However, a certain decrease in concentration levels appeared at the end of February, when the lockdown was already enforced in small municipalities in the middle of the Po Valley. Conversely, in March and April, they were always below q2
14–19 (<30 µg m
−3), and even below q1
14–19 (<15 µg m
−3) in 50% of the days, with the differences between mean values usually being in the 5−10 µg m
−3 range, but occasionally as high as 12–15 µg m
−3. Maximum daily values of 2020 were practically always lower than in the past, lying in the q3
14–19–max
14–19 range, and even within the IQR
14–19 sometimes, with values getting closer and closer to q3
14–19, down to around 30–40 µg m
−3, in late April. In May, and most of all, in June, concentrations progressively rose, but still mainly remained below the median values of the previous years (96.7% in May, 86.7% in June). Indeed, except for the first days of May, when lockdown measures were still in force, the differences between daily mean values became smaller and smaller, mostly in the 3–6 µg m
−3 range in May and in the 2–5 µg m
−3 range in June. Nevertheless, the pattern of the maximum values showed a weak growing trend, with values up to 40–50 µg m
−3 in the end of June, that is, about 20 µg m
−3 less than in the previous years.
The stratified analyses of the time patterns give further evidence to the piece of information obtained from monthly averaged data, namely, as far as the difference between traffic and background stations is concerned. Larger concentration reductions were consistently observed at traffic stations during the whole lockdown period, namely, in April (
Figure 7). Daily means were at least 10 µg m
−3 lower than the average of the previous years—up to 20 µg m
−3 lower in some periods; in these periods maximum values were 50–60 µg m
−3 lower than their 2014–2019 means (
Figure 7 top left panel). At background stations the reductions were less relevant and had reduced fluctuations, regarding both daily mean and maximum values: daily means were usually 3–8 µg m
−3 lower than in 2014–2019, but up to 10 µg m
−3 on the central days of the month, as for the traffic stations; daily maximum values showed an appreciable decrease only in the second half of the month, when they were about 20 µg m
−3 lower than the average of the previous years (
Figure 7 top right panel). Focusing the analysis on urban agglomerations, the effect of the lockdown on traffic emissions was further evidenced: at traffic stations, the daily concentrations usually showed reductions in the 15–20 µg m
−3 range, but also as high as 30 µg m
−3; at background stations, reductions were most frequently in the 5–15 µg m
−3 range only (
Figure 7 bottom panels). Interestingly, at the traffic stations of the largest urban agglomerates (cities of Milano and Torino) the difference between 2020 and the previous years was even larger, systematically in the order of 20–30 µg m
−3, showing the dominating role of traffic emissions in urban traffic hotspots. However, the role of traffic emissions in NO
2 ambient levels was even more highlighted by the reductions observed at the mentioned Milano-Torino motorway station, where daily means in April were 20–30 µg m
−3 lower than in the reference period for 12 days but up to 30–40 µg m
−3 lower for 18 days.
Regardless for the zone and station type, the 2020 time patterns displayed a rather clear 7-day cycle, corresponding to the weekly cycle, with the lowest concentrations on Sundays (e.g., March 8th and 15th or April 5th and 12th). However, such a temporal scheme was more evident at traffic stations, which are more directly influenced by traffic emissions, than at background stations, and specifically, at rural background stations (
Figure 7). Indeed, even though strongly reduced during the lockdown period, road traffic was not completely gone, as alimentary goods delivery was active and personal travel for some workers (e.g., sanitary operators, alimentary markets staff, and post deliveries) was allowed. In the panels of
Figure 6 and
Figure 7, 2020 Sundays’ data are compared with the working days’ average of the previous years; the weekly cycle was specifically investigated in order to have proper day-of-the-week comparisons. These analyses confirmed the systematically lower values throughout the lockdown and post-lockdown weeks (
Figure S3), and the reduction of concentration levels on Sundays. On average, Sundays’ concentration was 6.0 µg m
−3 (−28.4%) lower than on weekdays in March, 5.1 µg m
−3 (−35.9%) in April, and 5.6 µg m
−3 (−40.3%) in May; corresponding figures for the 2014–2019 period are 7.3 µg m
−3 (−22.4%) in March, 6.7 µg m
−3 (−28.1%) in April, and 5.8 µg m
−3 (−29.1%) in May, all larger in absolute terms, but they are 6% to 11% smaller in relative terms given the normally higher ambient levels. For the spring period, Sundays’ concentrations during lockdown could be regarded as an indicative value of the NO
2 background level in the Po Valley, in the order of about 10 µg m
−3 in urban agglomerates and 6–7 µg m
−3 at rural sites.
Finally, inspection of the daily patterns on an hourly basis showed that during lockdown, traffic emissions, even though strongly reduced, were still present, as suggested by the typical rush-hour concentration peaks in the morning and in the evening (
Figure S4). Actually, the concentration levels were almost uniformly reduced during the whole day, in the order of 8–10 µg m
−3 in March and April (−36% and −40%, respectively), and 4–6 µg m
−3 (−33%) in May. However, the peak during the evening rush hours was more reduced and progressively smoothed, especially in April and May: during these hours, reductions as high as 15–18 µg m
−3 (−41%) in March, 11–14 µg m
−3 (−51%) in April, and 7–9 µg m
−3 (−39%) in May were observed. Consistently with their direct exposure to traffic emissions, traffic stations showed larger reductions in both morning and evening rush hours: in urban agglomerations, namely, reductions in the order of −55% to −35% in the morning and −60% to −50% in the evening, exceeded those of the background stations, which were only in the order of −40% to −30% in the morning and −50% to −40% in the evening.
All these results show that the measures enforced during the lockdown period had a significant and prolonged impact on NO
2 concentration levels, resulting in their generalized reduction all over the Po Valley. However, the magnitude of the observed reductions was strongly related to the geographical location and the exposure of the monitoring station to the emission sources, especially road traffic. Indeed, as the reduction of the regional background was about −20%, urban areas had reductions in the order of −50% to −40%. These numbers are consistent with estimates reported for Milan metropolitan area only [
25], with the reduction of tropospheric NO
2 in Milan (−47 ± 15%) estimated by columnar data [
37], and with data from other European cities such as Barcelona (−51.4% to −47.0%) [
18]. The observed reductions were the consequence of the reduced traffic flows all over the region, with emission reductions estimated to be in the order of −35% compared to the regional average, but these were even higher at urban traffic sites during the full lockdown period. The substantial agreement between the emission reductions and the generalized air quality improvement confirms the reliability of the inventory data used to assess the emissions scenario for the lockdown period. This agreement also confirms the source apportionment results for NO
2 in large urban areas [
38], where concentration levels are mostly determined by very local and urban traffic emissions, but with an important contribution from regional background too.
3.2. Benzene
In general, the major findings resulting from the analyses of the benzene datasets are similar to those of NO
2, as far as the monthly mean variations and their time patterns are concerned and regarding the different extents to which stations have been affected by the emission reductions. Ambient concentrations in January 2020 were in agreement with the levels recorded in 2014–2019, with similar distributions and the same overall mean of 2.4 µg m
−3 (
Table 4). Positive relative deviations of the monthly means slightly prevailed over negative variations (55% vs. 45%), with 39% of the stations being in the ±10% range. From February on, we can see the progressive shift of the 2020 datasets towards lower concentration levels, with both significantly lower overall means and different data distributions, namely, as far as maximum values and high percentiles are concerned (
Figure 8 and,
Figure S5). The reductions of the overall mean were roughly −0.4 µg m
−3 in February and March (−25.4% and −33.7%, respectively), −0.25 µg m
−3 in April and May (−35.1% and −43.3%, respectively), and −0.15 µg m
−3 in June (−33.9%). Since February, relative reductions of the monthly mean were observed at about 90% of the stations (
Figure S6); however, the extent of these reductions was greater in March, April and May—when reductions in the −50% to −30% range were observed at 70% of the stations (−0.4 µg m
−3 on average, −1.1 to −0.1 µg m
−3 range)—than in February (40%) and June (50%). The outcome of the stratified dataset analysis for benzene confirmed that the Po Valley was rather homogeneously affected by the emission reduction due to the March–May lockdown, again with slightly higher average relative reductions (−40%, −0.40 µg m
−3) in the urban agglomerates than at the stations of the plains and hills sites (−30%, −0.24 µg m
−3); smaller and fairly constant reductions, in the order of −20%, were observed at mountain stations during the whole period (
Figure 9). The impact of traffic emissions on benzene ambient levels was highlighted by the larger reductions generally observed at traffic stations (−41% on the average, −0.34 µg m
−3) than at background stations (−32%, −0.20 µg m
−3), especially when the urban agglomerates were concerned: traffic stations showed lockdown reductions of around −45% (−0.48 µg m
−3), and background stations, around −35% (−0.31 µg m
−3). The peculiar features of the monitoring station located near to the Milano-Torino motorway emerged from benzene data too; however, the difference with respect to the other traffic stations was less marked than for NO
2.
The comparison of time patterns of the overall daily mean and median concentrations in 2020 with the corresponding values of 2014–2019 highlighted once again the progressive reduction of the concentration levels over the whole Po Valley (
Figure S7). In January, the overall mean concentrations were usually (65%) within the 2014–2019 range (roughly 1.8–3.0 µg m
−3), with a time pattern that mirrored the behavior of NO
2. In February, concentrations were still mostly (55%) within the IQR
14–19, but below q1
14–19 for the remainding 45%, namely during the low-concentration periods driven by the meteorological conditions already observed for NO
2 (e.g., February 4th–5th and 11th–13th) and in the end of February, when the very first local lockdown measures were already enforced in small municipalities of Lombardy. From March on, the overall means were always below q2
14–19 (<1.5 µg m
−3 down to <0.5 µg m
−3), and even below q1
14–19 (<1.0 µg m
−3 down to <0.2 µg m
−3) for more than 50% of days (
Table 5), with the difference between mean values usually being around 0.3 µg m
−3, but sometimes as high as 0.5–0.6 µg m
−3. The decreasing trend affected the maximum daily values too, almost always lying within the q3
14–19-max
14–19 range, down to around 0.7–1.0 µg m
−3, in May. In May and June, concentration levels remained fairly stable around 0.3 µg m
−3, always below the median values of the previous years (30% lower, roughly), and did not show the slight progressive increase observed for NO
2. In general, during the lockdown period the weekly patterns for benzene did not display an evident cycle and were less identifiable than for NO
2: indeed, daily concentrations remained basically constant during the whole week, and Sundays’ levels did not decrease with respect to weekdays. This pattern suggests that benzene levels are mainly driven by the regional background and less affected by contributions from local emissions. However, stratified analyses showed that a weak weekly pattern could still be recognized at traffic stations in general, and at those located in urban agglomerates in particular, with respect to background stations.