3.3.1. Particulate matter
The levels of air pollutants, namely PM10, PM2.5, NO2, SO2 and O3 in seven Portuguese regions are summarized Fig. 1, which show the statistics across all monitoring stations for the three types of zoning (rural – urban). Specifically, in 2019 daily levels of PM10 measured at 68 monitoring stations demonstrated large variations of the obtained data (Fig. 1a), with detailed descriptive statistics summarized in Table 3S. Average daily PM10 means were between 12 and 20 µg m− 3 (absolute range 1–74 µg m− 3) in rural zone of the five regions of the Continental Portugal, 18–21 µg m− 3 (2–82 µg m− 3) in suburban zones and 20–24 µg m− 3 (1–116 µg m− 3) in urban ones. Regions of Portuguese islands showed lower concentrations, especially at rural zones with the corresponding means of 11 µg m− 3 (Madeira, range 2–71 µg m− 3) and 7 µg m− 3 at Azores (2-–17 µg m− 3). These results demonstrated that for all 7 regions daily PM10 concentrations were the lowest at rural zones being significantly (p < 0.05) different (approximately 15% in Algarve – 60% for North) that the respective means at urban zones (or suburban for Alentejo). Regarding the EU limits daily (50 µg m− 3), for urban zones the exceedances were observed in 30% of the rural stations: two in Centre, one in Lisbon TV, one in Madeira one in Alentejo (the respective monitoring station is with industrial influence due to the industrial power plan). It though necessary to point out that EU legislation stipulates a tolerance of exceedance 35 per year and in that regard all the monitoring stations fulfilled the conditions as the registered exceedances occurred 1–7 times per the same station (Table 3S). Furthermore, it is necessary to enhance that the raw data were considered in this work and the possibility of subtracting contributions to the measured concentrations from natural sources and winter road sanding/salting has not been considered. In all the other regions (North, Algarve and Azores islands), no concentrations higher than daily limit were registered in rural zones. On the contrary, suburban and urban zones of Portugal exhibited exceeded the daily limits in all regions/zones. These were especially high for North (total of 57) and in Lisbon TV (64), which were the regions with the higher number of stations. In Centre, the limits were approximately 3 times less (21 times), whereas in Alentejo and Algarve the exceedance were even less frequent (6 and 12 times, respectively); in all stations the margin of tolerance (35 exceedance) fulfilled in all monitoring sites.
In 2020 the levels of PM10 (Fig. 1a, Table 4S) were slightly lower in a comparison with the previous year. Daily means of PM10 for Continental Portugal were observed as the following: 8 µg m− 3 (North) to 17 µg m− 3 (Algarve) in rural zones, 17 µg m− 3 (Alentejo) – 24 µg m− 3 (Centre) in suburban one, and 17 µg m− 3 (North) and 25 µg m− 3 (Algarve) for urban zones. These results showed that in rural zones, PM10 concentrations in 2020 were significantly lower than in the previous year (p < 0.05; overall mean of 12 µg m− 3 vs. 17 µg m− 3 in 2020), with the respective percentage being between 30% (Lisbon TV) and 80% in Alentejo. In urban zones the respective PM were lower in 2020 only in Centre and Lisbon TV regions (~ 30%), whereas PM no differences were observed in North and Alentejo region. Within the urban areas, ambient air pollution is often dominated by motor vehicles traffic ), but due to the variables such as number of junctions, distance to roadways, traffic flows, surrounding road length, and others the respective pollution may vary greatly37,38, 39. It is assumed that the lesser traffic in 2020 (Fig. 3S) might be the cause for the lower PM levels in some of the urban zones.
The analysis of PM10 levels across the urban zones with traffic emissions specifically (i.e. 22 monitoring urban-traffic sites) showed that in terms of monthly evolution (Fig. 2a) PM10 started to decrease in February 2020, with the minimal levels observed in April (when the state of emergency was implemented) and then increased in May (state of emergency ended). However, when comparing both years (Fig. 2b), the largest concentration drops were observed, as expected in month of April (10–70% in Madeira and Algarve) but also February (up to 70% in North). These data may be in agreement with the road transport trend that shows a significant decrease during the first trimester (January– March; Fig. 3S). However, correlations between the road transport changes on monthly basis (when available) and traffic-related pollutants would provide much deeper understanding of respective association. At the same time, as the traffic density decreased on the international level (due to the restrictions taking place in majority of European countries), PM10 levels at rural ones were most likely affected due to the lesser transport of long-distance emissions.
On the contrary, in suburban sites, PM10 in 2020 (Fig. 1) were higher (overall 21 µg m− 3 vs. 20 µg m− 3), though these differences were not statistically significant, with the highest changes observed in North and Centre region (20 and 30%). On European level road transport contributes only ~ 11% of PM in EU, the main sources of PM1040 are commercial, institutional and households sector (39%) and industrial processes (20%), which could be linked with the unchanged trends of PM in suburban zones. In terms of PM10 legislation, 24 h limit was exceeded in all three types of zones in 2020. However, majority of the exceedances were observed in urban zones (87%, Table 4S) and furthermore, 80% of these exceedances occurred in January and February (i.e. before the state emergency regulations took place). In addition, it is necessary to highlight that in 2020, for the respective period of 5 months, PM10 daily limits were exceeded approximately 50% less (107 vs. 218 in 2019). Thus, the results indicate that PM concentrations were positively influenced in 2020, most likely also by the lower vehicle road traffic. It is necessary to highlight that Algarve was the only region that exhibited in 2020 very different evolution of PM at urban traffic sites (and higher concentrations at urban traffic sites during all 5 months of 2020) than rest of the territory (Fig. 2a). While previous work emphasized the impacts of long–range transport of mineral dust from North Africa with high frequency and prevalence namely in southern parts of Portugal41, 42 it needs to be highlighted that the respective data (urban traffic) is based on 1 monitoring station (Table 1). Thus these values will need to be confirmed when the final registry of APA is released.
Concerning the fine fraction, 2019 average daily PM2.5 means (Fig. 1b) were between 5 and 9 µg m− 3 (absolute range 1–47 µg m− 3) in rural zone of the five regions of the Continental Portugal, 9–21 µg m− 3 (1–60 µg m− 3) in suburban zones and 5–15 µg m− 3 (1–53 µg m− 3) in urban ones. In agreement with PM10, these results demonstrated that for all 7 regions daily PM2.5 concentrations were the lowest at rural zones being significantly (p < 0.05) different (approximately 15% in Alentejo – 90% for Centre) that the respective concentration at urban zones. In 2019, for the considering period, the overall mean (7 µg m− 3 across 68 monitoring stations) was well below the annual target (Table 1S), though these results need to be implicated carefully, once the considered work of this study included 5 months (i.e. 42% of the calendar time).Worldwide, Portugal is among the countries with the better air quality in terms of PM2.543; in 2017 it ranked as 7th country in European Union with the lowest PM2.5 across 27 members40. Furthermore, it is noteworthy that for fine fraction, that out of the three different zones, suburban areas presented the highest PM2.5 (overall mean of 16 µg m− 3 ) with 80–265% higher concentrations than the respective levels at each regions of the rural zones. Still, the exposure concentration obligation (20 µg m− 3; calculated based on the levels of PM2.5 at suburban and urban background sites) are typically obliged40.
In 2020, The lowest PM2.5 concentrations were observed in rural zones where they ranged between 4 µg m− 3 (Algarve) and 7 µg m− 3 (Centre and Lisbon TV). The corresponding levels in suburban and urban areas were 30–90% higher with, respectively, overall means of 6 µg m− 3 (Alentejo) and 12 µg m− 3 (Lisbon TV) in suburban and 5–13 µg m− 3 (Madeira and Centre region) in urban zones. In comparison with 2019, PM2.5 emissions decreased in year after. Whereas the changes were statistically insignificant in rural zones (overall means of 6 µg m− 3 vs. 7 µg m− 3), the highest differences was observed in suburban (10 µg m− 3 vs. 16 µg m− 3 in 2020), and urban zones (8 µg m− 3 vs. 10 µg m− 3) being especially substantial for North (95%) and Lisbon TV region (~ 40%). While the data for urban traffic zones are limited (Fig. 5S), in agreement with the previous results, April was the month the lowest concentrations.
3.3.2 Gaseous pollutants
For the gaseous pollutants the overall mean of NO2 concentrations in 2019 (Fig. 1c) were 12 µg m− 3 (range of Azores 3 µg m− 3 – North 16) for rural zones, 42 µg m− 3 for suburban (33 µg m− 3in Centre to 64 µg m− 3 in North) and 54 µg m− 3 (37 µg m− 3 in Algarve − 74 µg m− 3 in North) for urban zones. These results shown the strong impact of anthropogenic emissions of level of NO2, being typically considered as indicator of traffic emissions44, 45. On European level, approximately 40% of NOx emissions are contributed by road transport sector40. The population exposure to ambient NO2 concentrations is especially relevant in urban areas because its emissions are close to the ground and are distributed across densely populated areas. Furthermore, the highest concentration of NO2 were observed in suburban and urban zones of North region (i.e. 40% of coverage for traffic emissions monitoring in Portugal; Table 1). Concerning the limits for heath protection, 9 exceedances of hourly limit value in 2019 were registered in 5.9% (4 stations) of all monitoring station (North and Lisbon TV region), all of them being urban sites (and 3 traffic influence).
In 2020, the mean of NO2 concentrations at rural zones (Fig. 1c) were between 2 µg m− 3 (Algarve) and 10 µg m− 3 (North). In suburban zones, depending on each region the respective levels were 3–7 times higher, with means between 14 µg m− 3 in Alentejo and 64 µg m− 3 in North, whereas in urban zones the respective NO2 levels were even higher (4–18 times in a comparison with in rural zones) with range of 29–53 µg m− 3 in Algarve and North, respectively. In agreement with the previous year, the highest levels of NO2 (up to 6 times) were observed for the zones (all) of North region. However, in 2020 NO2 levels were significantly (p < 0.05) lower in a comparison with the previous year, being approximately half for the rural and 30% lower in suburban and urban zones as follows: overall mean 6 µg m− 3 vs. 12 for rural zones, 33 µg m− 3 vs. 43 µg m− 3 in suburban and 43 µg m− 3 vs. 56 µg m− 3 in urban ones. Thus in 2020, NO2 pollution was significantly lower (p < 0.05) in all types of zones and in all regions of Portugal. NO2 was the pollutant with more significant changes during the two year and the restrictions associated with the COVID-19 pandemic seemed to have significant implications for NO2 levels in air, both on local (direct) and international level, the latter being demonstrated by the much lower concentrations of the pollutant observed at rural sites in 2020. Concerning the urban traffic zones specifically (Fig. 2c), the concentrations of NO2 were lower than in previous year in all regions between February and May. A significant decrease of NO2 levels was though registered in March, April was the month with the minimal means in 2020 in all the regions (range 17–33 µg m− 3). It is noteworthy that in North the NO2 levels were still almost twice higher (mean of 57 µg m− 3) during the state emergency period than in the rest of country. In addition, evaluating then decrease of NO2 (Fig. 2d), the biggest changes between the two years were observed in March and April of 2020 when NO2 decreased by 15% (North) and 240% (Algarve). Considering the two largest and most populated urban areas in country (Lisbon MA and Oporto MA in Lisbon TV and North region, respectively; Table 1S), NO2 cumulative decrease was 55% (40 and 15%, respectively) which from the national perspective may represent several health benefits46. These results clearly confirm that NO2 levels were significantly lowered during restrictions associated COVID-19 outbreak (especially in months of March and April). Finally, in 2020 over the period analysed in this work all monitoring stations fulfilled the limit value for the health protection and no exceedances were observed, unlike the previous year.
SO2 (Fig. 1d) maximum of 1 h mean concentrations ranged between 2–12 µg m− 3 in rural zones, 3–13 µg m− 3 and 3–29 µg m− 3 in suburban and urban zones, respectively. The levels of SO2 were especially high in North region, where for suburban and urban zones concentrations were 3–6 times higher than in the other regions. However, across all monitoring stations, 1 h limit alert threshold (500 µg m− 3) and 1 h limits value (350 µg m− 3) of SO2 concentrations were fulfilled. In addition, in general SO2 levels were below the 24 h limit value; in 2019 only 3 stations (North region) registered 1 h maximum concentrations above the daily limit value, but the 24 h concentrations during those exceedance were fulfilled. In 2020 (Fig. 1d) the 1 h maximum means ofSO2 were 5 µg m− 3 (range of 1–10 µg m− 3) in rural zones, and 9 µg m− 3 (3–23 µg m− 3) and 10 µg m− 3 (5–18 µg m− 3) in suburban and urban zones, respectively. For all three types of zones, the highest SO2 were observed in North region (up to 6 times for suburban zones and 9 times for urban ones) than in other regions. The North region was also the only one where 1 h maximum concentrations exceeded one time the 24 h limit value (on urban industrial site). Finally, 1 h alert and 1 h limit were obliged in all 68 monitoring stations. Though SO2 is a not a pollutant associated with traffic emissions, in 2020 the overall levels were approximately 65% lower than in the period of the previous year with overall means of 5 vs. 8 µg m− 3 which could be due to suspended industrial emissions. Nevertheless, evaluating the industrial sites specifically (suburban and urban Table 1), the means obtained between both years were not significantly different (6.3 vs. 6.9 µg m− 3in 2019). In addition, the monthly evolution trend of did not show any change of patterns in the lock down period, however, assessment of 24 h means (oppose to 1 maxima used in this work) should be conducted when available.
Data for ozone in 2019 (Fig. 1e) that maximum 1 h mean ranged between 74 µg m− 3 (North) and 104 µg m− 3 (Algarve) of rural zones, 62 µg m− 3 (North) and 90 µg m− 3 (Alentejo) in suburban and 67 µg m− 3 (North) and 101 µg m− 3 (Algarve) in urban ones. These results show that registered 1 h maxima concentrations were higher (p < 0.05) at rural sites than those in suburban and urban ones, in agreement with other studies reporting the “ozone paradox” . Production of background ozone exhibits both long-term trends and substantial annual variability  due to the variations in air-flow, air pressure or temperature49, 50, 51. In addition, peak ozone episodes are strongly influenced by emissions of its precursors, such as nitrogen oxides and volatile organic oxides, the latter being relevant in rural zones52 and by meteorological conditions (being favoured by warm, stagnant high-pressure conditions). For all zones, the registered 1 h maxima were the lowest in north of country consistently increasing towards the south, being the highest in southern regions of country (Algarve and Alentejo for suburban zones, Fig. 1e); in agreement the north of the country being the coldest and south being the warmest with mean air temperature as the follows: 13.8°C in North, 15.1°C for Centre, 16,8 Lisbon TV, 16.9°C in Alentejo and 17.0°C in Algarve53. From the legislative perspective, the European hourly alert of 180 µg m− 3 was exceeded once (North regions), whereas 1 h information threshold of 180 was reached once in Lisbon TV region (120 µg m− 3, expressed as daily 8 h mean) though could not be clearly assessed, once the continuous measurements of ozone are not public yet. Finally, in 2020 the overall levels of ozone slightly (though not significantly) increased (83 vs. 80 µg m− 3). One h maximum levels ranged between 79 µg m− 3 (North) and 92 µg m− 3 (Alentejo) in Continental Portugal whereas levels in islands were higher (92–97 µg m− 3). In agreement with previous year, suburban and urban sites of Continental Portugal exhibited, respectively, significantly lower concentrations of ozone as follows: 59–80 µg m− 3 (North and Alenetejo), and 70–80 µg m− 3 (Centre and Lisbon TV). However, no differences were observed between the levels during the two years with the means as 87 µg m− 3 vs. 89 µg m− 3 in 2020 and 2019 in rural sites, 73 µg m− 3 vs. 79 µg m− 3 for suburban, and 85 µg m− 3 (in both years) at urban zones. The European hourly alert of 240 µg m− 3 was not exceeded in 2020.