Introduction

The new coronavirus discovered in Wuhan in China in late December 2019 has spread easily and sustainably. This infectious virus has caused many effects in almost all countries around the world. Because of the high propagation rate of COVID-19 between people, several countries have taken measures of safety to limit the spread of the virus. Accordingly, several human activities were suspended including industrial activities, tourism, and transport, while all scientific and cultural meetings were postponed across the globe. On 15th June the number of COVID-19 confirmed cases in Italy has reached 236,989; in Spain 243,928; in The UK 295,893; in India 332,424; in the USA 2,057,838; in Brazil 850,514; and in France 152,767 (WHO 2020).

The coronavirus disease has reached France on 24, January 2020 and the first imported cases in Europe were also detected in France (Bernard et al. 2020). The number of confirmed cases was progressively increased which lead to the announcement of limited safety measures on February 28th. While on March 17th, 2020, the French authority has implemented a nationwide lockdown as a response to the rapid spread of COVID-19 (Pullano et al. 2020). These strict safety measures have impacted negatively almost all economic activities in the whole world. In contrast, the environment has taken advantage of lockdown as the activities generating pollutants were reduced or suspended such as industrial activities, local transport and travel in and out the home country, and population mobility (Lal et al. 2020). Accordingly, several studies were carried out to evaluate the impact of imposed lockdown on the environment (Agarwa et al. 2020; Mandal and Pal 2020; Kerimray et al. 2020; He et al. 2020a, b; Bao and Zhang 2020; Zhu et al. 2020; Yao et al. 2020; Chu et al. 2020; Shakoor et al. 2020; Gautam 2020; Pata 2020).

The present study investigates the impact of imposed lockdown on air quality using the main atmospheric pollutants including the nitrogen dioxide (NO2), ozone (O3), particulate matter (PM2.5 and PM10), volatile organic component (VOC), sulfur dioxide (SO2), and carbon monoxide (CO), retrieved from 79 air quality monitoring stations of Auvergne-Rhône-Alpes region, France. This will allow identifying the principal sources of pollution and helping regulators to set pollution reduction targets at a level that would minimize the risk to the health of the exposed population. This study was focused more precisely on Lyon city (center of Auvergne-Rhône-Alpes region), as it is considered to be the most urbanized area with a high vehicle density (Anzivino and Venzac 2018) and which was largely affected by air pollution.

Methodology

Study area

Auvergne-Rhône-Alpes region is one of the most populated regions in Europe and the leading French industrial region. It is characterized by high population density and the large expansion of its territory combining large agglomerations of 5 metropolises: Lyon (1,622,331; region prefecture), Grenoble (510,368), Saint-Etienne (372,308), Clermont-Ferrant (264,704), and Chambéry (186,355), and 12 departments namely: Ain, Allier, Ardèche, Cantal, Drôme, Isère, Loire, Haute-Loire, Puy-de-Dôme, Rhône, Savoie, and Haute-Savoie (Fig. S1). It is located in the southeastern quarter of France, bordering on Switzerland and Italy. Auvergne-Rhône-Alpes occupies an area about 69,711 km2, and a population of 8,037,059 inhabitants, with a density of 115 inhabitants/km2. It is bordered by five other administrative regions: Bourgogne-Franche-Comté to the north, Centre-Val de Loire to the northeast, Nouvelle-Aquitaine to the west, Occitanic to the south-west, and Provence-Alpes-Côte d'Azur to the south-east. It is also bordered by Italy (Aosta Valley and Piedmont) to the east and Switzerland (Cantons of Geneva, Valais, and Vaud) to the north-east.

The average annual temperatures for this region are ranged from 5 to 15 °C, while the highest average annual temperatures are characterizing the south part of the region, which is under the Mediterranean influence. Auvergne Rhône-Alpes region is the sunniest French region (1976 h/year), it is characterized by very variable climate, the summers are pretty hot and quite humid because of some Mediterranean influences. Besides, the winters are very cold due to the presence of the mountains and dry in the south. Because of the different climates around the region, the vegetation is miscellaneous, there are numerous regional and national parks and lush forests but also southerner plants.

Pollution data sources

The data covering the whole area of the Auvergne-Rhône-Alpes region were analyzed, in terms of mass concentration of the different pollutants between 3rd February and 15th June 2020. The daily average mass concentrations of several air pollutants, including nitrogen oxides (NOx, NO, and NO2), sulfur dioxide (SO2), particulate matter (PM2.5 and PM10), carbone monoxide (CO), ozone (O3) (8 h diurnal average, from 08:00 AM until 16:00 PM), and volatile organic component (VOC) from 79 monitoring stations spread over the region were extracted and used (Fig. S1). Data used in the present study is available in Atmo-Auvergne-Rhône-Alpes online portal for air quality data dissemination (Atmo 2020a).

Meteorological conditions

The meteorological conditions strongly influence the formation and the evolution of many atmospheric pollutants (Zhang 2019; Chen et al. 2020a, b). Meteorological conditions data used in the present study including maximum and minimum temperatures, relative humidity, wind speeds, solar radiation, sunshine duration, and rainfall, for the period extending from 3rd February to 15th June in the Lyon, were downloaded from the weather station of Lyon-Bron (Info climat 2020).

Meteorological parameters influencing air pollution

The wind is a fundamental element for the dispersion, dilution, and orientation of pollutant plumes. Under high wind speed, the dispersion of the pollutant is greater, while, in the lower wind speed is likely for air pollution accumulation. Strong winds can direct a plume to a specific area, thereby concentrating pollution. Rain cleans the atmosphere from several pollutants because the falling water interacts with the pollutants present during its fall and then transform or deposit them on the ground. For nitrogen oxides, leaching is an efficient phenomenon permitting the decrease of NOx concentrations; however, chemical interaction between NO2 and water leads to the formation of acid rain. Likewise, the particle matter (PM) can be cleaned from the air by the rain. Concerning ozone, despite his very low soluble in water, leaching contributes also to reduce its concentrations. Both high and low temperatures influenced the air atmospheric pollution emissions, formation, and evolution, for example, the volatility of organic species like VOC increase with temperature, which may increase their ratio in the gas phase and thus their condensation and conversion especially into particles (Kourtchev et al. 2016). Sunshine duration, solar radiation, and relative humidity (RH) can strongly affect atmospheric photochemistry such as O3 and OH radical formation, and also particle process formation including nucleation, condensation, and growth (Li et al. 2018; Lu et al. 2019a, b).

Results and discussion

The normal situation of NO2, O3, PM10, and PM2.5 mass concentration over Auvergne-Rhône-Alpes region

Figure 1 shows the mapping of the normal situation of NO2, O3, PM10, and PM2.5 mass concentration distribution over Auvergne-Rhône-Alpes. The highest NO2, PM2.5, and PM10 levels are recorded in the middle at Rhône and Loire departments, which are characterized by high population and car densities (Anzivino and Venzac 2018). Nevertheless, only NO2 levels exceed European limit values (40 μg/m3), especially in Rhône and Loire departments and in the north of the Ain department. The industrial activities located in the south of the Lyon metropolis contribute significantly to the increase in PM level, either by direct emissions or by VOC conversion via photochemical reactions (Kourtchev et al. 2016; Palm et al. 2016; Sbai et al. 2020). Thus, sulfate particles can represent a significant amount of PM in the center of the region due to the SO2 emission by industrial activities located in several areas in the Rhône department (Fig. S2). The moderate ozone mass concentration observed in Rhône and Loire departments can be especially due to its consumption by NO2 because this area is characterized by high levels of NO2 (Fig. 1). The departments affected by the highest O3 mass concentration are Allier, Cantal, and Puy de Dôme; they are characterized by low population and car density. Moreover, higher solar radiation and sunshine in this area promote the formation of ozone (O3), but stay below European Limit values (120 μg/m3 daily 8 h mean) (European Commission 2019).

Fig. 1
figure 1

Normal spatial distribution for NO2, O3, PM10, and PM2.5 mass concentrations for March 10th, 2020

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Spatial changes for main air pollutions (NO2, O3, PM2.5, and PM10) affecting the air quality during COVID-19 pandemic lockdown

Spatial pattern of nitrogen oxides (NO2) mass concentration

The NO2 mass concentration before, during, and after lockdown is presented in a form of maps in Fig. 2. The reduction of road activities during lockdown reached 80% in the Auvergne-Rhône-Alpes region (Fig. S3); hence, atmospheric level of NO2 may be affected. On March 10th, the NO2 mass concentrations were high in the center (Rhône and Loire departments) and medium in the west of the region. Since the lockdown was implemented by the French authority on March 17th, 2020 (Pullano et al. 2020), the NO2 level has decreased by 67.9% (at Saint Etienne) and 53.6% (at Annecy) during the lockdown (Table 1), because almost all the human activities were halted except power generation. It is also noticed that the NO2 levels tend to increase, which is due to the beginning of easing the imposed lockdown restrictions on 11th and May 27th. Thus, these results indicate the improvement in air quality and prove that human activities are the main source for nitrogen dioxide rather than natural events and that lockdown some time will be mandatory to reduce pollution and improve air quality.

Fig. 2
figure 2

Spatial distribution for NO2 mass concentrations before (March 10th), during (March 17th, 19th, and 26 th), and after lockdown (May 18th and 27th)

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Table 1 Summary of air pollution concentrations before and during COVID-19 pandemic for nitrogen dioxide (NO2), particulate matter (PM2.5 and PM10), and ozone (O3) for main cities in the region
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Several studies were carried out around the worldwide cities and have provided the positive impact of enforced lockdown as a response to the COVID-19 outbreak on air quality. In 44 cities located in the northern part of China, the NO2 values were decreased by 24.7% because of the drop in human mobility by 69.8% and suspension of industrial activities (Bao and Zhang 2020). The same case is observed in Almaty (Kazakhstan), where the NO2 levels were reduced by 35% (Kerimray et al. 2020) while in Barcelona (Spain), it was reduced by half during the lockdown period (Tobías et al. 2020). In the whole world, the restriction of human activities has led to an apparent reduction of nitrogen dioxide (Dantas et al. 2020; Bao and Zhang 2020; Ogen 2020).

Spatial pattern of ozone mass concentration

Figure 3 represents the spatial evolution of the ozone mass concentration over the Auvergne-Rhône-Alpes region from March 10th to May 27th, 2020. The results indicate an increase of O3 levels from the beginning of the lockdown (March 17th) and this increase has become clearer on May 18th, especially in the center and south of Auvergne-Rhône-Alpes region. The results show that ozone mass concentration increased by 120.7, 109.5, 105.7, 102.6, 94.4, and 75.9% in Saint Etienne, Annecy, Lyon, Grenoble, Chambery, and Valence, respectively (Table 1). This behavior could be attributed to the drastic drop in NO levels (Fig. S4), which leads to the reduction of the O3 consumption (Sharma et al. 2020). The ozone destruction flux via the main NOX photochemical reactions has been calculated (Table 2); the flux has been reduced from 1.51 × 1023 (molecule cm−3 s−1) before the lockdown to 6.28 × 1022 (molecule cm−3 s−1) during the lockdown, which represent a reduction of 40% of O3 destruction. This partially explains the increase in ozone during the lockdown. Similar results were found in almost all studies done in worldwide cities (Mahato et al. 2020; Siciliano et al. 2020). Besides, the significant ozone levels detected in rural areas can be explained by the transport of air masses over long distances from urban areas. In addition, the presence of methane (CH4), an important ozone precursor with a long chemical lifetime (about 9 years), contributes significantly to O3 formation in rural areas (Atmo 2020b).

Fig. 3
figure 3

Spatial distribution for O3 mass concentrations before (March 10th), during (March 17th, 19th, and 26th), and after lockdown (May 18th and 27th)

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Table 2 Ozone production and depletion flux via NOx photochemical reactions under low an high NOx levels
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Spatial pattern of particulate matter PM10 and PM2.5 mass concentration

Figures 4 and S5 show the spatial distribution of PM2.5 and PM10 mass concentrations. Conversely to the most pollutants, the PM2.5 and PM10 have increased during the lockdown and show the same trend while the lower concentrations were registered on March 10th (before lockdown), May 18th and 27th (after lockdown), and the higher levels were recorded during the lockdown on March 17th, 19th, and 26th. The results show also that the PM2.5 mass concentration increased by 97, 52.9, 35.3, 32.8, and 20.7% in Chambery, Lyon, Saint Etienne, (Grenoble and Valence), and Annecy, respectively. Furthermore, the PM10 mass concentration increased by 77.6, 24.7, 22.5, 14.9, 11.3, and 4.5% in Chambery, Valence, Lyon, Grenoble, Annecy, and Saint Etienne, respectively (Table 1).

Fig. 4
figure 4

Spatial distribution for PM10 mass concentrations before (March 10th), during (March 17th, 19th, and 26th), and after lockdown (May 18th and 27th)

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Residential wood heating which is the main emitter of PM10 and PM2.5 in cold weather can contribute to 80% of total particle emissions in this region (Atmo 2020c). Thus, domestic housing could contribute to emissions of PM during the lockdown where the people spent more time confined in their homes, which increases the consumption of wood heating and consequently much PM emission. In addition to primary sources (heating, vehicles exhaust, and industrial activities), numerous secondary sources can also contribute to the PM emission, such as the atmospheric photochemistry (Ortega et al. 2016; Huang et al. 2015; Sbai and Farida 2019a). The SOA formation is enhanced during the lockdown period because of the increase in O3 that can produce the OH radical in the presence of humidity, according to the two reactions O3 + hν → O2 + O(1D) and O (1D) + H2O → 2OH (Peng et al. 2015).

In the Rhône department, which is characterized by high levels of PM10 and PM2.5 (Figs. 4 and S5), industrial activities were not significantly affected by lockdown, since VOC levels in three areas of this region (Fayzin ZI, Saint-Fons ZI, and Vernaison ZI) remain almost stable (Fig. 5e). This clearly shows that O3/OH oxidation of VOCs is an important secondary source of PM10 and PM2.5 in the metropolitan Lyon. In addition, in the Auvergne-Rhône-Alpes region, there are several parks and important forest areas (Fibois 2018), which may represent an important source of biogenic VOCs (BVOC), which can contribute to the PM formation. In our previous study, we found that urban air in Lyon can effectively contribute to the SOA formation via OH oxidation and ozonolysis (Sbai et al. 2020). Moreover, Sea salt especially iodine can contribute also to PM formation via OH and O3 oxidation mainly under high O3 levels (Saiz-Lopez and Plane 2004; Gómez Martín et al. 2013; Sbai and Farida 2019b). These particles could be transported by the wind towards the urban environment during the lockdown.

Fig. 5
figure 5

Trend of average concentrations at Lyon Downtown for a NO2 (24 h average), b ozone (8 h average daily maxima), c PM2.5 (24 h average), d PM10(24 h average), e VOC FZI (Feyzin Zone Industriel), VOC SZI (Saint- Fons Zone Industriel), VOC VZI (Vernaison Zone Industriel), 10, 15, and 6 km from Lyon downtown (at South), respectively, and f CO. Between 3rd Feb and 15th Jun, the horizontal red lines for NO2, PM2.5, and PM10 indicate the limit values; for the other pollutants, there is no exceedance of limit values (the dense area represented lockdown period)

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These results indicate that this region has experienced spatial PM10 and PM2.5 level increase during the lockdown period, in contrast to the similar studies carried out in other cities around the world where the PM2.5 and PM10 levels were declined during the enforced lockdown (Chauhan and Singh 2020; Dantas et al. 2020; Zambrano-Monserrate et al. 2020).

The evolution of air pollutions (PM10, PM2.5, NO2, O3, CO, and VOC) affecting the air quality in Lyon

Figure 5 summarizes the evolution of the PM10, PM2.5, NO2, O3, CO, and VOC in Lyon (center of the Auvergne-Rhône-Alpes region) between February 3rd and June 15th, 2020, to assess the effects of lockdown on air quality. We noticed a strong decrease in NO2 during the first 2 weeks of lockdown. However, a slight upward trend has emerged since the start of progressive removal of lockdown from May 11th. It should be noted that the NO2 level dropped since February 23th, due to decreased road densities a week before strict lockdown (Fig. 5a). The O3 levels have increased steadily since February 23th and remain almost stable during the lockdown (Fig. 5b). However, a significant decrease in ozone was observed after April 18th due to the consecutive rainy days and moderate sunshine (Fig. 6b and d). The atmospheric oxidation of VOCs can represent an important source of ozone in Lyon since the VOC levels remained almost stable during lockdown (Fig. 5e) (Zhang et al. 2020).

Fig. 6
figure 6

Meteorological parameters pattern including temperature, humidity, rainfall, wind speed, sun radiation, and sunshine, at Lyon downtown between February 3rd and June 15th, 2020 (the dense area represented lockdown period)

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Meteorological conditions such as solar radiation, humidity, temperature, sunshine, rainfall, and wind speed can affect ozone formation through modulating the rate of chemical kinetics reaction, the partitioning of reaction pathways, and efficiency of dry and humid deposition (Lu et al. 2019a, b). Lyon region is very sunny and characterized by a hot continental climate, which is more sensitive to ozone pollution. Solar radiation has increased regularly from 3rd February to 15th June (Fig. 5c), which can lead to an increase of atmospheric photochemistry and ozone formation. Figure 5a shows a continuous increase in temperature, which can lead to an increase in natural emissions of BVOC, in particular isoprene (Fig. S6). The atmospheric oxidation of BVOC contributes to the formation of ozone (Allison 2020). On other hand, despite the reduction in the carbon monoxide mass concentration (Fig. 5f), the O3 formation may be influenced by CO reactivities, because under low NOx level, ozone depletion can occur; however, under high NOx level, CO can contribute to ozone formation according to the reaction mechanism depicted in Table 3.

Table 3 Carbon monoxide (CO) reaction mechanism showing O3 depletion under low NOx level and O3 formation under high NOx level
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Figures 5c and d show the evolution of PM10 and PM2.5, respectively, in Lyon between 3rd February and 15th June. PM10 and PM2.5 present the same trend and show a continuous increase during the lockdown. However, this trend was perturbed by successive rainy days from 25th April (Fig. 6d), which removed all the suspended particles (Info climat 2020). The decrease of ozone, solar radiation, and the sunshine between 25th April and 11th May (Fig. 6a, b, and c) can limit) can limit, and c) can limit, b, and c) can limit the formation of secondary particles resulting in the decrease of PM levels. Chen et al. (2020a, 2020b) show also an increase in PM during lockdown; they have found two pollution episodes with PM2.5 exceeding 100 μg/m3 in Shanghai, China.

Air quality index change during lockdown in Lyon

Air quality index (AQI) has been used in several cities around the world to quantify the presence of certain pollutants in ambient air. Their main purpose is to inform the public about air pollution and the associated potential health risk (Atmo 2020d). The AQI calculation is defined at the national level on the basis of regulatory thresholds (Table 4). Three pollutants were considered for AQI estimation, including NO2, O3, and PM10. These species are considered as the main indicators of air pollution. For each of these pollutants, a sub-index is determined using daily mass concentration between February 3rd and June 15th, the final index corresponding to the highest sub-index. The air quality sub-index associated with O3, NO2, and PM10 at Lyon downtown shows that PM10 represents the main pollutant and the AQI is often attached to the PM10 mass concentration (Fig. S7). In addition, the AQI of the region generally varies between good and moderate, which indicates that there is no possibility of affecting public health. Despite the decrease of NO2 level, the AQI level is not reduced during the lockdown due to a notable increase in PM. However, other studies have shown a drop in AQI; this decrease has been explained as a consequence of halting the human activities (Mahato and Ghosh 2020; Bao and Zhang 2020; He et al. 2020a, 2020b) (Table 3).

Table 4 Correspondence grid for the index values with the thresholds of the inter-prefectural decree for the management of pollution episodes
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Air pollution in 2020 vs 2019

We have performed a comparison with data for main air pollution (NO2, O3, PM10, and PM2.5) of previous year 2019 and this year 2020 over the same period (17th Mars to 11th May) corresponding to the lockdown period for six cities in the region (Lyon, Saint Etienne, Grenoble, Annecy, Gaillard, and Valence) (Fig. S8). The results show that for all the cities, the NO2 decreased whereas O3 and the particles (PM10 and PM2.5) increased. The difference between the level of pollutants in 2019 and 2020 changes depending on each city; this can be explained by the meteorological conditions and the industrial activities of each zone and also the lockdown measures (strict or loose).

Conclusion

In order to assess the air quality status of Auvergne-Rhône-Alpes during the lockdown period, we explored data related to atmospheric pollutants extracted from 79 air quality monitoring stations covering the whole region. The daily and hourly concentrations of air pollutants including PM2.5, PM10, NOx (NO2 and NO), CO, O3, SO2, VOC, and isoprene have been obtained from the online portal for air quality data dissemination.

The results revealed a substantial change in all studied atmospheric pollutants except the VOC and SO2 during the lockdown, as NO2, NO, and CO have decreased, owing to restriction of road traffic. However, O3, PM2.5, and PM10 have increased during the lockdown. The increase in ozone is attributed to the decrease in its titration by NO because the flux of ozone depletion via a photochemical reaction involving NO decreased by 40%. On the other hand, the reduction in ozone consumption by other gases is of anthropic origin. Moreover, weather conditions (temperature, solar radiation, RH, sunshine) have favored the formation of ozone during the lockdown. Thus, the oxidation of VOC and BVOC also can contribute significantly to O3 and PM formation, because they have not been reduced during the lockdown. The AQI which represents good and moderate categories remains almost stable despite the decrease of NOx because PM and O3 increase. It is noted that AQI shows that PM10 is the main pollutant in this region.