Impacts of the COVID-19 lockdown on air pollution at regional and urban background sites in northern Italy

. The COVID-19 lockdown measures gradually implemented in Lombardy (northern Italy) from 23 February 2020 led to a downturn in several economic sectors with possible impacts on air quality. Several communications claimed in the ﬁrst weeks of March 2020 that the mitigation in air pollution observed at that time was actually related to these lockdown measures without considering that seasonal variations in emissions and meteorology also inﬂuence air quality. To determine the speciﬁc impact of lockdown measures on air quality in northern Italy, we compared observations from the European Commission Atmospheric Observatory of Ispra (regional background) and from the regional environmental protection agency (ARPA) air monitoring stations in the Milan conurbation (urban background) with expected values for these observations using two different approaches. On the one hand, intensive aerosol variables determined from speciﬁc aerosol characterisation observations performed in Ispra were compared to their 3-year averages. On the other hand, ground-level measured concentrations of atmospheric pollutants (NO 2 , PM 10 , O 3 , NO, SO 2 ) were compared to expected concentrations derived from the Copernicus Atmosphere Monitoring Service Regional (CAMS) ensemble model forecasts, which did not account for lockdown measures. From these comparisons, we show that NO 2 concentrations decreased as a consequence of the lockdown by − 30 % and − 40 % on average at the urban and regional background sites, respectively. Unlike NO 2 , PM 10 concentrations were not signiﬁcantly affected by lockdown measures. This could be due to any decreases in PM 10 (and PM 10 precursors) emissions from trafﬁc being for by in from and/or from changes in formation The also led to an increase in the highest O 3 at both urban and regional sites resulting from reduced The relaxation measures beginning in May resulted in close-to-expected NO 2 concentrations in the urban background and to signiﬁcant increases in PM 10 in comparison to expected concentrations at both regional and urban background sites.

Abstract. The COVID-19 lockdown measures gradually implemented in Lombardy (northern Italy) from 23 February 2020 led to a downturn in several economic sectors with possible impacts on air quality. Several communications claimed in the first weeks of March 2020 that the mitigation in air pollution observed at that time was actually related to these lockdown measures without considering that seasonal variations in emissions and meteorology also influence air quality. To determine the specific impact of lockdown measures on air quality in northern Italy, we compared observations from the European Commission Atmospheric Observatory of Ispra (regional background) and from the regional environmental protection agency (ARPA) air monitoring stations in the Milan conurbation (urban background) with expected values for these observations using two different approaches. On the one hand, intensive aerosol variables determined from specific aerosol characterisation observations performed in Ispra were compared to their 3year averages. On the other hand, ground-level measured concentrations of atmospheric pollutants (NO 2 , PM 10 , O 3 , NO, SO 2 ) were compared to expected concentrations derived from the Copernicus Atmosphere Monitoring Service Regional (CAMS) ensemble model forecasts, which did not account for lockdown measures. From these comparisons, we show that NO 2 concentrations decreased as a consequence of the lockdown by −30 % and −40 % on average at the urban and regional background sites, respectively. Unlike NO 2 , PM 10 concentrations were not significantly affected by lockdown measures. This could be due to any decreases in PM 10 (and PM 10 precursors) emissions from traffic being compensated for by increases in emissions from domestic heating and/or from changes in the secondary aerosol formation regime resulting from the lockdown measures. The implementation of the lockdown measures also led to an increase in the highest O 3 concentrations at both the urban and regional background sites resulting from reduced titration of O 3 by NO. The relaxation of the lockdown measures beginning in May resulted in close-to-expected NO 2 concentrations in the urban background and to significant increases in PM 10 in comparison to expected concentrations at both regional and urban background sites.

Introduction
The COVID-19 pandemic is an epidemic of the coronavirus disease 2019 , of which the outbreak was first identified in Wuhan, China, in late December 2019. The World Health Organization declared COVID-19 a pandemic on 11 March 2020. The first case of COVID-19 in northern Italy was detected on 20 February 2020 in Codogno, about 60 km south-east of Milan (Fig. 1). To reduce the virus spreading, the Italian government quickly adopted a series of measures, such as the quarantine for 10 municipalities, the cancellation of all main public events, and the closure of schools and universities in northern Italy (DL, 23 February 2020a). The lockdown started in all of Italy on 9 March 2020 (DPCM, 8 March 2020a). All commercial and Published by Copernicus Publications on behalf of the European Geosciences Union.
retail activities were closed on 11 March, except for grocery shops and pharmacies (DPMCM, 11 March 2020b), and it was forbidden to move outside the place of residence, except for health issues or work. Further lockdown measures were decreed on 22 March 2020 (DPCM, 22 March 2020c), including the suspension of all non-essential industrial production activities. The lockdown lasted until 4 May 2020 (DPCM, 26 April 2020d), when a gradual relaxation of the measures was decided by the government. The reopening of manufacturing industries and construction sites was allowed, but schools and universities as well as some commercial activities such as restaurants remained closed. Movements from a region to another were still forbidden, but moving short distances to work and to visit relatives was possible. From 18 May 2020, most commercial businesses could reopen, and free movement was granted within regional borders (DL, 16 May 2020b). This lockdown provided a unique opportunity to determine how such dramatic measures can eventually influence air quality. This is the focus of this paper.
Lombardy, Piedmont and Emilia-Romagna in northern Italy produce roughly 50 % of the national gross domestic product (GDP), with Lombardy alone producing 22 % of the national GDP (data from ISTAT, 2020). This economic dynamism (mainly linked to industrial production and service-related activities) is associated with significant pollutant emissions, which together with unfavourable conditions for pollution dispersion (due to low wind speeds and particular orography) cause high pollution levels, leading to exceedances of the EU standards for nitrogen dioxide (NO 2 ), particulate matter (PM 10 ) and ozone (O 3 ) in northern Italy (European Environment Agency, 2019). In this area, the impact of the lockdown on economic activities was quite important, as illustrated by data relative to the production of electricity and energy for heating and to transport-related activities (ARPA Lombardia, 2020a). Compared to 2019, the Italian thermal electricity production ( Fig. 2) fell in March (−18 %), April (−24 %) and May 2020 (−16 %). The consumption of natural gas by the industrial sector as reported by the Italian natural gas provider (https://www.snam.it, last access: 9 October 2020) also fell by roughly −30 % at the end of March in comparison to the beginning of March 2020.
Regarding transport, the Monitoring of Polluting Vehicles project (MOVE-IN) managed by the Lombardy region provided data on the traffic changes derived from its monitoring of "vehicle km" (the sum of kilometres travelled by all vehicles in the area) driven by light-duty vehicles and passenger cars (for a small number of vehicles compared to the full fleet circulating in the region though). MOVE-IN data show that the number of vehicle km driven by lightduty vehicles remained quite constant till 9 March 2020, then dropped by −75 % to reach a minimum between 16 March and 13 April 2020 before returning to "usual" (i.e. as before the lockdown period) values after 4 May 2020 (ARPA Lombardia, 2020a). For private cars, the number of vehi-cle km driven also decreased by roughly −70 % between the beginning and the end of March and started increasing again after 4 May but with a slower recovery than for lightduty vehicles. The number of requests for driving directions (https://www.apple.com/covid19/mobility, last access: 9 October 2020) showed similar variations (Fig. 2).
Numerous early communications based on preliminary measurement data analyses associated observed improvements in air quality with the lockdown measures taken to contain the spread of the COVID-19 epidemic. In Brazil, the lockdown in São Paulo was followed by drastic reductions in NO (up to −77 %) and NO 2 (up to −54 %) and by an increase in O 3 (approximately +30 %) compared to the previous 5year means for the same period (Nakada et al., 2020). In the Yangtze River Delta region (China), Li et al. (2020) showed that concentrations of PM 2.5 , NO 2 and SO 2 decreased by 32 %, 45 % and 20 % during the first lockdown phase and by 33 %, 27 % and 7 % during the second lockdown phase, compared with the 2017-2019 average for the same period. O 3 also increased in that region. Across Europe, Grange et al. (2021) estimated that NO 2 and O 3 concentrations at urban background sites were 32 % lower and 21 % higher than expected, respectively, when maximum mobility restrictions were in place. A clear decrease in NO 2 concentrations in Barcelona and Madrid (Spain) during the lockdown was also described by Baldasano (2020). In France, the analysis by INERIS (Institut national de l'environnement industriel et des risques) compared air pollution forecast data (calculated without incorporating changes in emissions due to lockdown measures) with adjusted simulations performed a posteriori by assimilating observation data influenced by the lockdown measures. They estimated that NO 2 concentrations were on average approximately 50 % lower than expected in France's largest cities (INERIS, 2020).
Regarding Italy, maps of NO 2 surface concentrations estimated from satellite data (e.g. Sentinel-5p) were published by several websites and media showing large reductions in NO 2 concentrations over northern Italy in March 2020 as compared to the previous months and to March 2019 (e.g. Copernicus Atmosphere Monitoring Service headlines published on 17 and 26 of March 2020; CAMS, 2020a, b). Observations and models were also combined in the analysis from the German Aerospace Center (DLR) which estimated a decrease of about 40 % in the total columnintegrated NO 2 tropospheric concentrations over norther Italy due to the lockdown measures using Sentinel-5p data. They also estimated reductions in ground level NO 2 concentrations of about −20 µg m −3 (−45 %) by comparing ground-based observations from 25 stations in Lombardy to a model simulation with pre-lockdown emission levels (German Aerospace Center, 5 May 2020). In situ observations also showed reduced ground level NO 2 concentrations as lockdown measures were implemented. The environmental protection agency ARPA Lombardia showed that March 2020 NO 2 concentrations were below the stan-   (2020). Percentages are calculated in comparison with 2019 data for thermal energy production (source: https://www.terna.it, last access: 9 October 2020) and in comparison with data from the third week of February 2020 for mobility data (source: https://www.apple.com/ covid19/mobility, last access: 9 October 2020). dard deviation calculated from previous years, indicating a possible signal of reduced emissions from traffic and economic sectors (ARPA Lombardia, 2020a). The European Environment Agency (EEA) developed a viewer that tracks NO 2 and particulate matter (PM 10 and PM 2.5 ) weekly average concentrations (https://www.eea.europa.eu/themes/air/ air-quality-and-covid19, last access: 9 October 2020). It shows that NO 2 concentrations in Milan were at least 24 % lower after the lockdown implementation than during previous weeks and 21 % lower compared to the same period in 2019. Similar trends were found in other cities of northern Italy and European countries where strong measures were taken to contain the epidemic. In contrast, no consistent effect of the lockdown measures on particulate matter (PM 2.5 and PM 10 ) could be observed in the main European cities (European Environment Agency, 2020).
Air pollution did decline in northern Italy from February to May in 2020 as it does every year, mainly due to seasonal variations in emissions and weather conditions. The strength of certain sources does indeed change during the course of the year, like, for example, domestic heating, while weather conditions influence pollution concentration in diverse ways: advection and dispersion of pollutants resulting from horizontal winds; dilution of pollutants throughout the mixed boundary layer resulting from convection; and pollutant lifetimes resulting from photochemical reactions (sun radiation), wet removal (clouds and rain), etc. It is therefore not straightforward to disentangle the effects of changing emissions due to lockdown measure implementation from those of seasonal changes in emissions and variability in meteorological conditions between different seasons and different years. In the present study we determine how much of the changes in air pollution observed during the lockdown period in northern Italy were actually due to lockdown measures, independently from expected variations in pollutants' emissions, lifetime and dispersion. Our results are based on comparisons between air pollution observation data from Ispra (regional background site) and the Milan conurbation (urban background sites) with CAMS regional ensemble (hereafter ENSEMBLE) model forecast data for the same sites. To help understand the effect of the lockdown measures in the regional background area, we also use 4 years of specific aerosol measurements from Ispra.

Material and methods
Model and observation air pollution data from four sites located in Lombardy covering the time periods 17 February-24 May 2019 and 2020 were collected and analysed. We selected three sites located in the Milan conurbation as representative of the Milan urban background and the site of Ispra as representative of the regional background of the upper Po Valley (Fig. 1). Ground level concentrations of NO, NO 2 , SO 2 , O 3 and PM 10 as measured in situ at the monitoring stations and as calculated by the ENSEMBLE model forecast were considered. Particle number size distribution and aerosol light absorption Ångström exponent data from Ispra for the 2017-2020 period were also utilised.

Site description
The European Commission Atmospheric Observatory (ECAtmO) has been operated in Ispra (45.815 • N, 8.636 • E; 209 m a.s.l.) since November 1985. It has contributed to the CLRTAP-EMEP (co-operative programme for monitoring and evaluation of the long-range transmission of air pollutants in Europe under the Convention on Long-range Transboundary Air Pollution) and WMO-GAW (World Meteorological Organization -Global Atmosphere Watch) air pollution measurement programmes for several decades and to the European Research Infrastructures ICOS (Integrated Carbon Observation System) and ACTRIS (Research Infrastructure for the observation of Aerosol, Clouds and Trace Gases) for several years. ECAtmO is located on the north-western edge of the Po Valley, 20-60 km away from major pollution point sources but still in a densely populated area (ca. 500 km −2 ) with significant economic activity (GDP per capita = EUR 29 000; EUROSTAT, 2017). Wood burning for domestic heating is also an important source of particulate matter during the cold period of the year (Gilardoni et al., 2011). Past measurements of HCHO / NO 2 ratios compared to the threshold values proposed by Tonnensen and Dennis (2000) suggest that the photochemical production of O 3 is limited by the availability of volatile organic compounds (VOCs) in February-May in Ispra.
The Milan metropolitan area is the second most densely populated area in Italy (ca. 2300 km −2 ), with a GDP per capita of about EUR 54 000 (EUROSTAT, 2017) and about 4100 circulating vehicles km −2 (ISTAT, 2020). Three stations in the Milan conurbation were selected as representative of the urban background in Milan city, namely "Milan via Pascal" (45.478 • N, 9.236 • E; 122 m a.s.l.), "Cormano" (45.548 • N, 9.167 • E; 155 m a.s.l.) and "Limito di Pioltello" (45.483 • N, 9.328 • E; 123 m a.s.l.). All three stations are operated by ARPA Lombardia. We selected only urban background stations because pollutant concentrations at traffic sites are hardly reproducible by regional air quality models with a horizontal resolution of about 10 km. The station in Milan via Pascal is located near the university, and it is considered to be the urban background station of the city, while the other two stations are located in the hinterland, near (<500 m) two main roadways used by commuters at the northern (Cormano, with about 75 000 vehicles per day) and eastern (Limito, 20 000 vehicles per day) entrances of the city. Average population densities are 7500, 4500 and 2800 km −2 in Milan, Cormano and Limito di Pioltello, respectively (ISTAT, 2020).

Measurements
At ECAtmO in Ispra, online in situ air pollution measurements are performed from appropriate inlets located at 6.5 and 9 m above ground level for gaseous and particulate pollutants, respectively. The inlet for reactive gas is made of PTFE (inner diameter: 2.7 cm). The sample residence time in the inlet tube is ca. 2 s. Each analyser samples from the main inlet through a Nafion dryer. In 2019-2020, the measurement programme included CO, NO, NO x , NO 2 , SO 2 , O 3 , non-methane hydrocarbons (until 6 March 2020) and NH 3 (since 28 January 2020) as gaseous pollutants. The NO x (i.e. NO and NO 2 ), SO 2 and O 3 data reported in this work were obtained with trace level instruments based on infrared (IR) (1200 nm) chemiluminescence and a Mo converter (Thermo Fisher 42iTL), ultraviolet (UV) (214 nm) fluorescence (Thermo Fisher 43i TLE), and UV (254 nm) absorption (Thermo Fisher 49C), respectively. These instruments are calibrated every 3 months using zero air and certified gas cylinders (NO and SO 2 ) or a primary standard ozone generator (O 3 ). In 2019, annual average concentrations of NO, NO 2 , SO 2 , O 3 and PM 10 were 4, 16, 0.4, 38 and 21 µg m −3 , respectively. Particulate matter is sampled through metal-made inlets characterised by negligible losses. Each instrument samples isokinetically from the main aerosol inlet through Nafion dryers. In 2019-2020, the aerosol online in situ measurement programme included PM 10 mass concentration, particle number concentration and number size distribution, particle light extinction, absorption, scattering, and backscattering at several wavelengths. The PM 10 mass, particle number concentration and light absorption data reported in this work were obtained with a TEOM-FDMS (Thermo Fisher 1405-DF), a differential mobility particle sizer (home-made Vienna-type differential mobility analyser + TSI 3772 condensation particle counter) covering the particle mobility diameter range 10-800 nm and a seven-wavelength aethalometer (Magee AE31), respectively. The TEOM has been calibrated using a standard filter provided by the manufacturer, while the differential mobility particle sizer (DMPS) and the aethalometer are operated, maintained and controlled according to ACTRIS guidelines (https://www.actris.eu, last access: 9 October 2020). They were both calibrated at the specific ACTRIS central facility (https://www.actris-ecac.eu, last access: 9 October 2020) on 3-7 June 2019. Near-real-time data are available from the JRC data catalogue at http://data.jrc.ec.europa.eu/collection/ abcis (last access: 9 October 2020).
The three stations in the Milan conurbation are part of the ARPA Lombardia air quality network, compliant with Directive 2008/50/EC requirements in terms of measurement methods, macro and micro localisation, and data coverage. Inlets are located at 2.5 m above ground level for all pollutants. The measurement programmes comprise NO, NO x , NO 2 , SO 2 and O 3 at all three sites. Additional measurements include benzene, toluene, xylenes, PM 10 , PM 2.5 , benzo[a]pyrene and NH 3 in Milan and CO and PM 10 in Limito. Each gas analyser samples from the main inlet through a Nafion dryer. The NO x data reported in this work for the Milan conurbation were obtained with trace level instruments based on IR (1200 nm) chemiluminescence and a Mo converter (Teledyne API 201E, Thermo Fisher 42i and Thermo Fisher 42c in Milan Pascal, Limito and Cormano, respectively), and O 3 was measured by UV (254 nm) fluorescence (Thermo Fisher 49i at all three sites). All measurements are performed according to a specific quality assurance and quality control programme. All gas monitors are calibrated every 3 months using zero air and certified gas cylinders (NO) and every 6 months using a primary standard ozone generator for O 3 . The PM 10 mass concentrations in the Milan conurbation reported in this work were measured using beta absorption analysers (FAI SWAM DC and 5A models in Milan Pascal and Limito, respectively). The PM analysers are checked for temperature, pressure, flow rates, leaks and other operational parameters every 3 months. A periodical comparison with gravimetric samples has been performed once yearly in Milan Pascal and upon a specific audit programme in Limito.

CAMS regional ensemble forecast description
The Copernicus Atmospheric Monitoring Service (CAMS) provides 4 d ahead air quality forecasts daily for Europe from currently nine different regional air quality models (CHIMERE, DEHM, EMEP, EURAD-IM, GEM-AQ, LOTOS-EUROS, MATCH, MOCAGE, SILAM). Hourly pollutant concentrations are calculated for altitudes ranging from the 40 m thick surface layer to 5 km. The outputs of the different individual models are interpolated on a common regular 0.1 • × 0.1 • latitude × longitude grid (about 10 km × 10 km) over the European domain (30-72 • N, 25 • W-45 • E). Forecasts are performed independently by all the individual regional air quality systems: each air quality model is based on different chemical (gas and aerosols) and physical parameterisations but uses the same meteorological drivers as input (the ECMWF Integrated Forecasting System, IFS) and the same anthropogenic emissions data (Kuenen et al., 2014;Denier van der Gon et al., 2015) based on 2011 emission inventories until June 2019 and on 2016 emission inventories afterwards. An ensemble (named "ENSEMBLE forecast") is calculated from individual model outputs with a median approach (Marécal et al., 2015). This method provides an optimal estimate (Riccio et al., 2007) which is rather insensitive to outliers and generally yields better estimates than the individual models (Galmarini et al., 2018). CAMS regional air quality forecasts are routinely quality-controlled, and dedicated evaluation reports are published every third month for both individual and the ENSEMBLE models (see https://atmosphere.copernicus.eu/ regional-services, last access: 9 October 2020). In this work, we used daily averages of the ENSEMBLE surface concentration forecast each day for the next 24 h (D0). For the period March-May 2019, the differences between daily mean D0 forecasts and measurements performed at various reference stations across northern Italy (expressed as median of the root mean square errors, RMSEs) were 10.5, 10.6 and 24.5 µg m −3 for NO 2 , PM 10 and O 3 , respectively. Additional statistical scores are available in quarterly CAMS reports (CAMS, 2019(CAMS, , 2020c. Note that the actual ENSEMBLE RMSEs relative to the stations and time periods we analyzed are part of our statistical analysis described in Sect. 2.4.1.
As the anthropogenic emissions used by the individual models did not change to account for any lockdown measure, the ENSEMBLE model continued to forecast pollutants' concentrations as if the COVID-19 epidemic had not occurred in 2020.

Pollutant concentrations
To determine the specific impact of lockdown measures on concentrations of air pollutants, we compared daily observations (Obs 2020 ) with daily expected concentrations (Exp 2020 ) for the period 17 February-24 May 2020, which comprises the 8 lockdown weeks (D = 9 March-3 May 2020), the 3 weeks before the beginning (A = 17 February-8 March 2020) and the 3 weeks after the end of the lockdown period (P = 4-24 May 2020). NO 2 , PM 10 , NO, O 3 and SO 2 observed and expected concentrations are shown in Fig. 3. Expected concentrations were derived from 2020 EN-SEMBLE forecasts, which account for variations in meteorological conditions and seasonal changes in emission source strengths in a "business as usual" world, i.e. without lockdown measures. However, since data from 2019 show that the agreement between ENSEMBLE forecasts (CAMS 2019 ) and observations (Obs 2019 ) improves from February to May (see Figs. S1-S3 in Supplement), CAMS 2020 was corrected for this seasonality. Thus, 2020 daily expected pollutant concentrations (Exp) were calculated as follows: The comparison of observations with these expected concentrations for 2020 has the great advantage of being insensitive to the fact that the emissions inventories used to calculate ENSEMBLE forecast data for 2019 and 2020 were different. The disadvantage of this approach is that Obs and Exp cannot be compared to each other on a daily basis since Exp values are affected by random variations in the Obs 2019 / CAMS 2019 ratio. Therefore Obs 2020 and Exp 2020 data were compared statistically for the three periods A, D and P . Since changes in pollutant emission rates are expected to result in changes in pollutant concentrations in terms of percentages or ratios, statistical analyses were performed on Obs 2020 / Exp 2020 daily ratios. We calculated occurrence frequency distributions of the Obs 2020 / Exp 2020 ratio using eight class bins ranging from <0.25 to >2, all equally wide on a logarithmic scale (except the last one when specifically indicated). Cumulative frequencies of occurrence were also plotted to facilitate comparisons (Fig. 4). To detect possible specific impacts of lockdown measures on the highest concentrations, specific occurrence frequency distributions were also calculated by selecting the 28 d on which ENSEMBLE forecast data were greater than the median during the lockdown period. These days are different for each pollutant and each site. The statistical significance of the differences in Obs 2020 / Exp 2020 ratios during the lockdown period in comparison with before and after the lockdown period (i.e. between A and D or P and D) was assessed by applying a t test assuming unequal variances to the means A, P and D, defined as follows: (2) The null hypotheses (D = A and D = P ) were tested at the 95 % confidence level, and results were used to determine if differences between D and A and D and P were statistically significant.

Intensive aerosol variables
To complement our analyses based on pollutant concentrations, we also looked at two characteristics of the atmospheric aerosol measured at ECAtmO in Ispra. The first one is the percentage of number of tiny particles with mobility diameters (D p ) between 15 and 70 nm as compared with the "total" number of particles with mobility diameters between 15 and 800 nm. This percentage was calculated from full particle number size distributions (10<D p <800 nm). The smallest particles (10<D p <15 nm) were not considered because their measurement is affected by larger uncertainties (Wiedensohler et al., 2018) and by nucleation particle bursts. The range 15<D p <70 nm was selected as representative of particles emitted by primary sources (Giechaskiel et al., 2019;Giechaskiel, 2020;Ozgen et al., 2017;Tiwari et al., 2014). The second variable is the aerosol light absorption Ångström exponent (AÅE). It represents the wavelength dependence of light absorption by aerosol particles. AÅE values vary with particle sources and have commonly been used to apportion pollution particles between, for example, traffic and wood burning (Sandradewi et al., 2008). Traffic-emitted particles (mainly from diesel engines) have an AÅE close to 1, while particles from wood combustion have more variable AÅEs around 2 (Sandradewi et al., 2008). The mixture of pollution particles with primary or secondary aerosol of biogenic origin can also lead to AÅE values much greater than 1. Since both variables are insensitive to air pollution dispersion, they are much less variable than the extensive variables (i.e. atmospheric concentrations) they are derived from (e.g. Putaud et al., 2014). The values expected for these so-called intensive variables were calculated as the arithmetic averages observed during the 2017-2019 period.

Results and discussion
The observation and ENSEMBLE forecast data used to estimate the values expected for the air pollution variables discussed in this section are described in Sects. 1 and 2 of the Supplement to this article. The high PM 10 concentrations observed at all sites on 28 and 29 March 2020 were related to desert dust advection from the east (see maps from the World Meteorological Organisation Sand and Dust Storm Warning Advisory and Assessment System, 2021). The data from these two dates were not excluded from our statistical analysis since they did not affect its results.

Regional background (Ispra)
The trend in AÅE observed in Ispra in 2017-2019 (and also in 2020) is consistent with a decreasing contribution of wood burning to particulate pollution from winter to summer. The AÅE values measured in 2020 can of course not be compared point to point to the 2017-2019 average in Fig. 5 because the use of wood fuel for domestic heating also depends on weekend and cold-evening occurrences. However, the clear increase in the AÅE average between 9 March and 4 May 2020 compared to the 3 weeks before, the 3 weeks after and the corresponding period in 2017-2019 undoubtedly shows a change in particle sources related to lockdown measures (Table 1). A specific analysis focused on the first 4 weeks of the lockdown period (before significant numbers of biogenic aerosols are expected) suggests a −45 % reduction in aerosol from traffic (and a concomitant +45 % increase in aerosol from wood combustion) during that period. Particle number size distribution measurements in Ispra typically show modes at 25-50 nm during morning rush hours as well as in the evening in winter. Particle primary sources include fuel combustion by thermal engines and liquid (oil) or solid fuel (e.g. wood) combustion for domestic  Table 1. Observed/expected concentration ratios for pollutant concentrations and aerosol characteristics before, during and after the lockdown measures in Ispra (regional background) and Milan (urban background).

Ispra (regional background)
Milan (   heating. The ultrafine mode diameters of primary particle emissions range from 50 to 100 nm for domestic heating (e.g. Tiwari et al., 2014;Ozgen et al., 2017) and range from 10 to 90 nm for engines (e.g. Giechaskiel et al., 2019;Giechaskiel, 2020). Measurements also show that peaks in the number of 15-70 nm particles can result from the growth of nucleation particles in the afternoon. The percentage of 15-70 nm particles generally increased from mid-February till end of May in 2017-2019 (Fig. 6). Considering that (1) wood burning combustion for domestic heating did not decrease during the lockdown period, (2) nucleation and growth of secondary aerosol particles were observed on sunny days during the lockdown period from 6 April 2020, and (3) that mostly morning peaks in particle number diminished during the lockdown period especially from 11 March to 13 April 2020, the relative "disappearance" of 15-70 nm particles during the lockdown period (Fig. 6) can be attributed to a decrease in traffic related to lockdown measures. Since the atmospheric lifetime of 15-70 nm particles is 3-12 h, local to regional traffic was concerned. Although it significantly increased after 4 May 2020, the percentage of 15-70 nm particles did not get back to the level observed before the lockdown as lockdown measures were relaxed. The decrease in emissions from local traffic indicated by the drop in the percentage of the smallest particles (Fig. 6) is the most probable cause for the decrease in NO related to the lockdown measures in Ispra (Fig. 4). NO daily mean concentrations are indeed dominated by their morning peak corresponding to traffic rush hours (which disappears during weekends). During daytime, NO is rapidly converted to NO 2 , and NO concentrations reach very low steady-state values. Decreased NO emissions should therefore result in decreases in NO 2 . Such a decrease in NO 2 (−40 % on average) actually occurred in Ispra as a result of the lockdown measures from 9 March 2020 (Fig. 4). In contrast, NO 2 did not "recover" from the lockdown measures, unlike NO, of which concentrations increased again in comparison to expected concentrations as lockdown measures were relaxed on 4 May 2020  (Table 1). Due to its lifetime of about 1-2 d (Seinfeld and Pandis, 2016), NO 2 can travel rather long distances. Nitrogen oxides are also emitted by large combustion sources like thermal power plants, which also emit SO 2 . However, our analysis of SO 2 data also reveals that sources of SO 2 that affect concentrations in Ispra decreased due to lockdown measures (Figs. 3 and 4). The fact that NO 2 observed/expected concentration ratios remained as low after as during the lockdown period could be explained by a slower increase in traffic on the regional scale as compared to the local scale.
Regarding secondary pollutants, the highest O 3 concentrations significantly increased compared to expected concentrations during the lockdown period in comparison with the 3 weeks before (Figs. 7 and 4). This suggests that O 3 peaks are usually diminished by NO titration during this period of the year in Ispra and that the abatement in NO x emissions revealed by NO and NO 2 data analyses led to a reduction in this effect. The relaxation of lockdown measures led to a further increase in O 3 . Since O 3 production is generally VOC-limited in May in Ispra, this increase in O 3 is probably due to an increase in anthropogenic emissions of VOCs from, for example, local traffic. In the case of PM 10 , which is mainly composed of secondary particulate species in Ispra (Larsen et al., 2012), no significant decrease compared to expected concentrations could be identified as lockdown measures were implemented (Figs. 3 and 4). This is because the decrease in PM 10 related to traffic was compensated by the increase from wood burning for domestic heating, at least during the first half of the lockdown period. In contrast, PM 10 significantly but marginally increased as lockdown measures were relaxed on 4 May 2020, at a time of the year (from May onwards) when wood burning combustion for domestic heating is largely reduced. Figure 7. Changes in observed and expected concentrations for the 3 weeks before and after the lockdown in comparison with the 28 d of lockdown corresponding to the CAMS daily forecasts above the median for each pollutant. Filled bars represent statistically significant differences. Empty bars represent differences that are not significantly different from zero.

Urban background (Milan conurbation)
To represent the Milan urban background, we used data from three urban background sites located in Milan's hinterland and in Milan's city centre (Fig. 1). NO 2 significantly decreased (−30 % on average) compared to expected concentrations as lockdown measures were implemented (Figs. 3 and 4). NO 2 significantly but not totally "recovered" when lockdown measures were relaxed (Table 1), which suggests that not all sources determining NO 2 concentrations in Milan were fully reopened. However, the increase in NO after the end of the lockdown period suggests that local traffic largely resumed. Perhaps NO x emissions on a broader scale did not yet reach their usual intensity during the first 3 weeks after the end of the lockdown period, as already suggested by NO 2 data from Ispra. Regarding NO, it should be noted that a significant decrease in comparison to the weeks before the lockdown period could only be detected at the city centre station (Fig. S8). Both sites in the hinterland are much closer to highways and might reflect more NO emissions from heavyduty vehicles, whose traffic did not decrease that much, at least during the first weeks of the lockdown period.
As in Ispra, the implementation of lockdown measures on 9 March 2020 led to increases in O 3 in the Milan conurbation compared to expected concentrations (Figs. 3 and 4). This can be explained by the decrease in O 3 titration by NO in a pollution regime where photochemical O 3 production is limited by the availability of volatile organic compounds. The relaxation of lockdown measures did not lead to the expected decrease in O 3 (Fig. 4), perhaps because NO x emissions did not fully recover during the 3 weeks following the end of the lockdown period.
Again, as in Ispra, no significant change in PM 10 could be detected when lockdown measures were implemented. This is very likely due to the fact that decreased emissions of PM 10 (and PM 10 precursors) were compensated by increases from other sources like domestic heating and enhanced formation of secondary PM. In particular, Huang et al. (2021) reported that increased oxidative capacities of the atmosphere (e.g. higher O 3 concentrations) resulted from the drastic reductions in NO x emissions following from the lockdown measures in China, which in turn lead to an increase in the formation rate of nitric acid (HNO 3 ). Such a phenomenon in northern Italy, together with sustained emissions of ammonia from agriculture, which was not affected by the lockdown (ARPA Lombardia, 2020a), could have resulted in increased formation of particulate ammonium nitrate (NH 4 NO 3 ) and therefore an increase in PM 10 concentrations beyond expected concentrations in the Milan conurbation. For the 3 weeks following 4 May 2020, the relaxation of lockdown measures led to a further increase in PM 10 in comparison to expected concentrations in Milan. This might be attributed to the upturn in traffic and particularly to the re-suspension of dust from roads that had been little used for several weeks.

Conclusions
Northern Italy has been an air pollution hotspot for decades. Northern Italy also hosted the very first clusters of the COVID-19 epidemic in Europe, and from February 2020, containment measures were gradually implemented, culminating in strict lockdown measures in force between 9 March and 4 May 2020. We isolated specific impacts of the lockdown measures on air pollution by comparing observed with expected data at one regional background site (Ispra) and three urban background sites (in the Milan conurbation) across the period 17 February-24 May 2020. All four stations were in the COVID-19 "red zone". Expected pollutant concentrations were derived from ENSEMBLE forecasts, which are based on actual meteorological conditions and historical emissions estimates that ignored the COVID-19 epidemic and related lockdown measures. Changes in observed versus computed expected concentrations for the lockdown period and the 3 weeks before and after the lockdown period should therefore directly reflect the impact of lockdown measures on air pollution.
We showed that lockdown measures had statistically significant impacts on concentrations of most gaseous pollutants (Table 1). However, we were not able to highlight systematic significant effects on PM 10 concentrations.
Focusing on those days for which the ENSEMBLE model forecast concentrations were above the median for the lockdown period (Fig. 7), the following can be said: -NO 2 concentrations decreased by about −30 % and −50 % at the urban and regional background sites, respectively, as a result of the lockdown implementation on 9 March 2020. The relaxation of lockdown measures on 4 May led to a partial recovery in NO 2 concentrations in Milan (urban background) but not in Ispra (regional background).
-Unlike NO 2 , PM 10 concentrations were not significantly affected by the lockdown measures. We showed that the decrease in traffic-related PM 10 was compensated by an increase in PM 10 associated with wood burning for domestic heating in Ispra. PM 10 concentrations in Milan are to a great extent influenced by PM 10 "non-urban" and "non-traffic" sources (Thunis et al., 2018), including the formation of secondary aerosol. Sustained regional background PM 10 concentrations and a modified HNO 3 production regime associated with continuing NH 3 emissions from agriculture could explain the lack of decrease in PM 10 resulting from the lockdown measures in Milan too. In contrast, the relaxation of lockdown measures led to an increase in PM 10 concentrations at both urban and regional background sites (+30 % and +20 %, respectively) in May, when domestic heating is much reduced.
-The lockdown measures led to an increase in the highest O 3 concentrations at both the urban and regional background sites.
The sad experience of the COVID-19 epidemic and subsequent lockdown measures shows that drastic changes in mobility and economic activity can lead to 0 % (insignificant) to −30 % reductions in air pollution in urban background areas. These figures suggest that the abatement of air pollution down to levels that do not have adverse effects on human health in northern Italy may require structural changes in other sectors, including energy production, domestic heating and agriculture in addition to transport.
Author contributions. JPP, EP and LP contributed to conception and design. SMDS, FL and UDS contributed to acquisition of data. JPP, EP, LP, GL and AC contributed to analysis and interpretation of data. JPP, EP, LP, GL and AC drafted the article.
Competing interests. The authors declare that they have no conflict of interest.
Acknowledgements. The authors thank the Copernicus Atmosphere Monitoring Service Information, in particular the Regional Production Service. Jean-Philippe Putaud, Luca Pozzoli, Enrico Pisoni, Sebastiao Martins Dos Santos and Friedrich Lagler thank their colleagues from JRC for fruitful tele-discussions during the whole lockdown period and for helpful comments on the manuscript.
Review statement. This paper was edited by Rolf Müller and reviewed by S. Fadnavis and one anonymous referee.