Copernicus confirms a reduction of NO2 levels over Northern Italy since the lockdown


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    photo: Windy.com; licence: cc

    Amid the COVID-19 measurements, Copernicus monitors a 10% per week reduction in NO2 surface concentrations since mid-February.

    While the question of improved air quality is a secondary concern with the world facing the COVID-19 crisis, satellite observations detected abrupt changes in activity levels in Northern Italy, where dense surface observation of the pollutant nitrogen dioxide (NO2) shows a gradual reduction trend of about 10% per week over the last four to five weeks.

    The Copernicus Atmosphere Monitoring Service (CAMS), implemented by the European Centre for Medium-Range Weather Forecasts on behalf of the European Commission, provides daily analyses of hourly concentrations of regulatory air pollutants, which can serve as “ground truth” to assess quantitatively and in more details the changes in concentrations identified from satellites and attributed to the effects of COVID-19 measures across the world.

    The CAMS regional air quality analyses come in the form of hourly gridded maps and are obtained by combining satellite observations, surface in situ observations and numerical modelling. Taking the point covering the city of Milano, the hourly timeseries of surface concentration of NO2 since January 1st is shown below.

    photo: Copernicus; licence: cc

    While no clear trend is visible over the first five weeks of the year, a decreasing trend can be seen starting during week 6 and continuing to date. Average NO2 concentrations were around 65 mg.m-3 in January, 45 mg.m-3 in February, and around 35 mg.m-3 for the first half of March. The linear decreasing trend on the daily average since week 6 is of the order of -4 mg.m-3 per week. Similar decreasing trends are found for other cities in Northern Italy, such as Torino or Bergamo. In the East, there seems to be rather a step change: in Bologna, concentrations were on average around 30 mg.m-3 in January and are on average around 15 mg.m-3 since the beginning of February (35 mg.m-3 and 15 mg.m-3 for Venezia).

    “What the satellite observes is proportional to the vertically integrated amount of pollutants from the ground up to the top of the atmosphere”, explains Vincent-Henri Peuch, Director of Copernicus Atmosphere Monitoring Service. “This is quite different from the concentrations at the surface and even more so from the emissions. Given these limitations, it is quite remarkable that a signal of decreasing activity levels could be detected. This shows the extent of the measures taken by Italy.”

    NO2 is a short-lived pollutant. Once emitted, it will stay in the atmosphere generally less than a day before being deposited or reacting with other gases. This means that this pollutant remains fairly close to where it has been emitted. Most of its emissions sources are located at the surface and are generated by human activities such as traffic, energy production, residential heating, industries, etc. The sources vary greatly during the day, as well as depending on the day in the week and the month. It also depends on factors such as weather, as a cold spell will trigger increased residential heating and energy demand.

    CAMS is continually monitoring the air quality over Europe and on a global scale, providing the data to most leading websites and smartphone applications such as Windy, The Weather Channel, Breezometer or Plume Labs.

    What is CAMS?

    CAMS is implemented by ECMWF on behalf of the European Commission.
    CAMS regional air quality analyses are operationally produced by a consortium of contractors led by Météo-France and INERIS (France). These partners rely on surface air quality observations provided by the European Environment Agency, which gathers them from each individual country.

    The CAMS regional air quality analyses come in the form of hourly gridded maps and are obtained by combining satellite observations, surface in situ observations and numerical modelling that mimic the laws of physics and chemistry in the atmosphere. The data collected is of high quality and can be considered as “ground truth”. The main limitation of this dataset is that it cannot account for fine spatial variability and local effects that occur within a few meters to hundred meters near a source, like a road segment with heavy traffic or a power plant chimney stack. The data is representative of the average background values over a grid with a scale of 10km by 10km. The data can be obtained openly and freely from CAMS’ Atmosphere Data Store.

    Can satellites really observe air pollution at the surface?

    The European Commission’s Copernicus Sentinel-5 Precursor (S-5P) TROPOMI instrument benefits from an unprecedented spatial resolution of 3.5km x 5km, which allows to see fine maps of a number of air pollutants such as NO2. Satellite observations have however some limitations on the level of detail at which they can be used for monitoring air pollutants.

    Firstly, they make their measurements from satellites in Low-Earth-Orbit and provide only one measurement per day for each location on Earth: for example, the overpass time of S-5P is 13:30 Mean Local Solar time. Second, the physics of the atmosphere is such that for pollutants like NO2, measurements are not possible or significantly degraded in the presence of clouds and of fine particles suspended in the air (aerosol). During the winter in Europe, this means that only about 25% of the data can be used.

    To overcome this issue, observations can be averaged over longer periods of time (typically a running average of 10 days), in order to have at least one or two measurements for each location in each average period. Finally, and most importantly, what the satellite observes is proportional to the vertically integrated amount of pollutants from the ground and up to 10 km (tropopause); this is obviously different from the concentrations at the surface, let alone the emissions by human activities. Given these limitations, it is striking that a clear signal of decreasing activity levels could be detected. Over Europe, high-quality surface observations are made by each individual country and gathered by the European Environment Agency in Near-Real-Time. These observations can serve as “ground truth” to assess the reality of the signal detected by satellite.

    What is the situation in other parts of Europe?

    In other parts of Europe, the detection of statistically robust trend is yet more challenging, as shown below for some capitals in North-Western Europe. Weather-related episodes of high (between weeks 3 and 4; week 6) and low NO2 surface concentrations are the main features that can be seen.

    photo: Copernicus; licence: cc

    What are the plans of Copernicus for monitoring emissions using satellites in the future?

    As part of the Copernicus programme, CAMS is currently developing a capacity to derive emissions directly, using further advanced modelling techniques. An essential ingredient for this will be the new generation of satellites monitoring air pollutants. These will be embarked on geostationary platform and will provide hourly observations over selected part of the world.

    The Korean Aerospace Research Institute’s Cheollian 2B satellite, which was launched into orbit aboard an Arianespace Ariane 5 rocket on February 18th, hosts the first instrument of this kind, GEMS (Geostationary Environment Monitoring Spectrometer); scientifically useful data covering parts of Asia is expected to be available at the end of this year. Similar instruments are planned over the US (TEMPO) and Europe. As part of Copernicus, the Sentinel-4 instrument onboard Eumetsat’s Meteosat Third Generation will launch in late 2022 or 2023. Sentinel-4 will be the backbone for the planned emissions information service of CAMS, which will be operated by ECMWF.



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