Toward the next generation of air quality monitoring

Humans are very skeptical and they will only believe something if they see proof of it. Air pollution is something that we usually don't see with the naked eye, except in very extreme cases, like for instance when you have smog situations. Yet even minute quantities of these toxic gases cause severe deterioration of nature and affect humans' health. Sentinel-4, Sentinel-5 Precursor and Sentinel-5, we also refer them to as the atmospheric sentinels, are dedicated to observing atmospheric composition, and we want to know about trace gases and particles, how much is there, where is it and when is it there. We are focusing on three themes, one is climate, one is air quality and one is the state of the ozone layer and the hard UV radiation. Only if we have precise and trustworthy data, then we can confidently develop the correct strategy to improve air quality and mitigate the negative effect of greenhouse gases emission. The European Copernicus Programme has been created to answer this fundamental need. I'm Giorgio Bagnasco and I'm the project manager for Sentinel-4. In 2017, ESA has launched the first precursor satellite, Sentinel-5P. Probably you have seen in the news continental level maps showing the significant improvement of air quality across Europe during the pandemic lockdown. This was due to the significant drops of nitrogen dioxide levels across densely populated and industrialized areas. This data was coming precisely from the Copernicus Sentinel-5 Precursor. We put to the test some of the most cutting-edge instrument technologies and achieved exceptional scientific results in global air quality and atmospheric chemistry monitoring. The data for Sentinel-5P were for this reason really groundbreaking for the scientific community. The Copernicus Sentinel-4 mission and Sentinel-5 mission are collectively going to take these capabilities to the next level as a complete operational system. Their overarching objective is to measure independently and at the same time collectively, as a single virtual constellation of instruments, the quality of the air we breathe. Together, they aim to provide long-term, accurate, reliable data about the minute concentration of pollutant gases. The atmospheric sentinels form a constellation, so there's a distribution of tasks and Sentinel-5 and 5 Precursor are on a low Earth orbit. The so-called polar orbit at about 800 kilometers altitude. Polar orbit means that it will circulate around the north and the south pole and the Earth will be rotated underneath the rotation of the satellite and with that, we are mapping the full Earth. It's a little bit like peeling a potato, so you go round and round and round and at some point you have covered it all. And on the other side, we have Sentinel-4 that is flying much higher, staying over the same spot on ground all the time and a scanning mirror, it achieves an hourly revisit over Europe. In order to achieve data delivery every hour, required that Sentinel-4 is placed in the so-called geostationary orbit, 36,000 kilometers away from the Earth. If our eyes had the same capabilities as the optics of Sentinel-4, we would be able to read tiny letters on a target placed over 150 meters away. With the high spatial resolutions, we will be able to pinpoint the actual sources of this pollution, for instance, individual industrial areas and quantify the levels of their emissions. In addition, since we map the complete Earth, we will also see how these gases are moving in the various atmospheric layers and feed this into climate models. It will allow the decision-making authorities to set the goals on the limits of pollutant emissions. Of course, it will also allow us to ensure that the agreed and reduced levels of emissions are being adhered to across the boundaries of countries. My name is Ben Feilman. I work here at ESA ESTEC and I'm supporting the Sentinel Missions 4 and 5. The topics that we are studying with these, air pollution, climate and the ozone layer, they are global problems and they require a global coverage altogether. We have air pollution changes that are very fast. So emissions are changing fast, chemistry is changing fast and transport is also changing fast. And that's why the system needs to provide observations with an hourly revisit. So imagine being able to receive air quality analysis and forecasts in the same way you receive weather forecasts. It's not just a luxury anymore. It's a must for our health. My name is Didier Martin and I'm the Sentinel 5 project manager here at ESTEC at ESA. What we're doing is we're developing new satellites and in particular my task is to help develop Sentinel 5. The Sentinel 4 and Sentinel 5 missions rely on spectroscopy. In essence, when atoms and molecules interact with light, they emit or absorb a set of colours. We call them wavelengths, frequency, which make up what we call a spectrum. Each molecule and atom interacts with light in a unique way as it will absorb very specific colours. This means that they leave a distinguishing fingerprint of their presence. The light path is from the Sun through the atmosphere and then the light is reflected at the Earth and the light is then coming up again towards the satellite and on that path the trace gases imprint their specific fingerprints on the light by swallowing light at very specific wavelengths. The detection of the trace gases is extremely difficult. For each hundred million molecules of air there are, in average, only two molecules of dry nitrogen dissolved in it. The real challenge is to develop the technologies which would allow to achieve the kind of accuracy and sensitivity needed to detect so few pollutant molecules dispersed in an ocean of air. All the optical elements, lenses and mirrors alike, need to be polished to extreme smoothness. To give you an idea, the precision is better than one nanometer that is 50,000 times smaller than the diameter of a hair or, if you wish, only a few atoms thick. Also the detectors on board the Sentinel-5 mission are state-of-the-art. We have, for instance, five detectors on board, like one of these. While the number of pixels is between one and two millions for each of them and we have five of these kind of detectors, they are much better at distinguishing contrast than standard cameras. To give an example, in your regular phone camera you may have eight or sometimes ten bits of resolution for each pixel. Our detectors instead are measuring using 16 bits which is more than a hundred times more sensitive. Also, in order for Sentinel-5 to be able to map the full Earth each day, its telescopes need to have a very wide viewing angle of about 110 degrees. And this is 20 times more than your typical binocular and puts very stringent constraints on the mirrors and lenses used. Sentinel-5 has a fantastic heritage with the Sentinel-5 precursor with the Tropomy instrument. While Sentinel-5 builds on this, we are also pushing the boundaries even further by embarking novel technologies. For instance, in the areas of various detectors, key optical components or lightweight and highly stable carbon fibre structures. With these, we can provide state-of-the-art equipment that will deliver valuable data for the years to come. The overall optical design of Sentinel-4 instrument is extremely innovative with a very compact design featuring one common telescope and two separate spectrometers. It took more than 10 years to complete it. To reduce the complexity and the weight of the optical system, we had to minimise the number of lenses. One way to do this is to manufacture so-called aspheric lenses. We need to have very large diameter lenses to be able to collect the spectra of these trace gases from geostationary orbit. If the actual shape is only a few microns off the nominal one, the lens is not usable. All the optical elements need to be integrated and aligned in a perfect way with accuracies again of a few microns. It will allow Sentinel-4 to determine the concentration of the trace gases with a spatial resolution of only 8 by 8 kilometres from 36,000 kilometres. Far better than any other instrumentation currently available. A truly incredible result. This is exactly why Europe is on the forefront of Earth observation and a gold standard for the world. Weather satellites are built to answer questions like do I need an umbrella today? Or a farmer would ask, shall I harvest now? Or maybe in a week time from now? The atmospheric sentinels are built to answer a different set of questions related for example to air quality. How is the air quality today? So the data are primarily used by data assimilation and that works with atmospheric models that are a combination of forecast models that are beefed up with chemistry processes. These models are running alongside with reality and whenever an observation from the satellites is coming in the observation gives a little push in the right direction and when the observation error is small the push is big. If the observation error is large the push is small and altogether this enables to create one consistent picture ingesting many observations from many systems at the same time. For Sentinel-5 to provide all the details of the atmosphere it will generate about a terabyte of data each and every day and that for more than 21 years. This data also needs to then be converted into the final pollutant maps that will be used by the end users. And powerful computers at Jumitsat will crunch this data in near real time providing the results latest two and a half hours after they have been measured. And this is key then for asthma patients and it's also relevant for joggers who all might be concerned about ozone or smog. Our data will also be used by volcanic ash advisory centres and they are triggered by the detection of any ash cloud coming from a volcanic eruption and they immediately warn air traffic controllers that are then reacting by guiding pilots to avoid mineral ash clouds. The data from the atmospheric sentinels are also used by a very active science community who is chasing what is going on in the atmosphere including trends and events and this also enables then the research on complex processes in the atmosphere. Both Sentinel-4 and 5 will be able to measure nitrogen oxides emissions from combustion processes associated with industry, traffic, household, biomass burning sulphur dioxide emissions from volcanic emission plumes large coal-fired power plants and aerosols and substances called volatile organic compounds like ozone, formaldehyde or glyoxal which are major health concerns. With this extended wavelength range Sentinel-5 will provide in addition data on methane emissions from fossil fuels agricultural landfills oil and gas extraction permafrost thawing and carbon monoxide emissions from combustion processes associated with industry, traffic, households biomass and fossil fuel burning and finally also UV index which is the amount of harmful UV light reaching the surface of the earth. These missions are operational missions they are spanning a very long term and the lifetime of the Sentinel-5 precursor mission is a long seven and a half years. Sentinel-4 is implemented as a series of two instruments with a lifetime of seven to eight years each spanning a total lifetime of 15 years and Sentinel-5 with three instruments in series will span a lifetime of 21 years. We have a fully committed data availability 24 hours a day and the data are freely available to each and everyone all the time. We need accurate and reliable data because as Dutch people say neten is weten to measure is to understand. When we say long term we mean the ability to monitor and deliver data for several decades since this is the only way to understand properly the evolution of a complex system like our atmosphere. The main contractor for the Sentinel-5 development is Airbus Defence and Space with its outlet located near Munich. The overall industrial consortium consists of about 50 different companies from 13 countries around Europe with many more suppliers even and I've been counting the number of people that I know of and it's more than 500 of highly skilled engineers and scientists that have been working or are working still on the project. In average about 30 engineers, staff and contractors have been working on Sentinel-4 for the past 10 years. Similarly more than 200 engineers worked at our prime contractor Airbus Munich for the Sentinel-4 development. Airbus was selected because of its very strong experience and legacy from previous similar complex instrumentation like NIRSPEC, a near-infrared spectrometer now flying on board the James Webb Space Telescope and providing incredible scientific results. A huge number of people contributed to Sentinel-4 and Sentinel-5 over more than a decade. Sentinel-4 has completed the so-called phase D of its development life. In our jargon this means that the Sentinel-4 flight model has been built, tested and calibrated to verify its functionalities and performances. We are currently in the final phase of the Sentinel-5 mission development. The first Sentinel-5 instrument is fully assembled and is currently undergoing an extensive series of tests. So far we successfully completed the vibration tests which simulate the harsh environment during the launch of the rocket. We also completed the so-called electromagnetic compatibility tests and this is to ensure that the instrument can work without interfering or being interfered by other spacecraft equipments and instruments. After all these tests, Sentinel-5 will then be mounted at the beginning of 2024 onto the spacecraft. It will then undergo final functional tests where all the systems including the ground systems and also the data analysis systems will have to work together. Once all of this is done the spacecraft will be shipped finally to its launch site where we expect to lift off in early 2025. And yet the launches of the Sentinel-4 and 5 missions will not be the end of these incredible journeys. They will actually only be the beginning. We are all extremely excited and we look forward to seeing Sentinel-4 and Sentinel-5 delivering for the next 20 years those long-awaited atmospheric data which we hope will contribute to measure the quality of the air we breathe for the benefit of Europe and the whole planet. Do I feel if we make a potential positive impact on humanity? Definitely. And humanity is facing tremendous challenges with climate change. I always like to refer as an example to the ozone hole which was discovered in 1985 through similar measurement techniques. The lack of this crucial protection layer in our atmosphere was soon linked to the use of fluorocarbons used for instance in air conditionings and fridges. It was only after we were able to measure and quantify the effects that eventually these gases were banned and the continuous monitoring of the ozone layer proved that it had a positive effect. Providing accurate measurements of the atmospheric pollution and greenhouse gases will definitely help future generations. The Sentinel-5 precursor is detecting methane super emitters. More than a thousand plumes of this kind within one year and there have been examples when super emitters have been cut down after the observations by the Sentinel-5 precursor have been published. We are all aware, we all see that unfortunately our planet, its atmosphere, its land, its ocean are in a precarious status. Now if we want that Europe, its citizens, its public institutions as well as the private entities develop the correct strategies and take the right decision to address this enormous challenge then job number one must be to understand the phenomena which are at the root of this worrying state of affair. For 26 years I worked in the ESA science directory. Eventually I thought that my experience, my knowledge, my passion for instrumentation could have been used for even more relevant, practical and pressing challenges affecting our planet. I hope that the Sentinel-4, Sentinel-5 and Sentinel-5B missions will ultimately help save lives. This is to me the most vital, significant and personally motivating aspect of the Sentinel missions.