Satellites That Observe the Earth’s Atmosphere
The topic of the today’s podcast is using satellites to monitor atmospheric pollutants. Our guest is Mark Parrington, a senior scientist at the Copernicus Atmospheric Monitoring Service (CAMS) – one of the six earth observation services of the European Union. Mark holds a PhD in Atmospheric physics, and has done scientific research on satellite observation of the atmosphere for over ten years. He will help to explain to us how they make atmospheric observations from space, the kind of pollutants they are able to monitor, and how these atmospheric datasets are useful in various applications.
Remote Sensing and the Atmosphere
Satellites are quite the buzz in the geospatial field, with space technology improving over time and new, more efficient ways of observing the Earth emerging constantly. Right now, there are satellites that can look specifically at the Earth’s atmosphere. Unlike observing the Earth’s surface, monitoring the Earth’s atmosphere is a more complex process.
In order to observe the atmosphere, the satellites need to be able to model how electromagnetic radiation at different wavelengths interacts with different molecules in the atmosphere.
A lot of prior information about the atmosphere is needed to be able to accurately interpret what the satellite is measuring. Representative data obtained from these measurements exposes the composition of the atmosphere.
What is Limb Sounding?
We are used to satellites that are taking a nadir view of Earth. There are also satellites that make observations at a tangent to the Earth, or a more offset angle. Most satellites that observe the atmosphere do so through a technique called limb sounding. Using this technique, the satellites look across the Earth’s limb to determine the distribution of molecules present in the atmosphere.
The Earth’s limb is the edge of the planet, it is that halo-like region where the atmosphere transitions into space.
Observing the atmosphere with a limb sounder gives a higher vertical resolution on the distribution of molecules than can be obtained from nadir looking satellites. A higher vertical coverage of the atmosphere also gives a much clearer picture of the stratosphere – and it is at this altitude that the concentration gradients are very sharp for key atmospheric components, like Ozone.
Which Atmospheric Measurements are Taken from Space?
One of the key measurements taken by satellites that observe the Earth’s atmosphere is that of the ozone layer. Ozone was one of the main reasons that satellites began to measure the atmosphere’s composition to begin with. The information collected helps in tracking and understanding the changes in the ozone layer.
Some of the main pollutants relevant to the ozone layer, which is in the stratosphere, are nitrogen dioxide, methane, and water vapour. In the last 20 to 30 years, there has been a shift towards understanding actual atmospheric pollution caused by smoke from wildfires, carbon monoxide, dust from deserts, and more.
A tracer is kind of like a chemical finger print, or indicator of a certain type of activity or event. Carbon monoxide, for instance, is a tracer of pollution and wildfire emissions.
How Long Can a Pollutant Stay in the Atmosphere?
The period which a pollutant persists in the atmosphere mainly depends on its reactive properties. Molecules that are more chemically reactive usually have quite a short lifetime in the atmosphere because they are lost out of the atmosphere through different processes such as rain, interacting with surfaces, or degradation by sunlight. Particulate matter and nitrogen dioxide are examples of molecules that have short lifetimes from about a few days, to a week.
There are pollutants whose lifetime increases as they get further away from the Earth’s surface. One of them is carbon monoxide, and that is why it is a key tracer for understanding where atmospheric pollution goes after emission. In the troposphere, carbon monoxide has a typical lifetime of 30 or 40 days. In some cases, it persists for so long that plumes of pollution can be observed moving between continents, and around the world.
The Effect of Clouds on Atmospheric Measurements from Satellites
The presence of clouds, is a major challenge for earth observing satellites, especially for nadir-looking satellites. Satellites with limb viewing are somewhat less affected by clouds because they are mostly looking at the stratosphere, while clouds are most heavy in the troposphere.
Clouds are a challenge because of the water vapour that limits the amount of electromagnetic radiation that is emitted.
Some radiation is absorbed, some scattered, and some is reflected back into space without getting bounced back to the satellite.
When clouds are present, there are often gaps in observations because the satellite cannot see through, so data will be incomplete. Since the impact of clouds can influence the process of estimating the pollutants from observations, cloud screening must be done on the data to remove its influence.
How Do Satellites Obtain a Global Coverage of the Atmosphere?
In order to achieve an optimal global coverage of the Earth’s atmosphere, the participating satellites are designed to be sun synchronous. This means the satellites make observations at the same local time every time that they pass over the same area, chasing daylight hours. Using this design, the satellites do not observe the full daily picture of what is going on, but they can get a fairly full global coverage within a few days; or even more frequently like with the Sentinel 5p satellite that captures near daily collections due to its wider swath.
Gaps in Atmospheric Datasets
Observation gaps in atmospheric datasets are largely caused by the impacts of clouds, which obstruct the satellite’s vision.
At the same time, the concept of sun-synchronous orbits, where satellites taking measurements over a geographic area at the same time every day, introduces temporal gaps in the data since the other times of the day are totally left out.
One of the methods developed to try and fill these gaps is A-Train. This is a concept developed by NASA that involves flying several satellites along the same orbit path, passing the same geographical area at slightly different times. Making observations of the area at different times helps to fill the temporal gaps.
Geostationary Satellites and Observing the Atmosphere
A new development in the technology of observing the Earth’s atmosphere from space is using geostationary satellites. Considering that geostationary satellites are a long way away from Earth, the biggest challenge has always been how to get the right degree of signal to noise to accurately extract information on atmospheric pollutants.
A big advantage of geostationary satellites is that they maintain a constant view of a specific geographic area at all times, without temporal gaps as in low earth orbiting satellites.
This of course excludes nighttime collections for satellites that collect traditional imagery.
Applications of Atmospheric Datasets
The datasets collected by CAMS is completely open access, and freely available. Some general uses of this data are mobile phone apps that show atmospheric pollution forecasts, while possibly providing a different perspective on the data through additional overlays.
In solar energy production, atmospheric observation provides information on the amount of aerosols in the atmosphere that could attenuate the amount of sunlight reaching the solar farm. It also helps forecast the deposition of dust upon a solar farm, which helps in making early decisions on whether the solar panels will need cleaning or other maintenance.
The aviation industry benefits from information on particulate matter in the atmosphere, which could have an impact on jet engine maintenance. Other uses include looking at fire emissions data to understand the impacts of fire on air pollution in different parts of the world.
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