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In these times of self-quarantine and stay-at-home orders, satellites continue to orbit the Earth, taking the pulse of our planet in many ways.
One of these satellites, the European Space Agency’s Copernicus Sentinel-5P, has an imaging spectrometer on board making accurate measurements of upwelling light from the Earth’s surface. By analyzing the spectral content of this reflected light, tropospheric concentration maps of environmentally significant gases can be generated.
One such gas is nitrogen dioxide, a key pollutant generated by the burning of fossil fuels from sources including industrial installations, power plants, transportation and forest fires.
Significant changes in levels of nitrogen dioxide, particularly around major cities, are being reported this year compared to 2019 (see the satellites images at the ESA website). These differences are attributed, in part, to changes in human behaviour related to the coronavirus outbreak, especially the reduction in vehicle traffic and industrial activity.
It is universally acknowledged that inter-causal relationships in atmospheric chemistry are complex and rarely linear. For example, concentrations of nitrogen dioxide vary a great deal from day to day due to changes in the weather. Nonetheless, it appears that year-over-year atmospheric changes are visible in time-averaged data across cities studied in Italy, France, Spain, China and elsewhere.
Further analysis is forthcoming. Scientists from the Royal Netherlands Meteorological Institute and others are now embarking on building more complex models to understand the changes in greater depth and context. This unprecedented period will provide a very interesting dataset for the study of anthropogenic effects on air quality well into the future.
Ground-based measurements contribute valuable data to urban pollution models, providing a means to calibrate across atmospheric scales and to address heterogeneities that can be challenging to resolve by satellite data alone.
Differential Optical Absorbance Spectroscopy (DOAS) has long been a fundamental technique used to understand air pollution at ground and atmospheric levels alike, and it’s a technique where Ocean Insight spectrometers are often specified (this maritime exhaust emissions application is a good example).
DOAS was first proposed by Prof. Ulrich Platt of the Institute of Environmental Physics, Heidelberg University, Germany. The basic principle is to use the narrowband absorption characteristics of gas molecules in the air to identify the gas composition, and according to the narrowband absorption intensity, the concentration of trace gases is derived.
DOAS can be used to monitor harmful gases in cities, underground passages, and industrial mines. The method has the advantages of simple principle, simple structure, fast response and high precision. Also, DOAS can effectively eliminate the influence of particulate matter and water vapor on the measurement by means of filtering, which is very helpful for the monitoring of flue gas emissions, for example.
Many institutions around the world have developed ultra-low emission monitoring equipment based on DOAS technology. At the heart of this DOAS equipment is a UV-Visible spectrometer.
Ocean Insight customers have used our UV-Vis spectrometers for various DOAS applications, with instruments being deployed in measurement regimes for tropospheric halogens, NOx and SOx, volcanic emissions, polar atmospheric chemistry and industrial emissions. In China, many well-known companies have actively entered the field and achieved good results.
DOAS systems can take many forms, but the basic premise is to measure and compare different optical paths to measure the relative concentrations of target gas molecules. By careful attention to system calibration, absolute gas concentrations also are derived.
The oxides of nitrogen and sulfur -- so called NOx and SOx -- are an important group of atmospheric gases with absorbance spectra in the UV and Visible parts of the spectrum. By using a broadband UV spectrometer and a broadband xenon or deuterium light source it is possible to measure multiple gas species simultaneously along a pathlength of interest. This avoids the need to use lasers tuned to the specific absorbance of a single gas, another popular gas monitoring technique for smaller molecules.
Whether it’s providing spectrometers for DOAS or insight for other monitoring solutions, Ocean Insight is privileged that our spectrometers may play a small part in the global effort to understand the direct impact of human activity on air quality and the environment. Part of our mission is to offer the spectral tools, knowledge and processes to monitor air, water and soil samples, providing researchers, government regulators and manufacturers with the insight necessary to preserve and maintain a clean, healthy environment.