Airborne and Ground Measurements for Carbon Dioxide Detection in Deserts

By Usman AhmedOct 26 2022Reviewed by Sophia Coveney

Accurate measurement of atmospheric carbon dioxide is crucial for studying the carbon cycle. Airborne observations cover a larger region than terrestrial observational stations and are more useful for tracking the spread of carbon dioxide sources and sinks. A recent study published in Remote Sensing demonstrates the use of an aircraft campaign to measure atmospheric carbon dioxide concentration over the Dunhuang desert site in western China. 

Impact of Atmospheric Carbon Dioxide  

Atmospheric carbon dioxide is the most significant human greenhouse gas. Carbon dioxide is a major contributor to climate change and global warming. It contributes significantly to changes in sea ice, sea level, hydrology, and atmospheric temperature. There has been a steady rise in the amount of carbon dioxide in the atmosphere since the industrial revolution, mostly due to human activity such as burning fossil fuels, deforestation, and cement production.

Knowledge of carbon dioxide sinks, transport sources, and flux is essential for designing policies that stabilize carbon dioxide emissions. Current methodologies and advanced technologies can continuously monitor and assess atmospheric carbon dioxide. 

Sources for Atmospheric Carbon Dioxide Detection 

Many ground stations, such as those in the Global Atmospheric Watch network and the Total Carbon Column Observing Network sites keep close tabs on atmospheric carbon dioxide. Airborne observations provide a further source for monitoring atmospheric carbon dioxide levels.

The best technique to monitor atmospheric carbon dioxide with high spatiotemporal resolution is using satellites. The Circling Carbon Observatory 2 (OCO-2), GOSAT-2, and OCO-3 are among the satellites orbiting the Earth and are solely responsible for measuring atmospheric carbon dioxide content. 

Limitations of Current CO₂ Detection Techniques 

The observations from the ground for monitoring carbon dioxide have major drawbacks. For instance, the data gathered from the Global Atmospheric Watch stations only represent the lower atmosphere. The lack of night-time and seasonal high-latitude observations are some spatiotemporal constraints of passive spectrometers that rely on sunlight.  

The satellites capture the spectrum radiance fluctuation of sunlight reflected from the Earth's surface through their passive spectrometers. Due to aerosols, clouds, and other artifacts, carbon dioxide retrievals from satellites are uncertain.

It is essential to continuously evaluate satellite-derived carbon dioxide estimations to construct an accurate long-term atmospheric data record and to use several satellite measurements for joint flux inversion. 

Light Detection and Ranging (LIDAR) Techniques for CO₂ Detection 

Active remote sensing based on Light Detection and Ranging (LIDAR) techniques helps to overcome constraints affecting the effective monitoring of carbon dioxide. The differential absorption lidar (DIAL) technology converts the active remote sensing sensors' radiation sources into target-specific air molecular absorption features. 

The unique DIAL known as an Integrated Path Differential Absorption (IPDA) lidar gives a weighted average gas measurement. Several researchers have used the IPDA lidar systems to monitor the amount of carbon dioxide in the atmosphere.

Numerous research institutions and organizations have performed feasibility and sensitivity assessments of airborne atmospheric carbon dioxide monitoring using LIDAR devices.  

Airborne and Ground Measurements of CO₂ Detection in Chinese Desert  

China is the country that emits the most CO₂, accounting for 30% of the increase in the global carbon budget over the previous 15 years. The Chinese government is actively aiming to reach peak carbon emissions globally by 2030 and to reduce CO₂ emissions per unit GDP by 60–65%.

Large uncertainties in the available datasets make it challenging to evaluate the effectiveness of these reduction strategies and conduct ongoing monitoring of atmospheric carbon dioxide. It is required to compare the current datasets to precise and reliable observations to monitor atmospheric carbon dioxide with better technologies. 

Wang et al. developed an experiment that integrated ground-based and airborne components at the desert site of Dunhuang in western China. An IPDA LIDAR and a commercial instrument called the UGGA were used to measure airborne observations, while a portable Fourier Transform Spectrometer (EM27/SUN) and a different UGGA were used to measure atmospheric carbon dioxide on the ground. The results from the airborne flying equipment were contrasted with those from measurements made on the ground.  

Research Findings 

In this study, an aerial campaign was conducted in June and July 2021 over Dunhuang's western Chinese desert site. The experiment's goals were to examine atmospheric carbon dioxide distribution properties and assess how well a recently created ACDL system performed over a desert region. Ambient carbon dioxide was also measured simultaneously using portable instrumentation set up at the CRCS ground station in the desert. 

The UGGA and the ACDL systems were used to conduct the airborne observations. The UGGA measured the in-situ carbon dioxide concentration and the XCO₂ (mole fraction of carbon dioxide) was calculated using the ACDL observations. The XCO₂ was derived from the spectra recorded by the EM27/SUN spectrometer.  

Results for XCO₂ based on ACDL were understated when compared to other datasets. The vertical profile measurements made with the aircraft-based UGGA devices were compared to the vertical profiles acquired from satellite and model datasets. The vertical distribution of atmospheric carbon dioxide was similar across all datasets, however, there were minor discrepancies in their magnitudes.

The uncertainties in the input datasets that power the estimating models were most likely to blame for the disparities between the satellite and model dataset magnitudes. The atmospheric carbon dioxide content was lower near the surface and steadily increased with increasing height, according to the vertical distribution over the desert region. This might result from the experiment site having very low anthropogenic emissions and vegetation.

The results of a study on the daily changes of atmospheric carbon dioxide showed that the concentration gradually declined from 8:00 to 10:00. The value then turned around and began to rise, reaching its initial peak between 13:00 and 14:00. The carbon dioxide content then started to drop once more until 17:00. 

Overall, a sinusoidal pattern could be seen in the diurnal changes of atmospheric carbon dioxide. Ground-based and airborne observations were also used to study the relationship between atmospheric carbon dioxide and aerosol optical depth. The two variables had a strong positive correlation coefficient and a comparable trend. This study has processed and examined a small amount of ACDL data. Future expositions will be able to include more findings from the ACDL system and airborne observations. 

Reference

Wang, Q., Mustafa, F., Bu, L., Yang, J., Fan, C., Liu, J., & Chen, W. (2022) Monitoring of Atmospheric Carbon Dioxide over a Desert Site Using Airborne and Ground Measurements. Remote Sensing, 14(20), Article 20. https://www.mdpi.com/2072-4292/14/20/5224/htm

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