N2O, a less known greenhouse gas now monitored from space

Nitrous oxide, N2O, is the third most important anthropogenic greenhouse gas (after carbon dioxide, CO2, and methane, CH4) in terms of contribution to radiative forcing, with its long atmospheric lifetime of about 120 years and a continuous concentration increase since the 1970s. During recent years, we developed a method to retrieve the N2O concentration from satellite observations in the thermal infrared. The IASI instrument allowed scientists to obtain global data twice a day since 2006, enabling us to derive and analyse long-term trends of N2O.

N2O, a less known greenhouse gas

Amongst greenhouse gases, N2O has been given less attention than its two big brothers CO2 and CH4. Its anthropogenic emissions amount to as high as 40% of its total emissions and are linked mostly to:

  • production of agriculture fertilisers
  • burning fossil fuels and biomass including biofuel
  • industrial processes including waste management.

N2O is also a catalyser of stratospheric ozone depletion. Because of all that, it is important to include N2O in climate change mitigation strategies and therefore to monitor its sources and evolution.

New global good quality data

For that purpose, we have developed a new retrieval method to obtain atmospheric concentrations of N2O from observations by the IASI (Infrared Atmospheric Sounding Interferometer) instrument flying on board the 3 Metop satellites (launched in 2006, 2012 and 2018). Using those instruments allows to obtain almost global data twice per day (day and night).

The obtained N2O concentrations were validated against reference data and show a small positive bias of 2 to 5%. This is considered a very good quality, knowing that the reference data also bear some uncertainties and biases.

An even better quality of data is expected in the future when the next instrument, IASI-NG (New Generation) will be available – probably in 2025.Indeed, that instrument will offer increased spectral resolution (allowing to better observe the N2O atmospheric signature) and lower noise.

Long-term trends: soon…

However, there remain issues to be tackled before we can use this data for long-term analyses.

For example, the N2O data obtained is very sensitive to the quality of the atmospheric temperature profile used to describe the atmosphere. In our current data, the quality of these temperature profiles evolves in time, creating tiny inconsistencies in the N2O time series. Because the long-term trends in atmospheric N2O concentration are less than 0.5% per year (this may seem low, but think about what happens in 20 years!), extracting them requires very stable data sets over time (no inconsistencies or small jumps).

Therefore, work is now underway to find fully consistent and stable time series of all input data required by our N2O retrieval algorithm, to allow to derive fully consistent N2O time series which we will then analyse for long-term trends.

 

Reference

Vandenbussche, S.; Langerock, B.; Vigouroux, C.; Buschmann, M.; Deutscher, N.M.; Feist, D.G.; García, O.; Hannigan, J.W.; Hase, F.; Kivi, R.; Kumps, N.; Makarova, M.; Millet, D.B.; Morino, I.; Nagahama, T.; Notholt, J.; Ohyama, H.; Ortega, I.; Petri, C.; Rettinger, M.; Schneider, M.; Servais, C.P.; Sha, M.K.; Shiomi, K.; Smale, D.; Strong, K.; Sussmann, R.; Té, Y.; Velazco, V.A.; Vrekoussis, M.; Warneke, T.; Wells, K.C.; Wunch, D.; Zhou, M.; De Mazière, M. Nitrous Oxide Profiling from Infrared Radiances (NOPIR): Algorithm Description, Application to 10 Years of IASI Observations and Quality Assessment. Remote Sens. 2022, 14, 1810. https://doi.org/10.3390/rs14081810

World map N2O average partial column heating
N2O average partial column (800-80hPa) for June 2019 – with schematic representation of the heating due to the greenhouse gas effect and the thermal radiation escaping to outer space and observed with the satellite.