Measuring air pollution from thermal infrared satellite observations: when, where and how?
Résumé
Air quality has become a major concern in many megacities and highly populated areas. In those
regions, there is a need to properly quantify the emissions of pollutants and to understand their
transport pathways and their reactivity in the atmosphere. In recent years, there has been important
progress in the monitoring of air quality from satellites. Among the instruments currently in orbit,
those operating in the thermal infrared have usually their maximum sensitivity in the midtroposphere
and were thought to be inadequate for probing the near-surface atmospheric
composition. This has, however, been challenged recently with IASI measurements of surface air
pollution in specific geophysical conditions, characterized in particular by large temperature
differences between the ground and the air above it.
This work will review the principal results obtained with IASI on the sounding of the near-surface
atmospheric composition, especially above pollution hotspots. The presentation will then address the
following questions: where and when is IASI sensitive to the near-surface atmosphere? How large
and how variable is the sensitivity to near-surface pollutants? Which are the parameters that drive
this variability? Answers to these questions will be given for sulfur dioxide (SO2) and carbon
monoxide (CO) on the basis of theoretical sensitivity studies (radiative transfer simulations and
retrieval diagnostics) and real test cases. Based on the results, the capability of IASI to monitor air
quality will be revisited.