Questions: Remote Sensing of the Atmosphere

Question 1

The TROPOMI instrument on Sentinel-5P measures nitrogen dioxide (NO2) columns by detecting specific absorption features in reflected sunlight. What spectral region does it use for NO2 retrieval?

• A. Thermal infrared (10-12 um)
• B. Visible blue-violet (400-465 nm), where NO2 has strong absorption bands
• C. Microwave (10-30 GHz)
• D. Near-infrared (1.0-1.3 um)

Answer: (B)

NO2 absorbs strongly in the blue-violet portion of the visible spectrum. By measuring the depth of these absorption features in sunlight reflected from the surface and atmosphere, TROPOMI retrieves the total column amount of NO2. This has revealed urban pollution hotspots, shipping lanes, and the dramatic drop in NO2 during COVID-19 lockdowns at unprecedented spatial detail.

Question 2

Weather satellites in geostationary orbit can directly measure atmospheric temperature profiles from space.

• T. True
• F. False

Answer: True

Infrared sounders on geostationary satellites measure thermal emission at multiple wavelengths in CO2 absorption bands. Because CO2 is well-mixed and its concentration is known, the emission at different wavelengths originates from different altitudes (stronger absorption = higher altitude of emission). By measuring emission in a series of channels with different CO2 absorption strengths, the instrument retrieves temperature at multiple atmospheric levels -- a technique called thermal sounding.

Question 3

Explain how satellite-based lidar (like CALIPSO's CALIOP) provides information that passive atmospheric sensors cannot.

Model answer: CALIOP transmits laser pulses and measures the backscattered return as a function of time (altitude), producing vertical cross-sections of the atmosphere. This reveals the vertical distribution of clouds and aerosol layers -- their altitude, thickness, and optical properties. Passive sensors view the atmosphere from above and can measure column-integrated quantities but cannot determine the vertical structure. CALIOP can distinguish high thin cirrus from low thick stratus, identify elevated dust plumes versus boundary-layer pollution, and detect the altitude of volcanic ash layers critical for aviation safety.

Active lidar provides vertical resolution that passive sensors fundamentally lack, resolving the 3D structure of the atmosphere rather than just column-integrated properties.