TEMPO slant column density and vertical column density

[ASDC - ghayescrepps]

What is the difference between slant column density and vertical column density in the TEMPO data?

[ASDC - ghayescrepps]

TEMPO makes measurements by looking down at solar light absorbed by and scattered off the atmosphere and clouds, and reflected off the Earth’s surface. For each observation, this light has traveled over a slant path through the atmosphere that depends on the angle of the Sun and the satellite viewing geometry. A photon’s path and penetration depth additionally depends on the properties of the clouds, aerosols, and atmospheric gases that interact with the incoming solar light.

The slant column density is derived by fitting a modeled spectrum to the measured spectrum, and represents the amount of the trace gas in this path. To derive the vertical column density for an observation, which is independent of measurement geometry, the slant column is divided by an “air mass factor.” This air mass factor is calculated using the measurement geometry and a radiative transfer model that considers TEMPO’s cloud observations and assumptions about the state of the atmosphere and surface.