MODTRAN is impressively nuanced sometimes. Ordinarily MODTRAN is used on a macro scale at plausible altitudes and greenhouse gas concentrations, but it definitely resolves at single meters of altitude and single ppm of greenhouse gasses.
Here we explore one ppm CO2 with all the other GHG’s zeroed out. We look both up and down from one meter, and 70 kilometers.
Beginning at one meter, we see little difference from the Planck curve looking down. This is no different from what we see at 400 ppm looking down up to about 400 meters altitude, except that there is .31 W/m2 MORE upward radiation at 400 ppm.
Looking up from one meter the reach of a single ppm of CO2 is astonishing. The 667.4 fundamental bend does not quite kiss the downward looking curve, meaning that it is seen radiating from some small distance above one meter. Only the fundamental bend can be seen radiating from essentially the surface to an altitude of about six kilometers.
At the temperature of six kilometers the additional bending vibrations at 647.1 and 688.7 kick in, along with the P and R rotations dependent on the fundamental bend. At a temperature 15 kilometers we see the additional vibration at 618 and the bend to stretch transition at 720.8
Interestingly, at 1 ppm you get almost a “chromatograph” of the relative intensities, with the factor of progressive movement away from top dead center of the Planck curve with increasing altitude omitted.
The view of 1 ppm from 70 kilometers is I’m many ways the inverse of the view from one meter. Looking up from 70 km there is essentially no IR coming down from above. Looking down we see the “effective radiative levels” of the CO2 transitions to space. The fundamental bend at 667.4 is seen radiating at a temperature of 12 km, and once again it shows extraordinary reach, being the only signal from 6 to 12 kilometers.
At 1 ppm, there is essentially no difference in either upward or downward radiation above 25 km elevation.
Just for kicks, below is 1 meter looking up vs 70 km looking down at 1ppm.
Let’s compare the 1 meter up and down view at 400 ppm.
Looking down, there is no discernable difference from 1 ppm, and the total upward IR flux is only .31 W/m2 more at 400 ppm.
There are lots of differences looking up. The fundamental bend and rotations, and the nearby three order of magnitude weaker transition at 647.1, have melded into a zone radiating at the Plank temperature of the ground (299.7 K). Notably, this is above the temperature seen looking down. The upwelling radiation deviates from the Planck curve more at top dead center than elsewhere along the curve. The downwelling radiation is seen at a higher temperature at the ground BB curve.
The transitions at 618 and 688.7 radiate at about the upwelling (down looking) curve, 597.3 and 720.8 are seen radiating at the temperature of perhaps 800 meters, and a crop of extremely weak transitions is seen radiating above 9 kilometers.
Astonishingly, there is a 60.5 W/m2 increase of downward flux between 1 ppm and 400 ppm, as seen from one meter elevation.
Here is 400 ppm up and down from 70 km.
We have the usual cast of transitions, but their intensities (and temperatures) change with altitude and CO2 concentration. The difference in upwelling radiation between 1 meter and 70 km is 118.75 W/m2 at 400 ppm.
A transition can be deemed “saturated” when it radiates at a temperature conforming to the Planck curve. At 400 ppm from 70 km, the fundamental bend rotations are radiating at the 220K (12 km) curve, but the 667.4 bend itself reaches back down to a temperature of 9 km.
For kicks again, 1 meter looking up vs 70 km looking down at 400 ppm.
The fundamental bend is saturated looking up from a meter, but not above 9 km in the atmosphere.