Vertical structure of the atmosphereAn atmosphere interacts with radiation. We have seen this for the outgoing IR radiation in the discussion of global warming, but the same is of course true for the incoming stellar radiation, with the important difference that the incoming spectrum is from a hotter source and hence much richer in the UV.The latter has an important implication, as radiation in the UV range has enough energy to sever molecular bonds, which gives rise to photochemical reactions in the atmosphere. By virtue of the same property, UV radiation reaching the surface of a planet is also harmful to living organisms there. When stellar radiation is absorbed in the atmosphere, this locally heats the atmosphere. This is the reason that Earth's thermosphere above 90 km altitude is rather hot, and also that there exists a temperature rise above the troposphere into the stratosphere - in the former the low wavelength most energetic radiation is absorbed and in the stratosphere the mid-wavelength UV is captured. The simulation does not consider much of thermosphere physics, but as the stratosphere is important it contains a simulation of the Chapman mechanism. The Chapman mechanismIn very simple terms, the Chapman mechanism is a two-step photochemical process. In its first step, hard UV radiation breaks oxygen into single atoms, O2 -> O + O. These free radicals are very reactive and quickly form ozone O3 with the surrounding oxygen. In a second step, this ozone reacts with soft UV radiation and is broken up in the process, oxygen can be formed again, starting the cycle anew.The importance of the process lies in the fact that there are very few molecules of common gases which have a significant absorption cross section between 200 and 300 nm wavelength, so if no ozone can be formed, radiation at this wavelength can be expected to reach the surface. To capture the essentials of the Chapman mechanism, the local production of ozone is crucial. For that reason, the simulation ignores the amount of ozone specified in the atmosphere declaration and computes directly from the oxygen content at various altitudes and the UV photon flux. Due to the existence of the reactions O + O3 -> O2 + O2 and O3 + photon -> O + O2, one can't easily have independent amounts of ozone and oxygen in the first place when a photon flux is present. Using the following plot request for the standard Earth configuration, the thermal energy heating the atmosphere as a function of altitude can be obtained.
The result looks as follows:
Using the keyword ozone_production, the altitude dependence of ozone production (which peaks at a lower altitude) can be obtained as well. The keywords int_range_min and int_range_max serve to limit over which range in wavelength the process is considered. Since the simulation does not consinder a proper treatment of processes in the thermosphere and above, the lower integration limit restricts the calculation ot radiation which actually reaches the stratosphere and can produce ozone there. The upper limit is of less importance - towards the optical window, practially all cross sections drop dramatically and moreover the energy availabe to actually dissolve a molecule is reduced - proper treatment here would require including the measured quantum yields of the reaction. As it stands, the indicated values are defaults. Using the computed surface temperature, the adiabatic lapse rate (cooling of expanding air with altitude) and a heuristic model that is based on scaling the atmosphere thermal balance similar to blackbody radiation (without using the precise constants), a thermal profile of the atmosphere can be extracted using the keyword atmosphere_uv_T for var_y. The result reasembles the temperature of Earth atmosphere reasonably well in that it shows a stratosphere extending from about 10.000 to 50.000 m altitude, beyond which a mesosphere is found.
Of course the truly interesting insights appear when the code is applied to situations where e.g. the stellar spectrum or the oxygen content of the atmosphere is different to begin with, and as we will see there might be situations in which no stratosphere appears at all.
Surface radiation levelsThe same code can also be used to assess the surface radiation levels at different wavelengths (and so compute the UV habitability of a world). The following plot request generates the radiation spectrum at a certain altitude level:
If the keyword alt_level is simply omitted, the spectrum at the actual surface will be computed. The resulting spectrum clearly shows how the various IR bands of water and CO2 cut significant chunks out of the long-wavelength spectrum and how the UV yield smaller than 300 nm is nearly completely removed by the Chapman mechanism and the absorption of even lower wavelength in the upper atmosphere.
Actually, nearly 20% of the incoming radiation are absorbed in the atmosphere (and a significant part of that is radiated back into space without ever reaching the surface), so this gives rise of an atmosphere albedo. However, as this number is pretty much the same everywhere on the planet, it makes no computational difference if it is absorbed in e.g. a re-scaling of ground albedos.
Continue with Feedback loops. Back to main index Back to science Back to worldbuilder Created by Thorsten Renk 2023 - see the disclaimer, privacy statement and contact information. |
