Atmospheric Photolysis and its relation to Absorption Cross Section

[EMH121]

Dear Forum:

I have a question about atmospheric photodissociation. I use methane as an example, but any atmospheric gas molecule would suffice.
Methane, CH4, has a photodissociation energy of 439 kJ/mole at 298oK, meaning that
CH4 + hv --> CH3 + H , hv<274nm.
It also has an “absorption cross section” that starts at ~170nm and increases at lower wavelengths.
My puzzlement is as follows:1) If the CH4 molecule is decomposed at <274nm, why does it have an absorption profile at lower wavelengths (the molecule should no longer be intact, correct?)?2) If the absorption cross section profile has some kinetic function, why doesn’t the profile start near 274nm?
I know that the questions are rooted in my ignorance, so could you please help enlighten me. Thank you in advance. EMH121

 

[Chestermiller]

Can you give another example besides methane.? The overriding sink for methane in the atmosphere is reaction with OH radical.

 

[EMH121]

Thanks for replying Chestermiller.

Several more examples are:
O2: BDE <242nm; ACS increases for wavelengths <200nm.
CO2: BDE<258nm; ACS increases for wavelengths <190nm
H2O: BDE<242nm; ACS increases for wavelengths <200nm

Data for the BDEs (Bond dissociation energies) is from Handbook of Chemistry & Physics, 97ed, 9-73 - 9-86.
Data for ACS (Absorption Cross sections) are from "The Photochemistry of Atmospheres", Joel E Levine, 1985, p17-19

 

[Chestermiller]

The main absorbing species (that have large enough cross sections and concentrations in the atmosphere to cause solar photon flux attenuation) are oxygen and ozone. The absorption cross section of oxygen increases substantially with decreasing wavelength below 242 nm, while the solar photon flux in this region increases with increasing wavelength. At the lower end of the wavelength scale (below the Schumann Runge bands), the attenuation is so substantial that negligible solar radiation survives to the stratosphere. The oxygen photolysis rate at different altitudes is a combination of the effect of the photon flux surviving attenuation to that altitude (as a function of wavelength), the concentration of oxygen at that altitude, and the oxygen cross section spectrum. You need to integrate with respect to wavelength to get the photolysis rate. Another contributor to the photolysis rate is Rayleigh scattering.

 

[EMH121]

Sorry for my delayed response; it is because I needed to do some reading to generate a response. FYI, my interest is in ancient atmospheres when oxygen and hence ozone were insignificant.

My understanding is that Absorption Cross Sections (ACS) are measured at a specific T (298 is the standard ref T), for a specific gas, one wavelength at a time, and reported values are (usually?) extrapolations to dilute pressures. The ACS describes the amount of incident radiation of a given wavelength that does not reach a universal energy detector. Its scale typically involves 5 or more orders of magnitude, and ACS can be displayed in both log and linear graphs (see MPI-Mainz Spectral Atlas).

My understanding of Bond Dissociation Energies (BDE) is that they are measured at a specific T (298 is standard ref T), and that in their most precise form are reaction specific. They are very close numerically to reaction enthalpies, and can be determined from the Gibbs free energy equation, as, Enthalpy = Free energy + T*entropy, or by Photoionization Mass Spectrometry (PIMS). PIMS is based on the energy required for the ONSET of radical ion and neutral formation, and the radical ion reaction energy must be canceled by a corresponding balanced neutral reaction, for example (AB + hv --> A. + B+ + e- and B+ + hv + e- --> B., so that overall AB --> A. + B.). See Bond Dissociation Energies of Organic Molecules, 2002; SJ Blanksby & GB Ellison, Accounts of Chemical Research 10:1021/ar020230d CCC. BDE = <275nm comes from Handbook of Chemistry & Physics, 97ed.

Restricting discussion to diatomic molecules, and using H2,g as an example, my original question could be rephrased as, "If H2 decomposes at energies >275nm (wavelengths <275nm) based on BDE, why does its ACS show its absorption edge beginning at 125nm? I can speculate that absorption at lower wavelengths is due to scattering and/or H. absorption and/or fluorescent/phosphorescent/luminescent emission, but why does the ONSET of absorption start at 275nm per BDE but at 125 nm per ACS methodologies?

I have attached the MPI-Mainz log display ACS for H2,g.

Please correct any misunderstandings that I have. If possible, answer my primary question, "why does the ONSET of absorption by H2,g start at 275nm per BDE but at 125 nm per ACS methodologies?". Thanks in advance for your reply, EMH121.

 

[Chestermiller]

To get a better idea of how radiation transport and chemistry in the atmosphere is handled in practice, see Aeronomy of the Middle Atmosphere: Chemistry and Physics of the Stratosphere and Mesosphere by Guy Brasseur and Susan Solomon.

When I was working in this area around 1980, we obtained a computer data base of solar photon flux and photodissocation cross sections for atmospheric gases from Lawrence Livermore Laboratory. The data was in 126 wavelength bins (intervals) running from about 150 nm to about 700 nm. The cross sections were not very sensitive to temperature, but, in some cases, the temperature sensitivity was included. I doubt if the contacts we had then are still at Livermore, but, one of the guys, Donald Wuebbles, is head of the atmospheric science dept at the University of Illinois. I'm sure that he still has access to the data base.

 

[EMH121]

Thanks for your comments!