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Radiative forcings and global warming potentials of 39 greenhouse gases

A.K. Jain, B. P. Briegleb, K. Minschwaner, D.J. Wuebbles

J. of . geophys. Research, Vol 105, no. D16 p. 20,773-20,790 August 27, 2000

http://climate.atmos.uiuc.edu/atuljain/publications/2000JD900241.pdf

On p. 20,779 they write:

> "In the BBM and NBM the shortwave radiative fluxes are obtained from the Delta-Eddington model (Briegleb 1992b). Eighteen spectral intervals are used from 0.2 to 5.0 mikrometers with seven shortwaves with 0.35 mikrometers to represent ozone absorption: one in the visible (0.35 to 0.7 mikrometers) and ten in the near infrared (0.7 to 5 mikrometers) for O2, H20 and CO2 absorption. Molecular and cloud droplet scattering are included. The model gives an accurate approximation to shortwave scattering and absorption based on comparision with higher resolution and more accurate scattering calculations."

By this paragraph it is unclear to me which spectral data entered the CH4 calculations - by this paragraph it doesn't sound as if any shortwave absorption for CH4 was taken into consideration, even not the near infrared but as said this is a bit unclear. But in particular the paragraph says that no spectral data below 200 nm= 0.2 mikrometers, like the above ones for CH4, was taken into account.

The result in the radiative forcing calculations are in this article at the end of the article:

>"Radiative forcing based on the UARS-measured distributions of CH4 and N2O are 3% higher,..."

The spectral data of HITRAN 2004 which includes near infrared spectral data

is summarized here:

http://vpl.astro.washington.edu/spectra/ch4hitran2004images.htm

One sees that the sizes of the bigger peaks in the 7000 and 8000 cm^-1 range are very roughly a factor 1000 smaller than the 1300 cm^-1 band (while the width is somewhat similar), so this doesn't look as if there is a big influence from near infrared, but then this is at higher energies. As said I don't know anything about the models so I can't really tell what difference this makes.

But now Methane has also bands in the UV range.

I have sofar though no comparision of the absorption strengths with spectrum the UV regime, since I haven't yet figured out the HITRAN units. Here is an image of UV absorption crosssections of methane in the UV regime (110 nm translate to approx 90000 cm^-1:

http://joseba.mpch-mainz.mpg.de/spectral_atlas_data/cross_sections_plots/Alkanes+alkyl%20radicals/Alkanes/CH4_VUV_110-165nm_lin.jpg

Here even shorter wavelengths:

http://joseba.mpch-mainz.mpg.de/spectral_atlas_data/cross_sections_plots/Alkanes+alkyl%20radicals/Alkanes/CH4_VUV_2-165nm_log.jpg

Concluding my question: Given that my impression that higher frequency spectral data is in current calculations of radiative forcings not included is correct then are there any evaluations concerning the influence of the higher frequency spectral data on the radiative forcing of methane? Is a factor 10 for the global warming potential possible?

Radiative forcings and global warming potentials of 39 greenhouse gases

A.K. Jain, B. P. Briegleb, K. Minschwaner, D.J. Wuebbles

J. of . geophys. Research, Vol 105, no. D16 p. 20,773-20,790 August 27, 2000

http://climate.atmos.uiuc.edu/atuljain/publications/2000JD900241.pdf

On p. 20,779 they write:

> "In the BBM and NBM the shortwave radiative fluxes are obtained from the Delta-Eddington model (Briegleb 1992b). Eighteen spectral intervals are used from 0.2 to 5.0 mikrometers with seven shortwaves with 0.35 mikrometers to represent ozone absorption: one in the visible (0.35 to 0.7 mikrometers) and ten in the near infrared (0.7 to 5 mikrometers) for O2, H20 and CO2 absorption. Molecular and cloud droplet scattering are included. The model gives an accurate approximation to shortwave scattering and absorption based on comparision with higher resolution and more accurate scattering calculations."

By this paragraph it is unclear to me which spectral data entered the CH4 calculations - by this paragraph it doesn't sound as if any shortwave absorption for CH4 was taken into consideration, even not the near infrared but as said this is a bit unclear. But in particular the paragraph says that no spectral data below 200 nm= 0.2 mikrometers, like the above ones for CH4, was taken into account.

The result in the radiative forcing calculations are in this article at the end of the article:

>"Radiative forcing based on the UARS-measured distributions of CH4 and N2O are 3% higher,..."

The spectral data of HITRAN 2004 which includes near infrared spectral data

is summarized here:

http://vpl.astro.washington.edu/spectra/ch4hitran2004images.htm

One sees that the sizes of the bigger peaks in the 7000 and 8000 cm^-1 range are very roughly a factor 1000 smaller than the 1300 cm^-1 band (while the width is somewhat similar), so this doesn't look as if there is a big influence from near infrared, but then this is at higher energies. As said I don't know anything about the models so I can't really tell what difference this makes.

But now Methane has also bands in the UV range.

I have sofar though no comparision of the absorption strengths with spectrum the UV regime, since I haven't yet figured out the HITRAN units. Here is an image of UV absorption crosssections of methane in the UV regime (110 nm translate to approx 90000 cm^-1:

http://joseba.mpch-mainz.mpg.de/spectral_atlas_data/cross_sections_plots/Alkanes+alkyl%20radicals/Alkanes/CH4_VUV_110-165nm_lin.jpg

Here even shorter wavelengths:

http://joseba.mpch-mainz.mpg.de/spectral_atlas_data/cross_sections_plots/Alkanes+alkyl%20radicals/Alkanes/CH4_VUV_2-165nm_log.jpg

Concluding my question: Given that my impression that higher frequency spectral data is in current calculations of radiative forcings not included is correct then are there any evaluations concerning the influence of the higher frequency spectral data on the radiative forcing of methane? Is a factor 10 for the global warming potential possible?