Anthropogenic changes to the atmospheric cycles of carbon and hydrogen affect air
pollution, global warming and stratospheric water vapor. I will present our recent work
on photochemical processes involving the key species of the atmospheric C and H
cycles: methane, methoxy, formaldehyde and hydrogen.
It is known that atmospheric methane is depleted in deuterium, and that methane is a
large source of atmospheric hydrogen:
CH4 to CH3 to CH3O2 to CH3O to CH2O to CO and H2
However, analysis of the isotope budget shows that ‘photochemical hydrogen’ produced
from methane must be enriched in deuterium. (Formaldehyde is also produced
from isoprene and other VOCs, and these are also likely to be depleted in D).We conducted
an experiment to measure the relative photolysis rate of HCHO and HCDO in
natural sunlight, and found that HCDO is photolysed much more slowly than HCHO,
and that even when it is photolysed, it produces much less molecular hydrogen product,
HD. This result would appear to contradict modeling studies and isotope budgets.
However, in another study, we demonstrated that the CD bond is much less reactive
than the CH bond. This means that the reaction steps leading from methane through
methoxy to formaldehyde cause an enrichment in deuterium of up to ca. 1000%¸ that
more than compensates for the depletion occurring in the photolysis step.
Formaldehyde photolysis is also interesting from the standpoint of chemical dynamics.
A study of the UV spectrum showed that there is essentially no difference in the
integrated absorption cross sections of HCHO, HCDO or DCDO, and that they all absorb
sunlight at the same rate. However, they have very different photolysis rates and
different efficiencies for producing the molecular hydrogen product.
References
E. J. K. Nilsson, L. Bache-Andreassen, M. S. Johnson and C. J. Nielsen, Relative
Tropospheric Photolysis Rates of HCHO and DCDO Measured at the European Photoreactor
Facility, Physical Chemistry and Chemical Physics, manuscript, 2008.
A. Gratien, E. J. K. Nilsson, L. Bache-Andreassen, J.-F. Doussin, M. S. Johnson, C. J.
Nielsen, Y. Stenstrøm and B. Picquet-Varrault, UV and IR absorption cross-sections
of HCHO, HCDO and DCDO, Journal of Physical Chemistry A, 111, 11506 - 11513,
2007.
E. J. K. Nilsson, M. S. Johnson, F. Taketani, Y. Matsumi, M. D. Hurley and T. J.
Wallington, Atmospheric Deuterium Fractionation: HCHO and HCDO Yields in the
CH2DO + O2 Reaction, Atmospheric Chemistry and Physics, 7, 5873 – 5881, 2007.
K. L. Feilberg, M. S. Johnson, A. Bacak, T. Röckmann and C. J. Nielsen, Relative
tropospheric photolysis rates of HCHO and HCDO measured at the European Photoreactor
Facility, Journal of Physical Chemistry, A, 111 (37), 9034 -9046, 2007.
K. L. Feilberg, B. D’Anna, M. S. Johnson and C. J. Nielsen, Relative tropospheric
photolysis rates of HCHO, H13CHO, HCH18O and DCDO measured at the European
Photoreactor Facility (EUPHORE), Journal of Physical Chemistry A, 109, 8314
- 8319, 2005.
K. L. Feilberg, M. S. Johnson and C. J. Nielsen, Relative rates of reaction of
13C16O,12C18O, 12C17O and 13C18O with OH and OD radicals, Physical Chemistry
and Chemical Physics, 7(11), 2318 – 2323, 2005.
K. L. Feilberg, David W. T. Griffith, M. S. Johnson and C. J. Nielsen, The 13C and
D kinetic isotope effects in the reaction of CH4 with Cl, International Journal of
Chemical Kinetics, 37(2), 110 - 118, 2005.
K. L. Feilberg, M. S. Johnson and C. J. Nielsen, Relative reaction rates of HCHO,
HCDO, DCDO, H13CO and HCH18O with OH, Cl, Br and NO3 radicals, Journal of
Physical Chemistry A 108(36), 7393 - 7398, 2004.