The pressure dependence of the relative photolysis rates of HCHO and HCDO has
been investigated using a new photochemical reactor at the University of Copenhagen.
The relative photolysis rate of HCHO vs. HCDO under UVA lamp irradiation was mea-
5 sured at total pressures of 50, 200, 400, 600 and 1030 mbar. The relative dissociation
rate kHCHO/kHCDO was found to depend strongly on pressure, varying from 1.1±0.1 at
50 mbar to 1.75±0.10 at 1030 mbar. The products of formaldehyde photodissociation
are either H2+CO (molecular channel) or HCO+H (radical channel). The partitioning
between the channels has been estimated using available values for the absorption
10 cross section and quantum yield. As a result of the change in pressure with altitude
the isotope effect for production of molecular hydrogen is found to change from a value
of kH/kD=1.8±0.2 at the surface to unity at 50 km. The relative importance of the two
product channels changes with altitude as a result of changes in both pressure and
actinic flux. The study concludes that the D of photochemical hydrogen produced in
15 situ will increase substantially with altitude.