On the photodissociation of water vapour in the mesosphere

Abstract

The photodissociation of water vapour in the mesosphere depends on the absorption of solar radiation in the region (175–200 nm) of the O 2 Schumann-Runge band system and also at H-Lyman alpha. The photodissociation products are OH + H, OH★ + H, O + 2H and H 2 + O at Lyman alpha; the percentages for these four channels are 70, 8, 12 and 10%, respectively, but OH + H is the only channel between 175 and 200 nm. Such proportions lead to a production of H atoms corresponding to practically the total photodissociation of H 2O, while the production of H 2 molecules is only 10% of the H 2O photodissociation by Lyman alpha. The photodissociation frequency (s −1) at Lyman alpha can be expressed by a simple formula J Lyα H 2O=4.5 ×10 −6 1+0.2 F 10.7−65 100 exp[−4.4 ×10 −19 N 0.917] where F 10.7 cm is the solar radioflux at 10.7 cm and N the total number of O 2 molecules (cm −2), and when the following conventional value is accepted for the Lyman alpha solar irradiance at the top of the Earth's atmosphere ( Δλ = 3.5 A ̊ ) q Lyα,∞ = 3 × 10 11 photons cm −2 s 1̄. The photodissociation frequency for the Schumann-Runge band region is also given for mesospheric conditions by a simple formula J SRB ( H 2 O) = J SRB,∞ ( H 2 O) exp [−10 −7N 0.35] where J SRB,∞(H 2O) = 1.2 × 10 −6 and 1.4 × 10 −6 s −1 for quiet and active sun conditions, respectively. The precision of both formulae is good, with an uncertainty less than 10%, but their accuracy depends on the accuracy of observational and experimental parameters such as the absolute solar irradiances, the variable transmittance of O 2 and the H 2O effective absorption cross sections. The various uncertainties are discussed. As an example, the absolute values deduced from the above formulae could be decreased by about 25-20% if the possible minimum values of the solar irradiances were used.