Calculated spectroscopy and atmospheric photodissociation of phosphoric acid

Abstract

Detection of phosphine (PH₃) gas in the upper troposphere suggests that the biogeochemical P cycle also includes an atmospheric component that consists of volatile phosphorus-containing molecules. A reasonable end product for the oxidation of PH3 in the atmosphere is phosphoric acid (H₃PO₄). We propose that H₃PO₄ may be photodissociated into HOPO₂ and H₂O in the stratosphere, where H₃PO₄ is likely to be present in gaseous form. We have calculated the energy barrier of this reaction and show that in addition to electronic transitions, OH-stretching overtone transitions can also provide the necessary energy. OH-stretching fundamental and overtone transitions were calculated with the use of an anharmonic oscillator local mode model. The probability of overtone induced photodissociation was estimated with molecular dynamical reaction coordinate simulations. Electronic transitions were calculated with the equation of motion coupled cluster singles doubles method. We have calculated the photodissociation rate constants for absorption of visible, UV, and Lyman-α radiation at altitudes from 20 to 100 km. We show that at altitudes between 30 and 70 km, the photodissociation of H3PO4 is likely to proceed via absorption in the UV region by electronic transitions.