Photochemical reactions initiated by and influencing ozone in unpolluted tropospheric air

Abstract

This paper presents theoretical arguments in favour of OH-concentrations larger than 106 molecules cm-3 in the sunlit lower troposphere even if allowance is made for heterogeneous removal of OH, HO2 and HNO3 molecules. The OH-concentrations, presented in this paper, were calculated considering for the first time the photochemical effect of both scattered and direct radiation at ground levels and adopting recent accurate data for the quantum efficiency of O(1D) production in the ozone photolysis near 310 nm. The significant role played by OH in the atmospheric carbon, nitrogen and possibly sulphur cycles is discussed. Low background mixing ratios of NOx molecules (< 10-9) in the unpolluted atmosphere are calculated. The oxidation of NH3, initiated by the reaction with OH, may provide a significant source of tropospheric NOx-molecules, if its rate constant is about 10-13 cm3 s-1 near the ground. The conversion of NO2 into HNO3, followed by heterogeneous removal of HNO3, is the most important tropospheric sink for NOx-molecules. The photodissociation of ozone in ultraviolet light between 300 and 330 nm, which leads to the formation of the excited O(1D) atom, “drives” several sequences of reactions treated in this study. Several chains of reactions, involving carbon and nitrogen compounds, lead to large rates of formation and destruction of ozone in the troposphere. These rates partly balance each other. Whether there is a net production or destruction of ozone in the troposphere cannot be determined at present. Ozone is a very active, catalytic component in the tropospheric chemical systems. Globally, the combustion source of CO for the year 1970 is more than 20% of the natural source of CO as provided by the oxidation of biologically produced CH4. For middle latitudes of the Northern Hemisphere the combustion source is about equal to the natural source. However, too little is known about homogeneous and heterogeneous reactions affecting the methane oxidation products.DOI: 10.1111/j.2153-3490.1974.tb01951.x