Modelling the role of nitrogen oxides,hydrocarbons and carbon monoxide in the global formation of tropospheric oxidants

A new zonally averaged two-dimensional chemical and transport model of the lower atmosphere has been used to study the budgets of photochemical oxidants on a global scale in the troposphere. The sensitivities of these budgets to controls on the emissions of NO_x, CO and hydrocarbons from a variety of sources have also been considered. The species studied include O₃, PAN, H₂O₂ and organic peroxides, as well as the global budget of the hydroxyl radical. The results show the relative impacts of emissions from natural and from man-made sources. Overall, some 75% of the turnover of tropospheric ozone results from in situ chemical production, with only 25% having its origins in the stratosphere. The respective fractions for chemical loss and dry deposition are similar. Of the ozone formed in situ, approximately one-third is formed from natural emissions, and two-thirds from man-made emissions. Over half of this man-made contribution is due to industrial society, with much of the rest being caused by biomass burning and methane emitted from paddy fields. Although reducing emissions of NO_x is the most effective way of controlling tropospheric O₃, this also increases the inventories of peroxides and the rate of increase in the concentration of methane, due to a reduction in the inventory of the OH radical. It is, therefore, necessary to control the emissions of a wide range of species in order to reduce the budget of all photochemical oxidants without adversely perturbing the budget of CH₄.