The impact of human activities on the photochemical production and destruction of tropospheric ozone

The spatial and temporal distribution of tropospheric ozone is controlled b y transport processes, including advection, convection and dispersion and b y stratosphere-troposphere exchange, surface deposition to the earth's surf ace and by photochemical production and destruction within the troposphere itself. These processes have been represented in some detail in a global th ree-dimensional Lagrangian chemistry (STOCHEM) model which has been used to construct global ozone budgets and identify the main features in the spati al distribution of daily ozone tendencies. The annually and spatially integ rated net chemical production of ozone is about twice as large as the strat osphere-troposphere exchange flux, so that the concentration-dependent surf ace deposition balancing term is about three times larger than the stratosp here-troposphere exchange term. The total production and loss terms for ozo ne by tropospheric photochemistry are much greater than the net chemical pr oduction, with total chemical production about five times larger than the n et term. The ozone turnover time is therefore around 30 days, around one te nth of the turnover time due to stratosphere-troposphere exchange alone. Hu man activities may influence future tropospheric ozone levels through at le ast two distinct mechanisms: first, increasing emissions of tropospheric oz one precursor gases: methane, oxides of nitrogen, carbon monoxide and hydro carbons, leading to increased ozone levels; second, human-induced climate c hange may lead to increased temperatures and water vapour concentrations, a nd hence increased ozone destruction and decreased ozone concentrations.