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

Abstract

AbstractThe spatial and temporal distribution of tropospheric ozone is controlled by transport processes, including advection, convection and dispersion and by stratosphere‐troposphere exchange, surface deposition to the earth's surface and by photochemical production and destruction within the troposphere itself. These processes have been represented in some detail in a global three‐dimensional Lagrangian chemistry (STOCHEM) model which has been used to construct global ozone budgets and identify the main features in the spatial distribution of daily ozone tendencies. The annually and spatially integrated net chemical production of ozone is about twice as large as the stratosphere‐troposphere exchange flux, so that the concentration‐dependent surface deposition balancing term is about three times larger than the stratosphere‐troposphere exchange term. The total production and loss terms for ozone by tropospheric photochemistry are much greater than the net chemical production, with total chemical production about five times larger than the net term. The ozone turnover time is therefore around 30 days, around one tenth of the turnover time due to stratosphere‐troposphere exchange alone. Human activities may influence future tropospheric ozone levels through at least two distinct mechanisms: first, increasing emissions of tropospheric ozone precursor gases: methane, oxides of nitrogen, carbon monoxide and hydrocarbons, leading to increased ozone levels; second, human‐induced climate change may lead to increased temperatures and water vapour concentrations, and hence increased ozone destruction and decreased ozone concentrations.