Biospheric trace gas fluxes and their control over tropospheric chemistry

Terrestrial and marine ecosystems function as sources and sinks for reactive trace gases, and in doing so, profoundly influence the oxidative photochemistry in the troposphere. Principal biogenic processes include microbial methane production and oxidation, the emission of volatile organic compounds from forest ecosystems, the emission of nitric oxide from soils, the emission of reactive sulfur compounds and carbon monoxide from marine ecosystems, control over the production of hydroxyl radical concentration by regional hydrologic processes, and deposition of ozone and nitrogen oxides to ecosystems. The combined influence of these processes is to affect the tropospheric concentrations of ozone, hydroxyl radicals, reactive nitrogen oxides, carbon monoxide, and inorganic acids, all of which constitute fundamental components of oxidative photochemistry. In this review we discuss the recent literature related to the primary controls over the biosphere-atmosphere exchange of reactive trace gases, and also to efforts to model the dominant biospheric influences on oxidative dynamics of the troposphere. These studies provide strong support for the paradigm that biospheric processes exert the dominant control over oxidative chemistry in the lower atmosphere. Improvements in our ability to model biospheric influences on tropospheric chemistry, and its susceptibility to global change, will come from inclusion of more explicit information on the processes that control the emission and uptake of reactive trace gases and the impact of changes in ecosystem cover and land-use change.