Review: Exchanges of O3, NO, and NO2 between forest ecosystems and the atmosphere

Abstract

The emission and absorption of trace gases in the biosphere affect atmospheric chemistry and influence the potential indirect effects on the carbon and nitrogen cycles, air pollution, and climate. Ozone (O3 ) and nitrogen oxides are important for the atmosphere and have adverse effects on plant growth and human health. In this review, the observed fluxes and deposition velocities of O3, NO, and NO2 in various forest ecosystems and the commonly used measurement methodologies were summarized and compared. Canopy O3 fluxes have been reported in evergreen coniferous and deciduous broadleaf forests. O3 deposition was observed in the forest ecosystems during daytime and nighttime, because O3 is absorbed through stomata, deposited on the surface of the plant and the ground, and lost through forest chemical reactions. There were no significant differences in O3 fluxes and deposition velocities in broadleaf and coniferous forests during the summer. The O3 flux in the forest at night were 15±8% and 22±7% of those at daytime for broadleaf and coniferous forests, respectively. The observed NO flux was both negative (deposition) and positive (emission), whereas that of NO2 was only positive during the daytime in summer- autumn. NO2 emissions mechanisms have been discussed in terms of NO emissions from forest soil, NO2 absorption and emission by plant leaves through stomata, and NO and NO2 concentration gradients caused by photochemical reactions according to the differences in sunlight intensity inside and outside the forest. Nitrates attached to leaves are possible sources of NO2. Simultaneous measurements of O3, NO, and NO2 fluxes are important in different vegetation types for investigating the differences between vegetation types and for developing a global forest inventory of NO and NO2 fluxes, and O3 deposition because of their closely related exchange mechanisms.