Deposition of Gases Controlled by Stomatal Resistance

Ozone belongs to the group of gases whose deposition over vegetation is controlled by stomata resistance. It has been concluded that the majority of the daytime flux is absorbed via stomata with minimum canopy resistance. In contrast, the nocturnal deposition is nonstomatal with large leaf surface resistance. The surface resistance for nonstomatal ozone deposition decreases with temperature.

Stomatal uptake provides a convenient means of calculating deposition rates, assuming that there is no internal mesophyll resistance. The ozone flux assessment requires the models to calculate stomatal uptake from readily available land-use data and meteorological variables.

The ozone-deposition process and budget may be altered appreciably over agricultural areas by the reaction with NO formed during the nitrification and denitrification processes caused by soil bacteria. The scale of this alteration is variable and uncertain. Studies on this subject were carried out within the EUROTRAC project (http://www3.gsf.de) on Transport and Transformations of Pollutants in the Atmosphere.

Sulfur and nitrogen dioxides are very important gases with regard to the acidification of the environment and related environmental effects, including mobilization of toxic heavy metals in the aquatic ecosystems. Therefore, the deposition of these two gases, as precursors of acidification, have been intensively studied in both the laboratory and field measurements. It was concluded that the deposition of SO2 onto short vegetation is determined largely by stomata uptake within a small rate of deposition onto external surfaces. This deposition has a diurnal cycle with the maximum occurring in daytime and a nocturnal minimum. Deposition of SO2 to forest is further complicated by the emission of reduced sulfur compounds. Thus, it can be concluded that the rates of SO2 deposition are controlled mainly by the chemistry at the vegetation-atmosphere interface and that, as the surfaces are wet most of the time, the processes are regulated by the chemistry of this thin film of moisture. Many compounds influence this chemistry, including plant exu-dates and soil-derived compounds, but the key reactant is ammonia. The ambient concentrations of sulfur dioxide and ammonia regulate the pH of the surface moisture and thus control the uptake of SO2. These conclusions were proven by studies carried out with the EUROTRAC

Emission Deposition ^

Atmosphere Forest canopy

Temperature- NO -Soil N

Soil water

Soil

Figure 2 The interaction of soil emission of NO with NO-O3 reaction within canopy and NO2 uptake by stomata and the net exchange of NOx above forest canopies. Reproduced from Midgley PM and Reuther M (eds.) (2003) Towards Cleaner Air for Europe - Science, Tools and Applications. Part 2: Overviews from the Final Reports of the EUROTRAC - 2 Subprojects. Leiden: Margraf Publishers, with permission from Margraf Publisher.

project BIATEX-2 on Biosphere/Atmosphere Exchange of Pollutants.

Deposition of nitrogen dioxide is more complicated than the deposition of sulfur dioxide. Two processes need to be considered: (1) long-term emission of NO from forest soils, and (2) the interaction of chemical processing of NO in vegetation canopies with the deposition of NO2 to foliar surfaces and emissions of NO from soil. These processes are presented in Figure 2.

As concluded in the BIATEX-2 project, the consequence of soil emissions of NO and within-canopy conversion of NO to NO2 by reactions with ozone is that these processes produce a within-canopy source of NO2. This acts in a parallel way to the compensation point of ammonia to determine the net rate of exchange of NO2 above the forest. At low ambient concentrations of NO2, the forest can be a source of NO2. At large concentration of NO2 above the forest canopy, the forest becomes a net sink of NO2, as the stomatal uptake exceeds the NO soil emission.

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