The effect nitrogen oxides on vegetation depends on the action of NO and NO2. The

ratio of NO2 to NO is very important for the deposition rate, since NO2 is deposited

much faster than NO. Indirectly, NO may also have a positive effect on vegetation by

consuming O3. The NO2 has a lower phyto-toxic effect than O3.

 

Critical levels for oxides of nitrogen in the atmosphere and critical loads for nitrogen

deposition have been developed within UN ECE (Caporn, 1992) to protect

ecosystems. Guidelines to protect ecosystems have also been adopted by WHO,

based on the UNECE work. It is pointed out that synergistic effects (or multi pollutant

effects) are very important and the guideline is given for NOx rather than NO2 alone.

WHO recommended a guideline to protect vegetation as an annual average of 30

Tg/m; for NOx, calculated as the sum of NO and NO2 in ppb and expressed as NO2

in mg/m3. Existing guidelines and limit values for protection of vegetation and

sensitive ecosystems are summarised in Annex 2.

 

 

 

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