Pine forest: a negligible sink for atmospheric NO_x in rural Sweden

Abstract

A chamber technique was used to study the uptake of NO and NO₂ by branches of Scots Pine trees at concentrations ranging from about I ppbv (ambient levels) up to about 50 ppbv. The measurements were performed under field conditions of light intensity and temperature. Rates of photosynthesis and transpiration were determined from continuous measurements of CO₂ and water vapour.

The uptake of NO was small at all concentrations. The deposition rate per projected needle area was less than 0.1 mm s-¹. For NO₂, the uptake varied linearly with concentration. At high levels, the uptake was limited both by a stomata1 and an internal resistance and a marked diurnal variation was observed. Typical deposition rates per projected needle area at concentrations larger than 10 ppbv were 1-2 mm s-¹ during the day and 0.05-0.2 mm s^-l during the night. The internal resistance constituted between 3% and 60% of the total resistance to NO₂ uptake.

Contrary to observations made at high concentrations, no uptake of NO₂ could be detected when the concentration was decreased to 1 to 3 ppbv. Using a leaf area index of 3 m² m-², which is a typical value for a pine forest in Sweden, a deposition velocity per ground area of < 1 mm s-¹ is obtained. This is significantly lower than other values reported in the literature. Since the NO₂ concentration in the atmosphere of large rural areas covered with coniferous forest is of the same order of magnitude as that where no uptake of NO₂ was observed, this finding is of great importance for modelling of atmospheric chemistry and transport in the boundary layer. A comparison with the uptake of SO₂ and O₃ shows that daytime deposition velocities for these gases are a factor 10 larger than those for NO₂.