Deposition of Acidifying Compounds

Inputs of acidifying compounds to terrestrial ecosystems include deposition of the gases NO 2 NO, HNO 2, HNO 3, PAN, NH 3, SO 2 and the ions NO 3-, NH4 +, SO4 2- and H + in precipitation, cloud droplets and particles. Recent research has identified particular ecosystems and regions in which terrestrial effects are closely linked with specific deposition processes. The air pollution related forest health problems at high elevation in the Appalachian mountains for example are primarily due to the cloud water deposition while the health of Dutch heathlands appear to be linked primarily with inputs of ammonia. Areas of large wet deposition in northern Europe show large spatial gradients in inputs and peak values up to a factor of 2 in excess of those provided by long range transport models as a consequence of seeder-feeder scavenging. Aerosol fluxes to shoot vegetation are small relative to gaseous deposition and seldom provide deposition velocities larger than 2 mms-i. There is also evidence of aerosol production in the few metres of the boundary layer close to the surface. Dry deposition rates for gases onto vegetation are controlled by atmospheric processes for the very reactive gases HNO3 and HC1. For NO 2 uptake has been shown to be entirely stomatal with deposition increasing from zero at night to 6 mms 1 over rapidly growing vegetation during the day. Rates of SO2 deposition are also regulated by stomatal uptake in the absence of surface water or ammonia. In the presence of NH3 (r c 0-200 sm-i) rates of deposition may reach the upper limit set by turbulent exchange processes (Vg = (r a + rb) 1 but the chemistry of NH3 and SO2 interactions on natural surfaces and its dependence on surface condition has not been quantified. Ammonia exchange over natural plant communities has been shown to be bidirectional, with NH3 emission over polluted Dutch heathland in dry conditions and deposition in wet conditions and is very similar to agricultural surfaces. For the cooler, wetter, less polluted regions such ecosystems represent a very efficient sink for NH3. Mechanistic models of deposition processes to extrapolate to regional scales have now been developed for SO 2, NO 2, cloud droplet and wet deposition processes, but they do not satisfactorily incorporate NH3, SO2 co-deposition and are not yet adequate to extrapolate NH3 fluxes to the landscape scale. © 1992 Elsevier Science Publishers B.V. All rights reserved.