Atmospheric ammonia at a moorland site. II: Long‐term surface‐atmosphere micrometeorological flux measurements

Abstract

AbstractLong term micrometeorological measurements of the net vertical exchange of atmospheric ammonia (NH3) over moorland vegetation at a site in southern Scotland from February 1995 through to February 1996 are reported. The measurments, made using a combination of eddy covariance and aerodynamic flux‐gradient methods, provided satisfactory 30‐minute average fluxes approximately 50% of the time using continuous annular wet denuders for NH3 sampling at three heights between 0.4 and 3.4 m above the ground.The moorland vegetation was a net sink for atmospheric NH3 as 93.4% of all half‐hourly measured fluxes indicated dry deposition to the surface with mean flux of −5.2 ng m−2 S−1 and a mean deposition velocity Vd of 11.7 mm S−1 at (z ‐ d) = m. Despite the dominance of deposition to the moorland, emission fluxes were observed approximately 6.6% of the time with a median of +1.1 ng m−2 S−2, and most commonly resulting from drying water films on foliar surfaces.The mean aerodynamic resistance at a reference height of (z ‐ d) = 1 m and the mean viscous sub‐layer resistance were 36.6 s m−1 and 11.6 s m−1 respectively, and of a similar magnitude to the mean canopy or surface resistance Rc of 37 s m−1. When the moorland surfaces were wettend by rain or dew, canopy resistances to NH3 deposition were generally smaller (averaging 23 s m−1) than in dry conditions (when Rc was 61 s m−1) or when surfaces were covered in snow (Rc = 56 s m−1. Therefore, moorland surfaces cannot be treated as perfect sinks for NH3 deposition.Diurnal and seasonal patterns in rates of deposition and net fluxes to the moorland surface area were determined mainly by a combination of wind speed, surface wetness and ambient NH3 concentration. Thus, during winter, depositions rates expressed as a flux or deposition velocity were generally much larger than in summer by factors of 2 or 3 respectively, as a result of stronger winds and the presence of surface water for extensive periods which kept canopy resistances small except in frozen conditions. At temperatures below 0°C, the surface became progressively dry and canopy resistances were large (50–100 s m−1).The measurements provided direct estimates of the net annual gaseous NH3 input to the moorland, which was 2.5 kg NH3‐N ha−1 a−1 and was almost identical to the measured wet deposition input at the site of 2.4 kg NH4+‐N ha−1 a−1.