Atmospheric ammonia at a moorland site. I: The meteorological control of ambient ammonia concentrations and the influence of local sources

Ammonia (NH3) concentrations were continuously monitored at a moorland site in southern Scotland for 13 months from February 1995 to February 1996, providing about 13 500 half-hourly average measurements (71% of the time). Concentrations were found to be log-normally distributed (with a geometric mean of 0.43 g m-3 and a geometric standard devation of 3.35), and were strongly dependent on wind direction because of local agricultural point soources within 3 km of the site. These sources contributed approximately 60% of the NH3 advected over the measurement site, while more distant sources accounted for the remaining 40%. Meteorological conditions also influenced the concentration of NH3 because of their effects on emissions from vegetation, on dispersion by the atmosphere, and on rats of removal from the atmosphere. The largest concentrations were measured in warm (>15 °C), dry, summer conditions and in cold (90%) relative humidities, while evaporation from aerosols seemed more likely at lower (40-90%) relative humidities. Seasonal variations in meteorological patterns resulted in marked seasonal variations in concentrations, with the largest mean-values observed during the warmer and drier summer months, and the smallest values measured in humid and windy, atumn and winter conditions. Measurements indicated a short atmospheric residence time for NH3, of the order of 1-2 hours. The atmospheric residenc time for dry deposition of gaseous NH3 was estimated from these data and ranges from 2 to 20 hours depending on atmospheric and surface conditions, with a mean value of 3.5 hours for the 13 months of monitoring