Ammonia concentrations at a site in southern Scotland from 2 yr of continuous measurements

Atmospheric ammonia (NH3) concentrations were measured using a continuous-dow annular denuder over a period of 2 yr at a rural site near Edinburgh, Scotland. Meteorological parameters as well as sulphur dioxide (SO2) concentrations were also recorded. The overall arithmetic mean NH3 concentration was 1.4 mu g m(-3). Although an annual cycle with largest NH3 concentrations in summer was apparent for seasonal geometric mean concentrations, arithmetic mean concentrations were largest in the spring and autumn, indicating the increased importance of occasional high concentration events in these seasons. The NH3 concentrations were influenced by local sources as well as by background concentrations, dependent on wind direction, whereas SO, geometric standard deviations indicated more distant sources. The daily cycle of NH3 and SO2 concentrations was dependent on wind speed (u). At u <1 ms(-1), NH3 concentrations were smallest and SO2 concentrations were largest around noon, whereas at u > 1 m s(-1) this cycle was less pronounced for both gases and NH3 concentrations were largest around 1800 hours. These opposite diurnal cycles may be explained by the interaction of boundary layer mixing with local sources for NH3 and remote sources for SO2. Comparing the ammonia data with critical levels and critical loads shows that the critical level is not exceeded at this site over any averaging time. In contrast, the N critical load would probably be exceeded for moorland vegetation near this site, showing that the contribution of atmospheric NH3 to nitrogen deposition in the long term is a more significant issue than exceedance of critical levels. (C) 1998 Elsevier Science Ltd. All rights reserved.