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

Flechard, C.R.; Fowler, D.. 1998 Atmospheric ammonia at a moorland site. I: The meteorological control of ambient ammonia concentrations and the influence of local sources. Quarterly Journal of the Royal Meteorological Society, 124 (547). 733-757.

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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 (<°C) weather, typically 1-5 g m-3. Smaller concentratins, typically in the range 0.1-1 g m-3 were found in intermediate (1-15 °C), humid and wind conditions. Ammonia concentrations generally decreased with increasing wind speed and were very variable at windspeeds <3 m s-1. Concentrations also decreased with increasing relative humidity, reflecting the partitioning of NHx toward the liquid phase at large (>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.

Item Type: Publication - Article
Digital Object Identifier (DOI):
Programmes: CEH Programmes pre-2009 publications > Biogeochemistry
UKCEH and CEH Sections/Science Areas: _ Atmospheric Sciences
ISSN: 0035-9009
Additional Keywords: Ammonia, Atmospheric chemistry, Boundary layer, Deposition, Dispersion, Surace emissions
NORA Subject Terms: Ecology and Environment
Atmospheric Sciences
Date made live: 21 May 2009 16:18 +0 (UTC)

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