Fluxes of NH 3 and CO 2 over upland moorland in the vicinity of agricultural land

Intensive field measurements of NH 3 and CO 2 exchange were made over a wet heathland in the vicinity (<500 m) of sheep pastures in the Cairngorm mountains of Scotland for a two‐week period in the summer. Fluxes of NH 3 were determined using the aerodynamic gradient method with a 3‐height continuous denuder system; fluxes of CO 2 were determined using eddy correlation, while sensible and latent heat fluxes were determined by both methods. Few studies have measured NH 3 and CO 2 fluxes simultaneously, making these measurements relevant to compare exchange dynamics. Both NH 3 and CO 2 exchanged bidirectionally, in response to a combination of biological (foliar, soil) and physico‐chemical controls (solubility). NH 3 was deposited rapidly to leaf surfaces, although during warm, dry daytime conditions periods of emission occurred, explained by the existence of a compensation point concentration for NH 3 . By contrast, CO 2 followed a characteristic pattern of absorption during the day associated with net photosynthesis and emission at night. Both gases showed net uptake from the atmosphere, at 30 μmol NH 3 m −2 d −1 and 74 mmol CO 2 m −2 d −1 . In southeast winds, NH 3 emissions from the sheep pasture caused a significant advection error to the measured fluxes (>10%). Corrections were applied using a local‐scale dispersion‐exchange model. The analysis highlights how advection modifies the classical one‐dimensional inferential resistance approach. It is concluded that ecosystems in the vicinity of agricultural land receive more dry deposition than would be estimated using NH 3 concentration monitoring and standard inferential models. In the present study, this effect represented an overall increase in total NH 3 deposition of 32%.