Directional passive samplers have been developed by EA/LEC (Lin et al, 2010a; Lin et al, 2010b, Lin et al, 2011; Ferranti 2012) and one application they could be potentially used for is ammonia source monitoring. The Whim ammonia release experiment site was used to test the EA/Lancaster directional passive ammonia samplers (DPAS) downwind of a line source of ammonia gas.
The aim of the experiments was to assess the performance of the DPAS systems for monitoring the directional distribution of ammonia at this site. Two types of ammonia samplers were tested in the DPAS systems, both of which have been developed by CEH Edinburgh: the CEH Adapted Low-cost Passive High Absorption (ALPHA) sampler (Tang et al., 2001) and the CEH Mini Annular Denuder (MANDE) flux sampler. Testing was carried out over a period of six months with 5 exposure periods. This report summarises the results from these experiments and presents some discussion and interpretation.
ALPHA samplers proved not well suited for use in DPAS systems for two reasons: Firstly ALPHAs are designed to monitor concentration, whereas the DPAS system is designed to monitor fluxes. Effectively, ALPHAs use diffusive membranes to control the collection of gaseous species so that collection is by diffusion rather than by interception of fluxes. The second, related drawback was that the continuous diffusional sampling by the ALPHA samplers: ammonia as it disperses throughout the internal DPAS chamber becomes well mixed and hence each DPAS direction sampled slowly but continuously leading to a high “background” contribution to the measured ammonia mass in addition to the ammonia brought in with the specific wind direction.
MANDE samplers were much more successfully used in the DPAS. Though they also have a level of ammonia diffusion within the DPAS housing samplers leading to an above background ammonia measurement in all directions, this is an order of magnitude smaller effect than with the ALPHA. The direction in which the ammonia is coming from is clearly identifiable: in this case the line source release directions. Using the frequency of wind direction and the sampling rate of the MANDE as a function of wind speed, a wind run concept developed by EA/LEC has been applied to the data to calculate a “weighted ammonia wind run” which clearly shows a good reflection of the directions from which the ammonia was emitted.
Overall the DPAS-MANDE combination shows significant potential for studying the directional variation of ammonia in the environment when deployed in conjunctional with meteorological measurements. One caveat to be highlighted is that further work is required to assess sampling rate – wind-speed variation in order to move from a weighted ammonia wind run measurement to interpretation in terms of atmospheric ammonia concentrations.