Comparison of low cost measurement techniques for long-term monitoring of atmospheric ammonia

An inter-comparison of techniques for long-term sampling of atmospheric amm onia (NH3) was conducted with a view to establishing a national network wit h > 50 sites. Key requirements were for: a low cost system, simplicity and durability to enable a postal exchange with local site operators, a precisi on of < +/- 20% for monthly sampling at expected NH3 concentrations of 1-2 <mu>g m(-3), a detection limit sufficient to resolve the small NH3 concentr ations ( <0.2 <mu>g m(-3)) expected in remote parts of the UK, and a quanti tative means to establish quality control. Five sampling methods were compa red: A, a commercially available membrane diffusion tube (exposed in tripli cate). with membranes removed immediately after sampling; B. the above meth od, with the membranes left in place until analysis; C, open-ended diffusio n tubes (exposed with 4 replicates); D, a new active sampling diffusion den uder system, and E, an active sampling bubbler system. Method D consisted o f two 0.1 in acid coated glass denuders in series with sampling at approxim ate to0.31 min(-1). These methods were deployed at 6 locations in the UK an d the Netherlands and compared against reference estimates. Method D was th e most precise and sensitive of the techniques compared, with a detection l imit of < 0.1 <mu>g m(-3). The bubbler provided a less precise estimate of NH3 concentration, and also suffered several practical drawbacks. The diffu sion tubes were found to correlate with the reference at high concentration s (> 3 mug m(-3)), but were less precise and overestimated NH3 at smaller c oncentrations. Of the passive methods, A was the most precise and C the lea st precise. On the basis of the results, method D has been implemented in t he national network, together with application of method A to explore spati al variability in regions with expected high NH3 concentrations.