Ammonia miniaturized sensors: Are they ready to be used in outdoor environments?

NH₃ is the major alkaline gas in the atmosphere and the third most abundant N-containing species, after N₂ and N₂O. It is an important target pollutant due to its role in N deposition processes impacting over ecosystems, and it is also a precursor of fine particulate matter (PM), known to cause several impacts on human health. Being able to detect and quantify NH₃ is essential for determining the best mitigation policies to reduce these impacts, yet this is challenging given the high spatial and temporal variabilities of this pollutant.

Miniaturized sensors in theory combine the high time resolution with the flexibility in size and cost, however they have associated challenges including high or undefined LODs, variable response times, unspecified cross–interferences with other pollutants and degradation with usage time. The NH₃ sensor market is less developed than some pollutants for ambient air applications, with most suppliers offering indoor applications at ppm level concentrations, while ambient NH₃ concentrations are in the ppb range.

The results of a campaign comparing five NH₃ sensors (four electrochemical and one chemiresistive) against a Picarro G2103 are reported. The campaign was carried out over 1 month at the Whim Bog, Scotland, which releases NH₃ if specific wind speed and direction conditions are fulfilled simulating a small chicken farm. Local ambient concentrations are ~1-2 ppb. The commercial suppliers do not provide practical guidelines on how to properly use, maintain and clean the data for these sensors however the sensors were set-up accordingly to their technical capabilities, either sampling from a weatherproof enclosure or directly outdoor air. NH₃ concentrations were <10 ppb up to 3 ppm from the Picarro data. The sensors had variable responses, with only two correlating with the NH₃ release and the Picarro data (R²= 0.59 and 0.70). However, they underestimated the concentration levels (slopes = 0.26 and 0.6) and response times are still not satisfactory.

The study shows that only two out of five sensors were fit for measuring NH₃ in the ambient air. Nonetheless, these two sensors data are to be used only as qualitative information and would need significant improvement before use in situations which require quantitative data. Specific technical challenges need to be addressed, including sensor orientation, housing, and the airflow inside of and quantification of the concentration range.