Atmospheric ammonia in-situ long-term monitoring: review worldwide strategies and recommendations for implementation

Ammonia (NH3) is the major alkaline gas in the atmosphere and the third most abundant N-containing species, after N2 and N2O. It plays an important role in N deposition processes, responsible of several damages on ecosystems, and it is also a precursor of fine particulate matter, known to cause numerous impacts on human health. Despite this, not many countries have implemented long-term monitoring of NH3 in their air quality programs due to the lack of consensus on limit values for ambient levels and a reference method of measuring this gas. In the climate change context, governments and health organizations are increasingly concerned about NH3 and its effects. As a proof, the revision of the EU air quality directives proposes the inclusion of NH3 as a mandatory pollutant for several urban and rural supersites for all member states.Currently, there are only 12 long term programs worldwide dedicated specifically to measure NH3 or including gas-phase measurements of NH3. The longest NH3 time series come from UK and Africa, where measurements start in mid-1990. The rest of locations have started after 2000 and they have lower temporal coverage, between 5 and 22 years. The objectives pursued by these networks are to follow long term spatio-temporal trends, assess the N deposition on sensitive ecosystems, validate emission and/or chemistry transport models and help to understand the effectiveness of air pollution control and mitigation policies. Most of these networks operate using a combination of low-cost samplers with a high spatial density with few collocated sites with high time resolution instrumentation to help calibrate passive samplers and to better monitor the fine temporal variability of NH3. This combined approach has proven to be successful for most of the proposed objectives.However, there are several differences that may difficult harmonizing the information at both the technical and scientific level. At the technical level these include type and number of passive samplers per site, calibration protocol, data control and quality analysis, exposure duration and type of high time resolution sampling method. On the scientific level, increased difficulty understanding the operative parameters and scientific results may come from language barriers (non-English reports), availability of the data (whether it is public or not), and gaps on the knowledge of NH3 levels on a spatial scale due to differences in the implementation of monitoring strategies within the same country.This work aims to review synthetically the world current long-term NH3 networks and provide some insight and recommendations for other countries and supranational programs aiming to establish long term monitoring networks of NH3, based on cost-effective, technical, and operational criteria.