Dr Natalie Meades: IBERS, Aberystwyth University.
December 2023
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Ammonia is a colourless gas with a distinctive odour that is classified as an air pollutant.
- Ammonia can have negative effects on human and livestock health following inhalation. Moreover, ammonia can react with other compounds within the atmosphere to form ammonium-containing secondary particulate matter which can have negative effects on respiratory and cardiovascular health.
- Ammonia can also have damaging effects on the environment, where it can contribute to the acidification of soils, eutrophication and loss of biodiversity within ecosystems.
- A large proportion of ammonia emissions are produced by the agricultural industry from livestock (excreta and subsequent storage of muck and slurry) and use of organic and inorganic fertiliser application. As such, there is drive to reduce ammonia emissions within the agricultural industry in the UK and Wales.
Ammonia Emissions in the UK and Wales
Ammonia (NH3) is a colourless gas with a distinctive odour that is produced from natural (degradation of organic matter, volcanic activity) and anthropogenic (agriculture, industrial, waste treatment) processes. Moreover, ammonia is classified as an air pollutant that can have damaging effects on health and the environment (Figure. 1). It is therefore important that strategies are put in place to reduce ammonia emissions along with other air pollutants. One way of doing this is by addressing and reviewing anthropogenic activities.
The UK government has set NECR (National Emission Ceilings Regulations) emission reduction targets to reduce air pollutants. For the UK to meet the 2030 NECR targets, ammonia emissions need to be 16% lower in comparison to baseline levels recorded in 2005. According to the Air Pollutant Inventories for England, Scotland, Wales and Northern Ireland: 2005-2021 ammonia emissions in Wales were estimated to be 23 kt in 2021, which was thought to account for 9% of total ammonia emissions within the UK that year. The measures the Welsh Government propose to make to help the UK Government achieve NECR targets can be found within the Clean Air Plan for Wales.
Figure 1: The effect of ammonia on health and the environment AHDB.
A large proportion of ammonia emissions are produced from the agricultural industry, where it was estimated that agricultural activities generated 87% of total ammonia emissions in the UK in 2021 (Figure. 2). According to The Clean Air Plan for Wales, agriculture is thought to contribute to 85% of Welsh ammonia emissions. Therefore, there is great interest in reducing ammonia emissions within the agricultural industry. The breakdown of the ammonia emissions by sector can be found in Figure 3, which shows cattle (beef and dairy) to be one of the largest contributors to ammonia emissions within the UK agricultural industry. This is largely associated with intensive and indoor reared animals and manure and slurry management which will be discussed later in this article. It is therefore important that strategies are taken to reduce ammonia emissions on farm regarding the effects of ammonia on health and the environment. A previous technical article discusses various options for reducing ammonia emissions through fertiliser storage and land application techniques, click on the link to read more about this.
How is Ammonia Produced?
Within the agricultural industry ammonia is largely produced from livestock excreta (urine, faeces) and the subsequent storage and spreading of muck or slurry on the land. Excess dietary nitrogen or nitrogen that is not able to be digested or incorporated into production (milk, eggs) is excreted from the body as urea in the urine of mammals, as uric acid in bird excreta or as organic nitrogen in the faeces. Urea and uric acid can be converted into ammonia by enzymes that are produced by certain microorganisms within the faeces. As such, the mixing of urine and faeces to create muck or slurry helps increase this process. Therefore, a large amount of ammonia is produced from housed animals (on floors, within bedding and under slats), from muck or slurry stores and following the spreading of muck or slurry as fertilizer on the land. Ammonia emissions are less for livestock reared outdoors due to animals not typically defecating and urinating in the same locations. However, ammonia is still produced and is often seen around urine patches. Where urea within the urine can be converted into ammonia by certain urease producing microorganisms within the soil. Furthermore, the use of inorganic nitrogen fertilisers (especially urea based) also contributes to ammonia emissions.
The biochemical processes involved can be found in Figure 4. Briefly, uric acid in the presence of oxygen and water can be converted into carbon dioxide and ammonia by the enzyme uricase and urea can be degraded by the enzyme urease to carbon dioxide and ammonia. Furthermore, any undigested proteins within the faeces can be degraded into amino acids and ammonia by uricase, urease or via microbial metabolism.
Figure 4: Equations as presented by Behera, et al. (2013)
Ammonia and Health
Ammonia can be described as an irritant that can be damaging to health when over exposure occurs. Typical symptoms of ammonia exposure have been reported to include, eye, nose and throat irritation, headaches, nausea, diarrhoea, hoarseness, sore throat, cough, tight chest, nasal congestion, palpitations, shortness of breath, drowsiness and alterations to mood. Moreover, it has been reported that mild symptoms of ammonia exposure can occur as low as 5 ppm with more noticeable symptoms occurring at 30 ppm. Therefore, it is important that working environments and livestock housing units do not contain unsafe levels of ammonia for both human and livestock health.
Ammonia is also an air pollutant and contributes to poor air quality. Ammonia is a volatile compound that can evaporate into the atmosphere, where it has a relatively short life span of a few hours. However, the life span of ammonia can be extended to a few days when it mixes with other gases within the atmosphere, such as sulphur dioxide (SO2) and nitrous oxides (NOx). The mixing of these gases results in the formation of ammonium-containing secondary particulate matter (PM), such as ammonium sulphate and ammonium nitrate. This particulate matter consists of tiny particles (solid, liquid or droplets) that are suspended within the air and can be classified into two groups based on size. Where PM10 represents particles ≤10μm in diameter and PM2.5 represents particles ≤2.5μm in diameter. In the case of PM derived from ammonia, particulates are classified as PM2.5. This PM can be transported over large distances and therefore have effects in locations far from the source of origin (regionally). Moreover, there is great concern with regards to PM and health. The long-term inhalation of PM has been demonstrated to have negative effects on respiratory and cardiovascular health, where PM2.5 is thought to be able to penetrate deep within the lungs and possibly enter the blood stream, therefore having negative effects on other organs within the body.
Ammonia and the Environment
As previously discussed, ammonia can be generated from livestock excreta, where certain microorganisms in the faeces or in the soil produce enzymes that can transform nitrogenous compounds into ammonia. Ammonia is a volatile compound and can evaporate into the atmosphere under certain conditions. Ammonia can then be deposited back on the land after a few hours by dry deposition. Furthermore, ammonia can react with other pollutants within the atmosphere to form ammonium containing PM. This has a longer life span within the atmosphere and can be carried for longer distances before being deposited on the land by wet deposition as precipitation. Furthermore, ammonia and ammonium ions (NH4+) within the soil can be transformed into nitrite ions (NO2-) and then into nitrate ions (NO3-) in a process called nitrification. These processes are all part of the nitrogen cycle (Figure. 5), which is a perfectly normal biological process, where nitrogen can then be used by plants in a process called assimilation. However, issues arise when nitrogen within the system is excessive. Due to ammonia containing nitrogen it can therefore contribute to nitrogen pollution.
Figure 5: The nitrogen cycle Aczel, (2019).
High nitrogen levels in the soil can be toxic to certain plant species. Moreover, high nitrogen levels can result in alterations to plant species within communities, with nitrogen loving (nitrophilic) plants outcompeting plants that are less tolerant of nitrogen. As such this can lead to a loss of biodiversity within ecosystems. One particular ecosystem that is under threat are ancient woodlands, where an increase in nitrogen deposition has been demonstrated to have negative effects on certain important plants, fungi, lichens and mosses, thereby effecting the biodiversity of woodlands. Moreover, excessive ammonia concentrations within the soil can also have damaging effects to soil health. During nitrification hydrogen ions (H+) are produced which subsequently causes the pH of the soil to lower resulting in the acidification of soils. Furthermore, excess nitrogen within the soil can be lost by leaching into ground water sources or via run-off and enter water courses. The nitrogen can then be used as a substrate for certain plants and algae and contribute to eutrophication.
Summary
Ammonia is a colourless gas with a distinctive odour that is produced by natural and anthropogenic processes. Ammonia itself is classified as an air pollutant and can therefore compromise air quality. Moreover, ammonia is classified as an irritant and can react with other gasses within the atmosphere to form particulate matter and therefore have damaging effects to human and livestock health. Excessive ammonia can contribute to nitrogen pollution and therefore have negative effects on the environment, where it can contribute to the acidification of soils, eutrophication and loss of biodiversity within ecosystems. A large proportion of anthropogenic ammonia emissions are produced by the agricultural industry, where livestock account for a large proportion of emissions. Furthermore, inorganic and organic fertilizer application also contributes to ammonia production. As such there is need to reduce ammonia emissions through alterations to anthropogenic activities.