Ammonia is a colorless, toxic, irritating gas with a pungent odour released from decaying organic matter, including plants, animals, animal wastes, and fertilizer use. Prolonged exposure of the NH3 can quickly deaden a person’s sense of smell, making the odor of NH3 an unreliable indicator of its presence. Hence, other means such as use of NH3 monitors is a viable solution to provide adequate warning of hazardous exposure. This article covers information on ammonia, its sources in the ambient air, permissible levels, health and environmental impact, possible corrective measures, need for ammonia monitors as well as different methods of NH3 monitoring.
What is NH3?
Ammonia is a colorless gas at room temperature with a very pungent irritating smell detectable at 25 ppm consisting of one nitrogen atom bonded to three hydrogen atoms. It is toxic gas which is irritating to the eyes, nose, throat, and respiratory tract and if inhaled in great quantities, can lead to death. NH3 is a highly reactive, corrosive, alkaline gas that dissolves easily in water to form ammonium hydroxide. It is not highly flammable, but it may explode when exposed to high heat.
NH3 gas is lighter than air and hence when released, will rise and dissipate. However, in the presence of moisture (e.g. during conditions of high relative humidity), it reacts to form vapors of the liquefied anhydrous ammonia gas. These vapors are heavier than air and may spread along the ground, into low-lying areas with poor ventilation where there is risk of exposure.
Ammonia in Atmosphere
NH3 is the most abundant alkaline gas in the atmosphere released naturally from soil from bacterial processes and from decaying organic matter, including plants, animals and animal wastes.
Once released, the resident time of NH3 in the atmosphere is of only a day or less (i.e. can travel for <10-100 kms), before either getting deposited back onto terrestrial surfaces by dry deposition or being converted to ammonium (NH4+) particulate matter through reaction with atmospheric acids (H2SO4 and HNO3) which resides in the atmosphere for long periods of time before being removed from the atmosphere as wet deposition by rain or snow. Ammonium particulate matter having larger residence time i.e. can travel to 100-1000 kms, make NH3 responsible for long-range transport of reactive nitrogen.
The transformation of ammonia gas in the atmosphere is based on the chemical reaction with acidic gases species that form NH4+ salt aerosols, also known as secondary inorganic PM2.5. Gaseous ammonia first reacts with sulfuric acid (H2SO4) to form ammonium sulfate ((NH4)2SO4) and if more amount of ammonia is available, it then reacts with nitric acid (HNO3) to form ammonium nitrate (NH4NO3). These reactions are highly dependent on the atmospheric concentration of ammonia (NH3), nitrogen oxides (NOx), sulfur oxides (SOx), temperature and humidity. Due to lower SOx emissions in the urban areas, ammonium nitrate formed by reaction with HNO3 is the major source of fine particulate matter, specifically released from agricultural sources.
The largest source of ammonia emission, globally is agricultural (~85%) including NH3 emissions from ammonia-based fertilizers applications and livestock waste. Other sources of NH3 include vehicular emissions (specifically catalytic converters in petrol cars), biomass burning, composting of organic matter, landfills, sewage treatment plants, and volatilization from soil and oceans. Industrial sources of NH3 include the fertilizer manufacturing, coke manufacture, pharmaceutical and cleaning products, fossil fuel combustion, livestock management, and refrigeration methods.
Permissible exposure limits for NH3
The breakpoints concentrations describing the quality of air based on the ammonia concentrations have been derived in India are given below. Here, the daily average NH3 levels of up to 400 µg/m3 (i.e. 0.574 ppm) are considered satisfactory.
Table: Breakpoints of NH3 (µg/m3) – 24hr in India
|AQI Category||Breakpoint concentration|
|Moderately polluted (101-200)||800|
|Very poor (301-400)||1800|
Apart from ambient limits, the permissible exposure limits of NH3 in terms of continuous occupational exposure indoors are given below:
|8-hr. TWA||STEL||Ceiling Limit|
|Federal OSHA PEL||50 ppm||NA||300 ppm|
|NIOSH REL||25 ppm||35 ppm||300 ppm|
|ACGIH TLV (2010)||25 ppm||35 ppm||300 ppm|
Permissible Exposure Limit (PEL) given by OSHA (Occupational Safety and Health Administration) defines the maximum concentration of NH3 to which an unprotected worker may be exposed to. PEL may reference an eight-hour time-weighted average (TWA), a 15-minute short term exposure limit (STEL) or an instantaneous ceiling limit (CL) concentration that cannot be exceeded for any period of time. Similarly, Recommended Exposure Limit (REL) are the occupational exposure limit recommended by NIOSH (National Institute for Occupational Safety and Health) and the TLVs (i.e. threshold limit values) are the exposure guidelines given by ACGIH (American Conference of Governmental Industrial Hygienists).
Health & Environmental Impact of NH3
Exposure to low levels of NH3 can cause coughing as well as irritation to eyes, nose, throat and respiratory tract. NH3 presence is detectable by its pungent odor providing adequate early warning signs, however NH3 also causes adaptation or olfactory fatigue on prolonged exposure thereby damaging one’s sense of smell.
Exposure to high levels of NH3 causes immediate burning of the nose, throat and respiratory tract which can cause airway destruction resulting in respiratory distress or failure and can also lead to death. Children are the most vulnerable to NH3 exposure, due to their greater lung surface area-to-body weight ratios. Additionally, their short height makes them susceptible to inhalation of higher amounts of NH3 vapors which are initially found near ground.
The toxic effects associated with exposure to NH3 concentration is given below:
|NH3 (ppm)||Effect on human health|
|50||Irritation to eyes, nose and throat (2 hours’ exposure)|
|100||Rapid eye and respiratory tract irritation|
|250||Tolerable by most people (30 – 60 minutes’ exposure)|
|700||Immediately irritating to eyes and throat|
|>1500||Pulmonary edema, coughing, laryngospasm|
|2500-4500||Fatal (30 minutes’ exposure)|
|5000-10,000||Rapidly fatal due to airway obstruction, may also cause skin damage|
The two main environmental impacts of NH3 in the atmosphere are:
- Formation of Secondary Inorganic Aerosols (SIA): Ammonia reacts with H2SO4 and HNO3 present in the atmosphere to form ammonium salts that remain in the atmosphere for a few days up to a week as particulate matter before depositing back to ground impacting human health and environment over large scales.
- Eutrophication: The wet as well as dry deposition of NH3 emissions back to the landscape, contributes to the eutrophication of soil and surface water thereby affecting the plant and animal species.
Possible corrective measures
The primary action is NH3 monitoring i.e. to measure how much of NH3 concentrations you are exposed to. In addition to this following corrective measures can be taken:
- Avoid going to or staying in low lying areas where NH3 is produced or used such as agricultural or poultry farms
- Avoid use of ammonia-containing products, specifically cleaning products in poor ventilated areas.
- Always use appropriate personal protective equipment (PPE) while using NH3 containing products or while going to places where presence of high levels of NH3 is suspected.
- Also, if the presence of NH3 is detected, immediately vacant the area and provide proper ventilation to remove the gas.
Measurement methods of NH3 monitoring
Different working principles for ammonia monitoring in the ambient environment are chemiluminescence, semiconductor, and electrochemistry.
Chemiluminescence method based NH3 monitors continuously measure NH3 in addition to NOx with just some modifications. In NH3 monitor, a thermal NH3 converter module is placed upstream of the conventional chemiluminescence NOx analyzer. Here, NH3 and NO2 transform into NO in one part of the air stream while the other part contains the original NO content. Both the streams are passed to react with Ozone (O3). Further, the light produced by the chemiluminescent reaction of O3 and NO is measured photometrically and recorded. The ammonia monitor subsequently detects the total NOx after being passed through NH3 scrubbers in a separate channel, such that the difference in the both signals is proportional to the total NH3 concentration in the air sample. For the NH3 monitors based on this principle, specific precautions should be taken to address the interferences caused by other organic nitrogen compounds, if present can transform to NO resulting in the inaccurate results.
When a metal oxide semiconductor-based ammonia monitor is exposed to an air sample, the NH3 molecules react on the metal oxide surface of the sensor and dissociate into charged ions that alter the resistance of the film. This interaction is measured as a signal and is converted to the gas concentration. However, the energy consumption of such NH3 monitors is higher compared to others.
NH3 monitors working on the electrochemical principle are operated based on the diffusion of ammonia gas into the sensor which results in the production of electrical signals proportional to the NH3 concentration. Therefore, it allows accurate measurement of even low concentrations of NH3, which is essential in NH3 monitoring in the ambient air.
Among all the above principles of NH3 monitoring, applications like ambient air monitoring prefer ammonia monitors based on electrochemistry. This is because they yield more accurate NH3 concentrations and are inexpensive in comparison with the others.
Oizom’s working principle for NH3 monitoring
Oizom’s ODOSENSE is the real-time odour emission tracking solution. It continuously detects, measures, and monitors the odourful gaseous contaminants including hydrogen sulfide, ammonia, sulfur dioxide, methyl mercaptan, TVOC, formaldehyde, methane, and weather parameters like temperature, humidity, wind speed, and wind direction. The sensor that measures NH3 works on the principle of electrochemical sensing. With the help of meteorological data, Odosense can trace the odourant dispersion plume incited by conditions like wind speed and wind direction. Odosense is a proactive approach to measure real-time odour emissions. This makes it an ideal choice for landfill sites, wastewater treatment facilities, fertilizers, paper-pulp industries, and soil-treatment sites, etc.
Reasons why NH3 monitoring is important
- NH3 is a very soluble, colorless gas with a strong pungent smell primarily released from decaying organic matter, including animal wastes and fertilizer use.
- NH3 forms secondary particulate matter of ammonium salts (NH4+) by reacting with the acids of SOx and NOx, which move large distances along with the air impacting human health and environment of both local as well as international (transboundary) scales.
- NH3, on contact or when inhaled, rapidly reacts with the moisture containing parts of the body and causes irritation, damage to the cells of eyes, nose, throat and respiratory tract. Additionally, it plays a significant role in atmospheric deposition of nitrogen to sensitive ecosystems causing acidification and eutrophication of soils and natural waters.
- Prolonged exposure of the NH3 can quickly deaden a person’s sense of smell, making the odor of NH3 an unreliable indicator of its presence. Hence, other means such as use of NH3 monitors is a viable solution to provide adequate warning of hazardous exposure
- Real-time monitoring of NH3 levels helps in calculating air quality index to deliver health advisories as well as formulating an action plan to meet standards.