SO₂ monitoring is an efficient way to detect and reduce exposure to sulfur oxides which further benefits in reducing the formation of particulate sulfur pollutants like fine sulfate particles, acid rain, and smog.  This article covers information on sulfur oxides, their sources, permissible levels in the ambient air, health, and environmental impact, possible corrective measures, the need for sulfur dioxide monitors as well as different methods of SO₂ monitoring.

What is sulfur dioxide?

What Does Sulphur Dioxide Cause in Atmosphere

 

SO_x is naturally present in the atmosphere at very low concentrations. About 99% of SO₂ is released into the atmosphere through human activities. High levels of SO₂ are often found at industrial sites or in densely populated urban areas. The lifetime of SO₂ in the atmosphere is about a few days and is removed by reacting with various other compounds.

It reacts in the atmosphere to form sulfur trioxide (SO₃), and dissolves readily in water vapor, forming sulfuric acid(H₂SO₄) which forms condensation nuclei for aerosols and clouds and acidifies the rain. It can also attach to dust and soot particles in the atmosphere to form particulates. These reactions form from other secondary pollutants such as smog, and influence both human health and environmental processes.

Sulfur Dioxide Sources

 

Sulfur oxides are released into the atmosphere naturally from volcanic eruptions. SO_x, especially SO₂, is emitted by the burning of fossil fuels such as coal, oil, and diesel, or any other materials that contain sulfur.  Emissions that lead to high concentrations of SO₂ generally also lead to the formation of other sulfur oxides. 

The largest source of SO₂ in the air is the combustion of fuels in power plants and other industrial facilities. Other smaller sources include industrial processes such as extraction of metal from ore, aluminum smelting, refining of oil, pulp mills, and emission from locomotives, ships, and other vehicles and heavy equipment that burn sulfur-containing fuels. Diesel vehicles and equipment using high sulfur fuels were a major source of SO₂ in the past, but the federal regulations to reduce the sulfur content of diesel fuels have made a significant improvement in emissions from the transportation sector.c power plants and refineries.

The breakpoints concentrations describing the quality of air based on sulphur dioxide concentrations for different countries are given below. In India, the satisfactory levels of daily SO2 are up to 80 µg/m³.

Table: Breakpoints of sulphur dioxide (µg/m³)

India (24-hr)		US (24-hr)		China (24-hr)		EU (8-hr)
AQI Category	Breakpoint concentration	AQI Category	Breakpoint concentration	AQI Category	Breakpoint concentration	AQI Category	Breakpoint concentration
Good	40	Good	50	Excellent	89	Very low	50
Satisfactory	80	Moderate	150	Good	377	Low	100
Moderately polluted	380	Unhealthy for sensitive	475	Lightly Polluted	587	Medium	350
Poor	800	Unhealthy	800	Moderately Polluted	797	High	500
Very Poor	1600	Very Unhealthy	1600	Heavily Polluted	1583	Very high	500
Severe	1600+	Hazardous	1600+	Severely Polluted	1583+

Health & Environmental Impact of SO₂

Sulphur Dioxide Health Effects

SO₂ is soluble in aqueous media and hence when inhaled, it affects the mucous membranes of the nose and upper respiratory tract. The short-term effects of SO₂ exposure are felt very quickly. Most people feel symptoms within minutes of breathing it in. Moreover, the long-term effects are quite severe, sometimes resulting in decreased lung capacity. It irritates the respiratory system and causes inflammation and tightening of airways leading to difficulty in breathing. Longer exposures can aggravate existing heart and lung conditions. People with asthma, the elderly, and children are even more sensitive to SO₂ exposures. 

Sulfur Dioxide Environmental Effects

SO₂ has a direct negative impact on the natural environment. At high concentrations, SO_x can harm trees and plants by damaging foliage and limiting plant growth. When deposited, SO₂ turns the leaves of the trees and plants yellow.

 

SO₂ present in the atmosphere can mix with moisture and other chemicals in the air forming acids, which fall to the earth as acid rain.

Acid rain can harm sensitive   ecosystems:

• It can cause direct harm to trees and plants by damaging exposed tissues and, subsequently,  decreasing plant growth
• Also affects aquatic ecosystems such as streams and lakes turning them acidic and toxic to aquatic life.
• It changes the makeup of the soil affecting its quality and fertility.

Particulate matter formed in the atmosphere due to SO_x emissions, when inhaled, can penetrate deeply into the lungs and in sufficient quantities can cause severe health problems including damaging lungs, heart, and even the brain. The deposition of these particles can also stain and damage stone and other materials, including culturally important objects such as statues and monuments. Additionally, the sulfate-containing particles can contribute to the formation of smog and thick haze that reduces visibility in addition to impacting health. 

Possible corrective measures

 

The primary step is SO₂ monitoring to identify the areas with high SO₂ levels or areas where air quality does not meet the national SO₂ standards. In addition, here are some of the corrective measures:

• Promote the use of alternative renewable energy such as solar, wind, or hydro to produce power.
• Promote the use of alternative fuels i.e. fuels with lower or no sulfur content
• Avoid living in the areas downwind of industrial sites and coal-based power plants.
• Also, avoid any physical activity outside when pollution levels are high, especially on hot days. Stay indoors until it cools down outside and the air is fresh.

How to measure SO2 in air

 

They are methods for accurately measuring SO2 levels in the air. and It is critical for regulatory, environmental, and public health assessments.

How to Measure SO2 in Air

 

Different working principles for sulfur dioxide monitoring in the ambient environment are U.V. fluorescence, semiconductor, and electrochemistry.

U.V. Fluorescence

 

The SO₂ monitor working on UV fluorescence is based on the emission of a characteristic fluorescence when SO₂ molecules are irradiated by U.V. light. When the air sample taken by the SO₂ monitor is exposed to the beam of U.V. radiation in the region of 190-230 nm,  SO₂ gas present in the sample gets excited by the absorption of this light. They emit characteristic fluorescence radiation of 320-380 nm when they return to their ground state which is passed through a filter that only allows the radiation of this particular wavelength to pass through and is recorded by the detector (photomultiplier tube). The higher the SO₂ concentration in the air sample, the greater will the fluorescence be. This method for SO₂ monitoring is the most widely used principle in analyzers. 

 

Semiconductor method for SO₂ monitoring

 

When a metal oxide semiconductor-based SO₂ monitor is exposed to an air sample, the sulphur dioxide molecules react on the metal oxide surface of the sensor and dissociate into charged ions which alter the resistance of the film. This interaction is measured as a signal and is converted to the respective gas concentration. However, when compared to other SO₂ monitors, the energy consumption of such sulphur oxides monitors is higher.

Electrochemical method for SO₂ monitoring

 

The electrochemical principle works on the diffusion of sulfur dioxide gas into the respective sensor which results in the production of electrical signals proportional to the SO₂ concentration. It allows accurate measurement of even low concentrations of SO_x, which is essential in SO₂ monitoring for the ambient air. 

 

Among all the above principles of SO_x monitoring, applications like ambient air monitoring prefer sulphur dioxide monitors based on electrochemistry. This is because they yield more accurate SO_x concentrations and are inexpensive in comparison with the others.

Oizom’s working principle for SO₂ monitoring

 

5 Reasons Why SO2 Monitoring is Important

 

• Sulfur dioxide is one of the critical air pollutants. The combustion of sulfur-containing fuels, majorly emitted from coal-based energy production results in poor air quality.
• SO₂ is a precursor of other sulfur oxides such as SO₃ and many secondary air pollutants including particulate matter, smog, acid rain, etc. which have a negative impact on human health and the environment.
• Exposure to high levels of SO2 can lead to long-term effects on the respiratory system, especially in children, the elderly, and people already suffering from respiratory issues.  
• SO2 monitoring is an efficient way to detect and reduce exposure to sulfur oxides which further benefits in reducing the formation of particulate sulfur pollutants like fine sulfate particles, acid rain, and smog. 
• Real-time monitoring of SO2 levels helps in calculating the air quality index to deliver health advisories as well as formulating an action plan to meet standards.  

FAQs

 

1)Where does sulfur dioxide come from?

Sulfur dioxide (SO2) is primarily produced from the burning of fossil fuels like coal and oil in power plants and industrial processes. It can also be released during volcanic eruptions and from certain chemical reactions involving sulfur-containing materials

2)what is so2 in air quality

SO2, in terms of air quality, refers to sulfur dioxide, which is a harmful gas present in the atmosphere. It is one of the key pollutants monitored to assess air quality and its impact on human health and the environment.

3)What is the full form of SO2?

The full form of SO2 is “Sulfur Dioxide.” It is a chemical compound composed of one sulfur atom and two oxygen atoms, represented by the molecular formula SO2.