Sources of Air Pollutants

Depending on their origins or the sources, air pollutants might have a natural, man-made, or mixed origin. These pollutants, as well as their precursor gases, can come from both natural and man-made sources.

Natural sources of air pollutants include:

• Volcanic eruption – Volcanic eruptions release various hazardous gases (such as sulfur and chlorine) and particulate debris (ash particles). However, they are generally limited to a small region.
• Forest fires – Wildfires release carbon monoxide and particulate matter into the atmosphere (which can contain organic pollutants like PAHs); they can damage large areas, although they are usually limited and can be contained.
• Radioactive decay activities – Natural decay processes of the Earth’s crust release radon gas, which has the potential to collect in confined locations such as basements.
• Winds and air currents can mobilize pollutants from the ground and carry them over huge distances.
• Microbial decaying processes – Microorganisms found in all environments play an important role in the natural decaying of living creatures and environmental toxins, resulting in the natural release of greenhouse gases, particularly methane gas.
• Increasing temperatures cause an increase in the number of toxins released into the air from contaminated soil and water.

Man-made sources of air pollutants include: 

• Factory activities – release a range of metals into the air, which is absorbed by particulate matter dispersed in the air as a result of the processing of metallic raw materials (including the use of furnaces).
• Mining and smelting – release a range of metals adsorbed on particulate matter dispersed in the air as a result of the crushing and processing of mineral resources.
• Transportation — Internal combustion of various fuels generates a variety of air pollutants (gases such as carbon monoxide, sulfur oxides, and nitrogen oxides, as well as particulate matter). Also, through exhaust gases (usually gases such as oxides of carbons, sulfur, nitrogen, as well as organic chemicals as PAHs)
• Mine deposits – owing to their minor particle nature (coming from the crushing and processing of mineral ores). Mine tailings are a source of metals in the ambient air that the wind across broad distances may transport.
• Accidental spills and leaks of stored chemicals and the handling and storage of chemicals – particularly volatile inorganic compounds – can result in the release of organic and inorganic pollutants.
• Construction and demolition activity – numerous construction materials harm the air. The destruction of ancient buildings, which may contain various prohibited substances such as PCBs, PBDEs, and asbestos, poses a particular danger.
• Coal Power Plants — when burning coal, various gases and particulate matter, including metals (such as As, Pb, and Hg) and organic compounds (particularly PAHs), are released. Building heating releases a variety of gases and particulates as a result of the combustion of fossil fuels.
• Burning garbage – Various harmful gases and particulate matter are discharged into the atmosphere as a result of waste incineration, depending on the waste composition.
• Landfill disposal procedures – typically create methane when natural microbial decomposing activity in the disposal region intensifies.
• Controlled burning in forest and farm management – this includes gas and particulate matter emissions from controlled burning (similar to wildfires described above)
• Military operations – may expose people to harmful chemicals as a result of drills and training;
• Smoking releases various harmful substances, including organic and inorganic compounds, some of which are carcinogenic.
• Storage and usage of home items containing organic solvents that volatilize in the air (thus the odor we all sense while using them).

Classification of Air Pollutants

Based on sources, pollutants in the air can be classified as primary or secondary. 

Primary pollutants – Primary pollutants are directly discharged into the environment. (NOX), ammonia (NH3), methane (CH4), carbon monoxide (CO). Some non-methane volatile organic compounds (NMVOCs), including benzene, certain metals, and polycyclic aromatic hydrocarbons, including benzo[a]pyrene, are all examples of primary air pollutants.

Secondary pollutants – Pollutants generated in the environment from precursors of chemical reactions and microphysical processes. PM, ozone (O3), NO2, and many oxidized volatile organic chemicals are important secondary air pollutants. Sulfur dioxide (SO2), nitrogen oxides (NOX), ammonia (NH3), and volatile organic compounds (VOCs) are all important precursors gases for secondary pollutants.

The relationship between air pollution emissions in the atmosphere and air quality is complicated. Height of emission, chemical changes, solar interactions, extra natural and hemispheric input, and the influence of weather and geography are all factors to consider. The following are some of the most hazardous air pollutants:

• Particulate matter – Particulate matter is a combination of solid and liquid aerosol particles that come in various sizes and chemical compositions. PM is either directly discharged as primary particles or develops in the atmosphere due to precursor pollutants such as SO2, NOX, NH3, and NMVOCs being released into the sky. PM is released by various anthropogenic sources, including combustion and non-combustion. PM’s natural sources include sea salt and Saharan dust carried by the wind.
• Hydrocarbons – Automobiles that operate on these hydrocarbon-based fuels are responsible for 60 percent of air pollution. The incomplete combustion of hydrocarbon fuels is now the major source of pollution. Hydrocarbons react with Nitrogen Oxides as a result of this (NO2). When the gases that arise are exposed to the sun’s rays, ozone forms. In this situation, ozone is a contaminant that creates smog. This incomplete combustion of hydrocarbons has a significant impact on air quality, generating a variety of concerns ranging from illnesses to visibility challenges.
• Carbon monoxide – CO is a colorless, odorless gas that may be dangerous when inhaled in excessive volumes. CO emits from combustion. Cars, trucks, and other fossil-fuel-burning vehicles and machinery are the most significant emitters of CO in the outdoor air. A range of household equipment releases CO. Inhaling CO-containing air is hazardous. CO limits the quantity of oxygen in the bloodstream and gets transferred to vital organs like the heart and brain. CO can induce dizziness, disorientation, unconsciousness, and death at very high levels, conceivable indoors or in other confined places.
• Oxides of nitrogen – Nitrogen oxides are released into the atmosphere by industrial sites and the transportation sector because of burning fuel. Nitrogen monoxide (NO) and nitrogen dioxide (NO2) are two gases that make up the NOX group (NO2). The bulk of NOX emissions is made up of NO. NOX is a component of ozone and particulate matter production.
• Oxides of sulfur – Sulphur dioxide is produced and released by burning fossil fuels (mainly coal and oil) to create electricity. SO2 levels beyond a certain threshold are linked to various health and environmental consequences. SO2 levels are higher in the proximity of big industrial operations. Because SO2 emissions are a substantial precursor to ambient PM2.5 concentrations, they are a significant environmental concern.
• Photochemical smog – When nitrogen oxides and volatile organic compounds (VOCs) combine with sunlight, photochemical smog creates a brown cloud over cities. It happens more frequently in the summer since that is when we get the most sunshine. Photochemical smog may hurt the environment, people’s health, and even materials. The dark haze observed above numerous cities is the most noticeable impact of light scattering by extremely minute liquid and solid particles from the brown tint.

Meteorology and Air Pollution

Warm air rises over colder air, preventing vertical mixing and resulting in stable atmospheric conditions. An atmospheric inversion is a term used to describe this situation.

While wind speed and direction are concerned with air movement horizontally, atmospheric stability is concerned with the factors that move air vertically. High- and low-pressure systems that lift air over topography and mix it with the upper atmosphere have the greatest impact on vertical air movement or atmospheric stability.

 Atmospheric temperature and pressure are the processes that are especially responsible for vertical airflow. The sun’s energy is absorbed, stored, and reradiated by everything on Earth. Different sections of the planet and different types of surfaces heat up more quickly than others. This is called Differential heating.

The air above the ground is affected by the Earth’s differential heating. As heat flows away from a hot surface, the air directly above it becomes heated. Conduction and convection are the two primary mechanisms that cause this warmth. Conduction is the heat transmission that occurs when anything comes into contact with a hot surface. In this situation, the air comes into contact with the hot soil and absorbs part of the heat. The upward mixing of air is known as convection. When a parcel of air is warmer than the surrounding air masses, it expands, rises, and cools—the temperature and pressure of the airdrop as it expands. Cool air, on the other hand, behaves oppositely.

The Earth’s air flows in a three-dimensional movement. Turbulence is the term for this type of movement. Turbulence can be caused by one of two processes: mechanical or thermal turbulence. 

Thermal turbulence is caused by atmospheric heating, whereas mechanical turbulence happens due to air passing through an object. Both forms of turbulence commonly occur during atmospheric air movements, albeit one type may predominate under particular conditions.

 Thermal turbulence dominates on bright sunny days with little breezes; for example, mechanical turbulence dominates on a windy night with neutral atmospheric stability. Turbulence has the net impact of speeding up the pollution dispersion process. On the other hand, mechanical turbulence can create downwash from a pollution source, resulting in high pollutant concentrations immediately downstream.

Lapse Rate 

Up to a height of roughly 10 kilometers, the temperature in the troposphere declines with altitude. The lapse rate is the rate at which temperature drops as you get higher. The usual lapse rate is defined as a decline of –0.65°C/100 m on average. An adiabatic process occurs when a parcel of air is hoisted into the atmosphere and then allowed to increase and cool or compress and warm with a change in atmospheric pressure but no heat exchange. The air parcel must also be unsaturated, with a consistent adiabatic cooling or warming rate. The dry adiabatic lapse rate is defined as the rate of heating or cooling for unsaturated air at a temperature of 10°C/1000 meters, with the water staying in a gaseous form.

Individual vertical temperature readings can differ significantly from the usual or dry adiabatic lapse rate. The environmental lapse rate is variations in temperature with height for a certain observed site. Environmental lapse rates are used to determine atmospheric stability and directly influence vertical air movement and pollutant dispersion.

Plume Behaviour

Between atmospheric stability and pollution concentrations, there is a vital link. Pollutants that cannot be carried or dispersed into the higher atmosphere get stuck at ground level, posing serious health and environmental concern. The behavior of emission plumes from industrial smokestacks exemplifies this connection. Looping plumes, coning plumes, lofting plumes, fanning plumes, fumigating plumes, and trapping plumes are six forms of air pollution plumes that show the link between atmospheric stability and pollutant emissions.

Looping plume – Pollution discharged into an unstable environment results in looping plumes. Plumes that seem billowing and puffy may occur from rapid changes in temperature and pressure. High quantities of air pollution brought down by cooling air might be dangerous if confined at ground level, even though unstable circumstances are normally advantageous for pollutant dispersion. This can happen on bright days with light to moderate breezes, causing the stack gases to travel up and down in a wavy fashion, resulting in a looping plume.

Coning plume – Large billows or puffs of pollutants characterize a coning plume created by neutral or somewhat unstable circumstances. Coning plumes are most common on partly cloudy days when the atmosphere alternates between warming and cooling. Warm gases mixed with chilly surrounding air expand and climb to the higher atmosphere.

Lofting plume – Warm air lingers above chilly air when the atmosphere is reasonably steady, forming an inversion layer. Pollutants emitted below the inversion layer will remain confined at ground level, preventing upward transfer in the absence of any atmospheric instability. Pollutants tend to develop in large concentrations at ground level when there is little or no vertical mixing. Stack gases above the inversion layer generate a lofting plume that can efficiently disseminate pollution into the upper atmosphere when conditions are unstable or neutral above that level.

Fanning plume – A fanning plume is characterized by lengthy, flat streams of pollutant emissions that occur during steady circumstances. Because air pressure is steady, there is no tendency for emissions to increase or fall, allowing the pollutant to be transported and dispersed by (horizontal) wind velocity. Fanning plumes are observed in the morning hours, just when the sun begins to warm the atmosphere, and the winds are still mild.

Fumigating plume – A severe air pollution incident might occur if the plume is released just below the inversion layer in the early morning. Gaseous emissions quickly cool and drop to ground level when pollutants are discharged below the inversion layer. This process is known as fumigation, and it results in a high level of pollution that may be harmful to both individuals and the environment. This atmospheric state describes the most damaging form of air pollution episode.

Trapping plumes – However, a trapped plume is generated on bright, sunny days or clear nights with little breezes. An unstable air mass causes an inversion layer both above and below the plume, resulting in a trapped plume. In contrast to a fumigating plume, a trapped plume is one of the most beneficial forms of plume for pollution dispersal. Ground sources are protected from possible exposure by temperature inversions above and below the plume, while winds at altitude scatter and dilute the pollutant.

                                                                                                    Classification of Plume

Plume Rise Estimation

When looking at smoke from a stack, you’ll see that it generally rises past the stack’s top. Plume rise is the height at which the plume rises over the stack. The distance from the top or lower border of the plume to the imaginary centreline of the plume is plume rise estimation.

 The physical parameters of the stack determine how high the plume rises. For example, the effluent feature of stack temperature about ambient air temperature is more important than the stack feature of height. The temperature differential between the stack gas (TS) and ambient air (Ta) determines plume density, influencing plume ascent.

Momentum is determined by stack parameters, while effluent qualities determine buoyancy. The stack provides the effluent’s initial momentum. It is determined by the effluent’s departure speed from the stack. As the effluent leaves the stack, atmospheric conditions begin to impact the plume. 

The plume’s ascension will be primarily determined by the state of the atmosphere, particularly the winds and temperature profile along its path. The wind speed over the stack top begins to skew the plume as it rises from the stack. The speed of the wind normally rises as you get higher above the Earth’s surface. The stronger winds tip the plume even more upward as it rises. This procedure is repeated until the plume looks horizontal to the Earth. The point at which the plume seems to be horizontal might be a long way downwind from the stack. The plume will lean over quicker as the wind gets stronger.

The total of the starting height of the discharge (e.g., the top of the chimney) plus the plume rise owing to buoyancy and initial momentum of the discharge equals the height of the centreline of the plume H at a downwind distance, X. The model calculates chimney plume ascent using Briggs’ (1975) technique, using two formulas for (1) neutral and unstable situations; and (2) stable conditions.

Effluent Dispersion Theories

Wind direction and speed, atmospheric stability, plume ascent, and terrain all play a role in air pollution transmission and dispersion. The development of pollutant dispersion modeling is immensely beneficial for observing the results of these complex interactions and gathering data traceable to numerous natural and anthropogenic factors, and calculating the quantity of ground-level pollution at diverse distances from the source. 

As a result, modeling is a mathematical depiction of pollutant dispersion and the variables that affect it. Scientists utilize computers to create visual representations of air pollution transport and dispersion as an extension of these mathematical models. 

We require information regarding the pollutant source to construct an accurate model of how air pollution is carried and disseminated for a specific location. Surrounding geographic elements, features, quantity, and types of pollutants discharged, effluent gas conditions, stack height, and important climatic aspects are all examples of this information. Scientists can accurately anticipate how contaminants will be spread into the atmosphere using this sort of data as input to a computer model. Furthermore, pollutant concentration levels may be determined at various distances and orientations from the smokestack’s location.

Types of Pollutant Dispersion:

(a) Pollutant injection distribution within and outside the air cavity.

(b) The impact of streamlining an impediment during the effluent stack design phase.

The permitting procedure for new and existing industrial sites heavily relies on air dispersion modeling. Dispersion modeling can estimate if a proposed source would exceed its portion of an authorized air increment within the facility’s Air Quality Management Area for New Source Review (NSR) standards.

Model for Population Exposure Assessment (ASPEN) – Based on meteorological, chemistry, and the rates at which air toxins are discharged into the atmosphere, the Assessment Population Exposure Model determines ambient air levels. The Hazardous Air Pollutant Exposure Model (HAPEM4) is now utilized in conjunction with ASPEN’s ambient concentration. Which outputs as a screening tool to analyze national exposure levels of certain harmful air pollutants. Estimated exposures can then be paired with quantitative health effect data to estimate population-level health risk.

Model of Industrial Source Complexity (ISC) – The HEM is a general tool, but the Industrial Source Complex Model is more particular and exact. It forecasts pollution levels at specific places based on local data. The Industrial Source Complex Model (ISC) is a steady-state Gaussian plume model. That may predict air pollutant concentrations from a range of sources connected with an industrial source complex. On the other hand, both models are only tools for scientists to use when assessing air pollution dispersion. The models’ accuracy is restricted by the inherent difficulties of simplifying complex and linked processes that impact air pollution transport and dispersion.

Also Read:How Can I Contribute To The Reduction of Air Pollution?