Reasons for Toxic Air

1. TRAP- Traffic Related Air Pollution

TRAP is a major source of air pollution in India, caused by slow moving traffic, which exacerbates pollution emissions due to heightened fuel consumption and diminished engine efficiency.

Researchers have found an 80 % increase in tailpipe emissions due to traffic congestion and that when vehicle speeds decline from 50 km/h to 20 km/h during congestion, emissions such as CO and CxHy can surge by approximately 50% due to partial burning of fuel.

Also, constant braking and friction with the road surface results in creation of NEEs (Non Exhaust Emissions) releasing PM 2.5 and PM 10 into the atmosphere containing brake dust, fine rubber particles, heavy metals and VOCs.

2. BUILDING DOWNWASH - A Civil Engineering Challenge

Building Downwash is a wind-related phenomenon where wind striking tall buildings and/or structures, is deflected downwards, creating strong downdrafts at street level. The intensity of downwash is influenced by factors like building height, shape, orientation, and the surrounding urban environment.

Buildings and similar structures in the path of air flow create a turbulent wake region, which is a low pressure area on the leeward (i.e., downwind) side of the building. A plume caught in the path of this flow is drawn into this area called the wake, temporarily trapping it in a recirculating cavity.

This downwash effect leads to higher ground-level pollutant concentrations near the building than if the building was not present.

Downwash effects can be classified as:

Stack and building downwash cause an increase of ground-level concentrations, because of two concomitant phenomena:

Building downwash effects are the cause of maximum concentrations in approximately 90% of dispersion modelling studies.

3. STREET CANYONS- Poor Urban Design

A 'Street Canyon' is defined as a confined urban space surrounded by buildings on both sides of a street, where natural air ventilation is limited, leading to the accumulation of pollutants from vehicle emissions and other sources, posing health risks to pedestrians and residents.

Street canyons vary in structure, including the aspect ratio (AR), asymmetry etc, which significantly influence wind field and pollutant dispersion within the street canyons.

Radiative geometry of canyon contributes to lowering the effective albedo of the urban environment, which means it absorbs more incoming solar radiation, mainly because of the multiple reflection of radiation between the canyon surfaces.

This same phenomenon is also responsible for poor signal reception in cell phones and TV signals in densely populated areas.

4. THE CANOPY EFFECT - When Trees Increase Pollution

The canopy effect in urban areas refers to how trees and other vegetation influence air pollution levels by modifying wind patterns, temperature, and pollutant concentrations.

While vegetation can help remove pollutants like PM2.5 and ozone through processes like deposition (particles sticking to leaves) and stomatal uptake (gases entering leaves through small pores), it can also trap pollutants within street canyons, because of the geometric structure of urban canyons, with buildings and by hindering wind flow. Trees can also emit volatile organic compounds (VOCs), which can react with other pollutants like nitrogen oxides (NOx) to form ozone.

5. URBAN HEAT ISLAND (UHI) - A self-perpetuating problem

The Urban Heat Island (UHI), is a phenomenon where near-surface air temperatures are higher than their non-urban surroundings, particularly at night.

Built urban sites have large areas of exposed surface per unit area of ground cover. Therefore, potentially more solar radiation can be collected on a built urban site than on flat open terrain. In urban areas, the lack of vegetation converts more incoming energy into sensible heat whereas, in rural areas, the energy is transformed into latent heat. The sensible heat is stored in the urban fabric and released slowly causing an urban heat island in the nighttime.

The primary factors contributing to this phenomenon are the properties of urban infrastructures, reduced sky visibility, decreased relative humidity and latent heat flux, increased sensible heat flux, diminished vegetation coverage, and limited open space in urban environments.

Rising pollution levels in urban areas may amplify the UHI effect because of the radiative impacts of various atmospheric pollutants that harm human health and contribute to smog formation and sometimes acid rain.

Dense urban areas with concrete structures and limited greenery absorb and retain heat, raising local temperatures and altering air circulation. This traps pollutants like PM, NOx, and ozone, while promoting secondary pollutant formation (e.g., ozone via VOC-NOx reactions).

6. THERMAL INVERSION - Why pollution is visible in winters

A thermal inversion is a meteorological phenomenon where a layer of warmer air sits above a layer of cooler air.

7. PLANETARY POLLUTION - Poor Airshed Management

An airshed is a common geographic area where pollutants accumulate and impact air quality. It includes the entire area over which the pollutants disperse due to meteorological and geographical factors. India has 6 large airsheds.

During the southwest monsoon, the prevalent direction of the wind is easterly. The winds travel from over the Bay of Bengal, carrying moisture and rainfall to this part of the country. The Easterlies carry pollutants, too, along with moisture and rainfall.

Once the monsoon withdraws, however, the predominant direction of the winds changes to north-westerly. These winds carry particulate matter (essentially smoke, dust and soot), and Ozone from other countries of Europe and Central Asia, into India.

8. TOPOGRAPHICAL CONSTRAINTS- Location, Temperature and Humidity

India's geography, particularly the Indo-Gangetic Plains and Himalayan foothills, traps pollutants due to low wind speeds and basin-like terrain, limiting dispersion of PM, SO2, and ozone. This affects northern India, where flat plains and Himalayan barriers create a "pollution bowl", exacerbating winter smog with primary and secondary pollutants as air pollution becomes trapped due to a combination of factors, including topography and weather patterns

9. IMPROPER WASTE MANAGEMENT - Waste Burning and Open Landfills

Improper disposal of hazardous waste substantially threatens air quality, leading to significant air emissions that contain harmful collutants.

Waste burning emits a wide range of toxic compounds, including dioxins, furans, heavy metals, CO2, PM and volatile organic compounds (VOCS), into the air, which, upon inhalation, lead to respiratory problems, cardiovascular diseases, neurological issues and even cancer upon prolonged exposure.

Open improperly managed landfills present another significant source of hazardous waste air emissions. When hazardous waste is disposed of in open landfills without appropriate containment measures, it can generate LEACHATE and GAS EMISSIONS.

Hazardous waste materials may interact with moisture in the landfill, forming leachate, a toxic liquid that can percolate through the landfill layers and eventually seep into the surrounding soil and groundwater. Leachate can release hazardous chemicals into the air when it evaporates.

Anaerobic decomposition of hazardous waste in landfills can also generate landfill gas, mainly methane, carbon dioxide, and other volatile compounds. If not captured and treated properly, these gases escape into the atmosphere, contributing to greenhouse gas emissions and air pollution.

10. IMPORTED POLLUTION – Dirty Trade- ignorance, apathy and greed

Imported pollution refers to hazardous materials and fuels that enter India through global trade. Weak regulation, cost incentives, and limited domestic recycling capacity have turned India into a major destination for dangerous substances banned or restricted elsewhere. Some major such pollutants are as under.