The Role of Aerosols in Urban Air Pollution

On a clear morning, you can see the skyline of any major city for miles. But on a hazy, still afternoon, that same skyline disappears behind a brownish-grey veil. That veil has a name: smog. And at its core, driving its chemistry, its colour, and its danger to public health, are aerosols. Understanding the role aerosols play in urban air pollution isn’t just an academic exercise – it directly shapes how we regulate emissions, design cities, and protect human health.

What are atmospheric aerosols?
Aerosol formation during smog events
Sulfate aerosols from SO₂ oxidation
Nitrate aerosols from NOₓ chemistry
Secondary organic aerosols
Impact on visibility and urban air quality
Health consequences of aerosol pollution
Why aerosol chemistry matters for policy

What are atmospheric aerosols?

The term aerosol refers to any solid particle or liquid droplet suspended in the atmosphere. These particles range enormously in size – from coarse dust grains visible to the eye, down to ultrafine particles far smaller than the width of a human hair. In environmental chemistry, aerosols are broadly categorised as primary aerosols (emitted directly from sources like vehicle exhaust, industrial smokestacks, construction dust, and wildfire smoke) and secondary aerosols (formed in the atmosphere through chemical reactions involving gaseous pollutants).

In urban environments, secondary aerosol formation is especially significant. Gases such as sulfur dioxide (SO₂), nitrogen oxides (NOₓ), volatile organic compounds (VOCs), and ammonia react in the atmosphere to produce fine particulate matter. The resulting particles are classified by size: PM10 (particles with a diameter of 10 micrometres or less) and PM2.5 (particles 2.5 micrometres or less in diameter). PM2.5 is of greatest concern because its tiny size allows it to penetrate deep into the lungs and even enter the bloodstream.

Aerosols don’t just hover passively. They scatter and absorb sunlight, alter cloud formation, and – critically in the context of smog – serve as reactive surfaces where further atmospheric chemistry takes place. Airborne particulate matter reduces visibility and is the primary reason smog appears brown or grey over cities. It is one of the defining signatures of urban air pollution worldwide.

Aerosol formation during smog events

Smog is not a single substance – it is a complex, evolving mixture of gaseous and particulate pollutants. Its formation involves both primary pollutants (emitted directly) and secondary pollutants that are produced when primary emissions undergo chemical reactions in the atmosphere. Aerosols are central to this secondary formation process.

Sulfate aerosols from SO₂ oxidation

Sulfur dioxide is released primarily from the combustion of coal and fuel oil in power plants and industrial facilities. Once in the atmosphere, SO₂ is oxidised – through gas-phase reactions driven by hydroxyl radicals (OH), and through aqueous-phase reactions in cloud droplets and aerosol water – to form sulfuric acid (H₂SO₄). This highly hygroscopic compound readily condenses to form sulfate aerosols, which are a major component of fine particulate matter in polluted urban air.

Research published in Science Advances demonstrated that in the presence of high humidity, alkaline aerosol components trap SO₂, which is then oxidised by dissolved NO₂ to form sulfate, explaining the explosive growth of PM2.5 during severe winter haze events. This means that sulfate formation is not just a gas-phase process – it is strongly promoted by the aerosol particles already present, creating a feedback loop that worsens pollution episodes.

Nitrate aerosols from NOₓ chemistry

Nitrogen oxides – primarily nitric oxide (NO) and nitrogen dioxide (NO₂) – enter the atmosphere from vehicle exhausts, power plants, and industrial combustion. Trucks and automobiles account for roughly half of all nitrogen oxide emissions, with power generation contributing most of the rest. In the atmosphere, NOₓ undergoes oxidation to form nitric acid (HNO₃), which reacts with ammonia (NH₃) to produce ammonium nitrate aerosols – another key fraction of fine urban particulate matter.

During photochemical smog events, NOₓ also plays a dual role: it reacts with VOCs in the presence of sunlight to generate ground-level ozone, while simultaneously contributing to nitrate aerosol formation. The interaction is complex. NOₓ oxidation forming nitric acid, which then reacts with ammonia to produce ammonium nitrate aerosols, is one of the key pathways through which gaseous emissions are converted into solid and liquid particles that accumulate in the lower atmosphere.

Secondary organic aerosols

Beyond inorganic sulfate and nitrate, urban air also contains secondary organic aerosols (SOA). These form when VOCs – released from vehicle exhausts, solvents, paints, and natural vegetation – are oxidised in the atmosphere and produce low-volatility compounds that condense onto existing particles. Secondary pollutants formed in photochemical smog include sulfate aerosols, nitrate aerosols, and organic aerosols, and their formation is most intense during summer due to higher solar radiation driving the underlying photochemistry.

The presence of SO₂ accelerates SOA formation by increasing particle acidity, promoting heterogeneous reactions on existing particles, and facilitating new particle nucleation via sulfuric acid. In this way, different secondary aerosol types are chemically interconnected and mutually reinforcing during smog episodes.

Impact on visibility and urban air quality

One of the most immediate and visible consequences of aerosol loading in urban air is the reduction of atmospheric visibility. Aerosol particles – particularly fine PM2.5 – scatter and absorb sunlight, creating the characteristic haze that settles over cities during pollution events. Fine particles (PM2.5) are the main cause of reduced visibility (haze) in many urban and industrial regions. The brownish colour of photochemical smog is primarily attributed to nitrogen dioxide and the light-scattering properties of fine particulate matter.

Visibility impairment is not merely aesthetic. It affects aviation and road safety, and it serves as a direct, observable indicator of the concentration of harmful particles in the air. Studies from Wuhan, China, showed that during COVID-19 lockdowns – when industrial and vehicular activity dropped sharply – PM2.5 concentrations decreased significantly and visibility increased by over 100%, directly demonstrating the link between aerosol concentrations and atmospheric clarity. Ammonium nitrate, a secondary aerosol, was identified as the dominant driver of visibility impairment in that study.

Health consequences of aerosol pollution

The health impact of fine aerosols in urban smog is severe and well-documented. Because of their tiny size, PM2.5 particles bypass the body’s natural defences and penetrate deep into the respiratory tract. Short-term exposures to PM2.5 have been associated with premature mortality, hospital admissions for heart and lung conditions, asthma attacks, acute bronchitis, and restricted activity days, with the most vulnerable populations being infants, children, the elderly, and those with pre-existing cardiovascular or pulmonary conditions.

Long-term exposure carries even graver risks. Chronic exposure to PM2.5 has been linked to reduced lung function in children, cardiovascular disease, and lung cancer. The International Agency for Research on Cancer (IARC) concluded that outdoor particulate matter is a cause of lung cancer. A large study published in The Lancet Planetary Health found that approximately 86% of the world’s urban population lived in areas exceeding the WHO’s annual PM2.5 guideline, resulting in an estimated excess of 1.8 million deaths in 2019 alone.

The World Health Organization updated its air quality guidelines in 2021 to reflect the growing evidence of harm at lower concentrations than previously thought, setting a stricter annual guideline of 5 µg/m³ for PM2.5. Most cities in the developing world – and many in the developed world – still far exceed this threshold.

Urban geography compounds the problem. Tall buildings block wind, preventing pollutants from dispersing. Temperature inversions – where a warm layer of air traps cooler, polluted air near the surface – can cause aerosol concentrations to build for days. Cities in valley basins or surrounded by mountains, like Los Angeles or Delhi, are particularly susceptible. According to one estimate, air pollution causes around 10,500 deaths in Delhi every year, with aerosol-laden smog playing a central role during the post-monsoon to winter season.

Why aerosol chemistry matters for policy

The complex, multi-pathway nature of aerosol formation means that addressing urban air quality requires tackling multiple precursor emissions simultaneously. Reducing SO₂ alone won’t be sufficient if NOₓ and ammonia levels remain high – the chemistry simply shifts toward different secondary aerosol pathways. The US EPA estimates that its vehicle emissions standards will prevent tens of thousands of premature deaths annually by 2030, underscoring that regulating the sources of aerosol precursors – particularly from the transport sector – is one of the most direct levers available for protecting public health.

Effective policy requires understanding that aerosols are not static pollutants. They are chemically reactive, evolve over time, interact with humidity and temperature, and affect each other’s formation. Sulfate aerosols, for example, increase aerosol water content, which in turn accelerates nitrate formation, which further increases particle mass. This feedback means that small reductions in precursor emissions can have disproportionately large effects on fine particulate concentrations – and on the health of millions of people living in cities around the world.

What do you think? Given that aerosol formation involves multiple interacting pollutants – SO₂, NOₓ, VOCs, and ammonia – should air quality policies target all of these simultaneously, or is focusing on a single dominant precursor a more practical approach for cities in developing nations? And considering that secondary aerosols form through atmospheric chemistry rather than direct emission, how should regulators measure and attribute responsibility for aerosol-related health outcomes?

References

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Table of Contents

What are atmospheric aerosols? 🔗

Aerosol formation during smog events 🔗

Sulfate aerosols from SO₂ oxidation 🔗

Nitrate aerosols from NOₓ chemistry 🔗

Secondary organic aerosols 🔗

Impact on visibility and urban air quality 🔗

Health consequences of aerosol pollution 🔗

Why aerosol chemistry matters for policy 🔗