Atmospheric Aerosols

Introduction

Every breath of city air contains tens of thousands of particles per cubic centimetre, most of them far too small to see. These atmospheric aerosols — sea salt, dust, soot, sulfate, nitrate and organic matter — scatter and absorb sunlight, seed every cloud droplet on Earth and penetrate deep into human lungs. Outdoor fine particulate pollution is estimated to cause several million premature deaths worldwide each year. This page explains where particles come from, how their sizes evolve, and why a few micrograms per cubic metre matter so much for both health and climate.

Core explanation

Primary and secondary aerosol. Primary particles are emitted directly as particles: sea spray, wind-blown mineral dust, pollen, volcanic ash, and combustion soot (black carbon) from diesel engines and biomass burning. Secondary particles form in the air when gases are oxidised to products of low volatility that condense. The main examples are:

- sulfate, from SO₂ oxidised to H₂SO₄; - nitrate, from NOₓ oxidised to HNO₃, which reacts with ammonia to form ammonium nitrate: NH₃ + HNO₃ ⇌ NH₄NO₃(s); - ammonium sulfate, formed as NH₃ neutralises sulfuric acid; - secondary organic aerosol (SOA), from the oxidation of volatile organic compounds such as isoprene, monoterpenes and aromatic hydrocarbons by OH, O₃ and NO₃.

Across much of the continental lower atmosphere, secondary aerosol makes up the majority of fine particle mass.

Size modes. Particle diameters span from about 1 nm to over 10 µm, and the number distribution shows distinct modes:

Mode Diameter Origin Removal --- --- --- --- Nucleation below 0.01 µm new particles from H₂SO₄, NH₃, amines and organic vapours rapid coagulation Aitken 0.01–0.1 µm growth of nuclei, fresh combustion particles coagulation, growth Accumulation 0.1–1 µm coagulation and condensation, cloud processing mainly rain-out Coarse above 1–2.5 µm mechanical: sea spray, dust, pollen settling

Small particles dominate the number count, while accumulation and coarse particles dominate the mass . The accumulation mode is the longest lived — about a week — because these particles are too large to coagulate quickly and too small to settle, so they are removed mainly when they act as cloud nuclei and fall in rain.

Health effects. Deposition in the respiratory tract depends on size. Coarse particles are trapped in the nose and upper airways; PM2.5 reaches the alveoli; ultrafine particles (below 0.1 µm) can cross into the bloodstream. Long-term PM2.5 exposure is linked to heart disease, stroke, lung cancer and chronic respiratory disease. The World Health Organization's 2021 guideline for annual mean PM2.5 is 5 µg m⁻³, a level exceeded in most of the world's populated areas.

Climate effects. In the direct effect , particles scatter sunlight back to space (cooling) or, for black carbon, absorb it (warming). In the indirect effect , more cloud condensation nuclei spread the same water over more, smaller droplets, making clouds brighter and possibly longer lived. Overall, anthropogenic aerosol has a net cooling effect that offsets part of greenhouse warming, but its magnitude is the largest single uncertainty in estimates of human radiative forcing.

Step-by-step reasoning

To predict how a new particle source will affect air quality and climate:

1. Decide whether the particles are primary or formed from precursor gases. 2. Estimate the size range, which controls lifetime, removal route and lung penetration. 3. Identify the composition: scattering (sulfate, nitrate) or absorbing (black carbon). 4. Consider ability to act as cloud nuclei, which depends on size and solubility. 5. Combine these to judge local health impact and net direct and indirect climate influence.

Visual explanation

Draw a graph of particle number against diameter on a logarithmic axis from 0.001 to 10 µm. Mark a tall narrow peak near 0.01 µm (nucleation), a broader peak near 0.05 µm (Aitken), a peak near 0.3 µm (accumulation) and a low hump near 5 µm (coarse). Add arrows showing condensation and coagulation moving particles to the right, and label removal by settling and rain-out.

Real-world analogy

The accumulation mode is like a queue of mid-sized boats in a harbour. Tiny dinghies quickly bump into larger boats and are tied on, while heavy ships sink or run aground. The mid-sized boats neither merge readily nor sink, so they stay afloat longest and are only cleared when a storm (rain) sweeps them away.

Real-world example

In winter, cities such as Delhi and Beijing have experienced daily PM2.5 above 200 µg m⁻³. Analysis of the particles shows large fractions of sulfate, nitrate, ammonium and organic matter formed in the air from traffic, industry, crop burning and agricultural ammonia, so controlling precursor gases — not just visible smoke — is essential.

Why?

Why are secondary particles so abundant even where few particles are emitted directly? Oxidation adds oxygen-containing groups to gas molecules, sharply lowering their vapour pressure. Once a product such as H₂SO₄ or a highly oxidised organic compound is far less volatile than its parent, it condenses onto existing particles or nucleates new ones.

Common misconception

"Air that looks clear contains no harmful particles." Visibility is reduced mainly by accumulation-mode particles at high concentrations. Air can look reasonably clear while containing enough PM2.5 and ultrafine particles to exceed health guidelines, and invisible nanoparticles dominate the number count.

Worked example

Question: A spherical sulfate particle has diameter 0.5 µm and density 1.8 g cm⁻³. What is its mass, and how many such particles per cubic metre give PM2.5 = 10 µg m⁻³?

Reasoning: Radius = 0.25 µm = 2.5 × 10⁻⁵ cm. Volume = (4/3)π(2.5 × 10⁻⁵)³ ≈ 6.5 × 10⁻¹⁴ cm³. Mass = 1.8 × 6.5 × 10⁻¹⁴ ≈ 1.2 × 10⁻¹³ g = 1.2 × 10⁻⁷ µg. Number = 10 ÷ 1.2 × 10⁻⁷ ≈ 8 × 10⁷ m⁻³.

Answer: About 1.2 × 10⁻¹³ g per particle; roughly 8 × 10⁷ particles m⁻³ (about 80 per cm³).

Quick check

1. Why do accumulation-mode particles stay in the atmosphere longer than either ultrafine or coarse particles? Answer: They are too large to coagulate rapidly and too small to settle, so they are removed mainly by rain.

Exam focus

Define primary and secondary aerosol with examples, sketch and label the size modes, and link each mode to its formation and removal. Explain the direct and indirect effects and state that net aerosol forcing is negative but highly uncertain.

Advanced insight

New-particle formation was long attributed to H₂SO₄ and water alone, but chamber experiments at CERN showed that ammonia, amines and highly oxygenated organic molecules greatly enhance nucleation, and that pure biogenic organics can nucleate without sulfuric acid. This matters for estimating the pre-industrial aerosol background against which human forcing is measured.

Summary

Aerosols are primary (sea salt, dust, soot) or secondary (sulfate, nitrate, ammonium, organic aerosol formed by gas oxidation). Particles fall into nucleation, Aitken, accumulation and coarse modes, with the accumulation mode longest lived. Fine particles penetrate deep into the lungs and are a major health risk. Aerosols cool the climate by scattering and brightening clouds, with black carbon warming, and remain the largest uncertainty in radiative forcing.

Practice questions

1. Give two examples of primary aerosol and two of secondary aerosol. Answer: Primary: sea salt and black carbon soot (also dust, pollen). Secondary: ammonium sulfate and secondary organic aerosol (also ammonium nitrate). 2. Explain why controlling agricultural ammonia can reduce urban PM2.5. Answer: Ammonia neutralises HNO₃ and H₂SO₄ to form ammonium nitrate and ammonium sulfate particles, so less NH₃ means less secondary inorganic particle mass. 3. Describe the aerosol indirect effect on clouds. Answer: More cloud condensation nuclei divide the available water among more, smaller droplets, making clouds more reflective and possibly longer lived, which cools the climate. 4. Why are ultrafine particles of particular health concern despite contributing little mass? Answer: They are extremely numerous, have a large surface area, penetrate to the alveoli and can enter the bloodstream.