The invisible
atmosphere.

Aerosols begin as dust, spray, smoke and gases. Their story is a story of size, chemistry and change.

5 MIN READEXPLANATION · METHODS · EVIDENCEBEGIN READING ↓

THE QUESTION

What is in the air, and what happens to it?

A suspension, not a substance

A clear view is not a particle-free view. Between you and the horizon, air carries a shifting population of solids and liquid droplets.

An aerosol is a suspension of particles in a gas. In atmospheric science, the word often also refers to the suspended particles themselves. Mineral dust, sea salt, organic material, sulphate, nitrate and black carbon are common constituents. The air contains gases too, but a sulphur dioxide molecule is not a sulphate particle. That distinction matters: a gas can become the raw material for particles through atmospheric chemistry.

Clouds contain suspended droplets and ice crystals, but atmospheric researchers usually distinguish cloud particles from the smaller, unactivated aerosol population. A particle can pass from one category to the other when it becomes the nucleus of a cloud droplet.

MINERAL DUSTLifted from land

SEA SALTReleased by spray

BLACK CARBONFormed in combustion

SULPHATEFormed from gases

02The invisible atmosphere

Emitted, or made in the air?

Primary particles enter the atmosphere as particles. Wind lifts mineral grains from exposed surfaces; breaking waves produce spray; combustion emits soot and organic particles. Secondary particles form from gases. Oxidation can turn sulphur dioxide into sulphuric acid, while reactions involving organic gases produce compounds that condense into the particle phase.

Nature and human activity both contribute, often within the same air mass. Dust emissions depend on land condition as well as wind; fires may be natural or human caused; combustion produces gases and particles together. A source label alone does not specify optical behaviour or chemical composition.

Nor does a particle keep its original identity intact. A soot aggregate can acquire a coating. Dust can mix with soluble material. A fresh emission measured beside a source may behave differently after hours of sunlight, humidity and chemical processing.

Particle life cycle, schematic. Circle sizes indicate growth conceptually; no size scale or fixed lifetime is implied.

03The invisible atmosphere

Size changes the question

Aerosols span a broad size range, from newly formed particles only a few nanometres across to coarse particles measured in micrometres. One micrometre (µm) is a thousand nanometres (nm). A diameter tells us something different from a radius: the diameter is twice the radius.

Number, surface area and mass weight that size range differently. Many tiny particles can dominate a number count while relatively few large particles carry much of the mass. For spherical particles of equal density, mass increases with the cube of radius. Doubling radius multiplies a particle’s mass by eight.

Optical measurements add another weighting, because interaction with light depends on the size relative to wavelength and on the material. Two populations with the same total mass can therefore have different numbers, surface areas and optical depths.

Student note: what does PM₂.₅ mean?

PM₂.₅ is a size-selected mass measurement based on aerodynamic diameter with a nominal 2.5 µm cut. Aerodynamic behaviour depends on density and shape as well as geometric size. An optical particle diameter, an aerodynamic diameter and a mobility diameter need not be identical. Always check how a reported size was defined.

04The invisible atmosphere

A particle is a process

Condensation adds material from the gas phase. Coagulation joins particles when they collide: total particle number decreases, even though the process by itself conserves particle material. Evaporation can return volatile compounds to the gas phase. These changes alter the size distribution, not just one representative diameter.

Water is particularly important. Hygroscopic particles take up water as relative humidity rises, increasing their wet size and often their scattering of light. A humid afternoon can therefore change an optical signal without an equal increase in dry aerosol mass. The response depends on composition and humidity history.

During the Pinatubo balloon investigations over Hyderabad, the researchers used the angular pattern of scattered sunlight to infer changes in particle size. Growth was an interpretation of optical observations and a scattering model. It was not a direct film of individual particles merging. Read the study ↗

05The invisible atmosphere

The atmosphere also removes particles

Particles do not simply accumulate forever. They deposit on vegetation, water and land; larger particles settle under gravity. Clouds can incorporate particles, and precipitation can remove material from the atmosphere. The efficiency depends on particle size, cloud processes and the conditions along the journey.

These competing processes help explain why aerosols vary so sharply across space and time. A plume may dilute, grow, change composition or be rained out as it travels. A measurement is a sample of that evolving population at a particular place and time.

Persistence high in the stratosphere differs from persistence near the surface because the transport and removal environments differ. The long-lived volcanic layer and a shallow winter haze are both aerosol problems, but they call for different measurements and explanations.

06The invisible atmosphere

Ask what was measured

Before interpreting an aerosol number, attach a quantity and a unit. Number concentration counts particles per volume. Mass concentration reports material per volume, often in µg m⁻³. Extinction describes loss from a beam per distance, often in km⁻¹. Optical depth integrates extinction through a column and has no unit.

Composition and size determine how these quantities relate. There is no universal conversion between an optical depth and a ground-level mass concentration. A high layer can alter sunlight while leaving a surface instrument outside the layer.

At Gadanki, researchers combined spectral absorption and optical-depth measurements to investigate absorbing aerosols. The value of combining methods was precisely that one instrument could not describe the entire population. Read the study ↗

PAPERS & FURTHER READING