UISH

The analysis, validation and processing pages are full of acronyms whose meaning is taken for granted. This page explains them once: what each quantity is, in which units it is expressed, where the data comes from and why we look at it.

First of all: what “concentration” means

Almost everything that follows is a concentration, that is how much of a substance sits in a given volume of air. The usual unit is the microgram per cubic metre (µg/m³): one millionth of a gram in a thousand litres of air. These are tiny amounts — the daily limit for PM₁₀ is 50 µg/m³, fifty millionths of a gram per cubic metre — and yet at those levels the effects on health are measurable.

There are two other ways of expressing the same thing, and confusing them is the commonest mistake.

• Parts per billion (ppb) count molecules rather than mass. Converting ppb to µg/m³ depends on temperature, pressure and molecular weight: it is not a fixed factor. European networks publish in µg/m³, American ones often in ppb.
• The column (mol/m²) is what a satellite measures: not the concentration at a point, but the total along the whole height of the atmosphere. It is a different quantity, not an approximate version of the other — see below.

Carbon monoxide is the exception among the gases: being far more abundant it is expressed in milligrams per cubic metre (mg/m³), a thousand times the microgram. The processing chain converts it to µg/m³ for uniformity.

The gaseous pollutants

Nitrogen oxides: NO, NO₂, NO_x

They come from any high-temperature combustion, because the heat makes the nitrogen and oxygen of the air itself react. Traffic is the dominant source in cities.

What leaves an engine is largely monoxide (NO), which in the atmosphere oxidises into dioxide (NO₂) within minutes. The dioxide is the one regulated by law: it irritates the airways and is a precursor of both ozone and secondary particulate matter. The sum of the two is written NO_x and is used when what matters is the total emitted, regardless of how much has already transformed.

Nitrogen dioxide is the quantity best suited to studying the urban scale: it has a short life — a few hours — so its variations tell you what is happening nearby, not what arrived from far away.

Ozone (O₃)

It is the only one on the list that nobody emits: it forms in the atmosphere when sunlight makes nitrogen oxides and volatile organic compounds react. That is why it is called a secondary pollutant, and why its maximum is in the early afternoon in summer rather than at rush hour.

It behaves in a way that surprises newcomers: in a city centre ozone is often lower than in the suburbs or on the hills, because the nitrogen monoxide freshly emitted by traffic consumes it. It rises instead where the air has had time to react while moving away from the sources. This is why the stations measuring it are usually background ones, and fewer.

Sulphur dioxide (SO₂)

It comes from burning fuels that contain sulphur. In Europe it has collapsed over recent decades thanks to fuel desulphurisation, and today urban levels are very low; it remains relevant near industrial plants, ports and volcanic areas — which is why it appears in the Etna case studies.

Carbon monoxide (CO)

Produced by incomplete combustion. It is the classic indicator of old-generation traffic: catalytic converters brought it down a great deal, and today it serves mainly as a tracer — where CO rises, something is burning nearby.

Benzene (C₆H₆) and formaldehyde (HCHO)

Two volatile organic compounds. Benzene is monitored because it is an established carcinogen, and comes from fuels and evaporation. Formaldehyde is of interest mainly as an indicator of photochemistry at work: it is an intermediate product in the degradation of other organic compounds, and it is measured from space to estimate how intensely ozone chemistry is operating over an area.

Particulate matter: PM₁₀ and PM_2.5

It is not a substance but a mixture: dust, soot, salts, organic compounds, biological fragments. It is classified by size rather than composition, because size is what decides how far a particle manages to get into the respiratory tract.

The number is the aerodynamic diameter in micrometres: PM₁₀ covers particles up to 10 µm, PM_2.5 those up to 2.5 µm. The second is therefore a subset of the first, not a separate thing: where both are measured, PM_2.5 is always less than or equal to PM₁₀.

Part of the particulate is emitted directly (primary: brake and tyre wear, combustion, construction sites, resuspension from the road); another part forms in the atmosphere out of gases such as nitrogen oxides, ammonia and organic compounds (secondary). The secondary fraction is the one that travels far, and it is what makes winter episodes over a plain a regional rather than a local phenomenon.

The meteorological quantities

Meteorology is not background decoration: for a given level of emissions it decides almost everything that is measured at the ground. Three quantities enter the models on this site.

Wind: the u and v components at 10 metres

Wind is supplied as two components — u towards the east, v towards the north, in metres per second — rather than as speed and direction. The reason is practical and important: directions are angles, and averaging them makes no sense (the mean of 350° and 10° is 180°, the exact opposite of the truth). Components, on the other hand, average and interpolate like any other number.

Speed (the root of the sum of squares) and direction are derived from u and v when needed. Wind matters because it dilutes and transports: doubling it roughly halves the concentration downwind of a source.

Temperature at 2 metres

Measured two metres above ground by international convention. It enters the models for three reasons: it governs the speed of photochemical reactions, it is tied to domestic heating in winter, and above all it is the clue to the stability of the atmosphere, which is the next quantity.

Boundary layer height (BLH)

It is the depth of atmosphere that mixes with the ground within a few hours. Above that height the air is effectively another world: what is emitted at the surface does not reach it. BLH is therefore the volume into which emissions are diluted, and it is measured in metres.

It is the least intuitive quantity and the most decisive. At night, when the ground cools, the air near the surface becomes stable and the layer can close down to a few hundred metres; in the afternoon, with the sun heating the ground, convection opens it up to one or two kilometres. For the same emissions, ground concentration goes roughly as the inverse of this depth: between night and afternoon there is a factor of five to ten.

From this comes a practical rule for reading any series: if concentration rises while emissions do not change — a winter morning, a windless evening — it is almost always the boundary layer that has closed, not someone emitting more.

The land quantities

These do not change over time, and describe the place rather than the air. They serve the models that estimate concentration where there is no station at all: if you know what makes a place more exposed, you can assess that place without measuring it.

• Elevation (metres above sea level): terrain steers local winds, and thermal inversions form in basins.
• Population (inhabitants per cell): a clue to domestic emissions and, at the same time, to the number of people exposed.
• Built surface (fraction of the cell that is built up): built surfaces retain heat and hinder mixing.
• Land use, as fractions of the cell: urban, vegetation, water. Vegetation removes particulate, water changes the heat balance.
• Road density by class (metres of road per cell), distinguishing major axes, medium and minor roads: it is the best available proxy for traffic emissions where no traffic count exists.

Satellite measurements

A satellite does not measure ground concentration. It looks at the light crossing the atmosphere and derives from it how much substance sits in the whole air column beneath it, from the ground to space. The result is a quantity per unit surface, in mol/m², and is called the tropospheric column when restricted to the lower atmosphere.

The two measurements are complementary, not alternative. The station is accurate and hourly but exists in few places; the satellite covers everything but passes once a day, has pixels several kilometres wide and cannot tell at what height the substance sits. A high column may mean dirty air at the ground, or a deep boundary layer diluting a lot over the whole depth: which is why the column is used as an additional clue in a model, not as a substitute for the ground measurement.

Every satellite value carries a quality index (qa_value, between 0 and 1) stating how reliable the retrieval is in that pixel: clouds, snow and unfavourable viewing geometries lower it. Official recommendations ask for values below 0.75 to be discarded for nitrogen dioxide, and below 0.5 for the other species.

Summary