What Is Air Quality? How We Measure, Model and Manage the Air Around Us

What is air quality? Start with the simple answer

Air quality describes what is in the air, how much of it is there and what those concentrations mean for people, ecosystems and the environment. That sounds simple.

The interesting part is that air is constantly moving and changing. Pollutants come from industrial facilities, vehicles, homes, agriculture, wildfires and natural sources. Wind carries them. Turbulence spreads them. Buildings and terrain can redirect them. Chemical reactions can change them. Rain can remove some of them. A release that creates a high concentration beside a source may be barely detectable several kilometres away.

So air quality is really a story:

Source → emission → atmosphere → concentration → exposure → effect

That chain is the key to understanding almost everything on this website about air pollution.

A gas plant might release sulphur dioxide. A stack might release nitrogen oxides. A vehicle might release carbon monoxide and volatile organic compounds.

A wildfire can send enormous quantities of fine particulate matter across hundreds or thousands of kilometres. A person then experiences the resulting air at a particular place and time.

That final concentration is what matters for ambient air quality.

Air Quality Basics: Clean Air, Pollutants, Health & Risk

What is air quality? Follow the chain

Imagine an industrial facility with a stack. Something comes out of the stack. That is an emission.

The amount released might be expressed as grams per second, kilograms per hour or tonnes per year.

The emission enters the atmosphere. Now the weather takes over.

Wind transports the material. Atmospheric turbulence disperses it. The release may rise because of its temperature and velocity. Nearby buildings can disturb the airflow. Terrain can change the wind and affect where the plume travels.

Emissions from pipe leaks and the flare stacks can affect air quality.

Eventually the pollutant reaches places where people, vegetation, animals or other environmental receptors may be present.

That is where concentration matters. A concentration tells us how much of a substance is present in a given volume of air, often expressed in units such as micrograms per cubic metre or parts per billion.

Concentration is not the same thing as emission rate. That distinction is fundamental. A large emission from a tall, hot stack may produce a relatively low ground-level concentration because the plume rises and disperses.

A smaller release close to the ground may produce a much higher concentration nearby. And an emission rate that looks impressive on paper may have little effect at a particular receptor.

This is why what is air quality cannot be answered by looking only at what comes out of a facility. We have to understand what happens after it leaves.

What is air quality? What is actually in the air?

Clean outdoor air is not a single chemical. It is a mixture of gases, particles and trace substances. Some occur naturally. Others are produced by human activity.

Here are some of the pollutants that commonly matter in air-quality work.

The details matter.

VOC is a large family of chemicals, not one pollutant with one set of health effects.

PM₂.₅ is defined by particle size rather than by a single chemical composition.

NOx can undergo atmospheric reactions that produce other pollutants.

H₂S has a particularly strong odour at very low concentrations, which means nuisance can become important well before acute toxicity becomes the issue.

That is another reason what is air quality has a different answer for different projects.

The contaminant of concern may be selected by a regulator, specified by a client or engineering firm or identified from the type of release and applicable industry and provincial requirements.

At Calvin Consulting (where I work) we are often asked to assess one or two contaminants from a much larger possible list.

The type of source can provide an important clue. A combustion source points us toward pollutants such as NOx, CO and sometimes SO₂. A sour-gas facility raises different questions. Storage, processing and fugitive sources can make VOCs or odorous compounds more important.

The job is to identify what actually matters rather than modelling every chemical in the periodic table.

What is air quality? Is it getting better?

There is good news here. There is also an important catch.

Canadian emissions of several major air pollutants have fallen dramatically over the long term. According to Canada's 2024 emissions inventory, human-caused emissions compared with 1990 were approximately:

• SOx: 81% lower
• NOx: 46% lower
• VOCs: 38% lower
• CO: 66% lower
• PM₂.₅: 22% lower

Ammonia went in the opposite direction, increasing by about 28% over the same period. These figures describe emissions from human activities and exclude natural sources such as wildfires.

Measured concentrations show a similarly encouraging story for several pollutants. From 2009 to 2023, Canada's national average and peak concentrations of NO₂, SO₂ and VOCs generally decreased. National ozone concentrations showed no significant overall trend. PM₂.₅ is much more complicated because severe wildfire seasons can produce enormous temporary increases.

It's getting better

So the honest answer to what is air quality is that it depends on which pollutant, which location and which time period you examine.

Cleaner engines and fuels, industrial emission controls and changes in electricity generation have made a real difference. At the same time, wildfire smoke can overwhelm those long-term improvements for days or weeks.

Alberta's 2024 ozone data provide a good example of that complexity. The province recorded 54 one-hour ozone AAAQO exceedances that year, with wildfire smoke responsible for 33 and summer smog episodes responsible for 21.

Improvement is real. 

So are the remaining problems.

That is a more useful story than either 'air pollution is getting worse' or 'the problem has been solved.'

What is air quality? Why does location matter?

Air does not respect property boundaries. A factory may sit on one parcel of land while its emissions move over neighbouring properties.

A wildfire may start hundreds of kilometres away. Traffic pollution can be concentrated beside a busy roadway.

A wind direction can carry a plume toward one neighbourhood while leaving another relatively unaffected. This is where the idea of an airshed becomes useful.

An airshed is a geographic area where emissions, terrain, meteorology and patterns of air movement influence regional air quality. It is useful to imagine an airshed as a shared atmospheric neighbourhood.

It is not a box in which the air behaves uniformly. Concentrations can vary considerably within an airshed.

Alberta currently has 10 regional airshed organizations that monitor, report and educate on ambient air quality. They include organizations such as the Calgary Region Airshed Zone, Alberta Capital Airshed, Palliser Airshed Society, Peace Airshed Zone Association and Wood Buffalo Environmental Association.

There is an important bit of history here. The Clean Air Strategic Alliance (CASA) was established in 1994 as Alberta's consensus-based multi-stakeholder air-quality partnership. It brought together government, Indigenous, industry and non-government representatives and played a major role in developing Alberta's air-quality management system.

CASA ceased operations in June 2026 after more than 30 years. Its website is now maintained as an archive of its reports and resources. The regional airshed organizations continue their monitoring and reporting roles.

That history is worth remembering because Alberta's air-quality system grew from the idea that government, industry, communities and environmental interests can learn from the same measurements and work on regional problems together.

What is air quality? What does the AQHI number mean?

If you have ever checked an Alberta weather app and seen a number such as 2, 6 or 9, you have encountered the Air Quality Health Index. The AQHI is designed for a different purpose from industrial dispersion modelling.

It is a public communication tool. The current AQHI uses a scale of 1 to 10+:

• AQHI Health risk
• 1–3 Low
• 4–6 Moderate
• 7–10 High
• 10+ Very high

The index is based on the combined relative health risks associated with ground-level ozone, fine particulate matter and nitrogen dioxide. During significant wildfire-smoke events, PM₂.₅ can drive an hourly AQHI calculation by itself so that rapidly changing smoke conditions can be communicated more effectively.

This distinction matters. The AQHI is not a measurement of every pollutant in the air. It is also not the same thing as an Alberta Ambient Air Quality Objective.

Think of it this way: 

• AQHI → What does today's air mean for my health?
• AAAQO → How does this ambient concentration compare with an Alberta regulatory objective for this substance?

Those are different questions.

The AQHI is particularly useful for people who need to decide whether to change outdoor activities. Children, older adults and people with pre-existing respiratory or cardiovascular conditions can be more vulnerable to poor air quality. People who spend long periods outdoors may also have greater exposure.

And an AQHI value is an area-wide communication tool. Individual locations can experience different concentrations because air composition changes across space and time.

That is another answer to 'what is air quality?' And it depends on where you are standing.

What is air quality? Monitoring versus modelling

This is one of the most important distinctions on the page.

• Monitoring measures: A monitoring station samples or continuously measures the air at a particular location.
• Modelling predicts: A dispersion model estimates concentrations at specified locations under defined assumptions and scenarios.

Neither one makes the other obsolete. Think of them this way:

Alberta's monitoring system includes stations operated by airshed organizations, government and industry. Industrial facilities may also conduct fenceline or other ambient monitoring and source emissions monitoring according to their approvals. The province's Air Data Warehouse provides access to ambient data.

Monitoring is especially valuable because it tells us what actually happened. But a monitor cannot be everywhere.

Suppose you are proposing a new industrial facility. There is no five-year measurement record of the air after the facility is built. Modelling can estimate the facility's contribution before construction.

That is one reason Alberta requires air-quality modelling for specified regulatory applications.

So when someone asks, What is air quality?, a useful answer includes both measurement and prediction.

What is air quality? When does Alberta use modelling?

Alberta's regulatory system uses Ambient Air Quality Objectives (AAAQOs) and Ambient Air Quality Guidelines (AAAQGs) to manage ambient air quality.

AAAQOs are developed to protect human health and the environment. AAAQGs have additional planning and environmental-management roles. Alberta also has specific odour-management guidelines that can be used to characterize odour issues, identify potential sources and support regulatory air-quality modelling or management plans where appropriate.

For activities requiring an EPEA approval, operating under an applicable Code of Practice or subject to modelling requirements from another Alberta regulatory agency, air-quality modelling is required according to the Alberta Air Quality Model Guideline.

The purpose is to demonstrate the project's impact relative to applicable AAAQOs and, where reasonable local concern exists, AAAQGs. 

This is an important correction to a common misconception: Modelling is not simply a substitute for monitoring when monitoring is unavailable.

A regulatory assessment may require modelling even when monitoring data already exist. Why?

Because modelling provides information about the relationship between a source and receptors. It can ask:

• What contribution could this proposed facility make?
• Where is the maximum predicted concentration?
• What happens if the facility operates at a particular rate?
• What if a building causes plume downwash?
• What if the stack is taller?
• What if emission controls are added?

Those are questions a monitoring station alone cannot answer.

What is air quality? What does a modeller actually do?

A client may arrive with a facility drawing and a spreadsheet full of emission rates.

The temptation is to start building the model immediately. Experienced modelling starts earlier. The first questions are:

• What is the project trying to demonstrate?
• Which contaminants matter?
• Which sources are important?
• Which operating conditions should be assessed?
• What information is reliable?
• What information is missing?

At Calvin, the contaminant of concern may already be specified by a client, engineering firm or regulator.

For other projects, the contaminant selection can follow from the type of release and applicable industry and provincial requirements. Then the source data have to be examined.

A stack is more than just a location. The model may need:

• emission rate
• release height
• diameter
• exit velocity
• temperature
• operating schedule
• source location

Nearby buildings may matter. Terrain may matter. Meteorology certainly matters. Receptors matter. Background concentrations may matter.

The modeller has to put those pieces together before trusting the output. That is where the answer to what is air quality becomes more than an environmental definition. It becomes a professional process.

What is air quality? A real Calvin example

A recent Calvin assessment for an Alberta gas plant provides a good illustration. The project evaluated nitrogen dioxide from multiple stationary combustion sources. And it used:

1. Five years of Alberta meteorological data → processed through AERMET
2. Terrain data → processed for use by AERMOD
3. Multiple receptor grids → from detailed near-facility spacing out to 10 km
4. Building geometry → processed with BPIP to evaluate downwash
5. Background NO₂ → developed from Alberta monitoring data
6. NOx-to-NO₂ conversion → using PVMRM
7. AERMOD → to predict ground-level concentrations
8. Regulatory statistics → to compare the predicted values with applicable AAAQOs

That sounds complicated. It becomes easier when viewed as the same chain:

Source → atmosphere → concentration → receptor → comparison

The result was a much more defensible answer to the question of what the facility would contribute to ambient air quality than any one piece of the puzzle could provide.

And the report did not simply say the computer says everything is fine. It identified the information used, its sources and assumptions.

That matters because a model is only as reliable as the story it is given.

What is air quality? It can even involve radioactive contaminants

Most industrial air-quality work concerns familiar pollutants such as NO₂, SO₂, H₂S, particulate matter and VOCs. Calvin has also worked on something much more unusual.

For a proposed Calgary radiopharmaceutical facility, we once performed dispersion modelling for airborne radioactive isotopes including:

• carbon-11,
• fluorine-18,
• gallium-68,
• nitrogen-13,
• copper-64 and
• zirconium-89.

The modelling used AERMOD with five years of meteorology, terrain processing, detailed building downwash and several types of receptors. Those receptors included nearby homes as well as specific locations on campus buildings such as intake vents and doorways.

That last detail is revealing. A generic grid alone was not enough.

The project needed modelling at places where people or building systems could actually interact with the release.

The predicted airborne concentrations were provided to radiation specialists who would determine the corresponding potential doses.

That division of responsibility is important. Calvin's job was the atmospheric dispersion assessment. Radiation specialists handled the dose interpretation. 

Good consulting includes knowing where your expertise ends.

What is air quality? It can include odour

Air quality is not solely about whether a pollutant is toxic. People notice smells.

A facility may comply with a health-based concentration objective and still create a nuisance if an odorous substance is detectable beyond the property.

Alberta recognizes odour management within its AAAQG framework. Odour guidelines can help identify and characterize an odour issue, identify potential sources and provide quantitative evidence associated with odour complaints. Where applicable, regulatory modelling and management plans can then be used to address the problem.

This is another reason contaminant selection requires judgement. A client might initially say: We only need to look at the regulated pollutant.

A complaint history might suggest another question. A sour-gas operation might make H₂S important. A storage facility might bring VOCs into focus.

A process with a distinctive odour might require a completely different investigation. What is air quality?

It includes what people breathe, what they smell and what the applicable regulatory framework requires us to understand.

What is air quality? What an experienced modeller notices

A model can run perfectly and still represent the wrong problem. That is why experienced review matters.

At Calvin, we routinely compare model inputs against the information supplied by the client and engineering team, against other available information and against basic physical expectations.

For some equipment we can compare proposed parameters with information in Alberta emissions inventories. We ask questions such as:

• Does the emission rate make sense for this equipment?
• Does the stack exit velocity look reasonable?
• Is the source location correct?
• Is that building really there?
• Could downwash matter?
• Does the wind rose make sense?
• Are the receptors in the places that matter?
• Is the selected operating scenario really the controlling case?
• Why is the predicted maximum located there?
• Could a lower emission rate produce a higher ground-level concentration because the plume behaves differently?

These are not questions the software answers for you. They are questions a professional modeller asks of the software.

What is air quality? Three levels of understanding

Level 1 — I just need the idea

Air quality describes what is in the air and what the concentrations mean for people and the environment.

Sources release substances. The atmosphere transports and disperses them. People and the environment experience the resulting concentrations.

Level 2 — I want to understand the science

Think about:

emissions

↓

meteorology

↓

dispersion

↓

concentration

↓

exposure

↓

effect

Change the source, weather, terrain or receptor and the answer can change.

Level 3 — I need to assess a real project

Now the questions become:

• Which contaminant?
• Which sources?
• Which emission scenarios?
• Which meteorology?
• Which terrain?
• Which buildings?
• Which receptors?
• Which background concentration?
• Which objective or guideline?
• Which model?
• Which averaging period?
• Which regulatory statistic?
• Which quality-control checks?

That's the difference between knowing what air quality means and conducting an air-quality assessment.

What is air quality? The professional lesson

The phrase can sound as though air quality is a property of the atmosphere, like temperature. In practice, it is an interaction between the atmosphere and everything putting material into it.

A good air-quality assessment therefore combines several kinds of knowledge:

Chemistry tells us what substances are present.

Engineering tells us how they are released.

Meteorology tells us how the atmosphere behaves.

Geography tells us what the terrain and land use are doing.

Monitoring tells us what has actually been measured.

Modelling estimates concentrations under defined scenarios.

Regulation tells us what comparisons and demonstrations are required.

Professional judgement ties the pieces together.

That is why air quality is such an interesting field. It sits somewhere between science, engineering, mathematics, environmental management and detective work.

What is air quality? The Calvin approach

At Calvin Consulting Group, we generally begin with the question rather than the software. A project may involve one contaminant or several. The contaminant may already be prescribed by a regulator or specified by the client.

For other projects, the source type and applicable industry and provincial standards point toward the appropriate contaminants.

From there, we evaluate the release and determine what information is needed to predict or measure its effect. Our work can include:

• emissions characterization
• air-quality dispersion modelling
• AERMOD
• AERflare and flare assessments
• meteorology
• terrain and receptor analysis
• building downwash
• ambient monitoring design
• monitoring-data interpretation
• regulatory applications
• odour assessment
• QA/QC and senior technical review

Calvin Consulting handles your air quality issues

The point is not to model everything. The point is to understand enough of the system to answer the question that actually matters.

What is air quality? The answer is bigger than one number

Air quality is the concentration of substances in the atmosphere at a particular place and time and what those concentrations mean for people, the environment and applicable standards.

• Sometimes the most important pollutant is PM₂.₅ from a wildfire.
• Sometimes it is NO₂ from combustion.
• Sometimes it is SO₂ from an industrial process.
• Sometimes it is H₂S because odour is the complaint.
• Sometimes it is a contaminant most people have never heard of.

Calvin has even modelled airborne radioactive isotopes for a Calgary facility. The common thread is the same:

• What was released?
• How did it move?
• Where did it go?
• What concentration resulted?
• What does that concentration mean?
• How do we know?

Those questions turn air quality from a vague environmental phrase into something we can investigate scientifically.

Need help figuring out your air-quality question?

You can buy a monitor. You can download a dispersion model. You can produce a concentration map.

The harder question comes first: Are you measuring or modelling the thing that actually matters? A good specialist can help define the problem before the project becomes expensive to change.

That might mean identifying the contaminants of concern, checking source information, deciding whether monitoring or modelling is appropriate, selecting the right model, designing a receptor network or reviewing an assessment prepared by someone else.

At Calvin Consulting Group, our job is to make that process understandable and defensible. The earlier the air-quality question is sorted out, the more options a project usually has.

That can mean fewer assumptions, less rework and a clearer path through regulatory review.

The computer can calculate the concentration while an experienced modeller helps make sure it is the right concentration, at the right place, for the right question.

Our air quality group at Calvin Consulting, (CCGL Services) specializes in dispersion modelling, determining the location and equipment to be used for ambient monitoring, and analyzing the data collected by the monitoring for operational purposes. If you would like more information on how we can evaluate and help you improve the impact of your industrial site on air quality, please contact Barry Lough by email.

What is air quality? Explore further

This page is the starting point. The deeper questions lead into the rest of the Stuff in the Air air-quality library.

Understand air pollution 

• Air Pollution Facts - The common pollutants and where they come from.
• Air Quality Statistics - What the measurements actually show over time.

Measure it

• Air Quality Monitoring - How ambient air is measured and what monitoring can and cannot tell you.
• Natural Gas Leak Detection - How fugitive releases can be found and investigated.

Model it

• Air Dispersion Model - How to choose the right modelling approach.
• AERMOD: Alberta's Workhorse Air Dispersion Model - How AERMOD works and what experienced modellers check.
• Air Quality Dispersion Modelling - The deeper technical guide.

Understand specialized sources

• What Is a Gas Flare? - How flares work and what they emit.
• Alberta Directive 060 Explained - How flaring, incineration, venting and conservation are regulated.
• Benzene Emissions - A closer look at a specific industrial contaminant.

Understand professional judgement

• Expert Judgement - Why the modeller still matters after the computer has finished calculating.

That network is the bigger resource. This page is the map.

What is air quality? A practical checklist

Whether you are a student, project manager, regulator or curious homeowner, these six questions are a useful starting point.

1. What is being released? - Identify the source and the contaminant.

2. How much is being released? - Determine the emission rate and relevant operating conditions.

3. Where does it go? - Consider wind, turbulence, terrain, buildings and atmospheric chemistry.

4. What concentration reaches the receptor? - This can come from monitoring, dispersion modelling or both.

5. What does that concentration mean? - Consider health, environmental effects, odour and applicable regulatory objectives or guidelines.

6. How confident are we? - Check the source data, measurements, assumptions, model setup and limitations.

That last question is easy to overlook. It may be the most important one.

Do you have concerns about air pollution in your area??

Perhaps modelling air pollution will provide the answers to your question.

That is what I do on a full-time basis.  Find out if it is necessary for your project.

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Thank you to my research and writing assistants, and the author remains responsible for the content.

What is air quality? Can you assess it yourself?

You can certainly learn a great deal yourself.

• You can read monitoring data.
• You can check the AQHI.
• You can study emission sources.
• You can learn AERMOD.
• You can download datasets and make maps.

That is excellent for education. A regulatory assessment is a different task.

• The hard part usually is not pressing Run.
• It is deciding what should be entered before pressing Run.
• A model will happily calculate an answer from an incorrect stack height.
• It will happily process an unrealistic emission rate.
• It will happily create a beautiful concentration map from a poorly chosen receptor grid.
• It will happily produce a very precise answer to a poorly framed question.

That is why early specialist involvement can save much more than modelling time.

It can prevent the project from being built around the wrong assumption.