What Is Acid Deposition?

How pollution from the air can reach soils and water, what is acid deposition and how it can be modelled

What happens to sulphur and nitrogen emissions after they leave an industrial facility? They do not simply disappear.

Defining acid deposition

Sulphur dioxide (SO₂), nitrogen oxides (NOx) and other acid-forming substances can be transported through the atmosphere, chemically transformed and eventually deposited onto forests, soils, wetlands and lakes. The resulting input helps define what is acid deposition.

That makes acid deposition different from an ordinary air-quality concentration assessment.

A conventional dispersion model might ask: What concentration of a pollutant occurs at ground level?

An acid-deposition assessment asks a longer question: How much acidifying material reaches an ecosystem, in what form and is that additional deposition enough to matter?

That distinction is the key to understanding what is acid deposition and why specialized modelling can sometimes be required.

Again, what is acid deposition?

Acid deposition is the transfer of acidifying substances and their reaction products from the atmosphere to the Earth's surface.

It can occur through: 

• Wet deposition - Acidifying material is incorporated into precipitation and reaches the surface through rain or snow.
• Dry deposition - Gases and particles are transferred directly from the atmosphere to surfaces without precipitation.

Acid deposition can contain both sulphur and nitrogen compounds. 

Once these materials reach an ecosystem, they can affect soil and water chemistry. The consequences depend on the amount deposited and, importantly, the ability of the receiving ecosystem to neutralize or otherwise accommodate the additional acidity.

So acid deposition is not simply a story about acid rain. It is a story that looks more like this:

Industrial emissions

↓

Atmospheric transport

↓

Chemical transformation

↓

Wet and dry deposition

↓

Soils, vegetation and water

↓

Ecological response

That entire chain is what a what is acid deposition assessment is trying to understand.

Why can emissions from the air affect a forest or lake many kilometres away?

An emitted pollutant does not necessarily stay close to its source. Wind can transport emissions over considerable distances. At the same time, atmospheric chemistry can change the form of the material while it is travelling.

For example, sulphur dioxide and nitrogen oxides released from a facility can participate in atmospheric reactions that produce other sulphur- and nitrogen-containing compounds.

The material that eventually reaches an ecosystem may therefore not be identical to what came out of the stack. This is one reason acid-deposition modelling is more complicated than simply plotting the concentration of SO₂ or NO₂ around a facility.

The modeller is interested in the eventual deposition of acidifying species, not just the concentration at a particular point in the atmosphere.

Why do SO₂, NOx and NH₃ matter?

Three pollutants commonly enter the discussion:

• Sulphur dioxide (SO₂) - SO₂ is an important acid-forming emission. Atmospheric oxidation can produce sulphate compounds that contribute to sulphur deposition.
• Nitrogen oxides (NOx) - NO and NO₂ can undergo atmospheric reactions that produce oxidized nitrogen species that contribute to nitrogen deposition.
• Ammonia (NH₃) - NH₃ is part of the atmospheric nitrogen system and also participates in the chemistry that determines how sulphur and nitrogen are distributed among gaseous and particulate forms.

A what is acid deposition assessment may require more atmospheric information than a conventional dispersion assessment. The model may need to know not only:

• How much did the facility emit? But also,
• What was already present in the atmosphere and what happens chemically after the emissions are released?

What is a critical load?

One of the most important ideas in acid-deposition work is the critical load.

In simple terms, a critical load is an estimate of the level of acidifying deposition that a particular ecosystem can receive without unacceptable long-term effects, based on the applicable ecological and chemical assessment. That introduces an important distinction. 

• The model might calculate: How much sulphur and nitrogen were deposited here?
• But the environmental assessment then asks: Is that amount greater than the ecosystem can tolerate?

Alberta's current Acid Deposition Management Framework uses updated critical-load science and a steady-state mass-balance approach to establish critical loads of acidity for sulphur and nitrogen deposition. The framework took effect in January 2023 and superseded the earlier 2008 framework.

This is an important reason not to rely blindly on old acid-deposition studies or old threshold values when assessing a new project.

The acid-deposition modelling chain

A useful way to understand the whole subject is to follow one emission from the facility to the ecosystem.

1. Emissions: How much SO₂, NOx and other relevant substances are released?
2. Atmospheric transport: Where does the material travel?
3. Chemical transformation: How does it change while it is in the atmosphere?
4. Deposition: How much reaches the surface through wet and dry deposition?
5. Background deposition: How much acidifying material is already arriving from other sources?
6. Ecosystem response: How sensitive is the receiving environment?
7. Critical-load comparison: Does the resulting deposition exceed the applicable critical load?

This is the difference between simply modelling an emission and assessing its potential ecological significance.

Can AERMOD handle acid deposition?

This is an excellent question—and the answer is yes, to a point.

AERMOD includes algorithms for calculating dry and wet deposition for applicable gaseous and particulate emissions. The current EPA AERMOD documentation includes deposition outputs and deposition/depletion options.

So AERMOD is capable of modelling deposition. However: Being able to calculate deposition is not the same thing as performing a complete regional assessment of what is acid deposition doing to our environment.

An acid-deposition assessment can require:

• transport over a larger area;
• atmospheric chemical transformation;
• sulphur and nitrogen speciation;
• appropriate deposition treatment;
• background chemistry or deposition;
• multiple years of meteorology; and
• comparison with ecosystem-specific critical loads.

Those requirements can take the problem well beyond a conventional AERMOD concentration assessment.

AERMOD can partially address acid deposition by modelling the local dispersion of primary acid-forming pollutants like SO₂ and NOx and estimating their local concentrations and dry deposition. However, it is not sufficient for a comprehensive acid deposition assessment, as it typically does not account for the complex chemical transformations of these pollutants into secondary acidic compounds, nor does it fully model wet deposition by precipitation, which are critical components of the full acid deposition process often requiring more specialized models like CALPUFF for a complete understanding.

For these reasons, Alberta's current list of recommended refined dispersion models includes both AERMOD-PRIME and CALPUFF, with CALPUFF specifically described as a non-steady-state puff model capable of handling time- and space-varying meteorological conditions.

The appropriate model is therefore a question to be answered from the current regulatory framework and the requirements of the particular assessment, rather than simply choosing the model most familiar to the modeller.

Why CALPUFF is often associated with acid-deposition modelling

CALPUFF can represent the transport of pollutants through changing meteorological conditions and includes chemical transformation and deposition capabilities.

For regional what is acid deposition applications, one commonly encountered approach is the RIVAD/ISORROPIA chemistry formulation, combined with aqueous chemistry and wet and dry deposition.

This is not just a matter of choosing a checkbox in CALPUFF. The modeller also needs appropriate inputs describing the atmospheric environment.

What happens chemically after the emissions leave the stack? A simplified picture looks like this:

Sulphur SO₂

↓

atmospheric oxidation

↓

sulphate-containing species

↓

wet and dry deposition

Nitrogen NO + NO₂

↓

atmospheric oxidation and transformation

↓

oxidized nitrogen species

↓

wet and dry deposition

Ammonia NH₃

↓

interacts with sulphur and nitrogen chemistry

↓

changes the partitioning and forms of atmospheric nitrogen and sulphur

↓

deposition

Actual atmospheric chemistry is much more complicated than this simplified diagram, but the point is important: The material deposited onto an ecosystem is the result of both transport and chemistry.

That is why acid-deposition modelling needs chemical transformation mechanisms rather than treating every pollutant as a passive tracer.

RIVAD/ISORROPIA and aqueous chemistry

For modelling approaches that use the RIVAD/ISORROPIA formulation, the CALPUFF configuration includes specific chemistry options. For the applicable Alberta/BC-style regional acid-deposition approach, settings commonly referenced include:

• MCHEM = 6
• MAQCHEM = 1
• MLWC = 1

The exact configuration should always be checked against the current provincial guidance and the version of the modelling system being used.

The model also needs to distinguish NO and NO₂ emissions, rather than simply treating all NOx as one undifferentiated emission value.

That may sound like a small input detail. It isn't. The chemical pathway depends on the form in which the nitrogen is initially released.

Why ozone H2O2 and NH3 matter

Ozone (O₃) can participate in atmospheric oxidation chemistry. Where representative hourly onsite ozone data are available, they can provide valuable information for the chemical transformation calculation.

Where such data are not available, the Alberta modelling framework provides alternative time series that can be used according to the project setting. 

The important modelling principle is: Use the best representative information available and understand what the default data are standing in for.

A modeller should not assume that a default value is automatically better than site-specific data merely because it is published in a guideline. The better question is whether the data represent the atmosphere in which the project is actually operating.

Ammonia and Hydrogen peroxide

Acid-deposition chemistry also depends on other atmospheric constituents.

Ammonia (NH₃) affects nitrogen and sulphur chemistry and gas-particle partitioning. Hydrogen peroxide (H₂O₂) can act as an important oxidant in the chemical treatment.

Where representative current data are available, they can be preferable to generic assumptions. Where they are not, the applicable provincial guidance provides default values or time series.

This is another example of what is acid deposition modelling  and why it can become data-intensive very quickly.

Why five years of weather can matter

What is acid deposition if not a long-term environmental issue? A single unusual year of wind, stability and precipitation may not represent the broader conditions affecting transport and deposition.

For Alberta assessments using the applicable framework, multiple years of meteorological information can be used to represent interannual variability. The resulting annual deposition can then be evaluated at the receptors or ecological locations relevant to the assessment.

The important idea is: A deposition assessment is not simply asking what happened during one bad weather event. It is trying to characterize longer-term deposition patterns.

What about background deposition?

A new facility is not being built into an atmosphere with zero existing deposition. Sulphur and nitrogen are already deposited from:

• other industrial facilities;
• transportation;
• agriculture;
• natural sources; and
• more distant sources.

For a cumulative assessment, the project contribution therefore needs to be considered together with the appropriate background.

Background deposition may be established using monitoring or modelling, depending on the available information and the requirements of the applicable framework.

This is an important distinction from a simple project-only calculation. The project contribution is only one part of the deposition an ecosystem experiences.

What about the opposite of acidity?

Acid-deposition assessment is not always as simple as adding up acidifying substances. Some substances can have a neutralizing effect.

Base cations, including calcium and magnesium, can offset acidity in soils and ecosystems. This can become particularly interesting in areas with significant crustal disturbance, such as mining or major construction, where dust can contribute additional mineral material.

Whether neutralizing deposition should be included in a particular assessment is not something to decide mechanically. It may require:

• monitoring data;
• site information;
• literature;
• knowledge of the receiving environment; and
• professional judgment.

The key idea is that the ecological balance can involve both acidifying and neutralizing inputs.

Alberta's current approach

Alberta's Acid Deposition Management Framework was revised in 2022 and took effect on January 3, 2023. It supplements the Alberta Air Quality Modelling Guideline. 

Predicting long-term environmental conditions

The revised framework was developed to reflect updated science and to provide greater clarity for government, regulators and industry.

It is important to distinguish this current framework from older Alberta acid-deposition guidance and studies.The current framework includes a Handbook for Assessing Regional Acid Deposition Exceedances, along with supporting tools for calculating exceedances.

The revised approach uses critical-load information developed with a steady-state mass-balance model. The critical-load work considers factors such as:

• soil characteristics;
• vegetation;
• runoff;
• biomass harvest and grazing;
• base-cation deposition; and
• other ecological and chemical factors.

The result is a more ecosystem-specific approach than simply comparing deposition against one provincewide number.

H+ equivalent method remains valid in Alberta

Calculation Method: The concept of calculating a total hydrogen ion (H+) equivalent to represent the combined acidifying potential of SO2, NOx and NH3 emissions is still a valid and widely used approach.

Where does this matter?

Acid deposition can be particularly relevant where substantial industrial emissions occur near sensitive ecosystems.

Alberta's major industrial regions, including the Athabasca Oil Sands Region and Industrial Heartland, provide obvious examples of areas where cumulative industrial emissions can warrant attention.

But an assessment should not be triggered simply because a facility happens to be in one of those regions. The appropriate question remains: What does the current Alberta framework require for this project and its receiving environment?

Solutions to acid rain

And, now for BC?

British Columbia has a long history of regional sulphur and nitrogen deposition modelling. BC provincial air quality modelling guidelines have specifically used CALPUFF with RIVAD/ISORROPIA for regional estimates of sulphur and nitrogen deposition, including wet and dry deposition.

The BC approach also illustrates an important modelling principle: Acid-deposition modelling needs appropriate atmospheric chemistry inputs, not simply the project's emission rates. The specific requirements and acceptable inputs should always be checked against the current BC modelling guidance rather than copied from an older assessment.

What about other models?

CALPUFF is not the only model capable of being used for deposition or regional atmospheric chemistry.

AERMOD can calculate dry and wet deposition for applicable source types and pollutants. It is highly useful for many local-scale regulatory dispersion problems.

But an acid-deposition assessment may require capabilities and scales beyond a conventional AERMOD concentration assessment.

The Community Multiscale Air Quality model (CMAQ) is a regional, three-dimensional photochemical model developed by the U.S. EPA.

It is capable of representing much more extensive atmospheric chemistry, transport and deposition than a conventional steady-state plume model.

CMAQ has also been used in Alberta research and regional acid-deposition assessments, including work in the Athabasca Oil Sands Region.

CALPUFF occupies a useful middle ground for many regional transport applications, particularly where the regulatory or project framework calls for a puff model capable of representing changing meteorological conditions and chemical transformation.

The appropriate choice depends on the assessment requirements.

Is acid-deposition modelling part of every air-quality assessment? No. Acid-deposition modelling is a specialized assessment. A project may require detailed concentration modelling for SO₂ or NO₂ without requiring a regional acid-deposition assessment.

Another project may need to consider deposition because of its emissions, the surrounding ecosystem, the applicable regulatory framework or cumulative effects. That is why it is worth establishing the modelling requirement before building a complicated regional model.

The main lesson

Acid deposition connects industrial emissions to ecosystem health.

Acid deposition is the story of what happens after pollutants leave the stack. The emissions are transported through the atmosphere and can undergo chemical transformation.

They are deposited through rain, snow and direct surface transfer.

The receiving ecosystem already has a background level of deposition and may have some ability to neutralize additional acidity. The assessment therefore has to connect:

emissions → atmosphere → chemistry → deposition → ecosystem

That is why acid-deposition modelling is different from simply predicting a ground-level SO₂ or NO₂ concentration. 

And it is also why the first question should not necessarily be: Which model should I run?

It should be: What environmental and regulatory question am I actually trying to answer?

Need to know whether your project needs acid-deposition modelling?

Calvin Consulting Group Ltd. has extensive experience in air-quality dispersion modelling, although acid-deposition assessments have been a relatively small part of our project work.

That does not mean we regard the subject as off-limits.

Where a project requires this type of evaluation, we have the modelling background and technical capability to investigate the applicable guidance, establish the required methodology, assemble the necessary inputs and carry out the assessment.

That may mean using an established CALPUFF-based approach, investigating whether AERMOD's deposition capabilities are appropriate to the question or determining that a broader regional modelling system is warranted.

Our approach is straightforward: First determine what the regulatory and environmental question requires. Then build the modelling approach around that question.

If your project involves significant SO₂, NOx or NH₃ emissions (or you have been asked to investigate potential acid-deposition effects) contact Calvin Consulting Group to discuss the assessment before committing to a complex modelling exercise.

We can help determine what needs to be modelled, what information is required and which modelling approach makes sense for the project.

Come on, let's get your project moving. We don't compromise on air quality.

​Clean air is our Passion...Regulatory Compliance is our Business.

Common mistakes in acid-deposition modelling

Assuming acid deposition means acid rain: It includes both wet and dry deposition. 

Assuming AERMOD cannot model deposition: It can calculate deposition, but that does not mean it automatically answers every regional acid-deposition question.

Assuming AERMOD is sufficient for every acid-deposition assessment: The required scale, chemistry and regulatory framework may point toward CALPUFF, CMAQ or another approach.

Using old Alberta thresholds: Alberta's 2022 ADMF superseded the earlier framework. Older studies can still be useful for history and context, but their regulatory thresholds should not automatically be carried into a new assessment.

Treating NOx as one chemical species: The model may need separate NO and NO₂ emissions because the chemical transformation pathway depends on the starting species.

Using default atmospheric chemistry without asking whether better data exist: Representative site or regional data may provide a better basis where available.

Ignoring background deposition: The ecosystem already receives deposition before the project begins.

Ignoring neutralizing inputs: Base-cation deposition can matter in some environments, particularly where mineral dust inputs are substantial.

Assuming more sophisticated modelling is always better: A complicated model is useful only when the additional complexity answers a question that matters.

What an experienced modeller looks at first

Acid-deposition modelling can become complicated very quickly. That makes the first step especially important.

Before setting up the model, I would want to know:

What is being emitted? - How much SO₂, NOx and NH₃ are actually associated with the project?

Where is it going? - What ecosystems or sensitive areas could receive the deposition?

What already exists? - What background deposition is occurring?

What chemistry needs to be represented? - Which chemical transformation mechanism is appropriate?

What data are available? - Do we have representative O₃, NH₃, H₂O₂, deposition or meteorological information?

What does the current regulatory framework require? - Is this a local dispersion question, a regional deposition question or both?

Does AERMOD answer the question? - Sometimes its deposition capabilities may be useful.

Sometimes a broader regional model is more appropriate. And sometimes the first job is simply to determine whether an acid-deposition assessment is actually required.

The most expensive modelling mistake is doing the wrong modelling very thoroughly.

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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A practical acid-deposition workflow

A useful way to organize an assessment is:

1. Identify acidifying emissions

SO₂ / NOx / NH₃

↓

2. Determine whether an assessment is required

Apply the current provincial framework.

↓

3. Identify the receiving environment

Soils / forests / wetlands / lakes / sensitive ecosystems

↓

4. Define the modelling approach

AERMOD / CALPUFF / CMAQ / other accepted approach

↓

5. Develop emissions and source information

Current, representative and defensible

↓

6. Develop atmospheric chemistry inputs

O₃ / NH₃ / H₂O₂ / NO and NO₂ / other required data

↓

7. Model transport and deposition

Wet + dry deposition

↓

8. Include appropriate background

Project + existing environment

↓

9. Evaluate critical-load exceedance

Compare the calculated deposition with the applicable ecological benchmark

↓

10. Interpret the result

Determine whether the project causes a meaningful change and what that means for the assessment.