Air Quality Dispersion Models

by Prasanna
(Kolhapur, India)

Do you know where I can find air quality dispersion models? Hi....im an environmental engineering student,looking for some data about air pollution modeling & simulation.

Barry's Response - I get asked about these a lot, so hang on for a wild ride. For an introduction to dispersion modelling, you'll find this overview helpful.

Gaussian air dispersion models are good

Gaussian air dispersion models predict how pollutants will spread in the air. They have these advantages:
- With Gaussian models, you can get a fairly accurate prediction of pollutant dispersion since they take into account variables like wind speed, atmospheric stability, and emission rate.
- Gaussian models are flexible and can be applied to a wide range of sources, including point sources (like smokestacks), area sources (like landfills), and complex terrain.
- Each model is transparent and well-documented, so regulators and stakeholders can easily understand and evaluate its predictions and assumptions.
- A Gaussian model is widely accepted and used by regulatory agencies around the world for assessing air quality and public health impacts of industrial emissions.

Ultimately, the Gaussian air dispersion model is a powerful tool for predicting and assessing air pollution's impact. I will introduce four of them now. The models and their user manuals are available for free from the USA government, at their Environmental Protection Agency (EPA) support center for regulatory air models.

What is the support center for regulatory air models?

EPA's support center for regulatory air models (SCRAM) provides technical support and information on using air quality models for regulation. In addition to technical guidance documents, software downloads, and training materials, SCRAM is a central repository for air quality modeling information.

SCRAM has a lot of resources for air quality modelers, like:
- Technical manuals and guidance documents for specific models
- Air quality modeling FAQs (common questions)
- EPA-approved air quality models at your fingertips
- Forums and email information services for user support
- Schedules and materials for training and workshops

EPA also offers technical support through a help desk and email support on the SCRAM website. Modelers can get help with technical issues related to air quality modeling from the SCRAM help desk, including model setup and configuration, data inputs, and model output. Now we will compare some models.

What's the difference between ISC3 and Screen3?

These first two models were important ones in their time, and have since been replaced by more modern ones for official purposes in the US. However, they are good learning instruments that are still available and are the easier ones to use.

Regulatory compliance and environmental impact assessments used air dispersion models like ISC3 and SCREEN3. They both have their strengths and weaknesses, though.
- ISC3 (Industrial Source Complex) is a more advanced model used for power plants, refineries, and chemical plants.
- Different factors can affect pollutant dispersion, like building downwash, terrain, and plume rise. Additionally, ISC3 can handle more complex terrain features and multiple emission sources.
- SCREEN3 (Screening Model, a simplified version of ISC3) is often used for smaller sources like gas stations, dry cleaners, and other small businesses. It provides quick estimates of pollutant concentrations and is easy to use. SCREEN3 uses a Gaussian dispersion model and takes basic meteorological data into account, but it doesn't take complex terrain or building effects into account. It is the easiest of these models to learn.
- ISC3 is a more sophisticated and complex model that's good for industrial sources and complex terrain, whereas SCREEN3 is a simpler model that's good for smaller sites with only one smokestack. This depends on the application, the complexity required, and the data available.

What's the difference between AERSCREEN and AERMOD?

For regulatory compliance and environmental impact assessments, AERSCREEN and AERMOD are the current popular Gaussian air dispersion models. The two models differ in these ways:
- AERSCREEN (AERMOD Screening Model) uses AERMOD as its computational engine. It's a simplified version of AERMOD that's used for screening-level assessments of sources. AERSCREEN is faster and easier to use than AERMOD, but its accuracy and applicability are limited.
- AERMOD is a more complex model used for detailed air quality assessments. There are a lot of factors that can affect pollutant dispersion, like building downwash, terrain, and plume rise. AERSCREEN isn't as precise as AERMOD for larger, more complex sources.

Other differences between the two models:
- AERMOD requires more detailed input data such as weather and environmental information, while AERSCREEN uses simplified input data.
- The AERSCREEN model is for flat terrain and sources far from the receptors (places where we want to know about the predicted air quality), while AERMOD can handle more complex terrain and sources closer to the receptors.
- In AERSCREEN, pollutant concentrations are estimated more conservatively, while in AERMOD, they're estimated with greater precision.

AERSCREEN is a screening-level model that's faster and easier to use than AERMOD, but it has some limitations in terms of accuracy and applicability. The AERMOD model is more complex and accurate, so it's used for more detailed assessments of air quality. Choosing between the two models depends on the application, complexity level, and data available.

Finally, let's compare ISC3 to AERMOD

AERMOD replaced ISC3 as the regulatory model. AERMOD and ISC3 estimate pollution levels from point sources. There are strengths and weaknesses to each model, and their suitability for a particular application depends on factors like source type, weather conditions, and regulations.

In complex terrain and urban environments, AERMOD is generally considered more accurate than ISC3. As AERMOD incorporates advanced algorithms for turbulence, atmospheric stability, and terrain, it's better suited to modeling complex scenarios. Thus, the US Environmental Protection Agency (EPA) prefers it for regulatory compliance.

As opposed to AERMOD, ISC3 is an older model that's simpler to use. More suitable for modeling relatively simple sources in flatter terrain, it can still provide reasonable estimates of air pollution concentrations for regulatory compliance.

To recap, SCRAM's website and support services are great for air quality modelers working in regulatory contexts, helping them do air quality modeling consistently and accurately. While AERMOD may be more accurate, its complexity and data requirements can make it harder to use. In contrast, ISC3 is less valid, but it's easier to use, so it's good for simpler applications and for introducing yourself to air quality dispersion models. In the end, it depends on your application's specific needs and requirements.

How to get started

Go to the USA government website https://www.epa.gov/scram, scroll down to ISC PRIME - the model I used for many years, and download the executable and the manual. Get to know how to use it. This is good for a variety of source types.

Most of my modelling these days is done with AERMOD - the current one, and CALPUFF when its use is justified. Both of these models are available from the same EPA website. Visit this page instead - https://www.epa.gov/scram/air-quality-dispersion-modeling-preferred-and-recommended-models

Introductory air quality modelling

For simple flare or other single-source input, download the screen3 model and https://www.epa.gov/scram/air-quality-dispersion-modeling-screening-models

I just mentioned CALPUFF. There is something unique about this model that sets it apart from ordinary Gaussian models. It's a LaGrangian puff transport and dispersion model that's a lot more advanced than the models above. SCRAM also offers it for free, but the learning and data requirements require more commitment.

These may be the one for your project here. Have fun with it.

If you need a professional air quality dispersion modeling assessment done, you may contact Barry at Calvin Consulting Group Ltd., 403-547-7557, and I will determine the appropriate model for your situation and complete it for you.

There is plenty of detail in the manuals for these models. Not always the easiest read, though, but worth it. Hope these help...

Barry Lough



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