We have all taken a shortcut or two. Looking back, did we really save anything or get anything extra out of it? Several years ago, on my commute home from work, I noticed a coworker about 3 or 4 cars ahead of me on the freeway. There was a little congestion, but traffic was still moving but slower than the posted speed limit. The coworker exited the freeway to the freeway frontage road to avoid the slower traffic, then exited the frontage road to return to the freeway. Now she was 3 to 4 cars behind me. That’s a typical shortcut. You think you saved some time but you rarely do. There is always a price to pay.
In the dispersion modeling world, there are many well established “shortcuts”, but as technology has evolved and modernized, we have to question whether these methods provide any real benefit.
To Screen or Not To Screen
Back in the “Good Old Days” (they really weren’t so good), setting up a dispersion modeling run was a huge chore. If you wanted a quick and very rough estimate of the air quality impact of just one emission source, you could run a screening model, like SCREEN3 or AERSCREEN. The advantages were there were fewer data inputs and the run time was usually just a few seconds. The disadvantages were the user is limited to only one source and the results could be extremely over-predictive, requiring the inputs to be tweaked again and again.
One hidden disadvantage is the review of SCREEN3 and AERSCREEN runs by regulators is completely manual and very time consuming on a per source basis. For example, screening models do not require a spatial coordinate for the source, but the user is required to provide a distance from the source to the nearest property line. To fulfill the requirement, the user has to provide the coordinates of the source, the a representation of the property line, and the nearest distance between the two. Though the screening model itself requires fewer input than a full dispersion model, the user ends up having to provide all the same inputs as required by the dispersion model plus the distance from the source to property line.
When you consider compiling all documentation necessary for an air quality analysis using a screening model, are you really saving any time or effort and does the analysis get through the regulator any quicker?
Wouldn’t it be helpful if you could easily access and reuse all the information from a previous full dispersion modeling analysis and rerun the full model with only the changes specific to your project? Another option is to use a screening technique with a full dispersion model called unitized emission rate modeling. For more detailed description see the article at http://www.naviknow.com/2018/04/18/unitized-dispersion-modeling/.
Another shortcut used with screen and full dispersion models is source collocating, i.e. combining multiple sources into one representative source. The advantages are only one set of source parameters needs to be entered as input and the run time is shorter than modeling all the sources involved. The disadvantages are performing the calculations to justify the one set of source parameters and the increased review time to validate the calculations. Also, if the over-estimated concentrations are too high, tweaking the inputs requires redoing the calculations for the source parameters.
As with using a screen model or any other screening technique, you will need to consider compiling all documentation necessary for the reviewing regulator to determine the appropriateness of your analysis.
Several regulatory agencies have pre-processed meteorological data that modelers can use. By having these datasets available, it saves regulator staff time by not having to review the processing of raw data since they have already done it for the user. If you have processed raw meteorological datasets using AERMET, you know it is easier and quicker to just download a dataset and not have to provide documentation as to why you selected the dataset used. In most cases, you will still have to use AERSURFACE to customize the dataset a little for your specific project. The effort for AERSURFACE should be less than an hour.
The downside to using agency-provided pre-processed meteorological datasets is the data are not specific to your project site and may provide an over-estimate of air contaminant concentrations.
If you choose not to take a shortcut on building your own site-specific meteorological dataset, you can save some valuable time by viewing a series of short mini-courses on AERMET at https://naviknow.teachable.com/p/aermet-made-easy-preview. By using the tips in the mini-course, building a site-specific dataset will take an hour or two on your time.
Wouldn’t it be nice if you were able to process raw meteorological data into a model-ready dataset with all the necessary documentation in the same amount of time it takes to download a pre-processed dataset? We are working on it. In the meantime, you can go to http://www.naviknow.com/aermet-data-preparation-service/ where we will prepare your custom meteorological datasets with a very quick turn around.
From the list of common dispersion modeling shortcuts provided in this article, it is clear that there is a cost of using a shortcut. Sometimes is it a small price, such as with the pre-processed meteorological datasets, or a much larger price as in the case of using a screen model or screening technique like source collocation.
NaviKnow has the attitude that you should never have to sacrifice time for refinement.
If you found this article informative, there is more helpful and actionable information for you. Go to http://learn.naviknow.com to see a list of past webinar mini-courses. Most every Wednesday (Webinar Wednesday), NaviKnow offers FREE webinar mini-courses on topics related to air quality dispersion modeling and air quality permitting. If you want to be on our email list, drop me a line at [email protected].
One of the goals of NaviKnow is to create an air quality professional community to share ideas and helpful hints like those covered in this article. So if you found this article helpful, please share with a colleague.