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New Issues in Dispersion Modeling

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Sulfur Dioxide (SO2 )

In 2010, the US EPA finalized a new 1-hour National Ambient Air Quality Standard (NAAQS) for SO2.  Since that time, a number of guidance documents have been issued by the US EPA to clarify the modeling procedures needed to demonstrate compliance.  The tighter SO2 NAAQS standards have resulted in more refined analyses of interactions between sources (culpability analysis) as well as a tiered approach regarding background monitor values. For example, in some cases hourly/seasonal background data is appropriate rather than a single background value.  In other cases, the background monitor data may be modified with agreement from the permitting agency to avoid “double counting” nearby sources.  These are challenges that must be navigated on a case-by-case basis.  To learn more about the EPA’s NAAQS standards click here. Please contact Shell Engineering if we can assist you with developing a compliance plan for SO2.

Particulate Matter

In 2013, the US EPA provided new guidance for PM2.5 (particulate matter < 2.5 microns) permit modeling.  This guidance encompasses a number of recent changes, and in particular addresses considerations of the secondarily formed component of PM2.5.  A facility that emits significant quantities of PM2.5 precursors such as SO2 and NOx, must account for the PM2.5 formed through chemical reactions with the atmosphere as a result of these emissions.  The methods for determining an appropriate value for the secondary impact vary and may include both qualitative and quantitative portions, because modeling tools for the chemical transformation are not readily available.  Shell Engineering has experience with demonstration of compliance for the current PM2.5 NAAQS, including the analysis of secondary impacts. To learn more about the EPA NAAQS standards click here. Please contact Shell Engineering if we can assist you with developing a compliance plan for PM2.5

Nitrogen Dioxide (NO2)

In 2010, the US EPA also finalized a new 1-hour NAAQS for NO2.  There are many similarities between the procedures for 1-hour modeling of NO2 and SO2.   However, the complexity of chemical conversions between NO and NO2 in the atmosphere may not be realistically accounted for in the traditional dispersion model.  In cases where further refinement is needed, the use of non-linear model options that require additional data, including ozone conditions and stack NO2/NOx ratios, may be justified.   Although simple Tier 1 approaches are preferred whenever possible, Shell Engineering has experience implementing more complex refinements when necessary.   Please contact Shell Engineering if we can assist you with developing a compliance plan for NO2. To learn more about the EPA NAAQS standards click here. Please contact Shell Engineering if we can assist you with developing a compliance plan for NO2