Establishing Representative Background Concentrations for Quantitative Hot-spot Analyses for Particulate Matter

Focus Area

Air Quality


Air Quality






Under 1 year

Research Idea Scope


Background concentrations representative of a project area are the most critical information needed for transportation projects subject to the new guidance for quantitative hot-spot analyses of particulate matter (PM).  The background component is added to the project component to determine the design value for comparison to each relevant National Ambient Air Quality Standard (NAAQS).  In almost all cases, the background concentration is the predominant component in the design value computation.  If the background concentration is greater than the NAAQS, then a build versus no-build analysis is required.

New skill-sets and transportation conformity tools are of critical importance for State DOTs to implement new transportation conformity guidance.  Specifically, DOTs need methodologies for establishing representative background concentrations based on existing ambient air quality monitoring and associated data or on forecasts using air quality models.  Establishing representative background concentrations based on monitoring or modeling is highly complex with multiple variables and limited expertise outside of state environmental agencies and EPA.  

EPA’s December 2010, Transportation Conformity Guidance for Quantitative Hot-Spot Analyses in PM2.5 and PM10 Nonattainment and Maintenance Areas (“EPA Guidance”) discusses a few factors to consider in establishing background PM concentrations needed for the analysis, whether selecting an existing representative monitor or set of monitors and data or relying on forecasts from an air quality model.  This research will identify and evaluate other methodologies not yet specified.   This EPA Guidance also indicated that EPA and the conformity consultation process will be used to determine analysis requirements and are needed for determining unique and case-by-case issues. Two unique issues state DOTs, EPA and conformity consultation partners may need to address is how to account for exceptional events (EEs) and/or pollution transport, so that representative background concentrations are not higher than required for demonstrating conformity to the State Implementation Plan (SIP).  This is because the SIP is allowed to limit demonstration of attainment by excluding certain events outside of state control.  This research shall provide methodologies and associated rationale to identify and account for EEs and pollutant transport when establishing background concentrations.  This research will be highly beneficial by adding tools to the conformity toolbox as well as increase the knowledge base for all transportation conformity consultation partners (e.g. FHWA, EPA, state DOT, MPO, state and local air quality agency).  Appropriate methodology that is acceptable to EPA and consultation partners will reduce transportation project development time (NEPA and transportation conformity) for projects subject to hotspot analyses, including reductions in both analysis time and agency reviews (EPA, FHWA, state DOT and air quality agencies).

Background Information

EPA did not implement the quantitative hotspot analyses requirements under (40 DFR 93- Transportation Conformity) until EPA released MOVES (model to calculate transportation emissions for conformity) in December 2010.  EPA’s Guidance appropriately affords flexibility to address analysis requirements and unique case-specific issues through the conformity consultation process including EPA.  Since this is a new area for many resource agencies that make up the consultation partners, this research will help expand the knowledge base for these various resource agencies.

Hotspot analyses are part of transportation conformity which demonstrates conformity to a SIP.  A DOT should not be subject to analysis that is more rigorous than or a background concentration that is higher than SIP requirements that may not have to account for EE and pollutant transport.  The basic hotspot analysis is:  

Background PM ug/m3 + preferred option project level contribution PM ug/m3 – mitigation PM ug/m3 (if needed) = X  PM ug/m3.  

X PM ug/m3 will need to be less than the PM NAAQS level or less than the no build option in order for the project to proceed.    

The EPA Guidance, Appendix K.5 provides an example for establishing  the 24-hour PM10 NAAQS background level as the highest 24-hour background concentration (from three years of monitored data), but what constitutes highest “background” concentration is not further defined. Methodologies for determining representative monitors and data for calculating background concentrations will benefit consultation partners when consulting on the project level hotspot analysis requirements.

Section 319(b)(3)(B) of the federal Clean Air Act (CAA) identified exceptional events as events for which the normal planning and regulatory process established by the Clean Air Act (CAA) is not appropriate.  The 3/27/2007 EPA Exceptional Events Rule (EER) implements CAA Sections 319(b)(3)(B) and section 107(d)(3).  These sections give authority to exclude air quality monitoring data from regulatory determinations related to exceedances or violations of the (NAAQS) and to avoid designating an area as nonattainment, redesignating an area as nonattainment, or reclassifying an existing nonattainment area to a higher classification if a State adequately demonstrates that an exceptional event has caused an exceedance or violation of a NAAQS.  The EER contains the following definitions at 40 CFR 50.1:  

• (j) Exceptional event means an event that affects air quality, is not reasonably controllable or preventable, is an event caused by human activity that is unlikely to recur at a particular location or a natural event, and is determined by the Administrator in accordance with 40 CFR 50.14 to be an exceptional event. It does not include stagnation of air masses or meteorological inversions, a meteorological event involving high temperatures or lack of precipitation, or air pollution relating to source noncompliance.

• (k) Natural event means an event in which human activity plays little or no direct causal role.

• (l) Exceedance with respect to a national ambient air quality standard means one occurrence of a measured or modeled concentration that exceeds the specified concentration level of such standard for the averaging period specified by the standard.

Wildfires, dust storms (high wind events) and other natural events and pollutant transport (e.g. multi-national pollutant transport, bi-national pollutant transport, inter-and intra-state transport) are beyond the ability for a state to control and may be excluded under certain circumstances from SIP attainment demonstrations. While the EER is specific to exceptional events that exceed a NAAQS, such events can also lead to monitor data that is higher than what is representative of  “background concentrations”.   DOTs need appropriate methodologies to include representative data and exclude non-representative data for project level hotspot analyses.  

Currently, state environmental agency development of an exceptional events package and EPA’s concurrence or denial of that package exceeds 18-48 months.  Approval of transport issues is also lengthy and highly variable and may require detailed multi-year analyses.  Since such decisions are likely to occur beyond the project development schedule, alternatives are necessary.  

Section 179B (International Border Areas) of the CAA allows a state to demonstrate attainment of a NAAQS, but for emissions originating from outside of the United States.  State environmental agencies and EPA track PM emissions from around the globe including, for example, speciation and identification of Saharan Desert particles.  State DOTs need methodologies for ensuring truly “representative” background concentrations for project level analyses.    

EPA’s Guideline on Air Quality Models” (Appendix W to 40 CFR Part 51, section 8.2) states background air quality includes pollutant concentrations due to (1) natural sources (2) nearby sources other than the one(s) currently under consideration and (3) unidentified sources.  While this guidance was prepared for prevention of significant deterioration (PSD) type point sources it identifies key parameters to consider with background concentrations.  It also identifies some of the challenges posed with the use of a single (multiple) ambient monitor(s) to determine background concentrations.  For example, in one urban PM10 nonattainment area all regulatory monitor for the area have an average 24-hour concentration over 4 years of 38+/- 50 ug/m3 (2 standard deviations), while the highest monitor reading for this time proposed was 249 ug/m3.  The state has flagged the 249 ug/m3 data as an exceptional event and is developing an exceptional events package for potential EPA concurrence.  In addition, there were 7 high wind events with high PM10 concentrations and wind gusts between 40 and 60 mph.  If those events are excluded, then the highest monitor reading drops from 249 to 73 ug/m3.  A NOAA report evaluating over 70 years of dust storm events in that area indicated wind speeds above 13 mph may cause dust storms in the areas. Options for establishing background concentrations and representative monitors is a departure from the historical regional transportation conformity process known by consultation partners, so methodologies and rationales can help consultation partners address this new requirement.  

Some options for research consideration may include, but is not limited to:

• Provide methodologies and rationales for how to exclude emissions from high wind events and other events that would be exceptional events if the NAAQS is exceeded.

• Identify what to consider for defining such events, such as wind gusts or other readily available information.    

• Provide methodologies and rationales for how to exclude monitoring data affected by pollutant transport. Identify what readily available information could be considered for defining such events.  

• Determine options for how to handle flagged exceptional events that EPA has not made a decision when assessing background concentrations.

• Identifying statistical analyses of monitoring data could be used to calculate background concentrations (e.g. average of data).



• Provide a synthesis report of how exceptional events and/or pollutant transport has been handled in SIPs or other state initiatives.

• Provide a summary of: what cautions or precautions should be considered if following the EPA Guidance and using a monitor remote to the area, just how representative would remote monitors be to a specific nonattainment and/or maintenance area, and what measures can be taken to ensure representativeness of remote monitors.

• Provide options and methodologies for establishing representative background monitors above and beyond that presented in EPA’s guidance.

• Develop options and methodologies for ensuring background concentrations are representative when evaluating ambient monitoring data that could lead up to an exceptional event (wildfires, high wind events that don’t exceed the standard, but are still uncontrollable by a state).

• Provide options and methodologies for how to ensure background concentrations are representative when evaluating ambient monitoring data that may include pollutant transport.

• Provide objective, defensible procedures and methodologies for use for determining representative ambient monitors for establishing base-year and future forecasted background concentrations for PM10 and PM2.5.  

• Provide objective, defensible procedures and methodologies that transportation consultation partners can use to determine future forecasted background concentrations for PM10 and PM2.5.  (MOVES and AP42 provide future emissions but not concentrations).

• Consistent with EPA Guidance, provide template language or an annotated outline (similar to CalTrans air quality annotated outlines located at, including but not limited to:  representative monitor selection, background concentrations, modeling results and any studies that support methodologies provided by the research.  

o Provide annotated bibliography of data, reports, and readily available reference information that can be used in establishing background concentrations when monitoring data captures “exceptional-type” events and/or pollutant transport  (e.g. SIPs, NOAA data – dust/smoke/fire events at, trends data that identify outliers).

o Identify and provide rationale and annotated bibliography for any suggested bright line thresholds that may constitute data that is not representative for use in background concentrations  (e.g. emissions when wind gusts exceed X mph).

• Provide rationale for each methodology and/or procedure provided.

Urgency and Payoff

If background concentrations are not “representative” and consistent with SIP requirements, then DOTs may be required to:

• conduct mitigation that is not necessary;
• conduct more analyses than necessary;
• not advance a project; and
• incur other project delays because the DOT and consultation partners have insufficient information or resources to determine “representative” background concentrations.

Suggested By

Jackie Ploch, Jill Schlaefer, Beverly Chenausky, Mike Brady, Michael Claggett; TxDOT, Colorado DOT, AZ DOT, CalTrans, FHWA

[email protected]