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Utilization of the U.S. EPA Exceptional Events Rule in the OVPA

This chapter discusses the applicability of the U.S. Environmental Protection Agency’s (EPA’s) Exceptional Events Rule (EER; 40 C.F.R. § 50.14) for excluding air quality monitoring data when determining the Owens Valley Planning Area’s (OVPA’s) compliance with the National Ambient Air Quality Standards (NAAQS; Box 1-1). Based on the statement of task and information provided by the sponsors, this chapter focuses only on PM₁₀ exceedances that were identified by the Great Basin Unified Air Pollution Control District (the District) as originating from local off-lake sources (i.e., those OVPA sources that are above the 3,600-ft-elevation regulatory shoreline) and the necessary criteria to demonstrate exceptional events due to high wind dust events. However, the panel recognizes that on any individual monitored exceedance day, dust from both on-lake and off-lake sources may contribute to PM₁₀ exceedances at monitoring sites in the OVPA to varying degrees. This chapter does not consider exceptional event demonstrations for wildfires or regional events, nor does it include findings from the agencies able to prepare and/or concur with exceptional events demonstration(s).¹ This chapter also does not provide approvable analysis results under the EER for the OVPA; only the air quality management agencies (hereafter air agencies) can make EER-applicable demonstrations as authorized under the federal Clean Air Act.

THE EXCEPTIONAL EVENTS RULE

An exceptional event is “an event and its resulting emissions that is not reasonably controllable or preventable, and is caused by human activity that is unlikely to recur at a particular location or a natural event” (40 C.F.R. § Part 50.1[j]). Exceptional events are considered to be “events for which the normal planning and regulatory process established by the Clean Air Act (CAA) is not appropriate” (Treatment of Data Influenced by Exceptional Events, 72 FR 13560, 2007). When an air agency demonstrates (and EPA concurs) that a PM₁₀ exceedance is an exceptional event, those data are excluded from specific decisions determined to be of “regulatory significance” to be made under EPA CAA authority related to NAAQS attainment and related planning requirements. These regulatory significant decisions include NAAQS area designations and redesignation decisions, classifications, determinations regarding whether a nonattainment area has reached attainment, certain findings of State Implementation Plan (SIP) inadequacy, and other actions on a case-by-case basis (40 C.F.R. § 50.14[a][1]). Due to the 3-year design value period, it often takes more than 3 years to determine if an EER demonstration would have the effect of changing

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¹ For the OVPA, these agencies are the U.S. EPA, the California Air Resources Board (CARB), and the Great Basin Unified Air Pollution Control District (GBUAPCD).

the rolling 3-year average of exceedances used to determine attainment at an individual monitor or monitors and whether that effort could support a change in nonattainment status or affect related planning requirements.

Although not necessarily intuitive from the name, the determination of exceptional events is unrelated to event frequency (or infrequency). Instead, in the case of high wind events, certain exceedance event data can be excepted (i.e., excluded) by demonstrating the following criteria:

• the high-wind event clearly caused or led to the monitored exceedance,
• the exceedance event “was both not reasonably controllable and not reasonably preventable,” and
• the exceedance “was caused by human activity that is unlikely to recur at a particular location or was a natural event,” among other requirements, including public notice (EPA 2019b, p. 2).

EPA (2019b, p. 4) defines the high wind threshold as “the minimum wind speed capable of causing particulate matter emissions from natural undisturbed lands in the area affected by a high wind dust event.” As specified in the Exceptional Events Rule, EPA accepts a threshold of a sustained wind² of 25 miles per hour (mph) for certain areas of the western U.S…” including California, “as long as the value is not contradicted by evidence in the record at the time the demonstration is submitted.” For California, the high wind threshold of a sustained wind of 25 mph is based on findings from James et al. (1999) and Wacaser et al. (2006).³ However, the EER specifies protocols for evaluating any given exceedance event based on three tiers: Tier 1) for high-energy dust events with ≥40 mph sustained wind speed and ≤0.5 mile visibility, Tier 2) for events with ≥25 mph sustained winds, and Tier 3) for all other high wind dust events with <25 mph sustained wind speeds. For events with lower wind speeds, increasing evidence is required to justify an exceptional event (EPA 2019b).

The EER defines a natural event as “an event and its resulting emissions, which may recur at the same location, in which human activity plays little or no direct casual role” (40 C.F.R. § 50.1[k]). EPA (2019b, p. 16) further clarifies that “an event involving windblown dust solely from natural undisturbed landscapes is considered a natural event and therefore not reasonably controllable.” Thus, according to EPA guidance, any PM₁₀ exceedance event caused by a source judged to be natural meets the definition as “both not reasonably controllable and not reasonably preventable.” EPA generally considers natural disasters (e.g., hurricanes, wildland fires) to be natural events (Treatment of Data Influenced by Exceptional Events, 72 FR 13560, 2007), even though some of them may have been exacerbated by human activities and human-induced climate change. Additionally, the rule states: “The Administrator will consider high wind dust events to be natural events in cases where windblown dust is entirely from natural undisturbed lands in the area or where all anthropogenic sources are reasonably controlled….” (40 C.F.R. § 50.14[b][5]). Thus, events that include emissions from mixed sources (i.e., both natural and anthropogenic sources) can be considered natural events only if reasonable controls have been applied to the contributing anthropogenic sources (Treatment of Data Influenced by Exceptional Events, 72 FR 13560, 2007).

As stated in 40 C.F.R. § 50.14(b)(8), an event from an anthropogenic source is considered not reasonably controllable if “reasonable measures to control the impact of the event on air quality were applied at the time of the event.” According to 40 C.F.R. § 50.14(b)(5)(iv), air agencies are not required to submit justification for a high wind dust event not being reasonably preventable. Documentation of Tier 1 high-energy dust storms (≥40 mph sustained wind speed) is generally considered sufficient for an event to be considered not controllable. For Tier 2 (≥25 mph sustained wind speed) events, source areas are presumed reasonably controlled if the control measures in place at the time of the event for all relevant anthropogenic source areas have been identified in a SIP that was approved by EPA within the past 5 years. If there is a SIP but it was approved more than 5 years prior to the event, some level of documentation is necessary to satisfy the requirement that the relevant sources were reasonably controlled. Otherwise, air agencies can submit a control analysis to demonstrate reasonable

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² EPA defines a sustained wind speed as one where there is at least one full hour in which the hourly average was at or above the area-specific high wind threshold. EPA may also consider a sustained wind speed based on shorter averaging times on a case-by-case basis (EPA 2019b).

³ Agencies can propose an alternate area-specific high wind threshold if it is shown to be more representative of the local or regional conditions. EPA guidance includes Ono (2006), which focuses on the Owens Lake region as an example of an area-specific calculation of a high wind threshold. Ono (2006) find a wind speed threshold (adjusted to 10-m height above ground level [AGL]) of 17 mph for the Keeler Dunes. Thus, local OVPA high wind thresholds may be slightly lower than the regional 25 mph criterion.

control measure implementation and enforcement. EPA (2019b, p. 11) notes that “the reasonableness of measures is case-specific and is evaluated based on information available at the time of the event” and that dust controls on anthropogenic sources are reasonable in any case where they “render the anthropogenic source as resistant to high winds as natural undisturbed lands in the area affected by the high wind event” (40 C.F.R. § 50.14 [b][5] [v]). Additional analyses are required for Tier 3 (<25 mph sustained wind speed) events to justify that they are not reasonably controllable (EPA 2019b).

When considering the EER, it is important to understand that EPA was instructed by Congress to follow several principles in implementation of Section 319 of the CAA, including that “public health protection is the highest priority,” and this applies even when attainment decisions are made with exceptions for high wind events. Section 319(b)(3)(A) states that “each state must take necessary measures to safeguard public health regardless of the source of the air pollution.” Timely public notification is required when an “event occurs or is reasonably anticipated to occur which may result in the exceedance” of PM₁₀. All data are required to be included in a publicly accessible air quality database, and any exceptional event demonstration is subject to a public comment period of at least 30 days (40 C.F.R. § 50.14[c]).

APPLICABILITY OF EXCEPTIONAL EVENTS RULE FOR OFF-LAKE SOURCES IN THE OVPA

As discussed in Chapter 3, the OVPA is currently classified as being in “serious nonattainment.” Since 2017, even excluding wildfire smoke, only two of the nine monitors—Lone Pine and Stanley—have consistently met the NAAQS criteria of no more than one exceedance per year averaged over 3 years (Table 5-1). Successful demonstrations for high wind exceedance events in the OVPA would remove those exceptional event PM₁₀ exceedances in the determination of NAAQS compliance at that monitoring site. The District could reach regulatory attainment if all monitoring sites achieve the design value of less than one event per year, averaged over 3 years, as required by the NAAQS. Once regulatory attainment is achieved, the air agencies would be required to request redesignation and submit a Maintenance Plan SIP for approval. Monitoring and maintenance of all source areas would continue to ensure compliance with the NAAQS.

In 2017–2024, approximately 74 percent of exceedances have been attributed to sources other than “primarily on-lake sources,” including wildfire smoke, regional events, and local off-lake sources (see Figure 3-4). However, the exceedance counts from recent years show that the on-lake exceedances alone at two monitors (Shell Cut, Dirty Socks) exceed the requirement of no more than one exceedance per year per monitor averaged over 3 years (see Figure 3-3). In 2021–2023 and 2022–2024, the Dirty Socks monitor recorded nine and eight exceedances, respectively, that have been attributed to on-lake sources (exceedance database, Chris Howard, GBUAPCD, personal communication, August 2024 and April 2025). These numbers are around triple those allowable for attainment. Some of the recent exceedances at Dirty Socks may have been sourced from an on-lake area (T1A4A), where the Los Angeles Department of Water and Power completed contingency controls in 2023. Continued monitoring of the effectiveness of these on-lake controls will be helpful in evaluating on-lake control and the value of potential future application of the EER. Utilization of tools that calculate design values—like those produced by the South Coast Air Quality Management District—can help the District calculate if a reasoned use of the Exceptional Events Rule might be applicable and worth the investment of time and resources.

The panel was asked to “discuss the applicability of U.S. EPA’s Exceptional Events [Rule] for excluding air quality monitoring data affected by dust from off-lake sources” during high wind events in the OVPA (Box 1-1, Figure 5-1). As described above, a key aspect of considering EER applicability for a given exceedance under high wind conditions is whether either of these conditions occurred:

1. the exceedance was a natural event, meaning that windblown PM₁₀ was entirely from natural undisturbed lands in the area, where “human activity plays little or no direct causal role,” or
2. reasonable control measures were applied at the time of the high wind event to control PM₁₀ emissions from all anthropogenic sources.

Off-Lake Sources: Natural Event or Affected by Human Activity

The panel first evaluated what local off-lake PM₁₀ exceedances might have been caused by a natural event, as defined by EPA. Based on the currently available evidence outlined in Chapter 4, there are a few PM₁₀ source categories in the OVPA that the panel judges to be entirely natural, undisturbed lands. These include alluvial fans and many of the flood deposits, excluding those that have otherwise been affected by human activity, such as highway infrastructure. The panel also judges that the Swansea Dunes are likely natural, based on position and general dune movement over the past century, although this is based on limited information (see Chapter 4).

Other sources of PM₁₀ emissions in the OVPA were deemed by the panel to be partially anthropogenic in origin. As described in Chapter 4, these include the Keeler Dunes, which were formed prior to lake drainage but were augmented by sediments from the dry lake surfaces that destabilized the vegetated dunes in a way that increased dust emission potential. Another likely source of partial anthropogenic origin are the flash-flood deposits south of Dirty Socks that were potentially enlarged in area by highway construction, which blocked the flow of floodwaters (see Figure 4-19).

As described in Chapter 4, the Olancha Dunes have somewhat expanded to the south in recent years, but it is unclear if this is related to drainage of the lake or recreation, including off-highway vehicle (OHV) use. Although no research has been completed on the impacts associated with decades of OHV activity at Olancha to PM₁₀ exceedances, research at other sites shows clear associations between OHV activity, decreasing vegetation cover, and increased dust emissions (Cheung et al. 2021; Gillies et al. 2022; Groom et al. 2007; Walker et al. 2023). Chapter 4 includes recommendations for additional analysis to determine the effect of human activity on emissions from the Olancha Dunes. If these areas that are affected by human activity contribute to off-lake exceedances, a control measures analysis would need to be conducted, and reasonable controls would need to be implemented before exceptions under the EER could be considered.

Finally, there are several flood deposits described in Chapter 4 (e.g., Keeler, Centennial Wash) that were influenced by highway berms or a limited number of culverts, but it remains unclear whether the highway infrastructure affected PM₁₀ emissions beyond what would be expected without that infrastructure in place. EER demonstrations for exceedances originating from flood deposits potentially influenced by the highway may benefit from additional analysis that demonstrates that PM₁₀ emissions from these flood deposits have not been exacerbated by roadway infrastructure.

Reasonable Control Measures Applied

The panel also evaluated the “reasonably controllable” criterion for off-lake sources. EPA (2019b, p. 16) states that “an event involving windblown dust solely from natural undisturbed landscapes is considered a natural event and therefore not reasonably controllable.” For sites affected by human activity, however, air agencies are required to demonstrate that an event is not reasonably controllable. As described previously in this chapter, an event is not reasonably controllable if reasonable measures were implemented on anthropogenic sources at the time of the event. It is beyond the charge to the panel to determine what controls are “reasonable measures” because “reasonable” includes policy judgments rather than purely scientific assessments. However, the panel notes that a thorough consideration of reasonable controls would consider resilience under the impact of climate change (e.g., planning for more intense flooding and prolonged drought). An assessment of reasonable controls could be part of a controls analysis that the EPA would require for the OVPA, as the latest approved SIP (GBUAPCD 2016) is over five years old (EPA 2019b). EPA (2019b) states that it will use the local list of reasonably available control measures (RACM) or best available control measures (BACMs), as applicable, as a reference point to review the reasonableness of new controls, and they note that more stringent controls may be reasonable for areas like the OVPA, which have frequent exceedances due to high winds.

Within the focus area of this study, the only off-lake source with current dust control measures is the Keeler Dunes, where emissions have been reduced by implementation of the approximately 170-acre Keeler Dunes Dust Control Project (see Chapter 6, Box 6-1), which was previously determined to be BACM (Federal Register vol 81, no. 238). A controls analysis would also be needed to determine if there are reasonable controls (Chapter 6) that

could be implemented for exceedances from other sources determined to be anthropogenically influenced (e.g., flash flood drainage pooling caused by the highway).

The above evaluation for any given exceedance event requires consideration of wind speed using the EPA (2019b) three-tiered approach that determines the level of justification needed to assess if reasonable measures were implemented. Few off-lake exceedances (7 of 94) between 2017 and July 2024 in the OVPA have ≥40 mph sustained wind speed, which is one criterion for Tier 1 events (Figure 5-2). Nearly all off-lake exceedances met the sustained hourly wind speed of ≥25 mph for Tier 2 (Figure 5-2). However, the panel did not do a thorough investigation of a potential causal relationship between wind speed and PM₁₀ emissions for every exceedance, which is required for an exceptional events demonstration (EPA 2019b). Only three exceedances between 2017 and July 2024 had sustained winds of <25 mph (Tier 3), and two of these had sustained windspeeds of over 24 mph.

PUBLIC HEALTH IMPACTS AND A REASONED USE OF THE EXCEPTIONAL EVENTS RULE

As described in Chapter 1, PM₁₀ particles can penetrate the lungs and cause or worsen a variety of health problems like asthma, bronchitis, chronic obstructive pulmonary disease, and respiratory infections. Section 319 of the CAA indicates that “public health protection is the highest priority,” and this applies even when attainment decisions are made with exceptions for high wind events. Section 319(b)(3)(A) states that “each state must take necessary measures to safeguard public health regardless of the source of the air pollution.” However, Congress has recognized that it “may not be appropriate for the EPA to use certain monitoring data collected by the ambient air quality monitoring network and maintained in the EPA’s AQS database in certain regulatory determinations” (40 C.F.R. § 52). Instead, Congress provided the statutory authority for the exclusion of data influenced by exceptional events (i.e., those events that are sourced from natural, undisturbed sources and anthropogenic sources that are not reasonably controllable). Exclusion of data from these PM₁₀ sources may not improve the quality of polluted air in the OVPA, although the CAA requires timely information to alert the public whenever air quality is considered unhealthy. Exclusion of exceptional events provides a means for areas with high natural background levels of PM₁₀ to attain the NAAQS, which can promote and encourage action on other reasonably controllable sources.

If PM₁₀ emissions from natural or not reasonably controllable areas increase in the future due to climate change, the OVPA could experience worsening air quality (see Chapters 2 and 4). Under the Exceptional Events Rule, determinations about what is “natural” or “reasonably controllable” need to be made with care to prevent worsening air quality from anthropogenically influenced areas. Since changes in EPA’s air quality regulations and exceptional events policies in 2007, which both increased stringency of air quality attainment and also included wildfires as natural events, there has been a marked increase in days that are submitted for exclusion of this data from regulatory determinations, and these are largely driven by wildfire and high-wind dust events (David et al. 2021). Reasoned use of the Exceptional Events Rule involves careful consideration of the anthropogenic drivers of PM₁₀ emissions and the policy, health, and economic implications of controls. Acknowledging the long-term management approaches that enhance wildfire and dust emissions in otherwise “natural” areas as anthropogenic drivers of PM₁₀ emission could incentivize local to regional efforts to proactively manage for ecosystem resilience in a changing climate and mitigate the effects of wind, fire, and storms on air quality (Clifford 2022; David et al. 2021; Richmond 2019). Such proactive management of natural lands can include prescribed fires to mitigate larger wildfires (EPA 2019a), changes in water management, and vegetation restoration.

CONCLUSIONS AND RECOMMENDATION

The Exceptional Events Rule for high wind dust events can be used to exclude unusual or naturally occurring exceedance event data from consideration in regulatorily significant decisions, as long as other public health protections are met. Nevertheless, the way the Exceptional Events Rule is interpreted and implemented has consequences for a region’s air quality and associated health impacts. Based on current EPA guidance and the panel’s evaluation of the origin and evolution of off-lake sources, some exceedances from off-lake sources in the OVPA may be considered natural events, as human activity played little or no direct causal role.

Some emissions from off-lake sources in the OVPA may be considered anthropogenically influenced. Since the OVPA has a State Implementation Plan that is over 5 years old, the Exceptional Events Rule requires an assessment of controls that could be applied to these anthropogenically influenced sources and the implementation of reasonable controls. It is beyond the charge to the panel to determine what controls are “reasonable” because that includes policy judgments rather than purely scientific assessments.

A thorough consideration of reasonable controls would consider resilience of the dust control measure under climate change. These considerations may include designing for more intense flooding and prolonged drought.