Coarse and Fine Particulate Matter Components of Wildland Fire Smoke at Devils Postpile National Monument, California, USA

Received: April 22, 2019
Revised: June 10, 2019
Accepted: June 14, 2019

Download Citation: RIS|BibTeX|https://doi.org/10.4209/aaqr.2019.04.0219  

Cite this article:

Schweizer, D., Cisneros, R. and Buhler, M. (2019). Coarse and Fine Particulate Matter Components of Wildland Fire Smoke at Devils Postpile National Monument, California, USA. Aerosol Air Qual. Res. 19: 1463-1470. https://doi.org/10.4209/aaqr.2019.04.0219

HIGHLIGHTS

• Smoke increased both fine and coarse components of particulate matter.
• Fine to coarse particulate matter median ratios were 1.37 without and .47 with smoke.
• Fine particulate matter dominated on days with the highest levels of smoke.
• Fine to coarse particulate matter ratios during all smoke days varied from 0.03–2.3.

Keywords: PM2.5; PM10; Wilderness air quality; Emission characterization.

INTRODUCTION

Wildland fire is increasing in size and intensity in the California Sierra Nevada from a history of fire suppression and climate change (Miller et al., 2009; Miller and Safford, 2017; Collins et al., 2019). Increased wildfire has brought increased awareness and interest in understanding public tradeoffs of different fire management strategies when assessing public health impacts (Black et al., 2017; Cascio, 2018).

Particulate matter (PM) is understood to be a significant pollutant in wildland fire emissions (Fowler, 2003) and is often used in exposure assessments of wildland fire smoke (Navarro et al., 2018; Sharma and Balasubramanian, 2018). Review studies have largely relied on combining studies of fine particulate matter less than 2.5 microns in aerodynamic diameter (PM_2.5), total particulate matter less than 10 microns in aerodynamic diameter (PM₁₀), and the coarse fraction of PM (PM_2.5-10) to obtain conclusions (Reid et al., 2016). Wildland fire PM_2.5 and PM_2.5-10 have both been shown to contribute similarly to lung damage in mice but the understanding of toxicity in dosage for PM of different size is largely an assumption with intratracheal instillation route masking PM_2.5-10 toxicity of actual PM dosage in the lungs by altering the delivery during inhalation (Wegesser et al., 2009). Air quality impacts from all wildland fire are similarly categorized whether they burn only timber or have a large component of emissions from structures or other toxins from manufactured items or industry (Landis et al., 2018; Wentworth et al., 2018).

While little study has been done to understand the physical and chemical characteristics and toxicity of PM, virtually none has been done to compare differences in smoke from wilderness fires versus wildland fires burning homes. Ignoring toxicity of various emission sources and grouping all wildland fire smoke into one category limits the veracity of current public health and exposure assessments.

PM is a complex mixture of chemical species from anthropogenic and natural emission sources and secondary pollutants which form in the atmosphere. PM_2.5-10 has been found to be dominated by crustal trace elements (e.g., Al, As, Cd, K, and V) except immediately adjacent to roads, where vehicle-generated emissions can dominate over these naturally occurring elements (Hueglin et al., 2005). Other trace elements of PM_2.5 (e.g., Ba, Ca, Ce, Cu, Fe, La, Mo, Mn, Pb, Sb, and Rh) are mainly from anthropogenic processes and gradually reduce from urban to rural areas (Hueglin et al., 2005). Secondary aerosol formation (sulfate and nitrate), while typically associated with the PM_2.5 component of PM, is variable and can have a significant fraction in PM_2.5-10 (Diapouli et al., 2017).

Understanding of accuracy and precision and the limits of agreement of varying PM monitoring methods to obtain data is integral to understanding comparability of conclusions (Schweizer et al., 2016). Review studies assessing smoke exposure often rely on PM monitoring data from varying methods without acknowledging this weakness or including analyses in the robustness of using specific measurements comparatively.

Emissions estimates suggest nearly 90% of wildfire emissions are PM_2.5 (Vicente et al., 2013; Huang et al., 2014). Meteorological conditions have a role in PM size distribution (Kozákovác et al., 2018; Thuy et al., 2018). Estimates of emissions are done in both lab and field settings but are often limited in time and number of samples. Chamber studies show high variability in emissions factors depending on many characteristics (Cereceda-Balic et al., 2017) while prescribed and wildland fire estimates are limited by monitor siting (Evtyugina et al., 2013; Reisen et al., 2018). The aging of wildland fire smoke plumes has long been understood to alter PM size distribution through coagulation and removal causing impacts of smoke to be mitigated more rapidly than predicted (Radke et al., 1995). Ground-level monitoring of PM_2.5 and PM₁₀ used to compare physical distribution of PM particularly in remote locations in fire-prone areas has been little documented.

The fire-prone landscape of the California Sierra Nevada has large areas of federal and state protected lands including federally designated wilderness areas. Devils Postpile National Monument (DEPO) in the Sierra Nevada of California has a history of air pollutant monitoring and assessments where wildfire has been shown to elevate an otherwise extremely low background concentration of PM_2.5 (Bytnerowicz et al., 2013; Burley et al., 2016). During 2018, smoke transported from local wildland fires and wildfires across California impacted the site. The objective of this study was to assess current background physical properties of PM when wildland fire smoke was not present and when smoke impacted PM during 2018 at DEPO. Sampling at DEPO provides empirical ground-based data of particle size dynamics in a fire-prone area with little anthropogenic source contribution more consistent with wilderness than rural or urban areas because of its location in wilderness far from anthropogenic sources of emissions. The PM_2.5 component was the primary constituent of PM₁₀ when wildland fire smoke was present. During smoke-free periods, the ratio of PM_2.5/PM_2.5-10 varied from over 4 times to as little as 0.37. In the presence of smoke, PM_2.5/PM_2.5-10 ratios were typical of background levels with no smoke. The high variability suggests the PM_2.5/PM_2.5-10 components at ground level in the field are much less static and consistent than currently expected when considering fire emissions alone.

METHODS

Site Description

DEPO is located along a section of the Middle Fork of the San Joaquin River at about 2,200 to 2,500 m in elevation. It is 322 ha in size, 302 ha of which is in the Ansel Adams Wilderness surrounded by the Inyo National Forest. Monitoring was conducted on the north end of DEPO approximately 7 km from the town of Mammoth Lakes (Fig. 1). DEPO is unique in that it is hydrologically located on the western slope of the Sierra Nevada near the lowest pass on the Sierra Divide and is in very close proximity to the eastern slope of the Sierra Nevada. Previous studies show that air quality in DEPO is impacted by the transport of pollutants from the Central Valley and farther west along with wildland fire smoke from the adjacent and local fire-prone landscape, particularly from the west (Bytnerowicz et al., 2013; Burley et al., 2016; Bytnerowicz et al., 2016, 2019).

Fig. 1. A. Monitoring and major fire overview, B. Devils Postpile and surrounding area map.

Particulate Monitor Settings and Quality Assurance

Two Met One Instruments, Inc. Environmental Beta Attenuation Monitors (E-BAMs) were co-located to collect hourly averages of fine particulate matter less than 2.5 microns in diameter (PM_2.5) and particulate matter less than 10 microns in diameter (PM₁₀) at Devils Postpile National Monument in California (Fig. 1) from June 13, 2018, to October 15, 2018. E-BAMs were installed 2 m distance apart with inlets within 0.5 m in vertical height. Internal settings were consistent between each E-BAM (e.g., date and time, tape advance, and internal relative humidity set points). E-BAMs were calibrated and audited for integrity of flow (leak check), temperature, and flow rate at a minimum of once every 2 weeks (Met One Instruments, Inc., 2008). Data validity required flow to be 16.7 ± 0.8 L min^–1, temperature ± 2.0°C, and atmospheric pressure ± 2%, and leak check < 1.5 L min^–1.

Data Compilation, Handling, and Quality Control

E-BAM hourly values were used to calculate daily 24-hour averages so agreement could be aligned to more robust monitors used in compliance monitoring for particulate monitoring and not limited to simple association with correlation subject to misinterpretation on accuracy and precision (Schweizer et al., 2016). Negative E-BAM hourly values were set as 0 and a minimum of 18 valid hourly values were required to get a valid daily mean concentration. All calculations were done using the R software environment (Adler and Kelly, 2018; R Core Team, 2018).

Presence of smoke from wildland fire used field observations (e.g., personal observations, photo documentation, field notes) at the site during the E-BAM deployment. Satellite imagery along with fire and smoke reports were included in the determination of smoke impacts from wildland fire impacting the monitors for fire size, location, and distance from the site.

The E-BAM data included hourly averages of the meteorological variables including wind speed and direction, relative humidity, and ambient temperature. Meteorological conditions were used to compare smoke and times without the presence of wildland fire smoke and assess likelihood of error from environmental factors especially relative humidity (Schweizer et al., 2016).

RESULTS

Site Meteorological Conditions

The DEPO monitor site is in the canyon of the Middle Fork of the San Joaquin River. The San Joaquin River flows generally north to south at the site with the steep canyon walls oriented similarly. The site was characterized by consistent ground-level winds. The wind pattern consisted of winds that flowed from the northeast from midnight until the site experienced a calm period around 08:00, followed by a change to winds that came from the southeast until another calm period near 23:00 or 00:00 indicated the shift from predominantly down-canyon to up-canyon winds and the cycle repeated. Median wind speed (0.6 m s^–1) was similar for both smoke and non-smoke times with the maximum wind speed of 5.3 m s^–1 during the time without smoke. When smoke was present, wind speed had a 2.4 m s^–1 maximum.

Maximum (34.6°C) and minimum (–3.7°C) temperature occurred when wildland fire smoke was not present. Because of seasonal normal, as expected, median and mean temperatures were higher when smoke was present (4.4 and 3.9°C respectively).

Relative humidity at the site varied from ~5% to 90% with both smoke and non-smoke times nearly identical. RH was highest overnight, lowering in the morning to lows around 14:00. 

Wildland Fire Smoke Impacts

Wildland fire smoke impacted the monitoring site during June 23–July 7, 2018; July 7–August 13, 2018; August 16–August 18, 2018; and September 1–2, 2018, periods. During these periods, smoke was present from a single hour to the entire day. A number of large fires produced smoke that impacted much of California including the monitor at DEPO. Large regional fires contributed to smoke at the site but smoke was primarily from the nearest fires. Large regional fires occurring at the same time include the County Fire (start: June 30, 2018; 37,000 ha; ~300 km west-northwest), Carr Fire (start: July 23, 2018; 93,000 ha; ~450 km northwest), Mendocino Complex (start: July 27, 2018; 186,000 ha; ~ 400 km northwest), and the Donnell Fire (start: August 1, 2018; 15,000 ha; ~110 km northwest).

The Lions Fire was ignited by lightning on or around June 1, 2018, in the John Muir Wilderness. Fire management strategy of this fire was largely indirect “confine and contain” because of the remoteness, high potential for resource benefit and fuel reduction, and the low potential of impact to life and property. Therefore, the Lions Fire primarily burned up to natural barriers and containment lines strategically placed at a distance from the fire. The Lions Fire had a maximum 1-day growth of 456 ha on August 10, 2018, with 6 additional days of fire growth >200 ha and 15 days where fire growth was 100–200 ha. The fire burned less than 4 ha until June 23, 2018, when the fire spread to an estimated 400 ha. Until that time, no smoke was detected at the DEPO site. The Lions Fire largely stopped spreading at approximately 1,600 ha by July 2, 2018, with smoke continuing to be present at the site until July 7, 2018, while fuels within the perimeter continued to burn. The Lions Fire began contributing to smoke at the site as the perimeter began to grow again starting about July 30 adding to smoke from the Ferguson Fire. The Lions Fire continued to move west and away from the site and contributed to smoke at the site until September 2, 2018.

The Ferguson Fire was a full-suppression fire that started July 13, 2018. Daily growth of the fire from July 15 to August 6, 2018, varied from 1,000 to 3,000 ha. The Ferguson Fire contributed smoke to DEPO from July 24 to mid-August. Other fires (e.g., Lions Fire) contributed to smoke at DEPO. The Ferguson Fire was the primary contributor to the DEPO site from July 24 to the end of July. Even though the Ferguson Fire was farther from the site than other fires that impacted DEPO, the high-intensity burn of this fire increased injection height of the emissions to upper air winds where predominant transport is to DEPO.

Smoke input from several other fires coincided with the smoke from the Lions Fire (5,400 ha), which was approximately 6 km south-southwest of the monitors and the Ferguson Fire (39,000 ha) approximately 80 km west. Dominant wind patterns largely from the west, including the fire-prone western slopes of the Sierra Nevada, typically transport smoke from these locations to DEPO, while smaller fires often have impacts enhanced locally by drainage transport and local ground-level wind direction. The Ferguson Fire being over 7 times larger and generally burning larger areas per day had similar impacts on the DEPO site (Ferguson/Lions Fires median: PM_2.5: 39.8/45.7 µg m^–3; PM₁₀: 66.0/80.4 µg m^–3) as the Lions Fire. The Lions Fire smoke impacts were more localized than the Ferguson Fire, but nearness and local transport in the surrounding drainages made smoke contributions similar at this location.

Particulate Matter Contributions

Daily mean concentrations of PM were calculated from June 13, 2018, to October 15, 2018 (n = 123 days), with 84 days of valid data for both PM₁₀ and PM_2.5. For all days with available data, the maximum daily mean PM_2.5 concentration was 200.8 µg m^–3 and the minimum 1.7 µg m^–3. PM₁₀ concentrations ranged from 5.2 to 229.0 µg m^–3.

Over the 123 days, there were 84 days where there was data for both PM₁₀ and PM_2.5. A Mann-Whitney U test showed a significant difference (p < 0.001) between both PM_2.5 and PM_2.5-10 on smoke free and smoke impacted days. Mann-Whitney U test estimated PM_2.5 increased 35.3 µg m^–3 (95% CI: 26.7–48.0 µg m^–3) and PM_2.5-10 increased 10.33 µg m^–3 (95% CI: 7.6–14.3 µg m^–3) with smoke.

PM_2.5-10 was calculated for 33 days that were impacted by wildland fire smoke and 51 days with no wildland fire smoke. Mean concentrations were similar between PM_2.5 (9.1 µg m^–3) and PM_2.5-10 (10.9 µg m^–3) concentrations when smoke was not present, while mean PM_2.5 was over 2.5 times greater (63.8 µg m^–3) than the concentration of PM_2.5-10 (24.1 µg m^–3) when wildland fire smoke was present (Table 1).

PM_2.5-10 increased in the presence of wildland smoke but the increase and distribution was primarily in the concentrations of PM_2.5 (Fig. 2). Smoke impacted PM_2.5-10 was above the maximum no-smoke PM_2.5-10 concentration for 4 days (37.5, 48.6, 58.0, and 60.9 µg m^–3) while smoke impacted PM_2.5 was above the maximum no-smoke PM_2.5 concentration for 21 of the 33 days. The PM_2.5 component consistently showed when smoke was present at the site, while the PM_2.5-10 correlated weaker to the presence of smoke.

Fig. 2. Boxplot with kernel density plots of daily PM_2.5 and PM_2.5-10 in the presence and absence of wildland fire smoke at Devils Postpile National Monument in 2018.

The mean percentage contribution of PM_2.5 to PM₁₀ when smoke was present was 66% whereas contributions to days without wildland fire smoke was 44%. Days without smoke frequently had a larger contribution of PM_2.5-10 than PM_2.5, but when daily mean concentrations of PM₁₀ in the presence of smoke reached 100 µg m^–3, the primary component was PM_2.5 (Fig. 3).

Fig. 3. The ratio of daily mean concentrations of the coarse particulate (PM_2.5-10) component to fine particulate (PM_2.5) component compared to coarse particulate (PM₁₀) daily mean with and without wildland fire smoke.

DISCUSSION

Background concentrations of PM when wildland fire smoke is not present are often low and evenly distributed between PM_2.5 and PM_2.5-10. When smoke from wildland fire was present, the PM_2.5 component was approximately two thirds of all PM₁₀. The component ratio of PM_2.5-10 to PM_2.5 was typically comprised more of PM_2.5 (mean: 1.54, SD: 0.91, max: 4.1, min: 3.7) on days with no smoke, while primarily (only 3 days 1 or greater) in the PM_2.5-10 component (mean: 0.66, SD: 0.59, max.: 2.3, min.: 0.03) when smoke was present (Fig. 4). The PM_2.5-10 did increase when smoke from wildland fire was present but mean change was almost 4 times larger for PM_2.5.

Fig. 4. Ratio of coarse particulate component (PM_2.5-10) and fine particulate matter component (PM_2.5) during days with no wildland fire smoke and smoke impacted days.

Smoke from the Lions and Ferguson Fires were at times mixed. Smoke almost exclusively from the Ferguson Fire was limited to 8 days with only 5 having both PM₁₀ and PM_2.5 measurements. Because of the overlap of smoke contributing to the site and the limited number of days there was not a statistically significant difference in fine and coarse particulate components. Smoke from the nearer Lions Fire was attributed to the highest PM_2.5-10 concentration days (Lions: 60.9 µg m^–3; Ferguson: 30.1 µg m^–3) and the highest component ratio of PM_2.5-10 to PM_2.5 (Lions: 2.3; Ferguson: 1.0).

The complexity of smoke’s physical and chemical properties is not unknown (Statheropoulos and Karma, 2007) with emission estimates also being dependent on vegetation and combustion efficiency (Hosseini et al., 2013). Limitations in understanding from users, and fire and weather models themselves can negatively impact the quality of models used for smoke emissions and dispersal (Coen, 2018). PM data is for one location during a single fire season and not a comprehensive analysis (including ultrafine particles and chemical speciation) with multiple monitors over many years. However, ground-level PM at DEPO shows a large variation from more static estimates and assumptions of wildland fire emissions. This area of understanding in smoke and human health exposure appears to be subject to error that has not been rigorously assessed and needs further comprehensive understanding of both emissions and plume dynamics.

Current health analysis depends on a static relation of PM to assess impacts from smoke. While emissions of PM_2.5 from wildland fire in California forests have been estimated to be around 90% of the PM₁₀ emitted (Stephens et al., 2007) and to range between ~60 and 75% for wildfire globally (Huang et al., 2014), DEPO monitoring showed ground-level concentrations ranging from 30 to 97% in the PM_2.5 component in this remote forested environment.

CONCLUSIONS

The PM monitoring at DEPO demonstrates a limitation in general modeling assumptions for the distribution of PM in wildland fire smoke and the resultant ground-level concentrations. Further field-measured validation studies are needed to determine the physical distribution of PM in varying meteorological and transport scenarios and to evaluate the efficacy of using emissions to estimate the health impacts from smoke. Ground-level PM_2.5 and PM_2.5-10 in the wildland smoke at DEPO varied more than expected based on the emissions estimates, with transported smoke further increasing the concentration of PM_2.5.

ACKNOWLEDGEMENTS AND DISCLAIMERS

The authors would like to thank the U.S. Forest Service Pacific Southwest Region Air Program for providing the monitoring equipment, the National Park Service at Devils Postpile National Monument for providing equipment power and siting, and the Student Conservation Association summer interns for their help in site maintenance. Reference to any companies or specific commercial products does not constitute an endorsement by the authors.