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This study examines major wildfires in the western United States between 2005 and 2015 to determine which species exhibit the highest percent change in mass concentration on day of peak fire influence relative to preceding nonfire days. Forty-one fires were examined using the Environmental Protection Agency (EPA) Interagency Monitoring of Protected Visual Environments (IMPROVE) data set. Organic carbon (OC) and elemental carbon (EC) constituents exhibited the highest percent change increase. The sharpest enhancements were for the volatile (OC1) and semivolatile (OC2) OC fractions, suggestive of secondary organic aerosol formation during plume transport. Of the noncarbonaceous constituents, Cl, P, K, NO3−, and Zn levels exhibited the highest percent change. Dust was significantly enhanced in wildfire plumes, based on significant enhancements in fine soil components (i.e., Si, Ca, Al, Fe, and Ti) and PMcoarse (i.e., PM10–PM2.5). A case study emphasized how transport of wildfire plumes significantly impacted downwind states, with higher levels of fine soil and PMcoarse at the downwind state (Arizona) as compared to the source of the fires (California). A global model (Navy Aerosol Analysis and Prediction System, NAAPS) did not capture the dust influence over California or Arizona during this case event because it is not designed to resolve dust dynamics in fires, which motivates improved treatment of such processes. Significant chloride depletion was observed on the peak EC day for almost a half of the fires examined. Size-resolved measurements during two specific fires at a coastal California site revealed significant chloride reductions for particle aerodynamic diameters between 1 and 10 μm.
The western United States is becoming increasingly vulnerable to the effects of wildfires owing to a warmer climate, drought, and fire control strategies over past decades resulting in conditions that promote larger and more frequent fires [Flannigan et al., 2000; Moritz et al., 2012; Dennison et al., 2014; Hallar et al., 2017]. Biomass burning leads to emissions of gaseous and particulate species that are complex in nature and evolve in unknown ways as they age and mix during their transport. Although wildfire plumes significantly impact regions near the fire source, they can reach high altitudes above the mixing layer and can be transported long distances [e.g., Pickering et al., 1996; Nisantzi et al., 2014] to impact global atmospheric chemistry and climate [e.g., Crutzen and Andreae, 1990; Reid et al., 2005]. Plume particles can act as both cloud condensation nuclei and ice nuclei, consequently impacting cloud properties and their precipitation potential. Wildfire emissions impact precipitation chemistry at downwind sites, in addition to affecting the radiative properties, oxidative capacity, new particle formation potential of plumes, and biogeochemical cycles [Echalar et al., 1995; Chalbot et al., 2013; Sorooshian et al., 2013]. Smoke plumes contain many nutrients (e.g., Fe) and contaminants (e.g., As) that can affect downstream ecosystems where they deposit.
Chemical measurements are important both for linking air masses to fires by using tracer species and for quantifying enhancements of various constituents as a result of fires. Identification of robust tracer species is challenging as they could depend on flame condition and fuel type [Andreae and Merlet, 2001]. For example, Echalar et al.  highlighted how different sets of species are enhanced for different fuel types such as savanna fires (K, Cl, Br, and Zn) and forest fires (Cr, Si, and Ca). Another example is that K, a commonly used tracer, is emitted more in flaming conditions as compared to smoldering fires [Cahill et al., 2008; Lee et al., 2010]. Additionally, some species such as K may be enhanced in smoke in submicrometer sizes while also having other important sources such as dust leading to elevated concentrations in coarser sizes [e.g., Eldred et al., 1997].
Levoglucosan is well known to be a marker for wood combustion as it is derived from the breakdown of cellulose and hemicellulose molecules, [Fine et al., 2001; Hays et al., 2002], but it is vulnerable to degradation after some time [Fraser and Lakshmanan, 2000; Hennigan et al., 2010; Hoffmann et al., 2010]. Unlike primarily emitted tracer species such as levoglucosan, secondarily produced species from precursors emitted by fires are more difficult to use as tracer species for fires as they require additional time for formation, have multiple sources, and may undergo additional transformations in order to be depleted. However, this does not undermine the importance of studying them since they contribute greatly to aerosol mass concentrations, including secondary organic aerosol [Spracklen et al., 2007].
As dust is one of the largest contributors to particulate matter mass concentrations in the western United States, especially the Southwest, it is important to consider how biomass burning contributes to dust concentrations in the atmosphere. Measurements at a high-altitude site in northwest Colorado (Storm Peak Laboratory) showed that the median levels of contribution from dust and biomass burning aerosols to total aerosol optical depth were comparable [Hallar et al., 2015]. Fine dust can contribute to over half of PM2.5 mass concentrations in the southwestern United States during spring months [e.g., Kavouras et al., 2007, 2009; Sorooshian et al., 2011], with the onset of the dust season being sooner in the spring based on analysis of data between 1995 and 2014 [Hand et al., 2016]. Soil dust is often associated with biomass burning plumes as a result of turbulent mixing near flames and the burn front [Palmer, 1981; Gaudichet et al., 1995; Maenhaut et al., 1996; Clements et al., 2008; Kavouras et al., 2012; Chalbot et al., 2013; Popovicheva et al., 2014; Sturtz et al., 2014; Maudlin et al., 2015]. Soil dust accounted for 75–99% of the coarse particle mass in the plumes of flaming African savanna fires [Maenhaut et al., 1996]. In another study focused on fires by the California-Oregon border, Maudlin et al.  observed a 513% and 408% enhancement in mass concentration for Si and Fe, respectively, in submicrometer aerosol during fire periods as compared to nonfire periods in the summer of 2013. They also observed these soil dust tracer species in stratocumulus cloud water.
The goal of this work is to identify what particulate species are most enhanced in concentration in biomass burning plumes based on analysis of long-term data across the western United States from the Environmental Protection Agency (EPA) Interagency Monitoring of Protected Visual Environments (IMPROVE) network. This work builds on previous studies relying on IMPROVE data to investigate other aspects of wildfires in the same study region such as how much organic carbon (OC) and PM2.5 are produced specifically from fires in the summer season [Jaffe et al., 2008]. Particular focus is placed in this work on the extent to which fine soil and coarse mass concentrations (PMcoarse: PM10–PM2.5) are enhanced in biomass burning plumes. Unique aspects of the subsequent analysis are the broad spatial and temporal coverage of data in a region of North America vulnerable to biomass burning, the large number of species considered, treatment of different fractions of OC and elemental carbon (EC) based on thermal/optical carbon analysis, and the use of detailed size-resolved chemical data during two particular case study events to support conclusions from the IMPROVE data set. While the absolute concentration of aerosol constituents during wildfires is useful to discuss as well, this work focuses mainly on changes relative to background conditions, which has implications for both tracer species identification and for impacts on aerosol properties senstive to ratios of different aerosol constituents (e.g., hygroscopic and radiative properties). For example, if the volume fraction of low-hygroscopicity aerosol constituents is initially negligible prior to a wildfire, for the same concentration increase between low- and high-hygroscopicity constituents during the fire, the overall hygroscopicity can be reduced assuming linear additive water uptake by different species. These aerosol responses to fires have implications for visibility, climate, and public health.
This study relies on aerosol composition data from the IMPROVE network [Malm et al., 1994; http://views.cira.colostate.edu/fed/], which includes monitoring stations primarily in National Parks and Wilderness Areas. Figure 1 shows locations of the IMPROVE stations used in the analysis, and Table 1 summarizes the site details. Particulate matter is collected on filters for 24 h every third day. Samples are analyzed for ions, metals, OC and EC, and gravimetric mass measurements of both PM2.5 and PM10. Four fractions of OC and three fractions of EC are also discussed here, which are operationally defined based on the temperatures at which they are detected using the thermal optical reflectance method of carbon analysis [e.g., Chow et al., 1993; Watson et al., 1994]. More specifically, a flame ionization detector quantifies methane produced via either (i) volatilization of particulate species at temperatures of 120°C (OC1; volatile), 250°C (OC2; semivolatile), 450°C (OC3; nonvolatile), or 550°C (OC4; nonvolatile) in pure helium or (ii) combustion at temperatures of 550°C (EC1), 700°C (EC2), or 800°C (EC3) in a 98% helium and 2% oxygen environment.
Speciated analysis is conducted for PM2.5, including ion chromatography (IC) for water-soluble ions and X-ray fluorescence (XRF) for elements. Fine soil concentrations reported in this study are calculated using the following equation [Malm et al., 1994]:
The components and their coefficients in equation (1) were previously confirmed in comparisons of local resuspended soils and ambient particles in the study region [Cahill et al., 1980; Pitchford et al., 1981; Malm et al., 1994]. As this study is concerned with relative changes in fine soil levels, it is expected that this equation can successfully capture all soil-rich air masses regardless of whether minor variations exist in the factors used in equation (1). Species mass concentrations discussed in this study are from the fine fraction of aerosols (PM2.5). All species concentrations analyzed fit in the category of the “V0” status flag of the IMPROVE data set, which is defined as “Valid value.” Sampling protocols and additional details are provided elsewhere (http://vista.cira.colostate.edu/Improve/sops/).
Multiple sources of information are used to identify the presence of a wildfire. The first source of information is EC mass concentration from the IMPROVE monitoring sites. As EC concentration typically increases suddenly to a level that is far larger than its preceding and subsequent data points (Figure 2), it is possible in this way to identify the day with the strongest plume influence. As the IMPROVE measurements are every third day, it is cautioned that the peak EC day in this study for each fire may not coincide with the actual day of most smoke influence at a particular site. The EC mass concentration on the peak EC day always exceeded the average plus 2 times the standard deviation for the 10 week period centered around this day.
As a secondary confirmation of fire activity, output is used from the Navy Aerosol Analysis and Prediction System (NAAPS) [Lynch et al., 2016; http://www.nrlmry.navy.mil/aerosol_web/]. NAAPS relies on global meteorological fields from the Navy Operational Global Atmospheric Prediction System (NOGAPS) [Hogan and Rosmond, 1991; Hogan and Brody, 1993] analyses and provides output at a spatial resolution of 1° × 1°, at 6 h intervals, and with 24 vertical levels reaching 100 mb. Smoke from biomass burning is derived from near-real-time satellite-based thermal anomaly data used to construct smoke source functions [Reid et al., 2009]. We use surface concentrations and optical depths associated with smoke to confirm that fires occurred during the time of IMPROVE EC peaks. If NAAPS did not reveal evidence of a major smoke event during the day of an IMPROVE EC peak, that particular event was omitted from the analysis.
The third form of confirmation for fires included visual inspection of satellite imagery provided by NASA (fires before 2011: https://modis-atmos.gsfc.nasa.gov/IMAGES/; fires after 2011: https://worldview.earthdata.nasa.gov). Figure 2 shows an example of how the three aforementioned criteria were applied to identify a representative fire at Bliss State Park (California). Because a model and satellite data were used as criteria to identify fires, the analysis is biased toward large fires as opposed to smaller ones that these two tools could not resolve.
It was important for the subsequent discussion to define a background period by which to compare data from the peak EC day. Data were analyzed for the 5 weeks before and after the day identified with the peak EC mass concentration of each fire. For cases when there were adjacent IMPROVE data points in time with comparably high EC levels at the time of the peak of fire influence, an average of the peaks is used as the peak of the fire. This was the case for 10 fires, where the EC concentration of adjacent data points were within an average of 30% ± 21%. Fires with multiple peaks that were not directly adjacent to one another in time are not included in the analysis. Fires were also omitted from the analysis in instances when concentrations were not reported for EC for any of the three sampling days before or after the day of the EC peak or when multiple species did not have the “V0” status flag indicative of “Valid value.” If there was more than one site that had the exact same EC peak date, the site was used with the most data coverage (i.e., some species concentrations are not reported) of species around the time of the peak. If data coverage was comparable, the site with the highest peak EC mass concentration was used.
In order to quantify enhancements of aerosol constituents during peak fire periods (i.e., day of highest EC mass concentration), four time periods are defined that are collectively meant to capture the strength of concentration enhancements. The three data points preceeding a peak are referred to as period “−1,” and the three points following were designated as period “1”; note that a 24 h data point is collected every third day and thus periods −1/1 represent over a week of time. The period extending from the start of period −1 to 5 weeks before the EC peak is denoted period “−2,” and the period following period 1 until 5 weeks after the peak was designated as period “2.” Percentage change is calculated as follows:
where [x] is the concentration of an aerosol constituent and “period” represents any of the four periods defined above. In order to qualify as a valid fire for analysis, a fire period could have missed a maximum of only 3 days of data (i.e., when a sample had no species concentrations reported) from the 5 week periods preceeding and following a fire’s peak. Only one of these missing days could be from period −1 or 1.
A total of 41 individual fires were analyzed with their dates and site details provided in Table 1. The state with the most fires was California (14 fires), followed by Oregon (12), Washington (7), Utah (3), Nevada (3), and Arizona (2). It is cautioned that the states have widely ranging site densities; for example, the number of IMPROVE sites collecting data during the study period in the various states was as follows: California (23), Arizona (17), Washington (11), Oregon (6), Utah (4), and Nevada (2). Idaho, with two active IMPROVE sites during the study period, registered no fires that met the study criteria. As subsequent discussion will compare different geographic categories, a region termed Northwest (NW) is defined for convenience that includes Oregon and Washington.
Table 2 summarizes how the percent change values vary between the four time periods used in equation (2) for 39 aerosol constituents available from the IMPROVE data set. Expectedly, there are strong correlations in the percent change values when intercomparing the four time periods. The slopes are important in showing how sensitive the percent change results are to the choice of the reference time period in equation (2). The slopes closest to unity were for the intercomparison of period −2 (ordinate) versus period 2 (1.05) and period −1 (ordinate) versus period 1 (1.03). The slope furthest from unity is 1.44 when comparing period −1 (ordinate) versus period 2. Table 2 indicates that enhancements are more depressed when using periods 1 and 2 as compared to −1 and −2, most likely owing to residual influence of the wildifre emissions after the peak EC day. Subsequent analysis will focus primarily on the −2 time period as the reference period. The general conclusions of this study are preserved regardless of the time period used (or using the overall average); the only differences are that the rankings of a few components vary slightly depending on the time period used, some of which are discussed below. Results for all periods are provided in Tables S1–S6 (see supporting information).
For a case study analysis with more detailed size-resolved data as compared to the IMPROVE data, size-resolved chemical measurements are presented from two separate summertime campaigns (July–August) based in Marina, California, which is a coastal site in central California approximately 5 km from the coast (36.7°N, −121.8°W). The Nucleation in California Experiment (NiCE) took place in 2013 [Coggon et al., 2014; Crosbie et al., 2016; Wang et al., 2016], while the Fog and Stratocumulus Evolution (FASE) took place in 2016. During both campaigns, surface-based composition measurements were conducted with a Micro-Orifice Uniform Deposit Impactor (MOUDI, MSP Corporation) [Marple et al., 1991] with aerodynamic cut point diameters of 0.056, 0.1, 0.18, 0.32, 0.56, 1.0, 1.8, 3.2, 5.6, 10.0, and 18.0 μm. Teflon filters were used for MOUDI sampling (PTFE membrane, 2 μm pore, 46.2 mm, Whatman). Extractions of one half of each filter was performed by using 10 mL of milli-Q water in sealed glass vials that were sonicated at 30°C for 20 min. Samples were chemically analyzed with ion chromatography (Thermo Scientific Dionex ICS—2100 system) and triple quadrupole inductively coupled plasma mass spectrometry (Agilent 8800 Series), details of which are reported elsewhere [Maudlin et al., 2015]. Table S7 summarizes the sample set details and whether they were impacted by biomass burning or not, as based on olfactory and visual evidence, and the remarkable enhancement observed in numerous species examined [i.e., Maudlin et al., 2015; Sorooshian et al., 2015; Youn et al., 2015].
Figure 3 summarizes the ranking of particulate species most enhanced in concentration on the peak EC day based on all fires in Table 1, ranked from highest to lowest, relative to the −2 time period. Table S1 (supporting information) lists the percent change values of all particulate species for each time period. PM2.5 was more enhanced as compared to PM10 (673% versus 376%) owing to some potential combination of secondary production of aerosol species during plume transport, less deposition of smaller particles (Dp < 2.5 μm) during plume transport, and more primary emissions of PM2.5 as compared to PM10.
Among PM2.5 constituents, carbonaceous constituents were the most enhanced as shown in past work for a variety of different fuels [e.g., Echalar et al., 1995; Schauer et al., 1996; Fine et al., 2004]. The order of the top two overall constituents are the same for the four periods, with OC1 being most enhanced (5247%) followed by OC2 (1753%). As OC1 and OC2 are the first fractions of carbon to be evolved at the lowest temperatures, the constituents comprising these fraction are volatile [e.g., Kavouras et al., 2012; Lim et al., 2012], which makes this result interesting as it would be expected that the heat of the fire source would reduce these volatile OC fractions and enhance the nonvolatile fractions (OC3–OC4) [Vergnoux et al., 2011; Kavouras et al., 2012]. In fact, the volatilization of primary organic aerosol emitted from North American trees/shrubs/grasses has already been characterized extensively by May et al. . The results of this study suggest that there could be formation of OC1 and OC2 during transport of plumes via gas-to-particle conversion from precursors emitted during fires, as has been demonstrated in laboratory [Grieshop et al., 2009; Hennigan et al., 2011] and field measurements [Yokelson et al., 2009].
The next most enhanced constituent was EC1 (1431%), which represents char EC and was previously noted to be emitted more strongly in smoldering conditions as compared to flaming conditions [Lim et al., 2012]. After EC1, the percent change of other carbonaceous constituents was as follows: total EC (1209%), total OC (1172%), OC3 (834%), EC3 (627%), OC4 (435%), and EC2 (208%). In contrast to the OC fractions that decrease in percent change in order from OC1 to OC4, the order for the EC components does not follow the same pattern from EC1 to EC3. EC3 has a higher percent change as compared to EC2, where both are representative of soot EC and thought to be emitted more strongly in flaming conditions [Lim et al., 2012]. That the OC and EC components were the most enhanced on the peak EC day relative to other species suggests that the hygroscopicity of those particles consequently was reduced (i.e., reduced inorganic mass fraction), as has been confirmed in field measurements over the western United States [e.g., Hersey et al., 2013; Shingler et al., 2016a] and laboratory experiments with biomass fuels common in this region [e.g., Lewis et al., 2009; Petters et al., 2009; Carrico et al., 2010].
In terms of noncarbaneceous constituents, chlorine (Cl) exhibited the highest percent change in mass concentration on the peak EC day (1162%). Biomass burning is one of the most significant sources of Cl-containing species to the atmosphere [Lobert et al., 1999]. The following four constituents were next most enhanced, all of which have been noted to be biomass burning tracers in past work [e.g., Fine et al., 2004; Mahowald et al., 2005]: NO3− (486%), P (469%), K (438%), and Zn (357%).
In decreasing order of percent change of water-soluble anions after NO3− were Cl− (99%) and SO42− (60%). Nitrate originates from NOx emissions in fires that are photochemically oxidized to HNO3 followed by neutralization with an alkaline species [e.g., Prabhakar et al., 2014]. While SO42− is primarily formed secondarily from gaseous precursors, mainly SO2 emissions, it typically requires high relative humidity for formation, especially in clouds [e.g., Reid et al., 2005]. One study showed that SOx is emitted most intensely during the flaming phase of fires [Ferek et al., 1998]. Another study showed that SO2 concentrations were similar between fire and nonfire periods during the time of a major fire in northern Alberta, Canada [Bytnerowicz et al., 2016]. Chloride is often in the form of KCl in biomass burning emissions [Reid et al., 2005], especially in young smoke [Li et al., 2003; Wonaschutz et al., 2011]. There is a large difference between Cl and Cl− enhancement, and while the species are quantified via different instruments (XRF and IC, respectively), it has also been shown that Cl− is the better proxy for fine-particle sea salt concentrations [White, 2008], which could at least partly explain why a difference exists.
Although NO3− exhibited the largest percent change in concentration among the water-soluble anions measured, its absolute concentration was not the highest on the peak EC day for most of the 41 fires. Sulfate was the most dominant water-soluble anion in 28 of 36 wildfires, followed always by NO3−; note that 5 of the 41 fires did not have SO42− NO3−, nor Cl− meet the “V0” status flag requirement to be included in this analysis. The remaining eight fires with valid data were characterized by NO3− having the highest mass concentration among anions on the peak EC day, followed always by SO42−, and then Cl−. Five of those fires were in California (C2, C4, C6, C9, and C11), with two in Oregon (O6 and O8), and one in Washington (W5). Reasons for variations in the dominant anion have been proposed to be differences in biomass density, fuel type, and amount of water present in the vegetation prior to burning [e.g., Li et al., 2003; Posfai et al., 2003; Ryu et al., 2004]. With regard to other global regions, savanna fires in the Amazon basin [Andreae et al., 1998; Yamasoe et al., 2000] and southern Africa [Gao et al., 2003] were characterized as having Cl− as the dominant anion, while SO42− was most enhanced during wildfire activity around Moscow [Popovicheva et al., 2014].
The most enhanced anion, using equation (2), is sensitive to background concentrations of pollutants and their precursors that already exist. The higher percent change value for NO3− versus SO42− can be attributed most likely to its lower overall background concentrations in the study region. Based on analysis of IMPROVE data available in each state at the sites with fires in Table 1 between 2005 and 2015, the average concentrations of SO42−/NO3−/Cl− are as follows: AZ = 0.54/0.13/0.02 μg m−3; CA = 0.66/0.50/0.04 μg m−3; NV = 0.35/0.09/0.01 μg m−3; UT = 0.48/0.17/0.01 μg m−3; OR = 0.35/0.15/0.05 μg m−3; and WA = 0.45/0.28/0.06 μg m−3. The higher frequency of nitrate being the dominant anion in fires in CA can likely be explained by how the long-term average concentration of NO3− in that state is the highest, and closest to SO42−, as compared to the other areas.
Figure 3 shows that fine soil and PMcoarse exhibit significant enhancements on peak EC days. It is assumed here that the main component of PMcoarse is dust as has been the case in many other IMPROVE studies. For example, Malm et al.  investigated the composition of coarse particles across the continental United States and showed that soil was the major component (61%), with primary organic matter (POM) and ammonium nitrate contributing 24% and 8%, respectively. They also showed that the western United States generally had the highest fractional contributions of soil to PMcoarse with the exception that soil contributed only 34% to PMcoarse at Mount Rainier National Park with POM accounting for 59%. In this study, PMcoarse exhibited a lower percent change (74%) as compared to PM10 and PM2.5. The percent change in fine soil levels was similar (66%) to that of PMcoarse.
Other notable species with average enhancements ≥100% included Mn (158%), Ca (148%), Br (146%), and Sr (103%). These species have origins in crustal matter [e.g., Ryu et al., 2004]. These results support the notion that turbulent mixing near flames and the burn front can lift soil dust particles, leading to the presence of soil dust with biomass burning plumes across the western United States.
Table 3 summarizes percent change rankings for species separately in the states of Arizona, California, Nevada, Utah, and the Northwest (Oregon, Idaho, and Washington) based on the average for all fires in each region. Specific details about percent changes for each of the four time periods is shown in Tables S2–S6 for each of the five subregions. The various OC and EC parameters are consistently among the top ranked in terms of percent change regardless of region, with OC1 usually being the highest with the exception of Utah where EC3 exhibited the highest percent change. Of the noncarbonaceous components, Cl exhibited the highest percent change value in Arizona, California, Utah, and the Northwest, while P was highest in Nevada. NO3−, Zn, and K had the next highest percent changes in each area.
Species exhibiting wide ranges in percent change among the five areas include all of the OC constituents (OC1–4) owing to their significantly higher values in Arizona versus other regions. Conversely, EC3 exhibited a percent change in Utah (2100%) that was a far larger than other regions (32–819%). Several other species exhibited significantly higher percent change values in Utah versus other regions including Cl, K, and Zn.
Fine soil exhibits the greatest percent enhancement in the Northwest (119%) as compared to Arizona (27%), California (22%), Utah (21%), and Nevada (4%). While the Northwest sites exhibited a lower overall altitude than other regions, no significant trend was found between the fine soil percent changes as a function of site altitude. The more important feature of the Northwest is the lower background concentrations of fine soil. More specifically, based on analysis of IMPROVE data available in each state at the sites with fires in Table 1 between 2005 and 2015, the average concentrations of fine soil are as follows: AZ = 0.69 μg m−3; CA = 0.59 μg m−3; NV = 0.98 μg m−3; UT = 0.64 μg m−3; OR = 0.36 μg m−3; and WA = 0.35 μg m−3. As a result, additions of fine soil aloft in the northwestern states from fire activity yielded a higher percent change value than other regions that already have more dust in the air year-round.
Wildfires significantly impact downind areas, and an example of this is a case from October 2003 where wild-fires from California impacted Arizona. This scenario was captured at Agua Tibia in California on 27 October and Saguaro West in Arizona on 30 October. Figure 4a illustrates the high density of fires erupting in Southern California, especially near Agua Tibia, based on data from Moderate Resolution Imaging Spectroradiometer (MODIS) Collection 5 near-real-time data (https://earthdata.nasa.gov/firms). Figure 4b shows wildfire emissions concentrated near Agua Tibia on 27 October, while Figure 4c shows the movement of the plumes 3 days later to the east toward Arizona. Figure 4d shows 96 h back trajectories from the NOAA HYSPLIT model [Stein et al., 2015; Rolph, 2016] ending at the point of the Saguaro West station on its peak EC day of 30 October. The California fires influenced much of the region offshore where trajectories traveled over before reaching Saguaro West, which is confirmed by NAAPS data in Figure 5 where the progression of fire influence is shown between 27 and 30 October. Smoke enveloped much of the greater Southwest including all of Arizona (Figures 5a and and5b5b).
A time series of numerous species, including biomass burning tracers, is shown for Agua Tibia and Saguaro West in Figure 6. PM10, PM2.5, PMcoarse, OC, EC, K, and fine soil exhibit enhancements on the peak EC day as compared to preceding and subsequent weeks, with the exception of fine soil being high on the sample day before the peak EC day at Agua Tibia. Of the two sites, the most significant increase in PMcoarse on the peak EC day was at Saguaro West (from 17 μg m−3 on 27 October to 50 μg m−3 on 30 October); however, this event was not identified as a local dust event with the similar IMPROVE data set in Tong et al.’s  study that constructed a long-term dust record in the western United States. Two of the five different criteria used in their method to detect dust events included low PM2.5:PM10 ratios and low contributions from pollutants such as, Zn, Cu, Pb, SO42−, NO3−, OC, and EC, both of which break down to some extent when dust is emitted during a fire based on the results of our study. This is because there are high levels in wildfire plumes of both PM2.5 and the anthropogenic pollutants selected in that study.
To further motivate greater attention to fires as a source of dust, NAAPS does not show any dust influence over the Southwest during this case event (Figures 5c and and5d).5d). This is because NAAPS is not designed to resolve dust dynamics in fires. Dust emission in NAAPS is a function of surface friction velocity, surface wetness, and surface erodibility [Westphal et al., 1988; Lynch et al., 2016]. Only when surface friction velocity or surface wind exceeds a threshold, and surface wetness is below a certain value over erodible surfaces, can dust be emitted. This type of parameterization is broadly used in dust models [Huneeus et al., 2011]. Consequently, whether or not an aerosol model can capture a dust event is highly dependent on the underlying simulated surface properties, especially surface wind. Turbulence, buoyancy, downdrafts, and changes in surface wind promoted by fires are not readily represented in meteorological models, except for a limited number of emerging research models that are focused on fire-atmosphere interactions (e.g., the Weather Research and Forecasting Model WRF-Fire [Coen et al., 2013]); fire-atmosphere interactions are an ongoing research topic [Potter, 2012]. There are coexisting dust and smoke plumes in NAAPS, but they tend to be caused by separate dust and smoke processes. An improved aerosol model would ideally rely on the driving meteorology to resolve surface wind characteristics associated with fires to more accurately capture fire-promoted dust emissions; this is currently a major challenge for a global model such as NAAPS.
An important feature of Figure 6 is Cl− reaching depressed levels on the peak EC day as compared to adjacent periods before and after the peak EC day. This was a common occurrence among the fires in Table 1. For specific time periods used in equation (2), the number of fires from Table 1 with negative percent change values for Cl− was as follows (values in parentheses represent total number of possible cases with the “Valid value (V0)” status flag according to IMPROVE for Cl−): −2 = 15 (33), −1 = 10 (33), 1 = 14 (33), 2 = 18 (34).
Other work has shown that Cl− is depleted in wildfire plumes, including over California [Zauscher et al., 2013; Maudlin et al., 2015]. This is due to the well-documented chloride depletion phenomenon promoted by acidic species (e.g., sulfuric and nitric acids and organic acids) [e.g., Gaudichet et al., 1995; Gao et al., 2003]. Measurements in biomass burning plumes over southern Africa suggested that KCl particles in young smoke were converted into K2SO4 and KNO3 in aged smoke via reactions with S- and N-containing species from biomass burning and other sources [Li et al., 2003]. Similar observations have been made in Southern California wildfires [Zauscher et al., 2013]. Although organic acid concentrations are not available in the IMPROVE data set, they are enhanced in wildfires [e.g., Gao et al., 2003] and can contribute appreciably to chloride depletion.
To probe deeper into the chloride depletion issue, size-resolved chemical measurements are examined here that were collected during the NiCE and FASE campaigns in July–August of 2013 and 2016, respectively. During NiCE, a series of wildfires based near the California-Oregon border began near the end of July (Big Windy, Whiskey Complex, and Douglas Complex forest fires), while during FASE, the Soberanes Wildfire was present just south of Marina in Big Sur, which impacted the study region for the majority of the field campaign. Figure 7 compares Cl− data in sample sets with and without biomass burning influence; fortunately, sample sets were collected in each campaign before the fires began (Table S7), to allow for a comparison between fire and nonfire periods.
The total Cl− mass concentration integrated over all MOUDI stages was 1.41 ± 0.68 μg m−3 and 0.33 ± 0.17 μg m−3 for nonfire and fire conditions, respectively. The difference was mainly in the supermic-rometer sizes, as evident in the mass size distributions, where the average Cl− concentration was 1.37 ± 0.65 μg m−3 and 0.31 ± 0.16 μg m−3 for nonfire and fire conditions, respectively. As sea salt is abundant in this coastal area, the Cl−:Na mass ratio provides an indication of the degree of chloride depletion, as the value for natural sea salt is 1.8. For the three MOUDI stages with the majority of the mass of Cl− and Na (1–5.6 μm), the Cl−:Na ratio was 1.57 and 0.59 for nonfire and fire conditions (Figure 7). Interestingly, NO3− and SO42− were not enhanced in those stages during fire periods: NO3− = 0.07 μg m−3 (fire) versus 0.10 μg m−3 (nonfire); SO42− = 0.06 μg m−3 versus 0.12 μg m−3; however, organic acids, especially oxalate, were significantly enhanced (0.02 versus 0.01 μg m−3) albeit at lower concentrations.
Processes that alter aerosol composition and morphology during transport and aging (e.g., chloride depletion, sulfate and nitrate formation, and organic aerosol evaporation and formation) of biomass burning plumes impact the hygroscopic and radiative properties of aerosol [e.g., Lewis et al., 2009; Engelhart et al., 2012; Shingler et al., 2016b]. Therefore, it is important to understand more clearly why the sharp reduction in Cl− levels only occurs in some fires and not others.
This study has examined aerosol composition data for major wildfires between 2005 and 2015 across the western United States with the goal of identifying what particulate species exhibit the higher percent change in concentration on the day of the peak fire influence relative to periods before that day. After a filtering technique of isolating wildfire events using IMPROVE, NAAPS, and MODIS data, a total of 41 fires were identified and studied.
Species exhibiting the highest percent change increase on the peak EC day include the various components of OC and EC with OC1 being the highest, followed by OC2 and EC1. The volatile nature of OC1 and OC2 suggests a secondary formation mechanism for these species during plume transport. Of the noncarbonaceous constituents, Cl, P, K, NO3−, and Zn followed next in terms of having the highest percent change in concentration. While NO3− exhibited the greatest percent change in mass on the peak EC day among water-soluble anions (versus Cl− and SO42−), it was only the dominant anion on an absolute mass basis in 8 of 36 fires with valid data since SO42− was usually more abundant.
Dust was significantly enhanced in study region plumes, with fine soil and PMcoarse exhibiting comparable percent change enhancements on the peak EC day. This included many trace metals associated with crustal matter (e.g., Mn, Ca, Fe, Al, Ti, and Sr). The percent change for dust was highest in the northwestern states of Oregon and Washington owing to lower background concentrations during nonfire periods as compared to the rest of the western United States. A case study emphasizes how transport of wildfire plumes can significantly impact downwind states. An important feature of the particular event examined is that there were higher levels of fine soil and PMcoarse at the downwind state (Arizona) as compared to the source of the fires (California). The results from IMPROVE, and also the inability of a global aerosol model such as NAAPS to accurately represent dust emissions during fires, warrant more attention to this issue.
Significant reductions in Cl− concentration are observed on the peak EC day of almost half of the fires identified in this study. Size-resolved measurements in coastal California during two different summers reveal that there is a major difference in Cl− levels between 1 and 10 μm during periods with and without fire influence. While sulfate and nitrate levels were not enhanced during the fire periods, those of organic acids instead were, which highlights the potentially large importance of organic species in altering the inorganic composition of plume aerosol particles and consequently their hygroscopic and radiative properties.
All IMPROVE, NASA, and NAAPS data used can be obtained from websites provided in section 2, while FASE and NiCE data can be obtained from the corresponding author. This work was funded by grant 2 P42 ES04940 from the National Institute of Environmental Health Sciences (NIEHS) Superfund Research Program and ONR grants N00014-16-1-2567 and N00014-10-1-0811. We acknowledge the use of data and imagery from LANCE FIRMS operated by the NASA/GSFC/Earth Science Data and Information System (ESDIS) with funding provided by NASA/HQ. We acknowledge Agilent Technologies for their support and Shane Snyder’s laboratories for ICP-MS data. IMPROVE is a collaborative association of state, tribal, and federal agencies and international partners. The U.S. Environmental Protection Agency is the primary funding source, with contracting and research support from the National Park Service. The Air Quality Group at the University of California, Davis, is the central analytical laboratory, with ion analysis provided by Research Triangle Institute and carbon analysis provided by Desert Research Institute.
Quantifying the emission, properties, and diverse impacts of wildfire smoke