A Laboratory Assessment of 120 Air Pollutant Emissions from Biomass

May 27, 2019 - Department of Mechanical Engineering, Colorado State University, 1374 ... School of Molecular Sciences, Arizona State University, 1604 ...
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Article Cite This: Environ. Sci. Technol. 2019, 53, 7114−7125

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A Laboratory Assessment of 120 Air Pollutant Emissions from Biomass and Fossil Fuel Cookstoves Kelsey R. Bilsback,† Jordyn Dahlke,† Kristen M. Fedak,‡ Nicholas Good,‡ Arsineh Hecobian,§ Pierre Herckes,∥ Christian L’Orange,† John Mehaffy,† Amy Sullivan,§ Jessica Tryner,† Lizette Van Zyl,† Ethan S. Walker,‡ Yong Zhou,§ Jeffrey R. Pierce,§ Ander Wilson,⊥ Jennifer L. Peel,‡ and John Volckens*,† Downloaded via NOTTINGHAM TRENT UNIV on August 12, 2019 at 02:38:53 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

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Department of Mechanical Engineering, Colorado State University, 1374 Campus Delivery, Fort Collins, Colorado 80523, United States ‡ Department of Environmental and Radiological Health Sciences, Colorado State University, 1681 Campus Delivery, Fort Collins, Colorado 80523, United States § Department of Atmospheric Science, Colorado State University, 1371 Campus Delivery, Fort Collins, Colorado 80523, United States ∥ School of Molecular Sciences, Arizona State University, 1604 Campus Delivery, Tempe, Arizona 85287, United States ⊥ Department of Statistics, Colorado State University, 1877 Campus Delivery, Fort Collins, Colorado 80523, United States S Supporting Information *

ABSTRACT: Cookstoves emit many pollutants that are harmful to human health and the environment. However, most of the existing scientific literature focuses on fine particulate matter (PM2.5) and carbon monoxide (CO). We present an extensive data set of speciated air pollution emissions from wood, charcoal, kerosene, and liquefied petroleum gas (LPG) cookstoves. One-hundred and twenty gas- and particle-phase constituentsincluding organic carbon, elemental carbon (EC), ultrafine particles (10−100 nm), inorganic ions, carbohydrates, and volatile/semivolatile organic compounds (e.g., alkanes, alkenes, alkynes, aromatics, carbonyls, and polycyclic aromatic hydrocarbons (PAHs))were measured in the exhaust from 26 stove/fuel combinations. We find that improved biomass stoves tend to reduce PM2.5 emissions; however, certain design features (e.g., insulation or a fan) tend to increase relative levels of other coemitted pollutants (e.g., EC ultrafine particles, carbonyls, or PAHs, depending on stove type). In contrast, the pressurized kerosene and LPG stoves reduced all pollutants relative to a traditional three-stone fire (≥93% and ≥79%, respectively). Finally, we find that PM2.5 and CO are not strong predictors of coemitted pollutants, which is problematic because these pollutants may not be indicators of other cookstove smoke constituents (such as formaldehyde and acetaldehyde) that may be emitted at concentrations that are harmful to human health.

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may react and form secondary organic aerosols10,11 or tropospheric ozone).12 PM 2.5 and CO are the most commonly measured constituents of cookstove emissions because (1) exposure to PM2.5 and CO has been linked to adverse health impacts, (2) they are the only pollutants that have standardized performance targets,13 (3) they constitute a large fraction of cookstove smoke on a mass basis, and (4) they are relatively straightforward and less costly to measure (than many other coemitted pollutants). In this study, we comprehensively characterized cookstove smoke profiles from a broad range of stove/fuel combinations to better understand cookstove

INTRODUCTION

Household air pollution from solid-fuel combustion within cookstoves is a leading cause of disease and premature death worldwide.1,2 Many constituents of cookstove smoke have known health and/or atmospheric effects. For example, exposure to fine particulate matter (PM2.5) has been linked to respiratory tract infections, chronic obstructive pulmonary disease, and cardiovascular morbidity and mortality; exposure to carbon monoxide (CO) has been linked to low birth weight and perinatal death; volatile organic compounds (VOCs) are associated with eye and respiratory-tract irritation;3−6 and many compoundssuch as benzene, formaldehyde, acetaldehyde, and some polycyclic aromatic hydrocarbons (PAHs) have been classified as carcinogens.7,8 Additionally, if cookstove emissions are injected into the atmosphere, they can impact climate and the environment9 (e.g., VOC emissions © 2019 American Chemical Society

Received: Revised: Accepted: Published: 7114

December 12, 2018 May 23, 2019 May 27, 2019 May 27, 2019 DOI: 10.1021/acs.est.8b07019 Environ. Sci. Technol. 2019, 53, 7114−7125

Article

Environmental Science & Technology emissions beyond PM2.5 and CO. Although previous works have characterized CO, PM2.5, and bulk PM2.5 composition (e.g., elemental carbon (EC), organic carbon,14−16 and/or particle size17−23), data for other constituents of cookstove smoke (e.g., speciated PAHs, VOCs, and carbonyl compounds) are not as widely available. For example, some studies report speciated emissions from stoves, but only characterize a limited number of compounds (e.g., only formaldehyde or benzo[a]pyrene24,25) emitted by a limited number of stove/ fuel combinations (e.g., one26) and/or report few metrics (e.g., per-mass-fuel basis27). Built-in coal heating stoves from China and Southeast Asia are one of the few stove types that have been characterized in depth,28−35 while comprehensive emissions data from wood, charcoal, and fossil fuel stoves are lacking. In this study, we measured 120 particle- and gas-phase smoke constituents, including organic aerosol, EC, inorganic ions, carbohydrates, ultrafine particles, PAHs, VOCs, and carbonyls, to gain insight into constituents of cookstove smoke that have garnered little attention. Given the lack of data from wood, charcoal, and fossil fuel stoves, we tested 26 stove/fuel combinations that represent a range of technologies including traditional wood cookstoves (i.e., open fires), improved wood cookstoves (i.e., stoves which have been modified to lower PM2.5 emissions by adding insulation and/or a fan), charcoal stoves, and fossil fuel cookstoves (i.e., kerosene and liquified petroleum gas (LPG) stoves). Furthermore, because many of the pollutants measured in this study are typically not measured during cookstove testing, we used leave-one-out cross validation to quantify the extent to which PM2.5 and CO can be used to predict emissions of other smoke constituents (both on their own and when accounting for stove type or fuel type). Our findings highlight the need to consider emissions beyond PM2.5 and CO when designing and characterizing improved cookstoves. Our observations are relevant for research and policies concerning the dissemination of cookstoves to communities, because an “improved stove” does not necessarily guarantee emissions (and therefore exposure) reductions when considering all harmful compounds that may be present in cookstove smoke mixtures.

Figure 1. Stove/fuel test matrix and categories into which each stove falls (i.e., biomass vs fossil fuel and traditional vs improved). Stove type is indicated at the top center of each cell and fuel types are listed on the right side of each cell. Values in parentheses indicate the number of replicates conducted with each stove/fuel combination (n = 87 total emissions tests). Makes and models of each stove design are presented in SI Section 1.

Testing methodologies differed between continuously fed biomass, batch-fed biomass, and fossil fuel stoves due to differences in typical operation between stove types. In contrast to Bilsback et al.,36 fuel batches were fed one after another rather than at designated time intervals. Samples for filter-based, cartridge-based, and canister-based emissions measurements were captured over the entire sweep (which did not include the stove’s start-up and shut-down), while time-resolved instruments were operated during the entire test (from the stove’s start-up through the stove’s shut-down). Emissions Measurements. Details of the test setup and instrumentation are provided in SI Section 3. Briefly, a customdesigned, total-capture hood was used for emissions testing. Sampling media for time-integrated measurements included Teflon filters (analyzed for PM2.5 mass), quartz filters (analyzed for organic carbon, EC, organic carbon absorption artifacts,37 inorganic anions and cations, particle-phase PAHs, and carbohydrates38,39), and polyurethane foam plugs (analyzed for gas-phase PAHs). To minimize contamination, the quartz filters were baked at 800 °C and the polyurethane foam filters were sonicated in acetone and then in a dichloromethane/methanol/hexane mixture (and then airdried) before testing. Filter housings and cartridges were cleaned first with dish soap and deionized water and then rinsed with a dichloromethane/methanol/hexane mixture before use. Filter blanks were collected daily and filter cartridges were leak checked daily. Other time-integrated sampling media included a vacuum canister fitted with a critical orifice (analyzed for VOCs)40 and dinitrophenylhydrazine (DNPH) cartridges (analyzed for gas-phase carbonyls) that were placed in-line behind an ozone scrubber. Two-hour

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MATERIALS AND METHODS Test Matrix. The stove/fuel test matrix and categories into which each stove falls (i.e., biomass vs fossil fuel and traditional vs improved) are provided in Figure 1. A minimum of three replicate tests were run for each stove/fuel combination. After the initial tests, some pollutant measurements were excluded due to experimental error. An additional nine tests were conducted to make up for some of the experimental issues during the primary testing; however, three successful measurements were not available for all stove/fuel/pollutant combinations after erroneous data were excluded; Supporting Information (SI) Section 2 provides the total number of successful measurements by pollutant. Test Protocol. The cookstoves were operated using the Firepower Sweep Test (FST); details of this method are provided in Bilsback et al.36 In contrast to commonly used laboratory protocols, which are primarily task based (i.e., boiling and simmering a pot of water), the FST protocol directs the user to operate the cookstove across a range of firepowers. Past work suggests that the FST protocol captures a more realistic range of emissions, relative to in-field use, than task-based laboratory protocols (e.g., the Water Boiling Test). 7115

DOI: 10.1021/acs.est.8b07019 Environ. Sci. Technol. 2019, 53, 7114−7125

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Environmental Science & Technology

combustion of biomass fuel,” referring the entire smoke mixture, as Group 2A. However, since the following analyses are focused on the constituents of cookstove smoke, we only included compounds that have been classified on a compound level in the carcinogenic compound analysis. Regression Analysis. PM2.5 and CO are the most frequently measured cookstove air pollutants; they also have voluntary performance targets (ISO 19867−3:2018).13 EC emissions are also measured frequently, although EC does not have a voluntary performance target. Leave-one-out cross validation45 was used to assess whether emissions of PM2.5 and CO, EC, stove type, or fuel type could be used to predict other coemitted pollutants. The predictive ability of EC was evaluated separately from PM2.5 and CO, because EC is measured less frequently. Six linear models were evaluated:

background measurements were conducted on a weekly basis (18 tests in total) for all time-integrated instrumentation. Time-resolved instrumentation included a scanning mobility particle sizer (SMPS); carbon dioxide (CO2), CO, and methane sensors; and thermocouples that measured the temperature of the water in the cooking pot and the temperature at the combustion chamber outlet. Ultrafine particles, defined here as particles with mobility diameters between 10 nm (the lower limit of the SMPS) and 100 nm, were measured using the SMPS (∼3 min scans). The SMPS was installed after a Venturi pump that provided secondary dilution. Secondary dilution ratios were determined by simultaneously measuring carbon dioxide (CO2) concentrations in the ambient air, emissions hood, and after the secondary dilution. Five-minute background measurements were conducted before the beginning and after the end of each test for time-resolved instrumentation. Data Analyses. Data processing and analyses were conducted in R (v3.4.1); the code is published on Github: https://github.com/nickgood/stoves_nih_2016_git/tree/ master_kb. The 120 cookstove smoke constituents quantified as part of this study are listed in SI Section 4. Emission factors were calculated per-energy-delivered (mg/MJd), per-mass-offuel-burned (mg/kg), per-energy-of-fuel-burned (mg/MJ), and per-time (mg/s).41 Emissions measurements were corrected for handling and background contamination. Sample concentration data that were below a given analytical method limit of detection (LOD) were replaced with LOD/ 2 , and background-corrected values that were less than or equal to zero were replaced with zeros. Particle size distribution data were corrected for secondary dilution on a test-by-test basis. Secondary dilution ratios ranged from 2.7 to 100. Particle losses in the venturi pump, placed ahead of the SMPS, were not corrected for in the postanalysis (venturi pump losses as a function of particle size are provided in SI Section 3). Particle-phase organic carbon measurements were converted to organic aerosol. Conversion factors of 1.5, 1.5, 1.2, and 1.2 were chosen for wood, pellet, charcoal, and fossil fuels, respectively; these factors fall within the range of organic− aerosol-to-organic−carbon ratios measured from biomass burning in the laboratory.42 See SI Section 3 for a mass balance and the digital repository for organic carbon emissions factors.43 In this work, emissions from improved wood stoves, charcoal stoves, and fossil fuel stoves are presented as percent and absolute differences in the replicate-averaged emissions (including replicates across all fuel types) relative to the threestone fire, because the three-stone fire is the most commonly used traditional cookstove.44 The interquartile range and raw data are provided to represent emissions variability within a given stove type. Several smoke constituents are presented as groups rather than as individual constituents. For example, inorganic ions are grouped together, carbonyls and VOCs are grouped by carbon bond structure, and PAHs are grouped by number of rings. We refer to compounds as carcinogenic if they have been classified as a “known” or “reasonably anticipated” human carcinogens by the National Toxicology Program7 and/or classified as a Group 1- (carcinogenic to humans) or Group 2A- (probably carcinogenic to humans) compound by the International Agency for Research on Cancer.8 See SI Section 4 for compounds and classifications. The IARC also classifies “indoor emissions from household

ln(Pi) = β0 + β1·ln(PM 2.5) + β2 ·ln(CO) + ϵ

(1)

ln(Pi) = αj + β1·ln(PM 2.5) + β2 ·ln(CO) + ϵ

(2)

ln(Pi) = αk + β1·ln(PM 2.5) + β2 ·ln(CO) + ϵ

(3)

ln(Pi) = β0 + β1·ln(EC) + ϵ

(4)

ln(Pi) = αj + β1·ln(EC) + ϵ

(5)

ln(Pi) = αk + β1·ln(EC) + ϵ

(6)

where Pi is a coemitted smoke constituent; PM2.5 is fine particulate matter; CO is carbon monoxide; EC is elemental carbon; β0, β1, and β2 are fixed intercepts or slopes; αj is a fixed stove-specific coefficient; αk is a fixed fuel-specific coefficient; and ϵ represents the model error. Models were developed using emissions on a per-energy-delivered basis. Continuous variables (i.e., PM2.5, CO, EC, and Pi) were log-transformed, because the assumptions of linear regression were not satisfied otherwise. Thus, the slope coefficients can be interpreted as a percent change in PM2.5, CO, or EC for a percent change in a coemitted pollutants (rather than an absolute change). Model 1 evaluates whether PM2.5 and CO emissions alone can predict coemitted pollutants, Model 2 evaluates if PM2.5, CO, and stove type can predict coemitted pollutants, and Model 3 evaluates if PM2.5, CO, and fuel type can predict coemitted pollutants. Models 4−6 are analogous for EC. For Models 1−3, only test replicates that had both PM2.5 and CO measurements (i.e., complete observations) were used in the analysis. As stated previously, background-corrected values that were less than or equal to zero were replaced with zeros. Since these zero values could not be log-transformed, they were excluded from the analysis for all models. Pollutants (Pi) for which more than 15% of the possible observations were missing (due to below-background measurements) were also excluded, leaving 82 pollutants for the regression analysis (Note: PM2.5, CO, and EC were never used as outcomes). To avoid overfitting and to ensure compatibility across all models, stove/fuel combinations that had fewer than three observations (due to below-background measurements and/or measurement error) were also excluded. Due to limited replicates and low emission rates, fossil fuel stoves were most frequently excluded. The pressure kerosene and forced-draft gasifier stoves were excluded from all models due to missing CO observations. The number of observations for each pollutant and the stove/fuel combinations that were excluded are listed in SI Section 5. 7116

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Figure 2. Emissions of fine particulate matter (PM2.5), elemental carbon, organic aerosol inorganic ions, and carbon monoxide. The height of each colored bar represents replicate-averaged emissions for each stove type (including replicates across all fuel types). The circular markers indicate the PM2.5 emissions (top panel) and carbon monoxide emissions (bottom panel) for each replicate test by stove type. Boxplots indicate the median and interquartile range of the PM2.5 emissions (top panel) and carbon monoxide emissions (bottom panel) across all replicates for each stove type (Note: Because the fan-rocket elbow/oak stove/fuel combination only had one good carbon monoxide measurement, this stove/fuel combination was excluded from the carbon monoxide plot.).

Figure 3. Emissions of ultrafine particles (10−100 nm) and polycyclic aromatic hydrocarbons (PAHs). The height of each colored bar represents replicate-averaged emissions for each stove type (including replicates across all fuel types). The circular markers indicate ultrafine particles (top panel) and total PAHs (bottom panel) for each replicate test by stove type. Boxplots indicate the median and interquartile range of ultrafine particles (top panel) and total PAHs (bottom panel) for all replicates on each stove type. For scaling purposes, extreme outliers are represented as numeric values at the top of each figure rather than being plotted.

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Root-mean-square-error (RMSE) ratio was used to assess how well Models 1−6 predicted each coemitted pollutant. RMSE ratio was calculated by dividing the out-of-sample RMSE of Models 1−6 by the out-of-sample RMSE of a model that always used the population average as the prediction (i.e., a model with no predictors). A RMSE ratio of one indicated that the model provided no improvement in prediction over the population average and a RMSE ratio of zero indicated that the model removed all the prediction uncertainty.

RESULTS AND DISCUSSION

We measured above-background levels for 119 of the 120 cookstove smoke constituents (all except inositol, a carbohydrate), demonstrating the diversity of pollutants present in cookstove smoke (a summary of nondetects and belowbackground measurements are provided in SI Sections 6 and 7, respectively). We found that the composition of cookstove smoke varied substantially between stove types and between test replicates. (Emissions levels are summarized in Figures 2, 3, 4, 5. Additionally, CO2, methane, and carbohydrates are 7117

DOI: 10.1021/acs.est.8b07019 Environ. Sci. Technol. 2019, 53, 7114−7125

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Figure 4. Emissions of volatile organic compounds (VOCs) and carbonyls. The height of each colored bar represents replicate-averaged emissions for each stove type (including replicates across all fuel types). The circular markers indicate total VOCs (top panel) and total carbonyls (bottom panel) for each replicate test by stove type. Boxplots indicate the median and interquartile range of total VOCs (top panel) and total carbonyls (bottom panel) for all replicates on each stove type (Note: ethyne was the only pollutant measured in the alkyne category.).

provided in a digital repository.43) The variability across repeated tests from the same stove type was likely due to differences in fuel properties (SI Section 8) and stove operation (SI Section 9). Emissions from replicate tests were less variable for fossil fuel stoves than for biomass stoves, because the operating conditions of the former were more controlled. In SI Section 10 we compare the PM2.5 and CO emissions levels measured in this study to emissions measurements from uncontrolled field tests15,46,47 and the Water Boiling Test48 (i.e., a task-based laboratory test) from previous studies. Overall, the PM2.5 and CO emissions levels measured in this study are higher and more variable than the Water Boiling Test and tend to agree better with field measurements. For some of the improved biomass stoves (i.e., the built-in plancha and gasifier), the Firepower Sweep Test misses some of the highest emissions events. This underestimation could be explained by the fact that there were differences in stove types between the studies being compared. Additionally, in this study we measured emissions integrated across the Firepower Sweep Test rather than as a function of firepower. Although the Firepower Sweep Test is not representative of a specific cooking event, overall these results, as well as previous work by Bilsback et al.36 suggest that the PM2.5 and CO produced during the Firepower Sweep Test may be more representative of the emissions during real world cooking than task-based laboratory tests. The figures that follow are pooled by stove type; results by stove/fuel combination are provided in SI Section 11. Emissions factors are presented here on a per-energy-delivered basis, while other metrics are provided in a digital repository.43 Note that a limited number of replicate tests were conducted for a given stove/fuel combination (typically three), as we chose to prioritize testing a wider range of stove/fuel combinations using our available resources. Previous work has demonstrated that more than three replicates may be needed to determine whether a stove has reached a performance target or whether one stove is cleaner than

another with adequate statistical power;49−51 thus, we caution interpretation of our results in these contexts. However, the major conclusions of our study are based on large (nonoverlapping) differences in emissions that are unlikely to be overturned with additional laboratory testing. PM2.5 Composition. Among the stoves tested, PM2.5 emissions were highest from traditional wood stoves and lowest from fossil fuel stoves; improved wood stoves and charcoal stoves fell in between (Figure 2). Relative to the three-stone fire, average PM2.5 emissions from improved wood stoves were 44−81% lower (294−545 mg/MJd), charcoal stoves were 70−75% (468−502 mg/MJd) lower, and fossil fuel stoves were >99% (662−669 mg/MJd) lower. The decreased PM2.5 emissions from fossil fuel stoves relative to biomass stoves are likely attributable to the higher volatility of kerosene and LPG, which, combined with the specific designs of these stoves, promotes more complete fuel-air mixing and, thus, more complete combustion. In particular, the pressurized kerosene and LPG stoves employ the venturi effect to premix vaporized fuel with air, which increases the homogeneity of the fuel-air mixture and (when stoichiometry is optimal) tends to result in more efficient combustion. Of the wood stoves tested, the insulated natural-draft stoves and insulated forced-draft stoves had substantially lower average PM2.5 emissions than the traditional stoves (Figure 2). The lower PM2.5 emissions from the insulated wood stoves, compared to traditional wood stoves, are likely attributable to better fuel−air mixing and reduced heat loss from the combustion zone, the latter of which helps maintain the high temperatures needed to oxidize particulate matter.52 Charcoal stoves also had substantially lower average PM2.5 emissions than the traditional wood stoves tested. This finding has been documented previously in the literature.36,46,48,53 Lower PM2.5 emissions from charcoal stoves were attributed to the lower volatile content of charcoal fuels compared to wood fuels (SI Table S11). Charcoal combusts primarily via surface oxidation of carbon to CO, whereas wood fuels undergo mixed pyrolysis and gas-phase combustion during which pyrolysis products can 7118

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Carbon Monoxide. Average CO emissions were highest for charcoal cookstoves (Figure 2). Carbon monoxide emission factors for the ceramic jiko and metal jiko stoves were 137% (+13.6 mg/MJ d ) and 135% (+13.4 mg/MJ d ) higher, respectively, than for the three-stone fire. The high CO emissions from charcoal-fueled stoves were likely attributable to the primary oxidation process of charcoal fuels. The charcoal fuels consisted of less volatile matter (19−31%) and more fixed carbon (50−62%) than the wood fuels (SI Section 8). When the fixed carbon fraction of charcoal is burned, oxygen reacts directly with the fuel surface to produce CO, often under conditions that yield lower heat-release rates than wood-based fuels or fossil fuels.52 Lower heat release rates likely result in lower temperatures in the combustion zone, which can inhibit oxidation of CO to CO2. This mechanism is supported by the lower firepowers observed for charcoal stoves relative to the wood stoves tested (SI Section 9) and has been observed in previous studies.36,46,48,53 Average CO emissions from all improved wood stoves, except the built-in plancha, were lower than from the threestone fire (rocket elbow: 20% (2.01 mg/MJd) lower; fan rocket elbow: 31% (3.08 mg/MJd) lower; gasifier: 87% (8.66 mg/ MJd) lower; built-in plancha: 42% (+4.12 mg/MJd) higher) (Figure 2). Lower average CO emissions, compared to the three-stone fire, from the rocket elbow, fan rocket elbow, and gasifier stoves were attributed to use of electric fans and/or improved insulation to promote fuel-air mixing and maintain the high temperatures needed to oxidize CO. Average CO emissions from all fossil fuel stoves were substantially lower than from the three-stone fire (wick kerosene: 67% (6.61 mg/ MJd) lower, pressure kerosene: 93% (9.24 mg/MJd) lower, LPG: 91% (9.00 mg/MJd) lower), likely do to the increased volatility of fossil fuels as well as use of the venturi effect to mix vaporized fuel and air in the pressure kerosene and LPG stoves. Ultrafine Particles (10−100 nm). Both the fan rocketelbow and gasifier stoves emitted more ultrafine particles (defined here as particles between approximately 10−100 nm) than the three-stone fire (fan rocket elbow: 15% higher (+1.96 × 1014 particles/MJd); gasifier: 9% higher (+1.23 × 1014 particles/MJd); Figure 3). Other improved wood and charcoal stoves emitted 27−42% (3.65 × 1014−5.63 × 1014 particles/ MJd) and 60−65% (8.07 × 1014−8.72 × 1014 particles/MJd) fewer ultrafine particles than the three-stone fire, respectively. The largest reductions in ultrafine particles, relative to the three-stone fire, were observed for the wick kerosene (97% (1.30 × 1015 particles/MJd)), pressure kerosene (95% (1.28 × 1015 particles/MJd)), and LPG stoves (89% (1.20 × 1015 particles/MJd)), respectively. When inhaled, ultrafine particles are more likely to deposit in and penetrate beyond the alveolar region of the lungs than larger particles.58,59 Thus, ultrafine particles may promote more systemic inflammation (compared to particles deposited in the upper airways) due to the close coupling of the alveoli with the pulmonary circulatory system.60,61 While the fan rocket-elbow and gasifier stoves reduced PM2.5 and CO emissions relative to the three-stone fire, ultrafine particle emissions increased. This finding is consistent with previous studies18,62 demonstrating that forced-air cookstoves may shift the particle size distribution toward smaller particles. This finding also illustrates that design features added to reduce PM2.5 emissions from improved stoves may lead to emissions trade-offs (i.e., decreases in one emission type and increases in another). Ultrafine particles form via nucleation and

form precursors to particulate matter. Note, however, the emissions factors presented here do not include emissions during the production of charcoal fuel, which may lead charcoal to have greater PM2.5 emissions across its lifecycle as compared to wood.54 On average, organic aerosol constituted the largest fraction of PM2.5 emitted for all stoves, except for the wick kerosene stove, which emitted more EC (Figure 2). Organic aerosol emissions were highest for traditional wood stoves. Relative to the three-stone fire, average organic aerosol emissions from improved wood stoves were 72−81% (510−578 mg/MJd) lower, charcoal stoves were 86−87% lower (611−620 mg/ MJd), and fossil fuel stoves were >99% lower (709−710 mg/ MJd). In contrast to organic aerosol, EC emissions were highest for the two insulated natural-draft wood stoves (organic-carbon-to-EC ratios are provided in SI Section 12). Relative to the three-stone fire, average EC emissions from the rocket-elbow and built-in plancha stoves were 124% (+81.8 mg/MJd) and 105% (+69.5 mg/MJd) higher, respectively. Average EC emissions from other improved wood stoves were 18−70% (11.7−46.2 mg/MJd) lower, charcoal stoves were 94% (62.1−62.2 mg/MJd) lower, and fossil fuel stoves were 91 to >99% lower (60.1−65.7 mg/MJd) (Figure 2). The traditional wood stoves tested in this study were uninsulated and thus had greater heat loss to the environment than improved wood stoves. The greater heat loss likely leads to regions with lower temperatures, which can promote organic aerosol formation.52 Meanwhile, insulated combustion chambers led to higher EC emissions from some stoves, likely due to their tendency to favor flaming (instead of smoldering) combustion, which promotes soot-particle formation and growth in fuel-rich regions of the flame zone.52 Currently, there is insufficient toxicological and epidemiological evidence to evaluate whether the EC or organic aerosol components of PM2.5 have more serious health effects. However, a review on the health effects of black carbon published by the World Health Organization cites some evidence that the black carbon fraction of total PM2.5 may be more strongly associated with short-term and long-health effects.55 Emitting more black carbon may also be problematic from a climate perspective.56 Lacey et al.,57 demonstrated that removing light-absorptive species, like EC, will lead to the largest climate-cooling reponse per kilogram of emissions (especially at high latitudes were emissions are likely to impact snow albedo). Removal of organic carbon aerosols, on the other hand, may produce a net warming effect.57 On average, the highest inorganic ion emissions came from biomass stoves that were tested with preprocessed fuels (e.g., charcoal and pellets). Relative to the three-stone fire, average inorganic ion emissions from the metal jiko, ceramic jiko, gasifier, and built-in plancha stoves were 303% (+75.1 mg/ MJd), 128% (+31.7 mg/MJd), 24% (+5.87 mg/MJd), and 3% (+0.67 mg/MJd) higher, respectively. Notably, the high inorganic ion emissions from charcoal stoves were driven by the coconut briquette fuel, which emitted more inorganic ions by mass than particle-phase organic aerosol and EC combined (SI Section 11). The inorganic ion emissions from the coconut briquette fuel were dominated by potassium and chloride (both biomass burning tracers) due to the high ash content of the coconut charcoal fuel (SI Section 8). Overall, fuel choice was a large driver of variability in ion emissions among the stoves tested (SI Section 11). 7119

DOI: 10.1021/acs.est.8b07019 Environ. Sci. Technol. 2019, 53, 7114−7125

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Figure 5. Particle- and gas-phase emissions that are classified as “known” or “reasonably anticipated” human carcinogens by the National Toxicology Program or International Agency for Research on Cancer. The height of each colored bar represents replicate-averaged emissions for each stove type (including replicates across all fuel types). The circular markers indicate total particle-phase carcinogens (top panel) and total gasphase carcinogens (bottom panel) for each replicate test by stove type. Boxplots indicate the median and interquartile range of the total particlephase (top panel) and total gas-phase (bottom panel) carcinogens for all replicates on each stove type. For scaling purposes, numeric values are sometimes provided at the top of the plot rather than being plotted.

to more flaming (instead of smoldering) combustion and thus promoted formation of PAH precursors and EC (under some test cases).52 On a mass basis, gas-phase PAH emissions were higher than particle-phase PAH emissions (Figure 3); this result has been reported previously for other combustion sources,64 while higher particle-phase than gas-phase PAH emissions have been measured from built-in heating stoves in China.30 Three-ring PAHs, which are primarily found in the gas phase, made up 41−48% of total PAH emissions for wood stoves; 61% and 67% for the ceramic jiko and metal jiko, respectively; and 26%, 44%, and 30% for the wick kerosene, pressure kerosene, and LPG stoves, respectively. Contrastingly, six-ring PAHs, which are primarily found in the particle phase, made up 1−4% of total PAH emissions for wood stoves; 3% for both charcoal stoves; and 17%, 39%, and 14% for the wick kerosene, pressure kerosene, and LPG stoves, respectively. Note that naphthalene results are provided in the repository43 but were not included in the analyses presented here due to high measurement uncertainties. Volatile Organic Compounds. We note that only a limited number of VOCs and carbonyls were measured as part of this study. Some oxygenated VOCs such as phenols and furans, and nitrogen-containing compounds (which tend to have shorter atmospheric lifetimes) were not quantified. One study by Stockwell et al.27 found that a three-stone fire emitted higher-levels of these types of VOCs than several improved biomass stoves. Of the VOCs measured in this study, average VOC emissions from improved wood and charcoal stoves were 72−92% (834−1066 mg/MJd) and 80−83% (929−962 mg/ MJd) lower than from the three-stone fire, respectively (Figure 4). Some VOCs can be emitted if biomass fuel that has been volatilized escapes the combustion zone without being completely oxidized.52 Lower combustion temperatures in traditional stoves (due to poor thermal insulation and high excess-air ratios) could contribute to higher emissions of unburned hydrocarbons. Reductions of average VOC

condensation of organic exhaust vapors or from incomplete oxidation of soot. Particles that originate from condensation of organic vapors are likely to form in regions with lower temperatures. Forced-draft biomass stoves (e.g., fan rocket elbow and gasifier) may have more low-temperature regions because internal fans push relatively cool ambient air into the combustion chamber to facilitate fuel-air mixing.59 Nucleation may also be more likely to occur in forced-draft stoves because, due to reduced soot formation, there are relatively fewer surfaces for the organic vapors to condense onto, leading to higher vapor saturation ratios.63 Thus, fans that are added to the stove to reduce PM2.5, by promoting fuel-air mixing, may lead to increased formation of ultrafine particles that are potentially harmful to health. Polycyclic Aromatic Hydrocarbons. The majority of improved stoves had lower PAH emissions relative to the three stone fire; PAH emissions from improved wood stoves (excluding the rocket-elbow stove) and charcoal stoves were, on average, 61−85% (8.54−11.8 mg/MJd) and 71−85% (9.94−11.9 mg/MJd) lower, respectively. Average PAH emissions from fossil fuel stoves were also consistently much lower than from the three-stone fire (wick kerosene: 87% lower (12.1 mg/MJd); pressure kerosene: 99% lower (13.8 mg/MJd); LPG: 97% lower (13.6 mg/MJd)). On average PAH emissions from the rocket-elbow stove, however, were 20% higher (+2.84 mg/MJd) than from the three-stone fire. Although this increase does not hold when comparing the median emissions from the three-stone fire and rocket elbow, this result is still concerning because two of the rocket elbow measurements are several times higher than the median three-stone fire measurements. Many PAH species are carcinogenic (see the Carcinogenic Compound section for further discussion). The high PAH emissions from the rocket-elbow stove represent another pollutant trade-off given that the rocket-elbow stove decreased PM2.5 emissions relative to the three-stone fire. In this case, the insulation added to the rocket-elbow stove (to reduce thermal losses and thus promote oxidation of CO and PM2.5) likely led 7120

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Figure 6. Leave-one-out cross validation using Model 1: ln(Pi) = β0 + β1·ln(PM2.5) + β2·ln (CO) + ϵ, Model 2: ln(Pi) = αj + β1·ln(PM2.5) + β2· ln(CO) + ϵ, and Model 3: ln(Pi) = αk + β1·ln(PM2.5) + β2·ln(CO) + ϵ where Pi is a coemitted smoke constituent; PM2.5 is fine particulate matter; CO is carbon monoxide; β0, β1, and β2 are fixed intercepts or slopes shared by all stove/fuel combinations; αj is a fixed stove-specific coefficient; αk is a fixed fuel-specific coefficient; and ϵ represents the error. The root-mean-squared-error (RMSE) ratio is the RMSE of Model 1, Model 2, or Model 3 divided by the RMSE from a model in which the population mean is always the prediction (i.e., a model with no predictors). RMSE ratio = 1 indicates that the relevant model (i.e., Models 1, 2, or 3) provides no improvement in prediction over the population mean (i.e., a poorly performing model) and a ratio of zero indicates that the relevant model removes all the prediction uncertainty (i.e., a highly performing model). RMSE ratios larger than one indicates that the larger model results in worse prediction than a model with no predictors.

ceramic jiko and metal jiko, respectively; and 42%, 53%, and 60% from the wick kerosene, pressure kerosene, and LPG stoves, respectively. Acetaldehyde also made up a large portion of the total carbonyl compounds, especially for charcoal stoves (ceramic jiko: 25%; metal jiko: 28%). Similarly, Zhang and Smith35 found that formaldehyde and acetaldehyde were the most abundant carbonyls across a variety of stoves and fuel types. Carcinogenic Compounds. Average emissions of particlephase carcinogenic compounds were highest for the mud chulha and rocket elbow (Figure 5). Emission factors for these stoves were 56% (+1.55 mg/MJd) and 38% (+1.07 mg/MJd) higher than the three-stone fire, respectively. (Although this increase does not hold when comparing the median emissions from the three-stone fire and rocket elbow.) Average particlephase carcinogen emissions from other improved wood stoves and charcoal stoves were 41−88% (1.14−2.44 mg/MJd) and 87−95% (2.41−2.63 mg/MJd) lower compared to the threestone fire, respectively, while average particle-phase carcinogen emissions from the wick kerosene, pressure kerosene, and LPG stoves were 87% (2.42 mg/MJd), 99% (2.73 mg/MJd), and 97% (2.69 mg/MJd) lower. All of the particle-phase carcinogens measured here were PAHs, thus particle-phase carcinogens generally followed the same trends as total PAHs. Average gas-phase carcinogen emissions were highest for the mud chulha and the three-stone fire. Relative to the threestone fire, average gas-phase carcinogen emissions from improved wood stoves and charcoal stoves were 30−74% (44.9−113 mg/MJd) and 68−73% (104−111 mg/MJd) lower, respectively, whereas gas-phase carcinogen emissions from the wick kerosene, pressure kerosene, and LPG stoves were 47% (71.2 mg/MJd), 96% (146 mg/MJd), and 94% (143 mg/MJd) lower, respectively. Benzene was the most abundant gas-phase carcinogen emitted from traditional wood stoves (three-stone fire: 66%; mud chulha: 63%), whereas the gas-phase carcinogens emitted by other stoves were dominated by the carcinogenic carbonyls (i.e., formaldehyde and acetaldehyde).

emissions from the wick kerosene stove (54% (623 mg/MJd)) and LPG stove (79% (918 mg/MJd)) relative to the threestone fire were smaller than the reductions of other pollutants for these stoves (e.g., PM2.5, ultrafine particles, and PAHs). The VOC emissions from the wick kerosene stove (534 (388− 608) mg/MJd) were higher than from any of the improved wood or charcoal stoves. Ethene made up 50% of the VOCs emitted from the wick kerosene stove on a mass basis. VOC emissions from the LPG stove (239 (103−328) mg/MJd) were higher than all improved biomass stoves except the rocket elbow. Propane, an alkane and a major constituent of LPG, was the most abundant VOC emitted from the LPG stove (75%), indicating that much of the VOC emissions from the LPG stove were from unburnt fuel. VOCs can react in the atmosphere to form secondary organic aerosol, which can contribute substantially to ambient PM2.5.65 Only considering primary organic aerosol may bias the total PM2.5 contribution of cookstoves to ambient aerosol.66 Carbonyl Compounds. Average carbonyl emissions were highest for the built-in plancha and the fan rocket elbow, which emitted 70% (+79.0 mg/MJd) and 25% (+28.1 mg/MJd) more carbonyls, respectively, than the three-stone fire (Figure 4). Increased aldehyde emissions from these stoves might have occurred because both stoves have design features that lead to higher excess air ratios (i.e., a chimney or a fan), which may lead to low-temperature regions where aldehydes are not completely oxidized.52 Reductions in carbonyl emissions from the wick kerosene stove were modest compared to other fossil fuel stoves (33% (37.3 mg/MJd) relative to the three-stone fire). Two of the aldehydes measured in this study (formaldehyde and acetaldehyde) are carcinogenic,7,8 indicating the importance of quantifying stove emissions beyond PM2.5 and CO (see the Carcinogenic Compounds section). Formaldehyde was the most abundant carbonyl compound emitted across all cookstoves, making up 39−44% of total carbonyl emissions, on average, from wood-fuel stoves; 25% and 20% from the 7121

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Environmental Science & Technology PM2.5, CO, and EC as Predictors for Coemitted Pollutants. Of the models that used PM2.5 and CO as predictors, we found that Model 2 (RMSE ratio: mean = 0.77 (range = 0.46−1.13)) and Model 3 (RMSE ratio: 0.79 (0.51− 1.15)), which assessed the predictive ability of PM2.5 and CO conditional on stove type and fuel type, respectively, performed better than Model 1 (RMSE ratio: 0.91 (0.58− 1.13)), which only used PM2.5 and CO as predictors (Figure 6). Across Models 1−3, the best predicted pollutants were biomass burning tracers (RMSE ratio using Model 2: CO2 = 0.46, levoglucosan = 0.48, galactosan = 0.48) that are closely tied to the amount of fuel burned. However, none of the models explained more than half of the out-of-sample variance relative to the population average model, meaning that PM2.5 and CO measurements alone are unlikely to provide adequate information to predict emissions levels of other coemitted pollutants even when stove type or fuel type is accounted for. Notably, none of the known-carcinogenic compounds (e.g., benzo[a]pyrene formaldehyde, acetaldehyde, and benzene) were well-predicted by the models. This finding is problematic, because PM2.5 and CO are the only pollutants with performance targets for cookstove emissions (ISO 19867− 3:2018)13 and are frequently the only pollutants measured in emissions, air quality, and health studies. Overall, we found that the average predictive ability of the EC models was similar to the average predictive ability of the PM2.5 and CO models (SI Section 13). However, when controlling for stove type (Model 5), EC was a strong predictor of several PAHs (perylene (RMSE ratio: 0.48); benzo[b]fluoranthene (0.49); benzo[j]fluoranthene (0.50); benzo[e]pyrene (0.50); benzo[c]phenanthrene (0.51); benzo[a]pyrene (0.51); benzo[k]fluoranthene (0.51)). Given that many of these PAHs are carcinogenic,7,8 EC may be a useful indicator of carcinogenic properties of cookstove smoke for a given stove type. To a lesser degree, EC also had predictive ability over several gas-phase carcinogenic compounds (formaldehyde (RMSE ratio: 0.56), styrene (0.62)) when controlling for stove type. Given that EC provides some predictive ability over these harmful compounds, measurement of EC may provide a less expensive, more straightforward alternative to measurement of the carcinogenic compounds themselves. This could be especially useful for field studies, where measurements of speciated compounds are unlikely to be collected due to costs and logistical issues. Implications for Cookstove Research. Recently, public health researchers have begun pivoting away from improved wood stoves and toward liquid or gas fueled stoves (e.g., ethanol, LPG), arguing that improved wood stoves do not reduce emissions substantially enough to provide meaningful health or environmental benefits.67 Our work supports this transition, because we found that emissions from the LPG stove were substantially lower than emissions from all wood and charcoal stoves (for all pollutants, except select VOCs, on a per-energy-delivered basis). Emissions of fuel mixture alkanes, such as propane, from the LPG stove exceeded emissions from most improved wood and charcoal stoves. Although more comprehensive exposure-response research (epidemiological and toxicological) is needed to quantify the relative health benefits of switching from traditional (i.e., threestone fire) to advanced (i.e., LPG) cookstoves, the few studies that have examined the relative toxicity of various cookstove technologies support the connection between reduced emissions and reduced toxicity.62,68,69

We also found that although improved biomass stoves tend emit less PM2.5 and CO, reduced emissions of other copollutants are not guaranteed. For several pollutants, improved biomass stoves had interquartile ranges that overlapped with traditional biomass stoves and for some pollutants (i.e., ultrafine particles, carbonyls, EC, PAHs, particle-phase carcinogens) the three-stone fire did not have the highest emissions on average. Given that substantial emissions reductions may be needed to have meaningful health benefits,70 the term “improved biomass stove” should be used with caution. For example, despite reducing PM2.5 and CO emissions, the wick kerosene stove emitted benzene, formaldehyde, and other VOCs in quantities that rival some traditional stoves. This finding supports the World Health Organization’s discouragement of kerosene stoves.71 Given that not all species of VOCs have equivalent impacts on human health and the environment, however, further work is needed to assess the relative levels of toxicity, ozone forming potential, and secondary organic aerosol forming potential of VOCs emitted from cookstoves.10,11 Finally, we found that measuring the emissions of PM2.5 and CO alone will likely not provide adequate information to predict the levels of coemitted pollutants even when stove type and fuel type are known. This finding is of concern, because there is evidence to suggest that the emissions factors reported here are sufficient (for several carcinogenic compounds) to create exposure levels that are harmful to human health. For example, Zhang and Smith35 reported that aldehyde emissions from solid- and liquid-fueled cookstoves in China were sufficiently high to produce (modeled) indoor exposure levels that exceed irritant threshold concentrations. Our emissions factors were similar those of Zhang and Smith,35 which ranged from 1.5 to 453 mg/MJd, suggesting that several of the compounds that are not well predicted by PM2.5 and CO (e.g., formaldehyde, acetaldehyde, acrolein) will likely be emitted at concentrations that are above the minimum risk levels published by the Agency for Toxic Substances and Disease Registry.72 Accounting for EC emissions removes half the prediction uncertainty for several pollutants including select carcinogenic PAHs. Given that EC can be measured through light-absorbing techniques at relatively low cost, we recommend including EC measurements in future laboratory and field studies. We also recommend research and development of fieldable low-cost sensors that can detect speciated compounds such as formaldehyde and benzene, both of which are carcinogenic constituents of cookstove smoke and likely to be emitted in quantities that are harmful to human health.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.est.8b07019. Stove type information (Section 1); completed test information (Section 2); information on instrumentation and emissions measurements (Section 3); cookstove smoke constituents and classifications (Section 4); information on the regression analysis (Section 5); limit of detection (Section 6) and laboratory background information (Section 7); fuel properties (Section 8); stove operating parameters (Section 9); comparison of PM2.5 and CO emissions with the literature (Section 10); results by stove/fuel combination (Section 11); 7122

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(10) Saliba, G.; Subramanian, R.; Saleh, R.; Ahern, A. T.; Lipsky, E. M.; Tasoglou, A.; Sullivan, R. C.; Bhandari, J.; Mazzoleni, C.; Robinson, A. L. Optical Properties of Black Carbon in Cookstove Emissions Coated with Secondary Organic Aerosols: Measurements and Modeling. Aerosol Sci. Technol. 2016, 50 (11), 1264−1276. (11) Reece, S. M.; Sinha, A.; Grieshop, A. P. Primary and Photochemically Aged Aerosol Emissions from Biomass Cookstoves: Chemical and Physical Characterization. Environ. Sci. Technol. 2017, 51 (16), 9379−9390. (12) Seinfeld, J. H.; Pandis, S. N. Atmospheric Chemistry and Physics: From Air Pollution to Climate Change; John Wiley & Sons: Hoboken, NJ, 2006. (13) Clean Cookstoves and Clean Cooking SolutionsHarmonized Laboratory Test ProtocolsPart 3: Voluntary Performance Targets for Cookstoves Based on Laboratory Testing; PRF TR 19867-3; International Standards Organization. (14) Habib, G.; Venkataraman, C.; Bond, T. C.; Schauer, J. J. Chemical, Microphysical and Optical Properties of Primary Particles from the Combustion of Biomass Fuels. Environ. Sci. Technol. 2008, 42 (23), 8829−8834. (15) Roden, C. A.; Bond, T. C.; Conway, S.; Pinel, A. B. O.; MacCarty, N.; Still, D. Laboratory and Field Investigations of Particulate and Carbon Monoxide Emissions from Traditional and Improved Cookstoves. Atmos. Environ. 2009, 43 (6), 1170−1181. (16) Johnson, M.; Edwards, R.; Alatorre Frenk, C.; Masera, O. InField Greenhouse Gas Emissions from Cookstoves in Rural Mexican Households. Atmos. Environ. 2008, 42 (6), 1206−1222. (17) Venkataraman, C.; Rao, G. U. Emission Factors of Carbon Monoxide and Size-Resolved Aerosols from Biofuel Combustion. Environ. Sci. Technol. 2001, 35 (10), 2100−2107. (18) Rapp, V. H.; Caubel, J. J.; Wilson, D. L.; Gadgil, A. J. Reducing Ultrafine Particle Emissions Using Air Injection in Wood-Burning Cookstoves. Environ. Sci. Technol. 2016, 50 (15), 8368−8374. (19) Just, B.; Rogak, S.; Kandlikar, M. Characterization of Ultrafine Particulate Matter from Traditional and Improved Biomass Cookstoves. Environ. Sci. Technol. 2013, 47 (7), 3506−3512. (20) Tryner, J.; Volckens, J.; Marchese, A. J. Effects of Operational Mode on Particle Size and Number Emissions from a Biomass Gasifier Cookstove. Aerosol Sci. Technol. 2018, 52 (1), 87−97. (21) L’Orange, C.; Volckens, J.; DeFoort, M. Influence of Stove Type and Cooking Pot Temperature on Particulate Matter Emissions from Biomass Cook Stoves. Energy Sustainable Dev. 2012, 16 (4), 448−455. (22) Shen, G.; Gaddam, C. K.; Ebersviller, S. M.; Vander Wal, R. L.; Williams, C.; Faircloth, J. W.; Jetter, J. J.; Hays, M. D. A Laboratory Comparison of Emission Factors, Number Size Distributions, and Morphology of Ultrafine Particles from 11 Different Household Cookstove-Fuel Systems. Environ. Sci. Technol. 2017, 51 (11), 6522− 6532. (23) Caubel, J. J.; Rapp, V. H.; Chen, S. S.; Gadgil, A. J. Optimization of Secondary Air Injection in a Wood-Burning Cookstove: An Experimental Study. Environ. Sci. Technol. 2018, 52 (7), 4449−4456. (24) Gupta, S.; Saksena, S.; Shankar, V. R.; Joshi, V. Emission Factors and Thermal Efficiencies of Cooking Biofuels from Five Countries. Biomass Bioenergy 1998, 14 (5−6), 547−559. (25) Kandpal, J. B.; Maheshwari, R. C.; Kandpal, T. C. Release of Air Pollutants in Indoor Air: Comparison of Traditional and Metallic Cookstoves. Renewable Energy 1994, 4 (7), 833−837. (26) Leavey, A.; Patel, S.; Martinez, R.; Mitroo, D.; Fortenberry, C.; Walker, M.; Williams, B.; Biswas, P. Organic and Inorganic Speciation of Particulate Matter Formed during Different Combustion Phases in an Improved Cookstove. Environ. Res. 2017, 158, 33−42. (27) Stockwell, C. E.; Yokelson, R. J.; Kreidenweis, S. M.; Robinson, A. L.; DeMott, P. J.; Sullivan, R. C.; Reardon, J.; Ryan, K. C.; Griffith, D. W. T.; Stevens, L. Trace Gas Emissions from Combustion of Peat, Crop Residue, Biofuels, Grasses, and Other Fuels: Configuration and FTIR Component of the Fourth Fire Lab at Missoula Experiment (FLAME-4). Atmos. Chem. Phys. Discuss. 2014, 14 (7), 10061−10134.

organic-carbon-to-EC (Section 12); and leave-one-out cross validation with EC as a predictor (Section 13) (PDF) The online digital repository43 contains disaggregated emission per-energy-delivered (mg/MJd), per-mass-offuel-burned (mg/kg), per-energy-of-fuel-burned (mg/ MJ), and per-time (mg/s) as well as stove operation parameters such as fuel use, test time, firepower, and modified combustion efficiency.

AUTHOR INFORMATION

Corresponding Author

*Phone: 970-491-6341; e-mail: [email protected]. ORCID

Kelsey R. Bilsback: 0000-0002-5996-1522 Pierre Herckes: 0000-0002-0205-3187 Jessica Tryner: 0000-0002-0522-4551 Jeffrey R. Pierce: 0000-0002-4241-838X John Volckens: 0000-0002-7563-9525 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge the National Institute of Environmental Health Sciences for their support of this research (Grant No.: ES023688) and the referees for their valuable feedback on this work.

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