Characterization of Ultrafine Particulate Matter from Traditional and

Mar 7, 2013 - Biomass combustion in cookstoves has a substantial impact on human health, affects CO2 levels in the atmosphere, and black carbon (BC) ...
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Characterization of Ultrafine Particulate Matter from Traditional and Improved Biomass Cookstoves Brian Just,*,† Steven Rogak,† and Milind Kandlikar‡,§ †

Department of Mechanical Engineering, The University of British Columbia, 6250 Applied Science Lane, Vancouver, British Columbia V6T 1Z4, Canada ‡ Institute for Resources, Environment and Sustainability, The University of British Columbia, 2202 Main Mall, Vancouver, British Columbia V6T 1Z4, Canada § Liu Institute for Global Issues, The University of British Columbia, 6476 NW Marine Dr, Vancouver, British Columbia V6T 1Z2, Canada S Supporting Information *

ABSTRACT: Biomass combustion in cookstoves has a substantial impact on human health, affects CO2 levels in the atmosphere, and black carbon (BC) and organic carbon (OC) affect the earth’s radiative balance. Various initiatives propose to replace traditional fires with “improved” (nontraditional) cookstoves to offset negative local and global effects. In this laboratory study, we compared the size, composition, and morphology of ultrafine particulate emissions from a “threestone” traditional fire to those from two improved stove designs (one “rocket”, one “gasifier”). Measurement tools included a scanning mobility particle sizer, PTFE and quartz filter samples, and transmission electron microscopy. In the improved stoves, particulate mass (PM) emissions factors were much lower although median particle size was also lower: 35 and 24 nm for the rocket and gasifier, respectively, vs 61 nm for the three-stone fire. Particles from improved stoves formed clearly defined chain agglomerates and independent spheres with little evidence of volatile matter and had a higher proportion of BC to total PM, although overall BC emissions factors were fairly uniform. The 3-fold increase in quantities of sub-30 nm particles from improved cookstoves warrants further consideration by health scientists, with due consideration to the higher combustion efficiencies of improved cookstoves.

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INTRODUCTION Globally, an estimated three billion people rely on biomass for cooking activities.1 In addition to emissions of CO2, biomass cookstoves release aerosols whose black carbon (BC) and organic carbon (OC) components affect the earth’s radiative balance. Factors such as land use change and altered surface albedo complicate the quantification of aerosol effects; the total direct aerosol radiative forcing (RF) effect is estimated at −0.5 (±0.4) W/m2, with biomass burning contributing +0.03 (±0.12) W/m2.2 Emissions from biomass-fuelled cookstoves represent approximately 20% of global pyrogenic BC and OC.3 BC may also affect regional climate by altering the hydrologic cycle4 and the control of biofuel combustion has been suggested as means to combat regional climate change,5 though in addition to particulate matter (PM) composition and morphology, models must carefully consider mixing state; for example, see ref 6. Products of incomplete biomass combustion include a wide range of health damaging pollutants ranging from CO to carcinogenic hydrocarbons (e.g., benzene), mutagenic organics, and PM; woodsmoke contains at least 26 hazardous air pollutants.7 It is estimated that in 2010, over 3.5 million deaths were caused by household air pollution from solid fuels.8 Low birth weight and stillbirth have also been linked to indoor air pollution from stoves.9 © XXXX American Chemical Society

The literature is limited regarding health effects from ultrafine particulates, that is, those with diameters smaller than 100 nm, but there is evidence that such particles may be a primary cause of negative health impacts. Alveolar macrophages, the prevalent mechanism for clearing larger particles from the alveolar region, may not efficiently clear particles smaller than 100 nm.10 One study instilled fine and ultrafine carbon black (mean diameters 260.2 and 14.3 nm, respectively) and titanium dioxide (250.0 and 29.0 nm) into rats; the ultrafine particles of both materials induced more inflammation and epithelial damage than the (larger) fine particles.11 In another study, human nasal epithelial cells were exposed to both diesel exhaust and Paris urban air particles. Internalization of particles was found to be restricted to nanoparticles smaller than 40 nm.12 It is not clear to what extent inhaled nanoparticles enter the bloodstream of humans, though evidence supports the occurrence in rodents (e.g., 30 nm gold particles were found in pulmonary capillary platelets within 30 min of exposure to intratracheal regions in rats13). These studies and a recent comprehensive review14 suggest that Received: October 24, 2012 Revised: February 28, 2013 Accepted: March 7, 2013

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Figure 1. (a) Three-stone fire, (b) Chulika (rocket), and (c) Oorja (gasifier) cookstoves.

the cookstove used in a CO2 reduction project in India financed by the Clean Development Mechanism (expected CO2 emissions reduction from the project is over 40 kilotons tons per year26); the Chulika shares design features with other stove designs, including StoveTec (Eugene, OR). The selected gasifier was the “Oorja,” a stove developed with funding from a multinational company, now with active private distribution (First Energy Ltd., Bangalore, India); it is designed for biomass pellets and batch loading with forced-air draft powered by a fan and a single AA battery. The three tested cookstoves are pictured in Figure 1. The focus of the study was the change in PM size and composition across cookstove types. Ultimately, we desire the response of different stoves to real-world fuel and operation variations; as a starting point we considered only stabilized operation with consistent fuels, while recognizing that future work should include a wide range of fuel types and operating modes. To minimize fuel variation, we used hemlock from a single kiln-dried timber cut into sticks of rectangular 15−20 mm rectangular cross section (see Figure 1 (a,b)). All pieces were cut on the same day and stored in identical manner and testing occurred over a minimal period of time. Moisture content prior to testing was 9.5% (dry basis) and an elemental analysis yielded a carbon content of 47.5% of dry mass. Since the Oorja cookstove is not designed to burn sticks, we used commercial wood pellets produced in British Columbia. Pellets measured approximately 6.5 mm diameter with an average length of 10 mm. Moisture content was 6.9% (dry basis) and carbon content was 47.7% of dry mass. The widely used Water Boiling Test (WBT) 4.1.227 captures three distinct combustion phases and is meant to be a replicable way to assess stove performance, but its focus on efficiency measurement rather than emissions characterization rendered it better as a guideline only for the present study. Since emissions properties vary depending on burn phases,25 we targeted a medium-power, “steady” combustion condition most similar to the “simmer” phase of the WBT (considerable variations in emissions still occur; this is discussed later). We found that the Chulika and three-stone fires reached relatively stable combustion about 20 min after the ignition of fresh fuel. When loaded with 700g of pellets (combustion chamber about 75% full), the batch-loaded Oorja reached a stable condition after 10 min. Following the warm-up period, the Chulika and three-stone fires were tended for 60 min while the Oorja’s fuel supply was sufficient for 30−35 min of steady combustion. Based on a requirement to test different stoves (including both batch and continuously fuelled designs), the hood method of emissions collection28 was implemented in a system designed in accordance with WBT guidelines. A stainless

emissions of particles smaller than 30−40 nm deserve greater attention. The potential to reduce environmental and health impacts by using “improved,” “clean,” or nontraditional cookstoves is well documented.15,16 Such improved stoves can reduce fuel consumption and emissions of PM and other pollutants through improving combustion and heat transfer efficiencies. There are numerous designs for improved cookstoves; a prominent example is the “rocket” design, characterized by a short insulated chimney serving as the combustion chamber with unobstructed air intake into the combustion zone. Another improved style, the “gasifier,” enables pyrolysis of biomass and combusts the resulting gaseous products in a secondary naturalor forced-draft region above the pyrolysis zone. While less than one-third of those who rely on solid fuels use improved stoves,1 numerous initiatives target the replacement of unimproved/ traditional methods by these and other designs; for example, the Global Alliance for Clean Cookstoves targets the distribution of 100 million cookstoves by the year 2020.17 Cookstove “change-outs” are also financed by the Clean Development Mechanism and other programs that trade carbon credits earned through emissions-reduction projects; in 2011, clean cookstove technologies transacted 3.2 MtCO2e.18 Previous cookstove studies investigated cookstove efficiencies and emissions factors for gaseous pollutants and PM (e.g., refs 19,20); Jetter et al.21 included ultrafine particulate counts, but without size distributions. Other studies compared traditional stoves with both new and “broken-in” improved cookstoves and in-use vs laboratory emissions measurements,22 correlated indoor and outdoor BC and OC to cookstove events,23 investigated BC in cookstove plume and breathing zones,24 and investigated the optical properties of PM from cookstoves.25 However, most studies did not focus on the size and morphology of ultrafine particulate emissions, details that are critical to a more complete understanding of the human health and environmental effects of both improved and unimproved cookstoves. Hence there is a clear need to determine whether PM from improved cookstoves may be smaller, more numerous, and/or morphologically different in a way that could adversely affect the expected benefits of emissions reductions.

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MATERIALS AND METHODS A three-stone fire comprised of three bricks was tested against two common types of improved cookstoves: a rocket stove and a gasifier stove. The three-stone baseline consisted of three bricks placed in symmetrical orientation. The selected rocket was the “Chulika” (distributed by iSquareD, Bangalore, India), B

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Figure 2. Schematic of emissions testing setup.

steel pot containing five liters of water was used as a heat sink/ flame deflector to simulate actual cooking activity. A thin-walled isokinetic sampling probe, sized and constructed based on the expected flow requirement, was located 15 diameters downstream of the hood inlet. From the probe, exhaust was routed to gaseous emissions and PM monitoring instruments as indicated by the schematic in Figure 2. To avoid intertest contamination from particles that may have settled on sampling system walls (e.g., during high-polluting stove startup and extinguishing events), dwell times of several minutes were allowed for between tests, before taking PM measurements. CO2, CO, NO, NOx, CH4, total hydrocarbons (THC), and O2 were measured at 1 Hz by an AVL CEB II Emissions Bench that was spanned and zeroed with calibration gases at the start of each day. Background CO2 in the test cell was continuously monitored. PM instruments were connected to the sampling probe via electrically conductive lines to minimize diffusion losses; residence time in lines was approximately six seconds. One branch of the circuit was connected to real-time instruments; a TSI DustTrak DRX Aerosol Monitor 8533 measured light scattering of aerosol and provided an indirect measurement of PM mass in the 0.1−15 μm size range, a TSI Scanning Mobility Particle Sizer (SMPS) model 3080 with a custom differential mobility analyzer (DMA) column measured particles in the 14.6−661.2 nm diameter (midpoint) range, a TSI Aerodynamic Particle Sizer (APS) model 3321 measured particles in the 500 nm to 20 μm aerodynamic diameter range by a time-of-flight technique, and a Magee Scientific Aethalometer model AE 21 measured BC via light transmission across a quartz fiber filter. In a separate branch of the test circuit, a pump drew air through several devices that allowed more detailed (offline) PM characterization. Aerosol was collected onto 47 mm diameter filters using modified stainless steel Gelman filter holders that accommodated backup filters. PTFE membrane filters were used for gravimetric PM mass measurement. Quartz fiber filters were collected for EC/OC analysis by Sunset Laboratory Inc.

(Tigard, OR) using the National Institute for Occupational Safety and Health (NIOSH) 5040 protocol;29 these employed a quartz backup filter behind the PTFE filter to correct for positive adsorption of organic gases.30 Critical orifices controlled flow rates (∼6.3 lpm) for 47 mm filters and a Honeywell AWM 5104 Venturi mass flowmeter enabled flow verification and accurate timing of PM collection periods (flow rates did not appreciably change as filters were loaded during our tests). A thermophoretic particle sampler (TPS) developed by the author at The University of British Columbia was used to collect material onto 3.05 mm transmission electron microscopy (TEM) grids (Ted Pella model 0813-F, Carbon Type B support film on 300 mesh copper) for subsequent microscopy work that provided PM morphology and supporting PM size information via image analysis. During tests, the emissions bench, DustTrak, aethalometer, and auxiliary instruments recorded continuously while SMPS and APS scans (135 s each) were initiated at three-minute (approximate) intervals. Filters and TEM grids were collected during a stable period of combustion within a test. Gaseous emissions were corrected by subtracting averaged background measurement data collected for 10 minutes prior to each test series. Differences in exhaust flow rate between tests (on the order of 2%) were ignored. We used a carbon balance in which combusted carbon from fuel was accounted for by CO2 and the measured carbon-containing products of incomplete combustion (PICs): CO, CH4, NMHC, and PM.

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RESULTS Results shown are based on data for three tests per stove. For clarity, the Chulika and Oorja cookstoves are referred to below as rocket and gasifier, though we acknowledge that these two models are not necessarily representative of all stoves in each respective class. The “steady” burn period for analysis purposes was determined post-test based on SMPS data (see Supporting Information (SI)). C

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There was a notable difference in the emissions characteristics of the three-stone fire and the improved cookstoves, with PM and CO emission rates drastically lowered during the progression from three-stone fire to rocket to gasifier. Spikes in gaseous emissions and DustTrak data for the three-stone and rocket were correlated with fire tending (adding/moving fuel); see SI. A carbon balance for the three-stone fire indicated 87.4% of measured carbon going to the formation of CO2, with the balance going to PICs CO, CH4, NMHC, and PM at 9.1, 0.9, 1.4, and 1.2%, respectively (in the carbon balance, carbon to PM ratio was cookstove dependent and was adjusted based on EC/OC and gravimetric data). The rocket and gasifier combusted carbon much more completely, with 97.3 and 97.6% of carbon, respectively, going to the formation of CO2; the balance went to CO (2.4 and 2.3%) and PM (0.3 and 0.1%), while hydrocarbon emissions were not clearly distinguishable from background data. While a quantitative study of efficiency was not the aim of this research, the increase in combustion efficiency with the improved cookstoves is relevant to subsequent results and discussion. Post-test weighing of PTFE filters provided a gravimetric measure of PM against averaged values from the DustTrak during the (filter) collection period; this provided a correction to the real-time data that was applied to each test prior to normalizing results to the carbon burn rate of the rocket (which had the highest average rate of carbon combustion during this set of tests). Results appear in the Concentrations section of Table 1. Although the difference in average PM levels between stove types was less pronounced with normalization and the gravimetric correction applied, the improved cookstoves still had much lower PM emissions. Aethalometer data provided a near real-time estimate of BC content and was compared to corresponding corrected DustTrak data; slope of least-squares-fit lines yielded BC/PM ratios 0.06, 0.20, and 0.41 for the three-stone, rocket, and gasifier, respectively. A measurement of EC was obtained via NIOSH 5040 analysis of the quartz filters and also compared to PM; both the aethalometer and NIOSH 5040 results are presented in the Form of Carbon section of Table 1. While BC and EC are different measures and are not necessarily expected to corroborate (see ref 31 for further discussion), the NIOSH 5040 analysis concurs with aethalometer data indicating that improved stoves produce more EC as a percentage of TC (or PM) than the three-stone fire. The gasifier cookstove produced the “blackest” PM while the three-stone fire produced the highest organic content. While a direct comparison should be avoided due to differences in fuels, stoves, and operational conditions, results are in rough agreement with field testing of 11 traditional Honduran cookstoves in which EC/TC ratios of 0.07−0.64 (average 0.27) were reported, with 0.64 ± 0.04 as the value for a single improved cookstove.25 Emissions factors for the OC and EC portion of PM are shown in the Emissions Factors section of Table 1. These results appear to be consistent with prior studies, as discussed below. For PM, Roden et al.25 reported field measurements of 8.5 g/kg of fuel for an average of 11 traditional (U-shaped, mud/clay) stoves; for OC and EC, measurements were 4.0 and 1.5 g/kg of fuel, respectively. In a laboratory study, MacCarty et al.20 reported EC emissions factors of 0.88, 1.16, and 0.28 g/kg of fuel for a three-stone, rocket, and gasifier (stick burning), respectively. Though overall PM emission factors vary significantly, EC emission rates are fairly uniform, as most PM from the highly

Table 1. Summary of Particle Measurements for Three Cookstoves stove

three-stone

DustTrak raw PM averages (mg/m3)a DustTrak “error”b corrected and normalized PM (mg/m3)c EC/TC (NIOSH 5040)d TC/PM (NIOSH 5040)e EC/PM (NIOSH 5040)e BC/PM (aethalometer)f PM (g/kg dry fuel) OC (g/kg dry fuel) EC (g/kg dry fuel) mobility diameter, SMPS (nm)g primary particle diameter, TEM (nm)

rocket

gasifier

Concentrations 28.5 ± 5.6 4.63 ± 0.84

1.07 ± 0.17

+172 ± 13% 8.25

−16 ± 10% 1.18

+21 ± 4% 3.81

Form of Carbon 0.15 ± 0.07 0.80 ± 0.05 0.56 0.67 0.08 0.53 0.06 0.20 Emission Factors 8.44 2.38 3.98 0.32 0.70 1.27 Particle Size 61 35 57.7 ± 23.6 (N = 52)

46.1 ± 19.2 (N = 83)

0.70 ± 0.04 0.86 0.60 0.41 0.86 0.22 0.52 24 27.2 ± 10.5 (N = 105)

a

Averages based on data for all tests per stove. Standard deviation based on per-test averages (three per stove). b(DustTrak − gravimetric measurement)/gravimetric measurement. Variability is standard deviation based on three filters per stove. cDustTrak values corrected using data from previous row, then normalized to the carbon burn rate for the rocket cookstove. dVariability is standard deviation of raw EC/TC values for each cookstove (includes backup filter subtraction), N = 3. eEC from NIOSH 5040 analysis of quartz filters; PM from PTFE filter. Calculation assumes uniform distribution of PM onto quartz and PTFE filters and an effective (measured) collection diameter of 35 mm for the PTFE filter. fBC from aethalometer; PM from DustTrak with gravimetric correction. gApproximate number mode diameter, from Figure 3.

polluting three-stone fire is attributed to OC. We fully acknowledge that the overall impacts of a woodstove depend on the emissions associated with the production of a cooking or heat delivery unit, but stove and cooking vessel geometry and the use of auxiliary devices such as pot skirts can greatly affect results; by eliminating efficiency as a factor in our presentation of data, we hoped to obtain quantities that are more intrinsic to the combustion process itself. To obtain particle size distributions, SMPS data were normalized by the mass of carbon emitted during each scan; the area under each curve in Figure 3 is proportional to the total number of particles emitted. The three-stone fire emitted the most particles, but quantities were well within an order of magnitude; the gasifier and rocket emitted approximately 40% and 70%, respectively, of the normalized particle count of the three-stone fire. There was, however, a clear shift to smaller particles with the improved cookstoves; the number mode of three-stone PM emissions was approximately 61 nm; the mode was about 35 nm for the rocket and 24 nm for the gasifier. The rocket and gasifier stoves produced 3.5 and 3.1 times as many 30 nm and smaller particles as the three-stone. Follow-up testing at a later date indicated that particle size with the gasifier correlates strongly with operational setting; see SI. APS data indicated little contribution from larger particles and is not reported here. For the imaging analysis, three TEM grids were analyzed for each cookstove. Five regions of each grid were investigated in D

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(Figure 4(e,f)). The improved cookstove images indicate a tendency for similarly sized agglomerates, though the gasifier appears to have the smallest primary particles as both independent spheres and components of agglomerates. Such spherical “primary particles” are composed of numerous lamellar crystallites, typically with 5−10 sheets containing a quantity of carbon atoms on the order of 100 each;32 some coagulate to form chain agglomerates and can also grow by the adsorption of vapor species on their surfaces. Our focus was on sizing primary particles that form the building blocks of agglomerates (since it was not easy to isolate a large number of individual primary particles for the threestone fire). We note that because it was not straightforward to distinguish small particles in some agglomerates (e.g., the midupper portion of Figure 4(e)), sizing may be biased toward (larger) particles with better-defined boundaries. Refer to SI for additional commentary on particles collected on TEM grids. Results of the manual sizing are displayed in the Particle Size section of Table 1. Because of the relatively small number of TEM grids/agglomerates analyzed, we cannot state that TEM imaging validates the particle size distributions reported by the SMPS. However, image measurements are in general agreement with the number size distribution summary of Figure 3 and most of the sub-100 nm content in the SMPS data is likely composed of particles that are essentially spherical and not part of agglomerates. High OC content from the three-stone is suggested by the “oiliness” of the images and the fact that there is visual evidence of much evaporated volatile material. In comparison, both improved stoves show primarily clearly defined chain agglomerates and independent spheres with little evidence of evaporated volatile matter at any magnification. The gasifier appears to produce more independent spheres (Figure 4(f)) than the other stoves. Relatively few of these were measured and composition cannot be determined from TEM; gasifier spheres were 12.4 ± 6.4 nm (N = 50) while rocket

Figure 3. Normalized SMPS number distribution; standard deviation of the average value for each size bin is represented by thin dashed lines of the corresponding color [gasifier: N = 36 (scans), rocket: N = 67; three-stone: N = 68]. Data for diameters smaller than 350 nm is displayed. All diameters are based on electrical mobility.

detail. Figure 4(a−c) shows representative images for each of the three cookstoves at 15 000× magnification. Qualitatively, there are obvious differences; three-stone images consistently indicate what appear to be “oily”, evaporated droplets of volatile material with darker PM left behind, while the improved cookstoves display well-defined spheres and agglomerates. Particles were manually measured with photo editing software at 300 000× and 500 000× magnification. Figure 4(d−f) shows representative examples. The three-stone agglomerate in Figure 4(d) is comprised of primary particles that appear larger than those in the rocket and gasifier images

Figure 4. Examples of TEM imaging at 15 000× magnification for the (a) three-stone, (b) rocket, and (c) gasifier; second rows shows primary particle sizing (within agglomerates, circles indicate selected particles) at high magnification for the (d) three-stone at 300 000×, (e) rocket at 300 000×, and (f) an image of gasifier particles at 500 000×. E

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combustion conditions in the open fire. Also, lower combustion temperature likely means the existence of more unburned solids in the emissions (larger size, more OC). Meanwhile, both improved stoves direct combustion products through a hotter combustion chamber (temperature, geometry) and for longer periods (time) that further improve combustion by “smoothing out” fluctuations that occur in the traditional fire and promoting more oxidation of carbon and decomposition of organics; improved mixing of combustion gases (especially with the forced-draft gasifier) is also a factor. The resulting combustion products are smaller and are composed of less OC (as a fraction of total carbon products). While the size and morphology trends observed in this study could influence particle climate and health impacts, climate impacts will be influenced by atmospheric aging processes while health impacts are influenced by real-world dilution processes that differ from the controlled process used in this research. This study focused on a comparative, repeatable analysis of emissions from steady-state burn events using a three-stone fire and two improved cookstoves. Future work should make efforts to mimic actual fuels and cooking events with consideration to the likelihood of successful adoption of specific cookstoves/ programs. Given improved cookstoves’ recent funding and attention, continued improvement in our understanding of emissions and end effects is important. The characterization of ultrafine particles under other cookstove operating modes that better encompass the full spectrum of a burn cycle is warranted.

spheres were 21.9 ± 5.0 nm (N = 8). It is beyond the scope of this work to comment on differences (if any, once accounted for statistically) in the size of the primary particles produced by different cookstoves.

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DISCUSSION AND CONCLUSIONS The improved cookstoves exhibited higher combustion efficiency and produced fewer PICs than the three-stone fire. Given that they are also designed to better focus energy onto a recipient vessel, the goal of reduction in biomass fuel usage is likely and must be taken into account when evaluating environmental and health effects. PM emissions factors were 8.44, 2.38, and 0.86 g/kg of dry fuel for the three-stone, rocket, and gasifier, respectively. Despite the reduction in PM for the improved stoves, the increased quantities of smaller particles (e.g., diameters under 30 nm) could present health concerns if these particles prove capable of deeper penetration into the cardio-respiratory system than larger particles. This issue is relevant to other combustion systems, such as gas burners, which have low mass emissions and a mean particle size less than 10 nm (though in this sub-10 nm range, particles are more effectively deposited by diffusion in nasal passages). Particles from the improved stoves were smaller than those emitted by the three-stone fire and were less likely to exist as chain agglomerates. How long these ultrafine particles remain in the atmosphere and retain their small size may be relevant to climate considerations, but regarding human health it must be emphasized that the path the particles took from the cookstove to PM monitoring instruments (via the exhaust system) may be longer than the typical path from cookstove to a human tending it or other humans in the same (possibly poorly ventilated) building/enclosure. In both our tests and in the field, dilution is sufficient to prevent downstream coagulation (i.e., formation of longer chain agglomerates), but the extent of condensation or nucleation of semivolatile material could be influenced by the details of the dilution process. We observed high OC content in the three-stone fire when compared to the improved cookstoves. From a climate perspective, this suggests that large-scale deployment of improved cookstoves has potential to influence the earth’s radiative balance due to the displacement of large quantities of highly scattering OC from three-stone fires, though any such effect is likely small compared to other global factors and must be considered with respect to particle size and morphology; brown carbon is an additional source of radiative forcing that we have not considered, but could offset some of the effects of OC scattering. Overall EC emissions factors were fairly uniform across all tested cookstoves; to determine whether improved cookstoves offer means to mitigate EC emissions, efficiency gains with the improved stoves must be considered. The large, stove-dependent differences between DustTrak readings and corrected PM results indicate a need to do a direct calibration for each stove tested in any cookstove study, given the role of both EC/OC content and particle size on light scattering-based PM estimation devices (such as a DustTrak or nephelometer). TEM images were consistent with the high measured OC content in three-stone fire samples. Fuel for all cookstoves had similar chemical content; given that the three-stone fire is uncontrolled in time and temperature, pockets of volatile and pyrolyzed material may be quenched before further oxidation, with resulting larger particle sizes and higher OC content. This could be especially true given the large fluctuations in

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ASSOCIATED CONTENT

S Supporting Information *

Supporting Information contains further detail on the fuels and experimental procedure; an overview of the thermophoretic particle sampler; additional discussion of real-time, SMPS, APS results; notes on DustTrak errors and SMPS repeatability; additional notes on cookstove operating modes; and TEM protocol, including discussion of possible collection bias. This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS B.J. was funded by a fellowship awarded by the National Sciences and Engineering Research Council of Canada (NSERC) under the Collaborative Research and Training Experience (CREATE) Atmospheric Aerosols Program. We thank Assistant Professor Andrew Grieshop of North Carolina State University for his substantive comments.

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REFERENCES

(1) Legros, G.; Havet, I.; Bruce, N.; Bonjour, S. A Review Focusing on the Least Developed Countries and Sub-Saharan Africa; United Nations Development Programme and World Health Organization: New York, 2009. (2) Forster, P.; Ramaswamy, V.; Artaxo, P.; Berntsen, T.; Betts, R.; Fahey, D. W.; Haywood, J.; Lean, J.; Lowe, D. C.; Myhre, G. et al. Changes in Atmospheric Constituents and in Radiative Forcing. In Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the InterF

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dx.doi.org/10.1021/es304351p | Environ. Sci. Technol. XXXX, XXX, XXX−XXX