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A Laboratory Assessment of 120 Air

Pollutant Emissions from Biomass is published and by the American Chemical Society. 1155 Sixteenth Fossil-Fuel Cookstoves Street N.W., Washington,

Kelsey Bilsback,DCJordyn 20036 Dahlke, Published by by American Subscriber access Nicholas provided Nottingham Kristen Fedak, Good, Chemical Society. Trent University

Copyright © American Chemical Society. However, no copyright

Arsineh Hecobian, Pierre Herckes, Christian L'Orange, is published by the John Mehaffy,American Amy Sullivan, Chemical Society. 1155 Sixteenth Jessica Tryner, Lizette Van Street N.W., Washington, Zyl, Ethan Walker, Yong Zhou, DC 20036 Published by by American Subscriber access provided Nottingham ChemicalTrent Society. University Copyright © American Chemical Society. However, no copyright

Jeffrey R. Pierce, Ander Wilson, Jennifer Peel, and John Volckens is published by the

Environ. Sci. Technol., Just American Chemical Society. 1155 Sixteenth Accepted Manuscript • DOI: Street N.W., Washington, 10.1021/acs.est.8b07019 • DC 20036 Publication Date (Web): 27 May 2019 Published by American Subscriber access provided by Nottingham ChemicalTrent Society. University Copyright © American Chemical Society. However, no copyright

Downloaded from http:// pubs.acs.org on June 1, 2019 is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, 20036 Just Accepted DC Published by by American Subscriber access provided Nottingham ChemicalTrent Society. University Copyright © American Chemical Society. However, no copyright

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Alkenes Page 1 ofEnvironmental 42 Science & Technology

Ultrafine particles Inorganic ions Ketones CO2 Carbohydrates Elemental carbon CO Aldehydes Alkanes PAHs Aromatics Alkynes Organic aerosol CH ACS Paragon Plus Environment4 PM2.5

Traditional Insulated natural-draft Insulated forced-draft

Biomass stoves

Three-stone fire Science & Technology Environmental Douglas fir (4) Eucalyptus (5) Oak (4)

Rocket elbow

Charcoal Kerosene

Douglas fir (5) Eucalyptus (3) Oak (3)


Fan rocket elbow


Forced-draft gasifier


Eucalyptus pellets (4) Lodgepole pine pellets (3)

Metal jiko

S. hardwood lumps (3) M. hardwood lumps (4) Coconut briquettes (3)

Wick kerosene


Pressure kerosene

Kerosene (3)

LPG stove LPG

Page 2 of 42

Built-in plancha

Ceramic jiko

Fossil-fuel stoves

Mud chulha

Liquified petroleum gas (3) ACS Paragon Plus Environment

Kerosene (3)

miligrams per Megajouledelivered

PM2.5 composition

Page 3 of 42

Environmental Science & Technology

2000 1500 1000 500 0 Rocket elbow

Built−in Fan rocket Gasifier plancha elbow

Ceramic jiko

Metal jiko

Wick Pressure kerosene kerosene

LPG

30

4 3

20

2 1

10

0

0 Three−stone Mud fire chulha

Rocket elbow

ACS Paragon Plus Environment


Ceramic jiko

Metal jiko


LPG

grams per Megajouled

grams per Megajouledelivered

organic aerosol PM2.5 mass

25 20 15 10 5 0 Three−stone Mud fire chulha

Carbon monoxide

inorganic ions elemental carbon

particles per Megajouledelivered

3e+14 2e+14

2e+15

1e+14

1e+15

0e+00

0e+00 Three−stone Mud fire chulha


Ultrafine Particles

3e+15

Rocket elbow


Ceramic jiko

Metal jiko

73

50 40


LPG

six rings five rings

four rings three rings

0.6

30

0.4

20

0.2

10

0.0


Rocket elbow



Ceramic jiko

Metal jiko


LPG

Polycyclic Aromatic Hydrocarbo miligrams per Megajoul

Polycyclic Aromatic Hydrocarbons

Page 4 of 42

cles per Megajoule



Volatile Organic Compounds


3000

300 200 100

1000

0

0 Rocket elbow


Ceramic jiko

Metal jiko


LPG

300 ketones aromatic aldehydes

unsaturated aldehydes saturated aldehydes

200

15 10

100

5 0


Rocket elbow



Ceramic jiko

Metal jiko


LPG

miligrams per Megajoul


alkenes alkanes

2000

Three−stone Mud fire chulha Carbonyl Compounds

aromatics alkyne (ethyne)

ligrams per Megajoul

Page 5 of 42


Particle−Phase Carcinogens

10.0

10.7

7.5

benz[a]anthracene benzo[a]pyrene benzo[b]fluoranthene

benzo[j]fluoranthene benzo[k]fluoranthene cyclopenta[cd]pyrene

Page 6 of 42

dibenzo[a,h]anthracene indeno[1,2,3−cd]pyrene

0.15 0.10

2.5

0.05 0.00

0.0 Rocket elbow


Ceramic jiko

Metal jiko styrene benzene

300

Wick Pressure kerosene kerosene isoprene acetaldehyde

LPG

formaldehyde

15

200

10 5

100

0


Rocket elbow



Ceramic jiko

Metal jiko


LPG

miligrams per Megajoul



5.0

Three−stone Mud fire chulha Gas−phase Carcinogens

11.9, 14.7

1.00

0.75

0.50

0.25

0.00 levoglucosan galactosan mannosan nitrate benzo[c]phenanthrene organic aerosol propionaldehyde potassium chrysene and triphenylene glycerol benzo[b]fluoranthene ultrafine particles benzo[j]fluoranthene benzo[e]pyrene benzo[a]pyrene 1,2−diethylbenzene benzo[k]fluoranthene perylene cyclopenta[cd]pyrene benzo[ghi]fluoranthene benz[a]anthracene naphthalene threitol 1,2,4−trimethylbenzene chloride butanone 1,2,3−trimethylbenzene o−tolualdehyde sulfate isovaleraldehyde acephenanthrylene 1,4−diethylbenzene ammonium phenanthrene 1−pentene 3−ethyltoluene acrolein anthracene hexaldehyde fluorene 2,5−dimethylbenzaldehyde acetaldehyde t,2−butene c,2−butene ethylbenzene t,2−pentene fluoranthene 1,3,5−trimethylbenzene 1−hexene n−propylbenzene i−propylbenzene benzene ethane formaldehyde i−butene c,2−pentene 1,3−diethylbenzene pyrene carbon dioxide n−octane 1−butene indeno[1,2,3−cd]pyrene ethyne benzo(ghi)perylene butyraldehyde o−xylene 2−ethyltoluene crotonaldehyde styrene 4−ethyltoluene toluene methacrolein m,p−xylene ethene propene benzaldehyde 2−methyl−2−butene i−butane m,p−tolualdehyde dibenzo[a,h]anthracene propane valeraldehyde

RMSE ratio Page 7 of 42

● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ●

poorly performing models

●

Model 1: PM2.5 and CO

Environmental Science & Technology ●

●

Model 2: PM2.5, CO, and stove type


●

● ● ● ● ● ●

●

●

Model 3: PM2.5, CO, and fuel type

highly performing models


1 2 3

Page 8 of 42

A Laboratory Assessment of 120 Air Pollutant Emissions from Biomass and Fossil-Fuel Cookstoves

4 5

Kelsey R. Bilsback1, Jordyn Dahlke1, Kristen M. Fedak2, Nicholas Good2, Arsineh Hecobian3,

6

Pierre Herckes4, Christian L’Orange1, John Mehaffy1, Amy Sullivan3, Jessica Tryner1, Lizette Van

7

Zyl1, Ethan S. Walker2, Yong Zhou3, Jeffrey R. Pierce,3 Ander Wilson5, Jennifer L. Peel2, John

8

Volckens1*

9 10

1Department

of Mechanical Engineering, Colorado State University, 1374 Campus Delivery, Fort

11

Collins, Colorado 80523

12

2Department

13

Campus Delivery, Fort Collins, Colorado 80523

14

3Department

15

Collins, Colorado 80523

16

4School

17

85287

18

5Department

19

Colorado 80523

of Environmental and Radiological Health Sciences, Colorado State University, 1681

of Atmospheric Science, Colorado State University, 1371 Campus Delivery, Fort

of Molecular Sciences, Arizona State University, 1604 Campus Delivery, Tempe, Arizona,

of Statistics, Colorado State University, 1877 Campus Delivery, Fort Collins,

20 21

Corresponding Author Information

22

*Phone: 970-491-6341; Email: [email protected]

23 24 25


1

Page 9 of 42


26

ABSTRACT

27

Cookstoves emit many pollutants that are harmful to human health and the environment.

28

However, most of the existing scientific literature focuses on fine particulate matter (PM2.5) and

29

carbon monoxide (CO). We present an extensive dataset of speciated air pollution emissions from

30

wood, charcoal, kerosene, and liquefied petroleum gas (LPG) cookstoves. One-hundred and

31

twenty gas- and particle-phase constituents—including organic carbon, elemental carbon (EC),

32

ultrafine particles (10-100 nm), inorganic ions, carbohydrates, and volatile/semi-volatile organic

33

compounds (e.g., alkanes, alkenes, alkynes, aromatics, carbonyls, and polycyclic aromatic

34

hydrocarbons [PAHs])—were measured in the exhaust from 26 stove/fuel combinations. We find

35

that improved biomass stoves tend to reduce PM2.5 emissions, however, certain design features

36

(e.g., insulation or a fan) tend to increase relative levels of other co-emitted pollutants (e.g., EC,

37

ultrafine particles, formaldehyde, or PAHs depending on stove type). In contrast, the pressurized

38

kerosene and LPG stoves reduced all pollutants relative to a traditional three-stone fire (≥93%

39

and ≥79%, respectively). Finally, we find that PM2.5 and CO are not strong predictors of co-emitted

40

pollutants, which is problematic because these pollutants may not be indicators of other cookstove

41

smoke constituents (such as formaldehyde and acetaldehyde) that may be emitted at

42

concentrations that are harmful to human health.

43 44 45


2


46

Page 10 of 42

TOC ART Alkenes Ultrafine particles Inorganic ions Ketones CO2 Carbohydrates Elemental carbon CO Aldehydes Alkanes PAHs Aromatics Alkynes Organic aerosol CH4 PM2.5

47


3

Page 11 of 42

48


INTRODUCTION

49

Household air pollution from solid-fuel combustion within cookstoves is a leading cause of

50

disease and premature death worldwide.1,2 Many constituents of cookstove smoke have known

51

health and/or atmospheric effects. For example, exposure to fine particulate matter (PM2.5) has

52

been linked to respiratory tract infections, chronic obstructive pulmonary disease, and

53

cardiovascular morbidity and mortality; exposure to carbon monoxide (CO) has been linked to low

54

birth weight and perinatal death; volatile organic compounds (VOCs) are associated with eye and

55

respiratory-tract irritation;3–6 and many compounds—such as benzene, formaldehyde,

56

acetaldehyde, and some polycyclic aromatic hydrocarbons (PAHs)—have been classified as

57

carcinogens.7,8 Additionally, if cookstove emissions are injected into the atmosphere, they can

58

impact climate and the environment9 (e.g., VOC emissions may react and form secondary organic

59

aerosols10,11 or tropospheric ozone).12

60

PM2.5 and CO are the most commonly measured constituents of cookstove emissions

61

because (1) exposure to PM2.5 and CO has been linked to adverse health impacts, (2) they are

62

the only pollutants that have standardized performance targets,13 (3) they constitute a large

63

fraction of cookstove smoke on a mass basis, and (4) they are relatively straightforward and less

64

costly to measure (than many other co-emitted pollutants). In this study, we comprehensively

65

characterized cookstove smoke profiles from a broad range of stove/fuel combinations to better

66

understand cookstove emissions beyond PM2.5 and CO. Although previous works have

67

characterized CO, PM2.5, and bulk PM2.5 composition (e.g., elemental carbon (EC), organic

68

carbon,14–16 and/or particle size17–23), data for other constituents of cookstove smoke (e.g.,

69

speciated PAHs, VOCs, and carbonyl compounds) are not as widely available. For example,

70

some studies report speciated emissions from stoves, but only characterize a limited number of

71

compounds (e.g., only formaldehyde or benzo[a]pyrene24,25) emitted by a limited number of

72

stove/fuel combinations (e.g., one26) and/or report few metrics (e.g., per-mass-fuel basis27). Built-

73

in coal heating stoves from China and Southeast Asia are one of the few stove types that have


4


Page 12 of 42

74

been characterized in depth,28–35 while comprehensive emissions data from wood, charcoal, and

75

fossil-fuel stoves are lacking.

76

In this study, we measured 120 particle- and gas-phase smoke constituents, including

77

organic aerosol, EC, inorganic ions, carbohydrates, ultrafine particles, PAHs, VOCs, and

78

carbonyls to gain insight into constituents of cookstove smoke that have garnered little attention.

79

Given the lack of data from wood, charcoal, and fossil-fuel stoves, we tested 26 stove/fuel

80

combinations that represent a range of technologies including traditional wood cookstoves (i.e.,

81

open fires), improved wood cookstoves (i.e., stoves which have been modified to lower PM2.5

82

emissions by adding insulation and/or a fan), charcoal stoves, and fossil-fuel cookstoves (i.e.,

83

kerosene and liquified petroleum gas [LPG] stoves). Furthermore, because many of the pollutants

84

measured in this study are typically not measured during cookstove testing, we used leave-one-

85

out cross validation to quantify the extent to which PM2.5 and CO can be used to predict emissions

86

of other smoke constituents (both on their own and when accounting for stove type or fuel type).

87

Our findings highlight the need to consider emissions beyond PM2.5 and CO when designing and

88

characterizing improved cookstoves. Our observations are relevant for research and policies

89

concerning the dissemination of cookstoves to communities, because an “improved stove” does

90

not necessarily guarantee emissions (and therefore exposure) reductions when considering all

91

harmful compounds that may be present in cookstove smoke mixtures.

92

MATERIALS AND METHODS

93

Test matrix

94

The stove/fuel test matrix and categories into which each stove falls (i.e., biomass vs.

95

fossil fuel and traditional vs. improved) are provided in Figure 1. A minimum of three replicate

96

tests were run for each stove/fuel combination. After the initial tests, some pollutant

97

measurements were excluded due to experimental error. An additional nine tests were conducted

98

to make up for some of the experimental issues during the primary testing; however, three

99

successful measurements were not available for all stove/fuel/pollutant combinations after


5

Page 13 of 42


100

erroneous data were excluded; Supporting Information [SI] Section 2 provides the total number

101

of successful measurements by pollutant.

102

Test protocol

103

The cookstoves were operated using the Firepower Sweep Test (FST); details of this

104

method are provided in Bilsback et al.36 In contrast to commonly-used laboratory protocols, which

105

are primarily task based (i.e., boiling and simmering a pot of water), the FST protocol directs the

106

user to operate the cookstove across a range of firepowers. Past work suggests that the FST

107

protocol captures a more realistic range of emissions, relative to in-field use, than task-based

108

laboratory protocols (e.g., the Water Boiling Test). Testing methodologies differed between

109

continuously fed biomass, batch-fed biomass, and fossil-fueled stoves due to differences in typical

110

operation between stove types. In contrast to Bilsback et al,36 fuel batches were fed one after

111

another rather than at designated time intervals. Samples for filter-based, cartridge-based, and

112

canister-based emissions measurements were captured over the entire sweep (which did not

113

include the stove’s start-up and shut-down), while time-resolved instruments were operated

114

during the entire test (from the stove’s start-up through the stove’s shut-down).

115

Emissions measurements

116

Details of the test setup and instrumentation are provided in SI Section 3. Briefly, a

117

custom-designed, total-capture hood was used for emissions testing. Sampling media for time-

118

integrated measurements included: Teflon filters (analyzed for PM2.5 mass), quartz filters

119

(analyzed for organic carbon, EC, organic carbon absorption artifacts,37 inorganic anions and

120

cations, particle-phase PAHs, and carbohydrates38,39), and polyurethane foam plugs (analyzed

121

for gas-phase PAHs). To minimize contamination, the quartz filters were baked at 800°C and the

122

polyurethane

123

dichloromethane/methanol/hexane mixture (and then air dried) before testing. Filter housings and

124

cartridges were cleaned first with dish soap and deionized water and then rinsed with a

125

dichloromethane/methanol/hexane mixture before use. Filter blanks were collected daily and filter

foam

filters

were

sonicated

in


acetone

and

then

in

a

6


Page 14 of 42

126

cartridges were leak checked daily. Other time-integrated sampling media included: a vacuum

127

canister fitted with a critical orifice (analyzed for VOCs)40 and dinitrophenylhydrazine (DNPH)

128

cartridges (analyzed for gas-phase carbonyls) that were placed in-line behind an ozone scrubber.

129

Two-hour background measurements were conducted on a weekly basis (18 tests in total) for all

130

time-integrated instrumentation.

131

Time-resolved instrumentation included a scanning mobility particle sizer (SMPS); carbon

132

dioxide (CO2), CO, and methane sensors; and thermocouples that measured the temperature of

133

the water in the cooking pot and the temperature at the combustion chamber outlet. Ultrafine

134

particles, defined here as particles with mobility diameters between 10 (the lower limit of the

135

SMPS) and 100 nm, were measured using the SMPS (~3 minute scans). The SMPS was installed

136

after a Venturi pump that provided secondary dilution. Secondary dilution ratios were determined

137

by simultaneously measuring carbon dioxide (CO2) concentrations in the ambient air, emissions

138

hood, and after the secondary dilution. Five-minute background measurements were conducted

139

before the beginning and after the end of each test for time-resolved instrumentation.

140

Data analyses

141

Data processing and analyses were conducted in R (v3.4.1); the code is published on

142

Github: https://github.com/nickgood/stoves_nih_2016_git/tree/master_kb. The 120 cookstove

143

smoke constituents quantified as part of this study are listed in SI Section 4. Emission factors

144

were calculated per-energy-delivered (mg/MJd), per-mass-of-fuel-burned (mg/kg), per-energy-of-

145

fuel-burned (mg/MJ), and per-time (mg/s).41

146

Emissions measurements were corrected for handling and background contamination.

147

Sample concentration data that were below a given analytical method limit of detection (LOD)

148

were replaced with

149

zero were replaced with zeros. Particle size distribution data were corrected for secondary dilution

150

on a test-by-test basis. Secondary dilution ratios ranged from 2.7 to 100. Particle losses in the

, and background-corrected values that were less than or equal to


7

Page 15 of 42


151

venturi pump, placed ahead of the SMPS, were not corrected for in the post-analysis (venturi

152

pump losses as a function of particle size are provided in SI Section 3).

153

Particle-phase organic carbon measurements were converted to organic aerosol.

154

Conversion factors of 1.5, 1.5, 1.2, and 1.2 were chosen for wood, pellet, charcoal, and fossil

155

fuels, respectively; these factors fall within the range of organic-aerosol-to-organic-carbon ratios

156

measured from biomass burning in the laboratory.42 See SI Section 3 for a mass balance and the

157

digital repository for organic carbon emissions factors.43

158

In this work, emissions from improved wood stoves, charcoal stoves, and fossil-fuel stoves

159

are presented as percent and absolute differences in the replicate-averaged emissions (including

160

replicates across all fuel types) relative to the three-stone fire, because the three-stone fire is the

161

most commonly used traditional cookstove.44 The interquartile range and raw data are provided

162

to represent emissions variability within a given stove type. Several smoke constituents are

163

presented as groups rather than as individual constituents. For example, inorganic ions are

164

grouped together, carbonyls and VOCs are grouped by carbon bond structure, and PAHs are

165

grouped by number of rings. We refer to compounds as carcinogenic if they have been classified

166

as a “known” or “reasonably anticipated” human carcinogens by the National Toxicology Program7

167

and/or classified as a Group 1- (carcinogenic to humans) or Group 2A- (probably carcinogenic to

168

humans) compound by the International Agency for Research on Cancer.8 See SI Section 4 for

169

compounds and classifications. The IARC also classifies “indoor emissions from household

170

combustion of biomass fuel,” referring the entire smoke mixture, as Group 2A. However, since

171

the following analyses are focused on the constituents of cookstove smoke, we only included

172

compounds that have been classified on a compound level in the carcinogenic compound

173

analysis.

174

Regression analysis


8


Page 16 of 42

175

PM2.5 and CO are the most frequently measured cookstove air pollutants; they also have

176

voluntary performance targets (ISO 19867-3:2018). EC emissions are also measured frequently,

177

although EC does not have a voluntary performance target. Leave-one-out cross validation45 was

178

used to assess whether emissions of PM2.5 and CO, EC, stove type, or fuel type could be used to

179

predict other co-emitted pollutants. The predictive ability of EC was evaluated separately from

180

PM2.5 and CO, because EC is measured less frequently. Six linear models were evaluated:

181

(1)

182

(2)

183

(3)

184

(4)

185

(5)

186

(6)

187

where

is a co-emitted smoke constituent; PM2.5 is fine particulate matter; CO is carbon

188

monoxide; EC is elemental carbon;

189

stove-specific coefficient;

190

Models were developed using emissions on a per-energy-delivered basis. Continuous variables

191

(i.e., PM2.5, CO, EC, and Pi) were log-transformed, because the assumptions of linear regression

192

were not satisfied otherwise. Thus, the slope coefficients can be interpreted as a percent change

193

in PM2.5, CO, or EC for a percent change in a co-emitted pollutants (rather than an absolute

194

change). Model 1 evaluates whether PM2.5 and CO emissions alone can predict co-emitted

195

pollutants, Model 2 evaluates if PM2.5, CO, and stove type can predict co-emitted pollutants, and

196

Model 3 evaluates if PM2.5, CO, and fuel type can predict co-emitted pollutants. Models 4-6 are

197

analogous for EC.

,

, and

are fixed intercepts or slopes;

is a fixed

is a fixed fuel-specific coefficient; and represents the model error.

198

For Models 1-3, only test replicates that had both PM2.5 and CO measurements (i.e.,

199

complete observations) were used in the analysis. As stated previously, background-corrected


9

Page 17 of 42


200

values that were less than or equal to zero were replaced with zeros. Since these zero values

201

could not be log-transformed, they were excluded from the analysis for all models. Pollutants ( )

202

for which more than 15% of the possible observations were missing (due to below-background

203

measurements) were also excluded, leaving 82 pollutants for the regression analysis (Note: PM2.5,

204

CO, and EC were never used as outcomes). To avoid overfitting and to ensure compatibility

205

across all models, stove/fuel combinations that had fewer than three observations (due to below-

206

background measurements and/or measurement error) were also excluded. Due to limited

207

replicates and low emission rates, fossil-fuel stoves were most frequently excluded. The pressure

208

kerosene and forced-draft gasifier stoves were excluded from all models due to missing CO

209

observations. The number of observations for each pollutant and the stove/fuel combinations that

210

were excluded are listed in SI Section 5.

211

Root-mean-square-error (RMSE) ratio was used to assess how well Models 1-6 predicted

212

each co-emitted pollutant. RMSE ratio was calculated by dividing the out-of-sample RMSE of

213

Models 1-6 by the out-of-sample RMSE of a model that always used the population average as

214

the prediction (i.e., a model with no predictors). A RMSE ratio of one indicated that the model

215

provided no improvement in prediction over the population average and a RMSE ratio of zero

216

indicated that the model removed all the prediction uncertainty.

217

RESULTS AND DISCUSSION

218

We measured above-background levels for 119 of the 120 cookstove smoke constituents

219

(all except inositol, a carbohydrate), demonstrating the diversity of pollutants present in cookstove

220

smoke (a summary of non-detects and below-background measurements are provided in SI

221

Sections 6 and 7, respectively). We found that the composition of cookstove smoke varied

222

substantially between stove types and between test replicates. (Emissions levels are summarized

223

in Figures 2-5. Additionally, CO2, methane, and carbohydrates are provided in a digital

224

repository.43) The variability across repeated tests from the same stove type was likely due to

225

differences in fuel properties (SI Section 8) and stove operation (SI Section 9). Emissions from


10


Page 18 of 42

226

replicate tests were less variable for fossil-fuel stoves than for biomass stoves, because the

227

operating conditions of the former were more controlled.

228

In SI Section 10 we compare the PM2.5 and CO emissions levels measured in this study

229

to emissions measurements from uncontrolled field tests15,46,47 and the Water Boiling Test48 (i.e.,

230

a task-based laboratory test) from previous studies. Overall, the PM2.5 and CO emissions levels

231

measured in this study are higher and more variable than the Water Boiling Test and tends to

232

agree better with field measurements. For some of the improved biomass stoves (i.e., the built-in

233

plancha and gasifier), the Firepower Sweep Test misses some of the highest emissions events.

234

This underestimation could be explained by the fact that there were differences in stove types

235

between the studies being compared and in this study we measured emissions integrated across

236

the Firepower Sweep Test rather than as a function of firepower. Although the Firepower Sweep

237

Test is not representative of a specific cooking event, overall this result, as well as previous work

238

by Bilsback et al.36 suggests that the PM2.5 and CO produced during the Firepower Sweep Test

239

may be more representative of the emissions during real world cooking than task-based

240

laboratory tests.

241

The figures that follow are pooled by stove type; results by stove/fuel combination are

242

provided in SI Section 11. Emissions factors are presented here on a per-energy-delivered basis,

243

while other metrics are provided in a digital repository.43 Note that a limited number of replicate

244

tests were conducted for a given stove/fuel combination (typically three), as we chose to prioritize

245

testing a wider range of stove/fuel combinations using our available resources. Previous work has

246

demonstrated that more than three replicates may be needed to determine whether a stove has

247

reached a performance target or whether one stove is cleaner than another with adequate

248

statistical power;49–51 thus, we caution interpretation of our results in these contexts. However, the

249

major conclusions of our study are based on large (non-overlapping) differences in emissions that

250

are unlikely to be overturned with additional laboratory testing.

251

PM2.5 composition


11

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252

Among the stoves tested, PM2.5 emissions were highest from traditional wood stoves and

253

lowest from fossil-fuel stoves; improved wood stoves and charcoal stoves fell in between (Figure

254

2). Relative to the three-stone fire, average PM2.5 emissions from improved wood stoves were 44-

255

81% lower (294-545 mg/MJd), charcoal stoves were 70-75% (468-502 mg/MJd) lower, and fossil-

256

fuel stoves were >99% (662-669 mg/MJd) lower. The decreased PM2.5 emissions from fossil-fuel

257

stoves relative to biomass stoves are likely attributable to the higher volatility of kerosene and

258

LPG, which, combined with the specific designs of these stoves, promotes more complete fuel-

259

air mixing and, thus, more complete combustion. In particular, the pressurized kerosene and LPG

260

stoves employ the venturi effect to premix vaporized fuel with air, which increases the

261

homogeneity of the fuel-air mixture and (when stoichiometry is optimal) tends to result in more

262

efficient combustion.

263

Of the wood stoves tested, the insulated natural-draft stoves and insulated forced-draft

264

stoves had substantially lower average PM2.5 emissions than the traditional stoves (Figure 2). The

265

lower PM2.5 emissions from the insulated wood stoves, compared to traditional wood stoves, are

266

likely attributable to better fuel-air mixing and reduced heat loss from the combustion zone, the

267

latter of which helps maintain the high temperatures needed to oxidize particulate matter.52

268

Charcoal stoves also had substantially lower average PM2.5 emissions than the traditional wood

269

stoves tested. This finding has been documented previously in the literature.36,46,48,53 Lower PM2.5

270

emissions from charcoal stoves were attributed to the lower volatile content of charcoal fuels

271

compared to wood fuels (Table S11). Charcoal combusts primarily via surface oxidation of carbon

272

to CO, whereas wood fuels undergo mixed pyrolysis and gas-phase combustion during which

273

pyrolysis products can form precursors to particulate matter. Note, however, the emissions factors

274

presented here do not include emissions during the production of charcoal fuel, which may lead

275

charcoal to have greater PM2.5 emissions across its lifecycle as compared to wood.54

276

On average, organic aerosol constituted the largest fraction of PM2.5 emitted for all stoves,

277

except for the wick kerosene stove, which emitted more EC (Figure 2). Organic aerosol emissions


12


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278

were highest for traditional wood stoves. Relative to the three-stone fire, average organic aerosol

279

emissions from improved wood stoves were 72-81% (510-578 mg/MJd) lower, charcoal stoves

280

were 86-87% lower (611-620 mg/MJd), and fossil-fuel stoves were >99% lower (709-710 mg/MJd).

281

In contrast to organic aerosol, EC emissions were highest for the two insulated natural-draft wood

282

stoves (organic-carbon-to-EC ratios are provided in SI Section 12). Relative to the three-stone

283

fire, average EC emissions from the rocket-elbow and built-in plancha stoves were 124% (+81.8

284

mg/MJd) and 105% (+69.5 mg/MJd) higher, respectively. Average EC emissions from other

285

improved wood stoves were 18-70% (11.7-46.2 mg/MJd) lower, charcoal stoves were 94% (62.1-

286

62.2 mg/MJd) lower, and fossil-fuel stoves were 91 to >99% lower (60.1-65.7 mg/MJd) (Figure 2).

287

The traditional wood stoves tested in this study were uninsulated and thus had greater heat loss

288

to the environment than improved wood stoves. The greater heat loss likely leads to regions with

289

lower temperatures, which can promote organic aerosol formation.52 Meanwhile, insulated

290

combustion chambers led to higher EC emissions from some stoves, likely due to their tendency

291

to favor flaming (instead of smoldering) combustion, which promotes soot-particle formation and

292

growth in fuel-rich regions of the flame zone.52

293

Currently, there is insufficient toxicological and epidemiological evidence to evaluate

294

whether the EC or organic aerosol components of PM2.5 have more serious health effects.

295

However, a review on the health effects of black carbon published by the World Health

296

Organization cites some evidence that the black carbon fraction of total PM2.5 may be more

297

strongly associated with short-term and long-health effects.55 Emitting more black carbon may

298

also be problematic from a climate perspective.56 Lacey et al.,57 demonstrated that removing light-

299

absorptive species, like EC, will lead to the largest climate-cooling reponse per kilogram of

300

emissions (especially at high latitudes were emissions are likely to impact snow albedo). Removal

301

of organic carbon aerosols, on the other hand, may produce a net warming effect.57

302

On average, the highest inorganic ion emissions came from biomass stoves that were

303

tested with pre-processed fuels (e.g., charcoal and pellets). Relative to the three-stone fire,


13

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304

average inorganic ion emissions from the metal jiko, ceramic jiko, gasifier, and built-in plancha

305

stoves were 303% (+75.1 mg/MJd), 128% (+31.7 mg/MJd), 24% (+5.87 mg/MJd), and 3% (+0.67

306

mg/MJd) higher, respectively. Notably, the high inorganic ion emissions from charcoal stoves were

307

driven by the coconut briquette fuel, which emitted more inorganic ions by mass than particle-

308

phase organic aerosol and EC combined (SI Section 11). The inorganic ion emissions from the

309

coconut briquette fuel were dominated by potassium and chloride (both biomass burning tracers)

310

due to the high ash content of the coconut charcoal fuel (SI Section 8). Overall, fuel choice was

311

a large driver of variability in ion emissions among the stoves tested (SI Section 11).

312

Carbon monoxide

313

Average CO emissions were highest for charcoal cookstoves (Figure 2). Carbon monoxide

314

emission factors for the ceramic jiko and metal jiko stoves were 137% (+13.6 mg/MJd) and 135%

315

(+13.4 mg/MJd) higher, respectively, than for the three-stone fire. The high CO emissions from

316

charcoal-fueled stoves were likely attributable to the primary oxidation process of charcoal fuels.

317

The charcoal fuels consisted of less volatile matter (19-31%) and more fixed carbon (50-62%)

318

than the wood fuels (SI Section 8). When the fixed carbon fraction of charcoal is burned, oxygen

319

reacts directly with the fuel surface to produce CO, often under conditions that yield lower heat-

320

release rates than wood-based fuels or fossil-fuels.52 Lower heat release rates likely result in

321

lower temperatures in the combustion zone, which can inhibit oxidation of CO to CO2. This

322

mechanism is supported by the lower firepowers observed for charcoal stoves relative to the wood

323

stoves tested (SI Section 9) and has been observed in previous studies.36,46,48,53

324

Average CO emissions from all improved wood stoves, except the built-in plancha, were

325

lower than from the three-stone fire (rocket elbow: 20% [2.01 mg/MJd] lower; fan rocket elbow:

326

31% [3.08 mg/MJd] lower; gasifier: 87% [8.66 mg/MJd] lower; built-in plancha: 42% [+4.12 mg/MJd]

327

higher) (Figure 2). Lower average CO emissions, compared to the three-stone fire, from the rocket

328

elbow, fan rocket elbow, and gasifier stoves were attributed to use of electric fans and/or improved

329

insulation to promote fuel-air mixing and maintain the high temperatures needed to oxidize CO.


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330

Average CO emissions from all fossil-fuel stoves were substantially lower than from the three-

331

stone fire (wick kerosene: 67% [6.61 mg/MJd] lower, pressure kerosene: 93% [9.24 mg/MJd] lower,

332

LPG: 91% [9.00 mg/MJd] lower), likely do to the increased volatility of fossil fuels as well as use

333

of the venturi effect to mix vaporized fuel and air in the pressure kerosene and LPG stoves.

334

Ultrafine particles (10-100 nm)

335

Both the fan rocket-elbow and gasifier stoves emitted more ultrafine particles (defined here

336

as particles between approximately 10-100 nm) than the three-stone fire (fan rocket elbow: 15%

337

higher [+1.96e+14 particles/MJd]; gasifier: 9% higher [+1.23e+14 particles/MJd]; Figure 3). Other

338

improved wood and charcoal stoves emitted 27-42% (3.65e+14-5.63e+e14 particles/MJd) and 60-

339

65% (8.07e+14-8.72e+14 particles/MJd) fewer ultrafine particles than the three-stone fire,

340

respectively. The largest reductions in ultrafine particles, relative to the three-stone fire, were

341

observed for the wick kerosene (97% [1.30e+15 particles/MJd]), pressure kerosene (95%

342

[1.28e+15 particles/MJd]), and LPG stoves (89% [1.20e+15 particles/MJd]), respectively.

343

When inhaled, ultrafine particles are more likely to deposit in and penetrate beyond the

344

alveolar region of the lungs than larger particles.58,59 Thus, ultrafine particles may promote more

345

systemic inflammation (compared to particles deposited in the upper airways) due to the close

346

coupling of the alveoli with the pulmonary circulatory system.60,61 While the fan rocket-elbow and

347

gasifier stoves reduced PM2.5 and CO emissions relative to the three-stone fire, ultrafine particle

348

emissions increased. This finding is consistent with previous studies18,62 demonstrating that

349

forced-air cookstoves may shift the particle size distribution towards smaller particles. This finding

350

also illustrates that design features added to reduce PM2.5 emissions from improved stoves may

351

lead to emissions tradeoffs (i.e., decreases in one emission type and increases in another).

352

Ultrafine particles form via nucleation and condensation of organic exhaust vapors or from

353

incomplete oxidation of soot. Particles that originate from condensation of organic vapors are

354

likely to form in regions with lower temperatures. Forced-draft biomass stoves (e.g., fan rocket

355

elbow and gasifier) may have more low-temperature regions because internal fans push relatively


15

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356

cool ambient air into the combustion chamber to facilitate fuel-air mixing.59 Nucleation may also

357

be more likely to occur in forced-draft stoves because, due to reduced soot formation, there are

358

relatively fewer surfaces for the organic vapors to condense onto, leading to higher vapor

359

saturation ratios.63 Thus, fans that are added to the stove to reduce PM2.5, by promoting fuel-air

360

mixing, may lead to increased formation of ultrafine particles that are potentially harmful to health.

361

Polycyclic aromatic hydrocarbons

362

The majority of improved stoves had lower PAH emissions relative to the three stone fire;

363

PAH emissions from improved wood stoves (excluding the rocket-elbow stove) and charcoal

364

stoves were, on average, 61-85% (8.54-11.8 mg/MJd) and 71-85% (9.94-11.9 mg/MJd) lower,

365

respectively. Average PAH emissions from fossil-fuel stoves were also consistently much lower

366

than from the three-stone fire (wick kerosene: 87% lower [12.1 mg/MJd]; pressure kerosene: 99%

367

lower [13.8 mg/MJd]; LPG: 97% lower [13.6 mg/MJd]). On average PAH emissions from the rocket-

368

elbow stove, however, were 20% higher [2.84 mg/MJd] than from the three-stone fire. Although

369

this increase does not hold when comparing the median emissions from the three-stone fire and

370

rocket elbow, this result is still concerning because two of the rocket elbow measurements are

371

several times higher than the median three-stone fire measurements. Many PAH species are

372

carcinogenic (see “Carcinogenic compound” section for further discussion). The high PAH

373

emissions from the rocket-elbow stove represent another pollutant tradeoff given that the rocket-

374

elbow stove decreased PM2.5 emissions relative to the three-stone fire. In this case, the insulation

375

added to the rocket-elbow stove (to reduce thermal losses and thus promote oxidation of CO and

376

PM2.5) likely led to more flaming (instead of smoldering) combustion and thus promoted formation

377

of PAH precursors to EC (under some test cases).52

378

On a mass basis, gas-phase PAH emissions were higher than particle-phase PAH

379

emissions (Figure 3); this result has been reported previously for other combustion sources64,

380

while higher particle-phase than gas-phase PAH emissions have been measured from built-in

381

heating stoves in China.30 Three-ring PAHs, which are primarily found in the gas phase, made up


16


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382

41-48% of total PAH emissions for wood stoves; 61% and 67% for the ceramic jiko and metal jiko,

383

respectively; and 26%, 44%, and 30% for the wick kerosene, pressure kerosene, and LPG stoves,

384

respectively. Contrastingly, six-ring PAHs, which are primarily found in the particle phase, made

385

up 1-4% of total PAH emissions for wood stoves; 3% for both charcoal stoves; and 17%, 39%,

386

and 14% for the wick kerosene, pressure kerosene, and LPG stoves, respectively. Note that

387

naphthalene results are provided in the repository43 but were not included in the analyses

388

presented here due to high measurement uncertainties.

389

Volatile organic compounds

390

We note that only a limited number of VOCs and carbonyls were measured as part of this

391

study. Some oxygenated VOCs such as phenols and furans, and nitrogen-containing compounds

392

(which tend to have shorter atmospheric lifetimes) were not quantified. One study by Stockwell et

393

al27 found that a three-stone fire emitted higher-levels of these types of VOCs than several

394

improved biomass stoves. Of the VOCs measured in this study, average VOC emissions from

395

improved wood and charcoal stoves were 72-92% (834-1066 mg/MJd) and 80-83% (929-962

396

mg/MJd) lower than from the three-stone fire, respectively (Figure 4). Some VOCs can be emitted

397

if biomass fuel that has been volatilized escapes the combustion zone without being completely

398

oxidized.59 Lower combustion temperatures in traditional stoves (due to poor thermal insulation

399

and high excess-air ratios) could contribute to higher emissions of unburned hydrocarbons.

400

Reductions of average VOC emissions from the wick kerosene stove (54% [623 mg/MJd]) and

401

LPG stove (79% [918 mg/MJd]) relative to the three-stone fire were smaller than the reductions of

402

other pollutants for these stoves (e.g., PM2.5, ultrafine particles, and PAHs). The VOC emissions

403

from the wick kerosene stove (534 [388-608] mg/MJd) were higher than from any of the improved

404

wood or charcoal stoves. Ethene made up 50% of the VOCs emitted from the wick kerosene stove

405

on a mass basis. VOC emissions from the LPG stove (239 [103-328] mg/MJd) were higher than

406

all improved biomass stoves except the rocket elbow. Propane, an alkane and a major constituent

407

of LPG, was the most abundant VOC emitted from the LPG stove (75%), indicating that much of


17

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408

the VOC emissions from the LPG stove were from unburnt fuel. VOCs can react in the atmosphere

409

to form secondary organic aerosol, which can contribute substantially to ambient PM2.5.65 Only

410

considering primary organic aerosol may bias the total PM2.5 contribution of improved biomass

411

stoves to ambient aerosol.66

412

Carbonyl compounds

413

Average carbonyl emissions were highest for the built-in plancha and the fan rocket elbow,

414

which emitted 70% (+79.0 mg/MJd) and 25% (+28.1 mg/MJd) more carbonyls, respectively, than

415

the three-stone fire (Figure 4). Increased aldehyde emissions from these stoves might have

416

occurred because both stoves have design features that lead to higher excess air ratios (i.e., a

417

chimney or a fan), which may lead to low-temperature regions where aldehydes are not

418

completely oxidized.52 Reductions in carbonyl emissions from the wick kerosene stove were

419

modest compared to other fossil-fuel stoves (33% [37.3 mg/MJd] relative to the three-stone fire).

420

Two of the aldehydes measured in this study (formaldehyde and acetaldehyde) are

421

carcinogenic,7,8 indicating the importance of quantifying stove emissions beyond PM2.5 and CO

422

(see “Carcinogenic compounds” section). Formaldehyde was the most abundant carbonyl

423

compound emitted across all cookstoves, making up 39-44% of total carbonyl emissions, on

424

average, from wood-fuel stoves; 25% and 20% from the ceramic jiko and metal jiko, respectively;

425

and 42%, 53%, and 60% from the wick kerosene, pressure kerosene, and LPG stoves,

426

respectively. Acetaldehyde also made up a large portion of the total carbonyl compounds,

427

especially for charcoal stoves (ceramic jiko: 25%; metal jiko: 28%). Similarly, Zhang and Smith35

428

found that formaldehyde and acetaldehyde were the most abundant carbonyls across a variety of

429

stoves and fuel types.

430

Carcinogenic compounds

431

Average emissions of particle-phase carcinogenic compounds were highest for the mud

432

chulha and rocket elbow (Figure 5). Emission factors for these stoves were 56% (+1.55 mg/MJd)

433

and 38% (+1.07 mg/MJd) higher than the three-stone fire, respectively. (Although this increase


18


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434

does not hold when comparing the median emissions from the three-stone fire and rocket elbow.)

435

Average particle-phase carcinogen emissions from other improved wood stoves and charcoal

436

stoves were 41-88% (1.14-2.44 mg/MJd) and 87-95% (2.41-2.63 mg/MJd) lower compared to the

437

three-stone fire, respectively, while average particle-phase carcinogen emissions from the wick

438

kerosene, pressure kerosene, and LPG stoves were 87% (2.42 mg/MJd), 99% (2.73 mg/MJd), and

439

97% (2.69 mg/MJd) lower. All of the particle-phase carcinogens measured here were PAHs, thus

440

particle-phase carcinogens generally followed the same trends as total PAHs.

441

Average gas-phase carcinogen emissions were highest for the mud chulha and the three-

442

stone fire. Relative to the three-stone fire, average gas-phase carcinogen emissions from

443

improved wood stoves and charcoal stoves were 30-74% (44.9-113 mg/MJd) and 68-73% (104-

444

111 mg/MJd) lower, respectively, while gas-phase carcinogen emissions from the wick kerosene,

445

pressure kerosene, and LPG stoves were 47% (71.2 mg/MJd), 96% (146 mg/MJd), and 94% (143

446

mg/MJd) lower, respectively. Benzene was the most abundant gas-phase carcinogen emitted from

447

traditional wood stoves (three-stone fire: 66%; mud chulha: 63%), while the gas-phase

448

carcinogens emitted by other stoves were dominated by the carcinogenic carbonyls (i.e.,

449

formaldehyde and acetaldehyde).

450

PM2.5, CO, and EC as predictors for co-emitted pollutants

451

Of the models that used PM2.5 and CO as predictors, we found that Model 2 (RMSE ratio:

452

mean = 0.77 [range = 0.46-1.13]) and Model 3 (RMSE ratio: 0.79 [0.51-1.15]), which assessed

453

the predictive ability of PM2.5 and CO conditional on stove type and fuel type, respectively,

454

performed better than Model 1 (RMSE ratio: 0.91 [0.58-1.13]), which only used PM2.5 and CO as

455

predictors (Figure 6). Across Models 1-3, the best predicted pollutants were biomass burning

456

tracers (RMSE ratio using Model 2: CO2 = 0.46, levoglucosan = 0.48, galactosan = 0.48) that are

457

closely tied to the amount of fuel burned. However, none of the models explained more than half

458

of the out-of-sample variance relative to the population average model, meaning that PM2.5 and

459

CO measurements alone are unlikely to provide adequate information to predict emissions levels


19

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460

of other co-emitted pollutants even when stove type or fuel type is accounted for. Notably, none

461

of the known-carcinogenic compounds (e.g., benzo[a]pyrene formaldehyde, acetaldehyde, and

462

benzene) were well-predicted by the models. This finding is problematic, because PM2.5 and CO

463

are the only pollutants with performance targets for cookstove emissions (ISO 19867-3:2018) and

464

are frequently the only pollutants measured in emissions, air quality, and health studies.

465

Overall, we found that the average predictive ability of the EC models was similar to the

466

average predictive ability of the PM2.5 and CO models (SI Section 13). However, when controlling

467

for stove type (Model 5), EC was a strong predictor of several PAHs (perylene [RMSE ratio: 0.48];

468

benzo[b]fluoranthene

469

benzo[c]phenanthrene [0.51]; benzo[a]pyrene [0.51]; benzo[k]fluoranthene [0.51]). Given that

470

many of these PAHs are carcinogenic,7,8 EC may be a useful indicator of carcinogenic properties

471

of cookstove smoke for a given stove type. To a lesser degree, EC also had predictive ability over

472

several gas-phase carcinogenic compounds (formaldehyde [RMSE ratio: 0.56], styrene [0.62])

473

when controlling for stove type. Given that EC provides some predictive ability over these harmful

474

compounds, measurement of EC may provide a less expensive, more straightforward alternative

475

to measurement of the carcinogenic compounds themselves. This could be especially useful for

476

field studies, where measurements of speciated compounds are unlikely to be collected due to

477

costs and logistical issues.

478

Implications for cookstove research

[0.49];

benzo[j]fluoranthene

[0.50];

benzo[e]pyrene

[0.50];

479

Recently, public health researchers have begun pivoting away from improved wood stoves

480

and towards liquid or gas fueled stoves (e.g., ethanol, LPG), arguing that improved wood stoves

481

do not reduce emissions substantially enough to provide meaningful health or environmental

482

benefits.67 Our work supports this transition, because we found that emissions from the LPG stove

483

were substantially lower than emissions from all wood and charcoal stoves (for all pollutants,

484

except select VOCs, on a per-energy-delivered basis). Emissions of fuel mixture alkanes, such

485

as propane, from the LPG stove exceeded emissions from most improved wood and charcoal


20


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486

stoves. Although more comprehensive exposure-response research (epidemiological and

487

toxicological) is needed to quantify the relative health benefits of switching from traditional (i.e.,

488

three-stone fire) to advanced (i.e., LPG) cookstoves, the few studies that have examined the

489

relative toxicity of various cookstove technologies support the connection between reduced

490

emissions and reduced toxicity.62,68,69

491

We also found that although improved biomass stoves tend emit less PM2.5 and CO,

492

reduced emissions of other co-pollutants are not guaranteed. For several pollutants, improved

493

biomass stoves had interquartile ranges that overlapped with traditional biomass stoves and for

494

some pollutants (i.e., ultrafine particles, carbonyls, EC, PAHs, particle-phase carcinogens) the

495

three-stone fire did not have the highest emissions on average. Given that substantial emissions

496

reductions may be needed to have meaningful health benefits,67 the term “improved biomass

497

stove” should be used with caution. For example, despite reducing PM2.5 and CO emissions, the

498

wick kerosene stove emitted benzene, formaldehyde and other VOCs in quantities that rival some

499

traditional stoves. This finding supports the World Health Organization’s discouragement of

500

kerosene stoves.70 Given that not all species of VOCs have equivalent impacts on human health

501

and the environment, however, further work is needed to assess the relative levels of toxicity,

502

ozone forming potential, and secondary organic aerosol forming potential of VOCs emitted from

503

cookstoves.10,11

504

Finally, we found that measuring the emissions of PM2.5 and CO alone will likely not

505

provide adequate information to predict the levels of co-emitted pollutants even when stove type

506

and fuel type are known. This finding is of concern, because there is evidence to suggest that the

507

emissions factors reported here are sufficient (for several carcinogenic compounds) to create

508

exposure levels that are harmful to human health. For example, Zhang and Smith35 reported that

509

aldehyde emissions from solid- and liquid-fueled cookstoves in China were sufficiently high to

510

produce (modeled) indoor exposure levels that exceed irritant threshold concentrations. Our

511

emissions factors were similar those of Zhang and Smith,35 which ranged from 1.5 to 453 mg/MJd,


21

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512

suggesting that several of the compounds that are not well predicted by PM2.5 and CO (e.g.,

513

formaldehyde, acetaldehyde, acrolein) will likely be emitted at concentrations that are above the

514

minimum risk levels published by the Agency for Toxic Substances and Disease Registry.71

515

Accounting for EC emissions removes half the prediction uncertainty for several pollutants

516

including select carcinogenic PAHs. Given that EC can be measured through light-absorbing

517

techniques at relatively low cost, we recommend including EC measurements in future laboratory

518

and field studies. We also recommend research and development of fieldable low-cost sensors

519

that can detect speciated compounds such as formaldehyde and benzene—both of which are

520

carcinogenic constituents of cookstove smoke and likely to be emitted in quantities that are

521

harmful to human health.


22


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522

Supporting Information

523

The Supporting Information (SI) for this manuscript has two parts: a PDF document and an online

524

digital repository of the emissions data. The PDF document (associated with the manuscript

525

website) includes the following: stove type information (Section 1); completed test information

526

(Section 2); information on instrumentation and emissions measurements (Section 3); cookstove

527

smoke constituents and classifications (Section 4); information on the regression analysis

528

(Section 5); limit of detection (Section 6) and laboratory background information (Section 7); fuel

529

properties (Section 8); stove operating parameters (Section 9); comparison of PM2.5 and CO

530

emissions with the literature (Section 10); results by stove/fuel combination (Section 11); organic-

531

carbon-to-EC (Section 12); and leave-one-out cross validation with EC as a predictor (Section

532

13). The online digital repository43 contains disaggregated emission per-energy-delivered

533

(mg/MJd), per-mass-of-fuel-burned (mg/kg), per-energy-of-fuel-burned (mg/MJ), and per-time

534

(mg/s) as well as stove operation parameters such as fuel use, test time, firepower, and modified

535

combustion efficiency.


23

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536

Acknowledgements

537

We would like to acknowledge the National Institute of Environmental Health Sciences for their

538

support of this research (Grant #: ES023688) and the the referees for their valuable feedback

539

on this work.

540


24


FIGURES

Insulated natural-draft Insulated forced-draft

Biomass stoves

Traditional

541

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Three-stone fire

Mud chulha



Rocket elbow

Built-in plancha



Fan rocket elbow

Forced-draft gasifier


Eucalyptus pellets (4) Lodgepole pine pellets (3)

Kerosene

Metal jiko


Wick kerosene


Pressure kerosene

Kerosene (3)

Kerosene (3)

LPG stove LPG

Fossil-fuel stoves

Charcoal

Ceramic jiko

Liquified petroleum gas (3)

542 543

Figure 1: Stove/fuel test matrix and categories into which each stove falls (i.e., biomass vs. fossil

544

fuel and traditional vs. improved). Stove type is indicated at the top center of each cell and fuel

545

types are listed on the right side of each cell. Values in parentheses indicate the number of

546

replicates conducted with each stove/fuel combination (n = 87 total emissions tests). Makes and

547

models of each stove design are presented in Supporting Information (SI) Section 1.


25

Page 33 of 42


548 549

Figure 2: Emissions of fine particulate matter (PM2.5), elemental carbon (EC), organic aerosol

550

inorganic ions, and carbon monoxide. The height of each colored bar represents replicate-

551

averaged emissions for each stove type (including replicates across all fuel types). The circular

552

markers indicate the PM2.5 emissions (top panel) and carbon monoxide emissions (bottom panel)

553

for each replicate test by stove type. Boxplots indicate the median and interquartile range of the

554

PM2.5 emissions (top panel) and carbon monoxide emissions (bottom panel) across all replicates

555

for each stove type. (Note: Because the fan-rocket elbow/oak stove/fuel combination only had

556

one replicate, this stove/fuel combination was excluded from the carbon monoxide plot.)


26


Page 34 of 42

557 558

Figure 3: Emissions of ultrafine particles (10-100 nm) and polycyclic aromatic hydrocarbons

559

(PAHs). The height of each colored bar represents replicate-averaged emissions for each stove

560

type (including replicates across all fuel types). The circular markers indicate ultrafine particles

561

(top panel) and total PAHs (bottom panel) for each replicate test by stove type. Boxplots indicate

562

the median and interquartile range of ultrafine particles (top panel) and total PAHs (bottom panel)

563

emissions for all replicates on each stove type. For scaling purposes, extreme outliers are

564

represented as numeric values at the top rather than being plotted.


27

Page 35 of 42


565 566

Figure 4: Emissions of volatile organic compounds (VOCs) and carbonyls. For each stove type,

567

total VOCs and total carbonyls are indicated by The height of each colored bar represents

568

replicate-averaged emissions for each stove type (including replicates across all fuel types). The

569

circular markers indicate total VOCs (top panel) and total carbonyls (bottom panel) for each

570

replicate test by stove type. Boxplots indicate the median and interquartile range of total VOCs

571

(top panel) and total carbonyls (bottom panel) for all replicates on each stove type. (Ethyne was

572

the only pollutant measured in the alkyne category.)


28


Page 36 of 42

573 574

Figure 5: Particle- and gas-phase emissions that are classified as “known” or “reasonably

575

anticipated” human carcinogens by the National Toxicology Program or International Agency for

576

Research on Cancer. The height of each colored bar represents replicate-averaged emissions

577

for each stove type (including replicates across all fuel types). The circular markers indicate total

578

particle-phase carcinogens (top panel) and gas-phase carcinogens (bottom panel) for each

579

replicate test by stove type. Boxplots indicate the median and interquartile range of the total

580

particle-phase (top panel) and total gas-phase (bottom panel) emissions for all replicates on each

581

stove type. For scaling purposes, numeric values are sometimes provided at the top of the plot

582

rather than an upper bracket.


29

Page 37 of 42

583 584


Figure

6:

Leave-one-out

cross

validation

585

using

,

586

Model

,

and where

587

3:

is a co-emitted smoke

constituent; PM2.5 is fine particulate matter; CO is carbon monoxide;

589

intercepts or slopes shared by all stove/fuel combinations; is a fixed fuel-specific coefficient; and

1: 2:

Model

588 590

Model

,

, and

are fixed

is a fixed stove-specific coefficient;

represents the error. The root-mean-squared-error

591

(RMSE) ratio is the RMSE of Model 1, Model 2, or Model 3 divided by the RMSE from a model in

592

which the population mean is always the prediction (i.e., a model with no predictors). RMSE ratio

593

= 1 indicates that the relevant model (i.e., Model 1, 2, or 3) provides no improvement in prediction

594

over the population mean (i.e., a poorly performing model) and a ratio of zero indicates that the

595

relevant model removes all the prediction uncertainty (i.e., a highly performing model). RMSE

596

ratios larger than one indicates that the larger model results in worse prediction than a model with

597

no predictors.

598


30


Page 38 of 42

599 600

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