Chemical composition
Average VOC and PM2.5 EFs (g kg-1 dry fuel) as well as MCE are given
in Table 1. The compounds are grouped by fuel–stove combination, with major
species (CO2, CO, CH4, and PM2.5) listed first, followed by
sulfur-containing compounds, halogen-containing compounds, organonitrates,
alkanes, alkenes, alkynes, aromatics, terpenes, and oxygenated compounds.
The sample size (n) used for calculating the average values and standard
deviations was n=18 for dung–chulha, n=14 for brushwood–chulha, n=13 for
mixed fuel–chulha, and n=10 for dung–angithi. For the majority of the compounds, the
standard deviations are smaller than or comparable to the average values,
indicating fair reproducibility. There are many factors that may lead to
variability in biomass burning emissions, including pyrolysis temperature (Chen and Bond, 2010), fuel moisture content (Tihay-Felicelli et al., 2017), and
the wind speed/direction (Surawski et al.,
2015), among others. Relationships between emissions and fuel moisture
content (Fig. S1) or meal cooked were not found to be significant for any
compounds (all p<0.05). This paper therefore focuses on the
relationships between emissions and fuel–stove combination.
Averaged emission factors and standard deviation of PM2.5 and
gas-phase species (g kg-1 dry fuel) for dung–chulha,
brushwood–chulha, mixed fuel–chulha, and dung–angithi
cook fires. Previously published emission factors (g kg-1 dry fuel)
from dung and hardwood cook fires are shown for comparison (Stockwell et al.,
2016). Sample sizes for the current study (n) were n=18 for
dung–chulha, n=14 for brushwood–chulha, n=13 for
mixed-chulha, and n=10 for dung–angithi.
Compound
Dung–chulha
Brushwood–chulha
Mixed fuel–chulha
Dung–angithi
Stockwell et al. (2016)
Stockwell et al. (2016)
(formula)
average (SD)
average (SD)
average (SD)
average (SD)
dung
hardwood
average (SD)
average (SD)
Modified combustion Efficiency
0.865 (0.014)
0.937 (0.035)
0.892 (0.021)
0.819 (0.031)
0.898
0.923
PM2.5
19.2 (7.1)
7.42 (5.67)
11.0 (2.0)
33.2 (7.6)
14.73 (0.33)a
7.97 (3.80)a
Carbon dioxide (CO2)
984 (23)
1242 (61)
969 (31)
888 (48)
1129 (80)
1462 (16)
Carbon monoxide (CO)
97.7 (9.5)
53.0 (30.1)
74.8 (16.0)
125 (20)
80.9 (13.8)
77.2 (13.5)
Methane (CH4)
6.92 (1.23)
4.80 (2.09)
4.84 (0.89)
15.1 (2.6)
6.65 (0.46)
5.16 (1.39)
Sulfur-containing
Carbonyl sulfide (OCS)
0.124 (0.040)
1.44(0.54)×10-2
8.50(2.42)×10-2
0.352 (0.217)
0.148 (0.123)
1.87(1.15)×10-2
DMS (C2H6S)
9.69(4.54)×10-3
1.39(1.34)×10-3
4.81(2.26)×10-3
4.34(3.11)×10-2
2.37(0.08)×10-2
0.255 (0.359)
Halogen-containing
Dichloromethane (CH2Cl2)
4.46(3.94)×10-4
2.18(3.13)×10-4
4.04(6.44)×10-4
4.56(2.73)×10-4
nmb
nm
Chloromethane (CH3Cl)
1.78 (0.70)
0.280 (0.157)
1.02 (0.42)
4.58 (1.89)
1.60 (1.53)
2.36(1.62)×10-2
Bromomethane (CH3Br)
6.57(2.78)×10-3
7.92(2.13)×10-4
4.35(1.81)×10-3
1.43(0.57)×10-2
5.34(3.02)×10-3
5.61(3.01)×10-4
Iodomethane (CH3I)
6.10(4.78)×10-4
9.62(2.31)×10-5
2.41(0.66)×10-4
8.83(1.62)×10-4
4.39(1.78)×10-4
1.23(1.11)×10-4
Ethyl chloride (C2H5Cl)
2.54(1.17)×10-3
4.22(3.72)×10-4
1.59(0.67)×10-3
9.11(3.50)×10-3
nm
nm
Dichloroethane (C2H4Cl2)
8.80(2.98)×10-4
2.55(2.17)×10-4
1.21(2.32)×10-3
1.47(0.91)×10-3
4.97×10-3c
1.24(0.30)×10-4
Nitrates
Methyl nitrate (CH3ONO2)
1.83(5.18)×10-3
5.34(14.4)×10-3
6.60(11.7)×10-3
0.170 (0.339)
1.46(1.94)×10-2
6.96(5.73)×10-3
Ethyl nitrate (CH3ONO2)
2.37(3.86)×10-4
5.54(10.2)×10-4
2.27(6.40)×10-3
4.53(11.6)×10-2
nm
nm
i-Propyl nitrate (C3H7ONO2)
1.90(1.61)×10-4
2.40(4.92)×10-4
4.10(8.38)×10-4
5.90(12.1)×10-3
nm
nm
n-Propyl nitrate (C3H7ONO2)
6.32(5.23)×10-5
9.01(14.1)×10-5
1.44(3.25)×10-4
1.82(4.35)×10-3
nm
nm
2-Butyl nitrate (C4H9ONO2)
2.69(2.14)×10-4
1.05(1.13)×10-4
7.10(20.3)×10-4
2.45(4.09)×10-3
nm
nm
3-Pentyl nitrate (C5H11ONO2)
4.75(1.61)×10-5
2.29(2.08)×10-5
3.13(1.97)×10-5
1.94(4.06)×10-4
nm
nm
2-Pentyl nitrate (C5H11ONO2)
2.37(2.10)×10-5
1.63(2.46)×10-5
1.25(1.26)×10-5
1.82(4.54)×10-4
nm
nm
Alkanes
Ethane (C2H6)
0.717 (0.193)
0.380 (0.247)
0.422 (0.096)
2.06 (0.69)
1.08 (0.30)
0.160 (0.122)
Propane (C3H8)
0.211 (0.073)
9.48(8.41)×10-2
0.116 (0.032)
0.819 (0.157)
0.457 (0.137)
0.202 (0.140)
i-Butane (C4H10)
1.73(0.71)×10-2
4.60(4.86)×10-3
9.51(2.75)×10-3
7.27(1.54)×10-2
0.215 (0.126)
0.406 (0.478)
n-Butane (C4H10)
4.71(1.88)×10-2
1.57(1.67)×10-2
2.68(0.88)×10-2
0.215 (0.047)
0.29 (0.09)
1.11 (1.48)
n-Pentane (C5H12)
2.01(0.98)×10-2
4.44(4.08)×10-3
9.12(3.71)×10-3
6.80(2.95)×10-2
0.190 (0.254)
2.18(1.73)×10-2
n-Hexane (C6H14)
1.03(0.47)×10-2
1.96(1.58)×10-3
5.31(1.87)×10-3
4.93(1.10)×10-2
0.291 (0.248)
1.85×10-2c
n-Heptane (C7H16)
7.21(3.43)×10-3
9.23(6.94)×10-4
3.92(1.23)×10-3
3.17(0.85)×10-2
0.114 (0.069)
1.01(1.35)×10-2
2-Methylpentane (C6H14)
6.21(2.81)×10-3
1.23(0.99)×10-3
2.57(1.61)×10-3
2.29(1.67)×10-2
0.231 (0.192)
9.93(12.9)×10-3
3-Methylpentane (C6H14)
3.71(1.70)×10-3
1.21(1.01)×10-3
1.57(0.76)×10-3
7.54(4.30)×10-3
0.155 (0.137)
6.79(6.63)×10-3
Alkenes
Ethene (C2H4)
1.86 (0.48)
0.626 (0.284)
1.13 (0.38)
1.77 (0.35)
4.23 (1.39)
2.70 (1.17)
Propene (C3H6)
0.807 (0.235)
0.286 (0.202)
0.417 (0.091)
1.61 (0.33)
1.47 (0.58)
0.576 (0.195)
1-Butene (C4H8)
0.158 (0.047)
6.32(4.59)×10-2
8.38(1.83)×10-2
0.366 (0.096)
0.399 (0.331)
0.726 (0.904)
i-Butene (C4H8)
0.133 (0.057)
3.46(2.50)×10-2
6.40(1.86)×10-2
0.353 (0.158)
0.281 (0.091)
0.846 (1.113)
trans-2-Butene (C4H8)
4.45(1.60)×10-2
2.00(1.27)×10-2
2.38(0.70)×10-2
0.151 (0.055)
0.151 (0.010)
6.78(5.98)×10-2
cis-2-Butene (C4H8)
3.38(1.19)×10-2
1.51(0.95)×10-2
1.80(0.52)×10-2
0.107 (0.047)
0.102 (0.016)
5.51(4.76)×10-2
3-Methyl-1-butene (C5H10)
1.46(0.48)×10-2
5.74(4.49)×10-3
7.30(1.94)×10-3
3.82(0.88)×10-2
5.58(3.50)×10-2
7.43(5.79)×10-3
2-Methyl-1-butene (C5H10)
2.71(1.28)×10-2
9.96(10.9)×10-3
1.19(0.42)×10-2
7.70(3.99)×10-2
nm
nm
2-Methyl-2-butene (C5H10)
2.51(1.26)×10-2
6.40(4.78)×10-3
1.10(0.47)×10-2
9.17(4.70)×10-2
nm
nm
Continued.
Compound
Dung–chulha
Brushwood–chulha
Mixed fuel–chulha
Dung–angithi
Stockwell et al. (2016)
Stockwell et al. (2016)
(formula)
average (SD)
average (SD)
average (SD)
average (SD)
dung
hardwood
average (SD)
average (SD)
1-Pentene (C5H10)
4.17(1.59)×10-2
9.65(6.55)×10-3
2.13(0.60)×10-2
0.122 (0.033)
0.168 (0.086)
1.43(0.94)×10-2
trans-2-Pentene (C5H10)
1.74(0.65)×10-2
8.89(5.77)×10-3
8.69(2.22)×10-3
5.14(2.70)×10-2
0.115 (0.035)
1.05(0.83)×10-2
cis-2-Pentene (C5H10)
1.00(0.36)×10-2
5.55(3.62)×10-3
4.98(1.26)×10-3
2.50(1.28)×10-2
5.14(0.76)×10-2
8.69×10-3c
1-Hexene (C6H12)
6.10(2.46)×10-2
1.26(0.73)×10-2
3.09(0.91)×10-2
0.167 (0.050)
nm
nm
1,2-Propadiene (C3H4)
3.76(1.69)×10-2
1.31(0.62)×10-2
2.32(0.86)×10-2
1.80(0.923)×10-2
7.15(6.76)×10-2
2.33(1.07)×10-2
1,2-Butadiene (C4H6)
5.54(1.68)×10-3
2.82(1.81)×10-3
3.10(1.06)×10-3
4.33(1.59)×10-3
nm
nm
1,3-Butadiene (C4H6)
0.203 (0.071)
7.44(3.99)×10-2
0.108 (0.061)
0.263 (0.082)
0.409 (0.306)
0.204 (0.144)
Isoprene (C5H8)
8.94(5.80)×10-2
1.98(1.48)×10-2
3.03(2.39)×10-2
0.188 (0.143)
0.325 (0.443)
4.16(2.23)×10-2
1,3-Pentadiene (C5H8)
1.96(1.05)×10-2
9.17(4.79)×10-3
9.39(6.43)×10-3
5.66(2.94)×10-2
nm
nm
Alkynes
Ethyne
1.13 (0.42)
0.467 (0.160)
0.890 (0.323)
0.325 (0.238)
0.593 (0.443)
0.764 (0.363)
1-Propyne
9.42(3.46)×10-2
3.82(1.76)×10-2
5.99(2.22)×10-2
5.20(2.83)×10-2
nm
nm
1-Buten-3-yne (C4H4)
5.04(1.72)×10-2
1.86(0.90)×10-2
3.46(1.53)×10-2
1.74(1.26)×10-2
nm
nm
1-Butyne (C4H6)
7.72(2.29)×10-3
4.07(2.24)×10-3
4.48(1.41)×10-3
5.97(1.93)×10-3
2.29(1.38)×10-2
1.28(0.47)×10-2
2-Butyne (C4H6)
4.31(1.15)×10-3
2.55(1.44)×10-3
2.47(0.70)×10-3
4.52(1.40)×10-3
1.86(0.91)×10-2
1.02(0.66)×10-2
1,3-Butadyne (C4H2)
6.07(2.66)×10-3
2.71(1.21)×10-3
5.43(2.01)×10-3
1.53(1.31)×10-3
nm
nm
Aromatics
Benzene (C6H6)
1.03 (0.33)
0.373 (0.149)
0.723 (0.218)
0.769 (0.175)
1.96 (0.45)
1.05 (0.19)
Toluene (C7H8)
0.483 (0.273)
0.221 (0.085)
0.297 (0.077)
0.860 (0.167)
1.26 (0.05)
0.241 (0.160)
Ethylbenzene (C8H10)
3.41(0.791)×10-2
1.25(1.20)×10-2
1.97(0.40)×10-2
9.78(1.66)×10-2
0.366 (0.085)
4.19(4.25)×10-2
m/p-Xylene (C8H10)
6.36(1.26)×10-2
2.78(1.56)×10-2
4.03(0.98)×10-2
0.148 (0.030)
0.601 (0.294)
9.57(7.99)×10-2
o-Xylene (C8H10)
2.38(0.76)×10-2
8.37(5.78)×10-3
1.44(0.41)×10-2
7.96(1.91)×10-2
0.228 (0.083)
3.93(4.31)×10-2
Styrene (C8H8)
5.88(1.58)×10-2
2.28(1.50)×10-2
3.40(1.90)×10-2
8.63(5.96)×10-2
0.255 (0.091)
8.71(6.69)×10-2
i-Propylbenzene (C9H12)
2.91(0.77)×10-3
1.20(1.11)×10-3
1.69(0.45)×10-3
9.30(4.90)×10-3
1.87(1.40)×10-2
1.70(1.67)×10-2
n-Propylbenzene (C9H12)
6.48(2.59)×10-3
1.84(1.65)×10-3
4.02(1.59)×10-3
3.95(2.69)×10-2
3.10(1.45)×10-2
1.78(1.58)×10-2
3-Ethyltoluene (C9H12)
1.44(0.48)×10-2
5.46(4.40)×10-3
8.59(3.26)×10-3
7.14(4.13)×10-2
5.61(2.38)×10-2
2.62(0.54)×10-2
4-Ethyltoluene (C9H12)
6.35(2.36)×10-3
2.54(1.81)×10-3
4.18(1.96)×10-3
3.71(2.30)×10-2
3.57(1.74)×10-2
2.07(1.19)×10-2
2-Ethyltoluene (C9H12)
6.89(2.50)×10-3
2.70(1.68)×10-3
4.63(2.07)×10-3
3.76(2.69)×10-2
3.39(1.34)×10-2
2.10(1.16)×10-2
1,3,5-Trimethylbenzene (C9H12)
3.87(1.71)×10-3
1.63(1.22)×10-3
2.65(1.43)×10-3
2.23(1.60)×10-2
1.79(0.83)×10-2
2.14×10-2c
1,2,4-Trimethylbenzene (C9H12)
1.04(0.46)×10-2
4.25(2.69)×10-3
7.52(4.28)×10-3
6.23(5.18)×10-2
3.91(1.65)×10-2
1.74(2.35)×10-2
1,2,3-Trimethylbenzene (C9H12)
4.76(2.59)×10-3
1.16(0.81)×10-3
3.84(2.69)×10-3
3.01(3.16)×10-2
2.34(0.43)×10-2
2.16×10-2c
Terpenes
α-Pinene (C10H16)
8.30(5.40)×10-4
5.38(6.94)×10-4
7.82(6.32)×10-4
2.26(2.53)×10-3
0.35 (0.49)
2.02(2.33)×10-2
β-Pinene (C10H16)
2.27(1.49)×10-3
1.37(0.91)×10-3
2.76(3.15)×10-3
2.89(3.56)×10-3
0.471c
4.67×10-2c
Oxygenates
Acetaldehyde (C2H4O)
0.805 (0.279)
0.334 (0.199)
0.447 (0.119)
1.70 (0.75)
1.88 (1.63)
0.541 (0.362)
Butanal (C4H8O)
4.28(1.50)×10-2
1.90(1.29)×10-2
2.68(1.05)×10-2
0.108 (0.047)
5.40(2.19)×10-2
8.28(6.27)×10-3
Acetone (C3H6O)
0.705 (0.219)
0.365 (0.226)
0.416 (0.108)
2.05 (0.52)
1.63 (0.38)
0.524 (0.256)
2-Butanone (C4H8O)
0.172 (0.057)
8.00(6.18)×10-2
0.103 (0.038)
0.498 (0.151)
0.262 (0.109)
0.232 (0.286)
2-Propenal (C3H4O)
0.186 (0.060)
0.127 (0.069)
0.127 (0.059)
0.295 (0.245)
nm
nm
MVK (C4H6O)
0.129 (0.040)
6.59(4.56)×10-2
6.31(2.76)×10-2
0.280 (0.147)
nm
nm
Furan (C4H4O)
0.109 (0.041)
5.98(3.37)×10-2
6.81(2.19)×10-2
0.379 (0.093)
0.534 (0.209)
0.241 (0.024)
2-Methylfuran (C5H6O)
0.117 (0.051)
5.92(4.77)×10-2
6.92(2.83)×10-2
0.488 (0.227)
nm
nm
Furfural (C5H4O2)
8.55(6.05)×10-2
4.28(5.51)×10-2
8.22(5.09)×10-2
0.316 (0.133)
nm
nm
Methanol (CH3OH)
2.09 (1.14)
2.03 (2.01)
1.18 (0.40)
4.23 (3.40)
2.38 (0.90)
1.92 (0.61)
Ethanol (CH5OH)
4.08(5.93)×10-2
2.18(2.00)×10-2
5.63(6.69)×10-2
7.62(9.08)×10-2
0.563 (0.589)
0.128 (0.017)
a From Jayarathne et al. (2018) but part of same
NAMaSTE study. b nm indicates the species was not measured.
c From Stockwell et al. (2016) indicates that the measurementwas not above background.
Pie charts showing the contribution of each species class to
gas-phase composition (a), OH reactivity (b), SOAP-weighted
emissions (c), and ozone-forming potential (d). For
panels (b) and (d), total aromatics are shown rather than
the breakdown of aromatics shown in (a) and (c). Sums of
all components are shown below the pie chart. 1-Buten-3-yne is grouped in
with alkynes.
Emission factors (g VOC kg-1 fuel C) for select compounds.
The mean differences between dung–angithi and dung–chulha
are shown and similarly for dung–chulha and
brushwood–chulha. The significance between fuel or stove and EF is
indicated with asterisks. Accompanying the mean differences is the average
emission factor (g VOC kg-1 fuel C) for dung cook fires and
chulha cook fires, as well as the overall average for all performed
cook fires.
Compound
Average EF for
Angithi–chulha
Average EF for
Dung–brushwood
Average EF for
all cook fires
average EF difference
dung fires
average EF difference
chulha cook fires
(g kg-1 fuel C)
(g kg-1 fuel C)
(g kg-1 fuel C)
(g kg-1 fuel C)
(g kg-1 fuel C)
Ethane
2.47 (2.16)
4.18∗∗∗
3.70 (2.43)
1.19∗∗∗
1.60 (0.744)
Propane
0.827 (0.866)
1.88∗∗∗
1.32 (0.976)
0.397∗∗∗
0.448 (0.256)
n-Butane
0.200 (0.236)
0.52∗∗∗
0.331 (0.271)
0.0568∗∗∗
0.097 (0.063)
Ethene
4.17 (2.02)
N/A
5.64 (1.32)
4.05∗∗∗
3.88 (2.07)
Propene
2.24 (1.61)
2.50∗∗∗
3.38 (1.48)
1.72∗∗∗
1.63 (0.93)
1-Butene
0.473 (0.373)
0.644∗∗∗
0.718 (0.377)
0.213∗∗∗
0.327 (0.180)
Ethyne
2.32 (1.41)
-2.46∗∗∗
2.58 (1.63)
2.21∗∗∗
2.61 (1.37)
1-Propyne
0.196 (0.108)
-0.129∗∗
0.244 (0.116)
0.187∗∗∗
0.204 (0.112)
1-Butyne
1.74×10-2
-0.101∗∗∗
0.219 (0.007)
0.105∗∗∗
0.017 (0.008)
(7.74×10-3)
∗ denotes p<0.05. ∗∗ denotes p<0.01.
∗∗∗ denotes p<0.001. “N/A” indicates that a significant
difference was not found.
Figure 2a visually shows the mass fraction attributed to each compound class
for the measured gas-phase emissions. The total EFs given below the pie
charts are normalized by fuel carbon in Fig. 2a in order to compare
between cook fires generated with dung, wood, and wood–dung mixtures, which
have different carbon contents. The total measured VOC emissions from
dung–angithi were roughly twice those of dung–chulha in terms of gram per kilogram fuel
carbon. Further, dung–chulha emitted more than twice the amount emitted by brushwood–chulha. The most
prominent difference is non-furan oxygenates, making up almost half of all
brushwood–chulha emissions and a smaller fraction for other fuel–stove
combinations. While oxygenates make up a higher fraction of
brushwood–chulha emissions, the absolute EFs for oxygenates from dung-burning and
angithi cook fires are higher as discussed later in more detail.
Table 2 shows EFs (g kg-1 fuel C) for select VOCs. The differences in mean
EFs for each fuel–stove combination are also included in Table 2. Mean
differences in EFs reported for chulha and angithi stoves were calculated for cook fires
utilizing only dung fuels. Likewise, mean EFs for wood and dung cook fires
only represent cooking events using the chulha. This was done to isolate a single
variable – either fuel or stove type. For all alkanes and most alkenes, we
measured higher emissions for dung–angithi cook fires (Table 2). Also, from the
mean differences in EFs, we found that stove-specific combustion conditions
impact emissions more than the selection of fuel type. The difference is so
dramatic for alkanes and most alkenes that the mean difference in EFs for
cookstoves burning dung is always larger than the mean EF of that compound.
For comparison, the mean difference in EFs for chulha cookstoves is always lower
than the overall mean EF. Ethene was an exception; there was no relationship
between ethene emissions and stove type. On the other hand, the mean EF of
ethene by dung cook fires was very large compared to mean EFs from brushwood
cook fires, with a mean difference in EFs of 4.05 g kg-1 fuel C. Some alkenes
with two double bonds were also exceptions. For 1,3-butadiene (p=0.06)
and 1,2-butadiene (p=0.089), stove and EF may or may not have a
significant relationship. 1,2-Propadiene emissions from chulha cookstoves are
higher (p<0.01). All three compounds still show a significant
relationship to fuel type, with EFs being higher for dung cook fires.
Similar to alkanes and alkenes, aromatics, oxygenates, and halogen- and
sulfur-containing compounds all had higher emissions per kilogram of fuel
carbon when dung fuels and angithi stoves were utilized compared to brushwood fuels
and chulha stoves, respectively. We focus on the behavior of the most interesting
groups of compounds in the discussion below.
The chlorine-containing organic compounds are
generally not expected to come from cook
fires in large quantities. However, we observed an interesting practice in
which the cook often used plastic bags to start the fire, which could be a
source of chlorine-containing compounds if composed of polyvinyl chloride.
Carbonyl sulfide (OCS) is largely responsible for the yellow
sulfur-containing fraction in Fig. 2a, and biomass burning is a well-known
source of OCS in the atmosphere (Crutzen et al., 1979). Similar to other
VOCs, OCS was significantly emitted in higher quantities when
angithi stoves and dung fuels were utilized.
Benzene had higher emissions from chulha stoves, which had higher MCEs when
cooking with dung fuels compared to angithi stoves (dung–chulha: 3.18 g kg-1 fuel C;
dung–angithi: 2.38 g kg-1 fuel C). As the simplest aromatic compound, benzene also
had the largest average difference in fuel type EFs compared to other
aromatics (2.18 g kg-1 fuel C, dung–wood). This information is relevant for
exposure assessment, as benzene is a known human carcinogen. Higher benzene
emissions from chulha cook fires could lead to higher benzene exposures, which is a
potential public health concern. However, it should be noted that the cook
usually cannot control the stove used, as the angithi and chulha are used to prepare
different types of meals, and exposure to benzene is not straightforward
from its emission factors.
Higher emissions of alkynes were observed from dung fuels and chulha cookstoves.
The latter observation is consistent with the literature showing flaming
combustion generates more alkynes (Barrefors and Petersson,
1995; Lee et al., 2005). Chulha cook fires always had higher MCE than angithi cook fires
(Table 1), which rely on smoldering combustion. Approximately the same
difference in alkyne emissions results from comparing the chulha to the angithi using
dung, in relation to using wood versus dung in combination with the
chulha. There were two exceptions in stove type for 1-butane (p=0.055) and
2-butane (p≫0.05). The former may or may not have a
relationship with stove type, while the latter does not. Emissions of some
compounds did not show a relationship with either fuel or stove type;
they are listed in Table S2.
VOC emissions from Stockwell
et al. (2016) are also provided in Table 1 for comparison of VOC EFs.
Samples in Stockwell
et al. (2016) were collected in April 2015 in and around Kathmandu and the
Tarai plains, which border India. While both are EFs from cookstoves using
similar fuels, there are differences in the studies that should be noted. Stockwell
et al. (2016) collected measurements of simulated cooking in a laboratory
and from cooking fires in households; it was not noted in the latter case
what meals were cooked. EFs were calculated from similar WAS measurements,
but as grab samples in an area of the kitchen away from the fire (as opposed
to the time-integrated approach used here). Emissions were assumed to be
well mixed in the kitchen prior to sampling. Stockwell
et al. (2016) also used a range of stoves, including the traditional
single-pot mud stove; open three-stone fire; bhuse chulo; and rocket, chimney, and forced
draft stoves. “Dung” cook fires sometimes used a combination of fuels,
such as wood. Finally, our study also has a larger sample size than Stockwell
et al. (2016), with n=49 versus n≈10.
Emission factors as a function of modified combustion efficiency
(MCE) for select species. Open circles indicate cooking events conducted with
angithi stoves, whereas filled squares indicate chulha
stoves. Color indicates fuel: brushwood (blue), dung (red), or mixed
(purple). 1-Buten-3-yne is grouped in with alkynes.
The emission factors for most compounds determined in this study were lower
compared to those reported in Table 4 of the Stockwell et al. (2016) paper.
Figure S2.1 visually shows that the EFs were generally lower in the present
study. In some cases, EFs in this study were an order of magnitude lower,
most notably n-pentane and n-hexane. We also found that our EFs were always higher for
dung–chulha compared to brushwood–chulha, which was not
always the case in Stockwell et al. (2016). The EFs in Stockwell et
al. (2016) could be biased high due to calculations rather than real
differences in emissions. For example, ignoring ash and char carbon and using
the same carbon content inflates the EFs reported in our paper by 7 % for
dung and 24 % for brushwood emissions. However, this is a small
percentage compared to the observed differences between EFs between the two
measurements. By examining the EFs reported in the supporting information
section of Stockwell et al. (2016), we found that the disagreement resulted
from the way the final recommended EFs were obtained from the measurements.
Because Stockwell et al. (2016) measured both laboratory and field cook
fires, they elected to adjust the laboratory EFs to account for the lower
MCEs observed in the field. It has been shown by Roden et al. (2009) and
Johnson et al. (2008) that cooking activities can strongly influence
emissions, for example due to the cook tending to the cook fire differently
and thus affecting combustion conditions. However, such adjustments should be
done with caution because EF and MCE do not always follow a linear trend, as
explained in the next section. Figure S2.2 plots the unadjusted laboratory
and field emission factors as a function of MCE for all the measurements in
Stockwell et al. (2016), as well as this study. Plotting the unadjusted EFs
resolves most of the differences in observed EFs between the two studies. The
data show an encouraging level agreement despite the differences in the
experimental design between the two studies.
Modified combustion efficiency
The use of dung and angithi, rather than brushwood and chulha, respectively, results in
lower modified combustion efficiencies as shown in Fig. 3. In general, at
lower MCEs we measured higher emissions of gas-phase compounds as discussed
in Sect. 3.1. For example, emissions of ethane (Fig. 3a) and other
alkanes increase with decreasing MCE. However the dependence of ethane EF on
MCE is not linear as observed in previous studies (Liu
et al., 2017; Selimovic et al., 2018). For other VOCs, the dependence of the
EF on MCE deviates from the linear trend even more, with the maximum EF
observed at intermediate MCE values. For example, the ethene EF (Fig. 3b)
increases with decreasing MCE at MCEs >0.85, but it has the
opposite trend at MCEs <0.85. Previously, we discussed that there
is no relationship between ethene EF and stove type, and we see this more
clearly in Fig. 3b. Alkynes have the same relationship to MCE as ethene,
but it is even more pronounced (Fig. 3c). Benzene (Fig. 3e) stands apart
from other aromatics with a relationship with MCE similar to ethene, while
other aromatics have an EF-versus-MCE curve similar to alkanes and most
other VOCs. In Fig. 3e, we see again that emissions from
brushwood–chulha and dung–angithi cook fires result in lower emissions of benzene
compared to dung–chulha. Alkenes with two double bonds generally have a negative
correlation between emissions and MCE, such as 1,3-butadiene in Fig. 3f.
The 1,3-butadiene EF-versus-MCE plot is not necessarily representative of
all analogous plots for alkenes with two double bonds, as they have
different shapes. 1,3-Butadiene was chosen as its emission is high compared
to other compounds in its subcategory, and it also has health implications.
It also happens to have a more linear relationship with MCE, albeit noisy.
It is of interest to compare EFs obtained from different fuel–stove
combinations but with the same MCE. In the case of ethane, different cook
fire types yield vastly different EFs at the same MCE. For example, at
MCE ≈0.87, mixed fuel–chulha has an EF of roughly 1.5 g kg-1 fuel C,
dung–chulha is 2.5 g kg-1 fuel C, and dung–angithi is 5.5 g kg-1 fuel C.
Knowledge of the cook fire MCE alone is not sufficient to determine the EF
of ethane. Combustion conditions specific to the fuel–stove combination are a
significant factor in determining cook fire emissions. A similar conclusion
can be reached for most of the measured gases, including non-ethene alkenes
in Fig. 3d.