Emission Factors and Rates

2.5 and CO are plotted in p < 0.05) EFs and ERs for both PM 2.5 and CO compared to the traditional stoves. Compared to previous field investigations of the Philips HD4012-LS forced-draft gasifier stove burning unprocessed wood, the Mimi Moto (pellet) PM 2.5 and CO EFs are much lower. For example, median pellet PM 2.5 EF (0.4 g kg–1) is nearly 10× lower than the Philips in Malawi and Ghana (ranging between 2.5 and 4.7 g kg–1).–1, respectively). This difference in emissions performance could be due to many factors including wood fuel size, shape, Emission factors and rates for PMand CO are plotted in Figures 1 a–d for the three stove/fuel combinations (pellet, wood, and charcoal). Pellet stoves had substantially (e.g., means reduced by 84–97% relative to wood) and significantly lower (Wilcoxon rank-sum test,< 0.05) EFs and ERs for both PMand CO compared to the traditional stoves. Compared to previous field investigations of the Philips HD4012-LS forced-draft gasifier stove burning unprocessed wood, the Mimi Moto (pellet) PMand CO EFs are much lower. For example, median pellet PMEF (0.4 g kg) is nearly 10× lower than the Philips in Malawi and Ghana (ranging between 2.5 and 4.7 g kg). (12,27) CO EF differences are less dramatic, but the Mimi Moto pellet stove was still 3× lower than the Philips (14 vs 45–49 g kg, respectively). This difference in emissions performance could be due to many factors including wood fuel size, shape, (43) loading, (12) and moisture content. The Philips stove can accommodate a variety of woody biomass fuels, (44) but is often used with inconsistently cut and loaded wood. This suggests that the homogenization of a solid fuel (e.g., wood pelletizing) may greatly improve the emissions performance of an already advanced cookstove design.

Figure 1 Figure 1. Box and whisker plots for PM 2.5 EF (a) and ER (b), CO EF (c), and ER (d), EC EF (e), and ER (f), EC:TC ratio (g), and SSA (h). Boxes and whiskers indicate 25th to 75th and 10th and 90th percentiles, respectively; central lines indicate median and dark circles indicate group mean; hollow circles are individual test data. Also shown with letters as markers are mean and standard deviations for controlled lab emissions test data reported for P 1 : Mimi Moto pellet-fed forced-draft semigasifier stove,(20) and field emissions test data for W 1 –W 7 : TSF/mud stove burning wood,(11−13,27,29,45,46) C 1 : Coalpot charcoal stove,(27) and C 2 –C 3 : Jiko-style charcoal stoves.(46,47)

2.5 and CO EFs for pellet stoves observed in this study are similar to lab results reported in the Clean Cooking Catalog (0.37 vs 0.54 g kg–1 for PM 2.5 ; 14 vs 5.9 g kg–1 for CO), suggesting pellet stove field performance, at least for PM emissions, is on-par with controlled laboratory test results. This is likely due in large part to the homogeneous fuel supply. 2.5 EFs, pellet stoves have significantly lower PM 2.5 ERs compared to both traditional stove types. Differences between pellet and wood PM 2.5 ERs are greater than for the respective EFs due to the lower fuel consumption of pellet stoves compared to wood (median fuel consumption of 0.5 vs 1.3 kg hr–1; Median PMand CO EFs for pellet stoves observed in this study are similar to lab results reported in the Clean Cooking Catalog (0.37 vs 0.54 g kgfor PM; 14 vs 5.9 g kgfor CO), suggesting pellet stove field performance, at least for PM emissions, is on-par with controlled laboratory test results. This is likely due in large part to the homogeneous fuel supply. SI Figure S3 shows pellet PM and CO EFs grouped by the year in which the stoves were acquired by the household, and shows generally that stoves more than one year old had significantly higher EFs. Similar to PMEFs, pellet stoves have significantly lower PMERs compared to both traditional stove types. Differences between pellet and wood PMERs are greater than for the respective EFs due to the lower fuel consumption of pellet stoves compared to wood (median fuel consumption of 0.5 vs 1.3 kg hr SI Figures S4 and S5 ).

2.5 and CO EFs for the pellet tests were consistent with ISO Tier-4 for PM 2.5 and Tier-5 (“best”) for CO (see 2.5 EFs for pellet tests which included a reload (i.e., refuel) event were significantly higher and met Tier-3 for PM 2.5 , while those with no reload met Tier-4 for PM 2.5 ( 2.5 ER exceeded WHO emission rate targets for unvented stoves (3.3 vs 0.23 mg min–1), whereas the median CO ER met the guideline (125 vs 160 mg min–1). EFs in terms of useful energy delivered (MJ-del) were calculated, assuming fuel energy contents ( SI Table S1 ) and stove thermal efficiencies ( SI Table S2 ). Median PMand CO EFs for the pellet tests were consistent with ISO Tier-4 for PMand Tier-5 (“best”) for CO (see SI Figure S6 for detail). The median PMEFs for pellet tests which included a reload (i.e., refuel) event were significantly higher and met Tier-3 for PM, while those with no reload met Tier-4 for PM SI Figure S7 ). Note that this tier system is intended for use with laboratory data, and is employed here for comparison purposes only. In comparison to World Health Organization (WHO) indoor air quality guidelines, the median pellet PMER exceeded WHO emission rate targets for unvented stoves (3.3 vs 0.23 mg min), whereas the median CO ER met the guideline (125 vs 160 mg min). (48) Although controlled laboratory testing has identified the potential of gasifier stoves to meet the top emissions tiers, (49) to our knowledge no published studies have observed a solid-fuel cookstove meeting or approaching the highest tier designations for emissions performance during uncontrolled in-use (i.e., field) testing.

2.5 and CO EFs for traditional wood and charcoal stoves were generally similar to previous field studies of wood-burning TSFs, as plotted in 90 : [11.3, 22.5] g kg–1) overlapped with specified ranges about the mean from W 2 , but was higher than those from the other studies 90 overlapped with ranges from all other studies except for W 3 . Therefore, compared to previous field investigations, the traditional wood stoves studied here emitted more PM, but operated at similar combustion efficiencies (arithmetically inversely related to CO EF when using carbon balance approach). This may be due to the distinct type of fuel burned in this study (predominately elephant grass), as opposed to fuel wood. PM 2.5 and CO EFs and ERs for wood homes burning different types of wood (elephant grass vs eucalyptus vs mix) are reported in 2.5 and CO ERs are significant greater (p < 0.05) for elephant grass versus mixed-wood homes; no significant differences were observed between homes burning only elephant grass and only eucalyptus. For charcoal homes, the PM 2.5 EF CI 90 overlapped with both C 2 and C 3 , but not C 1 . The CO EF CI 90 from our work overlapped only with C 2 , and was higher than C 1 and C 3 . Therefore, emissions from both traditional wood and charcoal in this study were generally slightly higher compared to field data from the literature. As expected, both traditional stoves were classified as Tier-0 (“no improvement over baseline”; SI Figure S8 ). Our field-based PMand CO EFs for traditional wood and charcoal stoves were generally similar to previous field studies of wood-burning TSFs, as plotted in Figure 1 a and c and cited in the figure caption. The wood PM EF 90% confidence interval about the mean (CI: [11.3, 22.5] g kg) overlapped with specified ranges about the mean from W, but was higher than those from the other studies (11−13,27,46) as listed in SI Table S7 ; wood CO EF CIoverlapped with ranges from all other studies except for W. Therefore, compared to previous field investigations, the traditional wood stoves studied here emitted more PM, but operated at similar combustion efficiencies (arithmetically inversely related to CO EF when using carbon balance approach). This may be due to the distinct type of fuel burned in this study (predominately elephant grass), as opposed to fuel wood. PMand CO EFs and ERs for wood homes burning different types of wood (elephant grass vs eucalyptus vs mix) are reported in SI Figure S9 ; PMand CO ERs are significant greater (< 0.05) for elephant grass versus mixed-wood homes; no significant differences were observed between homes burning only elephant grass and only eucalyptus. For charcoal homes, the PMEF CIoverlapped with both Cand C, but not C. The CO EF CIfrom our work overlapped only with C, and was higher than Cand C. Therefore, emissions from both traditional wood and charcoal in this study were generally slightly higher compared to field data from the literature.

EC EFs and ERs are plotted in Figure 1 e and f. Pellet EC EFs and ERs were significantly lower than for wood, but not for charcoal. Given the nature of charcoal combustion (surface oxidation of a pyrolyzed fuel vs flaming combustion of devolatilized organics), low EC emissions are expected. Wood and charcoal EC EFs observed in this study are similar to previous field test results for these traditional stove types. Ratios of elemental carbon to total carbon (EC:TC) are plotted in Figure 1 g. Pellet stoves had the highest EC:TC ratio, consistent with what has been observed in stoves operating at higher efficiency (and presumably combustion temperature). (12,45) Pellet EC:TC ratios were more variable than, and not significantly different from, those for wood stoves. Literature EC:TC ratios for traditional wood stoves are highly variable and span an order of magnitude (0.06–0.6), whereas the (more limited) literature data for charcoal stoves range between 0.1 and 0.2, indicating that charcoal PM emissions are dominated by OC vs EC.

SSA (λ = 880 nm) follows a similar trend as EC:TC ratio ( SI Figure S10 ), with lower SSA (i.e., more light absorbing particles) generally corresponding to the higher EC:TC ratios for pellet and wood stoves, as observed previously for biomass burning aerosol. (50) SSA is not significantly different between pellet and wood stove types. EC:TC ratio (SSA) for charcoal are significantly lower (higher) compared to wood and pellet. The climate benefits estimated to accompany mitigation of cookstove emissions are influenced by the aerosol EC:TC ratio assumed for the baseline technology. (36,51) Here, pellet stoves emit less particle mass that is relatively more light absorbing compared to wood and charcoal stoves. Both quantity and optical properties of emissions must be considered, as net radiative impacts are a function of both.

2.5 and CO, but that PM 2.5 EFs (and ERs) from high-emitting pellet tests overlap with those from low-emitting wood and charcoal tests. The distribution of pellet PM 2.5 EFs is strongly positively skewed (skewness, γ = 4.5), with a mean EF (0.96 g kg–1) nearly three times the median; wood and charcoal stoves had lower PM 2.5 EF skewness (γ = 1.4 and 2.1, respectively). This emphasizes that (a) ICS performance can be highly variable in the field, and (b) pellet stoves offer tremendous potential to reduce emissions, but only when operated properly. High-emitting pellet tests are discussed in more detail in Examining distributions of the integrated emissions quantities, cumulative distribution functions (CDFs) of EFs ( SI Figure S11 ) show that the majority of pellet tests have low EFs for PMand CO, but that PMEFs (and ERs) from high-emitting pellet tests overlap with those from low-emitting wood and charcoal tests. The distribution of pellet PMEFs is strongly positively skewed (skewness, γ = 4.5), with a mean EF (0.96 g kg) nearly three times the median; wood and charcoal stoves had lower PMEF skewness (γ = 1.4 and 2.1, respectively). This emphasizes that (a) ICS performance can be highly variable in the field, and (b) pellet stoves offer tremendous potential to reduce emissions, but only when operated properly. High-emitting pellet tests are discussed in more detail in SI Section 3.2