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Estimation of organic and elemental carbon emitted from wood burning in traditional and improved cookstoves using controlled cooking test Pooja Suresh Arora, and Suresh Jain Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/es504012v • Publication Date (Web): 17 Feb 2015 Downloaded from http://pubs.acs.org on February 18, 2015
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Graphical abstract
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Estimation of organic and elemental carbon emitted from wood burning in traditional and improved cookstoves using controlled cooking test
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Pooja Aroraa, Suresh Jaina, b1 a
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Department of Natural Resources, TERI University, Delhi, 10, Institutional Area, Vasant Kunj, New Delhi 110070, India b Department of Energy and Environment, TERI University, Delhi, 10, Institutional Area, Vasant Kunj, New Delhi 110070, India
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Abstract
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Emission of various climate and health related pollutant species from solid biomass
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burning in traditional cookstoves is a global concern. Improved cookstoves serve as a possible
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solution to mitigate the associated impacts. However, there is a need to intensify the efforts in
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order to increase the data availability and promote revision of existing metrics of cookstove
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testing. In this study, the effect of different phases of a cooking cycle of Northern India on
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emission factors of OC and EC (char and soot) was assessed for four cookstoves (advanced,
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improved and traditional) using Acacia nilotica. Lowest EFs for OC (0.04 g/MJ) and EC (0.02
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g/MJ) were observed in case of the forced draft cookstove; while the traditional and natural
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draft top feed cookstove emitted the highest OC (0.07 g/MJ) and EC (0.09 g/MJ),
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respectively. Variation in terms of EFs for OC and EC (char & soot) within the cooking cycle
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was also found to be significant.
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Key words: Biomass energy; Controlled cooking test; Black carbon; Regional cooking cycle;
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Regression analysis
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Introduction
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In India, ~66% percent of the households depend on solid biomass fuels such as wood,
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cow dung and crop residues in order to fulfil their cooking energy needs.1 Combustion of these
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solid biomass fuels in inefficient traditional cookstoves leads to deterioration of indoor air
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quality due to emission of both particulate matter (PM) and gaseous pollutants. A characteristic 1
Corresponding author, Department of Natural Resources, TERI University, 10, Institutional Area, Vasant Kunj, New Delhi-110070, India; Tel: +91-11-2612 2222; Fax: +91-11-2612 2874; Email:
[email protected];
[email protected] 1 ACS Paragon Plus Environment
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of carbonaceous part (organic and black carbon) of PM has a strong bearing on the atmospheric
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radiative balance. Black carbon (BC) or elemental carbon (EC) content of PM has been
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identified as the major precursor of climate change due to its high intensity radiative forcing
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despite its short residence time in the atmosphere.2 Organic carbon (OC), on the other hand, has
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been linked to light scattering properties3 leading to cooling effect in earth’s atmosphere. OC
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(also known as ‘brown carbon’) has recently been found to be a significant source of light
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absorption activity in the atmosphere.4,5 Apart from impact at the global level, chemical
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composition of PM is also responsible for altering the environmental conditions at regional and
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local levels, which primarily include visibility reduction and changed rainfall patterns in areas
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where biomass burning is a common practice.6,7 Chemical characteristics of PM emitted from
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inefficient cookstoves can also be linked to adverse effects on human health because of their
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toxicity.8-10
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Chemical composition of PM is highly dependent on the type of combustion device, fuel
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properties and type of activity leading to variation in emissions.11 Field studies have a definite
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advantage over laboratory studies when the primary aim of the study is to build emission
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inventories and assess the impacts on human health. There have been a few field studies, which
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focus on quantification of emissions of carbonaceous aerosols from cookstoves under actual
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operating conditions.12,13 However, due to high variation within the data, it is difficult to
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ascertain the variables responsible for a typical performance of a cookstove.14 One such variable
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is the “burn cycle” or ‘cooking cycle”, which has been found to influence cookstove
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performance.15-17 The effect of different cooking cycles can be captured using a laboratory based
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cooking test by controlling the fuel characteristics such as fuel size and moisture, which has also
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been found to affect the cookstove performance in a few studies.18-21 The laboratory based
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cooking tests, such as the controlled cooking test (CCT), can serve as a standard protocol to
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assess the performance of cookstoves under the influence of different cooking cycles. To the best
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of the author’s knowledge, no attempt has been made to use CCT as a test in order to identify
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and assess variation in chemical composition of PM with different cooking cycles associated
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with diverse food items consumed and the sequence in which they are cooked in different
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regions. Additionally, the present study has also quantified emission of char and soot-EC from
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biomass burning in cookstoves. Quantification of these light absorbing chemical species is
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important since both char and soot-EC act as light absorbing fraction of PM; however, soot-EC 2 ACS Paragon Plus Environment
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has a high light absorption potential compared to char-EC. Char-EC is generated during low
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temperature combustion while formation of soot-EC takes place under high temperature
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combustion conditions. Therefore, their quantification can also be used to identify dominant
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combustion conditions in a particular cooking activity in context of cookstoves.22 The studies
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conducted on carbonaceous aerosols emitted from cookstoves so far, have not considered char
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and soot-EC as a measure to predict the dominant combustion conditions in a particular cooking
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activity. Therefore, the aim of the present study is to quantify the OC and EC content of the PM
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and further analyse the char and soot EC fractions and their relationship with different
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combustion conditions in different cookstoves by using the cooking cycle of Northern India.
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However, the scope of the study is limited to three cookstoves selected under the improved and
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advanced category, which were readily available in India. Nevertheless, the distinctiveness in
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designs adopted in the selected cookstoves provides, to some extent, useful set of data on effect
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of some specific variables on the cookstove performance.
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Materials and methods
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Survey
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A survey was conducted in two villages, Thalan village in district Uttarkashi and Aam
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Kheri village in Roorkee lying in the plains of Uttarakhand. The households were selected using
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stratified random sampling with 50 households in each village. The survey was conducted using
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a semi-structured questionnaire, which comprised of questions related to primary food type, its
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quantity, frequency and sequence of cooking. However, tasks other than cooking food were not
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considered during the study. The data collected from the survey was used to standardize the
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amount and type of each food item cooked in a single meal.
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Traditional cookstove
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Traditional (TR) cookstove used in the study was constructed in the laboratory with
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dimensions and material same as those observed on the field. The cookstove was made up of six
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bricks covered with mixture of clay, hay and cow dung. The cookstove was constructed on a
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thick metal sheet to allow ease of movement under the hood during testing. The walls of the
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cookstove were ~6 cm thick and the height, width and length of the combustion chamber was
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~22, ~14 and 18 cm, respectively, giving it a U-shaped top view. The cookstove design was
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similar to the traditional cookstove used by Kar et al.23 in field based studies conducted in
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Northern India. The cookstove was recoated with clay slurry at the completion of each meal 3 ACS Paragon Plus Environment
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preparation. The traditional cookstove design selected for baseline study holds a great
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importance in order to assess the actual improvement in a particular study area. For example, the
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single pot traditional cookstove design in Maharashtra (Western region of India) as reported by
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Dutta et al. 24 has a V-shaped combustion chamber, which might perform differently from the
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traditional cookstoves used in Northern India.
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Improved cookstoves
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The improved cookstoves (ICSs) were selected for the study among some of the popular
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designs available in the Indian markets. ICSs can be placed in three common categories, i.e.,
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basic, intermediate and advanced depending on the technology used to upgrade its performance
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as described in a report by Dalberg Global Development Advisors.25 The forced draft cookstove
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(PF) used in this study is manufactured by Philips and can be placed under advanced cookstove
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category. The PF (Model-4012) cookstove works as a “quasi-gasifier” cookstove. The forced air
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supply in this cookstove (both primary and secondary) allows adequate mixing of air and
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combustible gases leading to efficient combustion of fuel.26,27 The other ICSs were; a top feed
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natural draft cookstove (PN) manufactured by Philips and a front feed natural draft (EN)
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cookstove manufactured by Envirofit International Ltd, Colorado, USA. The PN (Model-
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HD4008) and EN (Model-G3300) cookstoves can be considered as an intermediate technology
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where the former is also a “quasi-gasifier” but with natural air convection and latter is a rocket
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cookstove. Rocket cookstoves utilize “L” shaped structure in order to enhance the combustion
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process. The photographs of the cookstoves can be accessed in the research article by Arora et
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al.16
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Fuel used
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Acacia nilotica (Keeker) wood was used in the present study owing to its availability at
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the test location. Keeker was also found to be one of the commonly used wood species for
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cooking purposes in the surveyed areas. Since the aim of the study was to assess the relative
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performance of different cookstoves with a common cooking cycle of the Uttarakhand region,
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the use of different wood types could be overlooked. Still, the emission factors (EFs) generated
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in the present study will not be specific to Uttarakhand region in terms of fuel practice and may
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vary with other wood species used in the study area. In the study region, wood was collected
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from the closest forest area and gathered wood consisted of a combination of thin and thick
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stems, which were used for ignition and high power cooking, respectively. A similar pattern was 4 ACS Paragon Plus Environment
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followed in the laboratory experiments where wood was procured from a common source of fuel
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and only thick branches were used for each cooking task. However, the wood logs were cut into
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pieces according to the recommended size for the two top feed cookstoves. In case of PF and PN
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cookstoves, wood size was kept 2×3×10 cm while it was 1.5×1.5×25 cm for EN cookstove. In
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case of TR cookstove, the wood pieces were also cut into a uniform size of 3×3×25 cm (without
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bark). The wood moisture during the testing period was 10-18%, which is also the case in field
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during actual cooking. The calorific values for wood and charcoal were calculated using bomb
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calorimeter and were found to be ~21 MJ/kg and ~32 MJ/kg respectively.
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Test protocols
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The CCT Version 2.0 developed under Household Energy and Health Programme, Shell
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Foundation (28) was used as a test method to assess the cookstove performance in terms of mass
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emission factors (EFs) of OC, EC and PM. The EFs were calculated using the total fuel energy
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consumed for specified cooking task. CCT is a laboratory based international protocol, which is
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used to compare cookstoves in terms of fuel savings, and involves actual cooking of a meal
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dominant in the study area. The meal was prepared by a female cook who was a resident of the
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same region. The experiments were started after adequate training of the female cook with the
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new ICSs. All relevant parameters were measured during the test in order to calculate OC/EC
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EFs. During each test, sampling was started as soon as the vessel was put over the cookstove and
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start time was recorded, which also included the ignition phase emissions. The wood chips were
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prepared from the same lot of wood used as kindling fuel, in order to maintain consistency. At
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the end of each cooking phases, sampling was terminated followed by weighing of cooked food
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and wood left, and the end time was recorded. The charcoal formed was also weighed along with
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the cookstove and was used for next cooking phase. The graphic representation of steps carried
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out in CCT is presented in Figure S1 of SI.
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The Water Boiling Test (WBT) Version 4.1.229 was also used to compare performance of
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cookstoves with the results obtained from CCT. WBT is a simulation of actual cooking cycles
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designed principally to study the effect of design on the performance of cookstoves (for more
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details related to the protocols, please refer to research article by Arora et al.21
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Sampling set-up
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The experiments were performed at TERI University’s testing facility located in New
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Delhi. The testing facility consists of an extraction hood and a dilution tunnel with location 5 ACS Paragon Plus Environment
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provided for PM sampling. Size of the hood was large enough to accommodate the cookstoves
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manufactured for domestic cooking purposes. The PM sampling point was kept ~9 times the duct
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diameters from the closest point of disturbance and the PM deposited on the filter (Whatman, 47
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mm quartz Q-MA, with pore size 0.2 µm) consisted of total particulate matter (TPM). The
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details of the experimental setup and sampling procedure have been mentioned in detail by Arora
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et al.16, 17 The schematic diagram of the experimental set-up can be referred in the research article
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by Arora et al.16
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OC/EC analysis
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PM samples collected on quartz microfiber filters (pore size, 0.2 µm) were preserved at a
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temperature of around -20ºC until the OC/EC analysis was carried out. The filter samples for
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correction of OC artifacts were collected for few experiments; however, the values were not
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reduced from the actual concentrations. This was because of the variation of data observed in gas
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phase OC artifacts, which might result in over or underestimation of the actual OC concentration
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in the rest of the experiments for which the artifact samples were not collected.
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The analysis was conducted with the help of thermal-optical method using EC/OC
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analyser (DRI, Model 2001) using the IMPROVE-TOR carbon analysis.30 The PM mass
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deposited on the quartz filter is analysed using a 0.5 cm2 section of the filter. In the thermal-
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optical analyser, the filter punch is first heated in an inert atmosphere (100% Helium, He) which
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converts the OC fraction into carbon dioxide (CO2). The EC fraction is converted to CO2 in an
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oxidizing atmosphere (98% He/ 2% Oxygen, O2). The concentration of CO2 produced during
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both the conditions is analysed by reducing it to methane, which is eventually measured on a
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flame ionization detector (FID). Transmittance of laser light (633 nm) through the filter is
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measured to set the split between OC and EC. Further, the actual EC is calculated by correcting
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for the pyrolysed carbon formed during charring of OC in the first phase. EC data was also used
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to calculate the char and soot-EC concentrations using EC1, EC2 and EC3, which are produced
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at a temperature of 550, 700 and 800˚C respectively in 2% O2 and 98% He atmosphere during
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analysis. Char-EC was calculated by using EC1 minus pyrolysed carbon (PC) and soot-EC using
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sum of EC2 and EC3. However, limited studies are available in the literature where TOR method
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has been employed for estimation of char and soot-EC, and the method is still under scientific
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scrutiny.31
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Statistical analysis 6 ACS Paragon Plus Environment
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The experimental design for the study consisted of two factors i.e. cookstove types (four
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levels - TR, PF, PN and EN) and cooking cycle (three levels – pulse (legumes), rice and roti
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making). Both single and two-way analysis of variance (ANOVA) with replicates were used to
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confirm significant (α = 0.05) difference in average values of OC and EC EFs and interaction
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between cookstoves and burn cycle. Further Student’s-test was conducted in order to identify
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pairs with statistically significant difference in average OC and EC EFs with the help of
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Bonferroni correction method, where α=α/c (c is the number of paired means compared).32
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The relationship was established using the coefficient of determination (R2) and
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Pearson’s correlation coefficients, which confirm that variation in PM EFs was associated to the
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change in OC and EC EFs.
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Results and discussion
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Cooking cycles – Survey results
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The data collected through the survey conducted in the villages of Uttarakhand, showed
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that ~64% of the households show two prominent sequences in a single meal i.e. pulse-rice-roti
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(Indian wheat bread) or pulse-vegetable-roti. Rice and vegetable were found to be cooked
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alternatively in a single meal throughout the week. Roti's were cooked mostly at the end because
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of its tendency to loose moisture. Therefore, in the present study the cooking sequence followed
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an order in which pulse was cooked in the first place, followed by rice and roti, for all the
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experiments. The vegetable preparation was omitted from the cooking cycle because of the
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possible interference in the OC quantification in PM due to use of cooking oil. The survey results
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also showed that the average quantity of pulse, rice and wheat flour consumed in a single meal
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was around 250 ± 20 g, 300 ± 34 g and 500 ± 23 g, respectively for an average household size of
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five members. Detailed survey results are available in the study conducted by Arora et al.16
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Variation in emission factors for OC and EC
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The variation in EFs for OC and EC during different phases of cooking cycle and their
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average in the four cookstoves during CCT are presented in Figure 1 and Table 1, respectively.
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The average EF for OC was found to be the highest in TR cookstove followed by EN, PN and PF
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cookstoves and difference in average OC EF between TR, and three cookstoves was found to be
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statistically significant (p
