Determining Particulate Matter and Black Carbon Exfiltration Estimates

Apr 6, 2015 - A majority of black carbon (BC) emitted to the atmosphere in the Indo-Gangetic Plain (IGP) region is from burning biomass fuel used in t...
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Determining Particulate Matter and Black Carbon Exfiltration Estimates For Traditional Cookstove Use In Rural Nepalese Village Households Sutyajeet Inderjeet Soneja, James M. Tielsch, Frank C. Curriero, Benjamin Zaitchik, Subarna K. Khatry, Beizhan Yan, Steven N Chillrud, and Patrick N Breysse Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/es505565d • Publication Date (Web): 06 Apr 2015 Downloaded from http://pubs.acs.org on April 8, 2015

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Environmental Science & Technology

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Determining Particulate Matter and Black Carbon

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Exfiltration Estimates For Traditional Cookstove

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Use In Rural Nepalese Village Households

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AUTHORS: Sutyajeet I. SonejaŦ, James M. Tielsch§, Frank C. CurrieroΤ, Benjamin ZaitchikŤ,

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Subarna K. KhatryŢ, Beizhan YanĦ, Steven N. ChillrudĦ, Patrick N. BreysseŦ*

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Ŧ

8 9

§

Department of Environmental Health Sciences, Bloomberg School of Public Health, Johns Hopkins University, Baltimore, MD 21205, USA

Department of Global Health, Milken School of Public Health and Health Services, George Washington University, Washington, DC 20037, USA Τ

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Department of Epidemiology, Bloomberg School of Public Health, Johns Hopkins University, Baltimore, MD 21205, USA

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Ť

Department of Earth and Planetary Sciences, Johns Hopkins University, Baltimore, MD 21218, USA

14

Ţ

15 16

Ħ

Nepal Nutrition Intervention Project Sarlahi, Kathmandu, Nepal

Division of Geochemistry, Lamont-Doherty Earth Observatory, Columbia University, Palisades, NY 10964, USA

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KEYWORDS:

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Exfiltration fraction; air exchange rate; mixing factor; particulate matter; black carbon; cookstove; low resource environment; biomass burning; ventilation; village; household; Indo Gangetic Plain

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ABSTRACT

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A majority of Black Carbon (BC) emitted to the atmosphere in the Indo-Gangetic Plain (IGP)

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region is from burning biomass fuel used in traditional, open-design cookstoves. However, BC

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and particulate matter (PM) household emissions are not well characterized. Household emission

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information is needed to develop emission profiles to validate regional climate change models

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and serve as a baseline for assessing the impact of adopting improved stove technology. This

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paper presents field-based household PM and BC exfiltration (amount exiting) estimates from

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village homes in rural Nepal that utilize traditional, open-design cookstoves. Use of these stoves

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resulted in a 26% mean PM exfiltration, ranging 6% to 58%. This is a significant departure from

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an 80% estimate cited in previous literature. Furthermore, having a window/door resulted in an

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11% increase in exfiltration when an opening was present, while fuel type had marginally

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significant impact on emission. Air exchange rates (AER) were determined, with average (95%

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CI) AER of 12 (10-14) per hour, consistent with previous studies. Also, BC to PM2.5 mass-ratio

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composition during cooking was ascertained, with an average (95% CI) of 31% (24-39),

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agreeing with previous biomass fuel emission composition literature.

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ABSTRACT ART:

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Introduction

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The Himalayan glaciers provide fresh water via glacial melt to more than a billion people

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across South and East Asia1. These glaciers have begun melting at an alarming rate over the past

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decade, threatening the fresh water supply for this region1. Furthermore, alterations in the overall

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hydrological cycle in South Asia is becoming more apparent2. Monsoons are deviating from

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typical seasonal patterns, impacting a number of groups within this region including farmers who

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are responsible for feeding a significant portion of the world’s population3,4. While it is believed

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that glacial retreat is driven by global warming, the rapidity of the retreat indicate other factors

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may be at play1. Black carbon emitted into the atmosphere and subsequent deposition on glaciers

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and snow pack is thought to be a major contributor to the changing climate and accelerated

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glacial retreat in this region5.

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Black carbon (BC), a component of particulate matter (PM), is produced by incomplete

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combustion of fossil fuels, burning of biomass from forest fires, and from households and

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factories that use wood, dried animal manure, or crop residue for cooking or other energy needs6.

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Worldwide, more than three-quarters of the black carbon produced is thought to come from

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developing countries, generated from cookstoves, open burning, and older diesel engines4. It is

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estimated that around 2.7 billion people worldwide cook using biomass fuel, most of which are

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poor and in developing countries7. Twenty percent of the total global BC emissions has been

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attributed to emissions of BC from biomass burning in cookstoves8, and biomass cooking is

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believed to contribute about two-thirds of BC emissions within South Asia9.

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Within the Himalayas, BC may be responsible for as much as 50% of the total glacial retreat

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seen9. Furthermore, recent studies have estimated BC as a major contributor to global warming,

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surpassed only by CO2 and possibly methane10. Since BC has an average residence time in the

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atmosphere of a few days to weeks relative to greenhouse gases which range from years to

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centuries, mitigating BC emissions to combat climate change can produce almost immediate

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effects in terms of reduced radiative forcing, subsequently producing direct benefits for public

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health10,11. Regional scale BC emission estimates, however, have a significant, 2-5 fold,

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uncertainty5. The Indo-Gangetic Plain region (IGP) has been identified as a regional hotspot for

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BC emissions10. The IGP is one of the most densely populated regions in the world and large

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uncertainties exist for published BC emissions12,13.

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Background. Cookstove emissions create indoor exposures and contribute to ambient air

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pollution through passive exchanges between indoor and outdoor air (indirect venting) and

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direct, active venting (i.e., chimney) to the outdoors. Open-design cookstoves, where combustion

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byproducts are expelled directly into the indoor environment, result in high concentrations of

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pollutants inside the home14–18. BC and PM emission factors for different stove designs have

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been determined in laboratory settings19–21. These emission factors can vary by 5-6 fold

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depending on stove design and fuel type19. In addition, the BC mass-ratio component of PM

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under 2.5 micrometers in diameter (PM2.5) has been shown to vary from 5-52% using laboratory-

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based test burns19. While laboratory-based studies are valuable for estimating stove emissions,

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climate modeling requires estimates of household emissions. Characterization of cookstove

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emissions emanating from a home requires an assessment of the PM exfiltrating from the house.

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A fraction of PM emitted indoors during combustion will settle and deposit on indoor surfaces,

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while the remaining will exfiltrate to the outdoor environment. Studies translating laboratory-

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based cookstove emissions to ambient emissions from a house often use hypothetical air

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exchange and particle deposition rates resulting in fractional exfiltration estimates. For example,

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an 80% exfiltration estimate of PM emitted by biomass combustion19, a published value with no

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documentation based upon hypothetical particle deposition rates has been cited without any field

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validation. Furthermore, measurement of air exchange rates from housing in developing

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countries is extremely limited22–24 compounding our poor understanding of how much PM may

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be exfiltrating to the outdoor environment. Estimates of PM and BC exfiltration from homes are

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needed to reduce uncertainty in emission inventories. Exfiltration estimates can be used to

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evaluate the impact of cookstove interventions (i.e., use of a chimney), designed to improve

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indoor air quality, on outdoor air quality. These estimates can further inform studies assessing

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indoor air pollution in other homes, where exfiltration of biomass air pollution has been

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identified as a significant contributor to indoor PM pollution in non-biomass using homes25.

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This paper presents air exchange rates and PM exfiltration estimates from homes in rural Nepal

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that utilize traditional, open-design cookstoves. Estimates of variability are provided for

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exfiltration as a function of housing and fuel characteristics in a real-world setting. Furthermore,

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this paper assesses the BC to PM2.5 mass-ratio produced by biomass cooking in order to estimate

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BC exfiltration from homes. In combination with an assessment of indoor PM concentrations,

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PM and BC exfiltration fractions can be used to estimate house emissions to ambient air in order

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to better assess regional air quality and climate change impacts.

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Methods

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Study Overview. Located in Sarlahi District Nepal, this exfiltration assessment study is nested

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under a broader parent cookstove intervention trial known as the Nepal Nutrition Intervention

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Project - Sarlahi (NNIPS) setup by the Johns Hopkins Bloomberg School of Public Health

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(JHSPH). This community-based, cluster randomized, step-wedge trial is characterizing indoor

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PM exposure in ~3,000 homes from traditional and alternative cookstoves, assessing primary

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health outcomes that include acute lower respiratory illness in children 1-36 months of age

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(ALRI), as well as birth weight and preterm birth among newborn infants26. This cookstove

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intervention trial is underway in 4 areas, referred to as Village Development Committees

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(VDC’s), within the NNIPS site. Sarlahi is a rural area located in the Terai region of southern

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Nepal (bordering Bihar State in India) and is representative of southern Nepal and most of

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northern India with elevation approximately 200 meters above sea level26. Cooking with

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traditional stoves, comprised of clay, mud, bricks, rice husk, and cow dung, are common in this

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area26. The typical cooking style involves the burning of biomass fuels (wood, dried animal

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manure, and crop residue) in traditional, open-design mud cookstoves without direct ventilation

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to the exterior26.

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Mock and Occupied Home Sampling. In order to assess exfiltration estimates of PM, a mock

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house was built at the research site in Sarlahi District, Nepal. Based on parent trial study data, the

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house (Figure 1) was representative of a typical household kitchen in this area with respect to

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size, material, and number of openings. Homes in this region typically consist of mud, wood,

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brick or cement walls with thatch roofs26. The mock house consisted of a 1-room floor plan with

Figure 1. Mock house representative of standard kitchen built in study area

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the ability to close and open 1-window and door. Housing material consisted of bamboo with

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mud, logs, and tree branches, while roofing was half tile and thatch/grass. House dimensions

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were: length 3.85m, width 4.65m, ground to the lowest roof point 1.8m, ground to apex of the

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roof 2.7m, window 0.6m by 0.6m (located on the back wall), and door frame 1.28m width by

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1.64m height (located on the front wall). Both the window and door had a hinged, wood-framed

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metal panel that allowed for opening/closing. In addition, a traditional mud-based cookstove with

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two openings was built inside according to typical practices for stove construction, placed on the

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floor of the back wall.

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Air quality data in the mock house included PM concentration collected passively with a

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DataRAM pDR-1000AN (Thermo Scientific, Franklin, MA), carbon monoxide concentration

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with an EasyLog USB CO Monitor (Lascar Electronics, Eerie, PA), and relative

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humidity/temperature using the HOBO Data Logger (Onset Corp., Bourne, MA). All devices

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were co-located at the center of the house before, during, and after simulated cooking events

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(discussed later), and recorded data in 10-second intervals. The pDR-1000 was zeroed in the

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field prior to each deployment using procedures recommended by the manufacturer. The CO

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monitor was calibrated before deployment at JHSPH using standard procedures27.

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In addition, gravimetric PM2.5 was collected with a PM2.5 cyclone inlet (BGI, Waltham, MA)

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on Teflon filters (37mm 2.0µm pore PTFE Membrane Filter w/ PMP ring Pel Life Sciences, Ann

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Arbor, MI). A personal sampling pump (5400 BGI Inc., Waltham, MA) was utilized at a flow

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rate of 4 L/min. Flow rates were recorded before and after sampling using a Drycal Flowmeter

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(DC Light BIOS Intl., Butler, NJ). Filters were pre and post-weighed in a temperature and

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humidity controlled room using an XP2U Microbalance (Mettler Toledo, Columbus, OH)

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located at JHSPH. Pre-weighed Teflon filters were loaded into polypropylene filter cassettes

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(SKC Inc., Eighty Four, PA), along with filter pads (Pall Life Sciences, Ann Arbor, MI) and

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37mm drain discs (Model #230800 Air Diagnostics and Engineering, Harrison, ME) at the field-

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site in a field-developed clean box to minimize contamination during assembly while in Nepal.

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After sampling, filter cassettes were sealed in plastic bags until returned to the United States,

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where they were disassembled and post-weighed. Ten percent duplicate samples were collected

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and all filter weights were blank corrected.

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Fuels utilized during simulated cooking events in the mock house were derived from survey

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data from the parent cookstove trial. The most common types of fuel were wood and a mixture of

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wood/dung/crop waste28. Fuel sources were obtained from the same area, by the same person,

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and subsequently sun dried to a consistent moisture content. The same person performed all of

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the simulated cooking events in the mock house. Each fuel combination was tested three times.

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In addition, two boundary conditions were tested, where both the window and door were either

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open or closed during mock house testing.

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The same test protocol was performed in 50 occupied homes randomly selected from the 2,854

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households in the parent cookstove trial. Homes chosen reflected typical housing based on data

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from the NNIPS cookstove intervention trial study (i.e., have 1 window/door, similar in kitchen

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size and composition to the mock house). Eligible homes met the criteria of only using

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cookstoves for personal food production, agreeing to not have other sources of combustion

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(including tobacco) ongoing in the house during measurement. Testing was performed midday to

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minimize interference to participants’ daily routines as well as smoke infiltration from

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surrounding homes’ cooking activities. Fuel type utilized and window/door status were observed

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in order to allow cooking preferences of the home to remain intact. The individual in the home

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that did the most cooking performed the simulated cook session and was supervised by local

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staff for protocol compliance. Kitchen volume and number of windows/doors were also

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recorded.

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Exfiltration Fraction Determination. The air exchange rate (AER) and subsequent exfiltration 180

fraction (EF), or the amount of PM exiting a home

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via natural ventilation, were determined via the

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concentration decay method29. This method involves

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measuring pollutant decay over time once the source

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of the contaminant is terminated or removed in order

185 to determine passive air exchange rates between Figure 2. Hypothetical decay curves for indoor to outdoor29,30. Hypothetical decay curves for 186 PM and a non-reactive gas. 187

a non-reactive gas and PM illustrating the concentration decay method are displayed in Figure 2.

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Assuming that the house is a simple one-room structure (i.e., no tight internal partitions), it can

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be treated as a single zone and a mass-balance continuity equation can be used to estimate the

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exchange of air between indoors and outdoors. For non-reactive gases (i.e., no other mechanism

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of removal) this model assumes that the air exchange carries the pollutant from inside to out, or

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visa versa if one is interested in infiltration. The air exchange rate (AER) is calculated assuming

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perfect mixing conditions29 using Equation (1):

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  ∗ 



(1)



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Where Q is ventilation rate in m3/minutes, V is room volume (m3), C2 and C1 are concentrations

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at various points in time, t2 and t1 are the corresponding time points (minutes), and k is a factor

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used to account for imperfect mixing conditions. The AER, , is determined by regression of



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Ln( ) of the non-reactive gas against ∆ time. Equation (2) illustrates the simple linear regression 

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model used to obtain this value. Ln(C) = β0 + β1×t + ε, where ε ~ N(0,σ2)

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(2)

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Where C is the concentration of the tracer gas, t represents time (minutes), and β1 represents the

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AER. Ventilation studies typically utilize a tracer gas not commonly found in the environment,

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such as sulfur hexafluoride (SF6), to measure AER’s31. Given logistical limitations regarding

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acquisition and transportation of SF6 to the study site, carbon monoxide (CO), deemed a viable

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substitute32, was utilized as the tracer gas. CO was suitable because the concentrations produced

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inside the home during cooking were more than two orders of magnitude higher than

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background.

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Figure

3

summarizes

passive

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exfiltration pathways for PM from a

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house. PM exfiltrates through open

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windows/doors as well as through

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cracks in the roof and walls (PME in

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Figure 3). Exfiltration for PM is

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complicated by the fact that the decay

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over time will be governed by the Figure 3. Pathways for particulate matter exfiltration from a house losses due to settling, surface

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deposition, as well as exfiltration. Calculating the AER based on the PM decay curve will

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therefore overestimate the real passive air exchange. To address this problem, two decay curves

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were measured as indicated in Figure 2. The slope of the PM decay curve was calculated in the

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same manner as the CO decay curve, utilizing Equations (1) and (2). The fraction of PM decay

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associated with exfiltration was calculated by comparing the slopes of the decay curves (i.e., the

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AERs) for CO and PM. Since CO is a non-reactive gas, its AER will not be affected by surface

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deposition. As shown in Figure 2, the decay rate for PM will be faster than that for a gas. The

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ratio of the AERs, Equation (3), provides an estimate of the fraction of PM that exfiltrates to the

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ambient air: 

EF   

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(3)



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where SCO and SPM are the slopes of the respective decay curves, or the AERs, when calculated

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according to Equation (2). Conceptually, if both agents had the same pathways for exfiltration

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then the decay curves would overlap resulting in unity. Deviations from unity are accounted for

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by the faster decay of PM due to particle losses on surfaces.

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Exfiltration Fraction Test Protocol. A simulated cooking session was performed to create

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elevated concentrations of PM and CO within the home. Prior to ignition, air-sampling

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equipment was deployed in the mock house at a height of 1m in front from the edge of the stove

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and 1.8m off the floor to maintain consistency between readings. Placement of sampling

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equipment in occupied homes relative to the mock house was slightly altered due to logistical

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limitations.

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By generating PM and CO concentrations that were orders of magnitude higher than outdoors,

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PM and CO infiltration from outdoor air into the home (assumed to be low relative to the indoor

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generated contaminants) when using Equation (2) could be ignored. To determine PM and CO

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decay curves, a modified version of the Water Boiling Test (WBT) 3.0 was used33. A cooking

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session was simulated by bringing 2 pots of water (5 Liters each) to a rolling boil from ambient

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temperature. The fire was then extinguished and remnant fuel removed from the house to halt

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further emissions. Passive PM, CO, and RH sampling were initiated 30 minutes prior to the

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cooking session and continued for an additional two hours post-fire extinguishment. Gravimetric

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sampling for PM2.5 took place only during the active flame period. Passive PM and CO

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concentration decays were examined only after the flame was extinguished. Equations (2) and

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(3) were applied to the decay curves to determine the AERs for CO and associated exfiltration

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fraction. All tests were performed during the Spring of 2012, considered the dry season as the

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outdoor temperature is moderate to high and precipitation is minimal.

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Determining the Black Carbon Fraction of PM2.5. PM2.5 filter samples were collected during

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the active flame period of the simulated cooking session in both the mock house and occupied

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homes in order to assess BC content of the PM using a validated optical method34. It is assumed

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that BC is mostly contained on smaller particles (i.e., PM< 2.5 m) and it is these particles that

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drive the impacts to climate35, therefore measuring PM over a broader size distribution is not

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necessary.

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Mixing Factor Assessment. An additional analysis examined if further adjustments were

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necessary when determining air exchange rates. Methods, data analysis, and results are presented

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in the Supporting Information section.

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Data Analysis. Exploratory analyses included generating summary statistics for AERs, PM and

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CO ventilation rates, exfiltration fraction by fuel type and window/door status, and BC to PM2.5

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mass-ratios. A linear mixed effects model with a random intercept and slope was utilized to

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examine if household factors are predictive of PM exfiltration. This regression model took the

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form

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EFijk = (β0+ b0k) + β1×Fi + (β2+ b2k)WDj + εijk

(4)

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Where i, j, k index fuel type, window/door status, and house type respectively, and {b0k} are

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independent Normal (0,σ02), {b2k} are independent Normal (0,σ22), errors {εijk} are independent

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Normal (0,σe2), and {b0k}, {b2k}, and {εijk} are mutually independent.

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The effect of fuel type (Fi) is fixed while house type is random. The effect of status of window/door during cooking (WDj) varies by house type to attain the random slope model. All statistical analyses were performed in the R Statistical Computing Environment (version 3.0.2; R Project for Statistical Computing, Vienna, Austria).

276 Results and Discussion

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Example of decay curves for PM and CO

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post-fire extinguishment determined in an

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occupied house is presented in Figure 4. Both

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curves

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presented

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relationship is seen for all tests performed.

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The more rapid decay of PM indicates that

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surface losses are significant.

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mimic in

the Figure

hypothetical 2

and

curves

a similar

Figure 4. Example of PM (red) and CO (blue) decay curves post-fire extinguishment in an occupied house

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Table 1. Summary Statistics for CO Air Exchange and Ventilation Rates, and Particulate Matter Exfiltration Fraction (%)

Carbon Monoxide Air Exchange Rate (per hour)

Carbon Monoxide Ventilation Rate (m3/min)

Particulate Matter Exfiltration Fraction (%)

Data Type

N

All Data

50

Mock House

Mean (95% CI)

Range

10th%

25th%

Median

75th%

90th%

11.9 (9.7, 14.1)

2.3-41.8

4.2

6.8

10.6

15.5

20.5

20

9.6 (5.4, 13.8)

2.3-41.8

3.8

4.7

6.8

9.7

17.9

Occupied Homes

30

13.4 (11, 15.6)

4.2-35.2

7.5

9.1

11.7

15.8

21.7

All Data

50

6 (4, 7.9)

0.6-31.5

1

2.3

4

5.6

13.8

Mock House

20

6.4 (3.6, 9.3)

1.6-28.1

2.5

3.2

4.6

6.5

12

Occupied Homes

30

5.7 (2.9, 8.4)

0.6-31.5

0.9

1.7

3.2

5.1

15.3

All Data

50

26.4 (22.4, 30.5)

5.8-57.6

10.3

14.8

22.1

37.4

45.4

Mock House

20

23 (15.9, 30.1)

6.3-57.6

8.9

13.6

19

28.2

45.3

Occupied Homes

30

28.7 (23.7, 33.6)

5.8-53.6

10.7

16.6

30.9

38.7

45.4

288 289 290 291 292 293 294

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Air Exchange, Ventilation Rate, and Exfiltration Fraction. Summary statistics of AERs,

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ventilation rates, and exfiltration fraction, with AER and ventilation rates calculated from the

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decay curves, are provided in Table 1.

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Air exchange rates based on the CO decay curves are reported in Table 1, with a mean (95%

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CI) for all tests equivalent to 11.9 h-1 (9.7, 14.1). Examining CO AER by window/door status

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yielded an average of 5.9 h-1 (4.4, 7.3) for the closed category (n=10) and 13.4 h-1 (10.9, 16) for

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having an opening present (n=40). This trend follows expectations for window/door status, with

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an opening present allowing for increased airflow and hence a higher average AER. Limited

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field studies examining AER in other village housing have been conducted. Measured AER for

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Indian village homes have been recorded between 7 to 30 per hour22. A more recent study in

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village homes located in Peru reported a range of AER’s between 0.5 h-1 to 20 h-1 depending

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upon partition presence23. These ranges of values are in agreement with results found in rural

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Nepal village housing, which has similar housing style to much of the Indo-Gangetic Plain

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region. In addition to demonstrating similarities across village housing, these findings contribute

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to the expansion of field-based residential AER data. Accurate estimation of AERs is critical to

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understanding cookstove contributions to ambient air pollution as well as the broader climate

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change impacts that may be associated with traditional vs. alternative stoves. Ventilation rates,

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which are a function of the home volume and air exchange rate, are also presented for all tests in

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Table 1. The ventilation rates calculated from CO decay for the mock house under varying

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conditions averaged 6.4 m3/min and ranged from 1.6 to 28.1 m3/min. Occupied homes’ CO

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ventilation rates similarly averaged 5.7 m3/min and ranged from 0.6 to 31.5 m3/min. As

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expected, mock house ventilation rates were similar to the occupied homes.

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The average (95% CI) PM exfiltration fraction for all tests (n=50) was 26.4% (22.4, 30.5),

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with values ranging from 5.8% to 57.6%. Mock and occupied home EF averages (95% CI) were

319

23% (15.9, 30.1) and 28.7% (23.7, 33.6), respectively. EFs for the mock and occupied houses

320

were not statistically different, thus conforming to the expectation that the designed mock house

321

kitchen is representative of a typical kitchen in this area. The average (95% CI) EF for closed

322

window/door status (n=10 tests) was 18.2% (13.1, 23.2), while having an opening present (n=40

323

tests) resulted in an increase to 28.5% (23.8, 33.2). Furthermore, the range of EF data is less

324

variable when the window/door are both closed relative to having an opening present.

325

Comparing open vs. closed conditions yielded statistically different results. When partitions are

326

closed, airflow through the kitchen is reduced, resulting in less exfiltration and a range of values

327

with a relatively smaller spread in contrast to having openings.

328

EF distributions broken down by fuel type are presented in Figure 5. A wide range of fuel

329

types were tested in the occupied homes because these were the fuel types used by the families

330

during the testing period. They included wood, crop residue, mixtures of wood/dung/crop

331

residue, dung/wood, and crop residue/wood. Only wood and mixture of wood/dung/crop residue

332

were evaluated in the mock house because these were the two fuel types most prevalent in the

333

parent cookstove trial. Fuel categories consisting of a mixture still had wood as the primary fuel

334

source. EF for fuel types of wood and crop residue by themselves had similar averages (95% CI)

335

of 30.2% (25.6, 34.8) and 28.1% (13.9, 42.4), respectively.

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Figure 5. Particulate matter exfiltration fraction presented by fuel type 336

Crop residue is a mixture of materials with the potential to produce a wider range of particle

337

sizes when burned, hence the potential for particle size distribution to be broader relative to

338

wood. Subsequently, one expects that EF variability for crop residue should be larger than that

339

seen for wood, which is shown in Figure 5.

340

Figure 5 has a line indicating where 80% exfiltration is located, which is a hypothetical value

341

with no field validation that has been previously cited but not independently published19. The

342

80% exfiltration is significantly greater than the averages and upper limits of the EF data,

343

presented in the overall EF data (Table 1) and in Figure 5. It is difficult to account for the

344

difference between the EF presented in this paper and the 80% estimate previously used19

345

because the assumptions made in the derivation of the theoretical 80% EF value have not been

346

published. One possible reason why the hypothetical estimate is greater than our measured

347

estimates may be that the actual mass median size of particulate matter generated from cooking

348

is larger than previously assumed, which can impact the assumed deposition rate. It should be

349

noted that temperature for all tests were measured, with no significant difference found and thus

350

considered not a factor.

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351

Utilizing a linear mixed effects model, factors that may impact EFs were evaluated. Pooling of

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categories for fuel type was necessary due to the small sample sizes of certain fuel categories.

353

With wood being the dominant fuel source for the categories of dung/wood, wood/dung/crop

354

residue, and crop residue/wood, these three sources were combined into one category and called

355

wood/other. The regression model examined window/door status (everything closed vs. opening

356

present) and fuel type (wood only and wood/other), with crop residue excluded due to the small

357

sample size. Reference levels for the covariates were set as wood for fuel type and everything

358

closed for window/door status. Regression results indicate that having an opening present

359

resulted in an 11.37% (SE=4.51) increase in exfiltration (p=0.022), while switching fuel type

360

from wood to wood/other resulted in a 7.64% (SE=3.83) decrease in exfiltration (p=0.0621).

361

Thus, window/door status was deemed a significant covariate while pooled fuel type was

362

marginally significant.

363 364

Ratio of Black Carbon to PM2.5 Analysis. Of the 50 PM2.5 filters collected during exfiltration

365

fraction tests, 43 were overloaded with PM thus too saturated to estimate BC using the optical

366

analysis method. The excluded filters exceeded the optical method’s upper threshold detection

367

limit which results in under prediction of BC values34,36. Fuel categories for the 7 samples with

368

acceptable loadings for BC analysis were broken down into wood/other (n=6) and crop residue

369

(n=1).

370 371 372 373

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Table 2. Summary Statistics for Black Carbon to PM2.5 Mass-Ratio N

Mean (95% CI)

Median

Range

PM2.5 Concentration (ug/m3)

7

4206.6 (2342.1, 6071)

3427

2341.2-8382.3

Black Carbon Concentration (ug/m3)

7

1317.5 (671.9, 1963.2)

1197.5

630.8-2470.7

Black Carbon to PM2.5 Ratio (%)

7

31.2 (23.8, 38.6)

29.5

17.6-41.3

374 375

Concentration summary statistics of BC and PM2.5, along with ratio estimates are provided in

376

Table 2. Measured only over the active cooking period of the simulated cooking test, the mean

377

(95% CI) ratio was 31.2% (23.8, 38.6), ranging from 17.6% to 41.3%. For the oversaturated

378

filters that were excluded, the most that can be reported is that all the mean BC to PM2.5 ratios

379

were larger than 9%.

380

Multiple studies have reported BC to PM ratios. For example, Rau (2007) reported elemental

381

carbon values from wood smoke particles ranging from 3 to 38%37, while other studies of wood

382

smoke have reported ratios that range from 8 to 53%38. Our estimates agree well with these

383

published data, further validating estimates determined in laboratory and field settings.

384

This research has major implications with regards to epidemiological and climate change

385

models that utilize emission inventories of BC generated by cooking activities. Specifically, this

386

research contributes to developing more robust estimates of the amount of BC that exits the

387

home and into the atmosphere as a result of cooking indoors with biomass fuels in traditional

388

stoves. To better understand the black carbon exfiltration from a cooking event, simple

389

calculations can be conducted. To present upper and lower bounds, the range of average PM2.5

390

concentration values during cooking (1.8 mg/m3 to 17.3 mg/m3) in the mock house were

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391

extracted. These values were multiplied by the volume of the mock house (40 m3) and BC/PM2.5

392

ratio (31.2%), producing a range of 23 to 216 milligrams of black carbon generated indoors over

393

the cooking event (mg of BC/cooking event). Subsequently, the mass of black carbon generated

394

indoors over the cooking event was then multiplied by the average exfiltration fraction (26.4%),

395

yielding the mass of black carbon that exfiltrates to the atmosphere over the course of a cooking

396

event (6 to 57 mg/cooking event). This range illustrates a rough estimate of the amount of black

397

carbon that can be emitted into the atmosphere from a household per cooking event after

398

accounting for deposition indoors.

399 400

Limitations. There are a number of limitations for this work. While we tried to keep combustion

401

characteristics constant in the mock house, burn rate, air-fuel ratio, flame turbulence, and

402

combustion temperature were not measured. These factors can influence particle size

403

distribution, which was also not measured. Differences in particle size will impact the settling

404

and fraction of PM that may deposit in cracks and on other surfaces. Furthermore, BC particles

405

may deposit at higher or lower rates than PM2.5 overall, but this issue was not measured. Future

406

work should also seek to address the incorporation of activity patterns in real-world homes to

407

assess if multiple family members within a home create changes in air exchange rate estimates.

408

Seasonality may also play a role with certain fuel types being more prevalent and is an area

409

requiring further exploration as sampling took place during one season (Spring). Due to

410

extremely high PM concentrations, oversaturation for the BC analysis occurred, thus future

411

studies should consider utilizing methods with pumps set to lower flow rates (i.e., 1.5 LPM). In

412

addition, all data collected for BC analysis was conducted utilizing a simulated cooking event

413

(water boil test) and further assessment over the course of actual cooking events is needed to

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414

assess the generalizability of the mock house testing. A limitation of using CO as the tracer gas is

415

that the fire had to be extinguished so that impact of the cooking fire on AER could not be

416

determined. Finally, our testing was limited to one area and one stove type, thus results may not

417

extrapolate to other areas and stoves. Data are needed on different housing stock, stoves

418

(traditional and improved), and analysis is also needed to assess the impact chimneys may have

419

on exfiltration.

420 421

Author Contributions

422

The manuscript was written through contributions of all authors. All authors have given approval

423

to the final version of the manuscript. Specific contributions were as follows: SonejaŦ designed

424

the study, conducted primary data collection and sample processing, performed data analysis,

425

and was the main author of the paper; KhatryŢ assisted with primary data collection; YanĦ and

426

ChillrudĦ assisted with black carbon sample processing and analysis; CurrieroΤ and ZaitchikŤ

427

assisted with data analysis; BreysseŦ and Tielsch§ assisted in the design of the study, provided

428

oversight to data collection, and assisted with data analysis. All coauthorsŢĦΤŤŦ§ assisted with

429

writing of this paper.

430 431

Corresponding Author

432

Patrick N. Breysse, Ph.D.

433

Department of Environmental Health Sciences

434

Bloomberg School of Public Health

435

Johns Hopkins University

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436

615 North Wolfe Street, Room E6628

437

Baltimore, MD 21205

438

Phone: 1-410-955-3608

439

Fax: 1-410-955-9334

440

[email protected]

Page 22 of 26

441 442

Funding Sources

443

Funding sources included a grant from The Environment, Energy, and Sustainability Institute at

444

Johns Hopkins, grant support from the National Institute of Environmental Health Sciences

445

(ES015558; ES003819), and the Thrasher Research Fund (02830-4).

446 447

Acknowledgements

448

We would like to thank NNIPS field research supervisors and staff, Steven LeClerq, Pramod

449

Sah, Samjhana Magar, Ana Rule, D’ann Williams, Jana Mihalic, Lance Wallace, Andrew

450

Persilly, and Chen Chen. This manuscript is dedicated to Dr. Alison Geyh, a mentor and

451

inspiration.

452

Supporting Information

453

An additional analysis examining mixing factors assessed if further adjustment was necessary

454

when determining air exchange rates. This information is available free of charge via the Internet

455

at http://pubs.acs.org/.

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456

Abbreviations

457

BC, black carbon; PM, particulate matter; PM2.5, particulate matter with aerodynamic diameter

458

of 2.5 micrometers; EF, exfiltration fraction; AER, air exchange rate; IGP, Indo Gangetic Plain

459 460

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461 462

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