10th International Aerosol Conference September 2 - September 7, 2018 America's Center Convention Complex St. Louis, Missouri, USA
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Aerosol Optical Properties and Climate Implications of Emissions from Traditional and Improved Cookstoves
GEORGES SALIBA, R. Subramanian, Kelsey Bilsback, Christian L'Orange, John Volckens, Michael Johnson, Allen Robinson, Carnegie Mellon University
Abstract Number: 1412 Working Group: Carbonaceous Aerosol
Abstract Cookstoves are a substantial global source of black carbon (BC) and brown carbon (BrC) particles which are climate-warming agents. Additionally, exposure to particulate matter (PM) from solid-fuel cookstoves is responsible for more than 4 million premature deaths worldwide, according to the World Health Organization. To reduce health, climate, and other environmental impacts, decades of research has gone into developing “improved” which aim to reduce emissions and/or exposure; however, measurements of real-cooking events indicate that improved cookstoves emit a higher BC fractions in emission compared to traditional cookstoves. Furthermore, the potential climate implications of a large-scale deployment of improved cookstoves remain uncertain due to limited (and variable) published field data of optical properties from cookstove emissions.
We used state-of-the-art instrumentation to measure aerosol optical properties from fresh emissions from 18 different cookstove/fuel combinations; the cookstoves tested include wood-burning three-stone fires, natural draft rocket cookstoves, charcoal cookstoves, and forced-draft cookstoves. The cookstoves were tested in the laboratory using the Firepower Sweep Test to characterize emissions across a wide range of cookstove power outputs. We used our large dataset to investigate differences in optical properties with varying cookstove technology, fuel (wood versus charcoal), and operation conditions. The BC/PM metric explained the measured variability in optical properties (R2=0.9) and performed significantly better than other operational metrics, such as modified combustion efficiency (R2=0.3) and firepower (R2=0.1). We developed semi-empirical parametrizations (consistent with Mie theory) of the mass absorption cross-section per BC mass (MACBC), absorption angstrom exponent (AAE), and single scattering albedo (SSA) as a function of BC/PM. We measured increase in MACBC (from 10 m2/g to 40 m2/g), SSA (from 0.2 to 0.9), and AAE (from 1 to 5) with decreasing BC/PM, independent of cookstove technology. These measurements suggest that with decreasing BC/PM there is increased coating around BC particles and increased contribution from BrC to absorption. these findings are consistent with BC-mixing state from single-particle soot photometer (SP2). Finally, we found little dependence of the measured intensive optical properties on cookstove technology or fuel.
A key outcome from our analysis is that the measured increase in MACBC with decreasing BC/PM was inconsistent with current model treatments of the optical properties of cookstove emissions. Models assume internal mixture of BC and non-BC material and BrC absorption based on biomass burning parametrizations; however, our data indicate that this approach may not reflect real-world emissions. We compared measured MACBC with Mie theory predictions for different particle mixing-states and with and without BrC and found that treating BC and non-BC materials as internally mixed overestimates the measured MACBC. Our new parametrizations include contributions from lensing, mixing-state, and BrC to absorption and can be used in emission inventories, which contain BC/PM data. Furthermore, our data indicate that: (1) intensive optical properties measured in the laboratory using the Firepower Sweep Test are representative of field measurements, and (2) for relevant cookstove operation, the enhancement in MACBC is dominated by mixing-state and lensing and not BrC.
Finally, we combined our parametrizations of intensive optical properties with field emissions data from the literature to estimate the direct radiative effect of emissions for different cookstove technologies, using a simple forcing efficiency metric. We predict only 20% climate benefits from improved cookstoves compared to traditional ones (differences not statistically significant), but more than 50% reductions in PM mass emissions (statistically significant differences). We predict that the large-scale deployment of improved cookstoves will likely result in a modest reduction in the direct radiative effect from emissions.