Atmospheric Environment 43 (2009) 5674–5681 Contents lists available at ScienceDirect Atmospheric Environment journal homepage: www.elsevier.com/locate/atmosenv Radiative forcing from household fuel burning in Asiaq Kristin Aunan a, *, Terje K. Berntsen a, b, Gunnar Myhre a, b, Kristin Rypdal a, David G. Streets c, Jung-Hun Woo d, Kirk R. Smith e a Center for International Climate and Environmental Research – Oslo (CICERO), P.O. Box 1129, Blindern, N-0318 Oslo, Norway Department of Geosciences, University of Oslo, 0315 Oslo, Norway c Decision and Information Sciences Division, Argonne National Laboratory, DIS/900 9700 South Cass Avenue Argonne, IL 60439, USA d Department of Advanced Technology Fusion, Konkuk University, Seoul 143-701, Korea e Global Environmental Health, School of Public Health, 747 University Hall, University of California, Berkeley, CA 94720-7360, USA b a r t i c l e i n f o a b s t r a c t Article history: Received 11 March 2009 Received in revised form 20 July 2009 Accepted 22 July 2009 Household fuel use in developing countries, particularly as biomass and coal, is a major source of carbonaceous aerosols and other air pollutants affecting health and climate. Using state-of-the-art emission inventories, a global three-dimensional photochemical tracer/transport model of the troposphere, and a global radiative transfer model based on methods presented in the latest IPCC Assessment Report (2007-AR4), we estimate the radiative forcing (RF) attributable to household fuel combustion in Asia in terms of current global annual-mean RF and future global integrated RF for a one-year pulse of emissions (2000) over two time horizons (100 and 20 years). Despite the signiﬁcant emissions of black carbon (BC) aerosols, these estimates indicate that shorter-lived (non-Kyoto) air pollutants from household fuel use in the region overall seem to exert a small net negative RF because of the strong inﬂuence of reﬂective aerosols. There are, however, major uncertainties in emission estimates for solid fuel burning, and about the sustainability of household fuel wood harvesting in Asia (the carbon neutrality of harvesting). In addition, there is still substantial uncertainty associated with the BC radiative forcing. As a result we ﬁnd that the sign of the RF from household biomass burning in the region cannot be established. While recognizing the value of integrating climate change and air pollution policies, we are concerned that for a ‘Kyoto style’ post-Kyoto treaty (with global cap-and-trade and the Global Warming Potential as the metric) expanding the basket of components with a selection of short-lived species without also including the wider range of co-emitted species may lead to unintended consequences for global-scale climate. Additional measurement, modelling, and policy research is urgently needed to reduce the uncertainties so that the net impact on climate of emissions and mitigation measures in this sector can be accurately assessed. Ó 2009 Elsevier Ltd. All rights reserved. Keywords: Solid fuel burning Asia Emissions Radiative forcing Air pollutants 1. Introduction Solid fuel combustion in households in developing countries is a major source of black carbon (BC), an aerosol that has been identiﬁed as both a major threat to public health and an important global warming agent. Mitigation policies targeted at household fuel combustion are among the least expensive options for BC abatement, thus making them especially appealing (Bond and Sun, 2005). Moreover, the atmospheric concentration of BC would respond quickly to any emission reductions because of its short lifetime (few days in contrast to over 100 years for CO2). However, BC is not currently q The corrected proof posted online August 27 has signiﬁcant changes from the uncorrected proof posted online August 5. * Corresponding author. Tel.: þ47 22 85 87 63/50; fax: þ47 22 85 87 51. E-mail address: [email protected] (K. Aunan). URL: http://www.cicero.uio.no 1352-2310/$ – see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.atmosenv.2009.07.053 included in the basket of greenhouse species regulated through the Kyoto Protocol to the United Nations Framework Convention on Climate Change. Calls for widening the scope of post-Kyoto climate treaties to include additional air pollutants such as aerosols and tropospheric ozone are based on the increasing evidence that these air pollutants play an important role in the climate system on a global as well as on a regional scale (Haywood and Shine, 1995; Ackerman et al., 2000; Ramanathan et al., 2001). Including components that are also a direct threat to public health is also expected to increase the relevance of the climate regime to developing countries such as China and India, presumably increasing the likelihood of their participation and thus the overall effectiveness of the regime (Hansen et al., 2000; Holloway et al., 2003; Streets and Aunan, 2005). The objective of the present study is to estimate the importance of emissions from solid fuel burning in Asia for current and future global warming, and discuss the case for targeting these emissions from a global warming perspective. Drawing upon our own results and those of other authors examining K. Aunan et al. / Atmospheric Environment 43 (2009) 5674–5681 the effects on climate of the full range of emissions from speciﬁc sectors (Fuglestvedt et al., 2008; Unger et al., 2008; Shindell et al., 2008), we discuss the case for expanding the basket of components in post-Kyoto climate regimes. BC is claimed to be the second or third most important individual anthropogenic warming agent after CO2 and possibly methane (Jacobson, 2002; Hansen and Nazarenko, 2004). Household combustion probably represents the second largest major source category of BC, after open biomass burning (of savanna, forests, and agricultural residues), representing about one quarter of global BC emissions (Bond et al., 2004). We argue in the following that it is highly uncertain, however, whether targeting BC emissions from the combustion of solid fuels in Asian households – which include coal and biomass fuels such as wood, charcoal, crop residues, and dung – will in aggregate achieve a net beneﬁt in climate terms because most likely it will be accompanied by changes in other atmospheric constituents, such as organic carbon (OC), sulfate, and ozone precursors, which also have impacts on climate. While the direct effect of BC on climate is a warming of the atmosphere through absorption of shortwave solar radiation, the direct effect of OC and sulfate aerosols is a cooling due to scattering of solar radiation. Aerosols also exert indirect and semi-direct aerosol effects and impacts on snow and sea-ice albedo (Hansen and Nazarenko, 2004; Jacobson, 2004). Ozone precursors as NOx, CO, and NMVOC (non-methane volatile organic components), contribute to a warming because tropospheric ozone acts as a greenhouse gas. NOx, however, also reduces the lifetime of the greenhouse gas methane, thus NOx emissions will through this effect imply a cooling. Here we estimate (with uncertainty ranges) the impact on the global climate of household fuel use in the Asian region. We model the RF exerted by the portfolio of air pollution constituents and the greenhouse gases CO2 and CH4, in terms of current top-of-theatmosphere (TOA) global, annual-mean RF (measured in Wm2). The estimated current RF values correspond to the global, annual-mean radiative forcing values for the period from pre-industrial to present as estimated, for instance, by the Intergovernmental Panel on Climate Change IPCC (Forster et al., 2007). This metric is ‘backward looking’, depicting the RF exerted today by present and historical emissions. More relevant for policy makers, however, is the ‘forward looking’ perspective of the integrated RF values, depicting the future integrated impact of a one-year pulse of emissions over a given time horizon. This is consistent with the perspective adopted in the Kyoto Protocol through the application of GWPs (Fuglestvedt et al., 2008). We quantify the impact of one-year’s emissions from Asian households in terms of the future global integrated RF over two time horizon of, respectively, 100 and 20 years. The results can be compared with corresponding estimates for global annual emissions provided by IPCC (Forster et al., 2007, see its Fig. 2.22). 2. Materials and methods 2.1. Emissions from household fuel use in Asia The study region stretches from Pakistan in the West to Japan in the East and from Mongolia in the North to Indonesia in the South (Streets et al., 2003). Solid fuels are the main household fuels used by the rural populations in the region, but also continue to be important for many poor people living in towns and cities. Biomass is the dominant fuel, with some 70% of the total energy used by households. Coal constitutes around 10%. The remaining share is predominantly other fossil fuels such as liqueﬁed petroleum gas (LPG), kerosene, and natural gas. Nearly all (97%) of the household coal burning in the region takes place in China, particularly in the northern parts of the country (Streets et al., 2003). The inventory for year 2000 includes BC, OC, SO2, NOx, CO, NMVOC (17 species), NH3, CH4, and CO2. Monthly emissions are 5675 given on a 1 by 1 grid and were interpolated to the 1.875 by 1.875 grid in the photochemical tracer/transport model applied. Emissions outside the Asia region are from the EDGAR database (Olivier et al., 2001), except for BC and OC where we rely on Bond et al. (2004). Regarding the fossil fuels, about 45% of the CO2 emissions from these fuels are from coal burning. Practically all aerosols emissions from fossil fuels are from coal, and only a small fraction (3–5%) of methane and NMVOC emissions are from noncoal fossil fuels. The uncertainties in the emission estimates are large, particularly for OC and BC. Generally, emissions are known least well in India, the rest of South Asia, and Southeast Asia, as described in Streets et al. (2003). Measured as 95% conﬁdence intervals, the uncertainty in total emissions from all sources are 16% (SO2), 31% (CO2), 37% (NOx), 65% (CH4), 130% (NMVOC), 185% (CO), 360% (BC), and 450% (OC). Uncertainty in published estimates for regional scale BC emissions is typically a factor of two to ﬁve (Ramanathan and Carmichael, 2008) and emissions estimates are regularly revised. Propagation-of-error analysis in Streets et al. (2003) showed that uncertainties in emissions from the residential sector are generally higher than from other sectors. The range depends on how well we know the amounts of fuel used and how well we know the combustion characteristics that determine the emission factors. A major source of uncertainty particularly in emissions of products of incomplete combustion (PIC) is the large variations in emission factors reported in laboratory studies, and the lack of realistic measurements in ﬁeld studies (emission factors applied are mainly based on standard cooking protocols in simulated kitchens (Smith et al., 2000)). Few ﬁeld studies in representative homes and regions of Asia are available. Recent work in Mexico and Honduras, however, indicates that emission factors based on tests in controlled settings, such as those used here, may substantially underestimate actual emissions under ﬁeld conditions (Johnson et al., 2007; Roden et al., 2009). Household fuel burning, particularly as biomass, is a dominating source of several air pollutants, especially for PIC, such as BC, OC, NMVOCs, and CO. According to the emission inventory applied, household emissions overall contribute to 66% and 65%, respectively, of the Asian region’s total anthropogenic emissions of BC and OC. Using the global estimates for 1996 from Bond et al. (2004) and for 2000 from Dentener et al. (2006), BC and OC emissions from households in the Asian region represent approximately one-ﬁfth of the total global emissions of BC and OC. Moreover, Asian households accounts for about 11% of the SO2 emissions in the region, and the sector’s share of emissions of ozone precursors in the region is 35% for NMVOC, 37% for CO, and 7% for NOx. Finally, the sector is responsible for 9% of the emissions of methane and 27% of the CO2 emissions. Keeping the issue of land use change and forestry outside the scope of this paper, we in the modeling assume zero CO2 emissions from household burning of biomass, i.e., a 100% renewable harvesting of fuel wood. With this assumption, the sector’s contribution to CO2 emissions in the region is reduced to 7%. According to Streets et al. (2003) much of the harvesting of biomass fuels in Asia is non-sustainable. The real value of net CO2 emissions from the use of ﬁrewood and charcoal is unknown (crop residues and dung can be considered CO2 neutral), however, mainly because reliable statistical data for harvesting practices in rural areas are lacking. A study by FAO (2007) in seven Asian countries (India not included and only Yunnan province of China) indicates that about 50% of the rural population has some degree of supply/ demand deﬁcit when it comes to fuel wood, with large variability between countries. Below we report sensitivity estimates for the assumption that 10% of the harvesting of biomass fuels was nonsustainable in 2000, which is the share assumed in the EDGAR emission inventory (Olivier et al., 2005). 10% non-sustainable 5676 K. Aunan et al. / Atmospheric Environment 43 (2009) 5674–5681 harvesting implies that assumed CO2 emissions from biomass fuels increase from zero to 210 Tg and the total CO2 emission from the household sector in the region increases with about 37%. 2.2. Radiative forcing We use two global models in our calculation – one for modeling the atmospheric burden of the constituents and one for modeling the RF exerted by this burden. We estimate the RF exerted by aerosols of BC, OC, and sulfate (the direct effect related to absorption or scattering of shortwave solar radiation by the aerosols), tropospheric ozone, and the RF from changes in the CH4 lifetime and concentrations induced by ozone precursors. The experimental design was to model the RF metrics from emissions in the Asian region with and without the household emissions included. The difference in RF metrics obtained in the two model runs gives estimates of the contribution from the household sector emissions. The uncertainty analysis assumes uncertainty in emissions as cited above for the total emissions in the region, uncertainty in modeling of atmospheric dispersal and removal, and RF. The procedure of the Monte Carlo simulation and the uncertainties assumed for modeling parameters and RF are given in Fuglestvedt et al. (2008), except for CH4 where we rely on Prather et al. (2009). For CO2 we include uncertainty in today’s emission, not in history (see separate sensitivity analysis for assumption about non-renewable harvesting). For BC and OC emissions we draw from a lognormal distribution. Air pollutants have a short atmospheric lifetime and emissions during one year will have no impact on RF the next year. When calculating current RF from household sector emissions of air pollutants we therefore need not consider historical emissions and simply model the RF exerted by one-year’s emissions (for 2000). Regarding CH4, which is relatively short-lived (about 12 years), the approach in principle is the same, but since CH4 has an atmospheric live-time longer than one year, we approximate by calculating the steady-state RF from one-year’s emissions (for 2000). Due to its long atmospheric lifetime, historical emissions of CO2, on the other hand, need to be included in order to depict how the sector contributes to climate forcing today. RF from CO2 was estimated by assuming that the CO2 emissions from the household sector have been proportional to world emissions since pre-industrial time (1750). The simulation of CO2 was performed using the modules of a simple climate model (Fuglestvedt et al., 2000; Fuglestvedt and Berntsen, 1999). The atmospheric concentration of CO2 was calculated using an efﬁcient and accurate representation of the carbon cycle developed by Joos et al. (1996). A global, three-dimensional (19-layer) photochemical tracer/ transport model of the troposphere, the Oslo CTM2 (Berntsen et al., 2006), was used to model the contribution from household fuels to concentrations of the different constituents in different atmospheric layers and the total column burden. A global radiative transfer model was applied to estimate the RF associated with the modeled changes in atmospheric burden. The radiative forcing model is documented in Myhre et al. (2007, 2003). The Oslo CTM2 has aerosol abundances close to the average of 16 models in the intercomparison exercise AeroCom (Textor et al., 2006). In the AeroCom exercise 9 global aerosol models using identical emission data provided RF results (Schulz et al., 2006). For the aerosol components of fossil fuel origin, the results from the models used in the present study were close to the average (within 15–20%) of the 9 models. For the biomassburning aerosols, the results from the models used here gave more negative RF values than the other models, which is likely linked to the large variation in the vertical proﬁle of the aerosols in the AeroCom models (Textor et al., 2006; Schulz et al., 2006). Treatment of hygroscopic effects, size distributions, and assumptions of mixing of the aerosols in the model used here are documented in Myhre et al. (2007). The model is compared to aircraft measurements of the aerosol vertical proﬁle (also in regions with high BC content), showing that Oslo CTM2 reproduces the main characteristics of the vertical proﬁle (Myhre et al., 2008, 2003). A crucial parameter for the magnitude and even the sign of the radiative forcing due to aerosols is the single scattering albedo. In Table 1 the values for single scattering albedo in the radiative forcing model applied here is shown for various aerosol components. For OC a fraction has been suggested to contain an absorbing component (Andreae and Gelencser, 2007; Lukacs et al., 2007), especially at short wavelengths (Jacobson, 1999; Sun et al., 2007). However, it has also been measured that for the main chemical components of water soluble OC pure scattering is more likely (Myhre and Nielsen, 2004). Internal versus external mixing of BC has been shown to be important for the RF and a simple method is recently suggested to account for the internal mixture (Bond et al., 2006). This method proposes to increase the absorption by 50% for the hydrophilic BC particles (but with no change for the hydrophobic BC particles). Using this approach in a global sensitivity test with the Oslo CTM2 resulted in an increased RF for BC of 28%. Below we report the estimates obtained by applying such an increase for the RF from BC. Note, however, that secondary organic aerosols (SOA), which are not included in our study, have a fraction of the total OC that is non-negligible in this region (Hoyle et al., 2007). Human activity has increased the amount of SOA mainly due to larger abundance of organic aerosol to partition at as well as an increase in the oxidation gases (Hoyle et al., 2009). Inclusion of SOA would have strengthened the negative RF due to aerosols. In Myhre et al. (2007) the aerosol optical depth, radiative effect, and absorption by the aerosols are compared to satellite-derived products. The model results showed good agreement with the satellite data, including over the oceanic regions around China and India. 3. Results 3.1. Net global radiative forcing The modeled current RF attributable to the household sector is shown in Fig. 1. Best estimates and uncertainty ranges for current RF and integrated RF over two time horizons are given in Tables 2 and 3. The current global annual-mean RF from the sector as a whole is estimated at 34 mWm2. If we include only the Kyoto gases (CO2 and methane), the estimated forcing from the sector is 46 mW m2, which corresponds to about 2% of the global annual-mean RF from greenhouse gases included in the Kyoto Protocol for the period from pre-industrial (1750) to present (about 2000). The relatively small difference (12 mW m2) between the RF from the Kyoto gases only and the estimated RF from all components is the sum of a relatively large negative forcing (56 mW m2) from scattering aerosols and shortening of methane’s atmospheric lifetime from NOx and other ozone precursors and a relatively large positive forcing (45 mW m2) from BC and tropospheric ozone. Aerosols alone (BC, OC and sulfates) is estimated to exert an RF of 17 mW m2. The negative aerosol forcing is in accordance with a study using nine global aerosol models (Schulz et al., 2006). Note that the GISS model used in the study by Koch et al. (2007), which reports a net positive RF from Table 1 Single scattering albedo (550 nm) for anthropogenic aerosols used in this study. Single scattering albedo (550 nm) Sulfate Black carbon, fossil and bio-fuel Organic carbon, fossil and bio-fuel Aerosols from biomass burning 1.0 0.21 1.0 0.91 K. Aunan et al. / Atmospheric Environment 43 (2009) 5674–5681 40 30 20 mW/m2 10 0 -10 CO2 CH4 Sulfate BC -20 OC O3 Total net RF CH4 (NOX) Total net RF -30 Household fossil fuels -40 Household biomass -50 60 CH4 CH4 (NOX) 40 O3 OC BC mW/m2 20 Sulfate CO2 0 Household biomass -20 Household fossil fuels 5677 household fuel burning to be positive (Unger et al., 2008). As the balance between ozone precursors affects the resulting estimate, the differing results may be related to the CO/NOx ratio being approximately 40% lower in the current paper than the ratio assumed for household fuels in 2030 in Unger et al. (2008). The best estimate for current RF from household biomass burning is close to zero, since the positive and negative RF values essentially cancel each other out. The RF from air pollutants alone (non-Kyoto components) is estimated to be 12 mW m2 for biomass fuel burning. The total RF from fossil fuels (35 mW m2) closely resembles the RF exerted by the Kyoto gases CO2 and methane alone for this emission source, as the net RF from all non-Kyoto components is close to zero (Fig. 1 and Table 2). Applying the calculated 28% increase in the BC RF as described above (to account for a potentially higher absorption by hydrophilic BC particles), would imply that net RF from biomass fuels is positive, but still the RF from all non-Kyoto components and for aerosols would be negative (for current RF 3 mW m2 for non-Kyoto components and 8 mW m2 for aerosols). Moving on to the integrated RFs as calculated for the two time horizons of 100 and 20 years the pattern remains the same as for the current RF values both when it comes to the net effect of the two fuel types, the net effect of the subgroups of components, and the uncertainty ranges (Table 3). Assuming a 10% non-sustainable harvesting practice for biomass fuels gives RF values for CO2 of 18 and 5 mW m2 yr for a 100 and 20 years time horizon, respectively, hence the net RF from biomass gets positive. The uncertainty ranges calculated in the Monte Carlo simulation were, as expected, broad (Tables 2 and 3). The large uncertainties in RF from particularly BC and OC imply that the net RF from biomass burning and, as a consequence, for the whole sector in total, cannot be reliably established, mainly due to uncertainties in the emissions. -40 3.2. Regional radiative forcing due to the direct effect of absorbing and scattering aerosols -60 40 30 20 mW/m2 10 0 -10 Biomass Kyoto -20 -30 -40 -50 Fossil fuels Kyoto Biomass nonKyoto Fossil fuels nonKyoto CH4 (NOX) O3 Sulfate OC CH4 BC CO2 -60 Fig. 1. Modeled global annual-mean RF since pre-industrial times in 2000 from emissions from household fuel combustion in Asia (mW m2) shown for each component (upper), for the two main types of fuels (middle), and for Kyoto components (long-lived) versus non-Kyoto components (short-lived) (lower). residential sector emissions of aerosols in the Asian region, is one of the models in Schulz et al. (2006) which, contrary to the majority of the models included, estimates a positive RF from anthropogenic aerosols over signiﬁcant areas in Asia, and, as a consequence, estimates the least negative RF from the Asian region as a whole. Other researchers have found the RF from the indirect methane effect from Fig. 2 shows the spatial distribution of modeled annual average RF from BC, OC, and sulfate aerosols, as well as their net value in the Asian region. According to our model results, the positive forcing caused by BC emissions in the most densely populated regions in China and India is heavily counteracted by the negative forcing exerted by OC and sulfate. Whereas both OC and sulfate aerosols are important over China, the negative forcing over India is almost exclusively caused by OC because biomass fuels dominate in this region. The direct effect on climate of absorbing and scattering aerosols is highly dependent on cloud distribution and the surface albedo. Moreover, season and location of emissions inﬂuence RF due to changing meteorology and solar radiation (Berntsen et al., 2006). Emissions from households show a clear seasonal pattern in the higher latitudes, including most of China, Mongolia and North and South Korea, with considerably larger emissions during winter due to the need for heating. The seasonal pattern becomes weaker as one moves southwards, and is absent in the southernmost areas (Fig. 3 and Fig. S1). The RF exerted by short-lived air pollutants from household fuel use in higher latitudes is affected by the fact that these emissions are at their highest in winter when less sunlight is available for absorption and scattering and the solar angle is low. Moreover, the chemical features prevailing in winter affect the burden of pollutants in the atmosphere. For instance, formation of O3, being a photochemical oxidant, is less effective in winter. Although SO2 concentrations from household emissions are signiﬁcantly higher in winter than in summer at high latitudes, this pattern is less clear for sulfate aerosol concentrations attributable to these emissions. This is due to the suppressed formation of sulfate in winter (Fig. S1). 5678 K. Aunan et al. / Atmospheric Environment 43 (2009) 5674–5681 Table 2 Best estimates with uncertainty ranges (68% conﬁdence intervals) of global annual-mean RF since pre-industrial times in 2000 from household fuel combustion in Asia (mW m2). CO2 Biomass fuels Fossil fuelsc Total a b c a – 33 (27–39) 33 (27–39) CH4 Sulfate BC OC O3 CH4 (indirect)b Sum all 12 (10–14) 1 (0.8–1.2) 13 (11–15) 2 (3 to 1) 6 (9 to 3) 8 (12 to 5) 26 (9–72) 6 (2–15) 32 (11–89) 39 (128 to 12) 1 (4 to 0) 40 (131 to 12) 8 (5–8) 6 (4–7) 13 (9–17) 5 (8 to 2) 4 (6 to 1) 8 (13 to 3) 0 (83 to 48) 35 (26–47) 34 (51 to 92) Assuming 100% renewable harvesting of biomass fuels. RF from NOx, CO, and NMVOC-induced changes in the methane lifetime and concentrations. For all except CO2 coal is the dominant contributor to each of the calculated RF values (see text). 4. Major limitations and uncertainties The global mean TOA RF has been generally accepted as a reasonably adequate metric for assessing how emissions affect mean temperatures on a global scale. There are still, however, unresolved questions related to the adequacy of comparing RF values for the short-lived components with those of long-lived. Moreover, as pointed out by others (Bond and Sun, 2005), the metric of global mean TOA RF is not sufﬁcient for capturing regional climatic impacts related to the short-lived components. Neither the impacts related to changes in regional climate nor ‘cloud burning’ (Ackerman et al., 2000), nor the indirect impact related to cloud enhancement, are captured in our estimates. We note that it is extremely difﬁcult to simulate the effects on clouds in climate models. The negative forcing related to the indirect effect through cloud enhancement is, however, likely to be larger than the potential net positive forcing from cloud burning, thus an attempt to include clouds effects would likely have pushed the RF estimates from aerosols in a negative direction. The large variation in emission factors due to different fuel/stove combinations and burning conditions and a lack of realistic measurements in the ﬁeld are among the large sources of uncertainties in a study like ours. Another major source of uncertainty relates to the fuel use data, particularly for rural populations. Together these two sources of uncertainty give broad uncertainty ranges for emission estimates, and subsequently for RF estimates. Of particular importance is the balance of absorbing and scattering aerosols. While the BC/OC ratio in household fuels use emissions in Streets et al. (2003) is around 0.25, this ratio is higher in comparable inventories, e.g., around 0.31 in Shindell et al. (2008) for 2001 and 0.39 in Zhang et al. (2009) for 2001. Based on in-ﬁeld measurements in rural homes in China, Li et al. (2009) report higher BC/OC ratios for fuel wood combustion than most previous studies, while the ratios for crop waste combustion are comparable to previous studies. The overall estimated BC/OC ratio for biomass combustion in Chinese households is 0.47, compared to 0.20 for biomass fuels in Asian households in Streets et al. (2003). A ratio higher that 0.3 would result in the net RF from BC and OC from biomass in Tables 2 and 3 becoming positive. SO2 emissions (the precursor of scattering sulfate aerosols), on the other hand, are lower in Streets et al. compared to the EDGAR inventory (Olivier et al., 2005) and in Dentener et al. (2006), but higher than in Shindell et al. (2008) and Zhang et al. (2009). An evaluation of the Streets et al. (2003) inventory by Tan et al. (2004) indicates that especially CO and carbonaceous particulates (all sources) may be severely underestimated (the underestimation was within the uncertainties ranges in Streets et al., however). Finally, a third major source of uncertainty relates to the parameterization of (partly not well understood) processes important for radiative forcing calculations in the models used. For instance, results from Ramanathan and Carmichael (2008) indicate that models being applied in the IPCC assessment reports (ours being one of them) generally underestimate the heating from BC aerosols. The differing estimates for BC heating in Ramanathan and Carmichael (2008) and in IPCC models is partly a result of the manner in which BC from biomass burning (including both open burning and contained combustion) and fossil fuel burning is treated. While IPCC models separate the BC from these two sources, in Ramanathan and Carmichael (2008) contributions from fossil fuel and biomass burning are added together. IPCC models separate the BC from these two sources since BC from biomass burning are associated with co-emissions of large amount of scattering organic aerosols, while this is not the case to the same extent for fossil fuel BC. The models applied in the IPCC assessment models differ when it comes to the vertical distribution of aerosols and assumptions about how aerosols are internally or externally mixed, which affects their heating estimates for BC. Above, we used a simpliﬁed methodology to tentatively estimate how our estimates change when internal mixing is taken into account. 5. Discussion and conclusions 5.1. Short-term and long-terms effects of emission reductions Household solid fuel burning is among the most promising targets for reduction of carbonaceous particles (Streets, 2007). However, the BC/OC ratio signature of emission reductions is of importance for their net impact on global RF. A detailed bottom-up analysis addressing, e.g., stove technologies and fuel properties is needed to draw conclusions about the climate impacts of individual control options within the sector at given points of time in the future. Control options, like improving stove efﬁciencies and switching to Table 3 Best estimates with uncertainty ranges (68% conﬁdence intervals) of global integrated RF from emissions in 2000 from household fuel combustion in Asia (mW m2 yr). CH4 Sulfate BC OC O3 CH4 (indirect)b Sum all Time horizon 100 years Biomass fuels –a 48 (39–57) Fossil fuelsc Total 48 (39–57) 13 (11–15) 1 (0.8–1.2) 14 (11–17) 2 (3 to 1) 6 (9 to 3) 8 (12 to 5) 26 (9–72) 6 (2–15) 32 (11–89) 39 (128 to 12) 1 (4 to 0) 40 (131 to 12) 6 (4–6) 5 (3–6) 10 (7–13) 4 (7 to 1) 3 (5 to 1) 7 (12 to 3) 1 (83 to 48) 49 (38–63) 48 (37 to 106) Time horizon 20 years Biomass fuels –a 14 (11–17) Fossil fuelsc Total 14 (11–17) 11 (9–13) 1 (0.8–1.2) 12 (10–14) 2 (3 to 1) 6 (9 to 3) 8 (12 to 5) 26 (9–72) 6 (2–15) 32 (11–89) 39 (128 to 12) 1 (4 to 0) 40 (131 to 12) 6 (4–9) 5 (4–6) 11 (7–15) 3 (6 to 1) 3 (4 to 1) 6 (10 to 2) 2 (84 to 47) 15 (9–26) 13 (70 to 72) CO2 a b c Assuming 100% renewable harvesting of biomass fuels (see text for sensitivity estimate). RF from NOx, CO, and NMVOC-induced changes in the methane lifetime and concentrations. For all except CO2 coal is the dominant contributor to each of the calculated RF values (see text). K. Aunan et al. / Atmospheric Environment 43 (2009) 5674–5681 5679 Fig. 2. Geographical distribution of annual-mean RF exerted by aerosols from household fuels in Asia. A: Total net RF from aerosols. B: RF from black carbon aerosols. C: RF from organic carbon aerosols. D: RF from sulfate aerosols. cleaner fuel types (e.g., coal briquettes to replace raw coal and pellets to replace crop residues) may alter the emission factors and the ratio between emitted species (Edwards et al., 2007), thus detailed information about the speciﬁc emission factors associated with alternative fuel/stove combinations and -improvements is needed. Using the modeled integrated RF from the Asian household sector emissions as estimated for two different time horizons, and acknowledging the limitations of the concept of global mean RF and the large uncertainties in the estimates, we draw tentative conclusions about the case for targeting household solid fuel use in a global warming context. On an aggregate level it seems that abatement of these emissions will have only a modest impact on global warming, as their net contribution to integrated RF seems not to be large for either time horizon. Biomass burning appears to be approximately climate neutral on a global aggregate level when a range of components are considered (and when renewability of harvesting is not accounted for, see above). The integrated RF is modeled to be close to zero (on the negative side) for both a 100 and a 20 years time horizon (Table 3). Overall reduced household burning of biomass seen in isolation, i.e., without considering how the energy will be replaced, could entail a short-term warming due to reductions in short-lived species exerting a net negative RF. However, as the response time of CH4 is also relatively short (about 12 years as compared to up to a few months for air pollutants), the cooling resulting from reduced CH4 could relatively soon cancel out the positive RF from the removed air pollutants. Of course, the energy source substituted for biomass will determine the magnitude of the climate effect of a fuel switch. An overall switch from biomass to coal in households could increase the net integrated RF considerably, primarily due to the increase in longlived components (CO2 and CH4). As long as sulfur is not removed from the coal, the negative forcing resulting from increased sulfate, OC and reduced lifetime of CH4 from NOx and other ozone precursors might balance the positive forcing from increased BC and O3 associated with a switch from biomass to coal. As technologies to reduce sulfur in coals are being employed to an increasing extent, however, a short-term warming is likely to result if a large-scale switch from 14 % South (Latitude 10-20) 12 % Middle (Latitude 20-30) 10 % North (Latitude 30-45) 8% 6% 4% 2% 0% Jan Feb Mar Apr Mai Jun Jul Aug Sep Okt Nov Des Fig. 3. Monthly distribution of percentage contribution of submicron PM (BC, OC, and sulfate) from household sector emissions to the total regional surface concentration of submicron PM (BC, OC, and sulfate) in Asia. 5680 K. Aunan et al. / Atmospheric Environment 43 (2009) 5674–5681 biomass to coal in households takes place (because the positive RF from air pollutants then may exceed the negative RF). Leapfrogging the coal rung on the ‘household energy ladder’ (Smith et al., 1994) to advanced combustion renewable biomass technologies for example is thus valuable from a global warming perspective. Reduction in residential coal burning seen in isolation, again without considering the fuel alternatives, contributes unambiguously to mitigating global warming for both time horizons according to our results. In contrast to emissions reductions in coal-ﬁred power plants in this part of the world, which imply a substantial short-term global warming due to reduced cooling from sulfate aerosols (Berntsen et al., 2006), a corresponding reduction in coal use in households would not have such an effect. We draw this conclusion because the net integrated forcing from non-Kyoto air pollutants is modeled to be around zero for fossil fuel burning in household stoves for both time horizons (Table 3). 5.2. Broadening the ‘Kyoto basket’ Broader participation and deeper emissions cuts are essential for any post-Kyoto climate treaty to have a signiﬁcant effect. Agreements to reduce the shorter-lived climate-active pollutants potentially provide attractive initial incentives for such engagement because reductions tend to pose less economic, technical, and political difﬁculties than CO2 controls, as well as providing signiﬁcant local co-beneﬁts in health and ecosystem protection. The current study and previous studies, e.g., by Fuglestvedt et al. (2008), Unger et al. (2008), and Shindell et al. (2008), however, demonstrate that the estimated global climate impact of abatement policies for short-lived species using models based on methods presented in the latest IPCC Assessment Report is heavily inﬂuenced by the selection of radiatively active species being taken into account. Based on these ﬁndings we conclude that for a ‘Kyoto style’ post-Kyoto treaty (with global cap-and-trade and GWPs as the metric) expanding the ‘‘Kyoto-basket’’ of components with a selection of short-lived species without also including the wider range of co-emitted species may send the wrong signal about the net impact on global climate of planned interventions. To include the full range of components and types of effects would, on the other hand, pose serious challenges in choosing appropriate metrics for measurement and comparison. An unresolved matter, for example, is how to deal with components exerting a negative forcing. Currently, the large uncertainties and methodological complexity regarding metrics for measuring climate effects of an extended number of components would complicate veriﬁcation and compliance, and thus potentially compromise feasibility (Rypdal et al., 2004). 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