Black Carbon : Impacts on Local, Regional and Global Environment and Climate (Invited)
Black Carbon is one of the unique pollutants that has a large direct negative impact on human health, indoor and outdoor air quality, temperature, cloudiness, precipitation, mountain glaciers, sea ice, and snow packs. We are just beginning to unravel its impact on all these scales and phenomena. The lack of access to fossil fuels forces more than 3 billion from rural populations to burn biomass fuels such as dung, firewood, and crops. The resulting pollution indoors and outdoors kills over 2 million people annually in developing nations. It also contributes to the so-called atmospheric brown clouds (ABCs), which eventually become transcontinental plumes, with large impacts on clouds and rainfall patterns and which also contribute to glacier melting. In the industrial world, fossil fuel combustion is a major source of black carbon and ABCs, which contribute to global warming and retreat of arctic sea ice. There is now a compelling, if not convincing, case to regulate or eliminate black carbon emissions. Such measures can reduce global warming, improve health, improve air quality, and slow down glacier retreat.
A reflection on the nature of combustion and the search for short-lived climate warmers (Invited)
The possibility of controlling emissions with high black carbon (BC) fractions is being discussed as a partial, immediate response to climate change. Reductions in absorbing aerosol could rapidly reduce warming in both the atmosphere and on snow surfaces in sensitive regions such as the Arctic and the Himalayas. We review a recent community effort, entitled “Bounding the Role of Black Carbon Aerosol in the Climate System.” The goal of this work was to provide confidence boundaries for climate forcing by black carbon. Including effects on liquid and ice clouds, our estimate of climate forcing by BC alone is strongly positive. Direct radiative forcing by aerosols and gases from many BC-rich sources is also positive with high confidence. These sources include older diesel engines, wood for cooking and heating, and even open biomass burning. Inclusion of cloud forcing components, including microphysical response, makes net climate forcing more negative and increases uncertainty for sources with large fractions of co-emitted aerosol species. With our present understanding of aerosol-cloud interactions, we do not have high confidence that net forcing is positive for any source. Confidence is lowest for sources with relatively large quantities of co-emitted aerosols or precursors. We close with a reflection on the evolution of combustion of solid and liquid fuels throughout the Industrial Era, and how these changes affect the balance of positive and negative climate forcing from BC-rich sources.
On the black carbon problem and its solutions
Black carbon (BC) warms air temperatures in at least seven major ways: (a) directly absorbing downward solar radiation, (b) absorbing upward reflected solar radiation when it is situated above bright surfaces, such as snow, sea ice, and clouds, (c) absorbing some infrared radiation, (d) absorbing additional solar and infrared radiation upon obtaining a coating, (e) absorbing radiation multiply reflected within clouds when situated interstitially between cloud drops, (f) absorbing additional radiation when serving as CCN or scavenged inclusions within cloud drops, and (g) absorbing solar radiation when deposited on snow and sea ice, reducing the albedos of both. Modeling of the climate effects of BC requires treatment of all these processes in detail. In particular, treatment of BC absorption interstitially between cloud drops and from multiply-dispersed cloud drop BC inclusions must be treated simultaneously with treatment of cloud indirect effects to determine the net effects of BC on cloud properties. Here, results from several simulations of the effects of BC from fossil fuel and biofuel sources on global and regional climate and air pollution health are summarized. The simulations account for all the processes mentioned. Results are found to be statistically significant relative to chaotic variability in the climate system. Over time and in steady state, fossil-fuel soot plus biofuel soot are found to enhance warming more than methane. The sum of the soots causes less steady-state warming but more short term warming than does carbon dioxide. Thus eliminating soot emissions from both sources may be the fastest method of reducing rapid climate warming and possibly the only method of saving the Arctic ice. Eliminating such emissions may also reduce over 1.5 million deaths worldwide, particularly in developing countries. Short term mitigation options include the targeting of fossil-fuel and biofuel BC sources with particle traps, new stove technologies, and rural electrification. However, the real solution, to be implemented over a 20-40 year period is complete conversion of the combustion infrastructure to electricity and electrolytic hydrogen, where the electricity is all produced by near-zero emitting wind, water, and solar (WWS) based energy technologies. Such a conversion would reduce BC and greenhouse gases simultaneously with cooling aerosol particles. This would ramp down the presence of both warming and cooling agents, but still cause net reduction of global warming, while reducing devastating health impacts that are occurring from both warming and cooling aerosols.
Assessing the Climatic Benefits of Black Carbon Mitigation
To limit mean global warming to 2 °C, a goal supported by more than 100 countries, it will likely be necessary to reduce emissions not only of greenhouse gases but also of air pollutants with high radiative forcing (RF), particularly black carbon (BC). Although several recent research papers have attempted to quantify the effects of BC on climate, not all these analyses have incorporated all the mechanisms that contribute to its RF (including the effects of BC on cloud albedo, cloud coverage, and snow and ice albedo, and the optical consequences of aerosol mixing) and have reported their results in different units and with different ranges of uncertainty. Here we attempt to reconcile their results and present them in uniform units that include the same forcing factors. We use the best estimate of effective RF obtained from these results to analyze the benefits of mitigating BC emissions for achieving a specific equilibrium temperature target. For a 500 ppm CO2e (3.1 Wm−2) effective RF target in 2100, which would offer about a 50% chance of limiting equilibrium warming to 2.5 °C above preindustrial temperatures, we estimate that failing to reduce carbonaceous aerosol emissions from contained combustion would require CO2 emission cuts about 8 years (range of 1-15 years) earlier than would be necessary with full mitigation of these emissions.
Source Attribution of Light-absorbing Aerosols in Arctic Snow (Invited)
Abstract Light-absorbing aerosols (LAA) deposited on the arctic snow pack, in particular black carbon (BC), contribute appreciably to the arctic radiation budget and their reduction has been suggested as a means to attenuate warming in the arctic. Effective prediction and mitigation of Arctic snow LAA requires that the sources of the LAA be elucidated. To this end, receptor modeling in the form of Positive Matrix Factorization (PMF) has been exercised on a data set of chemical concentrations in snow of various species (including inorganic and organic acids, carbohydrates and selected other organics as well as LAA) derived from an extensive set of snow samples from locations in Russia (including Siberia), Canada, Greenland, the Arctic Ocean and Svalbard. The data were obtained in three distinct periods: spring of 2007, spring of 2008, and spring of 2009. Data from each period were analyzed separately (note that the Svalbard data were analyzed only recently and were not included in the published 2007 analysis). Aerosol light absorption was determined spectrophotometrically at multiple wavelengths on filters through which melted snow was filtered. Based on the Angstrom exponent of the light absorption, partitioning of the absorption between BC and other LAA species was estimated. Statistics of the LAA concentrations for the Arctic as a whole and the geographic distribution of BC and other LAA species are presented. PMF analysis of the filtrate and filters from the 2007 data set from western Siberia, the Canadian lower arctic and Greenland revealed four factors or sources: two distinct biomass burning sources, a pollution source and a marine source. The first three of these were responsible for essentially all of the black carbon, with the two biomass sources together accounting for > 90% of the black carbon. Geographically, the biomass sources were dominant for all regions except the Arctic Ocean near the North Pole. For the 2008 and 2009 data sets, from eastern Siberia and Greenland, and the Canadian high Arctic respectively, inclusion of an additional tracer species (vanillin) permitted a more definitive differentiation of biomass sources between crop and grass burning and boreal biomass burning. As with the 2007 data set, four sources were found for each respective data set (crop and grass burning, boreal biomass burning, pollution and marine). For both data sets, the crops and grass biomass burning was the main source of both BC and other LAA species, suggesting the non-BC LAA was brown carbon. Mineral dust was present in many of the snow samples but its contribution to light absorption was typically a few percent or less based on the Fe content of the samples. Depth profiles at most of the sites allowed assessment of the seasonal variation in the source strengths. The biomass burning sources dominated in the spring but pollution played a more significant (though rarely dominant) role in the fall, winter and, for Greenland, summer.
Pole-to-Pole Observations of Long-Range Transport of Black Carbon Aerosol
Efforts are now underway to expand global measurements of black carbon (BC) aerosol mass loadings to better assess the impact of fossil fuel combustion and biomass burning sources of BC on global air quality and climate. Understanding the processes controlling BC aerosol in the atmosphere is necessary to constrain transport and microphysics in global aerosol models, evaluate climate impacts, and develop mitigation strategies. Yet measurements of BC above the surface have been very limited until recently and measurement-model comparisons of BC often show large discrepancies. Recent measurements from the HIAPER Pole-to-Pole Observations (HIPPO) study add new insight into the global distribution of BC and greenhouse gas species at finer spatial resolution than obtainable from satellite measurements with the goal of assessing emissions, transport timescales, and removal processes. The HIPPO campaigns include over 140 vertical profiles in the remote Pacific and polar regions from 0.3 to as high as 14 km altitude in each of three missions covering three seasons between 85°N and 67°S latitude. In the Arctic, highlights include observations of persistent stratified pollution from boreal autumn through spring. In the northern Pacific midlatitudes and subtropics, very polluted conditions were encountered over a deep portion of the troposphere with BC mass loadings varying between 100 and 1000 ng/kg in large-scale plumes from anthropogenic and biomass burning sources in Asia in boreal spring. Some of the first airborne observations of BC mass in the southern hemisphere show large interhemispheric gradients in boreal spring.
Sensitivity of Surface Air Quality and Global Mortality to Global, Regional, and Sectoral Black Carbon Emission Reductions
Black carbon (BC), a component of fine particulate matter (PM2.5) released during incomplete combustion, is associated with atmospheric warming and deleterious health impacts, including premature cardiopulmonary and lung cancer mortality. A growing body of literature suggests that controlling emissions may therefore have dual benefits for climate and health. Several studies have focused on quantifying the potential impacts of reducing BC emissions from various world regions and economic sectors on radiative forcing. However, the impacts of these reductions on human health have been less well studied. Here, we use a global chemical transport model (MOZART-4) and a health impact function to quantify the surface air quality and human health benefits of controlling BC emissions. We simulate a base case and several emission control scenarios, where anthropogenic BC emissions are reduced by half globally, individually in each of eight world regions, and individually from the residential, industrial, and transportation sectors. We also simulate a global 50% reduction of both BC and organic carbon (OC) together, since they are co-emitted and both are likely to be impacted by actual control measures. Meteorology and biomass burning emissions are for the year 2002 with anthropogenic BC and OC emissions for 2000 from the IPCC AR5 inventory. Model performance is evaluated by comparing to global surface measurements of PM2.5 components. Avoided premature mortalities are calculated using the change in PM2.5 concentration between the base case and emission control scenarios and a concentration-response factor for chronic mortality from the epidemiology literature.