Black carbon aerosol plays a unique and important role in Earth's climate system. Black carbon is a type of carbonaceous material with a unique combination of physical properties. This assessment provides an evaluation of black-carbon climate forcing that is comprehensive in its inclusion of all known and relevant processes and that is quantitative in providing best estimates and uncertainties of the main forcing terms: direct solar absorption; influence on liquid, mixed phase, and ice clouds; and deposition on snow and ice. These effects are calculated with climate models, but when possible, they are evaluated with both microphysical measurements and field observations. Predominant sources are combustion related, namely, fossil fuels for transportation, solid fuels for industrial and residential uses, and open burning of biomass. Total global emissions of black carbon using bottom-up inventory methods are 7500 Gg yr−1 in the year 2000 with an uncertainty range of 2000 to 29000. However, global atmospheric absorption attributable to black carbon is too low in many models and should be increased by a factor of almost 3. After this scaling, the best estimate for the industrial-era (1750 to 2005) direct radiative forcing of atmospheric black carbon is +0.71 W m−2 with 90% uncertainty bounds of (+0.08, +1.27) W m−2. Total direct forcing by all black carbon sources, without subtracting the preindustrial background, is estimated as +0.88 (+0.17, +1.48) W m−2. Direct radiative forcing alone does not capture important rapid adjustment mechanisms. A framework is described and used for quantifying climate forcings, including rapid adjustments. The best estimate of industrial-era climate forcing of black carbon through all forcing mechanisms, including clouds and cryosphere forcing, is +1.1 W m−2 with 90% uncertainty bounds of +0.17 to +2.1 W m−2. Thus, there is a very high probability that black carbon emissions, independent of co-emitted species, have a positive forcing and warm the climate. We estimate that black carbon, with a total climate forcing of +1.1 W m−2, is the second most important human emission in terms of its climate forcing in the present-day atmosphere; only carbon dioxide is estimated to have a greater forcing. Sources that emit black carbon also emit other short-lived species that may either cool or warm climate. Climate forcings from co-emitted species are estimated and used in the framework described herein. When the principal effects of short-lived co-emissions, including cooling agents such as sulfur dioxide, are included in net forcing, energy-related sources (fossil fuel and biofuel) have an industrial-era climate forcing of +0.22 (−0.50 to +1.08) W m−2 during the first year after emission. For a few of these sources, such as diesel engines and possibly residential biofuels, warming is strong enough that eliminating all short-lived emissions from these sources would reduce net climate forcing (i.e., produce cooling). When open burning emissions, which emit high levels of organic matter, are included in the total, the best estimate of net industrial-era climate forcing by all short-lived species from black-carbon-rich sources becomes slightly negative (−0.06 W m−2 with 90% uncertainty bounds of −1.45 to +1.29 W m−2). The uncertainties in net climate forcing from black-carbon-rich sources are substantial, largely due to lack of knowledge about cloud interactions with both black carbon and co-emitted organic carbon. In prioritizing potential black-carbon mitigation actions, non-science factors, such as technical feasibility, costs, policy design, and implementation feasibility play important roles. The major sources of black carbon are presently in different stages with regard to the feasibility for near-term mitigation. This assessment, by evaluating the large number and complexity of the associated physical and radiative processes in black-carbon climate forcing, sets a baseline from which to improve future climate forcing estimates.

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Bounding the role of black carbon in the climate system: A scientific assessment

[1] The Lagrangian particle dispersion model FLEXPART was used to construct a global data set of 1.4 million continuous trajectories. At the model start, particles were distributed homogeneously in the atmosphere and were then transported for 5.5 years using both resolved winds from European Centre for Medium-Range Weather Forecasts analyses and parameterized turbulent and convective transport. On the basis of this data set, a climatology of transport in and to the Arctic was developed. It was found that the time air resides continuously north of 70°N, called its Arctic age, is highest near the surface in the North American sector of the Arctic. North of 80°N and near the surface, the mean Arctic age of air is about 1 week in winter and 2 weeks in summer. It decreases rapidly with altitude to about 3 days in the upper troposphere. In the most isolated regions of the Arctic, air is exposed to continuous darkness for, on average, 10–14 days in December. Transport from the stratosphere to the lower troposphere is much slower in the Arctic than in the middle latitudes. In the central Arctic, for instance, the probability that air near the surface was transported from the stratosphere within 10 days is only about 1% in winter and 0.3% in summer. Air pollution can be transported into the Arctic along three different pathways: low-level transport followed by ascent in the Arctic, low-level transport alone, and uplift outside the Arctic, followed by descent in the Arctic. Only this last pathway is frequent for pollution originating from North America and Asia, whereas European pollution can follow all three pathways in winter, and pathways one and three in summer. Sensitivities of Arctic air masses to emissions of air pollutants, based on transport alone, were calculated for times of up to 30 days before the air masses reached the Arctic. They were highest over Siberia and Europe in winter and over the oceans in summer. Using an inventory for anthropogenic black carbon (BC) emissions, it was found that near the surface and for transport timescales of 5 and 10 days, BC source contributions from south Asia are only 1.6% and 10%, respectively, of the corresponding European values, despite much higher emissions in south Asia. Using an inventory for BC emissions from forest fires, BC source contributions to the Arctic, particularly from fires in Siberia, were larger than anthropogenic BC source contributions in summer in years of average burning.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2005JD006888

Characteristics of atmospheric transport into the Arctic troposphere

Regional climate change can arise from three different effects: regional changes to the amount of radiative heating that reaches the Earth’s surface, an inhomogeneous response to globally uniform changes in radiative heating and variability without a specific forcing. The relative importance of these effects is not clear, particularly because neither the response to regional forcings nor the regional forcings themselves are well known for the twentieth century. Here we investigate the sensitivity of regional climate to changes in carbon dioxide, black carbon aerosols, sulphate aerosols and ozone in the tropics, mid-latitudes and polar regions, using a coupled ocean–atmosphere model. We find that mid- and high-latitude climate is quite sensitive to the location of the forcing. Using these relationships between forcing and response along with observations of twentieth century climate change, we reconstruct radiative forcing from aerosols in space and time. Our reconstructions broadly agree with historical emissions estimates, and can explain the differences between observed changes in Arctic temperatures and expectations from non-aerosol forcings plus unforced variability. We conclude that decreasing concentrations of sulphate aerosols and increasing concentrations of black carbon have substantially contributed to rapid Arctic warming during the past three decades. The relative importance of regional and global changes in atmospheric greenhouse gas and aerosol concentrations for regional changes in climate is not well known. A climate model analysis of tropical, mid-latitude and polar regions shows that the extratropics and, in particular, the Arctic region are sensitive to local changes in radiative forcing.

Climate response to regional radiative forcing during the twentieth century

. The natural environment is a major source of atmospheric aerosols, including dust, secondary organic material from terrestrial biogenic emissions, carbonaceous particles from wildfires, and sulphate from marine phytoplankton dimethyl sulphide emissions. These aerosols also have a significant effect on many components of the Earth system such as the atmospheric radiative balance and photosynthetically available radiation entering the biosphere, the supply of nutrients to the ocean, and the albedo of snow and ice. The physical and biological systems that produce these aerosols can be highly susceptible to modification due to climate change so there is the potential for important climate feedbacks. We review the impact of these natural systems on atmospheric aerosol based on observations and models, including the potential for long term changes in emissions and the feedbacks on climate. The number of drivers of change is very large and the various systems are strongly coupled. There have therefore been very few studies that integrate the various effects to estimate climate feedback factors. Nevertheless, available observations and model studies suggest that the regional radiative perturbations are potentially several Watts per square metre due to changes in these natural aerosol emissions in a future climate. Taking into account only the direct radiative effect of changes in the atmospheric burden of natural aerosols, and neglecting potentially large effects on other parts of the Earth system, a global mean radiative perturbation approaching 1 W m−2 is possible by the end of the century. The level of scientific understanding of the climate drivers, interactions and impacts is very low.

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A review of natural aerosol interactions and feedbacks within the Earth system

This is an open access article published under a Creative Commons Attribution (CC-BY) License, which permits unrestricted use, distribution and reproduction in any medium, provided the author and source are cited. Review pubs.acs.org/CR Multiphase Chemistry at the Atmosphere−Biosphere Interface Inﬂuencing Climate and Public Health in the Anthropocene Ulrich Po schl* and Manabu Shiraiwa* Multiphase Chemistry Department, Max Planck Institute for Chemistry, 55128 Mainz, Germany air contaminants (SHCC) and their multiphase chemical interactions at the atmosphere−biosphere interface, including human lungs and skin, plant leaves, cryptogamic covers, soil, and aquatic surfaces. After an overview of diﬀerent groups of SHCC, we address the chemical interactions of reactive oxygen species and reactive nitrogen species (ROS, RNS), primary biological and secondary organic aerosols (PBA, SOA), as well as carbonaceous combustion aerosols (CCA) including soot, black/elemental carbon, polycyclic aromatic hydrocarbons, and related compounds (PAH, PAC). ROS and RNS interact strongly with other SHCC and are central to both atmospheric and physiological processes and their coupling through the atmosphere−biosphere interface, for example, in the formation and aging of biogenic and combustion aerosols as well as in CONTENTS inﬂammatory and allergic immune responses triggered by air pollution. Deposition of atmospheric ROS/RNS and aerosols 1. Introduction and Motivation can damage biological tissues, modify surface microbiomes, and 2. Health- and Climate-Relevant Air Contaminants induce oxidative stress through Fenton-like reactions and 2.1. Reactive Oxygen and Nitrogen Species immune responses. The chemical mechanisms and kinetics are 2.2. Primary Biological Aerosols not yet fully elucidated, but the available evidence suggests that 2.3. Secondary Organic Aerosols multiphase processes are crucial for the assessment, prediction, 2.4. Carbonaceous Combustion Aerosols and handling of air quality, climate, and public health. Caution 2.5. Other Air Contaminants Linking Atmospher- should be taken to avoid that human activities shaping the ic and Physiological Chemistry Anthropocene create a hazardous or pathogenic atmosphere 3. Multiphase Chemical Reactions at Speciﬁc Bio- overloaded with allergenic, corrosive, toxic, or infectious logical Interfaces contaminants. 3.1. Lung Lining Fluid Multiphase chemistry deals with chemical reactions, trans- 3.2. Human Skin port processes, and transformations between gaseous, liquid, 3.3. Plant Surfaces and Cryptogamic Covers and solid matter. These processes are essential for Earth system 3.4. Soil and Aquatic Surfaces science and climate research as well as for life and health 4. Conclusions and Outlook sciences on molecular and global levels, bridging a wide range Author Information of spatial and temporal scales from below nanometers to Corresponding Authors thousands of kilometers and from less than nanoseconds to Notes years and millennia as illustrated in Figure 1. Biographies From a chemical perspective, life and the metabolism of most Acknowledgments living organisms can be regarded as multiphase processes References involving gases like oxygen and carbon dioxide; liquids like water, blood, lymph, and plant sap; and solid or semisolid substances like bone, tissue, skin, wood, and cellular 1. INTRODUCTION AND MOTIVATION membranes. Even primitive forms of life and metabolic activity Multiphase chemistry plays a vital role in the Earth system, under anaerobic conditions generally involve multiple liquid climate, and health. Chemical reactions, mass transport, and and solid or semisolid phases structured by cells, organelles, and phase transitions between gases, liquids, and solids are essential membranes. 2 On global scales, the biogeochemical cycling of for the interaction and coevolution of life and climate. chemical compounds and elements, which can be regarded as Knowledge of the mechanisms and kinetics of these processes the metabolism of planet Earth, also involves chemical is also required to address societally relevant questions of global reactions, mass transport, and phase transitions within and environmental change and public health in the Anthropocene, that is, in the present era of globally pervasive and steeply Special Issue: 2015 Chemistry in Climate increasing human inﬂuence on planet Earth. 1 In this work, we review the current scientiﬁc understanding and recent advances Received: September 1, 2014 in the investigation of short-lived health- and climate-relevant Published: April 9, 2015 © 2015 American Chemical Society DOI: 10.1021/cr500487s Chem. Rev. 2015, 115, 4440−4475

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Multiphase chemistry at the atmosphere-biosphere interface influencing climate and public health in the anthropocene.

Several short-lived pollutants known to impact Arctic climate may be contributing to the accelerated rates of warming observed in this region relative to the global annually averaged temperature increase. Here, we present a summary of the short-lived pollutants that impact Arctic climate including methane, tropospheric ozone, and tropospheric aerosols. For each pollutant, we provide a description of the major sources and the mechanism of forcing. We also provide the first seasonally averaged forcing and corresponding temperature response estimates focused specifically on the Arctic. The calculations indicate that the forcings due to black carbon, methane, and tropospheric ozone lead to a positive surface temperature response indicating the need to reduce emissions of these species within and outside the Arctic. Additional aerosol species may also lead to surface warming if the aerosol is coincident with thin, low lying clouds. We suggest strategies for reducing the warming based on current knowledge and discuss directions for future research to address the large remaining uncertainties.

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D. Koch

Papers

Short-lived pollutants in the Arctic: their climate impact and possible mitigation strategies