Interaction of internally mixed aerosols with light (original) (raw)

Aerosol Properties and Their Impacts on Climate

2008

This report critically reviews current knowledge about global distributions and properties of atmospheric aerosols, as they relate to aerosol impacts on climate. It assesses possible next steps aimed at substantially reducing uncertainties in aerosol radiative forcing estimates. Current measurement techniques and modeling approaches are summarized, providing context. As a part of the Synthesis and Assessment Product in the Climate Change Science Program, this assessment builds upon recent related assessments, including the Fourth Assessment Report of the Intergovernmental Panel on Climate Change (IPCC AR4, 2007) and other Climate Change Science Program reports. The result provides a synthesis and integration of current knowledge about climate-relevant anthropogenic aerosol impacts for policy makers, policy analysts, as well as the general public. ES 1. Aerosols and Their Climate Effects ES 1.1. Atmospheric Aerosols Atmospheric aerosols are suspensions of solid and/or liquid particles in air. Aerosols are ubiquitous in air and are often observable as dust, smoke, and haze. Both natural and human processes contribute to aerosol concentrations. On a global basis, aerosol mass derives predominantly from natural sources, mainly sea-salt and dust. However, anthropogenic (manmade) aerosols, arising primarily from a variety of combustion sources, can dominate in and downwind of highly populated and industrialized regions, and in areas of intense agricultural burning. The term "atmospheric aerosol" encompasses a wide range of particle types having different compositions, sizes, shapes, and optical properties. Aerosol loading, or amount in the atmosphere, is usually quantified by mass concentration or by an optical measure, aerosol optical depth (AOD). AOD is the vertical integral through the entire height of the atmosphere of the fraction of incident light either scattered or absorbed by airborne particles per unit length. Usually numerical models and in situ observations use mass concentration as the primary measure of aerosol loading, whereas most remote sensing methods retrieve AOD. ES 1.2. Radiative Forcing of Aerosols Aerosols affect Earth's energy budget by scattering and absorbing radiation (the "direct effect") and by modifying amounts and microphysical and radiative properties of clouds (the "indirect effects"). Aerosols influence cloud properties through their role as cloud condensation nuclei (CCN) and/or ice nuclei. Increases in aerosol particle concentrations may increase the ambient concentration of CCN and ice nuclei, affecting cloud properties. A CCN increase can lead to of-atmosphere (TOA). Applying various methods using MODIS, MISR and the Clouds and Earth's Radiant Energy System (CERES), the aerosol direct RF derived above ocean converges to-5.5±0.2 W m-2. Here, the uncertainty is the standard deviation of the various methods, indicating close agreement between the methods and satellite data sets. However, regional comparisons of the various methods show greater spread than the global mean. Estimates of direct radiative forcing at the ocean surface, and at top and bottom of the atmosphere over land, are also reported, but are much less certain. These measurement-based estimates are calculated for cloud-free conditions and comparing to an aerosol-free atmosphere. Although there are no proven methods exist for measuring the anthropogenic component of the observed aerosol over broad geographic regions, satellite retrievals are able to qualitatively determine aerosol type under some conditions. From observations of aerosol type, the best estimates indicate approximately 20% of the AOD over the global oceans is a result of human activities. Following from these estimates of anthropogenic fraction, the cloud-free anthropogenic direct radiative forcing at top of atmosphere is approximated to be-1.1±0.4 Wm-2 over the global ocean, representing the anthropogenic perturbation to today's natural aerosol. ES 2.2. Assessments of Aerosol Indirect Radiative Forcing Remote sensing estimates of aerosol indirect forcing are still very uncertain. Even on small spatial scales, remote sensing of aerosol effects on cloud albedo do not match in situ observations, due to a variety of difficulties with the remote sensing of cloud properties at fine scales, the inability of satellites to observe aerosol properties beneath cloud base, and the difficulty of making aerosol retrievals in cloud fields. Key quantities such as liquid water path, cloud updraft velocity and detailed aerosol size distributions are rarely constrained by coincident observations. Most remote sensing observations of aerosol-cloud interactions and aerosol indirect forcing are based on simple correlations among variables, which do not establish cause-and-effect relationships. Inferring aerosol effects on clouds from the observed relationships is complicated further because aerosol loading and meteorology are often correlated, making it difficult to distinguish aerosol from meteorological effects. As in the case of direct forcing, the regional nature of indirect forcing is especially important for understanding actual climate impact. ES 3. Model Estimated Aerosol Radiative Forcing and Its Climate Impact Just as different types of aerosol observations serve similar purposes, diverse types of models provide a variety of approaches to understanding aerosol forcing of climate. Large-scale Chemistry and transport models (CTMs) are used to test current understanding of the processes controlling aerosol spatial and temporal distributions, including aerosol and precursor emissions, chemical and microphysical transformations, transport, and removal. CTMs are used to describe the global aerosol system and to make estimates of direct aerosol radiative forcing. In general, CTMs do not explore the climate response to this forcing. General Circulation Models (GCMs), sometimes called Global Climate Models, have the capability of including aerosol processes as a part of the climate system to estimate aerosol climate forcing, including aerosol-cloud interactions, and the climate response to this forcing. Another type of model represents atmospheric processes on much smaller scales, such as cloud resolving and large Eddy simulation models. These small-scale models are the primary tools for improving understanding of aerosol-cloud processes, although they are not used to make estimates of aerosol-cloud radiative forcing on regional or global scales. ES 3.1. The Importance of Aerosol Radiative Forcing in Climate Models The IPCC AR4 reported estimates of surface temperature change due to anthropogenic forcing by greenhouse gases and aerosols from more than 20 participating global climate models. Despite a wide range of climate sensitivity (i.e. the amount of surface temperature increase due to a change in radiative forcing, such as an increase of CO 2) employed by the models, they all yield a global average temperature change similar to the observed change. This agreement across models appears to be a result of the use of very different aerosol forcing values, which compensate for the range of climate sensitivity. For example, the direct cooling effect of sulfate aerosol varied by a factor of six among the models. Even greater disparity was found in the model treatment of black carbon and organic carbon. Some models ignored aerosol indirect effects whereas others included large indirect effects. In addition, the aerosol effect on cloud brightness (reflectivity) varied by up to a factor of nine among models for those models that included the effect. ES 3.2. Modeling Atmospheric Aerosols Simulations of the global aerosol distribution by different models show good agreement in their representation of the global mean AOD, which in general also agrees with satellite-observed values. However, large differences exist in model simulations of regional and seasonal distributions of AOD, and in the proportion of aerosol mass attributed to individual species. Each model uses its own estimates of aerosol and precursor emissions and configurations for chemical transformations, microphysical properties, transport, and deposition. Multi-model experiments indicate that differences in the models' atmospheric processes play a more important role than differences in emissions in creating the diversity among model results. Although aerosol mass concentration is the basic measure of aerosol loading in the models, this quantity is translated to AOD via mass extinction efficiency in order to compare with observations and then to estimate aerosol direct RF. Each model employs its own mass extinction efficiency based on assumed optical and physical properties of each aerosol type. Thus, it is possible for the models to produce different distributions of aerosol loading as mass concentrations but agree in their distributions of AOD, and vice-versa. Model calculated total global mean direct anthropogenic aerosol RF at TOA, based on the difference between pre-industrial and current aerosol fields, is-0.22 W m-2 , with a range from-0.63 to +0.04 W m-2. This estimate does not include man-made contributions of nitrate and dust, which could add another-0.2 W m-2 estimated by IPCC AR4. The mean value is much smaller than the estimates of total greenhouse gas forcing of +2.9 W m-2 , but the comparison of global average values does not take into account immense regional variability. Over the major sources and their downwind regions, the model-calculated negative forcing from aerosols can be comparable to or even larger than the positive forcing by greenhouse gases. ES 3.3. Aerosol Effects on Clouds Large-scale models are increasingly incorporating aerosol indirect effects into their calculations. Published large-scale model studies report calculated global cloud albedo effect RF at top-ofatmosphere ranging from-0.22 to-1.85 W m-2 with a central value of-0.7 W m-2 .. Numerical experiments have shown that the...

Scattering and absorbing aerosols in the climate system

Nature Reviews Earth & Environment

Aerosols are small liquid or solid particles suspended in the atmosphere 1. They can be emitted directly (such as dust, sea salt, black carbon (BC) and volcanic aerosols) or formed indirectly through chemical reactions (including sulfate, nitrate, ammonium and secondary organic aerosols). Owing to their relatively short lifetime, aerosol concentrations typically peak near their sources. Desert regions (such as North Africa and the Middle East), industrial regions (such as East and South Asia) and biomass-burning regions (such as South America and South Africa) are, therefore, characterized by high mass concentrations (Fig. 1). Aerosols exhibit complicated compositions and vary substantially in shape and size, typically ranging between 0.01 and 10 μm (reF. 2). Depending on these structural and compositional characteristics, aerosols can scatter and/or absorb shortwave radiation, as quantified through the single-scattering albedo (SSA; Table 1). Purely scattering aerosols include sulfates, nitrates, ammonium and sea-salt particles, whereas absorbing aerosols are primarily BC, with dust and organic carbon partly absorbing in the ultraviolet (UV) spectrum 3. Aerosols have a direct bearing on Earth's energy balance and, therefore, on climate. For instance, aerosol scattering and absorption alters the radiation balance and atmospheric stability through perturbations to the vertical temperature profile. Aerosols can further serve as cloud condensation nuclei (CCN) or ice-nucleating particles (INPs), which modify the reflectivity and lifetime of clouds through microphysical processes. Collectively, these influences are quantified as aerosol forcing: the change of net radiative flux at a specified level of the atmosphere, often assessed relative to estimated pre-industrial conditions 4. Globally, anthropogenic aerosols are estimated to produce a net cooling ~−1.3 ± 0.7 W m −2 at the top of the atmosphere; −0.3 ± 0.3 W m −2 is attributed to the aerosol-radiation interaction (ARI), −1.0 ± 0.7 W m −2 to aerosol-cloud interactions, ~−1.15 W m −2 to total forcing from scattering aerosols and ~+0.12 W m −2 to BC 4. This combined aerosol forcing offsets roughly one-third of the warming from anthropogenic greenhouse gases (GHGs). However, the large spread in the estimated aerosol forcing leads to large discrepancies in climate sensitivity 5,6. Thus, aerosols are considered to be the largest contributor of uncertainty in quantifying present-day climate change 4. Much of this uncertainty in aerosol forcing arises from both the lack of separate global constraints on aerosol optical and microphysical properties (optical depth, size distribution, hygroscopicity and mixing state, among others) and the inaccurate representation of them in climate models 7-10. In particular, aerosol SSA is further

Atmospheric aerosols in the earth system: a review of interactions and feedbacks

Atmospheric Chemistry and Physics Discussions, 2009

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 forcings are potentially several Watts per square metre due to changes in these natural aerosol emissions in a future climate. The level of scientific understanding of the climate drivers, interactions and impacts is very low.

A review of potential radiative effect of aerosol on climate

2018

The study of physical and chemical properties of aerosol is of significant importance, because their radiative effects exert strong impact on Earth’s climate. Aerosols scatter and absorb solar radiation. Backscattering of solar radiation towards space results loss in surface reaching solar radiation leads to cooling of the climate system. Absorption of solar radiation is associated with heating within the aerosol layer, thereby modifies the vertical temperature profile, and this also results loss in surface reaching solar radiation. Such processes alter the radiative balance of Earth directly so-called direct effects. A subset of aerosols also alters the radiative balance of the Earth by modifying microphysical and radiative properties of clouds via so-called indirect effects. Based on observations and models studies present work suggest that the regional radiative perturbations are several Wm -2 due to changes in aerosol emissions. Furthermore, if the black carbon emission is check...

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

Atmospheric Chemistry and Physics, 2010

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.

AEROSOL, CLOUDS, AND CLIMATE CHANGE

2005

Earth's climate is thought to be quite sensitive to changes in radiative fluxes that are quite small in absolute magnitude, a few watts per square meter, and in relation to these fluxes in the natural climate. Atmospheric aerosol particles exert influence on climate directly, by scattering and absorbing radiation, and indirectly by modifying the microphysical properties of clouds and in turn their radiative effects and hydrology. The forcing of climate change by these indirect effects is thought to be quite substantial relative to forcing by incremental concentrations of greenhouse gases, but highly uncertain. Quantification of aerosol indirect forcing by satellite-or ground-based remote sensing has proved quite difficult in view of inherent large variation in the pertinent observables such as cloud optical depth, which is controlled mainly by liquid water path and only secondarily by aerosols. Limited work has shown instances of large magnitude of aerosol indirect forcing, with local instantaneous forcing upwards of 50 W m -2 . Ultimately it will be necessary to represent aerosol indirect effects in climate models to accurately identify the anthropogenic forcing at present and over secular time and to assess the influence of this forcing in the context of other forcings of climate change. While the elements of aerosol processes that must be represented in models describing the evolution and properties of aerosol particles that serve as cloud condensation particles are known, many important components of these processes remain to be understood and to be represented in models, and the models evaluated against observation, before such model-based representations can confidently be used to represent aerosol indirect effects in climate models.