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Cockroach Aerosols: Overview and Terms of Use

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An aerosol is a suspension of liquid or solid particles in a gas, with particle diameters in the range of 10−9 to 10−4 m.

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Vertical Profiling of Aerosol Optical Properties From LIDAR Remote Sensing, Surface Visibility, and Columnar Extinction Measurements


Atmospheric aerosols are generating from the Earth’s surface except from aviation emissions or meteorite debris. The height of aloft aerosol layers is a critical determinant of global aerosol transport and dispersion. Despite its importance in geo-locational characteristics of aerosols, aerosols vertical information is typically unknown. Aerosol vertical profile is an important parameter for understanding the radiative effects of aerosols and for generating more accurate aerosol models. With the development and improvement of the remote sensing techniques, the integrated dataset from both active and passive remote sensing techniques have been provided a comprehensive observation of aerosols and radiation which are urgently required for air quality and climatic study. Ground-based sensors or satellite observations are well-developed and used for measuring aerosol vertical profiles. In this chapter, measurement techniques for the aerosol vertical profiles is explained and discussed, which is estimated using integrated data from ground-based measurement, satellite observation, radiative transfer model (RTM) calculation.



Effects of Aerosols on Solar Radiation

Aerosols are suspensions of liquid and solid particles in the atmosphere, excluding clouds and precipitation. The aerosol particle sizes range from 10 −4 to 10 μm, falling under the following broad categories: sulfates, black carbon, organic carbon, dust, and sea salt. Aerosol concentrations and compositions vary significantly with time and location. Visibility measurements reflect the aerosol concentration at ground level. The visual range can vary from a few meters to 200 km, depending on the proximity to sources, the strength of the sources, and atmospheric conditions.

Aerosols scatter and absorb solar radiation. Sulfate aerosols scatter primarily solar radiation and cause cooling of the Earth–atmosphere system. The increase in the reflected solar radiation at the top of the atmosphere due to such nonabsorbing aerosols is nearly identical to the reduction in the solar radiation at the surface. Carbonaceous aerosols (black carbon and organics) absorb and scatter solar radiation. The presence of black carbon aerosols results in the absorption of solar radiation, which reduces the solar radiation reaching the surface. At the same time, these aerosols absorb the upward solar radiation reflected from below and reduce the solar radiation reflected to space. Therefore, the effect of black carbon aerosols opposes the cooling effect of other aerosols at the top of the atmosphere, whereas at the surface all aerosols reduce the solar radiation. The changes arising from the aerosol scattering and absorption of solar radiation are referred to as their direct radiative forcing. Aerosols can also modify the solar radiation through their role in cloud condensation and as ice nuclei, an effect known as aerosol indirect radiative forcing.

Aerosol particles in the atmosphere are produced both in nature and by people. A global aerosol optical depth of about 0.12 is suggested. These aerosols increase the reflected solar radiation at the top of the atmosphere by about 3 W m −2 globally. Anthropogenic sources contribute significantly to the global aerosol optical depth. Global anthropogenic emissions of sulfates, organics, and black carbon even exceed natural sources. Such a large perturbation of the global aerosol loading due to human activities may significantly modify regional and global climates.

Radiometry in the Optical Domain


Aerosols are defined as a combination of liquid or solid particles suspended in a gaseous or liquid environment. In the atmosphere, these particles are mainly situated in the low layers of the atmosphere ( 1.5 km) since aerosol sources are located on the terrestrial surface. However, certain aerosols can still be found in the stratosphere, especially volcanic aerosols ejected into the high altitude layers.

The origin of atmospheric aerosols is either natural or the result of anthropogenic activities. They are usually classified as a function of their composition and origin:

terrigenous aerosols or dust: these aerosols come from continental surfaces such as deserts or bare soil. They are mainly composed of clay, quartz, feldspar and calcite. They are injected into the atmosphere by the uplifting of particles on the ground due to wind. The smallest particles can travel distances higher than a thousand kilometers. These aerosols are found in the highest quantity in the atmosphere;

oceanic aerosols and sea spray: as their name indicates, these come from maritime regions. They are formed by waves (sea spray) and by the evaporation of water on the ocean surface;

soluble aerosols: this term designates a large number of aerosols, especially sulfates, which are very present in urban environments. Soluble aerosols also include nitrates. They mainly come from cities, industrial sites and vegetation;

combustion aerosols: these aerosols are a result of biomass and domestic fires, industrial emissions and traffic;

volcanic aerosols: these aerosols are mainly composed of sulfur dioxide emitted by volcanic eruptions and thrown into the stratosphere. As a result of their injection into this part of the atmosphere, they can progressively spread throughout the entire globe.

Atmospheric Aerosols and Their Role in Climate Change

1 Introduction

Atmospheric aerosols consist of solid/aqueous particles suspended in the atmosphere and are typically of sizes in the range 0.001 μm–10 μm. Aerosols are generated from a wide range of natural and anthropogenic sources. Natural aerosols include sulphate aerosols that are formed from emissions of sulphur dioxide from volcanic eruptions or from dimethyl sulphide emissions from phytoplankton in the ocean, sea salt particles emitted at the ocean surface, or mineral dust aerosol that is emitted by the effects of wind erosion on arid land. Anthropogenic aerosols, i.e. those that are generated from human activities, include sulphate, nitrate, black carbon and organic carbon aerosols from fossil fuel burning, deforestation fires and burning of agricultural waste.

Aerosols scatter and absorb sunlight and terrestrial radiation and therefore increased concentrations of atmospheric aerosols from anthropogenic activities such as fossil fuel burning and biomass burning lead to modifications of Earth’s energy balance and hence modulate climate. Atmospheric aerosols can also act as cloud condensation nuclei and can change the microphysical and radiative properties and hence the lifetime of clouds and further modulate climate. Furthermore, emissions of natural aerosols may be enhanced or reduced in response to changes in climate. For example, should future climate lead to a reduction in soil moisture in a particular area and consequently increase the spatial extent of arid areas, then mineral dust emissions may increase and dust particles will interact with sunlight and terrestrial radiation, further affecting climate. These responses to climate change are known as climate feedback mechanisms, and aerosols are explicitly coupled to the climate system through them.

Here we will describe the life cycle of aerosols in the lowest layer of the atmosphere known as the troposphere and present estimates of their current global distribution (stratospheric aerosols are presented in detail in Chapter 26 ). We will then use simple approximations to gain a physical understanding of the important parameters associated with atmospheric aerosols in quantifying aerosol-radiation interactions and aerosol-cloud interactions. The role of aerosols in climate feedback mechanisms will be discussed, and recent work on the potential role of aerosols in geoengineering schemes that aim to counterbalance the impacts of global warming will be presented.

Phase Separation

5.7.1 Aerosols

Aerosols are formed by three mechanisms: condensation, atomization, and entrainment. The relative sizes produced by these formation mechanisms are given in Fig. 5.7 .

Figure 5.7 . Aerosol types ( Brown et al., 1994 ).

Aerosols formed by condensation of a vapor into a liquid are the smallest and most difficult to remove contaminants having a size distribution in the range of 0.2–5 microns. Atomization creates aerosol drops by breaking up larger liquid drops through mechanical shear such as passing through a constriction in a valve under a high velocity. Atomization forms aerosols in the size range of 10–200 microns. Entrainment involves the movement of liquid slugs along pipelines, and here the liquid drop sizes are very large from 500 to 5000 microns. All three types of aerosol liquids are commonly found in gas systems. High-efficiency liquid–gas coalescers can effectively remove the fine aerosols created by the condensation mechanism.

AEROSOLS | Aerosol–Cloud Interactions and Their Radiative Forcing


Aerosols are essential for cloud formation. Every cloud droplet needs an aerosol particle, called cloud condensation nucleus (CCN), for activation. Likewise ice crystals either form on a subset of aerosol particles that act as ice nuclei (IN) or form by homogeneous freezing of supercooled solution drops (liquid aerosols that took up water). In the absence of aerosols, several 100% relative humidity (RH) would be necessary for cloud droplets to form by homogeneous nucleation from supersaturated water vapor. Due to the ubiquitous presence of CCN, the RH in water clouds hardly exceeds 101%. The situation is different for ice clouds because IN are sparse, and formation of cirrus clouds is dominated by homogeneous freezing of solution droplets. This takes place at relative humidities below 100% with respect to water but well above 100% with respect to ice (at −60 °C the RH for homogeneous freezing is 150%). At the same time, removal of aerosols by clouds and precipitation is the largest sinks for aerosols with diameters

Climate–Chemistry Interaction

13.4.1 The Role of Sulfate Aerosols as a Climatically-Active Compound

Sulfate aerosols , either naturally produced or anthropogenerated, have been studied extensively. The sulfur cycle involving production, chemical conversion, transport, and removal of sulfate aerosol exemplifies a typical lifecycle of trace chemicals in the atmosphere. Other types of aerosols may also experience a similar lifecycle, although without strong chemical conversions (such as mineral dust and BC), while other chemical conversions are too complicated and not fully understood (such as OAs). We use sulfate as an example to demonstrate aerosol climate–chemistry interactions, with an added note that, unlike mineral dust and BC aerosols, which have strong absorption of solar radiation, sulfate aerosols mainly reflect solar radiation.

AEROSOLS | Climatology of Tropospheric Aerosols


Atmospheric aerosols are solid and liquid particles in suspension in the air, ranging in size from a few nanometers to tens of microns (10 −9 –10 −5 m), with the exception of cloud droplets and ice crystals. Natural sources of aerosols include sea salt generated from breaking waves, mineral dust blown from the surface by wind, and organic aerosols from biogenic emissions. Artificial, also called anthropogenic, aerosols include sulfate, nitrate, and carbonaceous aerosols, and are mainly from fossil fuel combustion sources. The first aerosol studies were motivated by visibility, and in 1880 the British scientist John Aitken correctly postulated that aerosol particles act as nuclei for the condensation of water vapor to form fog and clouds. In the second part of the twentieth century, aerosol particles were linked to an increase in human respiratory diseases and acid rain, prompting the need to improve air quality by reducing emissions of aerosols and their precursors. At the same time, scientists started quantifying the impact of aerosols on the Earth’s energy budget and simple aerosol representations were introduced in weather forecast and climate models. Climatologies of aerosols in the troposphere have been developed over time, tailored to the diverse interests in aerosol properties. Climatologies of aerosol surface concentrations and chemical composition originate from air quality monitoring networks such as the European Monitoring and Evaluation Programme and the Clean Air Status and Trends Network of the United States Environmental Protection Agency. Dedicated instrumented aircraft campaigns provide regional snapshots of aerosol composition and size across the lower atmosphere, including information on aerosol microphysical properties and vertical profiles. Networks of remote sensing instruments have been deployed at selected sites worldwide, forming for example the Aerosol Robotic Network (AERONET). AERONET sun photometers provide measurements of the column-integrated extinction of visible or near-infrared radiation by aerosols, called optical depth, and these measurements can be inverted to provide an estimate of aerosol size. The launch of dedicated satellite instruments beginning in the late 1990s provides a global view of aerosol distributions of optical depth. More recently, vertical aerosol distribution has been routinely observed by lidars, located on the ground, aboard aircrafts, or on satellite platforms. In parallel to improved observational capabilities, numerical models have developed and provide daily aerosol forecasts, increasingly often initialized from assimilation of satellite observations, as well as centennial simulations for climate projections. Numerical models vary in the number of aerosol species they include and in the number of aerosol characteristics they are able to represent depending on the need for computational speed.

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In spite of those diverse sources of data, there are a number of difficulties associated with obtaining the aerosol climatology that fits a given application. By definition, aerosols encompass a large range of particle sizes and chemical compositions. Their sources are very diverse and unevenly distributed across the globe. Aerosols are removed from the atmosphere primarily by spatially inhomogeneous precipitation processes and, to a smaller extent, turbulence. Thus, aerosols have a relatively short residence time in the troposphere of typically up to 1 or 2 weeks. Consequently, aerosol concentrations vary greatly in time and space, both horizontally and vertically, in contrast to well-mixed greenhouse gases like carbon dioxide. Well-defined aerosol plumes can often be identified in observations, with aerosol number and mass varying by orders of magnitudes within a few kilometers in the horizontal and a few hundred or even a few tens of meters in the vertical across the plume. Measurements at the surface may therefore not be indicative of aerosol concentrations and properties higher up in the atmosphere. There are also several ways to characterize aerosols. Since aerosols are typically present in large numbers in the atmosphere, their population is described by its size distribution, which gives the number, surface area, or volume of the particles as a function of their radii. As depicted in Figure 1 , a typical tropospheric aerosol size distribution exhibits several maxima, called modes. The smaller particles, with radii smaller than 50 nm, belong to the nucleation and Aitken modes and provide most of the total aerosol number. The accumulation mode covers particles with radii up to 0.5 μm, which dominate the aerosol surface area. Both Aitken and accumulation modes are often called fine modes to contrast with the coarse mode, which comprises particles larger than 0.5 μm and makes up the bulk of aerosol volume. Minima in the size distribution arise from coagulation processes, where smaller particles of favorable sizes aggregate into larger particles. Air quality networks routinely report aerosol concentrations for particles with aerodynamic diameters smaller than 2.5 and 10 μm, labeled Particulate Matter (PM) 2.5 and PM10, respectively. Air quality regulations impose thresholds on both the annual average and daily average of PM concentrations in many countries. In Europe, daily average PM10 concentrations should not exceed 50 μg m −3 for more than 35 days per year. In newly industrialized countries, where air quality regulation is weaker, PM10 concentrations routinely exceed 100 μg m −3 on an annual average.

Figure 1 . Distribution of the volume of aerosol particles as a function of their radii, as sampled by the Facility for Airborne Atmospheric Measurements aircraft (black lines) and inverted from ground-based sun-photometer measurements of the Aerosol Robotic Network (gray lines). Panels gather measurements of generic aerosol types at different locations: (a) Industrial aerosol, (b) Mineral dust aerosol, (c) Biomass-burning aerosol, and (d) Marine aerosol.

Osborne, S.R., Haywood, J.M., 2005. Aircraft observations of the microphysical and optical properties of major aerosol species. Atmospheric Research 73, 173–201. © Crown.

Aerosols are also characterized by their production processes and chemical composition. Primary aerosols are emitted into the atmosphere directly as particles. They include the carbonaceous by-products of incomplete combustion processes and wind-blown mineral dust aerosols. In contrast, secondary aerosols originate from gas-to-particle conversions. Gaseous precursors, such as sulfur dioxide and ammonia, are oxidized in the dry atmosphere or in clouds and become soluble in water. Eventually, a large fraction of the emitted precursor enters the aerosol phase, dissolved in a droplet. As such, water represents a sizable fraction of the total aerosol mass. Gas-to-particle conversion processes also mean that aerosols are part of the chemistry of the atmosphere and indirectly affect the concentrations of gaseous species such as ozone and methane. Aerosol populations are typically of diverse chemical composition and their mixing state can be both internal, where the mixed composition lies within the same particle, and external, where each particle has a chemically distinct composition. Particle size can be a simplified marker of the production process. Fine-mode aerosols often originate from gas-to-particle conversion or combustion processes. Coarse-mode particles typically originate from mechanical processes such as wind friction. The shape of the particles also depends on the production process: aerosol solutions are spherical droplets. Combustion aerosols, grains of mineral dust, and pollens have more complex, irregular shapes.

Aerosols can finally be characterized by their natural or anthropogenic origin. This distinction is difficult to obtain from observations alone, but is important for climate studies. Aerosols emitted into the atmosphere by human activities since the start of the industrial revolution have scattered and absorbed additional solar and thermal radiation. They also have modified cloud droplet sizes and number, again affecting solar radiation, albeit indirectly. In effect, anthropogenic aerosols have changed the radiative balance of the Earth in a process called radiative forcing. Although uncertain, this forcing is estimated to be negative, and the climate system responds by cooling. Estimates for the year 2000 suggest that aerosol forcing compensates two-thirds of the positive forcing by long-lived greenhouse gases. Local climate changes driven by anthropogenic emissions can also affect emissions of natural aerosols, desertification being a typical example. In that case, the anthropogenic or natural classification of the aerosol is not obvious.

AEROSOLS | Role in Climate Change


Aerosols , the liquid and solid particles in suspension in the atmosphere, matter to the scientific study of climate change for three reasons. First, they are optically active species, interacting with solar and terrestrial radiation directly by scattering and absorption, and indirectly by modifying the microphysical properties of clouds and the radiative properties of surfaces covered by snow and ice. Perturbations to atmospheric distributions of aerosols therefore translate into perturbations of the Earth’s radiative budget, which can enhance or counteract the radiative perturbations exerted by changes in the concentrations of long-lived greenhouse gases such as carbon dioxide and methane.

Second, human activities have profoundly perturbed aerosol distributions. Since the start of the Industrial Revolution in the eighteenth century, road transport, aviation, maritime shipping, industries, power plants, domestic cooking stoves, and forest-clearing fires have all emitted aerosols and their gaseous precursors, increasing the amount of particles in the atmosphere and modifying their chemical composition. Figure 1 (a) shows a time series of globally averaged emissions of the main anthropogenic aerosol and precursor species since the year 1850. According to those estimates, anthropogenic emissions of sulfur dioxide, the gaseous precursor to sulfate aerosols, have risen from a global total of 2 Tg for the year 1850 to more than 120 Tg per year at the end of the twentieth century. Over the same period, primary emissions of carbonaceous aerosols from fossil fuel combustion, and emissions of ammonia, the precursor of nitrate aerosols, increased by a factor 3. Increases in emissions increase aerosol mass over the source region and transport pathways, as illustrated by Figure 1 (b), which shows changes in surface concentrations of sulfate aerosols over the industrial period. Increases do not cover the whole globe because strong sinks, dominated by washout by precipitation, prevent aerosols from accumulating in the atmosphere. The very heterogeneous distribution of aerosol concentrations and compositions in the atmosphere complicates the study of their climatic role compared to that of well-mixed and long-lived greenhouse gases such as carbon dioxide.

Figure 1 . (a) Left panel: Time series of global, annual anthropogenic emissions of sulfur dioxide, ammonia, and primary carbonaceous aerosols over the period 1850–2000 according to the CMIP5 data set. (b) Right panel: Modeled changes in column-integrated concentrations of sulfate aerosols, in mg[S] m −2 , due to changes in anthropogenic emissions of sulfur dioxide over the period 1850–2000.

The third and last reason why aerosols play an important role in climate change is the dependence of their sources and sinks on the state of the climate. As climate responds to an initial radiative perturbation, the distributions of natural and anthropogenic aerosols and their gaseous precursors change, giving rise to radiative feedbacks that either oppose or reinforce the initial radiative perturbation. The Introduction section of this article introduces the three important concepts of radiative forcing (RF), fast adjustments, and climate response, which are relevant to the multiple roles of aerosols in climate change. Section Instantaneous and Effective RF, and Climate Response then reviews the mechanisms through which aerosols perturb the Earth’s RF, while Section Mechanisms of Aerosol RF discusses how aerosols are in turn influenced by climate change. Finally, the proposed use of aerosols as tools for climate mitigation is discussed in Section Climate Response and Aerosol Feedbacks .

Remote Sensing of Aerosols From Space: Retrieval of Properties and Applications


Atmospheric aerosols are multicomponent mixtures typically composed of liquid and/or solid particles suspended in the atmosphere. Aerosols originate from numerous sources, having successively evolved through various microphysical processes before being removed either by wet or dry depositions. The implications of these airborne particulates on the regional and global climate are many but most notably through regulating the atmospheric heat budget either by absorbing/scattering insolation or by modifying cloud microphysical properties. Global distributions of aerosols are typically regional; thereby, they pose a strong regional signature that induces additional uncertainties in estimating aerosols, induced climate forcing. Satellite remote sensing of aerosols has extensive applications in identifying aerosol columnar properties, especially in terms of optical depth, composition, morphology, and vertical distribution. This ultimately provides evidence in establishing the source-transport-receptor relations of aerosols over a synoptic scale. Further, satellite data of atmospheric compositions are often used for identifying pollutant emissions, transboundary movement, forecasting air quality, and, more recently, in associating air quality with human health. The principles of remote sensing of aerosols are quite different from that of trace gases as the extinction of light by aerosols is a function of wavelength. With the gradual advancement of sensing technologies, monitoring of atmospheric aerosols has become more precise. Therefore, it has become more widely applied in various academic disciplines, studies, policies, and decision-making processes. This article emphasizes the state of the art in the field of satellite remote sensing, specifically in terms of polar-orbiting satellites for tropospheric aerosols including both active- and passive-based observations; associated complexities and uncertainties; brief descriptions of data products; and the subsequent applications in climate science.

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