Synonyms

Airborne particles; Dust; Particles; Particulate matter; PM; Smoke

Definition

Aerosols are solid or liquid particles suspended in the air, typically smaller in size than a twentieth the thickness of a human hair. There is a subtlety: In traditional aerosol science, “aerosol” refers to the particles and the medium in which they are suspended, whereas in remote sensing, the term often refers to just the particles. We use here the latter definition.

Introduction

Aerosols are of interest due to their impact on climate and health, as well as the role they might play in transporting nutrients and even disease vectors on planetary scales. Particles ranging in diameter from about 0.05 to 10 μm are studied most commonly, as they dominate aerosol direct interaction with sunlight, and are also thought to make up the majority of the aerosol mass. The particles are produced naturally by forest and grassland fires, volcanoes, desert winds, breaking waves, and emissions from living vegetation. Human activities, such as fossil fuel and agricultural burning and altering natural land surface cover, are estimated to contribute about 10 % to the global aerosol load, though these tend to concentrate near population centers where they can have both acute and long-term health consequences.

Aerosols are especially challenging to study because they originate from many, diverse sources and exhibit an enormous range of chemical compositions and physical properties. Unlike long-lived atmospheric gases, airborne particles are typically removed from the troposphere by precipitation or gravitational settling within about a week, so aerosol amount and type vary dramatically on many spatial and temporal scales. For this reason, the frequent global coverage provided by space-based remote sensing instruments has played a central role in the study of aerosols.

Aerosol remote sensing: the first global observations

The aerosol parameter most commonly derived from remote sensing data is aerosol optical depth. It is a measure of aerosol amount based on the fraction of incident light that is either scattered or absorbed by the particles. Formally, aerosol optical depth is a dimensionless quantity, the product of the particle number concentration, the particle-average extinction cross section (which accounts for particle scattering + absorption), and the path length through the atmosphere. It is usually measured along a vertical path. Particle physical and chemical properties, such as size, brightness, and composition, must also be either measured or assumed, to assess aerosol impact on climate and health. Measuring particle properties remotely remains a major challenge to the field.

Since late 1978, the Advanced Very High Resolution Radiometer (AVHRR) imagers have been collecting daily, global, multispectral data, from polar orbit. These instruments were designed primarily to observe Earth’s surface, and data analysis typically included an “atmospheric correction” aimed at eliminating surface feature blurring caused by the intervening gas and particles. However, over-ocean, total-column aerosol optical depth was also deduced, initially from single-channel observations, assuming a completely dark ocean surface in the red band (0.63 μm wavelength) and medium-sized, purely scattering particles.

Global, seasonal, and shorter-term aerosol distributions have been mapped. By associating observed particle concentrations with deserts, wildfire regions, and high-population areas on nearby land, dust, smoke, and aerosol pollution plumes were identified. The AVHRR maps showed that Saharan dust is routinely carried across the Atlantic Ocean and deposited in the Caribbean and, more generally, demonstrated the degree to which aerosols are carried long distances. The observations led to interest in the possible impact of aerosols on the energy balance and hence the climate of the planet, and the role desert dust might play in fertilizing iron-poor remote oceans and the Amazon basin.

Among the limitations of this early work was the inability to separate surface from atmospheric contributions to the observed scene brightness, which precluded retrievals over brighter coastal regions and most land. There was also a lack of direct information about particle properties in these retrievals, and an absence of constraints on vertical distribution, both of which are important for calculating aerosol impacts on climate and health. In addition, it was difficult to distinguish cloud from aerosol signals and, more generally, to assess the accuracy of the reported values. Enormous efforts and considerable progress were made in each of these areas over the subsequent 25 years.

One way to avoid significant surface contributions to the observed signal is to view the planet edge on, as was done by the Stratospheric Aerosol and Gas Experiment (SAGE) instruments, beginning in 1979. Observing the sun through the long slant path of the atmosphere as the satellite crossed to the night side of Earth, SAGE produced upper atmosphere vertical soundings, which proved immensely effective for monitoring the sulfate aerosols produced in the stratosphere by the Mt. Pinatubo eruption in 1991. But with no more than two occultations per orbit (about 30 per day), and limited ability to sound even as far down as the upper troposphere, other approaches were needed to address key aerosol-related climate and health questions.

Due to the intense scattering of ultraviolet (UV) light by atmospheric molecules, the surface is obscured when viewed from space in the wavelength range 330–380 nm. In the late 1990s, it was realized that the Total Ozone Mapping Spectrometers (TOMS), versions of which had already been orbiting for nearly 20 years, contained the spectral channels needed to retrieve aerosol amount over land as well as water, based on their ability to absorb the upwelling background UV light. This resulted in the Aerosol Index, a qualitative measure of UV-absorbing aerosols such as dust and smoke. The retrieval has limited sensitivity to near-surface aerosols and depends sensitively on the elevation of the particles and their optical properties, but the maps provided the first comprehensive, long-term record of aerosol source regions and overland transports.

One of the first and most widely used aerosol remote sensing techniques is surface-based sun photometry, which involves measuring the varying intensity of the solar disk as the sun changes elevation in the sky. The method predates satellite observations and relies on observing a systematic increase in atmospheric opacity (and the corresponding decrease in solar brightness) as the sun is viewed through longer atmospheric slant paths. Assuming aerosol horizontal homogeneity, column-integrated aerosol optical depth is retrieved, and if this is done at multiple wavelengths, some information about particle size can also be derived.

Beginning in 1991, with the European Space Agency (ESA) two-view-angle Along Track Scanning Radiometer (ATSR) series of imagers exploited the geometrically based approach from space. Unlike sun photometry, the satellite technique measures light scattered by the scene below, so additional assumptions about aerosol and surface optical properties are required to retrieve column-integrated aerosol optical depth. But the atmospheric contribution to the signal still increases systematically, relative to that of the surface, as the slant path increases, making surface-atmosphere separation possible. The steeper slant paths also improve sensitivity to thinner aerosol layers.

In 1996, the first of the French Space Agency (CNES) POLarization and Directionality of Earth’s Reflectance (POLDER) imagers began collecting multi-angle, multispectral polarization data from orbit. The polarization effects of many types of land surfaces are fairly independent of wavelength, making it possible to separate the more constant surface polarization contribution to the satellite signal from the spectrally varying atmospheric contribution. Aerosols are sometimes divided into two groups, depending on whether their effective diameter is greater or less than a certain size, usually taken to be around 1 μm for satellite observations and 2.5 μm for many health and direct sampling applications. The majority of smoke and aerosol pollution particles, the products of combustion and chemical processing, tend to fall into the smaller or “fine” mode. Mechanically produced desert dust and sea salt particles tend to be weighted toward the “coarse” mode. From its combination of optical measurements, POLDER maps column-integrated fine-mode and total aerosol optical depth over water, as well as fine-mode optical depth overland, and more advanced retrieval algorithms for POLDER data are under development.

Second-generation global measurements

The NASA Earth Observing System’s (EOS) Terra satellite, launched in late 1999 into a sun-synchronous orbit crossing the equator at about 10:30 local time, carries two instruments designed in part to make detailed aerosol measurements: the MODerate resolution Imaging Spectroradiometer (MODIS) and the Multi-angle Imaging SpectroRadiometer (MISR). MODIS follows the multispectral approach of the AVHRR instruments, but with higher spatial resolution (a maximum of 250 m, compared with up to 1 km for AVHRR), 36 spectral channels, and much higher radiometric calibration accuracy and stability. Total-column aerosol optical depth over global water as well as darker land surfaces is produced routinely every 2 days, along with fine-mode fraction over ocean. Thermal infrared channels are used to detect fires, which can help identify smoke plumes. Efforts are being made to extend the interpretation of MODIS data, for example, by using blue-band data to retrieve aerosol optical depth over bright surfaces (following the TOMS approach) and by developing semiempirical ways of deducing aerosol type from the combination of geographic location, season, and fine-mode fraction. A second copy of MODIS, flying on the EOS Aqua satellite, provides observations similar to MODIS/Terra, at about 1:30 equator crossing time.

MISR complements MODIS, acquiring four-channel, near-simultaneous multispectral views of Earth at nine angles and spatial sampling up to 275 m/pixel. Having a narrower swath than MODIS, MISR takes about a week to image the entire planet. With the multi-angle coverage, aerosol optical depth as well as surface reflectance are retrieved, even over bright desert surfaces. The nine views also sample light scattered in different directions, which yields some information about column-averaged particle size, shape, and brightness under good retrieval conditions; taken together, the retrieved information makes it possible to map aerosol air mass types, though not detailed particle microphysical properties. From the stereo viewing, MISR also derives the heights of clouds and aerosol plumes near their sources. The injection heights of wildfire smoke, volcanic effluent, and desert dust, produced by stereo imaging, are key quantities used in computer-based aerosol transport simulation models to predict plume evolution and aerosol climate impact.

The most accurate information about aerosol vertical distribution is obtained from lidars, which, unlike passive imagers that collect scattered sunlight, send out their own laser beam as the illumination source. Their ability to detect multiple, very thin aerosol layers, day and night, makes it possible for them to create a global, climatological picture of transported aerosols. But the lidar swath is the width of the laser beam, less than a 100 m in size, so coverage of specific sites or events is serendipitous. The technique was demonstrated in 1994, with the Lidar In-Space Technology Experiment (LITE) aboard NASA’s Space Shuttle Discovery, and has been followed by the Geoscience Laser Altimeter System (GLAS) and Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO) instruments on polar-orbiting EOS satellites. CALIPSO flies in a constellation of satellites called the “A-Train,” aimed at making complementary, near-coincident measurements. It includes Aqua carrying one MODIS instrument as well as PARASOL, and the Aura satellite bearing updated versions of POLDER and TOMS, respectively.

The second-generation measurements also represent a new era for assessing the accuracy of satellite aerosol retrieval results. Surface-based networks of autonomous sun photometers, such as the several-hundred-instrument AErosol RObotic NETwork (AERONET) federation, began producing very accurate, uniformly processed time series of aerosol column amount. The results are compared statistically and event by event with the satellite retrievals. By scanning across the sky in addition to making direct sun measurements, the surface stations collect the data needed to derive column-average particle size and brightness as well. Regional surface-based networks of radiometers also contribute to satellite retrieval validation, as well as surface lidar networks such as the NASA’s Micro Pulse Lidar Network (MPLNET) and European Aerosol Research LIdar Network (EARLINet).

Despite enormous advances in space-and ground-based aerosol remote sensing, some key measurements elude these techniques. Currently, detailed knowledge of particle composition, brightness, shape, and ability to absorb water is obtained only by collecting samples of the particles themselves. For near-surface aerosols such as urban pollution particles, surface stations are usually deployed, equipped to measure particle size and mass and to acquire samples for chemical analysis. But for more complete atmospheric characterization, intensive field campaigns are required, involving aircraft, satellites, and ground stations making coordinated observations, often with the help of predictions from aerosol transport models.

Aerosol environmental effects

Most aerosols scatter more than 90 % of the visible light falling upon them, whereas nearly 70 % of Earth’s surface is dark water. So overall, aerosols tend to cool the planet by making it more reflecting than it would otherwise be. Climate models play a central role in assessing the magnitude of this effect, as polar-orbiting satellites provide only snapshots of aerosol optical depth and limited information about their brightness, and aerosol data from geostationary satellites, which can make continuous observations of the sub-spacecraft region, are currently only qualitative. For radiative forcing calculations, models simulate the full diurnal cycle, along with uniform, global fields of aerosol amount, brightness, and vertical distribution. Remote sensing observations provide constraints on these models.

Current assessments suggest that the globally averaged aerosol direct radiative effect amounts to a cooling of about 0.1–0.9 W/m2, compared to the global warming by carbon dioxide of about 1.66 W/m2 (IPCC, 2007). Although the aerosol cooling might offset some greenhouse warming, aerosols, unlike long-lived greenhouse gases, are not distributed uniformly, so their regional effects are far more significant than the global mean. Preliminary assessments of observed trends in aerosol optical depth suggest that since the mid-1990s, aerosol particle pollution has decreased over Europe and eastern North America, whereas it has increased over east and south Asia, and on average, the atmospheric concentration of low-latitude smoke particles has also increased.

In addition to the direct effect they have on sunlight, aerosols play a role in the formation of clouds. Collecting the water molecules needed to make cloud droplets is accomplished with the help of aerosols, called cloud condensation nuclei (CCN). The concentration of CCN in the droplet-formation regions of clouds mediates the number of droplets that form. If a fixed amount of cloud water is divided into more droplets of smaller size, the overall reflectivity of the cloud will be greater. The “first indirect effect” of aerosols on clouds refers to the way increased CCN can lead to increased cloud reflectivity. As smaller-sized droplets are less likely to coalesce into raindrops and precipitate, smaller droplets can also increase cloud lifetime, increasing global reflectivity and slowing the cycling of water through the atmosphere, a process often called the second indirect effect.

Aerosols can weaken or strengthen clouds in other ways as well. If dark aerosols such as soot or smoke are present, they can absorb sunlight and evaporate cloud droplets. Aerosols dissolved in cloud droplets may postpone freezing as droplets are carried aloft in the cores of some types of clouds, invigorating their development. Phytoplankton in ocean surface water might actively regulate cloud amount by modulating their output of gaseous sulfur compounds, which in turn form sulfate aerosols in the atmosphere and serve as CCN in aerosol-poor skies over remote oceans. It is difficult to test these hypotheses on large scales with currently available measurements, and therefore to assess their environmental impacts, but observations indicate that such mechanisms must at least be considered as part of the global picture.

The magnitude of aerosol indirect effects on clouds is much less certain that that of aerosol direct radiative effects, in part because detailed particle size and composition, which determine their ability to absorb water, cannot be measured remotely with sufficient accuracy. In addition, most CCN are smaller than 0.05 μm, too small to be distinguished from atmospheric gas molecules by remote sensing techniques. The current consensus model-based estimate for global-average indirect effects is cooling of 0.3–1.8 W/m2, but with low confidence (IPCC, 2007). Narrowing the uncertainties is likely to require a combination of detailed chemistry and microphysics from aircraft measurements, satellite observations to provide broad statistical sampling, and modeling to generate an overall result.

To date, remote sensing has contributed to our knowledge of aerosol impacts on human health only qualitatively, identifying, with the help of aerosol transport models or lidar profiles, regions where near-surface particle concentrations are especially high. Surface stations are typically more effective in isolating the near-surface aerosols that matter most for these studies, and direct samples are required to provide detailed particle size and chemical composition. However, the spatial coverage of surface stations is very limited. As with climate effects, the future seems to point toward combining satellite and suborbital measurements with models.

Summary

Our appreciation of aerosols’ role in climate change has grown over the past 25 years, in part due to the contributions made by remote sensing. First estimates of the impacts transported aerosols have on the atmospheric energy balance, on clouds and the hydrological cycle, on larger-scale atmospheric circulation, and on human health have been made. An understanding has developed for the need to combine detailed physical and chemical measurements from aircraft and ground stations and extensive constraints on aerosol optical depth, type, and vertical distribution from satellites, with numerical models that can simulate present and predict future conditions.

However, much remains to be done. For planning purposes, the accuracy of measurements needed to assess aerosol direct radiative effect must be improved, and uncertainties in aerosol indirect effects on clouds must be reduced. Techniques for systematically constraining models with satellite and suborbital data need to be developed, both to test model parameterizations of aerosol sources, cloud processes, etc., and to assess the uncertainties in the resulting simulations. Based on past experience, this can be achieved, provided we continue to develop and deploy the instruments, improve the models, and maintain the research community, which have carried the field to this point.