Aerosols in the Atmosphere

By Maricela Reyes

This paper was written as a reference to provide scientific background to aid the Aerosols team in their research.

Introduction

When solar radiation passes through the atmosphere, it is absorbed and scattered, not only by atmospheric gases, but also by aerosols and clouds. The Aerosols research project is concerned with using measurements of solar radiation to infer properties of aerosols.

What is an Aerosol?

Aerosols are defined as suspensions of liquid or solid particles in the air, excluding cloud droplets and precipitation. The mean radii of aerosol particles range from about 10^-4 to 10 microns (µm). Aerosol particles in the atmosphere are mainly due to two processes:

1. Direct injection into the atmosphere such as the formation of dust, soot and sea salt particles from human and/or natural process; and
2. Chemical reactions of gaseous materials within the atmosphere, such as the transformation of SO₂ into HSO₄ or sulfates, NO_x into nitric acids, etc.

Aerosols have different sizes. They can be classified according to their origin, size, and atmospheric distribution. The very small particles with mean radius between 0.001 and 0.1 microns are called Aitken particles. Particles with radii between 1 and 10 microns are considered large or coarse particles. Particles with radii between 0.1 and 10 microns are responsible for the turbidity (haziness) of the atmosphere. The concentration of aerosols is usually greater over the continents than over the oceans. The concentration of Aitken nuclei ranges from values of 150000 cm^-3 in cities to 400 cm^-3 over the oceans. The concentrations of large and giant particles are much smaller than those of the Aitken particles, and strongly depend on the type of air mass, being greatest in moist, tropical air masses.

For most atmospheric applications we reference the size of the aerosol using the particle radius where in environmental and health research, the particle diameter is used as a reference. Thus PM 2.5 refers to aerosols (particulate matter) with diameters less than 2.5 microns.

Why Do We Care About Aerosols?

Aerosols affect planetary energy balance in two ways:

• Directly: aerosols scatter and absorb solar energy both in cloud-free and cloudy conditions; and
• Indirectly: via their role as cloud condensation nuclei (CCN), aerosols modify the optical properties and lifetimes of clouds playing an important role in the process of cloud formation and precipitation.

What Happens When Light is Absorbed?

Absorption of radiation causes local heating of the earth's atmosphere. As this happens, the stratosphere is locally heated by the absorption of aerosols that can generate winds and temperature inversions.

What Happens When Light is Scattered?

Scattering of radiation causes a cooling effect, altering the weather and climate. Scattering occurs because the index of refraction of the particles differs from that of the homogeneous medium in which they are imbedded (Houghton, 1985). Although the frequency of the scattered radiation does not change, its phase and polarization may change substantially from those of the incident radiation.

The radiation that finally reaches the surface is partly reflected and partly absorbed by ocean waters, soil, vegetation, snow, and ice. A large portion of the latter energy is used to evaporate water into the atmosphere, whereas the remainder is transferred down into the ocean by conduction and turbulent heat exchange and up into the atmosphere by similar processes and by the emission of long-wave radiation. The fraction of the total incident solar radiation that is reflected and backscattered is called the albedo, as we have seen before. The albedo of the earth, as measured at the top of the atmosphere, increases with latitude, and changes seasonally.

What Does Scattering Mean and Why is it Important in the Earth's Atmosphere?

Scattering is a rapid process whereby light is actually absorbed by a particle and then quickly emitted in another direction. Scattering particles can be air molecules, water droplets, or pollutants. The light in the atmosphere is diverted (scattered) from its direction of propagation when it encounters particles or inhomogeneities, such as air molecules, aerosols, and clouds (see figure 1 below). It is important because the materials that scatter light (e.g. clouds) can affect weather and hence can give us an indication of what is happening in the Earth's atmosphere.

Figure 1: How particles in the air scatter sunlight.

The Scattering Process

Scattering of solar radiation does not lead to a conversion of radiant energy into heat as does absorption. The radiant energy is merely dispersed in all directions as if the particles act as a new source of radiation. Because some of the solar energy is scattered backwards and sideways, the amount of energy that reaches the surface is partly reduced. Thus, both absorption and scattering lead to a depletion of solar radiation.

There are 2 kinds of scattering: Rayleigh Scattering and Mie Scattering.

Rayleigh Scattering

Rayleigh developed the scattering theory for light scattered by particles or molecules in the atmosphere with diameters smaller than the wavelength of incident light. He showed that the amount of scattering is inversely proportional to the fourth power of the wavelength (λ^-4). Therefore, the following rule comes into play:

The shorter the wavelength of the incident light, the more the light is scattered.

Light in the blue part of the spectrum is more intensely scattered than in the red part by atmospheric molecules, hence we see a blue sky. On the other hand, sunsets and sunrises appear reddish because the shorter (blue) wavelengths in direct light are removed by scattering through the long path in the atmosphere, leaving the remaining reddish colors of the spectrum (see figure 2 below).

Figure 2: Sunlight is scattered by particles in the earth's atmosphere.

The illustration above shows that the light arriving at position A travels a shorter distance through the atmosphere than light arriving at position B. Therefore, a person at position B would see red light that encounters more scattering molecules and aerosols due to a longer path. A person at position A would see blue light because the light travels a shorter distance through the atmosphere before arriving at the surface of the earth.

The shorter radiation is also scattered by particles (dust, smoke, and ions) and impurities that form aerosols. When the dimensions of these particles increase, the λ^-4 law ceases to be valid, dispersion is less selective with respect to λ, and Mie scattering theory (see below for explanation) should be used (Van de Hulst, 1957). Mie theory is more generally valid and contains Rayleigh scattering and geometrical optics as limiting cases. When the particles are sufficiently large, the dispersion of radiation approaches a 1/λ dependence, leading to diffuse reflection. This explains why cloud drops and ice crystals reflect or refract radiation in all directions.

In general, for large particles, the change in direction of the incident radiation may be explained by geometrical optics, such as diffraction, reflection, refraction, or a combination of these effects, producing coronae, haloes, rainbows, etc. The diffuse radiation of "white" sunlight is also white due to the multiple reflections and refractions, explaining the whitish color of the clouds. Volcanic eruptions cause colorful sunrises and sunsets due to the large amounts of aerosols they eject into the air.

Mie Scattering

Mie scattering occurs when the size of the particle is on the order of the wavelength. Mie scattering works only for spherical particles such as cloud droplets. This type of scattering is responsible for the white appearance of clouds because the cloud droplets scatter all wavelengths of visible light in all directions.

Aerosols and Health

Aerosols can also affect health. Recent epidemiologic studies have indicated associations between ambient particulate matter and increased mortality and morbidity. Aerosols may also be linked to the increasing incidence of asthma.

Written: August 1997: Maricela Reyes (student at Queens College). GISS scientist advisor: Dr. Barbara Carlson.