13 Aerosols

Thara Prabhakaran and Baban Nangare

1. Learning outcomes
2. Introduction
3. Types of aerosol particles
4. Formation and growth of aerosol
5. Lifecycle of aerosol
6. Methods of observations of aerosols
7. Satellite measurements of aerosols
8. Direct effect of aerosols
9. Aerosol-cloud interaction
10. Atmospheric aerosol and climate
11. Summary

1. Learning outcomes

In this module we will learn about:

• Aerosol in the atmosphere
• Aerosol in the atmosphere
• Aerosol and role in weather and climate Aerosol observations

2. Introduction

Aerosol particles are the dispersed solid or liquid particles in the carrier gas. The carrier gas in the atmosphere is the air. By definition everything which is non-gaseous form in the atmosphere are the aerosol particles. These tiny ubiquitous particles in the atmosphere have the size range from few nanometers (nm, nano= 10-9) to several micrometers (µm, micro = 10-6) and are responsible for the reduction in visibility and cloud formation. The aerosol particles have an impact on human health since we inhale them along with the air. Especially aerosol particles with diameter less than 2.5 µm (PM2.5), cause respiratory problems. Aerosol particles are generated naturally as well as by anthropogenic activities. The natural ways of aerosol generation are the wind-blown dust, seas sprays, pollen emissions from trees, forest fires, volcanic ash etc. The human activities which produce aerosol are the vehicular exhausts, industrial chimney exhausts, agricultural activities, wood stove burning etc. Some of the aerosol particles are nucleated from gas phase by gas to particles conversion. The aerosol particles which are released into the atmosphere (e.g. dust) are called primary aerosol particles and those formed in the atmosphere through gas to particles conversion are called as secondary aerosol particles. Physically and chemically aerosol particles are very complex. These particles can exit in liquid state, solid state or mixed of both liquid and solid. Chemically these particles can be organic, inorganic or mixed of both organic and inorganic.

The atmospheric concentration of aerosol particles is determined by the winds e.g. the dust of one area is transported to other area by the wind. The processes which can affect the aerosol concentrations are coagulation (two particles collide and stick together); growth of particles by uptake of water (swelling), chemical transformations, removal of aerosols by the rainfall, deposition through gravitational settling etc. These processes also have a profound impact on size the aerosol particles. The larger aerosol particles have higher sedimentation velocity so they settle on the surface faster than the smaller aerosol particles. The very small aerosol particles have very high Brownian motion and higher sticking rate with the other aerosol particles. Due to the high Brownian motion, small particles can also stick to the surface when they are near to the surface.

Aerosol particles play a very important role in climate system. Aerosol particles influence the climate system in different ways, directly and indirectly. Directly, the aerosol particles scatter and absorb the incoming solar radiation and can absorb outgoing thermal radiation. Indirectly the aerosol  particles help in cloud formation as some of the aerosol particles act as a seed for the droplets and ice crystals formation in the clouds.

3. Types of aerosol particles

     Aerosol particles are typically classified according to the origin or sources. Figure 1 gives an account of different aerosol particle diameter and picture of aerosol particles taken with a micro scope. Anthropogenic aerosols are mainly of size below 1µm and natural aerosols such as dust, sea salt, and pollen have more than 1 µm sizes. Aerosols also have a geographical origin. In a desert for example, due to dry soil and strong winds and convection, the dust particles can get lifted above the earth surface into the atmosphere and sometimes into the upper atmosphere. Near the sea shore, and over the ocean, sea spray aerosols are important in contributing to the weather and climate.

The satellite image over Indian region, especially near the foothills of Himalayas show very clear indications of such increase in the aerosol particles and sometimes they get trapped in the atmosphere due to stable conditions of the atmosphere. This condition has an important implication to our weather. The fog layer could reduce the radiation reaching the surface which can results in less surface heating.

Aerosol particles over a city may also be different as the road transportation, industries and human activities such as construction, space heating, etc. may cause significant contribution to the aerosol concentration over the city.

Major Aerosol types are listed below. The processes of their formation are given in brackets: Sea spray (The water buble bursting over the ocean results in emission of particles in to the atmophere) Mineral and agricultural Dust (The dust from dry land is lifted into the atmophere by the wind) Volcanic aerosols (The volcanos can emmit the materials from the interior of the earth to the atmophere. After cooling of the plume gas to particle conversion can also take place.)

Biogenic particles (The plants emmmit pollens into the atmophere. Many plants also emmit aromatic gases into the atmophere condensing of which can formVOCs i.e.Volatile Organic Compounds. The airborne viruses and bacteria also fall in this cateogory.)

Smoke (burning)

Natural gas to particle conversion (suphates ocean surface with dimethyl sulfide) Industrial emissions (soot, smoke road dust) Products of gas to particle conversion (secodary aerosol)

Aerosol particles are found in various sizes and from various sources as shown in Figure 1.Aerosol particles have size from few nanometers to tens of micrometers. It is useful to describe their number concentrations in terms of size ranges. The size ranges of aerosol particles are called as themodes. There are four modes of aerosol as follows: nucleation mode, Aitken mode, accumulation mode and coarse mode. Note that size of particles given on the X-axis of Figure 1 is in nanometer.

As aerosol particles are formed/released into the atmosphere, they are carried to higher altitudes by the wind, where their concentration is less compared to that near to the surface. So if we look at the vertical variation of cocentration of aersol particles into atmosphere, it shows a typical exponential decrease with height.

4. Formation and growth of aerosol

Background aerosol exists over land at heights above about 5 km, and over the oceans far from shore above about 3 km. Aerosol particles of terrestrial origin are formed by a) gas-to-particle conversion (GPC), b) drop-to-particle conversion (DPC) involving the evaporation of cloud and raindrops which contain dissolved and suspended matter, c) bulk-to-particle conversion (BPC) involving mechanical and chemical disintegration of the solid and liquid Earth surface. The secondary aerosol particles can form in the atmosphere by following ways:

• Homogeneous nucleation of new particles in supersaturated vapor: e.g. plant inhalations, combustion, volcanic activity etc.
• Condensation of vapor molecules on drops or solid particles close to the source: e.g. Industrial, manmade or natural fires, homogeneous nucleation by gas phase chemical reaction such as in the case of sulphate aerosol formation from acids.
• GPC may also involve pre-existing aerosol particles

Nucleation is a process in which gas molecules aggregate to form clusters as shown in Figure 2, until it reaches a critical size which is a critical cluster. This cluster then forms an aerosol particle which is of the size of a few tens of nanometers. The figure shown is a typical for sulfuric acid aerosol formation in the atmosphere. There are many species which can form in this way in the atmosphere, e.g. Secondary Organic Aerosols (SOA).

Homogeneous and heterogeneous nucleation of secondary aerosol particles:

Nucleation of the aerosol particles can be either homogeneous, where gas molecules do not require a surface or a particle to get nucleated or heterogeneous, where a surface is needed for gas to nucleateinto liquid or solid.In homogeneous nucleation, new particles are formed, while heterogeneous nucleation does not lead to formation of new particle. One of the important homogeneous nucleation processes in the air is binary nucleation, where molecules of two gases are involved. Figure 2 shows such a process where molecules of sulphuric acid gas and water vapor combined to for an aerosol particle. These sulfuric acid–water molecules are of 3 to 20 nm in diameter. In the remote oceanic atmosphere, such homogenous nucleation events can produce more than 104 particles in a volume of 1 cm3in short period. After the critical cluster has formed the aerosol particles can grow with by condensation of sulphuric acid molecules and water vapor molecules leading to growth of the particle.

Figure2: Schematic representation of the nucleation and subsequent growth process of atmospheric binary of sulfuric and water (Modified from Curtius, 2006).

Aerosol particle size distribution/modes:

In Figure 3, different modes of aerosol size distribution and associated processes are indicated. Aerosol particles with diameters ≤ 0.01 µm are called nucleation mode, those with diameters between 0.01-0.1 µm are Aitken mode, those with diameter between 0.1 and 1.0 µm are called accumulation mode, and those having diameter 1-10µmare called coarse particles. Note that sea salt, mineral dust and biological particles are in the coarse mode. If the size is greater than 1 µm, those are called giant particles. Particle with diameter less than 1 µm also are called fine particles. Formation and growth of aerosol particles is illustrated in Figure 3. Coarse mode particles are mostly directly emitted into the atmosphere. The nucleation mode particles are formed in the atmosphere by gas to particles conversion. The nucleation mode particles can further grow by condensation to form Aitken and accumulation mode particles.

The aerosol particle as it reaches the size close to 100 nm, through condensation and coagulation, can give rise to Aitken/accumulation mode particles. Some of these particles can act as cloud condensation nuclei (CCN) and form cloud droplets of the size of 1-10 µm. The coagulation process may lead to sticking together of two particles. The cloud droplets thus may have a few aerosol particles inside them. Larger aerosol particles acting as cloud condensation nuclei need only a small super saturation (saturation above 100 % relative humidity of ambient air) due to their surface area, to form cloud droplets. But smaller CCN needs much higher super saturations.

Brownian motion is the random movement of particles suspended in a fluid. Particles diffuse, collide and coalesce due to these random motions. Collision and coagulation are important processes for nucleation and Aitken modes. The aerosol particles collide with each other due to their Brownian motion in the air. Coagulation is a process by which two particles come together and stick together to form single particle. Biomass burning and fossil-fuel combustion contribute to small accumulation mode particles. However, the coagulation and gas-to-particle conversion increase size of these particles to mid-high accumulation modes. Coagulation can also move some particles to coarse mode.

5. Lifecycle of aerosol

Aerosol particles as they are emitted into the atmosphere, they get transported to upper layers in the atmosphere and can get into the free troposphere (FT), where they get into the long range transport and winds transport them globally. For example, dust particles emitted from the Thar Desert can come over peninsular India, especially due to the convective uplift due to intense heating over Thar Desert. As the aerosol particles get into FT due to the convection, can get into the anticyclone and northerly winds in the middle troposphere during the premonsoon conditions and can get transported to southern Indian regions. During the transportation, these particles can coagulate with other absorbing type of aerosols such as soot and black carbon aerosols and can become more absorbing due to the large surface area of dust particles. The interaction between aerosol particles and radiation is known as aerosol direct effect.

The subset of aerosol act as a cloud condensation nuclei when they get exposed to relative humidity more than 100 %. The effect of aerosol cloud interaction is called indirect effect of aerosols. Aerosol may impact precipitation processes through this indirect effect.

Removal of aerosol:

Aerosols are removed from the atmosphere by dry and wet deposition processes. This can happen due to sedimentation under gravity. Aerosols also get removed by collision with rain drops and snowflakes and through uptake of aerosols by droplets in cloud. Aerosol can get scavenged by precipitation and the process is called as wet scavenging. Wet scavenging is a major sink for aerosol globally, that removes nearly 80-90% of aerosol mass from the atmosphere. The process where aerosol particles acts as CCN and gets scavenged is called as nucleation scavenging.

The lifetime of aerosol particles depend on their properties e.g. chemical composition, size and height at which these particles are present in the atmosphere (Jaenicke, 1980). In the free troposphere, the lifetime of aerosol is usually of the order of 3 to10 days and they can be transported to long distances in that time. Near the earth surface in the boundary layer, the lifetime of aerosol is less than a week. In the stratosphere, the lifetime of aerosol can be of 1 year. The small aerosol particles are removed efficiently due to coagulation with the other particles and they have lifetime from minutes to  day. The larger particles sediments very fast and have very short lifetime. The accumulation mode particles where coagulation and sedimentation are not efficient have an average lifetime from 3 to 10 days and they can be removed from the atmosphere by the rain.

6. Methods of observations of aerosols

Physical and chemical characteristics of aerosol particles are measured in different ways. The size range of aerosol particles in the atmosphere varies between 1 nm to 100 µm and they have different shapes, chemical composition etc. Aerosol particles are categorized in three general sizes are PM10 (particles smaller than 10 µm), PM2.5 (particles smaller than 2.5 µm) and PM1.0 (particles smaller than 1 µm). In this type of aerosol measurements only the particles with diameter less than the specified diameter are allowed to pass through the inlet of the sampling system and then these particles are deposited on chemically inert filter papers. The weight of deposited particles can be found out by weighing the filters before and after. These filters can also be used for chemical and physical analysis.

Cascade impactors can be used to size segregate the particles to derive the information about the weight of particles of diameter larger than the specified diameters. The cascade impactor samples then can be used for further chemical and physical analysis. The chemical composition of the particles can be known with the different types of advanced mass spectrometers and chromatograms for the aerosols. Transmission Electron Microscopy (TEM) can be used to get the information about the shape and size of the aerosol particles. TEM attached with Energy Dispersive Spectroscopy and X ray diffraction pattern can be obtain to know elemental composition and crystallographic structures of the aerosol particles

The number concentration of aerosol is measured with the condensation particles counter (CPC). The particles are passed over a very large supersaturation where they can grow by condensation in order to get detectable by optical detection. The particles size of aerosol particles can be detected with scanning mobility particle sizer (SMPS). The other instruments that can be used are optical particles counters and aerodynamic particle sizer. The images of the particles can also be obtained with the electron microscopes. The measurement methods of measurements particles sizes and choice of instrument depend on the purpose and the diameters of interests.

7. Satellite measurements of aerosol

Satellite based instruments, and surface based networks of observations are used to make aerosol and cloud measurements. The satellite typically measures the integrated column concentrations (e.g. aerosol optical depth) and derives the particle sizes based on radiative transfer algorithms.

Radiant energy reflected/emitted by the Earth’s surface and atmosphere has signature of the atmospheric and surface properties. By measuring the reflected light’s spectral, angular and/or  polarization properties, satellite sensors can also quantify several atmospheric and surface properties. Human eye is sensitive to a narrow range of the solar spectrum, with receptors in the blue, green, and red while in remote sensing the invisible part of solar spectrum can also be used (e.g. infrared wavelengths). The TOMS instruments, flown since 1978 have two channels sensitive to ultraviolet (UV) light that were discovered to be excellent for observations of elevated smoke or dust layers above scattering atmosphere.

MODIS (MODerate resolution Imaging Spectroradiometer) and MISR (Multi-angle Imaging SpectroRadiometer) instruments on the Terra satellite have been measuring global aerosol

distributions and properties since year 2000 (Figure6, see website http://earthobservatory.nasa.gov/GlobalMaps/view.php?d1=MODAL2_M_AER_OD). A wide range (0.47 to 2.1µm) is used to distinguish small particles (high concentrations of anthropogenic pollution or smoke) from coarse particles such as sea salt and dust. Over land MODIS uses the 2.1µm to observe the surface cover properties, to estimate surface reflectance at visible wavelengths, and to derive the aerosol optical depth.

In Northern Hemisphere, most of the aerosol particle mass enters the atmosphere at latitudes between 30 and 60° N, and this latitude belt contains about 88% of all anthropogenic sources for aerosol particles.

GLAS profiles of aerosols on the region of the Himalayas indicate elevated layers of aerosol over that region. http://icesat.gsfc.nasa.gov/images/movies/india_noclouds_dates.mpeg

8. Direct effect of aerosols

Direct effect refers to the interaction between solar and terrestrial infrared radiation with aerosol particles before they become cloud particles. The scattering and absorption of solar radiation by aerosol particles reduce the amount of incoming solar radiation. The absorption and scattering of outgoing long wave radiation traps the energy in the Earth’s system. This magnitude of the interaction depends on the size, shape, chemical composition and mixing state of the aerosol particle. This produces a net cooling effect due to the solar radiation that is scattered back to space. However,  aerosol particles like black carbon can produce net warming effects in the atmosphere. The net effect of aerosol direct effect is negative radiative forcing as the scattering of solar radiation prevails over the absorption of the radiation by the aerosols.

9. Aerosol-cloud interaction

Evidence for the indirect effect (Twomey effect) in this satellite image of clouds off the coast of California is shown in Figure9. The ship tracks are a result of high reflectivity regions in the marine stratus clouds formed by increased concentrations of small droplets formed on the sulfate particles from emissions by ships. The concentration of water droplets depends directly on the concentration of aerosol particles that can form from cloud condensation nuclei (CCN) and the vapor pressure of water with respect to the equilibrium saturation vapor pressure.

An increase in anthropogenic sources of CCN can increase the reflection (albedo) of clouds, by increasing the droplet concentration while decreasing the average diameter. This effect was named the indirect effect of aerosols by Twomey (Twomey, 1974).

While less number of aerosol/CCN are present, they can form cloud droplets under supersaturated environment, which can grow by the expense of available water vapour in the surrounding air and grows until the surrounding environment becomes unsaturated. Some of these droplets grow to raindrops, which may fall out of the cloud as rain. Over marine environment, these processes take place at a lower altitude and raindrop forms at a low level. In a polluted cloud, more aerosols compete for uptake of water vapor; cloud droplet does not grow and remain small or grow much slower than in a clean cloud. As a result more numerous, smaller cloud droplets – have larger surface area than few large cloud droplets forms. These clouds are more reflective and can send more radiation back to space. This effect is called cloud albedo effect. Since the droplets are small, they do not make raindrops. Figure 11 shows the observations made for cloud droplet number concentration over the areas with different concentrations of the aerosol particles.

10. Atmospheric aerosol and climate

Atmospheric aerosol particles play an important role in the radiative budget in the climate system. The incoming solar radiation are scattered and can also be absorbed (e.g. black carbon) by the aerosol particles. These particles also scatter and absorb outgoing long wave radiation emitted by the Earth and the atmosphere. The term radiative forcing is defined as the difference between incoming solar radiation and outgoing radiation from the top of the atmosphere of the Earth to the space. The positive radiative forcing indicates the warming of the climate system while negative radiative forcing indicates cooling of the climate system. The findings of Inter governmental panel on climate change (IPCC) noted that the aerosol radiative forcing due to aerosol direct effect to be -0.45 Wm-2 with uncertainty range of -0.95 to +0.05 Wm-2and due to aerosol indirect effect -0.45 Wm-2 with uncertainty range of -1.2 to 0 Wm-2.

11. Summary

• Aerosols are tiny ubiquitous particles in the atmosphere, originated both from natural and anthropogenic sources. In the northern hemisphere, the latitudinal belt between 30° N to 60° N contribute as high as 88% of all anthropogenic aerosol particles in the atmosphere. 
• Aerosols from anthropogenic sources generally have sizes below 1µm whereas natural aerosols are mainly greater than 1 µm sizes. Example of aerosol particles include sea spray, mineral and agricultural dust, volcanic materials, biogenic particles like pollens, Volatile Organic Compounds (VOCs), airborne viruses and bacteria, smoke, soot, black carbon and road dust.
• Wet scavenging is the most important process for aerosol removal globally; this removes about 80-90% of aerosol mass from the atmosphere.
• Globally, aerosol has a net cooling effect. The radiative forcing due to aerosol direct effect is about -0.45 Wm-2.
• Atmospheric aerosol determines the radiation budget of the earth, global evapotranspiration and diffuse radiation dynamics among other things. Hence, aerosol has an important role in day today weather and climate system as a whole.
• The aerosol particles can adversely affect the human health, particularly the respiratory problems when exposed to particulate matters (PM) of less than 2.5 µm sizes.
• The cloud droplets are formed on the subset of aerosol particles called as cloud condensation nuclei.
• Aerosol particles are the one of the largest uncertainties in the climate prediction.