ATOC 5600 Physics and Chemistry of Clouds & Aerosols

  Aerosol properties

 

 

Aerosols are small (sub-micron to several microns) particles in suspension in the atmosphere. They can be in the solid phase or in the liquid phase. Aerosols originate both from natural and man-made (anthropogenic) sources (Seinfeld and Pandis, 1998, p.97). They can be directly emitted as particles (primary aerosols) into the atmosphere by volcanoes, through the effect of wind lifting dust particles in arid regions, from combustion during biomass burning, from sea spray, from vegetation etc. They can also be the result of chemical reactions (gas-to-particle conversion) (secondary aerosols). Table I summarizes the main sources of aerosols. It is estimated that 10% to 20% of the aerosols can be characterized as anthropogenic on the global scale. 


Montréal in fog. Photo by Robert Tardif.

 

Table I
Main sources of aerosols.

Natural Anthropogenic
Primary
 Mineral aerosol
Sea salt
Volcanic dust
Organic aerosols
 Primary
 Industrial dust
Soot
Biomass burning
Secondary
Sulfates from biogenic gases
Sulfates from volcanic SO2
Organic aerosols from VOCs
Nitrates from NOx
 Secondary
Sulfates from SO2
Organic aerosols from VOCs
Nitrates from NOx

Note : VOC = Volatile Organic Compound

Aerosols in the atmosphere have direct and indirect effects on the Earth's climate. The direct effect is related to their optical properties. Indeed, aerosols act to scatter and/or absorb solar and terrestrial radiation. The level of scattering and absorption depends on their physical and chemical characteristics. Consequently, aerosols act to modify the Earth's radiation budget and thus influence the warming/cooling of the planet. 

Aerosols can also act to modify the properties of clouds (indirect effect). Some aerosols in the ambient air act as cloud condensation nuclei (CCN). The formation of cloud droplets is thus promoted by the presence of these particles. The main results from this are clouds forming at lower supersaturation than if there were no aerosols. Homogeneous nucleation of water droplets (no aerosols) occur only at very high levels of supersaturations, which are not observed in the Earth's atmosphere. Consequently, heterogeneous nucleation (water drops nucleating on aerosols) is the main mechanism for the formation of clouds, such as fog. The nucleation and growth rates of cloud droplets depend on the physico-chemical  properties of aerosols. Thus aerosol properties have an impact on cloud properties. Here we focus on aerosols that are usually associated with fogs. 

In the troposphere, a significant portion of aerosols is anthropogenic in origin. Since fogs are close to the surface, they are affected by aerosols that are in the lower atmosphere, usually close to their source of emission. It has been observed that most fog events is characterized by the presence of haze (horizontal visibility roughly between 15 and 1km) before the onset of fog itself. Haze particles are aerosols that have "swelled" by picking up water even if the relative humidity is well below 100%. Observations show that sub-micrometer and micrometer size particles are often made up of more than 50% water. This leads to the conclusion that such small particles containing that much condensed water must be made of hygroscopic material, such as (Heintzenberg et al., 1998):

- mineral acids like sulfuric acid or nitric acid

- soluble inorganic salts such as magnesium chloride, sodium chloride, sulfates, nitrates or other organic material. 

The first group is produced mainly from gaseous precursors due to emissions from stationary (industry) and mobile (ground transportation) combustion sources. These are observed in higher concentrations in urban environments. Aerosols in coastal urban regions may contain some sea salt.  Soluble organic and inorganic material are major components of aerosols in the troposphere. 

Chemical composition of aerosols can be extremely variable, depending on the geographical region. Nevertheless, typical compositions can be determined. Usually, rural aerosols are assumed to be composed of ammonium sulfate ((NH4)2SO4) and insoluble mineral material (Bott, 1991).  The urban aerosol is assumed to be composed of 80% rural aerosols and 20% soot (Shettle and Fenn, 1979). Small maritime aerosols are assumed to be 100% ammonium sulfate (gas-to-particle conversion of SO2), while large particles are pure NaCl (sea spray evaporation). Detailed description of aerosol chemical composition is given by Warneck (1988). 

Aerosol size distributions describe the number of particles observed to have a certain radius, for various size ranges.  Model size distributions have been constructed by fitting log-normal functions to data representing typical aerosol concentration for urban, rural and maritime environments (Fig. 1). It is observed that urban environments are characterized by higher concentrations of aerosol  particles (as much as 105 particles per cm3), while the maritime environment is  characterized by the lowest particle concentrations (maximum of 100 particles per cm3).  

Figure 1. Model size distributions describing the aerosol concentrations for urban, rural and maritime environments (Data from Jaenicke (1993)). 

 

For the urban environment, the maximum particle concentration is observed in the small particle sizes (radius at 0.01 micron, in nuclei mode) and drops off significantly toward minimum values of just a few particles per cm3 for particles in the 10 micron range. Urban aerosols are a mixture of primary particulate emissions from industries, transportation, power plants etc., and from natural sources. Secondary material from gas-to-particle conversion also contributes to the urban aerosols. Generally, the size distribution of urban aerosols is very variable. High concentrations of small particles are found near sources, and fall off rapidly with distance from these sources. Also, chemical composition of small and coarse particles is different. Small particles are products of combustion and from chemical reactions (gas-to-particle conversion) of sulfate, nitrate, ammonium and secondary organics. Coarse particles result from mechanical processes and are composed of dust, sea salt, fly ash, tire wear etc. (Seinfeld and Pandis, 1998). 

Rural aerosols are mainly from natural origins, but a noticeable proportion is anthropogenic in nature. The size distribution is characterized by a lower concentration of particles compared to urban aerosols. The maximum is at a radius of about 0.01 micron. Aerosol concentration remains somewhat high up to a radius of 0.1 micron (accumulation mode). 

The maritime environment is typically characterized by aerosol concentrations of about 100 particles per cm3. The maximum concentration is within the accumulation mode at 0.1 micron. Coarse mode particles are mainly composed of sea salt, resulting from the evaporation of sea spay. Coarse particles represent about 95% of the total mass of maritime aerosols. 

Aerosols particles play an important role in radiative budget of the atmosphere. The scattering and absorption of radiation by a single aerosol particle is expressed by its complex refractive index (n = nr + ini), where the real part represents scattering and the imaginary part represents absorption. The refractive index is strongly dependent on the chemical composition of the particle. Following Shettle and Fenn (1979), the real and imaginary parts of the complex refractive index for urban, rural and maritime aerosols, as a function of wavelength, are shown in Fig. 2. The results for pure water are shown as references. Important variability in scattering and absorption is observed for different aerosol composition. Thus different radiative responses are expected for different aerosol models. 

Figure 2a. Real part of the complex index of refraction for different aerosol models. 

Figure 2b. Imaginary part of the complex index of refraction for different aerosol models. 


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