How Aerosols are Produced

Aerosols may be divided into two broad categories based on their method of creation: primary aerosols and secondary aerosols.

Primary Aerosols

Primary aerosols are solid or liquid particles that are that are ejected directly into the atmosphere. Primary aerosols are pre-formed by processes occurring on land or water. The processes may be natural or anthropogenic. The sources of the material may be living (such as plants, animals or microbes), or non-living (such as soil or volcanoes). Processes that introduce particles into the atmosphere include:
wind suspension (sometimes called deflation) wave breaking and bubble bursting leaf abrasion burning of vegetation burning of fossil fuels (eg. diesel smoke) volcanoes agriculture industry
Major global source for primary aerosols are, (1) Central South America (Brazil) near 10°S, (2) Africa near 0° to 20°S and 0° to 10°N, (3) the Saharan Desert and sub- Saharan region (Sahel), (4) Arabian Peninsula, (5) the northern border region of India, (5) agricultural burning in Indonesia, Eastern China and Indochina, and near the mouth of the Amazon River, and (5) coal burning and dust in northeastern China.

Secondary aerosols

Secondary aerosols are solid or liquid particles that are created in the atmosphere after chemical or physical transformations of precursor gases. The precursor gases become particles by phase changes (condensation from gas to liquid), adsorption onto pre-existing particles, chemical reactions with other gases that produce particles, and absorption into water droplets. The chemistry of pre-cursor gases is complex, and they are created by many natural and anthropogenic processes. Processes that introduce precursor gases into the atmosphere include:
burning of biomass burning of fossil fuels volcanoes and other geothermal emissions emission from ocean, lakes and rivers emission from soils emission from vegetation emission from construction materials and other synthetic products exhaust: jet contrails, automobiles, ship tracks
Precursor gases are converted to aerosols by two main pathways: gas to particle conversion (GPC) and cloud processing.

Gas to Particle Conversion (GPC)

Two types of GPC are recognized, homogeneous nucleation and heterogeneous nucleation. Homogeneous nucleation takes place when brand new particles are produced by condensation of precursor gases. This process generally requires that the involved gas vapors occur at a supersaturation of several hundred % in the air space. Hydrocarbons are an exception; they may change from the gas to liquid phase even in unsaturated conditions. Heterogeneous nucleation takes place when gas molecules condense onto pre-existing solid or liquid particles. Only minor supersaturation of gases required for this pathway to occur (a few % of supersaturation).

Sulfur compounds

Many reduced sulfur gases enter the atmosphere naturally or through human activities. Reduced sulfur gases must be oxidized to SO2, and then to sulfate (SO4-2) before condensing from vapor to liquid form. Sulfur compounds that lead to particle formation in the atmosphere include: SO2 (sulfur dioxide) H2S (hydrogen sulfide) CS2 (carbon disulfide) COS( carbonylsulfide) CH3SCH3 (dimethylsulfide or "DMS") CH3SSCH3 (dimethyl disulfide) It is now known that sulfate can condense onto mineral particles or be oxidized inside cloud droplets containing sea salts (examples of heterogenous nucleation). These larger aerosols are less efficient scatterers of light and have shorter residence times. This reduces the pool of sulfate ending up as tiny submicron aerosols, which are efficient scatterers.

Nitrogen compounds

NO (nitrogen monoxide) is emitted into the atmosphere during nitrification in soils and biomass burning, is created during lightning, and is released by fossil fuel use. NO is oxidized to NO2 when ozone and sunlight are present. NO2 may next react with OH radicals to produce HNO3 (nitric acid). The saturation vapor pressure of nitric acid is high, thus nitric acid generally condenses onto existing aerosol, such as alkaline mineral aerosols or sea salt aerosols. Nitric acid can be also be converted to nitrate-containing aerosol via these other two pathways: (1) Nitric acid vapor is scavenged by cloud droplets; if these droplets evaporate, nitrate aerosol particles are produced (an example of cloud processing). Evidence for this pathway is provided by the presence of a size mode of nitrate aerosol too large (>1 µm) to be explained by GPC. (2) If sufficient ammonia gas is present (NH3(g)), nitric acid vapor can change phase together with the ammonia, creating ammonium nitrate aerosol (NH4NO3). However, ammonium will preferentially take part in the neutralization of sulfate in (NH4)2SO4 aerosols, so its role in nitrate aerosol production requires an excess of ammonium in the air. Nitrate aerosols may become increasingly important if agricultural outputs of ammonium increase.

Hydrocarbons

Hydrocarbons of marine origin include organic coatings that get aerosolized by bubble bursting (these would have sodium in them) and marine organohalogens such as chlorine containing terpenes. Hydrocarbons of terrestrial origin are diverse, and are dominated by terpenes and related organic molecules that escape from living vegetation via stomata. The gaseous and particulate phases of volatile hydrocarbons can exist simultaneously. Only higher hydrocarbons are found on aerosols (C10-C28 n-alkanes). Alkenes and aromatic hydrocarbons require oxidation or ring opening before condensation into aerosol.

Cloud processing

Clouds serve both as chemical reactors for aerosols, but also as a source of particles. When cloud droplets evaporate (a continual process in clouds), aerosol particles are left behind suspended in the air. This source, which is on the order of 3000 Tg/yr, is comparable in strength to other major sources like oceans and deserts. The particles thus produced can be very different chemically than the original cloud condensation nuclei (CCN) because once a cloud droplet has formed, it can scavenge other gaseous compounds via condensation that may not have sufficient vapor pressure to form homogeneous CCN. In this way, mass is added to CCN but their number concentration does not change. Cloud processing is presumed to account for some types of large sized aerosols that are unlikely to be produced by primary processes.