Aerosols are fine, airborne particles consisting at least in part of solid material. Density of the basic materials of aerosols range from 1.0 g/cm3 (for soot) to 2.6 (for minerals). The ocean is a major source of natural aerosols. Air-sea exchange of particulate matter contributes to the global cycles of carbon, nitrogen, and sulfur aerosols, such as dimethylsulfide (DMS) produced by phytoplankton. Ocean water and sea salt are transferred to the atmosphere through air bubbles at the sea surface. As this water evaporates, the salt is left suspended in the atmosphere. Four other significant sources of aerosols are terrestrial biomass burning, volcanic eruptions, windblown dust from arid and semi-arid regions, and pollution from industrial emissions (Fig 1).
Clean continental air often contains less than 3,000 particles per cubic centimetre (of which half are water-soluble), polluted continental air typically 50,000/cm3 (of which two-thirds are soot, and the rest mostly water-soluble). Urban air typically contains 160,000/cm3, mostly soot, and only 20% is water-soluble. Desert air has about 2,300/cm3 on average, almost all water-soluble. Clean marine air generally has about 1,500/cm3, about all water-soluble. The lowest sea-level values occur over the oceans near the subtropical highs (600/cm3 on average, but occasionally below 300/cm3). Arctic air has about 6600/cm3 (including 5,300 soot) and on the Antarctic plateau only 43/cm3 occur (about all sulphate) (1).
Fig 1. Main sources and types of aerosols that affect climate.