This serves as an introduction to atmospheric aerosols to accompany my guest post on RealClimate. I’ll be discussing more about aerosols in the near future, including their potential role in GeoEngineering.
Atmospheric aerosol refers to the liquid or solid particles suspended in the air. They can be emitted directly into the atmosphere (primary aerosol), or they can be formed in the atmosphere by gas-to-particle conversion (secondary aerosol). The oxidation of precursors, such as SO2, NO2, and suitable organic compounds, can produce compounds with very low vapor pressures. These can condense to form a new particle, often in combination with other species, such as water vapor and NH3, a process called homogeneous nucleation. They can also condense on existing particles and contribute to them growing in size, referred to as condensation. Furthermore, aerosol particles can undergo evaporation, deposition, and coagulation, whereby two particles collide to form one larger particle. All these processes influence the evolution of the aerosol population in time.
Atmospheric aerosol particles span several orders of magnitude in diameter, from a few nanometer to hundreds of micrometer. Cloud droplets and raindrops can grow even larger, but are usually treated separately. Though not all particles are spherical, they are usually characterized according to their equivalent spherical diameter. Most of the properties, and thus effects, of particles depend on their size.
The tropospheric number concentration of aerosol particles ranges from several tens or hundreds per cubic centimeter of air in remote locations, to more than hundred thousand a million per cubic centimeter in polluted environments or after a strong nucleation event. The number concentration as a function of particle size is described by a size distribution. An important feature of atmospheric aerosol size distributions is their multimodal character: commonly found are the aitken mode, the accumulation mode, and the coarse mode. The nucleation mode consists of freshly nucleated particles and usually occupies diameters below a few tens of nanometers. The accumulation mode can vary in size between 50 nanometer and 1 micrometer, and its name comes from the fact that particles tend to accumulate in this size range, because they bear the longest lifetime (~2 weeks): smaller particles are efficiently removed by deposition and coagulation, and larger particles are efficiently removed by gravitational settling. The coarse mode usually refers to particles larger than 1 micrometer diameter. Sometimes an Aitken mode is defined as the mode that could exist between the nucleation and accumulation mode.
The number concentration of the atmospheric aerosol is usually dominated by sub-micrometer particles, while the total particle surface area and volume (and thus mass) are more influenced by larger particles. The exact shape of the size distribution depends on the environment, as it reflects the sources, sinks, and transformations of the particles.
Atmospheric particles have both natural and anthropogenic sources.
Examples of natural (primary) particle emissions are the shattering of sea-spray into tiny droplets, that evaporate before gravitating back towards the water surface, particles caused by forest fires, soil dust, pollens, etc. Examples of natural emissions of precursors to (secondary) aerosol are SO2 and H2S from volcanoes, dimethylsulfide (DMS) from phytoplankton, volatile organic compounds such as monoterpenes from vegetation, etc.
Examples of anthropogenic (primary) particle sources are vehicular emissions, industrial emissions, and some natural sources that are significantly enhanced by human activities, such as biomass burning and soil dust (due to e.g. erosion). Anthropogenic precursors to (secondary) aerosol are emitted by industry, burning of fossil fuels, usage of solvents, etc. Primary emissions contribute relatively more to the aerosol volume (because they’re often large), while gas to particle conversion (secondary aerosol) contributes relatively more to the aerosol number.
Aerosol particles are necessary for clouds to form: Dependent on their size, composition and hygroscopic properties, they can act as cloud condensation nuclei (CCN) on which the available water vapor condenses, when it reaches a certain critical super-saturation of a few tenths of a percent. This process is called (cloud droplet) activation. Without these substrates to condense on, the relative humidity would have to exceed several hundred percent in order for the water vapor to overcome the Kelvin effect and homogeneously nucleate into droplets.
Similarly, some aerosol particles act as ice nuclei on which water can freeze into an ice crystal. Aerosol particles also provide a surface for specific heterogeneous reactions to take place, and thus affect the chemistry of the atmosphere.
Atmospheric aerosols have potentially far reaching effects on health, ecosystems, and climate. Aerosol particles are a major component of smog, and act to reduce visibility. Their health effects are mainly due to their adverse effects on the respiratory function, and some also act as irritants for the eyes. Small particles can enter deep into the lungs, reaching the alveoli, where the transfer of O2 and CO2 takes place. H2SO4 is often a major component of these small particles, and dependent on their history other toxic compounds, such as heavy metals and PAH’s, could also have partitioned into the particle phase. Due to their low volatility, these species would otherwise not have reached that deep into the lungs. Atmospheric particles have been found to play a major role in excess mortality in epidemiological studies in several cities. Through their acid composition, aerosol particles also contribute to acidification of ecosystems.
Atmospheric particles contribute to climate change directly by scattering or, in the case of black carbon, absorbing sunlight, and indirectly by changing the radiative properties and lifetime of clouds. Both direct and indirect radiative forcing are influenced by particle size, composition and relative humidity. These processes are responsible for the largest amount of uncertainty in assessing climate change. The upper end of the estimated (direct and indirect) negative forcing of atmospheric aerosols could -at least on a global and annual average- balance the positive forcing of CO2. This does not necessarily mean that on a regional scale the opposing forcing mechanisms will balance each other. Aerosols are not homogeneously mixed throughout the globe, whereas the atmospheric lifetime of most greenhouse gases is much larger, so they will get distributed equally over the globe, despite large spatial differences in emissions.
The short lifetime (days to weeks) of aerosols is an important reason for the uncertainty in their role in climate change. It causes their concentration to be highly variable in time and space, and it’s hard to even know what the global concentration is, let alone what it was in the pre-industrial era. Add to that their variability in size and chemical composition, and the poorly understood role of clouds, and it’s clear that the uncertainty in aerosol radiative forcing will remain a steady feature of climate science for some time to come.