4. Tropospheric Aerosols - UCD · Atmospheric aerosols are suspensions of small solid and/or liquid particles in air that have negligible terminal fall speeds. Molecular aggregates

4. Tropospheric Aerosols

4. Tropospheric AerosolsAtmospheric aerosols are suspensions of small solid and/orliquid particles in air that have negligible terminal fall speeds.


Molecular aggregates that carry an electric charge are calledions.



The number density and type of ions in the air determinesthe electrical conductivity of the air which, in turn, affectsthe magnitude of the fair weather atmospheric electric field.




Ions in the lower atmosphere are produced primarily bycosmic rays, although very close to the Earth’s surface ion-ization due to radioactive materials in the Earth and atmo-sphere also plays a role.





Ions are removed by combining with ions of opposite sign.





Ions are removed by combining with ions of opposite sign.

The following Figure shows the size ranges of particles thatplay a role in the atmosphere.

Size range of particles in the atmosphered < 0.2µm: Aitkin. 0.2 < d < 0.2µm: Large. d > 2µm: Giant.

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Concentrations of small ions vary from about 40 to 1500cm−3 at sea level, while concentrations of large ions varyfrom about 200 cm−3 in marine air to a maximum value ofabout 8× 105 cm−3 in some cities.

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The electrical conductivity of the air is generally dominatedby small ions. However, when the concentrations of largeions and uncharged aerosols are large, as they are in cities,the concentration of small ions tends to be low due to theircapture by both large ions and uncharged aerosols.

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Consequently, the electrical conductivity of air is a mini-mum when the concentration of large ions is a maximum.

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Consequently, the electrical conductivity of air is a mini-mum when the concentration of large ions is a maximum.

The observed decrease of at least 20% in the electrical con-ductivity of the air over the North Atlantic Ocean duringthe 20th century is attributed to a doubling in the concen-tration of particles with diameters between 0.02 and 0.2 µm,probably due to pollution.

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Sources

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Sourcesa. Biological

Solid and liquid particles are released into the atmospherefrom plants and animals. These emissions, which includeseeds, pollen, spores, and fragments of animals and plants,are usually 1-250 µm in diameter. Bacteria, algae, protozoa,fungi, and viruses are generally <1 µm in diameter.

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Microorganisms live on skin: when you change your clothes,you can propel ∼ 104 bacteria per minute into the air, withdiameters from 1–5 µm.

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Microorganisms live on skin: when you change your clothes,you can propel ∼ 104 bacteria per minute into the air, withdiameters from 1–5 µm.

The oceans are an important source of biogenic aerosols.They are injected into the atmosphere by the bursting ofair bubbles and by sea foam.

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Production of film droplets and jet drops when an air bubble bursts at

the surface of water. The time between (a) and (d) is ∼2 ms.

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Smoke from forest fires is a large source of atmosphericaerosols. Small smoke particles and fly ash are injected di-rectly into the air by forest fires. It is estimated that about54 Tg of particles are released into atmosphere each year bybiomass burning.

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Smoke from forest fires is a large source of atmosphericaerosols. Small smoke particles and fly ash are injected di-rectly into the air by forest fires. It is estimated that about54 Tg of particles are released into atmosphere each year bybiomass burning.

The number distribution of particles from forest fires peakat ∼0.1 µm diameter, which makes them efficient cloud con-densation nuclei. Some biogenic particles (e.g., bacteriafrom vegetation) may nucleate ice in clouds.

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b. Solid EarthThe transfer of particles to the atmosphere from the Earth’ssurface is caused by winds and atmospheric turbulence.

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On the global scale, semi-arid regions and deserts (whichcover about one-third of the land surface) are the mainsource of particles from the Earth’s surface. They provide∼2,000Tg per year of mineral particles.

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On the global scale, semi-arid regions and deserts (whichcover about one-third of the land surface) are the mainsource of particles from the Earth’s surface. They provide∼2,000Tg per year of mineral particles.

Volcanoes inject gases and particles into the atmosphere.The large particles have short residence times, but the smallparticles can be transported globally, particularly if they areblasted to high altitudes.

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c. AnthropogenicThe global input of particles into the atmosphere from an-thropogenic activities is ∼20% (by mass) of that from nat-ural sources.

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The main anthropogenic sources of aerosols are dust fromroads, wind erosion of tilled land, biomass burning, fuelcombustion, and industrial processes.

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In 1997 the worldwide direct emission into the atmosphereof particles <10 µm diameter from anthropogenic sourceswas estimated to be ∼350Tg per year. Particle emissionsworldwide were dominated by fossil fuel combustion (pri-marily coal) and biomass burning.

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These emissions are projected to double by the year 2040,due largely to anticipated increases in fossil fuel combustion,with the greatest growth in emissions from China and India.

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These emissions are projected to double by the year 2040,due largely to anticipated increases in fossil fuel combustion,with the greatest growth in emissions from China and India.

It is projected that by 2040, anthropogenic sources of par-ticles could be comparable to those from natural processes.

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d. In situ formationIn situ condensation of gases (i.e., g-to-p conversion) is im-portant in the atmosphere. Gases may condense onto ex-isting particles, thereby increasing the mass (but not thenumber) of particles, or gases may condense to form newparticles.

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The quantities of aerosols produced by g-to-p conversion ex-ceed those from direct emissions for anthropogenically de-rived aerosols, and they are comparable to direct emissionin the case of naturally derived aerosols.

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Organic and carbonaceous aerosols are produced by g-to-p conversion from gases released from the biosphere, andfrom volatile compounds such as crude oil that leak to theEarth’s surface. Carbonaceous particles emitted directlyinto the atmosphere derive mainly from biomass fires.

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Organic and carbonaceous aerosols are produced by g-to-p conversion from gases released from the biosphere, andfrom volatile compounds such as crude oil that leak to theEarth’s surface. Carbonaceous particles emitted directlyinto the atmosphere derive mainly from biomass fires.

Summary: Table 5.3 in Wallace & Hobbs summarizes the magnitudes of

the principal sources of direct emission of particles into the atmosphere

and in situ sources.8

Chemical CompositionExcept for marine aerosols, the masses of which are domi-nated by sodium chloride, sulfate is one of the prime con-tributors to the mass of atmospheric aerosols.

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Ammonium (NH+4 ) is the main cation associated with SO2−

4in continental aerosol. It is produced by gaseous ammonianeutralizing sulfuric acid to produce ammonium sulfate.

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In marine air the main contributors to the mass of inorganicaerosols are the ions Na+, Cl−, Mg2+, SO2+

4 , K+ and Ca2+.Such particles are particularly effective in scattering light(reducing visibility) and as cloud condensation nuclei.

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In marine air the main contributors to the mass of inorganicaerosols are the ions Na+, Cl−, Mg2+, SO2+

4 , K+ and Ca2+.Such particles are particularly effective in scattering light(reducing visibility) and as cloud condensation nuclei.

Nitrate (NO−3 ) occurs in larger sized particles than sulfatein marine air. Since seawater contains negligible nitrate, thenitrate in these particles must derive from the condensationof gaseous HNO3 by g-to-p conversion.

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Many of the particles in urban smog are by-products of pho-tochemical reactions involving hydrocarbons and nitrogenoxides, which derive from combustion.

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Polycyclic aromatic hydrocarbons (PAHs) are of particularconcern because they are carcinogens. In the U.S.A. car-bonaceous materials can account for some 50% or more ofthe total dry aerosol mass.

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Polycyclic aromatic hydrocarbons (PAHs) are of particularconcern because they are carcinogens. In the U.S.A. car-bonaceous materials can account for some 50% or more ofthe total dry aerosol mass.

Elemental carbon (commonly referred to as “soot”) is acommon component of organic aerosols in the atmosphere,and is a strong absorber of solar radiation.

For example, in polluted air masses from India, elementalcarbon contributes about 10% to the mass of submicrometersized particles.

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TransportAerosols are transported by the airflows they encounter dur-ing the time they spend in the atmosphere. The transportcan be over inter-continental or even global scales.

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TransportAerosols are transported by the airflows they encounter dur-ing the time they spend in the atmosphere. The transportcan be over inter-continental or even global scales.

Thus, Saharan dust is transported to the Americas, anddust from the Gobi Desert can reach the west coast of NorthAmerica.

Sulpher dioxide (SO2) emitted from power plants in theUnited Kingdom can be deposited as sulfate far inland incontinental Europe.

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SinksOn average, particles are removed from the atmosphere atabout the same rate as they enter it.

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Small particles can be converted into larger particles by co-agulation. Although coagulation does not remove particlesfrom the atmosphere, it modifies their size spectra and shiftssmall particles into size ranges where they can be removedby other mechanisms.

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Small particles can be converted into larger particles by co-agulation. Although coagulation does not remove particlesfrom the atmosphere, it modifies their size spectra and shiftssmall particles into size ranges where they can be removedby other mechanisms.

Improvements in visibility that frequently follow periods ofprecipitation are due, in large part, to the removal (i.e.,scavenging) of particles by precipitation. It is estimatedthat, on the global scale, precipitation processes accountfor about 80–90% of the mass of particles removed from theatmosphere.

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The terminal fall speeds of particles >1 µm diameter aresufficiently large that gravitational settling (i.e., dry depo-sition) is important as a removal process.

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For example, the fall speeds of particles 1 and 10 µm indiameter are ∼ 3× 10−5 and ∼ 3× 10−3 ms−1, respectively.

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For example, the fall speeds of particles 1 and 10 µm indiameter are ∼ 3× 10−5 and ∼ 3× 10−3 ms−1, respectively.

Some 10–20% of the mass of particles removed from theatmosphere is by dry fallout.

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Concentrations.One of the oldest and most convenient techniques (which invarious forms is still in widespread use) for measuring theconcentrations of particles in the air is the Aitken nucleuscounter. For details, see Wallace & Hobbs.

The concentration of particles measured with this instru-ment is referred to as the Condensation Nucleus (CN) count.

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Concentrations.One of the oldest and most convenient techniques (which invarious forms is still in widespread use) for measuring theconcentrations of particles in the air is the Aitken nucleuscounter. For details, see Wallace & Hobbs.

The concentration of particles measured with this instru-ment is referred to as the Condensation Nucleus (CN) count.

Condensation nucleus counts near the Earth’s surface varywidely from one location to another, and they can also fluc-tuate by more than an order of magnitude with time at anyone site.

Generally, they range from average values of ∼103 cm−3 overthe oceans, to ∼104 cm−3 over rural land areas, to ∼105 cm−3

or greater in urban polluted air.

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Size Distributions

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Size DistributionsAtmospheric aerosol particles range in size from ∼10−4 µmto tens of micrometers. The averages of numerous measure-ments of aerosol size spectra in continental, marine, andurban polluted air are shown in the following Figure.

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Size DistributionsAtmospheric aerosol particles range in size from ∼10−4 µmto tens of micrometers. The averages of numerous measure-ments of aerosol size spectra in continental, marine, andurban polluted air are shown in the following Figure.

The measurements are plotted in the form of a number dis-tribution in which the ordinate dN/d(log N) and the abscissa(D) are plotted on logarithmic scales, where dN is the num-ber concentration of particles with diameters between D andD + dD.

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Number distributions of tropospheric particles in continental (red),

marine (blue), and urban polluted (black) air.16

Conclusions from results shown in Figure.

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• The concentrations of particles fall off rapidly as theyincrease in size. The total number concentration is domi-nated by particles with diameters <0.2 µm, called Aitkennuclei or condensation nuclei.

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• The concentrations of particles fall off rapidly as theyincrease in size. The total number concentration is domi-nated by particles with diameters <0.2 µm, called Aitkennuclei or condensation nuclei.

• Those portions of the number distribution curves thatare straight lines in the Figure can be represented by anexpression of the form

logdN

d(log D)= const− β log D

or, taking antilogs,

dN

d(log D)= CD−β

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• The number distributions of particles shown confirm CNmeasurements, which indicate that the total concentra-tions of particles are, on average, greatest in urban pol-luted air and least in marine air.

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• The number distributions of particles shown confirm CNmeasurements, which indicate that the total concentra-tions of particles are, on average, greatest in urban pol-luted air and least in marine air.

• The concentrations of particles with diameters >2 µm (gi-ant particles) are, on average, rather similar in continen-tal, marine, and urban polluted air.

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5. Air Pollution

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5. Air PollutionIn urban and industrialized locations anthropogenic emis-sions can become so large that the concentrations of vari-ous undesirable chemical species (pollutants) cause signifi-cant deterioration in air quality and visibility, and can posethreats to human health.

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Severe air pollution episodes, and associated visibility re-duction, occur when the rates of emissions or formationof pollutants greatly exceed the rates at which the pollu-tants are dispersed by winds and vertical transport or areremoved from the atmosphere by chemical reactions or de-position.

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Severe air pollution episodes, and associated visibility re-duction, occur when the rates of emissions or formationof pollutants greatly exceed the rates at which the pollu-tants are dispersed by winds and vertical transport or areremoved from the atmosphere by chemical reactions or de-position.

Dispersion in the vertical is particularly restricted understable conditions.

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Sources of PollutantsCombustion (in power plants, smelters, automobiles, and ofwood, vegetation, etc.) is the largest source of air pollu-tants.

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On a global scale fossil-fuel combustion is the major sourceof CO, CO2, NOx, and SO2. Many other pollutants arereleased into the air by combustion.

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The most common fuels are hydrocarbon compounds (oil,natural gas, coal, and wood). About 15% of the total emis-sions of hydrocarbons are from anthropogenic sources.

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The most common fuels are hydrocarbon compounds (oil,natural gas, coal, and wood). About 15% of the total emis-sions of hydrocarbons are from anthropogenic sources.

Ideal (or complete) combustion of a hydrocarbon fuel yieldsonly CO2 and H2O. However, for a given quantity of fuel, aprecise amount of oxygen is required for complete combus-tion, and this ideal combination of fuel and oxygen is rarelyachieved.

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Modern internal combustion engines with catalytic convert-ers that reduce the emissions of particularly undesirableemissions, have quite small emissions.

Older automobiles tended to run with a fuel-rich mixtureand these emit considerable pollutants.

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Even modern automobiles operate in a “fuel-rich mode” foralmost 1 minute after the engine is started, and for severalseconds when driven at full throttle, and they produce asignificant fraction of the CO.

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Emissions of NOx from automobiles play a key role in theformation of photochemical smog.

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Emissions of NOx from automobiles play a key role in theformation of photochemical smog.

There are also lower temperature sources of air pollutants;for example, leakages of hydrocarbons from natural gas lines,organics from the evaporation of solvents, and nitrogen gasesfrom fertilizers.

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Anthropogenic activities also emit large numbers of parti-cles into the atmosphere, both directly and through g-to-pconversion.

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For particles >5µm diameter, human activities worldwideare estimated to produce ∼15% of natural emissions, withindustrial processes, fuel combustion, and g-to-p conversionaccounting for ∼80% of the anthropogenic emissions.

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However, in urban areas anthropogenic sources are muchmore important.

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However, in urban areas anthropogenic sources are muchmore important.

For particles <5 µm diameter, human activities produce∼20% of natural emissions, with g-to-p conversion account-ing for ∼90% of the human emissions.

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SmogsThe term smog derives from smoke and fog. The term isnow applied to all forms of severe air pollution, particularlyin urban areas, that restrict visibility.

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a. “London smog”Prior to the introduction of air pollution abatement laws inthe latter part of the twentieth century, many large cities inEurope and North America regularly suffered from severesmogs.

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In the London (or classical) type of smog, particles swell insize under high relative humidity and some of the particlesserve as nuclei on which fog droplets form.

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In the London (or classical) type of smog, particles swell insize under high relative humidity and some of the particlesserve as nuclei on which fog droplets form.

Sulfur dioxide gas absorbs into the fog droplets where it isoxidized to form sulfuric acid.

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In December 1952, cold air moved from the English Chan-nel and settled over London. This produced a pollution-trapping inversion fog. Over the next five days London ex-perienced its worst air pollution episode.

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The smog was so thick that people had to grope their wayalong the streets, buses crawled along at a walking paceled by pedestrians with flashlights, and indoor events werecancelled because the stage could not be seen.

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4,000 people had died of respiratory problems, and the smogwas implicated in an additional 8,000 deaths in the monthsthat followed.

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4,000 people had died of respiratory problems, and the smogwas implicated in an additional 8,000 deaths in the monthsthat followed.

In the Great Smog, as it was called, SO2 reached peak mix-ing ratios of about 0.7 ppmv (compared to typical annualmean mixing ratios of about 0.1 ppmv in polluted citieswith large coal usage), and the peak particle concentrationswere 1.7mgm −3.

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After the Great Smog, laws were passed in Britain and else-where banning the use of coal on open fires for domesticheating and the emissions of black smoke, and requiring in-dustries to switch to cleaner-burning fuels.

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In particular, bituminous coal has been banned in Dublinfor about twenty years.

Thank you, Mary Harney!

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In particular, bituminous coal has been banned in Dublinfor about twenty years.

Thank you, Mary Harney!

Nevertheless, pollution is still prevalent in significant quan-tities in many cities in Europe and the United States. Also,many large cities, particularly in developing countries (e.g.,China, India), still suffer from London-type smogs due tothe burning of coal and wood and to the lack of strict airpollution controls.

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b. Photochemical (or Los Angeles) smogDuring the second half of the 20th century emissions fromautomobiles became increasingly a source of pollutants inmany urban areas.

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When subjected to sunlight, and stagnant meteorologicalconditions, the combination of chemical species in stronglypolluted urban air can lead to photochemical (or Los Angeles-type) smog.

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These smogs are characterized by high concentrations of alarge variety of pollutants, such as nitrogen oxides, O3, CO,hydrocarbons, aldehydes (and other materials that are eyeirritants), and sometimes sulfuric acid.

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These smogs are characterized by high concentrations of alarge variety of pollutants, such as nitrogen oxides, O3, CO,hydrocarbons, aldehydes (and other materials that are eyeirritants), and sometimes sulfuric acid.

The chemical reactions that lead to photochemical smogare complex, and still not completely understood. For anoutline, see Wallace & Hobbs.

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Regional and Global Pollution

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Regional and Global PollutionThe effects of anthropogenic pollution now extend to re-gional and global scales.

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Europe, Russia, the northeastern United States, India, andlarge areas of southeastern Asia, are regularly covered byenormous palls of polluted air that reduce visibility signif-icantly, produce acid deposition, soil and erode buildingsand other materials, and have deleterious effects on humanhealth, animals and plants.

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Europe, Russia, the northeastern United States, India, andlarge areas of southeastern Asia, are regularly covered byenormous palls of polluted air that reduce visibility signif-icantly, produce acid deposition, soil and erode buildingsand other materials, and have deleterious effects on humanhealth, animals and plants.

The fact that pollutants can be transported over large dis-tances is well illustrated by air pollution episodes in the Arc-tic, known as arctic haze, which can be as severe as those incities. The pollutants originate from fossil-fuel combustion,smelting, and other industrial processes in northern Europeand Russia.

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The pollutants are transported to the Arctic by synoptic-scale airflow patterns, primarily from December to April.Since the arctic atmosphere is generally stably stratifiedduring this time of the year, vertical mixing is limited; also,precipitation is rare so that wet removal processes are weak.

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Consequently, the pollutants can be transported over largedistances with relatively little dilution. A major contributorto arctic haze is SO2, which is converted to sulfate particlesover the long transport distances.

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Consequently, the pollutants can be transported over largedistances with relatively little dilution. A major contributorto arctic haze is SO2, which is converted to sulfate particlesover the long transport distances.

Glacial records show that air pollution in the Arctic hasincreased markedly since the 1950s, paralleling the increasesin SO2 and NOx emissions in Europe.

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Ice cores from Greenland show unusually high lead con-centrations from ∼500 B.C. to 300 A.D. This is attributedto Greek and Roman lead and silver mining and smeltingactivities, which apparently polluted large regions of theNorthern Hemisphere.

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Ice cores from Greenland show unusually high lead con-centrations from ∼500 B.C. to 300 A.D. This is attributedto Greek and Roman lead and silver mining and smeltingactivities, which apparently polluted large regions of theNorthern Hemisphere.

Anthropogenic influences are now apparent on a global scale,as illustrated by the world-wide increase in CO2 concen-trations since the Industrial Revolution (following Figure).Other trace gases (e.g., CH4 and N2O) also show increasingconcentrations worldwide over the past 150 years or so.

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Changes in the concentrations of (a) CO2, (b) CH4, and (c) N2O over

300 years deduced from analyses of ice cores.30

6. Tropospheric Chemical Cycles

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6. Tropospheric Chemical CyclesThe reservoirs of chemical species in the Earth system arethe solid Earth, the hydrosphere, the cryosphere, the bio-sphere and the atmosphere. Chemical species can be trans-ferred between these reservoirs.

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Since, under steady-state conditions, a chemical species can-not accumulate indefinitely in any of the reservoirs, theremust be continual cycling of species through the variousreservoirs. This is termed biogeochemical cycling.

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Since, under steady-state conditions, a chemical species can-not accumulate indefinitely in any of the reservoirs, theremust be continual cycling of species through the variousreservoirs. This is termed biogeochemical cycling.

Here we will consider the tropospheric portions of the bio-geochemical cycles of nitrogen and sulfur. We will be con-cerned with relatively rapid interchanges involving the at-mosphere and other reservoirs (generally the oceans and thebiosphere).

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Many aspects of global chemical cycles are not well un-derstood, therefore, in many cases, the magnitudes of thesources and sinks given are estimates.

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Many aspects of global chemical cycles are not well un-derstood, therefore, in many cases, the magnitudes of thesources and sinks given are estimates.

Also, the magnitudes of the various emission fluxes are notnecessarily equivalent to the importance of the species, sinceatmospheric residence times must also be taken into ac-count.

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The Nitrogen Cycle

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The Nitrogen CycleNitrogen gas (N2) constitutes more than 99.99% of the ni-trogen present in the atmosphere, and N2O makes up morethan 99% of the rest of the nitrogen.

The other nitrogen species in the atmosphere are thereforepresent in very low concentrations, but they are of crucialimportance in atmospheric chemistry.

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For example, NH3 is the only basic gas in the atmosphere.Therefore, it is solely responsible for neutralizing acids pro-duced by the oxidation of SO2 and NO2.

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For example, NH3 is the only basic gas in the atmosphere.Therefore, it is solely responsible for neutralizing acids pro-duced by the oxidation of SO2 and NO2.

Nitric oxide and NO2 play important roles in both tropo-spheric and stratospheric chemistry.

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The principal sources and sinks of nitrogen-containing speciesin the atmosphere are shown in the following Figure. Themain atmospheric sources of nitrogen-containing species arebiogenic emissions from the Earth and the oceans (NH3,N2O, and NOx), decomposition of proteins and urea fromanimals (NH3), biomass burning and fossil fuel consumption(NOx, NH3, and N2), and lightning (NOx).

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The main sinks are wet removal by precipitation (NH3 andNOx as NO−3 ), dry deposition (NOx and NH3), and thechemical breakdown of N2O in the stratosphere.

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The main sinks are wet removal by precipitation (NH3 andNOx as NO−3 ), dry deposition (NOx and NH3), and thechemical breakdown of N2O in the stratosphere.

Since anthropogenic sources of NH3, N2O, and NOx (fromfossil fuel consumption, biomass burning, and agriculturalnitrate fertilization) are appreciable, they may be causingsignificant perturbations in the budgets of these species inthe atmosphere.

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Principal sources and sinks of nitrogen-containing gases inthe atmosphere.

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The Sulfur Cycle

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The Sulfur CycleThe most important reduced sulfur gases in the air are H2S,DMS, COS, and CS2. Their main natural sources are bio-genic reactions in soils, marshland and plants.

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Biogenic reactions in the oceans, due primarily to phyto-plankton, are sources of DMS, COS and CS2. Estimatesof the fluxes of these natural emissions of sulfur gases, andtheir transformations to SO2 and SO2−

4 are given in the fig-ure below. DMS dominates the emissions of sulfur gasesfrom the oceans.

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Biogenic reactions in the oceans, due primarily to phyto-plankton, are sources of DMS, COS and CS2. Estimatesof the fluxes of these natural emissions of sulfur gases, andtheir transformations to SO2 and SO2−

4 are given in the fig-ure below. DMS dominates the emissions of sulfur gasesfrom the oceans.

The global sulfur budget is significantly affected by anthro-pogenic emissions. These are almost entirely in the formof SO2, and 90% are from the Northern Hemisphere. Themain sources are the burning of coal and the smelting ofsulfide ores.

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The main mechanisms for removing sulfur from the atmo-sphere are wet and dry deposition.

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The main mechanisms for removing sulfur from the atmo-sphere are wet and dry deposition.

For example, of the 80 Tg(S) per year of SO2 that are ox-idized to SO2−

4 , about 70 Tg(S) per year occurs in clouds,which is subsequently wet deposited.

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Principal sources and sinks of sulpher-containing gases inthe atmosphere.

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Residence Time. Spatial Scales

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Residence Time. Spatial ScalesIf the globally-averaged concentration of a trace constituentin the atmosphere does not change significantly over a giventime period, the rate at which the constituent is injectedinto (and/or produced within) the atmosphere must equalthe rate at which it is removed from the atmosphere.

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Under such steady-state conditions, we can define the res-idence time (or lifetime) τ of a trace constituent in the at-mosphere as

τ =M

Fwhere M is the amount of the constituent in the atmosphereand F the rate of its removal from the atmosphere.

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Under such steady-state conditions, we can define the res-idence time (or lifetime) τ of a trace constituent in the at-mosphere as

τ =M

Fwhere M is the amount of the constituent in the atmosphereand F the rate of its removal from the atmosphere.

An analogy is a tank of water, which can represent the at-mosphere. Assume water is pumped into and removed fromthe tank at rate F . The mean time spent by each small el-ement of water is M/F , where M is the volume of the tank.

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Spatial and temporal scales of variability for some atmospheric con-

stituents. The temporal scale is represented by residence time.40

If a chemical species has a very short residence time inthe atmosphere, significant variations in the concentrationof the species will generally occur over very short spatialscales.

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Species with short residence times will be present in highconcentrations close to localized sources, and in low concen-trations far removed from their sources.

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Species with short residence times will be present in highconcentrations close to localized sources, and in low concen-trations far removed from their sources.

The residence times of particles in the atmosphere as a func-tion of their size are shown below.

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Above: Schematiccurves of particle sur-face area distributionsfor urban polluted air(black line), continen-tal air (red line) andmarine air (blue line).

Below: principalsources and sinks ofatmospheric particlesand estimates of theirmean residence timesin the troposphere.

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4. Tropospheric Aerosols - UCD · Atmospheric aerosols are suspensions of small solid and/or liquid particles in air that have negligible terminal fall speeds. Molecular aggregates