I organic air pollutants I 1 Volatile Organic Compounds (vocs)
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I.1.2. INORGANIC AIR POLLUTANTSToday the atmosphere below 100 km contains only a few well-mixed gases that, together, make up more than 99 percent of all gas molecules in this region. These well-mixed gases are called fixed gases because their mixing ratios do not vary much in time or space respect to their magnitude. Nevertheless, it is the variable gases, whose mixing ratios are small but vary in time and space, that are the most important gases with respect to air pollution issues. I.1.2.1. Fixed GasesTable I.1.2 gives the volume mixing ratios of fixed gases in the atmosphere up to 100 km. At any altitude, O2(g) makes up about 20.95 percent and N2(g) makes up about 78.08 percent of all non-water gas molecules by volume. Although the mixing ratios of these gases are constant with increasing altitudes, their partial pressures decrease with increasing altitude because air pressure decreases with increasing altitude, and O2(g) and N2(g) partial pressures are constant fractions of air pressure. Table I.1.2.Volume Mixing Ratios of Fixed Gases in the Lowest 100 km of the Earth's Atmosphere
Together, N2(g) and O2(g) make up 99.03 percent of all gases in the atmosphere by volume. Argon (Ar) makes up most of the remaining 0.97 percent. Argon, the "lazy gas," is colorless and odorless. Like other noble gases, it is inert and does not react chemically. Other fixed but inert gases present in trace concentrations include neon, helium, krypton, and xenon. I.1.2.2. Variable GasesGases whose volume mixing ratios change in time and space are variable gases. Table I.1.3 summarizes the volume mixing ratios of some variable gases in the clean troposphere, the polluted troposphere (e.g., urban areas), and the stratosphere. Many organic gases degrade chemically before they reach the stratosphere, so their mixing ratios are low in the stratosphere. Table I.1.3. Volume Mixing Ratios of Some Variable Gases in Three Atmospheric Regions. Organic gases are included in the table although treated in section I.1.1.
In addition to the most important gases mentioned in table I.1.3., there are other gaseous compounds that can also induce air pollution problems that in the past where much more important in Europe, and now are relevant in certain non-developed countries in the world. This is the case of Hydrogen fluoride (HF(g)) and up to some regard Hydrogen Chloride (HCl(g)). | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Indoor Air Pollution | Outdoor Urban Air Pollution | Acid Deposition | Stratospheric Ozone Reduction | Global Climate Change |
| Gases | ||||
| Nitrogen dioxide Carbon monoxide Formaldehyde Sulfur dioxide Organic gases Radon | Ozone Nitric oxide Nitrogen dioxide Carbon monoxide Ethene Toluene Xylene PAH | Sulfur dioxide Sulfuric acid Nitrogen dioxide Nitric acid Hydrochloric acid Carbon dioxide | Ozone Nitric oxide Nitric acid Hydrochloric acid Chlorine; nitrate CFC-11 CFC-12 | Water vapor Carbon dioxide Methane Nitrous oxide Ozone CFC-11 CFC-12 |
| Aerosol Particle Components | ||||
| Black carbon Organic matter Sulfate Nitrate Ammonium Ammonium Allergens Soil dust Asbestos Sea spray Fungal spores Tobacco smoke | Black carbon Organic matter Sulfate; Nitrate Ammonium; Soil dust; Sea spray; Tire particles; Lead | Sulfur Nitrate Chloride | Chloride Sulfur Nitrate | Black carbon Organic matter Sulfur Nitrate Ammonium Soil dust Sea spray |
Water Vapor
The water vapor (H2O(g)) is the most important variable gas in the air. It is a greenhouse gas that readily absorbs thermal-IR radiation, but it is also a vital link in the hydrologic cycle on Earth. As a natural greenhouse gas, it is much more important than is carbon dioxide for maintaining a climate suitable for life on Earth. Water vapor is not considered an air pollutant; thus, no regulations control its concentration or emission.
The main source is evaporation from the oceans. Approximately 85 percent of water vapor originates from ocean-water evaporation. Table I.1.5 summarizes the sources and sinks of water vapor.
Table I.1.5. Sources and Sinks of Atmospheric Water Vapor
| Source | Sink |
| Evaporation from the oceans, lakes, rivers, and soil Sublimation from sea ice and snow Transpiration from plant leaves Kinetic reaction | Condensation to liquid water in clouds Vapor deposition to ice crystals in clouds Transfer to oceans, ice caps, and soils Kinetic reaction |
The water vapor has no harmful effects on humans. Liquid water in aerosol particles indirectly causes health problems when it comes in contact with pollutants because many gases dissolve in liquid water. Small drops can subsequently be inhaled, causing health problems in some cases.
Carbon Dioxide
Carbon dioxide (CO2(g)) is a colorless, odorless, natural greenhouse gas, that is pointed out as responsible for much of the global warming that has occurred to date. It is not an important outdoor air pollutant in the classic sense because it does not chemically react to form further products nor is it harmful to health at typical mixing ratios. CO2(g) plays a subtle role in stratospheric ozone depletion because global warming near the Earth's surface due to CO2(g) enhances global cooling of the stratosphere, and such cooling feeds back to the ozone layer. Mixing ratios of carbon dioxide are not regulated in any country. CO2(g) emission controls are the subject of an ongoing effort by the international community to reduce global warming.
Figure I.1.2. shows how outdoor CO2(g) mixing ratios have increased steadily since 1958 at the Mauna Loa Observatory, Hawaii. Average global CO2 (g) mixing ratios have increased from approximately 280 ppmv in the mid-1800s to approximately 370 ppmv today. The yearly increases are due to increased CO2(g) emission from fossil-fuel combustion. The seasonal fluctuation in CO2(g) mixing ratios is due to photosynthesis and bacterial decomposition. When annual plants grow in the spring and summer, photosynthesis removes CO2(g) from the air. When such plants die in the fall and winter, their decomposition by bacteria adds CO2(g) to the air. Typical indoor mixing ratios of CO2(g) are 700 to 2,000 ppmv, but can exceed 3,000 ppmv when un-ventilated appliances are used (Arashidani et al., 1996).
Figure I.1.2. Yearly and seasonal fluctuations in carbon dioxide mixing ratio at Mauna Loa Observatory, Hawaii, since 1958. Data for 1958-1999 from Keeling and Whorf (2000) and from 2000. Source: http://www.visionlearning.com/library/
Outdoor mixing ratios of CO2(g) are too low to cause noticeable health problems. In indoor air, CO2(g) mixing ratios may build up enough to cause some discomfort, but doses higher than 15,000 ppmv are necessary to affect human respiration. Mixing ratios higher than 30,000 ppmv are necessary to cause headaches, dizziness, or nausea (Schwarzberg, 1993). Such mixing ratios do not generally occur.
Carbon Monoxide
Carbon monoxide (CO(g)) is a tasteless, colorless, and odorless gas. Although CO(g) is the most abundantly emitted variable gas aside from CO2(g) and H2Og), it plays a small role in ozone formation in urban areas. In the background troposphere, it plays a larger role in ozone formation. CO(g) is not a greenhouse gas, but its emission and oxidation to CO2(g) affect global climate. CO(g) is not important with respect to stratospheric ozone reduction or acid deposition. CO(g) is an important component of urban and indoor air pollution because it has harmful short-term health effects.
Table I.1.6 summarizes the sources and sinks of CO(g). A major source of CO(g) is incomplete combustion in automobiles, trucks, and airplanes.
Table I.1.6. Sources and Sinks of Atmospheric Carbon Monoxide
| Sources | Sinks |
| Fossil-fuel combustion Biomass burning Photolysis and kinetic reaction Plants and biological activity in oceans | Kinetic reaction to carbon dioxide Transfer to soils and ice caps Dissolution in ocean water |
CO(g) emission sources include wildfires, biomass burning, non-transportation combustion, some industrial processes, and biological activities. Indoor sources of CO(g) include water heaters, coal and gas heaters, and gas stoves. The major sink of CO(g) is chemical conversion to CO2(g). It is also lost by deposition to soils and ice caps and dissolution in ocean water. Because it is relatively insoluble, its dissolution rate is slow.
Mixing ratios of CO(g) in urban air are typically 2 to 10 ppmv. On freeways and in traffic tunnels, they can rise to more than 100 ppmv. Typical CO(g) mixing ratios inside automobiles in urban areas range from 9 to 56 ppmv (Finlayson-Pitts and Pitts, 1999). In indoor air, hourly average mixing ratios can reach 6-12 ppmv when a gas stove is turned on (Samet et al., 1987). In the absence of indoor sources, CO(g) indoor mixing ratios are usually less than are those outdoors (Jones, 1999). In the free troposphere, CO(g) mixing ratios vary from 50 to 150 ppbv.
Exposure to 300 ppmv of CO(g) for one hour causes headaches; exposure to 700 ppmv of C0(g) for one hour causes death, CO(g) poisoning occurs when it dissolves in blood and replaces oxygen as an attachment to hemoglobin (Hb(aq)), an iron-containing compound. The conversion of O2Hb(aq) to COHb(aq) (carboxyhemoglobin) causes suffocation. CO(g) can also interfere with O2(g) diffusion in cellular mitochondria and with intracellular oxidation (Gold, 1992). For the most part, the effects of CO(g) are reversible once exposure to CO(g) is reduced. Following acute exposure, however, individuals may still express neurological or psychological symptoms for weeks or months, especially if they become unconscious temporarily (Choi, 1983).
Ozone
Ozone (O3(g)) is a relatively colorless gas at typical mixing ratios. Ozone exhibits an odor when its mixing ratio exceeds 0.02 ppmv. In urban smog or indoors, it is considered an air pollutant because of the harm that it does to humans, animals, plants, and materials. It is regulated in many other countries including the EU. In the stratosphere, the ozone's absorption of UV radiation provides a protective shield for life on Earth. Although ozone is considered to be "good" in the stratosphere and "bad" in the boundary layer, ozone molecules are the same in both cases.
Ozone is not emitted. Its only source into the air is chemical reaction. Sinks of ozone include reaction (with most of organic surfaces preferable), transfer to soils and ice caps, and dissolution in ocean waters. Because ozone is relatively insoluble, its dissolution rate is relatively slow. Table I.1.7 summarizes the sources and sinks of ozone.
Table I.1.7. Sources and Sinks of Atmospheric Ozone.
| Sources | Sinks |
| Chemical reaction of O(g) with O2(g) | Photolysis Kinetic reaction Transfer to soils and ice caps Dissolution in ocean water |
There is controversy about ozone background mixing ratios in the free troposphere, but they are in the range of 20 to 40 ppbv near sea level and 30 to 70 ppbv at higher altitudes. Ozone is a pollutant that is produced in the atmosphere and therefore picks are not necessarily related to urban or industrial areas, and may be seen in suburban or rural areas, downwind areas from where the precursors are emitted (Millán et al 1992, 1997, 2000, 2002). In urban air, ozone mixing ratios range from less than 0.01 ppmv at night to 0.50 ppmv (during afternoons downwind from the most polluted cities world wide, i.e. Los Angeles), with typical values of 0.15 ppmv during moderately polluted afternoons. It has a typical daily cycle that are characteristic of position with respect to the topography and the location where the precursors are emitted (Figure I.1.3.) Indoor ozone mixing ratios are almost always less than are those outdoors. In the stratosphere, peak ozone mixing ratios are around 10 ppmv.
Ozone causes headaches at mixing ratios greater than 0.15 ppmv, chest pains at mixing ratios greater than 0.25 ppmv, and sore throat and cough at mixing ratios greater than 0.30 ppmv. Ozone decreases lung function for people who exercise steadily for more than an hour while exposed to concentrations greater than 0.30 ppmv. Symptoms of respiratory problems include coughing and breathing discomfort. Small decreases in lung function affect people with asthma, chronic bronchitis, and emphysema. Ozone may also accelerate the ageing of lung tissue. At levels greater than 0.1 ppmv, ozone affects animals by increasing their susceptibility to bacterial infection. It also interferes with the growth of plants and trees and deteriorates organic materials, such as rubber, textile dyes and fibers, and some paints and coatings (U.S. EPA, 1978).
Figure I.1.3. Processes that result in typical Ozone cycles
Summary of recirculation processes in the East coast of Spain after MECAPIP (Meso-meteorological Cycles of Air Pollution in the Iberian Peninsula) and RECAPMA (Regional Cycles of Air Pollution in the West Central Mediterranean Area) projects as well as simplified model of Derwent & Davies (Derwent & Davies, 1994) relating NO emissions NO and Ozone production.
Ozone increases plant and tree stress and their susceptibility to disease, infestation, and death (Sanz & Millan, 2000). Ozone is absorbed through the leaf pores, and damages the cell membranes and the cells collapse as a result. Symptoms vary from stippling or flecking to bleached or dead areas. Upper leaf surfaces may have white to tan, purple or black flecking, which may also be visible on the lower leaf surface of some plants. Damage usually occurs when ozone concentrations are highest: in mid to late summer, during the early afternoon when the air is still. Symptoms are often more severe on leaves exposed to direct sunlight and on plants growing in moist, light soils with good fertility. Older leaves are most sensitive.
Sulfur Dioxide
Sulfur dioxide (SO2(g)) is a colorless gas that exhibits a taste at levels greater than 0.3 ppmv and a strong odor at levels greater than 0.5 ppmv. SO2(g) is a precursor to sulfuric acid (H2SO4(aq)), an aerosol particle component that affects acid deposition, global climate, and the global ozone layer. SO2(g) is now regulated in many countries, with effective measures that lead to decrease mixing ratios.
Some sources include coal-fired power plants, automobile tailpipes, but also natural sources like volcanoes. SO2(g) is also produced chemically in the air from biologically produced dimethylsulfide (DMS(g)) and hydrogen sulfide (H2S(g)). Sulfide is removed by chemical reaction, dissolution in water, and transfer to soils and ice caps. SO2(g) is relatively soluble. Table I.1.8. summarizes the major sources and sinks of SO2(g).
Table I.1.8. Sources and Sinks of Atmospheric Sulfur Dioxide
| Sources | Sinks |
| Oxidation of DMS(g) Volcanic emission Fossil-fuel combustion Mineral ore processing Chemical manufacturing | Kinetic reaction to H2SO4(g) Dissolution in cloud drops and ocean water Transfer to soils and ice caps |
In the background troposphere, SO2(g) mixing ratios range from 10 pptv to 1 ppbv. In polluted air, they range from 1 to 30 ppbv, and in extreme cases peaks of 5ppm (Millan,M & Sanz,MJ, 1993). SO2(g) levels are usually lower indoors than outdoors. The indoor to outdoor ratio of SO2(g) is typically between 0.1:1 to 0.6:1 in buildings without indoor sources (Jones, 1999). In one study, indoor mixing ratios were found to be 30 to 57 ppbv in homes equipped with kerosene heaters or gas stoves (Leaderer et al., 1984, 1993).
Because SO2(g) is soluble, it is absorbed in the mucous membranes of the nose and respiratory tract. Sulfuric acid (H2SO4(aq)) is also soluble, but its deposition rate into the respiratory tract depends on the size of the particle in which it dissolves (Maroni et al., 1995). High concentrations of SO2(g) and H2SO4(aq) can harm the lungs (Islam and Ulmer, 1979). Long-term exposure to SO2(g) from coal burning is associated with impaired lung function and other respiratory ailments (Qin et al., 1993). Injury to plants is also well documented, symptoms appear as ivory to brown areas between the veins (inter-vein) and along the leaf edges. Uninjured tissue next to the veins remains green. Injury is likely to occur at mid-day on plants growing in moist soil.
Nitric Oxide
Nitric oxide (NO(g)) is a colorless gas and a free radical. It is important because it is a precursor to tropospheric ozone, nitric acid (HNO3(g)), and particulate nitrate (NO3-). Whereas NO(g) does not directly affect acid deposition, nitric acid does. Whereas NO(g) does not affect climate, ozone and particulate nitrate do. Natural NO(g) reduces ozone in the upper stratosphere. Emissions of NO(g) from jets that fly in the stratosphere also reduce stratospheric ozone. Outdoor levels of NO(g) are not regulated in any country.
NO(g) is emitted by microbes in soils and plants during denitrification, and it is produced by lightning, combustion, and chemical reactions. Combustion sources include aircraft, automobiles, oil refineries, and biomass burning. The primary sink of NO(g) is the chemical reaction. Table I.1.9. summarizes the sources and sinks of NO(g).
Table I.1.9. Sources and Sinks of Atmospheric Nitric Oxide.
| Sources | Sinks |
| Denitrification in soils and plants Lightning Fossil-fuel combustion Biomass burning Photolysis and kinetic reaction | Kinetic reaction Dissolution in ocean water Transfer to soils and ice caps |
A typical sea-level mixing ratio of NO(g) in the background troposphere is 5 pptv. In the upper troposphere, NO(g) mixing ratios are 20 to 60 pptv. In urban regions, NO(g) mixing ratios reach 0.1 ppmv in the early morning, but may decrease to zero by midmorning due to the reaction with ozone.
Nitric oxide has no known harmful human health effects at typical outdoor or indoor mixing ratios.
Nitrogen Dioxide
Nitrogen dioxide (NO2(g)) is a brown gas with a strong odor. NO2(g) is an intermediary between NO(g) emission and O3(g) formation. It is also a precursor to nitric acid, a component of acid deposition. Natural NO2(g), like natural NO(g), reduces ozone in the upper stratosphere. It is now regulated in many countries.
Its major source is the oxidation of NO(g). Minor sources are fossil fuel combustion and biomass burning. During combustion or burning, NO2(g) emissions are about 5 to 15 percent those of NO(g). Table I.1.10. summarizes sources and sinks of NO2(g).
Table I.1.10. Sources and Sinks of Atmospheric Nitrogen Dioxide.
| Sources | Sinks |
| Photolysis and kinetic reaction Fossil-fuel combustion Biomass burning | Photolysis and kinetic reaction Dissolution in ocean water Transfer to soils and ice caps |
Indoor sources of NO2(g) include gas appliances, kerosene heaters, wood-burning stoves, and cigarettes. Sinks of NO2(g) include photolysis, chemical reaction, dissolution into ocean water, and transfer to soils and ice caps. NO2(g) is relatively insoluble in water.
Mixing ratios of NO2(g) near sea level in the free troposphere range from 20 to 50 pptv. In the upper troposphere, mixing ratios are 30 to 70 pptv. In urban regions, they range from 0.1 to 0.25 ppmv. Outdoors, NO2(g) is more prevalent during early morning than during midday or afternoon because sunlight breaks down most NO2(g) past midmorning, normally opposite to ozone. In homes with gas-cooking stoves or unvented gas space heaters, weekly average NO2(g) mixing ratios can range from 21 to 50 ppbv, although peak mixing ratios may reach 400-1,000 ppbv (Spengler, 1993; Jones et al., 1999).
Although exposure to high mixing ratios of NO2(g) harms the lungs and increases respiratory infections (Frampton et al., 1991), epidemiological evidence indicates that exposure to typical mixing ratios of NO2(g) has little effect on the general population. Children and asthmatics are more susceptible to illness associated with high NO2(g) mixing ratios than are adults (Li et al., 1994). Pilotto et al. (1997) found that levels of NO2(g) greater than 80 ppbv resulted in increased reports of sore throats, colds, and absences from school. Goldstein et al. (1988) found that exposure to 300 to 800 ppbv NO2(g) in kitchens reduced lung capacity by about 10 percent. NO2(g) may trigger asthma by damaging or irritating and sensitizing the lungs, making people more susceptible to allergic response to indoor allergens (Jones, 1999). At mixing ratios unrealistic under normal indoor or outdoor conditions, NO2(g) can result in acute bronchitis (25 to 100 ppmv) or death (150 ppmv). In plants, acute symptoms appear as ivory to brown areas between the veins (inter-vein) and along the leaf edges. Uninjured tissue next to the veins remains green and damage occurs at night. Nitrous oxide also causes yellowing of leaf margins and internervial chlorosis.
Lead
Lead (Pb(s)) is a grey-white, solid heavy metal with a low melting point that is present in air pollution as an aerosol particle component. It was first regulated as a criteria air pollutant in the United States in 1976. Many countries now regulate the emission and outdoor concentration of lead.
Lead is emitted during combustion of leaded fuel, manufacture of lead-acid batteries, crushing of lead ore, condensation of lead fumes from lead-ore smelting, solid-waste disposal, uplift of lead-containing soils, and crustal weathering of lead ore. Between the 1920s and the 1970s, the largest source of atmospheric lead was automobile combustion. Table I.1.11 summarizes the sources and sinks of atmospheric lead.
Table I.1.11. Sources and Sinks of Atmospheric Lead.
| Sources | Sinks |
| Leaded-fuel combustion Lead-acid battery manufacturing Lead-ore crushing and smelting Dust from soils contaminated with lead-based paint Solid-waste disposal Crustal physical weathering | Deposition to soils, ice caps and oceans Inhalation |
In December 1921, Thomas J. Midgley Jr. (1889-1944) discovered that tetraethyl lead was a useful fuel additive for reducing engine knock, increasing octane levels, and increasing engine power and efficiency in automobiles. Midgley experienced lead poisoning. In 1925, the U.S. head of the Public Health Service put together a committee to study the health effects of tetraethyl lead. Moreover, it was argued that because no regulatory precedent existed, the committee would have to find striking evidence of serious and immediate harm for action to be taken against lead (Kovarik, 1999). Based on measurements that showed lead contents in faecal pellets of typical drivers and garage workers lower than those of lead-industry workers, and based on the observations that drivers and garage workers had not experienced direct lead poisoning, they concluded that there were "no grounds for prohibiting the use of ethyl gasoline" (U.S. Public Health Service, 1925). He did caution that further studies should be carried out (U.S. Public Health Service, 1925). Despite the caution, more studies were not carried out for thirty years, and effective opposition to the use of leaded gasoline ended. By the mid-1930s, 90 percent of U.S. gasoline was leaded. Industrial backing of lead became so strong in 1936, that only in 1959 did the U.S. Public Health Service reinvestigate the issue of tetraethyl lead. At that time, they found it "regrettable that the investigations recommended by the Surgeon General's Committee in 1926 were not carried out by the Public Health Service" (U.S. Public Health Service, 1959). Despite the concern, tetraethyl lead was not regulated as a pollutant in the United States until 1976. In 1975, the catalytic converter, which reduced emission of carbon monoxide, hydrocarbons, and eventually oxides of nitrogen from cars, was invented. Because lead deactivates the catalyst in the catalytic converter, cars using catalytic converters could run only on unleaded fuels. Thus, the required use of the catalytic converter in new cars inadvertently provided a convenient method to phase out the use of lead. The regulation of lead as a criteria air pollutant in the United States in 1976 due to its health effects also hastened the phase out of lead as a gasoline additive. Between 1970 and 1997, total lead emissions in the United States decreased from 219,000 to 4,000 short tons per year. Since the 1980s, leaded gasoline has been phased out in many countries, although it is still an additive to gasoline in several others.
Ambient concentrations of lead between 1988 and 1997 decreased from about 0.17 to 0.06 g m-3, or by 67 percent (U.S. EPA, 1998). The highest concentrations of lead are now found near lead-ore smelters and battery manufacturing plants in the States. But developing countries still have high harmful concentrations.
Health effects of lead were known by the early Romans. Lead accumulates in bones, soft tissue, and blood. It can affect the kidneys, liver, and the nervous system. Severe effects of lead poisoning include mental retardation, behavior disorders, and neurological impairment. A disease associated with lead accumulation is plumbism. Symptoms at various stages include abdominal pains, a black line near the base of the gums, paralysis, loss of nerve function, dizziness, blindness, deafness, coma, and death. Low doses of lead have been linked to nervous system damage in fetuses and young children, resulting in learning deficits and low IQs. Lead may also contribute to high blood pressure and heart disease (U.S. EPA, 1998). Many plant species accumulate lead up to very high concentrations, and can be used for bioremediation. On the other hand, edible plants that accumulate lead can be harmful for humans.
Ammonia
Ammonia is a colorless alkaline gas (NH3(g)). It is a precursor to the formation of secondary particles in the atmosphere. Gaseous ammonia reacts chemically with other gases and particles which can produce particulate matter (PM) such as ammonium nitrate (NH4NO3) or ammonium sulfate ((NH4)2SO4) with diameters less than 2.5 µm (PM2.5) (CAC, 1995). These fine particles cause the greatest concern for human health. In low concentrations, it has a penetrating pungent sharp odor. In high concentrations, it causes a smothering sensation when inhaled. They can penetrate deep into the lungs where they may cause irritation and exacerbate lung disease. Particulate matter and ammonia are also linked to air quality issues such as reduced visibility.
Hydrogen Fluorine
Hydrogen fluoride (HFl (g)) is a colorless gas or fuming liquid (Hydrofluoric acid) and it is strongly irritant and very corrosive. It is used extensively in industry, especially as an intermediate in the manufacture of most fluoride-containing products. The odor detection limit is around 30-130 μg/m3. Humans are reasonable tolerant, but it is the most toxic pollutant where plants are concerned and it may also have profound effects on grazing animals if excessive amounts contaminate their forage (Weinstein and Davison, 2004). In plants visible injury is very characteristic as marginal chlorosis that can evolve to necrosis as concentric bands.
I.1.2.4. Aerosol Particles in Smog and the Global Environment
Although most regulations of air pollution focus on gases, aerosol particles cause more visibility degradation and possibly more health problems than do gases. Particles smaller than 2.5 m in diameter cause the most severe health problems. Particles enter the atmosphere by emissions and nucleation. In the air, their number concentrations and sizes change by coagulation, condensation, chemistry, water uptake, rainout, sedimentation, dry deposition, and transport. Particle concentration, size, and morphology affect the irradiative energy balance in urban air and in the global atmosphere.
Size Distributions
Aerosol and hydrometeor particles are characterized by their size distribution and composition. A size distribution is the variation of concentration (i.e., number, surface area, volume, or mass of particles per unit volume of air) with size. Table I.1.12 compares typical diameters, number concentrations, and mass concentrations of gases, aerosol particles, and hydrometeor particles under lower tropospheric conditions. The table indicates that the number and mass concentrations of gas molecules are much greater than are those of particles. The number concentration of aerosol particles decreases with increasing particle size. The number concentrations of hydrometeor particles are typically less than are those of aerosol particles, but the mass concentrations of hydrometeor particles are always greater than those of aerosol particles.
The aerosol particle size distributions can be divided into modes, which are region of the size spectrum (in diameter space) in which distinct peaks in concentration occur.
Table I.1.12. Characteristics of Gases, Aerosol Particles, and Hydrometeor Particles
| | Typical diameter (m) | Number Concentration (molecules or particles cm-3) | Mass Concentration (g m-3) |
| Gas molecules Aerosol particles Small Medium Large Hydrometeor particles Fog drops Cloud drops Drizzle Raindrops | 0.0005 <0.2 0.2-2.0 >2.0 10-20 10-200 200-1,000 1,000-8,000 | 2.45 x 1019 103 - 106 1 – 104 < 1 – 10 1 – 500 1 – 1000 0.01 – 1 0.001 – 0.01 | 1.2 x 109 < 1 < 250 104 – 106 104 – 107 105 - 107 105 - 107 |
Aerosol particle distributions with the 1, 2, 3, or 4 modes are called unimodal, bimodal, trimodal, or quadrimodal, respectively. Such modes may include a nucleation mode, two sub-accumulation modes, and a coarse mode. The nucleation mode (mean diameters less than 0.1 (m) contains small emitted particles or newly nucleated particles (particles formed directly from the gas phase). Small nucleated or emitted particles increase in size by coagulation (collision and coalescence of particles) and growth (condensation of gases onto particles). Only a few gases, such as sulfuric acid, water, and some heavy organic gases, among others, condense onto particles. Molecular oxygen and nitrogen, which make up the bulk of the gas in the air, do not.
Growth and coagulation move nucleation mode particles into the accumulation mode, where diameters are 0.1 to 2 m. Some of these particles are removed by rain, but they are too light to fall out of the air by sedimentation (dropping by their own weight against the force of drag). The accumulation mode sometimes consists of two sub-modes with mean diameters near 0.2 m and 0.5 to 0.7 m (Hering and Friedlander, 1982; John et al., 1989), possibly corresponding to newer and aged particles, respectively. The accumulation mode is important for two reasons. First, accumulation mode particles are likely to affect health by penetrating deep into the lungs. Second, accumulation mode particles are close in size to the peak wavelengths of visible light and, as a result, affect visibility. Particles in the nucleation and accumulation modes together are fine particles.
The coarse mode consists of particles larger than 2 m in diameter. These particles originate from windblown dust, sea spray, volcanoes, plants, and other sources. Coarse mode particles are generally heavy enough to sediment out rapidly within hours to days. The emission sources and deposition sinks of fine particles differ from those of coarse mode particles. Fine particles usually do not grow by condensation to much larger than 1 m, indicating that coarse mode particles originate primarily from emissions.
Sources and Compositions of New Particles
New aerosol particles originate from two sources: emissions and nucleation. Emitted particles are called primary particles. Particles produced by homogenous nucleation, a gas-to-particle conversion process, are called secondary particles. Primary particles may originate from point, mobile, or area sources.
The aerosol particle emission sources may be natural or anthropogenic. Natural emission processes include volcanic eruptions, soil-dust uplift, sea-spray uplift, natural biomass burning fires, and biological material release. Major anthropogenic sources include fugitive dust emissions (dust from road paving, passenger and agricultural vehicles, and building construction/demolition), fossil-fuel combustion, anthropogenic biomass burning, and industrial emissions. Table I.1.13 summarizes the natural and anthropogenic sources of the major components present in aerosol particles. These sources are discussed in more detail shortly.
Table I.1.13. Principal Sources of Major Components of Aerosol-Particles
| | Sea-spray | Soil-Dust | Vulcanic | Biomass Burning | Fossil-Fuel Combustion for Transportation and Energy | Fossil-Fuel and Metal Combustion for Industrial Process |
| Black carbon (C) Organic matter (C,H,0,N) Ammonium (NH4+) Sodium (Na+) Calcium (Ca2+) Magnesium (Mg2+) Potassium (K+) Sulfate (SO42-) Nitrate (NO3-) Chloride (Cl-) Silicon (Si) Aluminum (Al) Iron (Fe) | X X X X X X X | X X X X X X X X X X X | X X X X X X X X X X X | X X X X X X X X X X X | X X X X X X X | X X X X X X X X X X X X X |
Aside from emissions, nucleation is the only source of new particles in the air. Nucleation is a process by which gas molecules aggregate to form clusters. If the radius of the cluster reaches a critical size, the cluster becomes stable and can grow further. Nucleation is either homogeneous or heterogeneous. Homogeneous nucleation occurs when gases nucleate without the aid of an existing surface. Thus, homogeneous nucleation is a source of new particles. Heterogeneous nucleation occurs when gases nucleate on a pre-existing surface. Thus, it does not result in new particles. Homogeneous or heterogeneous nucleation must occur before a particle can grow by condensation, a process discussed shortly.
The most important homogeneous nucleation process in the air is binary nucleation of sulfuric acid with water. Homogeneously nucleated sulfuric acid-water particles are typically 3 to 20 nm in diameter. In the remote atmosphere (e.g., over the ocean), homogenous nucleation events can produce more than 104 particles cm-3 in this size range over a short period. Homogenous nucleation of water vapor does not occur under typical atmospheric conditions. Water vapor nucleation is always heterogeneous. Indeed, all cloud drops in the atmosphere consist of water that has condensed onto aerosol particles following the heterogeneous nucleation of these particles. Aerosol particles that become cloud drops following heterogeneous nucleation by and condensation of water vapor are called cloud condensation nuclei (CCN).
