18 Volatile Organic Compounds
Prof. Ashu Rani
Contents
- Introduction
- What are Volatile Organic Compounds (VOCs)
- Polycyclic Aromatic Hydrocarbons Sources of VOCs
- Global Scene
- VOCs Emissions – Indian Scene
- VOCs in Tropospheric Chemistry
- VOCs in Stratospheric O3 Depletion
- Green House Effect
- Damaging Effects on Humans
- Bhopal Disaster – Methyl Isocyanate
- Suggested Reading
Introduction
Increased pollution levels in hydrosphere, atmosphere and lithosphere is one of the serious drawbacks of human development generated by exploitation of natural resources and industrial growth to fulfill the demands of rapidly increasing population all over the world. Among group of pollutants the most harmful air pollutants are volatile organic compounds (VOCs), which can be evaporated and transport to atmosphere at even ambient conditions and are widely present in both outdoor and indoor air. The concentration of observed VOCs is higher in indoor air than outdoor air and depends mostly on temperature and humidity of the enclosed environment. VOCs are widely used in daily life as household and commercial products as cleansers, disinfectant, paint, varnishes, fuels, agrochemicals etc.
What are Volatile Organic Compounds (VOCs)?
VOCs are organic chemicals having high vapor pressure at room temperature resulted from low boiling points, causes large number of molecules to evaporate or sublimate from the liquid or solid form of the compound and to enter into the surrounding environment. These are a large group of organic chemicals that include any compound of carbon except carbon monoxide, carbon dioxide, carbonic acid, metallic carbides or carbonates and ammonium carbonate that participate in atmospheric photochemistry. A large number of VOCs such as aromatics, alcohols, terpenes, phenols and carbonyl compounds etc. as given in Table 1, are released globally/year contributing in hazardous effects on biosphere and global environmental consequences. VOCs are ubiquitous and have natural and anthropogenic sources. Emissions from natural processes such as plants, vegetation, forest, volcanic eruptions etc are dominant but their effects are less hazardous than compared to emitted by man-made activities. Use of fossil fuels by human beings for development has manifold increased the concentrations of VOCs in atmosphere and their visible consequences can be seen as global warming, ozone depletion and sudden changes in global climate.
Table 1 List of VOCs emitted from various natural and anthropogenic sources every year
(http://www.aqt.it/index)
|
VOC (CHEMICAL NAME& MOL. WT.) |
NAME |
| PROPANE (44.1) | PROPANE |
| N-BUTANE (58.12) | C-4 |
| N-PENTANE (72.15) | C-5 |
| N-HEXANE (86.18) | C-6 |
| N-HEPTANE (100.21) | C-7 |
| N-OCTANE (114.23) | C-8 |
| N-NONANE (128.26) | C-9 |
| N-DECANE (142.29) | C-10 |
| N-UNDECANE (156.31) | C-11 |
| N-DODECANE (170.34) | C-12 |
| N-TRIDECANE (184.37) | C-13 |
| N-TETRADECANE (198.4) | C-14 |
| N-PENTADECANE (212.42) | C-15 |
| N-HEXADECANE (226.45) | C-16 |
| N-HEPTADECANE (240.46) | C-17 |
| N-OCTADECANE (254.49) | C-18 |
| N-NONADECANE (268.51) | C-19 |
| N-C20 (282.54) | C-20 |
| N-C21 (296.57) | C-21 |
| N-C22 (310.59) | C-22 |
| ISOBUTANE (58.12) | 2-ME-C3 |
| NEOPENTANE (72.15) | 22-DM-C3 |
| ISOPENTANE (72.15) | 2-ME-C4 |
| 2,2-DIMETHYL BUTANE(86.18) | 22-DM-C4 |
| 2,3-DIMETHYL BUTANE(86.18) | 23-DM-C4 |
| 2-METHYL PENTANE(86.18) | 2-ME-C5 |
| 3-METHYL PENTANE(86.18) | 3-ME-C5 |
| 2,2,3-TRIMETHYLBUTANE (100.21) | 223TM-C4 |
| 2,2-DIMETHYL PENTANE (100.21) | 22-DM-C5 |
| 2,3-DIMETHYL PENTANE (100.21) | 23-DM-C5 |
| 2,4-DIMETHYL PENTANE (100.21) | 24-DM-C5 |
|
PENTADECANE (226.45) |
|
|
4,8-DIMETHYL TETRADECANE (226.45) |
48DM-C14 |
| BRANCHED C16 ALKANES (226.45) | BR-C16 |
| BRANCHED C17 ALKANES (240.46) | BR-C17 |
|
CYCLOPENTANE (70.14) |
CYCC5 |
| CYCLOHEXANE (84.16) | CYCC6 |
| CYCLOALKANES (84.16) | CYC-C6 |
| ISOPROPYL CYCLOPROPANE (84.16) | IPR-CC3 |
| ETHENE (28.05) | ETHENE |
| PROPENE (42.08) | PROPENE |
| 3-METHYL-1-PENTENE(84.16) | 3M1-C5E |
| 1-TRIDECENE (182.35) | 1-C13E |
| 1-TETRADECENE (196.38) | 1-C14E |
| ISOBUTENE (56.11) | ISOBUTEN |
| 2-METHYL-1 -BUTENE(70.14) | 2M-1-BUT |
| CIS-3-METHYL-2-HEXENE(84.16) | C3M2-C5E |
| TRANS-3-HEPTENE (98.19) | T-3-C7E |
| C6 CYCLIC OR DI-OLEFIN(82.15) | C6-OL2D |
| CYCLOPENTADIENE (66.1) | CYC-PNDE |
| A-PINENE (136.24) | A-PINENE |
| 3-CARENE (136.24) | 3-CARENE |
| D-LIMONENE (136.24) | D-LIMONE |
| SABINENE (136.24) | SABINENE |
| TERPENE (136.24) | TERPENE |
| C9 STYRENES (118.18) | C9-STYR |
| C10 STYRENES (132.21) | C10-STYR |
| STYRENE (104.15) | STYRENE |
| 2-BUTYNE (54.09) | 2-BUTYNE |
| ACETYLENE (26.04) | ACETYLEN |
| A-METHYL STYRENE(118.18) | AME-STYR |
| C8 DISUB. BENZENES(106.17) | C8-BEN2 |
| M-XYLENE (106.17) | M-XYLENE |
| O-XYLENE (106.17) | O-XYLENE |
| BENZENE (78.11) | BENZENE |
| TOLUENE (92.14) | TOLUENE |
| INDAN (118.18) | INDAN |
| TETRALIN (132.21) | TETRALIN |
| 2-METHYL NAPHTHALENE (142.2) | 2ME-NAPH |
| C-13 TRISUBSITUTED NAPHTHALENES (170.26) | C13-NAP3 |
| ETHYL ACETYLENE(54.09) | ET-ACTYL |
| CYCLOPENTANOL (86.13) | CC5-OH |
| T-BUTYL ALCOHOL(74.12) | T-C4-OH |
| ISODECYL ALCOHOL(158.29) | I-C10-OH |
| ETHYLENE GLYCOL(62.07) | ET-GLYCL |
| PROPYLENE GLYCOL(76.1) | PR-GLYCL |
| 1,2-BUTANEDIOL (90.12) | 12-C4OH2 |
| GLYCEROL (92.1) | GLYCEROL |
| TRIMETHYLENE OXIDE (58.08) | TME-OX |
| TETRAHYDROFURAN (72.11) | THF |
| CROTONALDEHYDE (70.09) | CROTALD |
| 4-METHYL-2-PENTANONE (100.16) | MIBK |
| BASE ROG MIXTURE (14.44) | ARBROG |
| TLEV EXHAUST — RFA (14.04) | RFA-TLEV |
| TLEV EXHAUST — PHASE2 (14.12) | PH2-TLEV |
| TLEV EXHAUST — LPG (14.86) | LPG-TLEV |
| TLEV EXHAUST — CNG (15.22) | CNG-TLEV |
| FINAL LEV — PHASE 2 (14.22) | PH2-LEV |
| MINERAL SPIRITS “D”(14.08) | (TYPEII-C)MS-D |
| MINERAL SPIRITS “A”(14.1) | (TYPE I-B,91% ALKANES) MS-A |
| FINAL LEV – RFA(14.03) | RFA-LEV |
| TLEV EXHAUST — M-85 (27.45) | M85-TLEV |
| TLEV EXHAUST — E-85(20.74) | E85-TLEV |
TLEV: transitional low-emission vehicle
LEV: low-emission vehicle
RFA: renewable fuel association
M-85: methanol upto 85%
E-85: ethanol upto 85%
CNG: compressed natural gas
LPG: Liquified petroleum gas
Polycyclic Aromatic Hydrocarbons
Another category of volatile organic compounds responsible for serious environmental concern due to their carcinogenic and mutagenic behavior includes Poly aromatic hydrocarbons (PAHs). PAHs are group of semi-volatile organic compounds and toxic chemicals present in the environment originated from natural and anthropogenic sources. Some of the important natural emission sources are volcanic eruptions, forest fires, degradation of biological materials, land plants, bacteria and macro and micro algae whereas anthropogenic sources includes coal refuse banks, coke production, commercial boilers and incinerators, wood gasifiers, industrial discharges and automobiles etc. Some examples are anthracene, Phenenthrene, Tetracene, Chrysene, Triphenylene, Pyrene, Corannulene, Benzoperylene, Ovalene, Coronene etc. It is well known that large quantities of PAHs are emitted during secondary thermal chemical reactions at temperatures over 700°C while evolution in small amounts has been reported from pyrolysis reactors at temperature range 350-600°C. Pakdel and Roy noted that hydrocarbons obtained temperatures around 500°C are highly branched structures such as anthracene, fluroanthrene, phenanthrene, chrysene and nonaecylbenzene etc. have lower environmental and toxicological impact. The compounds obtained by gasification processes at higher temperature around 700°C are highly condensed polyaromatic in structure and produce high level of mutagenic activity which includes dibenzofuran, fluroanthene, pyrene and ethylbenzene etc. Numerous reports have indicated that carcinogenic PAHs are immunotoxicants. By low solubility and hydrophobic nature they easily enter into the aquatic environment and rapidly become part with inorganic and organic suspended particles. Once deposited in sediments these are less available for photochemical and biological oxidation. Also found in phytoplankton, plant leaves which are food source for all aquatic animals, PAHs with lower molecular weight are less soluble are more prone to bioaccumulation from sediments by marine organisms. Some invertebrates such as fishes are able to degrade and excrete PAHs very efficiently metabolized by phase 1 enzymes of cytochrome P-450 in its liver and excreted in bile. But while metaolizing these compounds other carcinogenic and mutagenic intermediates such as diol and epoxides are produced by cytochrome P-450.
Sources of VOCs
VOCs are universal atmospheric species emitted by both natural and anthropogenic sources. On the basis of emission source, VOCs can be categorized into two types: Anthropogenic VOCs and Biogenic VOCs.
Anthropogenic VOCs
These are emitted from combustion sources such as emissions from vehicle and fossil-fueled power plants, fuel storage and transport, solvent usage, other industrial operations, landfills and hazardous waste facilities. The global emissions as a result of anthropogenic activities are by following main activities:
Indoor Emissions
Daily human activities are also an important source of VOCs emissions commonly known as ‘indoor VOCs’. Building materials including adhesives, paints and vinyl floors etc. emits typical solvents viz. aliphatic hydrocarbons, ethyl acetate, glycol ethers, and acetone and formaldehyde. Commonly used home and personal care products such as air cleaners that produce ozone, perchloroethylene used in dry cleaning and disinfecting chemicals releases VOCs. Benzene a well known carcinogenic VOC is emitted from tobacco smoke (cigarettes), stored fuels and vehicular exhausts. Environmental tobacco smoke emits benzene a known human carcinogen which is a chemical found in stored fuels, and exhaust from cars. As per studies Our routine works such as cooking, dry cleaning, newspapers, non-electric space heaters, photocopiers, smoking, stored paints and chemicals etc. increase level of VOCs in closed rooms, cars and in other closed surroundings. The level of VOCs in indoor air is generally two to five times higher than the outdoor air.
Automobile exhausts
Vehicular pollution is one of the key factors for large emissions of VOCs all over the globe. During the combustion process while a vehicle is operating mode, volatile organic compounds (VOC) are released along with oxides of nitrogen (NOX), particulate matter (PM), and carbon monoxide (CO). Exhaust emissions occur during two modes: Cold start – starting and driving a vehicle the first few minutes result in higher emissions because the emissions control equipment has not yet reached its optimal operating temperature and running exhaust emissions – pollutants are emitted from the vehicle’s tailpipe during driving and idling after the vehicle is warmed up. Several VOCs are released from gasoline, LPG, CNG and renewable fuels comprising gasoline and ethanol mixed blends generally known as ethanol fuel.
In static anthropogenic sources, fuel evaporation from the vehicles is another factor for VOC release into the atmosphere. Despite evaporative emissions controls, evaporative losses can still account, on hot days, for a majority of the total VOC pollution from current model cars. Evaporative emissions occur in several ways including running losses while the vehicle is running and hot engine and exhaust system can vaporize gasoline, by hot soak, when the car is parked while cooling down, the gasoline can evaporates as engine remains hot for a period of time after the vehicle is turned off, diurnal (while parked and engine is cool) even when the vehicle is parked for long periods of time, gasoline evaporation occurs as the temperature rises during the day, while the tank is being filled the refueling gasoline vapors escape from the vehicle’s fuel tank.
Solvent usage
e increasing solvent usage can be judged by the fact that global annual emissions of 26 Tg/year is only due to organic solvents such as alkanes, ketones, fluorinated hydrocarbons and esters (Koppmann 2007)..
Chemical process industries dealing with the manufacture, storage, handling and distribution of paints, lubricants and liquid fuels etc are the main sources of VOCs emissions. In these industries chlorinated solvents are widely used as degreasing fluids for many different purposes such as dry-cleaning clothes; de-caffeinating coffee, cleaning metal machinery and dissolving grease build up in septic tanks. Some solvents are used in common household products as spot removers, typing correction fluids, adhesives, automotive cleaners, inks, and wood furniture cleaners etc.. Fuel component chemicals found in products such as gasoline, kerosene, and heating oil are also used as solvents. For example, methyl tertbutyl ether (MTBE) is added to gasoline as an octane-booster and as an “oxygenator” (or “oxyfuel”) to reduce carbon monoxide emissions and also used as a laboratory chemical and in medicine to remove gallstones. Benzene, toluene, and xylenes have been used as solvents in the workplace and are components of glues, paints and cleaners.
Industrial processes
Industrial processes are another major source of VOCs emissions in atmosphere after vehicular pollution. Chemical inorganic industries including sulphuric acid plants, fertilizer manufacture, nitric acid and ammonia manufacturing emits SO2, HF, H2S and NOx while organic industries such as insecticide, synthetic rubber soap and detergent manufacturing releases organic intermediates, solvent vapors, odours, SO2 etc. Coal fired plants emits aromatic hydrocarbons including benzene, toluene, ethylbenzene and xylene and aldehydes including formaldehyde. Pulp and paper (kraft process) industries uses digester blow system, pulp washers, evaporators and oxidation towers emit odorous sulphur compounds (methyl mercaptan, dimethyl sulphide and H2S).
Oil refining
In petroleum refineries and petrochemical plants most of the organic compounds are derived from petroleum fractions, which are a few basic hydrocarbons such as methane, ethane, propane, benzene, toluene, and xylene. The emission of these organic compounds into the atmosphere mainly originates from the production processes, storage tanks and the waste areas.
Landfills waste
Disposal of mixed urban waste in a sanitary landfill or dump is one of the common methods of urban waste handling. Compact nature of waste delays the decaying of waste by oxidation processes hinders air transfer to bulk mass. So degradation process takes place via anaerobic oxidation producing methane and carbon dioxide as the major gases. Methane present underground is flammable, but it is not associated with odors or hazards once emitted into the air above the landfill. Other gases produced by landfill bacteria are termed reduced sulfur gases or sulfides (e.g., hydrogen sulfide, dimethyl sulfide, mercaptans and ammonia). These odorous gases give the landfill gas mixture its characteristic “rotting” smell.
Food manufacture
In food manufacturing industries such as baking, vegetable oil extraction, solid fat processing, animal rendering, fish meal processing, coffee production, sugar beet processing and brewing, spirit production and wine making use products that contain VOCs, such as flavorings, dyes, inks, adhesives, and other surface coatings which releases in the environment during production and processing.
Agriculture
In agriculture to increase crop yield and for pest control use of pesticides is mainly responsible of VOCs generation in land fields. Uses of non-aqueous liquid formulation have greatest potentials to emit VOCs due to organic solvent usage for commercial production.
Fugitive emissions
Such emissions by fossil fuels through their production, storage, transport, leak and supply is important point for serious concern. Petrochemical fractions typically produce limited number of compounds such as acyclic alkanes, cyclic alkanes, monoaromatics and diaromatics and each of these having number of isomers and homologues.
Biogenic sources of VOCs
These are mainly emitted from vegetation and natural processes including bushfires, lightning and the microbial processes that occur in soil generate nitrogen oxides etc.
Natural processes
In nature volcanic eruptions, bushfires, lightning and the microbial processes that occur in soil generate nitrogen oxides. These can actively participate in photochemical smog pollution, being extremely reactive with OH radicals and ozone.
Emissions from plants, trees, animals, forest fires and anaerobic processes
The VOCs emissions by plants are manifold higher than by animals and account for a relevant amount of carbon fixed by photosynthesis, especially under stress conditions. Almost about 99 per cent of the total biogenic non methane VOCs (NMVOCs) comprising isoprene, monoterpene, alkane, alkene, carbonyls, esters, alcohols and aromatic hydrocarbons etc are emitted from terrestrial sources including forests, grasslands, shrub-lands and croplands. The global emission inventories show isoprenoids (isoprene and monoterpene as the most dominant biogenic volatile organic compounds (BVOC) followed by alcohols and carbonyls as the predominant groups. Isoprene, predominantly emitted from deciduous (hardwood/ broad leaf) trees such as oak, poplar, aspens and willows is not stored in plants and emitted in sunlight during photosynthesis. On the other hand, monoterpenes are released from coniferous (softwood) trees such as pines, cedars and first are stored in plants and can be emitted both during day and night. The emission of both these VOCs is also found in several species such as spruce and eucalyptus. From livestock production the odor emission is another natural source of VOC emission. VOC emitted from the mixture of feces and urine, as well as feed and silage experiencing microbial fermentation. Odours are also associated with the storage and decomposition of manure as these are complex mixtures of volatile fatty acids, alcohols, aromatic compounds, amides and sulphides. The production of odor takes place during digestion and subsequent manure storage by incomplete anaerobic fermentation of substances mediated by microorganisms. The forest fire also contributes significantly to the levels of ozone in the region by reaction of anthropogenic NOx emissions and biogenic VOC emissions depending on type of vegetation. The rate of important VOCs emissions and their uncertainty range per year are given in Table 2.
Table 2 Rate of VOCs emissions and their uncertainty range per year (Ralf Koppmann 2007)
| Type of VOC | Emission rate | Rate of uncertainty |
| Fossil fuel usage | ||
| Alkanes | 28 | 15-60 |
| Alkenes | 12 | 5-25 |
| Aromatic compounds | 20 | 10-30 |
| Terrestrial Plants | ||
| Isoprene | 460 | 200-1800 |
| Monoterpene | 180 | 50-400 |
| Other VOC | 580 | 150-200 |
| Oceans | ||
| Alkanes | 1 | 0-2 |
| Alkenes | 6 | 3-12 |
Global Scene
Manifold increase in anthropogenic activities all over the globe has given a boost to the emission of various types of VOCs depending upon the type of origin source. With increasing rate of about 1% per year, since year 2000 the concentration of background tropospheric ozone has increased at northern mid-latitudes in all seasons. By background measurements of tropospheric ozone, approximately the same rate of increase has been observed all around the world defining it as a global phenomenon. Since the year 2000 ozone precursor emissions have increased for NOX (2.3 %/year), NMVOC (1.6 %/year), CH4 (2.5 %/year), and CO (2.5 %/year) worldwide which can also be noticed by general increase in ozone background concentrations of 1% per year.
According to the Task Force on Hemispheric Transport of Air Pollution (TF HTAP) (2010), rise in 43% to ozone levels in Europe is due to the transport of ozone from other source regions (North America, South Asia and East Asia) which is expected to have nearly equal weight. However, the highest emissions in ozone precursor emissions especially the increases in NOX is observed in Asia which has the strongest influence on the increasing ozone background levels in the Northern Hemisphere. Larger effect on ozone formation takes place by increase in the NOx background concentration than an equal increase in the VOC concentration due to the already large natural emission of VOCs, CH4 and CO. On further increase in the background NOx may lead to the chemical shift from ozone depletion to ozone formation. An estimated increase of ~10% in the ozone input of 50 (+/- 6 standard error) Tg/yr for the years 2000-2035 compared to the total estimated transport from the stratosphere of 552 +/- 168 Tg/yr has been reported in recently published work using a climate model (Bach et. al, 2014).
VOC Emissions – Indian Scene
Having an agricultural based economy, the biomass burning is a common and widely used practice in India. The major sources of biomass burning in India are forest fire, deforestation, agricultural waste and wood burning. Among the Asian countries, India is the second largest contributor to the emission of non-methane VOCs (NMVOCs). The major species emitted during combustion are C2H4, C2H4O, C2H6, C2H6S, C3H6, C3H6S, C3H8, C5H8, CH3OH, higher alkanes, higher alkenes, terpenes and toluene lumps. Agricultural waste burning (41%) and deforestation (47%) are the major contributors for non-methane hydrocarbons (NMHCs) emissions in India where agricultural waste burning mainly emits isoprene (80%), toluene (73%), formaldehyde (80%) and methyl alcohol (59%). The VOC emissions during biomass burning show large inter-annual variation. For example, in the period from 1997-2009 emission estimates of NMHCs and CH3OH varied in the range 100–470 and 46–211 Gg yr–1 respectively (Pandey and Sahu, 2014).
Another major concern for VOC emissions is the transport sector which is a major anthropogenic source of air pollution in the metropolitan cities of India. In urban atmosphere, benzene, toluene, ethylbenzene and xylene (BTEX), group of aromatic VOCs constitute up to 60% of non-methane VOCs group act as one of the major pollutant, proving it to be an efficient indicator of pollution arising from road traffic (because of increased global consumption of gasoline). Among all the aromatic hydrocarbons which are constituted in the class of BTEX, benzene has been chosen a prime target for assessment of pollution levels in the urban atmosphere as it is considered to be a genotoxic carcinogen which have fatal effect, mutagenicity. According to research study, in national capital Delhi among the seasons, the concentration of BTEX was observed highest during winter season followed by monsoon and then in summer. Fossil fuel combustion in vehicles is the biggest reason for these higher emissions. Delhi has the highest number of vehicles (9.4 millions as on 31.10.11) among the Indian cities and there is an exponential growth in the number of vehicles
(Saxena and Ghosh, 2012, Sharma et. al, 2011).
VOCS in Tropospheric Chemistry
Reactions with OH/NO3 Radicals
The most general degradation pathway of VOCs is the reaction with OH/O3/ NO3 to from radical, which further react with oxygen resulting in the generation of organic radicals as intermediates.
The formation of ozone depends on the concentration of VOCs and the percent of NOx in the atmosphere. If the concentration of VOCs is more than the concentration of NOx, the limiting reactant of the reaction is NOx and vice-versa. The formation and accumulation of ozone will depend on the concentration of VOCs.
The intermediates generated by reactions with radicals undergo further tropospheric reactions and form alcohols, aldehyde, ethers etc. These products on further photodissociation or by similar tropospheric reactions give aerosols. Reactions of aerosols with O3 play a significant role in the formation of smog, which being toxic creates health problems especially to old persons and children, and is injurious to plants.
Reactions of VOCs with Sulfate Ion Radicals
Sulfate ion radical is the intermediate in atmospheric aqueous phase oxidation of dissolved SO2. It is as strong oxidant as OH radical. The VOCs present in troposphere react with this radical. The reactions of sulfate radical ions have been studied in sufficient detail and following three mechanisms have been identified:
(i) One-electron oxidation
The reaction with benzene and its derivatives is believed to occur as in Eq.(19).
(ii) Addition to a double bond
(iii) Hydrogen abstraction
In the VOCs studied, in no case mechanism (ii) is applicable. The mechanisms (i) and (iii) are applicable to aromatic and aliphatic compounds, respectively. The VOCs scavenge the sulfate ion radicals and break the aqueous SO2 oxidation chain and therefore inhibit the acidification reaction and gets themselves degraded.
Peroxyacetylnitrate (PAN)
PANs are toxic VOCs, which are formed by the reaction of peroxyacyl radicals with NO2. The former is formed by the attack of OH radicals on aldehydes form acyl and ultimately peroxyacyl radicals. The aldehydes are formed from the oxidation of hydrocarbons.
Reactions of Olefins
Unsaturated hydrobarbons with double bonds are called olefins. These react with ozone to yield carbonyl compounds.
Sulfur –based VOCs
VOCs such as dimethyl sulfide are degraded by reaction with OH radicals ultimately to SO2.
VOCS in Stratospheric O3 Depletion
The much larger decrease in springtime in stratospheric ozone over Antarctica was noticed and later phenomenon was named as the ozone hole. The major cause of ozone depletion was reaction of chlorine/OH/NO with O3. The chlorine atoms in the stratosphere were produced due to photo-dissociation of CFCs, which are VOCs.
Chlorine atoms, released from CFCs/CCl4/other active chlorine compounds during photodissociation, react with ozone to form ClO and O2.
Green House Effect
Increasing concentrations of green house gases (GHGs), such as methane and carbon dioxide in atmosphere is responsible for global warming. The most important radiatively active trace gases in the atmosphere are water vapour and carbon dioxide. The relative effectiveness of a compound to cause global warming compared with carbon dioxide can be expressed in term of Global Warming Potentials (GWPs). It is defined as the ratio of the radiative forcing from a given mass emission of the trace gas compared to that from the same mass emission of carbon dioxide, integrated over a given time horizon. GWPs of some common VOCs are in Table 3.
Table 3: Global warming potential of some common VOCs in a 100-year time horizon (AEA group 2007)
| VOCs | GWPs |
| dimethylether | 1 |
| Dichloromethane | 10 |
| Chloromethane | 16 |
| Bromomethane | 5 |
| 1,1,1-Trichloroethane | 144 |
| Dichlorotrifluoroethane | 76 |
| Chlorodifluoromethane | 1780 |
| Chlorotetrafluoroethane | 599 |
| Dichlorofluoroethane | 713 |
| Chlorodifluoroethane | 2270 |
| Dichloropentafluoropropane | 120 |
| Dichloropentafluoropropane | 586 |
| Trifluoromethane | 14310 |
| Difluoromethane | 670 |
| Pentafluoroethane | 3450 |
| 1,1,1,2-Tetrafluoroethane | 1410 |
| 1,1,1-Trifluoroethane | 4400 |
| 1,1-Difluoroethane | 122 |
| 1,1,1,2,3,3,3-Heptafluoropropane | 3140 |
| 1,1,1,3,3,3-Hexafluoropropane | 9500 |
| 1,1,1,3,3-Pentafluoropropane | 1020 |
| 1,1,1,3,3-Pentafluorobutane | 782 |
| 1,1,1,2,3,4,4,5,5,5-Decafluoropentane | 1610 |
| ethane | 8.4 |
| propane | 6.3 |
| butane | 7.0 |
| ethylene | 6.8 |
| propylene | 4.9 |
Damaging Effects on Humans
Exposure to VOCs concentrations above the allowable limit usually result in acute and chronic health effects including eye, nose and throat irritation, headaches, vomiting, dizziness cancer, liver damage, kidney damage, nervous system damage and asthma exacerbation. With short-term exposure, the consequences can encompass eye and respiratory tract irritation, headaches, dizziness, visual disorders, fatigue, loss of coordination, allergic skin reactions, nausea, and memory impairment. For example, exposure to air containing 600 ppm of toluene for more than 8 hours causes headache and dizziness. The harmful effects of VOCs ate listed as Table 4.
Table 4: Some VOCs and their harmful effects (Navaladian et. al, 2007)
| VOCs | Harmful Effects |
| Benzene | Carcinogenic |
| Toluene | Headache and dizziness |
| Xylene | Eye and respiratory tract irritation, narcotic effect, nervous system depression and death |
| Chloroform | Affect central nervous system causing depression, dizziness, liver and kidney damages, skin infection |
| Ethylene, Styrene | Depletion of ozone layer |
| Acetaldehyde, acetone | Respiratory and eye irritation |
| Phenol | Offensive odour and toxicity |
| Epoxides | Toxic, carcinogenic, explosive |
| Ethers | Producing peroxides, affecting the reproductive system |
| Vinyl chloride, freon | Ozone hole formation, carcinogenic, toxic, greenhouse effect, climate changes |
| Nitrogen containing compounds (Amines) | Bad odour, carcinogenic (affects urinary bladder) |
Bhopal Disaster – Methyl Isocyanate
The Bhopal Gas Disaster, of 2nd-3rd December, 1984, is an example of the worst chemical disaster due to release of a highly toxic VOC, methyl isocyanate (MIC), stored in a SS tank, of UCIL (Union Carbide of India Ltd) factory. There was a massive release of toxic gases into the atmosphere which spread rapidly over a densely populated area of Bhopal City, the capital of Madhya Pradesh, India. Over 500,000 people were exposed to MIC gas and other chemicals. Soon the toxic substances dispersed in and around the areas located near the plant. This had serious health consequences.
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References:
- Volatile Organic Compounds in the Atmosphereedited by Ralf Koppmann, Blackwell publishing Ltd. 2007.
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