National Atmospheric Emissions Inventory

6.2 Persistent Organic Pollutants

Persistent organic pollutants (POPs) are found in trace quantities in all areas of the environment. They are extremely persistent, with half-lives in soils of the order of years, although they may be transformed both physically and chemically over long periods. Over recent years there has been a growing interest in these pollutants and in particular their potential chronic toxicity and impacts on human health. This is reflected by the recent international agreement to reduce releases of these chemicals under the UNECE Persistent Organic Chemicals Protocol and their consideration by the Expert Panel for Air Quality Standards (EPAQS) recommendations for air quality standards. A detailed methodology for the compilation of these inventories is documented in "Future UK Emissions of Persistent Organic Pollutants, Cadmium, Lead and Mercury" (Wenborn MJ 1998a).

6.2.1 Polycyclic aromatic hydrocarbons (PAHs)

The PAH inventory has been substantially improved during the past year, including the compilation of the first speciated inventory for the UK. Detailed methodology is published in "Speciated PAH Inventory for the UK" (Wenborn MJ 1998d). This is a major step forward and will allow more detailed understanding of the UK inventory in light of forthcoming international agreements and regulations.

The inventory focuses on the sixteen PAHs, designated by the United States Environmental Protection Agency (USEPA) as compounds of interest under a suggested procedure for reporting test measurement results (USEPA 1988). These are:

  USEPA Priority pollutants (16 PAH) IRAC Probable or possible Human carcinogens (6 PAH) Borneff (6 PAH) UNECE POPs Protocol Indicators for purpose of emission inventories
Napthalene   ü      
Acenapthene ü      
Acenapthylene ü      
Fluorene ü      
Anthracene ü      
Phenanthrene ü      
Fluoranthene ü   ü  
Pyrene ü      
Benz[a]anthracene ü ü    
Chrysene ü      
Benzo[b]fluoranthene ü ü ü ü
Benzo[k]fluoranthene ü ü ü ü
Benzo[a]pyrene ü ü ü ü
Dibenz[ah]anthracene ü ü    
Indeno[1,2,3-cd]pyrene ü ü ü ü
Benzo[ghi]perylene ü   ü  

A subset of six of these PAHs have been identified by the International Agency for Research on Cancer (IARC) as probable or possible human carcinogens (IARC 1987). The Borneff 6 PAHs have been used in some EC emission inventory compilations. Those PAHs to be used as indicators for the purposes of emissions inventories under the UNECE's Persistent Organic Pollutants Protocol are indicated in the final column.

The main environmental impact of PAHs relate to their health effects, focusing on their carcinogenic, mutagenic and teratogenic properties. The most potent carcinogens have been shown to be benzo[a]anthracene, benzo[a]pyrene and dibenz[ah]anthracene (APARG 1996). The semi-volatile property of PAHs makes them highly mobile throughout the environment via deposition and re-volatilisation between air, soil and water bodies. It is possible that a proportion of PAHs released in the UK are deposited in the oceans and/or subject to long range transport making them a widespread environmental problem.

Emissions of Benzo[a]pyrene and the total of the 16 PAH's are summarised in Table 6.2. Anode baking carried out for the aluminium industry is the largest source of PAH emissions in the UK, contributing 39 % of the total emissions of all PAHs in 1995 and 1996. Table 6.2 shows a reduction in emissions of benzo[a]pyrene from anode baking from 26% in 1995 to 17% in 1996. As a result of the implementation of the Environmental Protection Act 1990, some anode baking plant have dramatically reduced their emissions and others will follow shortly. Anode baking is also the largest single source of the emissions for all except three of the individual PAHs studied. The exceptions are naphthalene where industrial coal combustion is the largest source, acenapthylene where domestic wood combustion is the largest source and benzo(ghi)perylene where vehicles are the largest source.

Other major sources of all the PAHs are industrial and domestic coal combustion and domestic wood combustion. Collectively vehicles are a significant source of many PAHs with cars burning leaded petrol being the largest subgroup, reflecting both their higher emission factors and the large proportion vehicle miles associated with these cars.

Wood treatment is a significant source of some of the lighter PAHs such as acenapthene, fluorene and anthracene. The emissions from wood treatment might be higher than the estimates suggest as domestic use, where no abatement techniques are employed, has not been estimated separately due to a lack of data. Emissions from bitumen production and use have not been estimated due to a lack of data. It is likely that bitumen use is a significant source of benzo[a]pyrene and other PAHs.

Overall, the level of uncertainty associated with the PAH inventory is largely due to the lack of actual measurements. To significantly improve the quality of the inventory requires the measurement of emission factors and detailed assessment of the processes involved in the following areas:

Table 6.2 - Summary of Emissions (tonnes) of PAHs in UK


Year 1995 1996
  BaP(1) (tonnes) % (16) PAHs(2) (tonnes) % BaP(1) (tonnes) % (16) PAHs(2) (tonnes)  
vehicles - diesel 1 3% 30 1% 1 4% 30 1%
vehicles - gasoline 6 17% 200 5% 5 22% 150 7%
natural fires and open agricultural burning 3 9% 100 3% 3 13% 100 5%
creosote use 0.06 0% 100 3% 0.06 0% 100 5%
aluminium production (3) 1.3 4% 200 5% 1.3 6% 190 9%
anode baking (4) 9 26% 1500 39% 4 17% 860 39%
coke production 1 3% 90 2% 1 4% 90 4%
domestic wood combustion 1 3% 200 5% 1 4% 210 10%
industrial wood combustion 0.1 0% 20 1% 0.1 0% 24 1%
domestic coal combustion 2 6% 300 8% 2 9% 270 12%
industrial coal combustion 5 14% 600 16% 4 17% 460 21%
other sources (5) 0.4 1% 30 1% 0.4 2% 27 1%
Total UK Emission (6) 35   3800   23   2200  

Notes
  1. Benzo[a]pyrene
  2. The selected PAHs are listed above
  3. 'Aluminium production' includes production by the pre-baked anode and Soderberg processes.
  4. 'Anode baking' is the manufacture of carbon anodes for aluminium smelting
  5. 'Other sources' includes the electricity supply industry, waste incineration and iron & steel works.
  6. Total of above sources. For several potential sources information is not available to enable emission estimates to be made. However, it is likely that the above categories cover the majority of B[a]P emissions.

6.2.2 Dioxins (PCDD/F)

The term dioxin is used to refer to the polychlorinated dibenzo-p-dioxins (PCDD) and polychlorinated dibenzofurans (PCDF). The emissions for dioxins cover the 17 PCDD and PCDF congeners of concern as defined by the NATO/CCMS international toxic equivalent (I-TEQ) scheme. The I-TEQ scheme was developed to weight the toxicity of the less toxic congeners as fractions of the toxicity of 2,3,7,8-TCDD, the most toxic congener.

PCDD/Fs have been shown to possess a number of toxicological properties. The major concern is centred on their possible role in immunological and reproductive effects. The main sources of dioxins are thermal processes, but they can also be released to the environment from some chemical processes. Dioxins can potentially arise from any thermal process where chlorine, in any form is present. PCDD/Fs can themselves be present in the feed stock material. Also chlorinated impurities may be introduced into the feed stock of some thermal processes. The amount of chlorine required for PCDD/F formation need only be small, so many processes have the potential to emit these pollutants. They can also be emitted from the chemical production and use of polychlorinated aromatic pesticides and herbicides, many of which are now controlled. However, some chlorinated organic chemicals such as the wood preservative pentachlorophenol are still used in the UK and these have the potential to be sources of dioxins for example from the combustion of treated wood.

The PCDD/F emission estimates for 1990-1995 are derived from APARG (1996); these have been extrapolated to provide estimates for 1996. In addition, emission estimates have been improved, based on more recently available information for the following sectors:


The estimated PCDD/F emissions for 1990-1996 are summarised in Table 6.3.

Table 6.3 UK emissions of PCDD/Fs


  1990 1991 1992 1993 1994 1995 1996
               
Power stations (coal and oil) 46 42 37 32 28 27 25
Industrial coal combustion 19 20 20 18 17 15 13
Domestic coal combustion 30 31 28 28 21 15 16
Industrial wood combustion 2 2 2 2 2 2 2
Domestic wood combustion 10 10 10 10 10 10 10
Coke production 3 3 2 2 2 1 1
Sinter plant 42 42 41 40 41 42 42
Electric arc furnaces (iron & steel) 12 10 11 11 10 9 9
Non-ferrous metal production 27 24 24 24 23 24 24
Chemical industry 0 0 0 0 0 0 0
MSW incineration 602 602 602 602 521 416 184
Chemical waste incineration 6 6 5 5 5 4 4
Clinical waste incineration 53 53 53 53 53 46 37
Sewage sludge incineration 5 4 4 4 3 3 3
Other sources 101 93 79 46 45 46 46
Accidental fires 58 58 58 58 58 58 58
Vehicle - gasoline 28 25 23 20 18 16 14
Vehicle - diesel 0 0 0 0 0 0 1
Total PCDD/F Emissions 1047 1026 999 957 858 735 489

The largest sources of PCDD/F emissions is waste incineration. Together MSW, clinical, chemical and sludge incinerators account for around 47% of UK emissions. There has been a steady reduction in PCDD/F emissions between 1990 and 1996 due the closure of many MSW incinerators which did not meet the revised IPC emission limits which came into force in 1996. New designs of MSW incinerator will result in lower levels of PCDD/F emissions. The relatively low emissions from chemical incinerators reflects the use of rotary kilns and the incorporation of a secondary combustion chamber in the process to destroy organic contaminants. Other significant sources are accidental fires (12%), sinter plant (9%), power stations (5%) and non-ferrous metal production (5%).

Coal and other solid fuels contain trace amounts of chlorinated compounds which can under certain combustion conditions result in dioxin formation. Emissions from power stations are fairly low because the high temperatures in the boilers cause a large proportion of any dioxins formed to be destroyed. The emission factors associated with industrial and domestic coal combustion are significantly higher and result in a significant contribution from these sectors (2.7% and 3.3% respectively) even though the coal consumption is less. However, emissions from all three sectors have decreased with the reduction in the quantity of coal burned.

There are significant emissions from iron and steel production and sinter plants owing to high temperatures and consumption of coal and coke. Emissions from iron and steel plant are probably underestimated since only electric arc furnaces are considered. Scrap used in electric arc furnaces and secondary non-ferrous metal production will contain chlorinated impurities such as plastics and cutting oil which contribute to dioxin formation.

It is generally accepted that the source of PCDD/F emissions from road transport are the 1,2-dichloroethane scavengers added to leaded petrol. The reduction in the lead content of leaded petrol and the increase in the use of unleaded petrol which like diesel fuel is likely to contain only trace quantities of chlorinated impurities has resulted in a significant reduction in emissions. The contribution in 1996 to PCDD/F emissions from road transport was 3%.

The 'other sources' emissions include open agricultural burning and the large reduction between 1990 and 1993 is a result of the ban on stubble burning.

The priority areas to further improve the PCDD/F emission inventory are :

6.2.3 Polychlorinated biphenyls (PCBs)

PCBs are classified as probably carcinogenic to humans and have been linked with subtle sub-chronic effects such as reduced male fertility and long-term behavioural and learning impairment. PCBs are extremely persistent in the environment and possess the ability to accumulate in the food chain. They are highly insoluble, lipophilic, semi-volatile compounds.

The estimated PCB emissions for 1990 - 1996 are summarised in Table 6.4

Table 6.4 - Summary of PCB Emissions in the UK 1990-1996 (kg)


PCB Emissions 1990 1991 1992 1993 1994 1995 1996
               
Leaks from capacitors 5814 5343 4871 4400 3929 3457 2986
Leaks from transformers 97 90 82 75 68 60 53
Fragmentisers 317 291 266 240 214 189 163
Manufacture and combustion of RDF 10 10 10 10 9 8 7
Application of sewage sludge 70 71 68 71 64 56 48
Power stations 89 88 82 68 55 53 48
Industrial & domestic combustion 32 34 31 32 27 22 21
Iron & steel (inc. sinter plant) 529 457 474 494 464 431 411
Incineration 14 14 14 14 13 12 9
Other sources 3 3 3 3 3 3 3
Total PCB Emissions 6976 6400 5901 5407 4845 4291 3749

The emission inventory for PCBs is very uncertain as the information on the quantity of PCBs in installed electrical equipment is based on an estimate and the leakage rate from these dominating sources is also estimated.

Sales of PCBs in the UK were stopped in 1986. The total PCB emission in 1990 was dominated by leaks from capacitors (83% of total emission). The UK will phase out and destroy all electrical equipment known to contain PCBs by 1999. However, not all electrical equipment containing PCBs is readily identifiable and emissions from electrical equipment may continue beyond 1999, but will decrease as the electrical equipment concerned reaches the end of its working life.

PCBs are synthetic organic compounds that have mainly been used as dielectric insulating media in electrical equipment such as capacitors and transformers. They have not been manufactured and used in the UK for many years, but many old, PCB-containing appliances (mainly cars and household appliances) still exist. It is estimated that 81% of PCB emissions to the atmosphere are associated with such appliances, mainly as a result of leaks from poorly maintained in-service appliances but also leakage during their subsequent disposal. Large quantities of PCBs have been disposed of to landfill in the past, mainly in the form of electrical components or fragmentiser residues, but now such equipment containing PCBs are disposed of by chemical incineration.

PCB emissions from the iron and steel industry reflect the fact that PCBs like dioxins can be formed in trace amounts from chlorinated precursors in thermal processes such as scrap metal recycling. PCBs occur in sewage sludge due to their persistent nature. Not all the PCBs spread on land will volatilise but the potential is greater than that of landfill. The emission estimate comprises only 1% of the total and is highly uncertain. The other emissions appear to be very small. Emissions arise from waste incineration and refuse derived fuel production result from the PCB content of the waste. Emissions may also occur from building materials where PCBs were used as plasticisers or in paints however it is not yet possible to estimate the magnitude of this emission.

6.2.3 Pesticide Emissions

These estimates are a first attempt at quantifying pesticide emissions to air. As a result confidence in the accuracy of these estimates is low. Statistical information and relevant emission factors are currently scarce with the majority of emission factors coming from the USA or Europe. Application methods and atmospheric conditions may vary significantly from those at the time the emission factors were derived.

Emissions from pesticides occur predominately from their production, application and/or volatilisation after application. Tables 6.5, 6.6 and 6.7 show the estimated emissions of lindane (gamma-HCH), pentachlorophenol (PCP) and hexachlorobenzene (HCB) respectively.

Lindane (HCH)

Lindane is applied as an insecticide and fungicide in agriculture and is used for wood preservation and in domestic and veterinary formulations. Until 1990, lindane was also used as a remedial wood treatment. However, data on quantities used for a remedial wood treatment prior to 1990 are not available. The emission estimates, presented in Table 6.5, were made assuming that emissions occur during application in wood treatment, agricultural and domestic use. HCH emissions are dominated by wood preservation and treatment activities which account for 86% of 1996 emissions.

Emissions from agricultural activities are also significant, accounting for around 14% of total 1996 HCH emissions. These emissions are based on statistics on the use of pesticides containing lindane, obtained from the Pesticide Usage Survey Group (MAFF, 1991a,b,c; 1992a,b,c,d) The emission factors used are taken from van der Most et al (1989). Emissions from domestic applications are thought to be comparatively small. However, usage statistics are scarce and were only available for 1988 ( DOE, 1989). Emission factors are taken from van der Most et al (1989).

Table 6.5 - Summary of HCH Emissions in the UK 1990-1996 (tonnes)

Source 1990 1991 1992 1993 1994 1995 1996
Wood Preserving 36 28 21 17 13 10 8
Treated Wood 57 51 46 41 37 33 30
Wood Remediation 0            
Domestic Applications 1 1 1 1 1 1 1
Agriculture Pesticide 6 6 6 6 6 6 6
Total 99 85 74 64 56 50 44

Pentachlorophenol (PCP)

Pentachlorophenol is used as a biocide effective in destroying insect eggs. It is used in the timber and textile industries. The estimate also includes emissions of sodium pentachlorophenoxide (NaPCP) and pentachlorophenyl laureate (PCPL) as well as PCP since these are also included in the proprietary formulations.

The estimated PCP emissions for 1990 to 1996 are given in Table 6.6. As with Lindane the most significant emission sources are from treated wood and wood preserving with estimates of 501 and 60 tonnes respectively in 1996. Once again it is very difficult to be certain of these estimates due to the lack of research into emission rates and limited knowledge of quantities used both in the year of the estimate and in previous years. These were estimated in a similar way as lindane using an emission factor of 3% of the wood content per year.

PCP emissions from the textile industry arise mainly from volatilisation during application as a cotton preservative. The textile finishing usage data were an unofficial estimate from the Textile Finishing Association. Emission factors used were based on a study of PCP emissions in the UK (Wild, 1992) who report that approximately 30% of the applied PCP is lost through volatilisation.

Agricultural emissions of PCP occur from use of the active ingredient in mushroom farming to disinfect wooden trays. Usage statistics are reliable coming from the Pesticide Usage Survey Group (MAFF, 1991a,b,c; 1992a,b,c,d). The emission factor assumes 30% loss due to volatilisation (Wild, 1992).

Table 6.6 - Summary of PCP Emissions in the UK 1990-1996 (tonnes)


  1990 1991 1992 1993 1994 1995 1996
PCPL Formulation 0.00009 0.00009 0.00009 0.00009 0.00009 0.00009 0.00009
Wood Treatment 309 309 309 60 60 60 60
Cotton and Textiles 3 3 3 3 0 0 0
Previously treated wood 525 525 525 525 517 509 501
Agriculture 0.18 0.18 0.18 0.00 0.00 0.00 0.00
Other sources 0.44 0.44 0.44 0.44 0.39 0.32 0.18
Total 837 837 837 588 577 569 562

The emission inventory for PCP is very uncertain as limited emission factors are available on the release of PCP during agricultural activities and statistics are not actively collected on the extent of its usage. There is some data on release of PCP from combustion processes. However, the available studies are not consistent with each other. This suggests that the uncertainty may be considerable. However combustion processes are not significant sources.

Hexachlorobenzene (HCB)

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Studies in the USA have identified two main industrial sources of HCB (Mumma et al, 1975) (Jacoff et al, 1986). These are the manufacture of chlorinated solvents such as trichloroethylene, tetrachloroethylene and carbon tetrachloride and the manufacture of specific pesticides where HCB remains as an impurity. HCB emissions can also result from the use of hexachloroethane tablets as a degassing agent in secondary aluminium smelting (van der Most et al, 1992). Emissions may also arise from combustion sources, but other than waste incineration these could not be estimated though they are believed to be small.

Statistics for chlorinated solvent production in the UK are confidential, hence estimates were made based on UK solvent usage data from the Solvent Industries Association and import and export statistics. Emission factors were taken from van der Most et al (1989).

Although there is no UK manufacture of pesticides that results in the production of HCB, pesticides with HCB as an impurity are still imported and used in the UK for agricultural pest control. Statistics for the use of these pesticides is provided by the Pesticide Usage Survey Group (MAFF, 1991a,b,c; 1992a,b,c,d). Emission factors used are from van der Most et al (1989)

HCB emissions in secondary aluminium smelting result from the use of hexachloroethane (HCE) tablets as a degassing agent. Not all secondary aluminium is smelted using HCE degassing tablets. Estimates of the quantity of degassing agent supplied and industrial expert estimates of the quantity of HCE used per tonne of aluminium smelted were used to estimate the total aluminium smelted using HCE. Emission factors used are from van der Most et al (1989).

Emissions from chlorinated solvent production and pesticide application are the most significant sources in the UK (Table 6.7) and in 1996 were estimated to account for around 47% and 45%, respectively, of total emissions.

Table 6.7 - Summary of HCB Emissions in the UK 1990-1996 (tonnes)


Source 1990 1991 1992 1993 1994 1995 1996
Chemical industry              
Tetrachloroethylene Prod 0.08 0.08 0.08 0.08 0.08 0.08 0.08
Trichloroethylene 0.14 0.14 0.14 0.14 0.14 0.14 0.14
Carbon Tetrachloride 0.36 0.36 0.36 0.36 0.36 0.36 0.36
Secondary Aluminium Processing 0.10 0.10 0.12 0.12 0.10 0.11 0.11
Pesticide use              
Quintozine 0.00 0.00 0.00 0.00 0.00 0.00 0.00
Chlorthal-dimethyl 0.08 0.08 0.08 0.08 0.08 0.08 0.08
Chlorothalonil 0.47 0.47 0.47 0.47 0.47 0.47 0.47
Picloram 0.00 0.00 0.00 0.00 0.00 0.00 0.00
Pentachlorophenol (Na) 0.03 0.03 0.03 0.01 0.01 0.01 0.01
Waste Incineration - Old 0.01 0.01 0.01 0.01 0.01 0.00 0.00
Waste Incineration - New 0.00 0.00 0.00 0.00 0.00 0.00 0.00
TOTAL 1.27 1.26 1.28 1.26 1.24 1.25 1.24

6.2.4 Accuracy of POPs inventories

There are major uncertainties in the POPs emission inventories due to the lack of source measurements and emission factors. In general there is greater confidence in the percentage change in emissions from 1990 -1996 than in the individual estimates. In general emission estimates of persistent organic pollutants are only of order of magnitude accuracy.