National Atmospheric Emissions Inventory

Appendix 1

The Methodology of the National Atmospheric Emissions Inventory

2.11 Agricultural Emissions

 2.11.1 Livestock

The NAEI estimates emissions of methane from farm animals resulting from enteric fermentation and the storage and disposal of animal wastes. The methane emission estimates were supplied by MAFF (1997a).

2.11.1.1 Enteric Emissions of Methane
Methane is produced in herbivores as a by-product of enteric fermentation, a digestive process by which carbohydrates are broken down by micro-organisms. Emissions are calculated from animal population data collected by the June Agricultural census and published in MAFF (1997b) and the appropriate emission factors. Earlier years data are often revised so data was taken from the MAFF database. Table A29 shows the emission factors used. Apart from the dairy cattle, lambs and deer, the methane emission factors are IPCC defaults (IPCC, 1997) and do not change from year to year. The dairy cattle emission factors are estimated following the IPCC Tier 2 procedure (IPCC, 1997). The base data and emission factors for 1990-1995 are given in Table A30. For the years 1970-1989 emission factors are extrapolated backwards from 1990 based on the average annual change from 1990 to 1996 (e.g the 1990 factor is 1.011 times the 1989 factor and so on). The emission factor for lambs is assumed to be 40% of that for adult sheep. In using the animal population data it is assumed that the reported number of animals are alive for that whole year. The exception is the treatment of sheep where it is normal practice to slaughter lambs and other non-breeding sheep after 6 to 9 months. Here the procedure adopted is to use an average number of sheep calculated as:

        Total Breeding Flock
        + (Total other Sheep + Lambs under one year old)/2

Emissions from farmed deer are now included.

Table A29 Methane Emission Factors for Livestock Emissions.

  Enteric Methanea kg CH4/head/year Methane from Wastesa kg CH4/head/year
Dairy Cattle (in milk) 116b 12.1b
Dairy not in milk, beef and other cattle 48 6
Pigs 1.5 3
Breeding and Other Sheep 8 0.19
Lambs < 1year 3.2c 0.076c
Goats 5 0.12
Horses 18 1.39
Deer (stags & hinds) 10.4c 0.26c
Deer (calves) 5.2c 0.13c
Poultryd 0 0.078
  a IPCC(1997)
  b 1996 Emission Factor
  c Sneath et al (1997)
  d Fowls, turkeys, geese and ducks.

Table A30 Dairy Cattle Methane Emission Factors

  1990 1991 1992 1993 1994 1995 19961
Average Weight of cow (kg) 550 556 561 567 572 578 584
Average Rate of Milk Production (liter/d) 14.1 14.1 14.4 14.5 14.5 14.9 15.2
Average Fat Content (%) 4.02 4.03 4.07 4.08 4.07 4.05 4.07
Gestation Period (d) 281 281 281 281 281 281 281
Digestible Energy (%) 65 65 65 65 65 65 65
Methane Conversion rate (%) 6.5 6.5 6.5 6.5 6.5 6.5 6.5
Maximum Methane Capacity (m3/kg volatile solid) 0.24 0.24 0.24 0.24 0.24 0.24 0.24
Ash Content of Manure (%) 8 8 8 8 8 8 8
Enteric Emission Factor (kg CH4/head/y) 109 109 111 112 112 114 116
Manure Emission Factor (kg CH4/head/y) 11.3 11.3 11.5 11.6 11.6 11.8 12.1
1         1996 data are provisional.

2.11.1.2 Methane Emissions from Animal Wastes.
Methane is produced from the decomposition of manure under anaerobic conditions. When manure is stored or treated as a liquid in a lagoon, pond or tank it tends to decompose anaerobically and produce a significant quantity of methane. When manure is handled as a solid or when it is deposited on pastures, it tends to decompose aerobically and little or no methane is produced. Hence the system of manure management used affects emission rates. Emissions of methane from animal wastes are calculated from animal population data (MAFF, 1996b) in the same way as the enteric emissions. The emission factors are listed in Table A29. Apart from dairy cattle, lambs and deer, these are all IPCC defaults (IPCC, 1997). Emission factors for dairy cattle were calculated from the IPCC Tier 2 procedure using data shown in Tables A30 and A31 (MAFF, 1997a). For the years 1970-1989 emission factors are extrapolated backwards from 1990 based on the average annual change from 1990 to 1996 (e.g the 1990 factor is 1.011 times the 1989 factor and so on). The emission factor for lambs are assumed to be 40% of that for adult sheep. Emissions from farmed deer are now included.

Table A31 Dairy Cattle Manure Management Systems in the UK

Manure Handling System Methane Conversion Factor % Fraction of manure handled using manure system %
Pasture Range 1 43
Liquid Range 10 38
Solid Storage 1 10
Daily Spread 0.1 9

2.11.1.3 Emissions of Nitrous Oxide from Animal Waste Management Systems
Animals are assumed not to give rise to nitrous oxide emissions directly, but emissions from their wastes during storage are calculated for a number of animal waste management systems (AWMS) defined by IPCC. Emissions from the following AWMS are reported under the IPCC category, manure management.


According to IPCC(1997) guidelines, the following AWMS are reported in the Agricultural Soils category

Emissions from the combustion of chicken litter for power generation have yet to be included in the NAEI, and would be reported under power stations.

The IPCC(1997) method for calculating emissions of N2O from animal waste management systems can be expressed as:

N2O(AWMS) = 44/28 . å NT . Nex(T) . AWMS(T) . EF(AWMS)

where

N2O(AWMS) = N2O emissions from animal waste managment systems
    (kg N2O/yr)
NT = Number of animals of type T
Nex(T) = N excretion of animals of type T (kg N/animal/yr)
AWMS(T) = Fraction of Nex that is managed in one of the different
    waste management systems of type T
EF(AWMS) = N2O emission factor for an AWMS (kg N2O-N/kg of Nex in AWMS)

The summation takes place over all animal types and the AWMS of interest. Animal population data is taken from MAFF Statistics (MAFF, 1997b). Table A32 shows emission factors for nitrogen excretion per head for domestic livestock in the UK (Nex). The emission factors are for wastes spread on land so it is assumed that ammonia volatilisation from storage and housing has already occurred. This is a different basis from the IPCC default nitrogen excretion emission factors which express total N excreted before ammonia volatilisation. The conversion of excreted N into N2O emissions is determined by the type of waste management system used. The distribution of waste management systems for each animal type (AWMS(T)) is given in Table A33. Table A34 gives the N2O emission factor for each animal waste management system (EF(AWMS)). These are expressed as the emission of N2O-N per mass of excreted N processed by the waste management system.

Emissions from grazing animals (pasture range and paddock) and daily spread are calculated in the same way as the other AWMS. However, emissions from land spreading of waste that has previously been stored in liquid systems, other systems, solid storage and dry lot are treated differently . These are discussed in Section 2.11.2.6 on Organic Fertiliser.

Table A32 Nitrogen to Land from Animal Excretion in the UKa

Animal Type Emission Factor kg N/animal/yr
Dairy cows 96
Beef & other cattle < 2yr 34
Other cattle > 2yr 58
Pigs < 20kg 3
Other Pigs 20-50 kg 7
Fattening & Other Pigs > 50 kg 11.7
Breeding pigs > 50 kg 19.5
Adult Sheep 8.8
Lambs 2.4
Goats 8.8
Broilers 0.50
Broiler Breeders 0.98
Layers 0.66
Ducks, Geese & Guinea Fowl 0.9
Turkeys 1.0
Growing Pullets 0.125
Horses 40
Deer: Stagsb 21.9
Deer: Hindsb 14.6
Deer: Calvesb 10.8
a MAFF(1998)
b Sneath et al, (1997)


Table A33 Distribution of Animal Waste Management Systems used for Different Animal typesc
Animal Type Liquid System Daily Spread Solid Storage and dry lota Pasture range and paddock Otherb Fuel
Dairy cows 38 9 10 43 0  
Other cattle 14 9 27 50 0  
Fattening & Other Pigs > 50 kg, (1970-96)e 59 14 27 0 0  
Breeding sows (1990-96)e 41 10 19 30 0  
Weaner Pigs (1990-96)e 53 13 24 10 0  
All Pigs (1970-90); Finishing Pigs after 1990e 59 14 27 0 0  
Sheep 0 0 2 98 0  
Goats 0 0 0 96 4  
Broilers, Pullets(1970-91)f       1 99 0
Broilers, Pullets (1992-96)f       1 64 35
Layers (1970-91)f       10 90 0
Layers (1992-96)f       10 89 1
Ducks, Geese & Guinea Fowlf       50 50 0
Turkeysf       8 92 0
Horses       96 4 0
Deer: Stagsd       100 0  
Deer: Hinds & Calvesd       75 25  
aFarmyard Manure
bPoultry Litter, Stables
cADAS(1994/5)
dSneath et al (1997)
eAgricultural Economics Unit Exeter University (1996)
fTucker (1997)
        

Table A34 Nitrous Oxide Emission Factors for Animal Waste Handling Systemsa
   
Waste Handling System Emission Factor kg N2O-N per kg N excreted
Liquid System 0.001
Daily Spreadb 0
Solid Storage and Dry Lot 0.02
Pasture, Range and Paddockb 0.02
Fuel -
Other 0.005
aIPCC(1997)
bReported under Agricultural Soils

2.11.2 Agricultural Soils

Direct emissions of nitrous oxide from agricultural soils are estimated using the IPCC recommended methodology (IPCC, 1997) but using some recent UK specific emission factors. This involves estimating contributions from:

(i)        The use of inorganic fertilizer
(ii)        Biological fixation of nitrogen by crops
(iii)         Ploughing in crop residues
(iv)        Cultivation of histosols (organic soils)
(v)        Spreading animal wastes on land
(vi)        Manures dropped by animals grazing in the field

In addition to these, the following indirect emission sources are estimated:

(vii)        Emission of N2O from atmospheric deposition of agricultural NOx and NH3.
(viii)Emission of N2O from leaching of agricultural nitrate and runoff.

The methods used are described next.

2.11.2.1 Inorganic Fertiliser
Emissions from the application of inorganic fertiliser are calculated using the IPCC (1997) methodology and IPCC default emission factors. They are given by:

N2O(SN) = 44/28 . NFERT . (1-FracGASF) . EF1

where
N2O(SN) = Emission of N2O from synthetic fertiliser application
(kg N2O/yr)
NFERT = Total use of synthetic fertiliser (kg N/yr)
FracGASF = Fraction of synthetic fertiliser emitted as NOx + NH3
  = 0.1 kg NH3-N+NOx -N / kg synthetic N applied
EF1 = Emission Factor for direct soil emissions
= 0.0125 kg N2O-N/kg N input

Annual consumption of synthetic fertiliser is estimated based on crop areas (MAFF, 1997) and fertiliser application rates (BSFP,1997).

2.11.2.2 Biological Fixation of Nitrogen by Crops
Emissions of nitrous oxide from the biological fixation of nitrogen by crops are calculated using the IPCC (1997) methodology and IPCC default emission factors. They are given by:
        
N2O(BF) = 44/28 . 2. CropBF . FracNCRBF. EF1

where
NO(BF)= Emission of N2O from biological fixation (kg N2O/yr)
CropBF>= Production of legumes (kg dry mass/year)
FracNCRBF= Fraction of nitrogen in N fixing crop
= 0.03 kg N/ kg dry mass
EF1= Emission Factor for direct soil emissions
= 0.0125 kg N2O-N/kg N input

The factor of 2 converts the edible portion of the crop reported in agricultural statistics to the total biomass. The fraction of dry mass for the crops considered is given in Table A35

Table A35 Dry Mass Content and Residue Fraction of UK Crops

Crop Type Fraction dry Massb Fraction Crop Residue removed
Broad Beans, Green Peas 0.08 -
Field Beand, Peas(harvest dry) 0.86 -
Rye, Mixed corn, triticale 0.855a 0.3846
Wheat, Oats 0.855a 0.4348
Barley 0.855a 0.4545
Oil Seed Rape, Linseed 0.91a 0.45
Maize 0.50 0.5
Hopsc 0.20 0.45
Potatoes 0.20 0.7142
Roots, Onions 0.07 0.45
Brassicas 0.06 0.45
Sugar Beet 0.1 0.8333
Other 0.05 0.45
Phaseolus beans 0.08 0.3225
  a MAFF(1997b)
  b Burton (1982), Nix (1997) or MAFF estimates
  c Hops dry mass from Brewers Licensed Retail Association(1998)
  d Field beans dry mass from PGRE (1998)

The data for fraction removed is taken from IPCC(1997) defaults in Table 4.17 of the Agricultural Soils section, or derived from Table 4.17 of the Field Burning of Agricultural Residues section. Crop production data is taken from MAFF(1997b,1997d).The total nitrous oxide emission reported also includes a contribution from improved grass calculated using a fixation rate of 4 kg N/ha/yr (Lord, 1997).

2.11.2.3 Crop Residues
Emissions of nitrous oxide from the ploughing in of crop residues are calculated using the IPCC (1997) methodology and IPCC default emission factors. They are given by:

N2O(CR) = 44/28 . 2 . (CropO . FracNCRO . + CropBF . FracNCRBF )(1-FracR) . (1-FracB) EF1

where

N2O(CR) = Emission of N2O from crop residues (kg N2O/yr)
CropO = Production of non-N fixing crops (kg dm/yr)
FracNCRO = Fraction of nitrogen in non-N fixing crops
  = 0.015 kg N/ kg dm
FracR = Fraction of crop that is remove from field as crop
FracB= Fraction of crop residue that is burnt rather than left on field
EF1= Emission Factor for direct soil emissions
  = 0.0125 kg N2O-N/kg N input
CropBF= Production of legumes (kg dry mass/year)
FracNCRBF= Fraction of nitrogen in N fixing crop
= 0.03 kg N/ kg dry mass

Production data of crops is taken from MAFF(1997b, 1997d). The dry mass fraction of crops and fraction of crop removed from the field are given in Table A35. Field burning has largely ceased in the UK since 1993. For years prior to 1993, field burning data was taken from the annual MAFF Straw Disposal Survey. (MAFF, 1995)

2.11.2.4 Histosols
Emissions from histosols were estimated using the IPCC(1997) default factor of 5 kg N2O-N/ha/yr. The area of cultivated histosols is assumed to be equal to that of eutric organic soils in the UK and is based on a FAO soil map figure supplied by SSLRC. (Armstrong-Brown, 1997).

2.11.2.5 Grazing Animals
Emissions from wastes dropped by grazing animals are classified under agricultural soils by IPCC. The method of estimation is the same as that for AWMS in Section 2.11.1.3 but applying factors for pasture range and paddock.

2.11.2.6 Organic Fertilisers
Emissions from animal wastes used as organic fertilisers are classified under agricultural soils by IPCC. The procedure involves estimating the amount of nitrogen applied to the land and applying IPCC emission factors. For daily spreading of waste, the emission is given by:

N2O(DS) = 44/28 . å NT . Nex(T) . AWMS(T) . EF1

where

N2O(DS) = N2O emissions from daily spreading of wastes (kg N2O/yr)
NT = Number of animals of type T
Nex(T) = N excretion of animals of type T (kg N/animal/yr)
AWMS(T) = Fraction of Nex that is daily spread
EF1 = Emission Factor for direct soil emissions
= 0.0125 kg N2O-N/kg N input

For the application of previously stored wastes to land, a correction is applied to account for N2O losses during storage.

N2O(FAW) = 44/28 . å(NT . Nex(T) . AWMS(T) - N(AWMS) ) . EF1

where the summation is for all animal types and for liquid system, solid storage and other systems where wastes are stored.

N2O(FAW) = N2O emission from organic fertiliser application
N(AWMS) = N2O emissions from animal waste managment systems as
    nitrogen (kg N2O-N/yr)
NT = Number of animals of type T
Nex(T) = N excretion of animals of type T (kg N/animal/yr)
AWMS(T) = Fraction of Nex that is managed in one of the different
    waste management systems of type T

2.11.2.7 Atmospheric Deposition of NOx and NH3
Indirect emissions of N2O from the atmospheric deposition of ammonia and NOx are estimated according to the IPCC(1997) methodology but with corrections to avoid double counting N. The sources of ammonia and NOx considered, are synthetic fertiliser application and animal wastes applied as fertiliser.

The contribution from synthetic fertilisers is given by:

N2O(DSN) = 44/28 . (N(FERT) - N(SN)) . Frac(GASF) . EF4

where

N2O(DSN) = Atmospheric deposition emission of N2O arising from synthetic fertiliser application (kg N2O)
N(FERT) = Total mass of nitrogen applied as synthetic fertiliser (kg N)
N(SN) = Direct emission of N2 O(SN) as nitrogen (kg N2O-N)
Frac(GASF) = Fraction of total synthetic fertiliser nitrogen that is emitted
    as NOx + NH3
  = 0.1 kg N/ kg N
EF4 = N deposition emission factor
  = 0.01 kg N2O-N/kg NH3-N and NOx-N emitted

The estimate includes a correction to avoid double counting N2O emitted from synthetic fertiliser use.

The indirect contribution from waste management systems is given by

N2O(DWS) = 44/28. (N(EX)/(1-Frac(GASM)) -N(F) -N(AWMS) ) . Frac(GASM) . EF4

where

N2O(DWS) = Atmospheric deposition emission of N2O arising from animal wastes(kg N2O)
N(EX) = Total N excreted by animals
Frac(GASM) = Fraction of livestock nitrogen excretion that volatilises as NH3 and NOx
  = 0.2 kg N/kg N
N(F) = Total N content of wastes used as fuel (kg N)
N(AWMS) = Total N content of N2O emissions from waste management systems including daily spread and pasture range and paddock (kg N2O-N)

The equation corrects both for the N lost in the direct emission of N2O from animal wastes and the N content of wastes used as fuel. The nitrogen excretion data in Table A32 already includes volatilisation losses and hence a correction is included for this.

2.12.2.8 Leaching and Runoff
Indirect emissions of N2O from leaching and runoff are estimated according the IPCC methodology but with corrections to avoid double counting N. The sources of nitrogen considered, are synthetic fertiliser application and animal wastes applied as fertiliser.

The contribution from synthetic fertilisers is given by:

N2O(LSN) = 44/28 . (N(FERT) . (1-Frac(GASF) )- N(SN)) . Frac(LEACH) . EF5

where

N2O(LSN) = Leaching and runoff emission of N2O arising from synthetic fertiliser application (kg N2O)
N(FERT) = Total mass of nitrogen applied as synthetic fertiliser (kg N)
N(SN) = Direct emission of N2 O(SN) as nitrogen (kg N2O-N)
Frac(GASF) = Fraction of total synthetic fertiliser nitrogen that is emitted
    as NOx + NH3
  = 0.1 kg N/ kg N
Frac(LEACH) = Fraction of nitrogen input to soils that is lost through leaching and runoff
  = 0.3 kg N/ kg fertiliser or manure N
EF5 = Nitrogen leaching/runoff factor
  = 0.025 kg N2O-N /kg N leaching/runoff

The estimate includes a correction to avoid double counting N2O emitted from synthetic fertiliser use.

The indirect contribution from waste management systems is given by

N2O(LWS) = 44/28. (N(EX-N(F) -N(WS) ) . Frac(LEACH) . EF5

where

N2O(LWS) = Leaching and runoff emission of N2O from animal wastes
    (kg N2O)
N(EX) = Total N excreted by animals (kg N)
N(F) = Total N content of wastes used as fuel (kg N)
N(AWMS) = Total N content of N2O emissions from waste management systems including daily spread and pasture range and paddock (kg N2O-N)

The equation corrects both for the N lost in the direct emission of N2O from animal wastes and the N content of wastes used as fuel.

2.11.3 Field Burning.

The NAEI estimates emissions from field burning under the category agricultural incineration. The estimates are derived from emission factors calculated according to IPCC(1997) and from Larkin et al. (1985) shown in Table A36

Table A36 Emission Factors for Field Burning (kg/t)

  CH4 CO NOx N2O NMVOC BS PM10
Barley 3.05a 63.9a 2.18a 0.060a 7.5c 2.55b 0.132
Other 3.24a 67.9a 2.32a 0.064a 9c 2.55b 0.132
a        IPCC(1997)
b        Larkin et al. (1985)
c        USEPA(1995)

The estimates of the masses of residue burnt of barley, oats, wheat and linseed are based on crop production data (MAFF, 1997d) and data on the fraction of crop residues burnt (MAFF, 1997; ADAS(1994/5)). Field burning ceased in 1993 in England and Wales. Burning in Scotland and Northern Ireland is considered negligible as is grouse moor burning, so no estimates are reported from 1993 onwards. The carbon dioxide emissions are not estimated because these are part of the carbon cycle.

2.11.4 Forests

The NAEI reports an estimate of NMVOC emissions from forests and woodlands in the UK. The methodology is discussed by Eggleston et al. (1987). The estimate is very approximate and the same value is used for all years.