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Methodology for unit emissions of freight air traffic
Unit emissions (g/tkm) from air travel cover the amount of emissions released while an aircraft is being used. To calculate them, information is needed on how much fuel is used, how much emissions are released, how much freight is being transported and how long the route is. Then the amount of fuel used or emissions released can be divided by the amount of freight and kilometres flown.
Sources of information
There is very little information available on air freight traffic emissions in Finland. Even outside of Finland there are only a few public sources of information that could be applied under the conditions in Finland.
Because of the data shortage, LIPASTO unit emissions for air freight traffic are defined using Defra's (The Department for Environment, Food and Rural Affairs, the UK) carbon dioxide factors . Figures by Defra are meant to be used to measure and report greenhouse gas emissions for corporate or personal carbon footprint in the UK, but they are assumed to describe the situation in Finland as well to certain extent.
Energy consumption along with other emissions are calculated from Defra's carbon dioxide figures by multiplying them by factors derived from passenger air traffic unit emissions in Finland. Emission factors are thus "adjusted" to the situation in Finland in a way that leads to identical emission figures relative to carbon dioxide emissions for both freight and passenger air traffic.
The amounts of emissions released are reported as grams per tonnekilometre (g/tkm). The compounds of interest are carbon monoxide (CO), hydrocarbons including methane (HC), nitrogen oxides (NOx), sulphur dioxide (SO2) and carbon dioxide (CO2). Certain factors used in calculating emissions (see for example sources  and ) are shown in section Numeric values.
Methane and nitrous oxide along with carbon dioxide are greenhouse gases, contributing to climate change. One way to outline the combined heating effect of different greenhouse gases is to convert them to carbon dioxide equivalents. The Kyoto protocol suggests following factors found in IPCC Second Assessment Report : carbon dioxide 1, methane 21 and nitrous oxide 310.
Following methods and definitions have been used:
Domestic flights: Flights within Finland.
Following values for aviation fuel are used:Density 800 kg/m3
Heating value 43 MJ/kg
CO2 emissions* 3.169 kg/(kg fuel)
SO2 emissions* 0.001 kg/(kg fuel)
CH4 emissions 0.0005 g/MJ
N2O emissions 0.002 g/MJ
Energy 1 kWh = 3.6 MJ
* used only to cover missing raw data values
The best known factor in aviation induced greenhouse effect is carbon dioxide released from the fuel. The warming effect of carbon dioxide is independent of where it is being released, but for some other compounds the release in cruising altitude of over 10 km makes a big difference.
To estimate the total aviation induced warming effect on the atmosphere, the warming and cooling effects of each compound released have to be summed up. Carbon dioxide has a warming effect whereas nitrogen oxides have both a warming effect through increasing ozone and a cooling effect through decreasing methane. The warming effects caused by water vapour and sulphur oxide in cruising altitude are estimated to remain rather modest. One of the biggest uncertainties is, however, lacking knowledge of the impact of contrails and aviation induced cirrus clouds. If cirrus clouds are not included, the net impact of aviation according to different sources is estimated to be about two or three times the warming effect of carbon dioxide alone. This factor is known as radiative forcing index (RFI).
These pages only report the actual emissions that are released when the fuel is burnt. Because of the many uncertainties and undeveloped science, the use of a RFI factor is not yet justifiable.
 The Intergovernmental Panel on Climate Change, 2006 IPCC Guidelines for National Greenhouse Gas Inventories, Volume 2, Energy, Chapter 18.104.22.168, 2006.
 The Intergovernmental Panel on Climate Change, IPCC Second Assessment Report: Climate Change 1995, 1995.
 AERO2k Global Aviation Emissions Inventories for 2002 and 2025, p.102, 2004.
 2009 Guidelines to Defra / DECC's GHG Conversion Factors for Company Reporting,