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Methodology for unit emissions of air traffic
Unit emissions (g/pkm) 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 many passengers are being transported and how long the route is. Then the amount of fuel used or emissions released can be divided by the number of passengers and kilometres flown. In case of transportation of goods, the emissions are usually divided by the mass or volume of the freight and kilometres flown.
An example of the problems in calculating unit emissions is the diversity in the air fleet. Both the size of the aircraft and the technical solutions vary a lot, which makes it difficult to describe air traffic with average-based numbers in a just way. Also the cabin factor (use of seats available) can vary a lot depending on the season and the type of flight. On average the cabin factor is about 50 to 80 %: the longer the flight the bigger the cabin factor.
Sources of information
Unit emissions for scheduled and charter flights (passenger traffic) are calculated from the data delivered by three Finnish airlines. The data consist of nearly 130 000 flights to or/and from Finland between January 2008 and November 2008. The airlines (Finnair, Finncomm Airlines and Blue1) report the number of flights flown and the amount of fuel used and emissions released per passenger kilometre. The term passenger kilometre covers the transport of one paying passenger over one kilometre. In fact, the actual cabin factor is used for each flight, not some theoretical value.
The data from the airlines are based on route-specific, observed fuel consumption. The amounts of emissions released can then be derived from fuel consumption, using engine manufacturers' reports. The mass of emission is then divided by the number of passengers and the length of the flight. The compounds of interest are carbon monoxide (CO), hydrocarbons including methane (HC), nitrogen oxides (NOx), sulphur dioxide (SO2) and carbon dioxide (CO2). In a few cases a missing value had to be estimated by using average values for aviation fuel (Numeric values). Also, missing hydrocarbon values for turboprops were estimated using numbers from source  (References).
In addition to the compounds mentioned above, also methane (CH4) and nitrous oxide (N2O) emissions were calculated from fuel consumption using average values for aviation fuel. These values are found from source  (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.
Energy consumption and emissions from scheduled and charter flights in Finland are calculated from the data delivered by Finnish airlines as a weighted average (weighted by the numbers of flights).
Domestic, short-distance: Not more than 250 nm (nautical mile) = 463 km flights inside Finland.
Types of flights
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 22.214.171.124, 2006.
 The Intergovernmental Panel on Climate Change, IPCC Second Assessment Report: Climate Change 1995, 1995.
 AERO2k Global Aviation Emissions Inventories for 2002 and 2025, s.102, 2004.