Table 4.1 - Average percentage of main atmospheric constituents under clean air conditions, with mean residence times and current relative changes
Source: Compiled by EEA-TF from multiple sources * Note: For pollutants with short residence times, the indicated range goes from clean areas to polluted cities.Gases Mean percentage Mean residence time* Present change
(per cent per year)Nitrogen (N2) 78 106 years   Oxygen (O2) 21 103 years   Argon (Ar) 0.9   Water vapour (H2O) variable: 03 810 days   Carbon dioxide (CO2) 0.035 50200 years +0.4 Methane (CH4) 0.00017 710 years +1 (decreasing to 0.6) Hydrogen (H2) 0.00006 +0.6 Nitrous oxide (N2O) 0.000033 130 years +0.3 Carbon monoxide (CO) 420 x 10-6 0.4 year +12 Ozone (O3)   tropospheric 10-6 10-5 weeksmonths +1.5 stratospheric 10-5 5 x 10-5 months 0.5 Ammonia (NH3) 10-8 10-6 3 days   Sulphur dioxide (SO2) 10-7 5 x 10-5 3 days   Nitrogen oxides (NOx) 10-8 5 x 10-5 3 days   CFCs (Freons) 10-7 50150 years +510 PAN 10-7 5 x 10-6     Volatile organics (VOC) 10-5 10-4