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

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 ­  

* Note:

For pollutants with short residence times, the indicated range goes from clean areas to polluted cities.