1. Measurement context

1.1 Particularities of the atmospheric environment

Table 1 shows the average composition of dry air in unpolluted regions.

We note that only three compounds account for 1% or more, and that none of the compounds present at more than 0.3 ppmv are of significant importance in tropospheric pollution.

Concentrations of the most abundant pollutant species rarely reach one part per million by volume (ppmv) (case of SO ₂ and NO in critical situations) and are most often in the ppmv fraction range, from 0.001 to some 0.1 ppmv i.e., using a more suitable unit, 1 to 100 ppbv (part per billion by volume).