The atmospheric pollution concentration is usually determinated by the differential absorption (DA) method using spectral absorption lines of pollution. It is known the relationship between absorption and the phase refractive index (PRI) of medium is given by the Kramers-Kronig ratio. The maximal difference of PRI near spectral line will be more than DA sensitivity. Using the Raleigh's relation the group refraction index (GRI) may be obtained too. Investigations show that the GRI has a considerably much deviation (about million times) near absorption line then the PRI. It is proposed to use above deviation for atmospheric pollution determination by measuring propogating signals differential group delay (DGD) between extremal deviation points. It is shown that the new method sensitivity is 10-20 and more times higher than with DA method.
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