"In this process, oxygen from the inducted air reacts with the hot gases of the exhaust; thus, further partial oxidation of the unburnt hydrocarbons and carbon monoxide take place, and in consequence, converts these noxious gases into harmless carbon dioxide, water and traces of nitrogen as they are expelled-out clean and friendly to the atmosphere.

Hmmnn.... my opinion is that dilution is much more likely than oxidation........ and since the opacimeter measures the amount of light transmitted through the medium (exhaust gas) relative to the incident radiation, any amount of dilution with a transparent medium (i.e. air) will result in higher transmission
( Beer-Lambert's Law) .... so the venturi effect only serves to draw in fresh air (diluent).... (Bernoulli's Principle).

and dilution cannot clean the exhaust....it can only mask.... much like adding more water to orange juice until the color is barely visible...




btw, NOx can be converted to elemental nitrogen (N2) via a "reduction process" only and NOT by oxidation....(General Chemistry)

Cheers!