Nitrogen Dioxide is a pollutant.

Perhaps you meant NO3, nitrates? Yes, those are detected, but so are glycerins. Nitrates may be combined with other elements and molecules such as potassium and ammonium (ammonium nitrate was Timothy McVeigh's bomb) to form explosive compounds.

Glycerins not only indicate the possible presense of nitroglycerin, but also trinitrotolulene (TNT), as nitroglycerin will leach out of TNT if the TNT lies around for a bit. However, the ETD also detects TNT all by itself.