In the Emergency Responce Guide, or as we call it the ERG, the chemical placard number for liquid nitrogen is 1977. it is carried in pressurised containers such as the MC338.
This guide is given, free of charge, to all public safety (ems, police, fire, dispatch etc) divisions and is kept most often on the responding trucks. it is updated every 4 years and gets thicker every update.
Truckers also are known to carry these, they are not classified, or "secret". they describe the response procedures for the first responders in the event of a HAZ-MAT incident. They are one of the most important books we use.
We have come a LONG long way from the days of the "cop-o-meter". (police officers used to be known for getting too close to haz-mat spills and some even going as far as tasting the substance in an effort to identify it. we call a "cop-o-meter" the process of finding the dead police officer furthest from the scene and setting a perimeter 50 yards out from that. It is said in jest, but I have seen videos where fire chiefs and police officers have been killed because they got too close toa toxic substance, or, they accually tasted it.)