Also liquid ammonia is cryogenic. Cars would constantly be venting ammonia gas as it boiled off, assuming you didn't want the tanks to explode.

What? Put a cap on the tank. What I mean is you can store ammonia in pressure vessels, same as we do with LPG for cars or barbecues.

Edit: hit update accidentally.

You can't just cap cryogenic materials. As they heat they want to expand.

LNG is _compressed_ not _cooled_. If you're providing _compression_ for a liquid, it can be maintained.

Consider the equivalent: a sealed vessel in which you're boiling water. Eventually it's going to find a weak point. Just because the boiling point here is well below normal ambient temperatures doesn't mean you aren't still getting a 1000:1 expansion ratio from liquid to gas.

And again: venting ammonia is very fraught as it's corrosive and toxic at fairly low concentrations -- it'll burn your lungs off from the inside.

LNG is cooled and compressed to turn it from gas to liquid, and like the bulk cryogenic liquified "fixed gases", it is the cryogenic storage temperature which maintains the liquid state, not the pressure under which it is stored.

This is also true for liquid nitrogen, liquid oxygen, liquid argon & liquid helium.

At ambient temperatures the vapor pressures of these fixed gases are too high for the liquid to be contained by conventional pressure cylinders having tightly shut valves. Instead they are stored cryogenically in highly insulated dewars under low pressures of 35 to 100psi which vent pressures above that to the atmosphere continuously as the liquid slowly evaporates and maintains the reduced temperature of the remaining bulk by self-refrigeration.

That's why these tanks hiss as if they are leaking from the time you have one delivered until the liquid is fully evaporated whether you use it or not.

Carbon dioxide is a different fixed gas since it only has a vapor pressure less than 1000psi at room temperature. So it is available in regular tightly shut welding-style tanks containing 50pounds of liquid at room temperature, or it can be provided as a cryogenic liquid in low-pressure vented dewars like the LN2.

Liquid (anhydrous) ammonia has a vapor pressure less than 1800psi so it can be stored in a tightly shut pressure cylinder itself, stainless steel DOT cylinders are commonly used for lab samples. This is at room temperature, so it is not necessary to be handled cryogenically in order to transport or store ammonia, unlike LNG. As mentioned elsewhere, you wouldn't want to vent ammonia anyway.