why is ammonia gas has so much bad smell, is there any mathod 2 decrease its effect?

no idea Why it smells exactly, but part of it may be that it has a low vapor pressure and the gas is very basic (alkaline).

 

to lessen it you need to neutralise it with acid, HCl is often used for this.

Actually, it is good that it is so smelly. Before the gas becomes really dangerous for you, the stench is unbearable, so you will try to get away, before it can do serious harm (or even kill you).

 

Why does is smell like it does? I don't know. This question can be generalized. Many chemicals have smells, unique to those chemicals. The molecules of those chemicals trigger receptors and that causes the smell.

 

The pungent part of a smell can be understood (stinging and burning sense). If a chemical is pungent, then it starts damaging the tissue. Some chemicals are odourless on their own, they only give a burning sensation. One of them is HCl. If you breathe this, then you smell a very nasty burning/stinging thing, but besides that it has no real "juice" or "taste". Ammonia, however, has a certain "juice" besides the stinging/pungent feeling. Chlorine gas also has a certain "juice" as part of its smell.