挥发 means volatile or volatilization in English. In serious scientific Chinese, it is only used to describe volatile due to thermal motion of molecule
(a kind of 物理变化(physical change)). Substances like ammonia and vinegar can 挥发 into the air, so you can smell it.
But in this usage (apparently it is informal), it is not thermal motion. I searched the ingredient of 拉面剂 and shockingly found that many 拉面剂 contains Sodium Sulfide 硫化钠(Na2S). When dissolved in water and exposed to air, it would have chemical reaction and eventually turned into volatilizable Hydrogen Sulfide 硫化氢(H2S), so you can smell some bad smell (which is often described as the smell of 臭鸡蛋(bad eggs)).
Hydrogen Sulfide is very harmful to your health. As a result, there shouldn't be any Sodium Sulfide inside your 拉面剂. Also, it is not a food additives listed in Chinese National Standard for food additives (GB2760), so it is illegal to add it.
There is a lot of argument about the actual ingredient of 拉面剂, and I can't be sure which is right. But I found two articles from 人民网 and 人民日报(two of the Chinese official media), and they claimed that safe 拉面剂's main ingredient should includes 食用盐, 碳酸盐，磷酸盐 and 焦亚硫酸钠 (sorry for not being translated, I'm just lazy), which are just some common food-grade things.