The intensity of chemtrails are kept at a level just below the threshold for most people to smell. However, from time to time, they inadvertently spray an area heavily enough so that more of the public can smell it.
Some people with sensitive senses of taste and smell detect the chemical and metals aerosols being sprayed in the air 24/7. These sensitive people provide a warning that should be taken seriously by the public because long term exposure to small-particle metals dramatically shortens lifespans and induces common diseases and even death.
The first of the articles above say that the odor started in New York in 2005. Well that just happens to be the year that I first saw chemtrails in western New York.