Related:<p><a href="https://en.wikipedia.org/wiki/Operation_Sea-Spray" rel="nofollow">https://en.wikipedia.org/wiki/Operation_Sea-Spray</a><p>The journalist in OP chose not to report on it, but the mystery gas is <i>probably</i> one of the candidates summarized on pp. 3-5 of this 87 page PDF, which shares acronyms with OP ("Urban Threat Dispersion"),<p><a href="https://www.dhs.gov/sites/default/files/publications/ea_utd_cbt_v5.pdf" rel="nofollow">https://www.dhs.gov/sites/default/files/publications/ea_utd_...</a><p>The list includes perfluorinated gases (perfluorocarbons, SF6); several kinds of aerosolized microparticles with DNA sequence tracers; aerosols with amine dye or biological antibody tracers; and some proprietary fluorescent dye called "Visolite", "commonly used in HVAC tracing".