Deadly Fog

In my files, I have two reports of mysterious fogs that were injurious to all sorts of life.
The first incident occured in 1978 in Brazil. A deadly cloud killed sea lions, fish, horses, cows, dogs and a person. While most of the dmamage was centred around the city of Porto Alegre, dead sea life was reported along 286 miles of the Brazilian and Uruguayan coasts. Survivors reported the death-cloud as having an "ammonia like" odor. Fits of nausea and dizziness were common.
The second incident occured in 1980 in Vancouver, British Columbia. Around the northern end of Vancouver Island, a fog rolled in off the ocean and killed a large swath of vegetation. Environmental spokesman Lanny Hubbard said that the vegetation "curled up, turned brown and died for no apparent reason."
These two incident occured about two years from each other, with the first incident taking place on the Atlantic and the second taking place on the Pacific. The fact that 286 miles of Soth American coast were effected makes me think some sort of very large chemical explosion occured far out at sea, and the fumes slowly spread so that by the time they hit land they had a damage area of over 200 miles. Underwater volcano? Military experiment?