"Then, nowhere near a road, I had to ride through a layer of diesel fumes. I could see a layer of blue haze. Fortunately I broke out of it after about a quarter of a mile. Temperature inversion? Weird"

Possibly some farmer started up an old tractor & drove it off? Older tractors can produce what you describe if they have been sitting quite a while between startups. Mine has left a blue haze hanging low over our lake for the fishermen to avoid & in cold weather it can persist for quite a while. Don