If you're burning normal diesel, then it's just the H2O vapor from the combustion reaction, which immediately condenses in the freezing-cold air, making it white. Then it dissipates.
But check my latest publication about chemtrails. Nowadays, they lace diesel with Aluminum, Barium, and Strontium nanoparticles. They are poisonous and cause Alzheimer's and mess with the soil pH, which is deadly to most organisms. Again, check out my latest publication - it is quite good and answers your question directly.