Diesel is not really "less refined" it is a different distillate of crude oil, and contains heavier molecules. I think major contributor to its worse atomization is it's higher viscosity and lower vapor pressure at room temperature. It just evaporates a lot less readily. If you've never handled gas and diesel, diesel feels a lot oilier and if you soak a rag with diesel vs gas, the gas will evaporate a lot sooner.