In order to simulate/produce soot emissions during lab scale experiments.

I think that essence of question of Vishal Vasistha is better expressed in formulation No 3 which has been proposed by Remi Cornwall, namely: "As ethanol has low cetane number, low calorific value, increased NOx, increased ignition delay, then why is it widely used for research purposes with diesel in a mixture?". Really, the determination of optimal content of hybrid (ethanol + diesel) fuel allow us to increase the cetane number and calorific value, as well as to decrease NOx and ignition delay with benefits of renewable source of energy. Best regards 

I think your question should be framed as "As  ethanol possess low cetane number, low calorific and increased NOx increased ignition delay compared to diesel fuel, then why is it employed for research application?" 

The answer is ethanol can reduce reduce cold flow properties such as cloud and  pour  point , CCP when blended with diesel fuel. Hence, the introduction will pave way for wide application of alyernate fuel such as biodiesel from  animal fats and butter to be used in diesel engines in cold and artic regions.

Thanks everyone for your valuable answers. Mr.Serge V. Shil’ko, do you have any publication proof as you said in reply? I am eager to read that  optimal content.

Dear Vishal Vasistha, there are a lot of references on topic (finding of optimal content of mixture "ethanol - diesel") , f.e. valuable paper in Russian (attached). Good luck! 

You can see the paper attached too.