Diesel particulates have a bimodal size distribution which includes small nuclei mode particles and larger accumulation mode particles.
Most of diesel particle mass is contained in the accumulation mode while most of the particle number can be found in the nuclei mode. Although the exact composition of diesel nano particles is not known, it is believed that they are composed primarily of condensates (hydrocarbons, water, sulfuric acid).
The amount of these condensates and the number of nano particles depends significantly on the particulate sampling conditions, such as dilution ratios, which were applied during the measurement. Spark ignited engines also emit numbers of small particles which are comparable to those from diesel engines.
References;
https://www.
dieselnet.com/tech/dpm.php
This definition includes both solids, as well as liquid material which condenses ...Diesel particulates include small nuclei mode particles of diameters below 0.04 µm and ... The mass of particulate emissions is determined based on the weight of PM ... Since the atmospheric dilution ratios of PM (about 500-1000) are much ...
http://www.meca.org/resources/meca_ufp_white_paper_0713_final.pdf