# Oxygenated Diesel Fuels and Their Effect on PM Emissions
## 1 Introduction  
Particulate matter (PM) is one of the most toxic and, at the same time, most complicated components of diesel exhaust gas. Particulate matter is composed of a center core of elemental carbon (soot) and adsorbed organic compounds including polycyclic aromatic hydrocarbons (PAHs) and small amounts of sulfate, nitrate, metals and other trace components. PM components can be divided into nonvolatile or insoluble and volatile or soluble fractions [1]. The first group of components includes soot; the second group consists of organic substances deposited on it which are mainly compounds derived from unburned or partially burnt fuel and lubricating oil. The composition of the emitted PM depends on many factors, e.g., engine load, type of combustion chamber and fuel properties. However, for particulate matter formed in the combustion chamber of a diesel engine, soot is typically the primary component. The main component of PM from automotive diesel engines is carbon [2]. The combustion process in a diesel engine is very short, as is the process of soot formation. The fuel molecules contain essentially 12 to 22 carbon atoms and approximately twice as many hydrogen atoms. Within milliseconds, soot particles containing thousands of carbon atoms and about 10 times fewer hydrogen atoms are produced. A more detailed analysis of this process shows that it runs in several stages. The pyrolytic reactions lead to the decomposition of the fuel molecules and the production of soot precursors that are involved in the nucleation process of the first visible particles (nuclei). The process of the surface growth of the nuclei then takes place, associated with the attachment ofcarbon atoms and removal of hydrogen atoms. As a result, spherules with a diameter of 20-50 nm are created. Simultaneously with the increase in surface area, agglomeration of the spherules takes place, leading to the formation of particle clusters. Another process, i.e., oxidation, has the opposite effect to those mentioned above; namely, it removes soot at each stage of its development [3; 4]. Due to the widespread use of diesel engines, a large population of people is exposed to diesel exhaust and particulate matter. The high harmfulness to humans of particulate matter from diesel engines results not only from their chemical composition, but also from the ineffectiveness of the defense mechanisms of the human respiratory system against them [5]. Particles are not efficiently filtered from the air we breathe, so they penetrate deep into the respiratory tract, settle or penetrate the lung epithelium. By carrying highly toxic condensed compounds on its surface, soot allows semi-volatile and non-volatile chemicals to access organs, fluids and cells that they could not reach without soot as a carrier. The spectrum of harmful effects of particulate matter on human health is very wide. Acute or short-term exposure to PM may cause acute irritation (of eyes, throat and bronchi), neurophysiological symptoms (dizziness, nausea) and respiratory symptoms, as well as intensification of allergic reactions [6]. Long-term exposure to particulate matter increases the morbidity and mortality and inter alia increases the incidence of chronic bronchitis and chronic obstructive pulmonary disease. Long-term exposure to diesel particulate matter increases the risk of lung cancer [7]. There are many studies confirming the more frequent occurrence of this disease for people who frequently come into contact with exhaust gases from diesel engines [8; 9]. In 2012, the International Agency for Research on Cancer (IARC) officially recognized diesel exhaust as a carcinogen [10]. Recent studies show that a large number of ultrafine particles can pass through the brain-blood barrier, exerting direct detrimental effects on the central nervous system [11]. There is no doubt that the reduction of PM emissions should be achieved by all means. The particulate emissions from diesel engines can be influenced in a number of ways.