**1. Introduction**

Air quality has improved significantly in European cities over the past 20 years. The emission of sulfur oxides was mainly reduced: ~80%, nitrogen oxides: ~46%, non-methane volatile organic compounds: ~44%, particles with a diameter lower than 2.5 μm and 10 μm: 30% [1]. Nevertheless, European society is exposed to air pollution by PM2.5 and O3 at levels significantly exceeding the WHO limit values for the protection of human health [1]. This is related to premature mortality [2], estimated at 4.5 million deaths worldwide and approximately 0.5 million in Europe annually [3]. WHO, on their official website, states that 91% of people live in the area where air quality levels exceed WHO limits [4]. WHO, in Reference [5], says that the level of PM2.5 does not have a significant influence on human health in long-term time if it is 10 μg/m<sup>3</sup> per year. Today, according to the official data presented by WHO, the level of PM2.5 in all European countries is 25 μg/m3 per year (except Switzerland, where it is 10 μg/m<sup>3</sup> per year) [6].

Exposure to particles with diameter less than 2.5 μm for several hours may result in cardiovascular-related mortality. Long-term exposure (e.g., several years) increases the risk of cardiovascular mortality, can cause pulmonary inflammation, and reduces life expectancy in the more vulnerable segments of the population by several months to several years [7,8]. Atmospheric pollution with particles emitted by combustion engines

**Citation:** Jasi ´nski, R.; Strzemiecka, B.; Koltsov, I.; Mizeracki, J.; Kurzawska, P. Physicochemical Analysis of the Particulate Matter Emitted from Road Vehicle Engines. *Energies* **2021**, *14*, 8556. https:// doi.org/10.3390/en14248556

Academic Editor: Islam Md Rizwanul Fattah

Received: 9 November 2021 Accepted: 14 December 2021 Published: 18 December 2021

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adversely affects human health. Particles smaller than 1 μm in diameter can cause various diseases, especially heart and lung diseases, and related deaths. The worsening of disease is associated with the long-term effects of particles in the environment. They contribute to diseases, such as asthma and bronchitis. They are also one of the causes of heart arrhythmias and heart attacks. The most serious problems arise from the interaction of fine particles. People with heart and lung diseases, the elderly, and children have the lowest resistance to the negative effects of particulate matter. In impact analysis of air pollution on humans, in addition to common diseases, one should consider human cognitive abilities. Some scientists argue that air pollution negatively affects cognitive performance [9–11]. It was found that there are significant negative relationships between air pollutant exposure and cognitive function, especially PM2.5 exposure and long-term exposure. Additionally, it was found that the effect of PM on cognitive traits differs depending on the characteristics and attributes of the exposed person [12].

One of the main sources of particulate matter emissions in cities is road transport, especially vehicles with internal combustion engines (ICE), which are one of the main sources of particles emissions smaller than 1 μm [13,14]. Particles emissions emitted by compression ignition engines are from 6 to 10 times higher than emitted from spark ignition engines [15]. According to European Automobile Manufacturers Association (ACEA) Report for 2019, in European Union, 41.9% of passenger cars, 91.2% of light commercial vehicles and 98.3% of medium and heavy commercial vehicles were diesel vehicles [16]. The average retirement age of diesel cars in 2012–2013 was 14.0 years, and it increased by almost 12% since 2006. Lifetime mileage of diesel cars was approximately 208,000 km, based on data from 2012–2013 [17].

Particulate matter is one of the four main pollutants emitted by diesel engines, along with carbon monoxide (CO), nitrogen oxides (NOx), and hydrocarbons (HC). Particulate matter in internal combustion engines mainly arises as a result of incomplete combustion of hydrocarbons contained in the fuel and also from lube oil [15]. More than 90% of particles emitted by diesel engine are smaller than 1 μm, and most of the particles are between 15 and 40 nm in diameter [15,18]. Size distribution of particles emitted from diesel engines is bimodal, with mainly nucleation and accumulation mode [19]. There are many factors that influence the amount of particulate emissions from diesel engines. These are, among others: the age of the engine, the properties of the fuel supplying the engine, the level of maintenance of the vehicle and the driving style, environmental conditions, and the exhaust gas treatment technology used, but also the amount of oil scraped into the combustion chamber through the piston rings [14,20]. In order to reduce particulate emissions from internal combustion engines, particulate filters, such as the diesel particulate filter (DPF), are used. To function properly, filters used in diesel engines require periodic regeneration, which allows to remove accumulated solid particles. The efficiency of particle reduction for a properly functioning diesel particulate filter is about 95% [21]. In 2009, Europe established a new EURO 5 standard, and, since then, DPF is mandatory in new diesel cars [22]. In 2011, a new EURO 5b standard came into force for light passenger and commercial vehicles, which set a limit for the number of particles of 6 × 1011 per km, and was the first limitation of particles number [23]. The last established standard is EURO 6d, which has been in force since January 2021, but the limits for particles number and mass have not changed since EURO 5. As a diesel particulate filter is mandatory on new cars, many users of diesel cars remove the DPF to avoid the cleaning costs [22]. As a result of the removal of the DPF, the car no longer meets the applicable standards and is very harmful to the environment in terms of particulate matter. In many European countries, removal of DPFs is forbidden and punished with high fines [22,24].

One of the solutions to reduce pollutant emissions in cities is to completely ban or limit the entry of vehicles that do not meet the relevant standards. Currently, there are Low Emission Zones (LEZs) in many countries, such as Italy, Spain, France, Germany, Belgium, and others, where the most polluting vehicles are regulated [25]. More and more countries are considering a permanent ban on diesel and gasoline cars to favor electric cars. Thirteen

countries and several regions and cities announced ICE car bans until 2045 [26,27]. For this reason, development and researches of new technologies and trends in powertrains is inevitable. Currently, there are few configurations of electric powertrains: Plug-in Electric Vehicle (PEV), Plug-in Hybrid Vehicle (PHEV), Battery electric vehicle (BEV), and Extended-Range Electric Vehicle (EREV) [28–30]. Thanks to an electric or hybrid drive system, that does not generate exhaust gases during use, it is possible to reduce greenhouse gases and other harmful exhaust gas compounds arising during combustion in a conventional engine. While electric cars still need electricity, which, in many countries, is produced from fossil fuels, reduction of harmful exhaust gas compounds emission is not so significant as it can be when electricity is produced from renewable energy sources [31]. However, another alternative for combustion engines is the Hydrogen Powered Vehicle (FCV), which use hydrogen fuel, that can be produced from zero-emission energy source [31]. Alternative powertrains can be considered as a relatively quick solution for reduction of harmful emission from road transport in countries with well-developed infrastructure for electric cars, while, in many countries, the main barriers will be the lack of charging infrastructure, insufficient or lack of information and promoting programs on the national level, and also high price for electric powertrains [30].

The other problem of PM is the influence of the chemical composition on the human health. There are many questions and problems that concern the analysis of the chemical composition of the PM:


Generally, WHO states that the largest sources of PM emission are solid fuel, households, and transport.

In this paper, the authors take into account the transport factor and assess the influence of the engine type, the conditions of its work on PM amount emission, and chemistry. Compared to other studies that have been carried out in the past few years, this article includes both particulate matter data analysis obtained from the particles analyzer, but also three different chemical analyzes: Fourier Transform Infrared Spectroscopy (FTIR), Scanning Electron Microscope with Energy Dispersive Spectroscopy (SEM-EDS), and Evolution Gas Analysis (EGA), while most research focuses on only one chemical analysis [34–36]. The authors of this paper verify the hypothesis of whether the type of engine and its work conditions influence/does not influence the chemistry of PM and this chemistry difference are/are not significant. Moreover, this paper gives a brief answer as to what technique can be adequate for quick and accurate PM chemistry studying.
