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For many years, intensive development of the automotive industry has been observed, most noticeably in larger cities and urban agglomerations. In urban areas, where the land cover is conducive to the absorption of solar energy, a specific microclimate is created, characterized by higher temperature and lower humidity. Moreover, dense build-up results in the attenuation of wind strength, which limits the circulation of air masses and, thus, the ability to ventilate. All of this is compounded by a significant traffic volume; thus, road transport, as one of the main sources of air pollution, exerts a very negative impact on the environment. An important feature of road transport pollution is that – compared to sectors such as energy and industry – the amount of exhaust fumes emitted by vehicles is smaller, but the concentration levels of pollutants in the proximity of linear sources such as roadways can be significantly increased. Considering that these are often densely populated areas, the problem is really severe.
Typically, exhaust emissions are taken into account when considering air pollution from vehicles. The exhaust gases comprise carbon monoxide, nitrogen oxides, hydrocarbons, sulfur oxides and particulate matter. Exhaust emissions vary depending on the engine type. This results directly from the use of a different type of fuel but also from a different way of preparing the combustible mixture, and, finally, a different process of combustion itself. The movement of vehicles is also related to non-exhaust emissions, which should also be taken into consideration. These include the abrasion of the top layer of tires, wear of brake and clutch linings, as well as road surface abrasion and resuspension of the road dust.
Various actions are being taken to eliminate the harmful effects of transportation. On the one hand, the aim is to systematically reduce the amount of fuel consumed, while constantly improving the methods of its combustion. More and more modern vehicle constructions using many new pro-ecological solutions are being and have been introduced to the market. Furthermore, propulsion sources alternative to fossil fuels are commonly used. At the same time, however, in many European countries, the number of vehicles is increasing at a high rate and the fleet itself is largely obsolete. Nevertheless, new technological solutions make it necessary to look at the problem of the environmental impact of vehicles more widely. This is, for example, the case with electric cars, commonly referred to as zero emission cars. Although they do not emit fumes while driving, the way in which electricity is generated should be taken into account. Another environmental problem is the method of producing and, later, utilizing the batteries necessary for electricity storage. It should also be noted that all vehicles, regardless of the propulsion system used, are the source of non-exhaust emissions.
One of the most harmful and dangerous components of exhaust gases is carbon monoxide (CO), produced when carbon-containing compounds are incompletely burned. It is a colorless, odorless and tasteless gas that, after entering the human body, reacts with hemoglobin (Hb), leading to the formation of carboxyhemoglobin (COHb). As a result, the process of combining hemoglobin with oxygen is blocked, as the affinity of carbon monoxide for hemoglobin is 200–250 times greater than that of oxygen (Blumenthal 2001, Bleecker 2015, Rose et al. 2020). Depending on the concentration of CO in the air, hypoxia can manifest itself in the form of fatigue, weakness, headaches and dizziness, disorientation, and, in the case of higher concentrations, loss of consciousness and death by suffocation. Lower doses of CO combined with prolonged exposure may lead to damage to the central nervous system and, thus, decrease in visual perception and driving performance (Raub and Benignus 2002, Weaver et al. 2007).
In addition to nitrogen oxides and volatile organic compounds, CO is referred to as a ground-level ozone precursor because in the presence of sunlight, it takes part in a series of chemical reactions leading to the formation of photochemical smog. In vehicles, carbon monoxide is formed when fuel is burned with an insufficient amount of oxygen (partial oxidation). Comparing the CO emissions by engine type, the combustion of fuel in a diesel engine produces less CO due to the fact that diesel engines have a high excess air ratio. Much of the carbon monoxide is retained on the catalytic converter, where CO is converted into CO2.
Other environmentally important components of exhaust gases are nitrogen oxides (NOx). Inhalation of NOx leads to irritation of the throat mucous membranes and can be fatal at higher concentrations. Two compounds belong to this group: nitric oxide (NO) and nitrogen dioxide (NO2). Nitric oxide is a colorless and odorless gas that is slightly soluble in water. Its molecule is unstable and quickly oxidizes to nitrogen dioxide in the air. Nitrogen dioxide, in turn, is a brown gas that dissolves very well in water and has a pungent, characteristic odor. It is highly toxic and, even after a short-term exposure leads to irritation of eyes and respiratory system, which in turn results in breathing problems. Moreover, NO2 has the ability to oxidize iron in hemoglobin, as a result of which it loses its oxygen-carrying capacity. In the atmosphere, nitrogen dioxide forms nitric acid that is leached out as nitrates by rainfall.
In the case of road transport, nitrogen oxides are released during the high-temperature combustion of fuels. In spark-ignition engines, where fuel combustion occurs at low air–fuel equivalence ratio (lambda), the formation of nitrogen oxides is limited. The problem is much more acute in diesel vehicles where the combustion process takes place at high temperatures with a high excess air ratio.
Road transport is a major source of nitrogen oxides. In some EU countries, the share of emissions from road transport can exceed 50%. Despite this, emissions have been steadily decreasing over the last few decades, mainly due to new cars being fitted with catalytic converters or catalyst systems. In the case of diesel engines, an SCR technology, i.e. selective catalytic reduction of nitrogen oxides, has been used since the 1980s. Depending on the active catalytic components, vanadium and tungsten-vanadium catalysts are used, but manganese, molybdenum, platinum and palladium catalysts are also possible. With the catalyst, the reduction of NOx to molecular nitrogen N2 and water (water vapor) takes place with the aid of ammonia as a reducing agent. For safety reasons, a non-toxic and odorless aqueous urea solution of 32.5% is used. The solution decomposes into ammonia and carbon dioxide when exposed to high flue gas temperatures. In spark-ignition engines, on the other hand, so-called three-way catalysts are used, which enable the reduction of nitrogen oxides, with the simultaneous oxidation of carbon monoxide and hydrocarbons. In the case of the latter, which are also an important component of exhaust gases, a large reduction in their emissions is precisely due to the equipping of new vehicles with catalytic converters.
As a result of the combustion of fuel contaminated with sulfur, sulfur oxides are emitted. Among this mixture of compounds, the main share (about 90%) is sulfur dioxide (SO2). It is a colorless gas with a pungent and suffocating odor. Inhalation of sulfur oxides leads to irritation of the mucous membrane of the nose, throat and eyes, and even to death at higher concentrations.
In the air, SO2 is oxidized to SO3, and this in turn to sulfuric acid. The process takes place with the participation of catalysts in the form of elements such as chromium, aluminum, vanadium, or manganese adsorbed on particulates. Sulfur dioxide is a well-known compound that contributes to the formation of acid rain. Its presence in the atmosphere causes a number of negative effects, such as corrosion of buildings. In combination with nitrogen oxides, carbon oxides and particulates, it is responsible for the formation of London smog.
Over the years, as a result of stringent regulatory requirements, the sulfur content of fuel has been significantly reduced. This applies to both spark and compression ignition engines. As a consequence, sulfur oxides originating from motorization have a small share in total emissions and, therefore, do not constitute a significant environmental problem at present.
The use of fuels in motor vehicles leads to the formation of particulate matter (PM). PM has a liquid or solid state and includes elemental and organic carbon, nitrogen and sulfur compounds, hydrocarbons, as well as heavy metals. Metals can be released through the abrasion of engine parts, as fuel additives or as contaminants. For many years, there has been an increasing proportion of PM emissions from non-exhaust sources such as tire wear and brake and clutch lining wear.
One of the main criteria for classifying PM is size. Currently, the most commonly used classification includes total suspended particles (TSP), coarse particles (PM10), fine particles (PM2.5) and ultrafine particles. Definitions of PM10 and PM2.5 can be found in the Directive 2008/50/EC of the European Parliament and of the Council of 21 May 2008 on ambient air quality and cleaner air for Europe. PM10 is defined as a particulate matter that passes through a size-selective inlet according to the European Standard EN 12341 (current version EN 12341:2014), with a 50% efficiency cut-off at 10 μm aerodynamic diameter. On a similar basis, PM2.5 is defined as a particulate matter that passes through a size-selective inlet according to the European Standard EN 14907 (currently superseded by EN 12341:2014), with a 50% efficiency cut-off at 2.5 μm aerodynamic diameter (EC 2008).
In general, the formation of PM is accompanied by complex processes of a physicochemical nature. It is commonly accepted that diesel engines are more affected by this problem. Due to the lack of air in the combustion chamber, products of incomplete combustion are formed. The soot produced promotes the adsorption of hydrocarbons, nitrogen oxides and sulfur dioxide. Nowadays, in order to reduce particulate emissions, diesel particulate filters (DPFs) are commonly used. For spark-ignition engines, particulate emissions are less of a concern. Only a small amount of soot is formed due to the slower thermal decomposition of gasoline. However, an important source of particulate emissions in this type of engine is engine oil and the contaminants found in gasoline.
In European countries, emissions of major air pollutants from road transport place this sector among significant sources of pollution. Figure 1.1 presents the shares of emissions of some transport-related pollutants compared to total emissions in 2018 considering the 28 EU member states. The data was taken from EU emission inventory database on national emissions reported under the Convention on Long-Range Transboundary Air Pollution (LRTAP Convention). The “non-road transport” sector includes, inter alia, international and domestic aviation, railways, as well as international and domestic shipping. For all but one pollutant, road transport is the main source of emission. The situation is different only for SOx, for which international shipping is the dominant source. The percentage share of NOx emissions from road transport exhaust sources for all 28 EU countries reached 39% in 2018. During the same year NOx emissions from other transport sources amounted to 8%. A high share of emissions from road transport was also observed for black carbon, almost 26%, of which more than 22% represented exhaust emissions. An equally high share was registered for carbon monoxide, over 20%, with the emissions from non-transport sources accounting for only 3%. In addition, transportation by all modes was a notable source of emissions of non-methane volatile organic compounds (over 9%) as well as PM2.5 and PM10, 13% and 12%...
