Anthropogenic emissions of air pollutants into the atmosphere depend on the population and various human activities to improve the quality of life. These activities are mainly related to supply of energy and food. Human-made emissions are very divers. In the following, some major source types are presented.
Combustion processes are the major anthropogenic sources of air pollutants during the production and use of energy. Combustion process produces carbon dioxide (CO2) and water (H2O) as the main products. Additionally, combustion also produces several by-products, which originate either from incomplete fuel oxidation (e.g. CO, hydrocarbons, aerosol particles etc.) or from the oxidation of non-combustible species present in the combustion chamber (e.g. NOx, SOx etc.).
During combustion processes in energy and transformation industries, or in manufacturing industries, the emissions depend on the fuel and process activity. Relevant pollutants are generally CO2, H2O, SO2, NOx, CO, NMVOC, aerosol particles, heavy metals (HM), polycyclic aromatic hydrocarbons (PAH), polychlorinated dibenzo-dioxin and polychlorinated dibenzo-furans (PCDD/F) and, for some activities, polychlorinated biphenyls (PCB) and hexachlorobenzene (HCB).
Current anthropogenic emissions of CO2 are primarily the result of the consumption of energy from fossil fuels (IPCC, 2001). About 20–33% of total anthropogenic sources of methane (CH4) is also originated from combustion.
In the absence of flue gas desulphurisation (FGD) technology, the emissions of sulphur oxides (SOx) are directly related to the sulphur content of the fuel. The sulphur content of refined natural gas is negligible. The majority of SOx is sulphur dioxide (SO2) although small proportions of sulphur trioxide (SO3) can arise. For cement manufacture, some of the SO2 (and other acid gases) is absorbed through contact with alkaline media in the cement kiln and, in the dry process, the raw meal.
The emission of NOx is generally in the form of nitric oxide (NO) with a small proportion present as nitrogen dioxide (NO2). Nitric acid manufacture includes catalytic combustion of ammonia to provide NO2 for subsequent absorption. Emissions of nitrogen oxides arise from nitrogen in the fuel (mainly relevant to solid and liquid fuels) and from reaction of atmospheric nitrogen. Combustion control can provide a high degree of NOx emission control (low NOx burner technology) and this may be supplemented by use of selective catalytic reduction (SCR) or selective non-catalytic reduction techniques (SNCR).
Particulate matter (PM) emissions from large combustion plants (> 50 MW) burning solid fuels are often lower than emissions from smaller plants (per unit of energy input); the physical and chemical characteristics of the particulate matter also differ. This is because different combustion and abatement techniques are applied. Combustion of fuels can generate solid residues, which may be deposited within combustion chambers (furnace bottom ash) within the furnace, boiler surfaces or ducting (fly ash) or on heat exchanger surfaces (soot and fly ash). Coal and other fuels with significant ash content have the highest potential to emit particulate matter. Suspended ash material in exhaust gases may be retained by particulate abatement or other emission abatement equipment (abatement residues). Materials, which remain in the flue gases beyond the abatement equipment and passes to the atmosphere, are primary particles.
The emission of heavy metals (arsenic (As), cadmium (Cd), chromium (Cr), copper (Cu), mercury (Hg), nickel (Ni), lead (Pb), Selenium (Se), Zinc (Zn) and Vanadium (V)) during combustion processes strongly depends on their contents in the fuels and process feedstock. Most of the heavy metals considered are normally released as compounds (e.g. oxides, chlorides) in association with particulates. Only Hg and Se are at least partly present in the vapour phase. The content of heavy metals in coal is normally several orders of magnitude higher than in oil (except occasionally for Ni and V in heavy fuel oil) and in natural gas. For natural gas, only emissions of mercury are relevant. During the combustion of coal, particles undergo complex changes, which lead to vaporisation of volatile elements. The rate of volatilisation of heavy metal compounds depends on fuel characteristics (e.g. concentrations in coal, fraction of inorganic components, such as calcium) and on characteristics of technology (e.g. type of boiler, operation mode).
The emissions of dioxins and furans are highly dependent on the conditions under which combustion and subsequent treatment of exhaust gases is carried out. The sintering process in iron and steel manufacture has been identified as a significant source of dioxins.
The emissions of polycyclic aromatic hydrocarbons (PAH) results from incomplete (intermediate) conversion of fuels. Emissions of PAH depend on the combustion process, particularly on the temperature (too low temperature favourably increases their emission), the residence time in the reaction zone and the availability of oxygen.
Carbon monoxide is found in gas combustion products of all carbonaceous fuels, as an intermediate product of the combustion process and in particular for under-stoichiometric conditions. CO is the most important intermediate product of fuel conversion to CO2; it is oxidized to CO2 under appropriate temperature and oxygen availability. Thus, CO can be considered as a good indicator of the combustion quality. Substantial emissions of CO can occur if combustion conditions are poor.
Emissions of non-methane volatile organic compounds (NMVOC), e.g. olefins, ketones, aldehydes, result from incomplete combustion. Furthermore, unreacted fuel compounds such as ethane (C2H6) can be emitted. The relevance of NMVOC and CH4 emissions from boilers, which are often reported together as VOC, is very low for large-sized combustion plants.
Transport is another major source of air pollutants. Terrestrial, naval and aerial transportation use a huge amount of energy and during the combustion processes, various components are emitted into the atmosphere. In order to comply with emission legislation, vehicle manufacturers have installed various after-treatment devices, such as catalytic converters and diesel particle filters (DPFs) to reduce pollutant emissions. However, such devices may, as a result of their action, also produce small quantities of pollutants such as NH3 and N2O.
During terrestrial transport, the most important pollutants emitted by road vehicles are ozone precursor compounds, (CO, NOx, NMVOCs), greenhouse gases (CO2, CH4, N2O), acidifying substances (NH3, SO2), aerosol particles, carcinogenic species, such as polycyclic aromatic hydrocarbons (PAHs) and persistent organic pollutants (POPs), heavy metals and other toxic substances (dioxins and furans).
The emissions produced by railways arise from combusting the fuel in an internal combustion engine. Consequently, the principal pollutants are those from diesel engines, i.e. similar to those used in road transport. These are principally CO2, PM and NOx, plus to a lesser extent CO and hydrocarbons, together with SOx and heavy metals originating from the content of fuel in sulphur and metals, respectively.
The emissions produced by aviation come from the use of jet fuel (jet kerosene and jet gasoline) and aviation gasoline (used to fuel small piston engine aircraft only) that are used as fuel for the aircraft. Consequently, the principal pollutants are those common to other combustion activities, i.e. CO2, CO, hydrocarbons and oxides of nitrogen, with SO2 emissions being dependent of the level of sulphur in the fuel. Other important species (like PM, N2O, CH4) are emitted at relatively low concentrations. Air pollutants emitted by aircrafts can affects the atmospheric processes in the free troposphere and in the lower stratosphere too.
Water-borne navigation causes emissions of carbon dioxide (CO2), methane (CH4) and nitrous oxide (N2O), as well as carbon monoxide (CO), non-methane volatile organic compounds (NMVOCs), sulphur dioxide (SO2), particulate matter (PM) and oxides of nitrogen (NOx).
Agricultural activities can also result human-made emission of air pollutants into the atmosphere. Main sources of emission from animal husbandry and manure management are livestock housing and holding areas, manure storage, field-applied manure and manure deposited during grazing. During these activities NH3, NO and NMVOCs are emitted to the atmosphere. Additionally, livestock feeding is another source of particles.
Main sources of crop production and agricultural soils are fertiliser application (NH3), soil microbial processes (NO) crop processes (NH3 and NMVOCs) soil cultivation and crop harvesting (PM).
An additional, less important agricultural source of air pollutants is crop burning. During these processes ammonia (NH3), nitrogen oxides (NOx), non-methane volatile organic compounds (NMVOCs), sulphur dioxide (SO2), carbon monoxide (CO), particulate matter (PM), heavy metals (HM) and dioxin are released to the atmosphere.
Major emissions from waste disposal are emissions of greenhouse gases. Small quantities of NMVOCs, CO, NH3 and NOx may be released as well. Particulate matter (PM) emissions are also emitted from waste handling, but no emission factors are available.
The relative contribution of (open and domestic) biomass burning emissions to the global annual emission for CO is ~ 25 %, for NOx ~ 18 % and for non-methane volatile organic compounds (NMVOC) and CH4 ~ 6 % (Intergovernmental Panel on Climate Change (IPCC), 2001). In Europe however, the contribution to total emissions is much lower, since the vast majority of fires occur in tropical regions.
The air pollutants emissions vary greatly with time and space. The distribution among each sectors are also highly variable depending to the development level of each country. Here we focus only on the emission inventories of the European Union based on the Technical Reports of European Environment Agency (EEA, Technical Report, 2008, 2009, 2012). Further data can be available for example at http://www.epa.gov/air/emissions/.
The total greenhouse gas (GHG) emission in the Europen Union was 4720.9 Mt CO2 equivalent in 2010. The most significant sectors that release GHGs into the atmosphere are energy supply and use as well as transport, both in the whole European Union (Figure 2.5) and in Hungary (Figure 2.6).
Figure 2.7 and Figure 2.8. show the percentage of each emitted greenhouse gases in 2010 in EU, and in Hungary, respectively.
Nitrogen oxides (NOx):
The main anthropogenic sources of nitrogen oxides are transport and public electricity and heat sector (Figure 2.9). In combustion equipments the NOx are formed during the combustion process at high temperatures by the oxidation of nitrogen content of fuel and combustion air. In a first phase only NO is formed while the NO2 is mainly formed after the combustion in exhaust process when more O2 is present and into the atmosphere. The total yearly emission of Nitrogen oxides in EU27 was 11 199 Gg in 2006, which means a 35% reduction between 1990 and 2006 (EEA, 2008).
Carbon monodide (CO):
Carbon monoxide is mainly produced as an intermediary product of combustion processes. In the EU-27, total emissions of CO was 30 200 Gg in 2006. This is a result of a large decrease by just over 53% between 1990 and 2006 (EEA, 2008). Contributions to total CO emission by different sectors in the European Union can be seen in Figure 2.10. Main sources of carbon dioxide are transport and residential heating.
Non methane volatile organic compounds (NMVOC):
The Earth’s vegetation naturally releases huge amounts of organic gases into the air. As plants assimilate carbon dioxide into biomass through photosynthesis, a fraction of this carbon leaks out in to the atmosphere, predominantly in highly reduced forms such as isoprene and terpenes. In addition to emissions from natural sources, several anthropogenic processes result in the emission of organic compounds such as carbonyls, alcohols, alkanes, alkenes, esters, aromatics, ethers and amides. Biogenic sources in total are considered to be approximately ten times larger than the sum of anthropogenic emissions including fossil fuel emissions and biomass burning (Williams and Koppmann, 2007). At the same time, human made emission can play important role in local atmospheric chemistry, e.g. in ozone production (see Chapter 8). The anthropogenic contribution to organic emissions in the atmosphere is originated from transport, other explotation of fossil fuels, product use and paint applications (Figure 2.11).
In Europe, the total NMVOC emission of EU27 was 9 391 Gg in 2006 (EEA, 2008). The NMVOC emission is decreased by 44% in Europe since 1990.
Sulphur oxides (SOx):
Sulphur components are released into the atmosphere by both natural and anthropogenic sources. Human made emissions however are more significant than natural sources (Möller, 1994). Anthropogenic sulphur emission is principally originated from fossil fuel combustion. Total SOx emission was 7 946 Gg in 2006 in EU countries (EEA, 2008). Due to the rigid emission reduction strategies in Europe, SOx emission have decreased continuously in the last decades (about 70% decrease have realized in SOx emission since 1990, when total SOx emission was 26 217 Gg). Main anthropogenic sources of sulphur compounds is public electricity and heat production, which accounts for more than 58 % of total emissions, and manufacturing industries and constructions (about 14% of total SOx emissions) (Figure 2.12).
The major sources for atmospheric ammonia are agricultural activities. Close to the emission sources, acute exposures to NH3 can result in visible foliar injury on vegetation. NH3 is deposited rapidly within the first 4–5 km from its sources in the function of weather and plant conditions (see e.g.: Horváth, et al., 2005). However, NH3 is a very important alkaline constituent in the atmosphere. It reacts with acidic substances such as sulphuric acid (H2SO4), nitric acid (HNO3), nitrous acid (HNO2), or hydrochloric acid (HCl) to form NH4 ammonium salts that occur predominantly in the fine particle (size < 2.5 mm) fraction causing regional scale problems (see e.g.: Krupa, 2003).
Total ammonia emission was 4 001 Gg in 2006 in the European Union countries (EEA, 2008). This value shows a 22% decrease compared to the emission in 1990 (5 118 Gg). The two most important key categories of NH3 are manure management and direct soil emission, which contributed approximately 70% and 23% respectively of total EU27 emissions in 2006. Agriculture thus contributed more than 90 %of total EU27 ammonia emissions in 2006 (Figure 2.13).
Next to the natural sources of primary aerosol particles (see Chaper 9), a huge number of different size particles are also released into the atmosphere by several various anthropogenic processes. The total PM10 emission was 1 555 Gg in EU27, in 2006 (EEA, 2008). Almost 60% of this emission occurs in energy-related sectors, with a further 13% of emissions occurring in the agriculture sector (Figure 2.14).
Total PM2.5 emissions was 1 044 Gg in EU-7, in 2006 (EEA, 2008). In 2006, PM2.5 emissions from the residential category contributed approximately 30% to total emissions (Figure 2.15). Other main sources were road transportation (18%) and manufacturing industries (11%). Both PM10 and PM2.5 emission have decreased since 1990 with about a 10% in Europe.
EEA Technical report, 2008: Annual European Community LRTAP Convention emission inventory report 1990–2006 Submission to EMEP through the Executive Secretary of the UNECE. EEA Technical report. ISBN 978-92-9167-366-7.
EEA Technical report, 2009: EMEP/EEA air pollutant emission inventory guidebook 2009. Technical guidance to prepare national emission inventories. EEA Technical report. ISBN 978-92-9213-034-3.
EEA Technical report, 2012: Greenhouse gas emission trends and projections in Europe 2012. Tracking progress towards Kyoto and 2020 targets. EEA Report. ISBN 978-92-9213-331-3.
2005. Measurement of ammonia exchange over grassland in the Hungarian Great Plain In: Agricultural and Forest Meteorology. 130. 282-298.
IPCC, 2001: Climate Change 2001: The Scientific Basis. Contribution of Working Group I to the Third Assessment Report of the Intergovernmental Panel on Climate Change. (eds.). Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA. 881pp. ISBN 0521 80767 0.
2003. Effects of atmospheric ammonia (NH3) on terrestrial vegetation: a review. Environment Pollution. 124. 179-221.
1994. Global sulfur and nitrogen biogeochemical cycles. In: Boutron, F (ed) Topics in atmospheric and terrestrial physics and chemistry. 125-156. ISBN 2-86883-287-3.