Air Pollution

Sources, Impacts and Controls

Pallavi Saxena, Vaishali Naik, Pallavi Saxena, Vaishali Naik

• Publisher: CAB International
• Year: 2018
• ISBN:
  9781786393913
• Topic: Tecnología e ingeniería
• Subtopic: Gestión medioambiental

Air pollution has become a major global issue due to rapid industrialization, human population growth and increasing urbanization. The various sources of atmospheric pollutants, both those created by human activity and those from natural physical and biological processes, have become the focus of much scientific research and analysis. An understanding of how these many pollutants are affecting air quality is essential in order to design strategies to mitigate them.Written by a team of international experts, this book aims to provide a broad overview of the issues surrounding air pollution and how to control and monitor pollution levels. Beginning with a brief background on the subject, the book moves on to discuss global emissions, with an emphasis on megacities and their effects. Possible pollution control measures and methods of air pollution measurement and modelling are also explored. The book ends with descriptions of the various indices used for assessing air quality with a focus on human health impacts, and a discussion on policy making to control air pollution.The book will be useful to students of environmental science and atmospheric science, as well as environmental consultants and researchers interested in air quality.Key Features: Comprehensive introduction to the primary causes of air pollution today with an emphasis on growing urban populations and megacitiesDiscusses both anthropogenic and biogenic emissions and their effects on human health and the environmentGives an overview of indices used today for assessing air quality and describes current methods for air pollution monitoring and modellingDiscusses new technologies for mitigating the effects of air pollution and policy making for implementation of controls

1 Anthropogenic Sources of Air Pollution

Chinmay Mallik^*

Max Planck Institute for Chemistry, Mainz, Germany

^*E-mail: [email protected]

Abstract

While ‘Air Pollution’ is an alarming term for environmentalists, policy makers, governments and common people, its ‘anthropogenic sources’ make it a formidable hazard to deal with. The universal dependence of humanity on fossil fuels, which has changed the way human beings live and breathe on this planet, has been a catastrophe for human health and the Earth’s environment. Although there exists a myriad of anthropogenic air pollutants, their human-made sources, dominated by the combustion of fossil fuels, can be conveniently grouped into a few major sectors (energy, industry, agriculture and waste) for the purpose of comparison across various temporal and spatial domains, and the formulation of strategies to monitor and control their emissions. While hundreds of anthropogenic air pollutants are toxic, there exist six ubiquitous air pollutants which are regulated by the governments in most countries due to their significant harmful impacts on human health and the environment. The association of anthropogenic air pollutants and their emission sources is documented in the form of emission inventories, spanning local, regional and global domains. This chapter provides an overview of the types of air pollutants, their primary sources, and the estimate of their global emission strengths as represented in emission inventories.

1.1 Introduction

The word ‘anthropogenic’ refers to anything produced due to human activities. Thus, any biological, chemical, radioactive or physical substance that is emitted into the air as a result of human activities, and results in concentrations higher than that which would be present in the natural atmosphere, leading to adverse impacts on human, animals, vegetation and other biotic as well as abiotic components of the Earth and its atmosphere, would classify as an ‘anthropogenic air pollutant’. Specifically, anthropogenic emissions are those that are produced as a result of human activities but not necessarily those produced biologically from humans, e.g. emissions of ammonia (NH₃) from human breath/sweat is a natural source of NH₃. The clearest indication of anthropogenic influence is visible in the growing difference in carbon dioxide (CO₂) emissions and uptake, leading to a dramatic increase in measured atmospheric CO₂ levels, now crossing 400 ppm. The impacts of anthropogenic air pollution are clearly manifested in the form of global warming, premature mortality in the millions due to ozone (O₃) and particulate matter (PM), extreme precipitation and droughts leading to crop loss, intensification of cyclones, fog and haze jeopardizing daily lives, impacts on ecosystems, loss of flora and fauna, increased ecosystem carbon storage, and changes in soil nitrogen and phosphate, to name but a few (IPCC, 2013).

Air pollution is not only an emission problem but also a chemical problem. Hence, the sources of air pollution are both primary and secondary. ‘Primary’ pollutants are those that are directly emitted into the atmosphere from various emission sources, e.g. sulfur dioxide (SO₂) emissions from coal burning in power plants, and nitrogen oxides (NO_x: NO+NO₂) from the transport sector. The concentration of primary pollutants is likely to be greater near their emission sources, but depending on their chemical lifetime and meteorological conditions, they can be transported over long distances (thousands of kilometres) in a short time (days). ‘Secondary’ pollutants are produced in the atmosphere due to various physical and chemical processes involving atmospheric constituents (gases and particles) including primary pollutants, e.g. formation of ozone (O₃) from NO_x and hydrocarbons. High concentrations of secondary pollutants can occur even in places far removed from large emission sources, e.g. O₃ formation can take place in rural and even pristine remote areas due to photochemical reactions among O₃ precursors, as they are transported away from their emission sources (Mallik et al., 2013). Contemporary sources and impacts of anthropogenic air pollutants span all environment regimes, spanning urban, semi-urban and rural areas, farmlands, forests, lakes, mountains and even oceans (Fig. 1.1).

Fig. 1.1. The illustration shows the sources of various criteria and toxic air pollutants, as well as sources of greenhouse gases, encountered by a healthy farmer during his migration to a city. The various emission sources depicted are livestock, agriculture, industries, road dust, vehicular emissions and waste. As he experiences high levels of air pollution, his health deteriorates, cutting his journey short, and he is in a dilemma as to which path to tread next.

1.2 Anthropogenic Air Pollutants

Although a variety of chemical substances emitted into the atmosphere due to human activities exert significant harmful effects on human health and the environment, the United States Environment Protection Agency (USEPA) identifies six ubiquitous air pollutants that need to be regularly monitored and regulated, hence referred to as ‘criteria pollutants’ (USEPA, 1990). These criteria pollutants are ground level O₃, carbon monoxide (CO), nitrogen dioxide (NO₂), lead (Pb), particulate matter (PM) and SO₂. Regulation of criteria pollutants mandates development of air quality standards, which vary from country to country. The ‘non-criteria pollutants’ do not have an air quality standard assigned to them, and include the entire range of air contaminants including toxic and hazardous substances, most of which are volatile organic compounds (VOCs). Gases such as carbon dioxide (CO₂) and methane (CH₄), co-emitted with various criteria pollutants, are studied under the realm of ‘greenhouse gases’ because of their significant control on the radiative balance of the Earth’s atmosphere and, hence, its climate. Many short-lived air pollutants also influence the radiative balance of the Earth by absorbing terrestrial infrared radiation (e.g. tropospheric O₃), absorbing (e.g. black carbon) or scattering (sulfate aerosols) solar radiation or by interacting with clouds. These influences on radiation induce changes in the Earth’s climate. Therefore, such radiatively active pollutants are also known as short-lived climate forcers (IPCC, 2013).

1.2.1 Toxic pollutants

Toxic air pollutants, popularly known as ‘air toxics’ or as ‘hazardous air pollutants (HAPs)’, are known or suspected to cause serious health effects including cancer, reproductive and birth defects, or to cause adverse environmental effects (EPA, n.d.a). The USEPA identifies about 187 air toxics, emission sources for some of which are given in Table 1.1. The sources of air toxics can be classified as major and area sources. ‘Major source’ refers to a singular or a group of stationary sources juxtaposed in an area of common control, and can potentially emit 10 tons of a HAP or 25 tons of a combination of HAPs annually (USEPA, 1992). A ‘stationary source’ implies any standing structure e.g. building, stack, or set-up which emits or may emit an air pollutant. An ‘area source’ refers to a stationary source or an aggregate of stationary sources of HAP that do not constitute a major source but are still a threat to human health and the environment.

Table 1.1. Major sources and health effects of hazardous/toxic air pollutants. (Adapted from EPA, n.d.b)

1.2.2 Criteria air pollutants

1.2.2.1 Sulfur dioxide (SO₂ )

Sulfur is emitted into the atmosphere in various states of oxidation. Despite its ubiquity in all spheres of the globe, the most recognizable form of sulfur in the atmosphere is SO₂ as it is a precursor for sulfate aerosol – a key component of PM. The atmospheric sources of SO₂ are natural as well as anthropogenic but, over the years, the anthropogenic component has increased overwhelmingly. The primary source of SO₂ is combustion of coal and oil (which contain 1–2 % sulfur by weight) with smaller contributions from other industrial activities such as metal smelting and manufacture of H₂SO₄. The global SO₂ emissions were of the order of 115 Gg-SO₂ during 2005 with China contributing 32 Gg-SO₂ (~28%; Smith et al., 2011). Due to its profound impacts on human health, and aquatic and terrestrial ecosystems including acid rain, SO₂ has been regulated in power plants and transport sectors in various developed countries employing desulfurization and end-of-pipe abatement techniques. However, over the Asian region, anthropogenic SO₂ emissions are not well controlled and are projected to increase under current regulations (Wang et al., 2014). SO₂ can be toxic at high levels causing reduced respiration, inflammation of the airways, and lung damage (ATSDR, 1998). Plants exposed to high levels of SO₂ incur acute foliar injury, where it can be oxidized to sulfite, which is very toxic and can interfere with photosynthesis and energy metabolism. The SO₂ emissions over India were estimated at 8.8 Tg for 2010 with sector-wise contribution of 66% and 32% from power and industries, and fuel-wise contribution of 76% and 19% from coal and oil, respectively, to the national SO₂ emissions (Lu et al., 2011). High SO₂ levels have been detected in ambient air of megacities like Beijing (60 ppbv in winter; Sun et al., 2004) and Kolkata (6.4 ppbv in winter; Mallik et al., 2014). For India, the national ambient air-quality standard requires annual average SO₂ to be less than 19 and 7.6 ppbv for industrial/residential and sensitive areas, respectively (CPCB, 2013; EPA, n.d.c).

Several precursors of SO₂ in a lower oxidized state (reduced sulfur compounds, RSCs), including dimethyl sulfide (DMS), hydrogen sulfide (H₂S), carbon disulfide (CS₂) and carbonyl sulfide (COS), contribute significantly to the global sulfur budget. Once released into the atmosphere, these are oxidized to produce SO₂. Landfills are a major source of anthropogenic RSCs. Due to its comparatively long lifetime, COS is able to penetrate into the stratosphere where its photolysis and subsequent oxidation contributes to the stratospheric sulfate layer. Being the major precursors of sulfate aerosols which exert a negative radiative forcing on the atmosphere, sulfur gases indirectly play a crucial role in the Earth’s radiative balance and are of great interest to geoengineering (climate engineering) experts.

1.2.2.2 Nitrogen oxides (NO_x )

NO_x is composed of both NO and NO₂. NO has both natural (e.g. soils, lighting) and anthropogenic sources (e.g. vehicle exhaust). NO₂ is formed from the oxidation of NO. NO_x is the major precursor of tropospheric O₃. NO converts atmospheric HO₂ into OH, the most importa...