Air pollution, definition and impacts

Air pollution can be defined as a situation in which substances are present in the atmosphere at concentration sufficiently high above their normal ambient levels to produce a measurable and undesirable effect on humans, animals, vegetation, or materials (Seinfeld and Pandis, 2006). The concentrations of pollutants in the atmosphere are a measure of air quality.

The World Health Organization (WHO) estimates that exposure to ambient air pollution causes each year approximately 3.7 million premature deaths worldwide (WHO, 2013).

Under a business-as-usual socioeconomic scenario, it has been stated that the contribution of outdoor air pollution to worldwide premature mortality by 2050 could double (Lelieveld et al., 2014) and that air pollution will be the top environmental cause of premature mortality (OECD, 2012). Depending on the exposure, the effects of air pollution on human health range from subclinical and symptomatic events to increased morbidity and mortality (Figure 1-1) (Brunekreef and Holgate, 2002; Gurjar et al., 2010; WHO, 2013). The acute and chronic exposure to air pollutants – gases and aerosols – has been positively associated with respiratory and cardiovascular sicknesses, and lung-cancer (Pope et al., 2009; Shah et al., 2013; Beelen et al., 2014). Recently, several European studies have highlighted that there are statistically significant positive associations between nitrogen dioxide (NO₂) and sulphur dioxide (SO₂) concentrations with total, cardiovascular and respiratory mortality, particularly in urban areas (Samoli et al., 2006; Cesaroni et al., 2013;

Bentayeb et al., 2015). Long-term exposure to tropospheric ozone (O₃) and particulate matter (PM) has also been associated with increased death risk due to cardiopulmonary causes (Jerrett et al., 2009). O₃ is responsible of ~17 400 premature deaths each year in the European Union (EEA, 2014a). The degradation of air also results in an increase of the burden of other related diseases, a reduction in life expectancy, and an increase in the health care public spending which convey to air pollution considerable financial and life-quality costs (Künzli et al., 2010; Vedrenne et al., 2015).

The United Nation Framework Convention on Climate Change states – based on the fifth Intergovernmental Panel on Climate Change Assessment Report (IPCC, 2014) – that “the anthropogenic driven warming of the climate system is unequivocal, and that climate change is on top of the humankind major challenges”. Climate change and air pollution are

highly interlinked phenomena (Figure 1-2). The radiative forcing (RF) of the atmosphere – a measure of the planetary radiation balance of which climate change relies on – strongly depends on the concentration of greenhouse gases (GHG) such as carbon dioxide (CO₂) and methane (CH₄). However, RF is also sensitive to the concentration of short-lived atmospheric pollutants such as carbon monoxide (CO), non-methane organic volatile compounds (NMVOC), O₃ (which is considered both a GHG and an air pollutant), and aerosols (Jacob and Winner, 2009). Whereas CO, NMVOC, and O₃ have a positive RF that enhances climate change (Figure 1-3), the net contribution of aerosols is yet under discussion due to the complexity of the aerosol-cloud-radiation interactions that include light absorption and back scattering, and the formation of cloud condensation nuclei (Myhre et al., 2013; von Schneidemesser et al., 2015). It is noteworthy that the implementation of air pollution abatement plans may lead to climate impacts. For example, Baker et al. (2015) have recently demonstrated using three coupled climate-air quality models that a total reduction of global SO₂ emissions results in an increase of the surface temperature motivated by an asymmetric hemispheric warming, particularly in the Northern Hemisphere. On the other hand, climate change-driven processes can enhance surface pollution for example, through an increased frequency of deep stratospheric intrusions – that elevate surface O₃ – associated to modifications of the polar jet (Lin et al., 2015).

Figure 1-1. Pyramid of health effects associated with air pollution (WHO, 2006) Air pollution is a threat for the environment affecting a wide range of ecosystems through a variety of processes including acidification, eutrophication or vegetation oxidation that can ultimately lead to a biodiversity and ecosystem services loss (Lovett et al., 2009; de Vries et al., 2014). As an example, trees exposed to acute or chronic high O₃ concentrations can be affected by reduced photosynthesis, damage to reproductive processes, lowered carbon transport to roots, as well as visible physiological effects on leafs that correlate with

reductions in growth in deciduous and evergreen species such as poplar and pine trees (Felzer et al., 2007). As a consequence, O₃ pollution leads to a reduction on the provision of ecosystem services such as carbon sequestration – which in turn, enhances climate change (Figure 1-2).

Figure 1-2. Overview of the main categories of air quality and climate change interactions and feedbacks (von Schneidemesser et al., 2015)

Furthermore, high ambient concentrations of gaseous pollutants as NO₂, O₃, and SO₂ have been statistically correlated with a reduction in the productivity of several crops (wheat, mung beans, beetroot), particularly in suburban areas (Agrawal et al., 2003). In the case of O₃, the typically high concentrations registered over Europe in summer causes considerable damage to crops and pastures that lead to relevant economic impacts (up to 1 billion € in 2000 in Europe according to Van Dingenen et al., 2009). In the context of Climate Change, in a business-as-usual emission scenario, predicted high O₃ concentrations could lead to an increase in worldwide crop damage of up to 20% by 2050 and therefore affect the global food production (Chuwah et al., 2015).

Dry and wet deposition of atmospheric pollutants can cause substantial deterioration of a diversity of materials (plaster, bricks, glass, metals, limestone, etc.) by the effect of corrosion and weathering, even in indoor environments (Kucera and Fitz, 1995). Damage caused to materials exposed in the atmosphere constitutes one of the most important direct effects of acidifying air pollutants (Chen et al., 2005). It has been shown that there is a statistically significant correlation between the corrosion rate of copper by SO₂ and O₃; aluminium by NO₂ and particulate matter; and iron by SO₂ and NO₂ (Liu et al., 2014). In

urban areas, where there is a high density of building structures and cultural heritage (stone façades, bronze statues, etc.), the damage can be particularly significant (Nord et al., 2001).

Additionally, the rubber cracking is caused by oxidative degradation of natural and synthetic rubbers due to the ozonolysis of rubber chains. This process, enhanced by air pollution leads to a deterioration of the physical and mechanical properties of rubbers such as tires and rubber seals (Li and Koenig, 2005).

Figure 1-3. Radiative forcing (W m^-2) of climate change shown by emitted components and drivers relative to 1750. Horizontal bars indicate the overall uncertainty (IPCC, 2014)

The solar energy sector is also affected by air pollutants. High concentration of air pollutants reduces the sunlight radiation reaching solar cells ultimately leading to a decrease in the electricity generation efficiency (Chaturvedi and Shashank, 2015). Moreover, particulate matter (dust, sand, ashes) deposited over solar panels require permanent cleaning that increase the facilities maintenance costs (Mani and Pillai, 2010).

Visibility is another area impacted by atmospheric pollution during haze and smog events.

Particles and gases in the atmosphere interact with light via scattering and absorption reducing visibility (Figure 1-4) and thence, affecting citizen’s life quality and altering on-road, marine and aviation safety (Watson, 2002; Hyslop and White, 2008). It is worth mentioning that the modification of the visibility, as perceived by people, can be used as an

indicator of air quality and it is currently being tested as tool to quantify citizen’s willingness-to-pay for reducing urban air pollution (Yu et al., 2015).

Figure 1-4. Effect of air pollution on visibility: panorama of the city of Madrid on a clear (top) and a polluted (bottom) day. Photo credits: Jorge París (@Jorgeparis1)