A Review of Air Pollution and its Impact on the Environment  

What is Particulate Matter?

Particulate matter (PM) or aerosols, most ranging from between 2.5 and 10 micrometers in diameter, have many sources and can be anthropogenic or naturally occurring. Some examples include marine aerosols like salt, mineral dust from agricultural practices, biological particles such as microorganisms and their spores and pollen, wood combustion for residential heating, and transport-related aerosols (Fuzzi et al., 2015). The latter will be the focus of this section.

Composition and concentration of anthropogenic PM

According to the Environmental Protection Agency (EPA) website, www.airnow.gov, anthropogenic sources of PM2.5 (considered fine particles) are generally produced from all types of combustion including motor vehicles, power plants, and residential heating. Human-produced PM10 (considered coarse dust particles) are associated with airborne residue from rock and mineral processing operations like the mining and transport of coal.

In the urban setting, concentrations of PM emissions tend to spike during the morning and evening rush hours when modes of transportation reach their zenith (Fuzzi et al., 2016). The combustion of diesel fuel and its byproducts mixing with exhaust gas is a major contributor of sulfur (SO2 & SO3) emissions into the atmosphere (Merkisz et al., 2002). sulfuric oxides and heavy metals such as Pb, Ni, Zn, and Cu are known to be primary sources of PM2.5 in a region of Finland where copper, nickel, and industrial fertilizers are produced (Eeva et al., 1998). In their 2005 global update, the World Health Organization published a PM2.5 exposure guideline of 25 ?g/m3/d and a guideline of 50 ?g/m3/d of PM10.

 PM impacts on fauna

Airborne heavy metal pollution from an industrial copper and nickel smelter affected populations of lichens, mosses, pine trees, and the bird species Parus major L. (P. major) in the coniferous forests of southwest Finland. The study found a significant reduction in the density of green caterpillars and sawfly larvae, the birds’ primary food source, nearest the smelter. This set off a chain reaction within the P. major population as their characteristic bright yellow plumage began to fade. Dulling of plumage led to increased competition for brightly colored mates and consequently produced faded coloring in the nestlings. In addition to disruptions to the mating/offspring cycles, male-male competition, battles for social dominance, and the decreased ability to use crypticism in avoiding predators may be attributed to PM pollution. Conditions slowly improved as radial distance from the smelter increased (Eeva et al., 1998).

PM pollution studies carried out on mice and metropolitan transportation emissions have produced startling results in the area of reproductive cycles. As Carré et al. writes, “Two studies were carried out on mice in the city of Sao Paulo, Brazil, which has a high level of air pollution. Mohallem et al. found a significant reduction in the number of newborns per mouse and a significant increase in the embryo implantation failure rate in female mice exposed as newborns for 3 months to the city polluted air and then mated with non-exposed males as adults” (Carré et al., 2017, p.2). 

The other study in Sao Paulo was conducted by Veras et al. They found, in adult mice exposed to urban traffic pollution (PM2.5) compared to mice exposed to filtered air, significantly more time was needed to mate, decreased number of pregnant females as a percentage of total number of females, and an increased occurrence of spontaneous abortion (Carré et al., 2017). The pollution pathology is a bit unclear but the work of Maluf et al. may shed some light in this case. Carré writes of their work, “...they [Maluf et al.] did observe a significant effect of PM2.5 exposure on the cell lineage allocation at the blastocyst stage between inner cell mass (ICM i.e. cells participating in the ontogeny of the future fetus) and trophectoderm (TE i.e. cells participating in the ontogeny of the future placenta)” (2017, p. 5). Cells in ICM were greatly decreased and cells in TE greatly increased in the mice exposed to urban automobile pollution. The opposite is expected for healthy pregnancy.

Impacts on the climate system

Light absorbing PM has been identified as a major climate warming agent, second only to carbon dioxide (Bond et al., 2013; IPCC, 2013). Carbon-rich soot particles from industrial and residential combustion have been shown to increase warm and mixed-phase cloud development (and therefore drizzle and rain development) and negatively modify the ability of clouds to reflect incoming solar radiation back into space. These dark colored aerosols transfer solar radiation into the atmosphere increasing its thermal energy and gather in concentrations dense enough to produce an apparent dimming effect of the sun’s visible light (Fuzzi et al., 2015).

What is Tropospheric Ozone?

While it is a naturally occurring greenhouse gas with dense concentrations located in the Earth’s Stratosphere, the focus of this section will be the ozone appearing in the lower Troposphere of our atmosphere. Ozone is most commonly emitted from automobile exhaust and is the main component of smog. Other sources of tropospheric ozone include biomass burning, chemical manufacturing, and fossil fuel combustion (Logan, 1983). Tropospheric ozone formation is the result of complex chemical reactions between nitrogen oxides, hydrocarbons, and carbon monoxide and is driven by exposure to sunlight. It forms most frequently on the sunny, hot days of summer when the sinking air of high pressure prevents atmospheric mixing (Mauzerall and Wang, 2001). According to its website, the Environmental Protection Agency (EPA) has recently set new, strict standards for ozone exposure to not exceed 0.070 ppm average per 8-hour period. This is the second standards revision since 1997.

On crop yields

Most crops around the world are grown in the late-spring through summer months. As ozone concentration peaks during this time, it is more reason to consider crop damage by O3. Studies conducted in the United States and Europe are progressing our understanding of the physiological mechanisms and effects of ozone pollution on crops. In both studies, a variety of common crops including beans and wheat were grown in open-top chambers to allow for control and monitor of O3 levels.

In the US, the results were varied based on the plant tested. According to Heck, soybean yields decreased by 40% when exposed to O3 concentrations of 70-90 ppb. However, broccoli yields appeared unaffected when exposed to 63 ppb of O3 (Heck, 1989). This seemed to indicate different crops have ranging levels of natural sensitivities to O3 exposure. The times of day when experiments were being conducted were of special note as 8:00am-8:00pm is when the leaf stomata remain open and gas exchange between the atmosphere and crops is maximized (Lee and Hogsett, 1999).

In Europe, studies included crops such as barley, beans, wheat, and even grazing pasture and exposure methods were the same as previous studies. A cumulative reduction in crop yields was observed over the exposure threshold of 30-40 ppb per hour during daylight hours (Fuhrer, 1994). It was noted that environmental factors tend to play a role in O3 uptake in sensitive plants including soil moisture, temperature, and vapor pressure deficit (Grunhage et al., 2001). At the time of publication, Mauzerall and Wang report no Asian government had yet to undertake an organized effort to study the effects of O3 concentrations on crops (2001).

On temperate forests

Aerodynamically speaking, forests are rough terrestrial surfaces where large amounts of mass, heat, and atmospheric gases exchange by frictional drag (Fowler et al., 1999). These take place by an order of magnitude or more than over lands covered in grasses. With such concentrations of O3 potentially harming crops which tend to be short in stature, it is valid to wonder if the taller, more exposed trees of the world’s forests are also susceptible. According to Lapinski et al., “in the mountains near Los Angeles, millions of ponderosa pines have been severely damaged by ozone pollution” (361).

Just as in the crops studies, it seems degree of O3 effects on trees depend on the sensitivity of each species. The free-air O3 exposure experiments conducted by Matyssek et al. showed a higher level of ozone sensitivity in pioneer trees than in climax species, although variations may exist within different genotypes of the same species (2010). In Open-top chambers where the concentrations of O3 are controlled and monitored, there is noticeable impact in the trees of the temperate forest (Skarby et al., 1998). In their study, Karenlampi and Skarby used an O3 concentration of 40 ppb because it has become a benchmark in developing a dose-effect relationship in biomass yield (1996). “Elevated ozone levels reduce the supply of carbohydrates to roots reducing their biomass”, (Bytnerowicz et al. 441). Grulke noted a similar danger to that of the crop studies in that toxic levels of O3 exposure directly impair stomatal function diminishing regulation of water loss in trees (2010). If current trends in ozone formation and concentration continue, Fowler et al. predict nearly half of the world’s forests will be exposed to phytotoxic levels of O3 by the year 2100 (1999).

More study is needed to more fully establish the links between biotic infections and climatic factors and O3 uptake and defense in conifers.

On boreal forests and possible mitigation

A team of Finnish researchers tested two native European species of aspen trees and eight hybrid clones and their response to ozone exposure. These trees are typically found in the boreal and temperate seasonal forests or northern and central Europe and central Russia. Free-oxygen generated the ozone compound used in testing. Fumigation of trees in the field ran 24 hours a day except in very high or very low winds, in heavy rain, or if the ambient ozone concentration was greater than 10 ppb. Two hybrid aspen species experienced decreased leaf-level net photosynthesis by 40% early in the growing season as a result of elevated ozone exposure. The hybrid clones were the most ozone-tolerant. Researchers suggest cross-breeding ozone sensitive species with tolerant clones as a way to mitigate O3 damage in the boreal and temperate forest tree population. Researchers found no changes in total biomass as a result of ozone exposure in either native or hybrid aspens (Häikiö et al., 2007).

On pollinators

A team of researchers at the University of Virginia used the behaviors of bees combined with historical ozone data to model how air pollution could be impacting floral pollination. Computer simulations run with this data modeled a sharp decrease in the distance floral scents travel compared to the 1840s (Mitchell, 2008). Under historic conditions, researchers found only 20% of scents would have been altered by air pollution and could have been picked up by pollinators for kilometers around (Fuentes et al., 2008). Modeling the conditions of many of today’s urban centers where air pollution consisting of hydroxyl radicals, nitrate radicals, and ozone can reach concentrations of 120 ppb by volume, Fuentes et al. report a meager 25% of floral scents would survive past 300 meters downwind. This is likely impacting bees’ ability to find and pollinate flowers as they use both sight and scent during daylight hours to pollinate. Moths, however, are expected to be most negatively affected as they only use scent because they are nighttime pollinators (Fuentes et al., 2008).

Connection to climate change

A survey of available literature indicates there is a connection between carbon monoxide and climate change (Ramanathan and Feng, 2009). While not a greenhouse gas, CO is considered a chemical precursor and a secondary climate forcing agent (AMS, 2016). In the late 70s, Weng et al. were the first to discover the roles methane and nitrous oxide played as greenhouse gases (1976). Carbon monoxide chemically reacts in the atmosphere with methane and other trace gases (AMS, 2016). This increases lower tropospheric ozone which is a primary warming agent of the lower atmosphere. Air pollution like CO and nitrous oxides are important contributors to global warming due to these atmospheric interactions (Fishman et al., 1980). Sources of carbon monoxide include biomass and fossil fuel combustion (AMS, 2016).

On animals

Carbon monoxide is known to bind to hemoglobin cells in blood. Hemoglobin is responsible for carrying oxygen in the body via red blood cells. If hemoglobin is carrying carbon monoxide, it can’t carry oxygen through the the bloodstream which can stress the heart and cause headaches (Lapinski et al. 360). Animals can be affected by air pollution in the same way humans can. Animals can suffer from such symptoms as lung and eye irritation, cancer, bronchitis and , at high levels, even death (Lapinisk et al. 361).  

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