Oil production from oil sands has raised numerous environmental concerns, but the contribution of oil sand exploration to secondary organic aerosol (SOA) formation is significantly high. SOA is an important component of atmospheric particulate matter that adversely affects air quality. The evaporation and atmospheric oxidation of low-volatility organic vapors (VOCs) from mined oil sands are responsible for most of the observed SOA mass. The findings suggest that tar sands gas pollution is 81 percent higher than average conventional oil [1]. The scope of this paper is to study the sources of secondary aerosols in the oil sands industry and their formation mechanisms. In addition, a brief review of SOA control technologies and solutions is also covered. The concluding notes highlight the future recommendation on how to control the aerosol emission and enhance removal efficiency, and the deficient areas to obtain a more comprehensive understanding of the SOA emission resources.

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INTRODUCTION:

It is estimated that 96 percent of Canada’s oil reserves, out of total 170 billion barrels, is located in the oil sands, which amount to 164 billion barrels. Oil sands are a mixture of mineral solids (83%–88%), water (3%–5%) and a heavy, thick fluid called bitumen (8%-14%) [2]. These oil sands reserves cover an accumulated area of almost 142,200 km²in northern Alberta. Oil is being produced through these reserves for more than five decades. Two main methods employed for oil sands recovery are drilling (in situ) and mining. The recorded peak oil sands production in 2014 was 2.3 million barrels per day [3]. Surface mining is used for the oils sands that are closer to the earth surface, within 75 meters. Oil sands are extracted, crushed and mixed with water to form a mixture for ease of transportation. Then at the processing facility, hot water is used to recover the bitumen. Various additives such as NaOH and Na₃C₆H₅O₇ (sodium citrate) and diluents (naphtha or paraffinic solvents) are used to maximize the separation rate [4]. In situ techniques are used to recover bitumen that is deeper than 75 meters below the surface. Steam Assisted Gravity Drainage (SAGD) and Cyclic Steam Stimulation (CSS) are the most applied in situ technologies. Once the bitumen has extracted out of the sand, then it is upgraded into the synthetic crude oil via four main processes. First, long hydrocarbon molecules are broken down with the aid of thermal conversion or coking. Then catalytic conversion further breaks down the oil molecules into smaller hydrocarbons that are then distilled to separate different component in the bitumen. In the end, hydro-treating adds hydrogen to stabilize unsaturated molecules and to remove Sulfur and Nitrogen impurities [5]. These mining and in situ operations utilize large amounts of water during the extraction and upgrading of the crude bitumen. An estimated 2.4 to 4 barrels of water is used for each barrel of oil in the mining operation. Although the SAGD process recycles some of its used water, however still a large volume of water goes to the tailing ponds. These ponds not only create water pollution, in fact, they are also considered a source of air pollution [6].

Several pollutants are produced during the above-mentioned processes. However, the focus of this report is to mainly discuss the secondary organic aerosols (SOA) produced by the oil sands industry.

ORGANIC AEROSOLS:

Aerosols are microscopic solid and/or liquid particles scattered in the atmosphere. About 20-90% of these fine particle are organic, particularly in the lower troposphere. Ambient organic aerosols (OA) are found in two forms: Primary and Secondary. Primary organic aerosols (POA) are directly introduced into the air by the fossil fuel combustion and mechanical processes. Whereas the secondary organic aerosols (SOA) are formed in the atmosphere when volatile organic compounds (VOCs) emitted directly by the oil sands are exposed to sunlight and react with oxygen and other compounds. The POA is water-insoluble or hydrophobic while the SOA compounds are more water soluble or hydrophilic [9].

SECONDARY ORGANIC AEROSOLS (SOA):

SOAs are harmful pollutants. As per the findings of the World Health Organization, particulate matter is causing the respiratory problems such as asthma, cardiovascular diseases and lung cancer [7]. SOAs become a significant component of pollution known as “particulate matter,” or PM. A new study reports that Alberta’s oil sands generate 45 to 84 tons of SOAs a day [8].

SOURCES OF SOA FORMATION:

There are multiple primary sources for the particulate matters (PM) emissions into the environment. Removal of vegetation cover from topsoil, crushing and grinding of rocks, soil dust raised by the transportation trucks on unpaved roads and petcoke dust (a byproduct of bitumen upgrading process) are major sources of particulate emissions. Additionally, SOx, NOx, NH₃ and volatile organic compounds (VOC) are also generated during the oil sands

_{3. 1.} SO_x

SO_xare inorganic air pollutants having sulfur with varying oxygen amounts. The most common SO_x pollutant is sulfur dioxide (SO₂) that commonly produces by the combustion of sulfur-containing materials. Hydrogen sulfide (H₂S) emitted during oil sands extraction, processing of bitumen and heavy oil upgrading is usually oxidized to SO₂ during high-temperature combustion. In 2014, 67 670 metric tons of SOx was produced by the oil sands industry, accounting for 12% of the total SOx emissions in Alberta.

_{3. 2.} NO_x

NOx are inorganic air pollutants having nitrogen with varying oxygen amounts. In the presence of sunlight and VOCs, NOx can take part in the formation of ground-level ozone (O₃) that is harmful to human health. The two most common pollutants under this class are Nitrogen dioxide (NO₂) and nitric oxide (NO). Heavy-duty equipment such as trucks in the sands region form higher amounts of NOx.

3. 3. NH₃

NH₃ is used for separating and recovering bitumen from oil sands. Ammonia (NH₃) emissions can result from various sources such as the hydrotreating process (in which N is removed as NH₃), and volatilization from tailing ponds that are contaminated with NH₃. In Alberta, 1065 metric tons of ammonia (NH₃) was released to the air from oil sands operations in 2014.

3. 4. VOCs

VOCs are organic chemicals that have high volatility and evaporate into the atmosphere as a gas at ambient temperatures. VOCs discharges from oil sands can be categorized into two classes: fugitive emissions from the oil sands and its products used in bitumen operations including emissions linked with the mining processes. The three primary facets associated with the VOCs release in the oils sands region ae bitumen extraction techniques, tailing treatment processes, and tailing properties. Multiple known factors contribute to the VOCs discharge at the oil sands industry, some of these are described below.

3.4.1.1. SVOC & IVOCs

Semi-volatile organic compounds are contaminants with saturation concentration in the range of 10⁻¹ to10² μg.m⁻³. Examples of semi-volatiles compounds include hydrocarbons, aldehydes, and amines [10]. Intermediate volatile organic compounds are contaminants having a saturation concentration in the range of 10³ to10⁶ μg.m⁻³. The average IVOC contribution to SOA formation is estimated to be 7 % [11]. It was determined that SVOCs and IVOCs were the predominant elements of SOA formation during the 4.9 million barrels±10% Deepwater Horizon (DWH) oil spill in April 2010 at the Gulf of Mexico [8].

3.4.1.2. Diluents in the Tailing Ponds

Diluents are organic solvents, which are used to decrease the viscosity of the bitumen for the purposes of purification and easier transportation. Though the major part of used diluents is recovered however a small fraction (<1 % mass) is swept away in the tailing ponds [11]. Oil sand tailing ponds are engineered dams to contain by-products and leftovers of the mining and bitumen extraction resources. Compounds present in the tailing ponds can undergo further chemical reactions and can emit some pollutants in the air. For example, the BTEX compounds (benzene, toluene, ethylbenzene, and xylenes) that are present in naphtha diluent are considered relatively volatile and would contribute to VOC emissions. Freshly discharged tailings can have a temperature up to 60 °C and therefore, higher VOC emissions are expected to occur at a tailings outfall. These emissions can increase during the spring or summer seasons when ambient temperatures are greater [12].

3.4.1.3. Aerosolization

Aerosolization at the surface of tailing ponds may result in VOCs emissions. Physical processes such as splashing at the tailing discharge and environmental factors such as wind-generated waves eject tiny droplets into the air that evaporate [12].

3.4.1.4. Microbial Biodegradation

The microbial biodegradation of tailing ponds contaminants such as naphtha (containing n-alkanes and BTEX) will lead to the production of VOCs. Thermal insulation of tailing ponds’ surface and heat added due to freshly entering tailings can cause the VOCs emissions into the atmosphere round the year [8].

3.4.1.5. Bitumen

Bitumen has higher molecular weight and leftover bitumen in the tailing ponds is not considered an initial source of VOCs emissions. However, with the passage of time, it keeps accumulating at the bottom surface of the tailing ponds. In the absence of air, anaerobic microbes might utilize Bitumen as a source of Carbon and produce biogenic gasses (VOCs) [8].

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4. Formation Mechanisms of Secondary Organic Aerosols

The formation of SOA is a complex process following multiple micro physicochemical reactions. Oxidation plays an important role in SOAs formation and the oxidation agents present in the lower atmosphere such as O₃ and its photodissociated by-products (Hydroxyl radical-OH and hydrogen peroxide-H₂O₂) determine the SOAs concentration. The three main steps in the formation of SOA are Nucleation, Condensation, and Coagulation.

4.1. Nucleation

Nucleation is gas-to-particle conversion, in which low volatile gas phase species such as VOCs, SO_x, NH₃, and NO_x are converted to aerosol phases. Nucleation is the source of new particles in the atmosphere. Sulfuric acid and nitric acid are two most common and well-studied nucleating species due to their lower boiling points. As a result, Nanoparticles called as critical nuclei are formed [13].

Figure 1. Birth and Growth of an Aerosol (Aurelia et al., 2016)

4.2. Condensation

In the atmosphere, condensation is one of the primary mechanisms of pollutant formation and growth. The fresh vapors condense on the surface of the nucleated aerosols and thus make it grow in size [14].

4.3. Coagulation

When two particles collide in a process called coagulation, they also merge to form a bigger particle by decreasing their number concentration. Coagulation is a common particle growth phenomenon for Nano-particles. It causes the bigger particles to become even bigger while making the freshly nucleated smaller one disappear completely [14].

5. SOLUTIONS FOR SOA POLLUTION CONTROL

5.1.Bio-filtration

Biological air contaminant removal is a well-established air pollution control (APC) technology in the European Countries for eliminating the VOCs, toxic discharges and odors from both civic and industrial sources with a control efficiency more than 90%. Polluted off-gas is vented from the emitting source through the biologically active filter. Given sufficient residence time, microorganisms at the biofilm metabolize the pollutants. End products from the complete biodegradation are CO₂, water, and microbial biomass [19].

5.2.Flue Gas Recirculation

Flue gas recirculation recycles flue gasses from steam generators commonly used in the steam-assisted gravity drainage (SAGD) process. The recirculation decreases the emission of nitrogen oxides into the air from the combustion process of fuel. Cenovus Energy Inc. is successfully using this technology in the Christina Lake oil sands. Additionally, the reintroduction of the exhaust also cools the burner flame. Results show a drop of emission from 40 ppm (parts per million) to 20 ppm that is a 50 percent reduction of discharged pollutants [22].

5.3.Photocatalytic Degradation of Air Pollutants

Photocatalytic degradation of organic pollutants uses a catalyst such as TiO₂ for the VOCs and NOx removal from the air. Basically, the titanium dioxide, in the presence of ultraviolet light, oxygen, and water, will produce hydroxyl radicals that will react with NO_x compounds, producing nitrates ( $N O_{3}$

), which are much less harmful to the environment and, in fact, can be used for agriculture [17]. On the other hand, reacting with the Organic compounds, CO₂ and H₂O will be the product. Therefore this method can be utilized to economically control VOCs pollution at the oil sands industry. The titanium dioxide can be applied to many different surfaces, such as building concrete, concrete tiles and any type of pavement.

Figure 2. Photocatalytic treatment of VOC (Aymen et al., 2015)

5.4.Sulfide Scavengers

During oil and gas operations, hydrogen sulfide is found in the formation gases, dissolved water, hydrocarbons, or sometimes as liquid Sulphur. Thermal degradation of organic materials and sulfate-reducing bacteria (SRB) can create hydrogen sulfide along with other gases. The term “sulfide scavenger” is applied for any chemical that reacts with sulfide species converting them to a more inert form. Currently, two types of widely used scavengers in the drilling industry are: 1) zinc – containing chemicals; and (2) iron oxide, Fe₃O₄.

Figure 3. Metal Oxide H₂S Scavenging Process (Gatekeeper 2014)

5.5.Particulate Emission Controls

Various equipment such as Electrostatic Precipitator is used to lower particulate emissions with 99% particle removal efficiency from the flue gasses. Particulate emission control is applied to the upstream of the boilers which are the major contributors of VOC’s SO_xand NO_x emissions [20].

6. CURRENT CHALLENGES

Cutting the SOA emissions may not be an easy step due to the fact that chemistry happening in the atmosphere cannot be stopped. However, experts believe that the only applicable approach is to reduce oil sands emissions that ultimately react to form SOAs, but even that is tricky because many of them are thought to originate from open-pit mines, where they would be difficult to contain.

7. CONCLUSION

The tar sands are Canada’s fastest growing source of gaseous pollution. Open-pit mining and processing, in situ extraction and processing of bitumen, result in significant secondary aerosol formation. If Alberta was a country, its per person emissions would be the second highest in the world after Qatar [1]. Formation of SOAs takes place at far distances from the source-Oil Sands and therefore creates health problems to the nearby civic population. The composition of the tailings ponds also influences the biodegradation rates of organic pollutants. Completing cutting the VOC emissions may not be possible, however with the aid of modern technologies and a better understanding of the pollution chemistry at Oil sands, particulate emission and formation of secondary organic aerosols can be significantly controlled.

8. RECOMMENDATIONS FOR FUTURE SCOPE

More experimental work and understanding is needed to know about emissions sources. As it is clearly more economical and convenient to control the pollution at their initial sources, such as mining regions and bitumen upgrading facilities [7]. It is also recommended to investigate the alternative of tailing ponds as tailing ponds are considered the largest source of VOCs discharge. The focus should also be put on the reduction of uncertainties in calculating the PM discharge factors associated with the oil sands industry.

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