The Critical Role of Chromatography in Addressing Urban Air Pollution

Chromatographic techniques have been used judiciously to track air pollution and its sources more effectively. For example, a recent study used comprehensive two-dimensional gas chromatography coupled with mass spectrometry (GC×GC-MS) to identify and classify thousands of organic compounds in the air, including those emitted by diesel vehicles (1). Hydrophilic interaction liquid chromatography (HILIC) is another method being used, especially to measure organosulfates, which are significant components of atmospheric aerosols (2). Additionally, GC has proven helpful in monitoring volatile organic compounds (VOCs), which are emitted by various consumer products and contribute to urban air pollution (3,4).

A panorama of a city skyline blanketed by smog, illustrating the impact of air pollution on urban environments. Generated with AI. | Image Credit: © Plaifah - stock.adobe.com

These techniques are being used to solve a global issue that is negatively impacting many cities around the world. Thanks to rapid industrialization, vehicle emissions, and inadequate or nonexistent environmental regulations, cities such as Cairo, Beijing, and Delhi, among others, contain an abundance of harmful pollutants in their air that their citizens breathe in on a regular basis (5). These harmful pollutants include particulate matter (PM_2.5 and PM₁₀), nitrogen oxides (NO_x), sulfur dioxide (SO₂), and ground-level ozone (O₃) (5,6). When ingested by humans, pollutants can cause serious health problems, including various respiratory diseases (6). Air pollution is estimated to be the primary cause for 6.5 million deaths each year around the world (6).

In this article, we examine how chromatography can play a meaningful role in combatting urban air pollution, and how advancements in techniques and instrumentation have helped nations reverse the dangerous trend of air pollution around the world.

How Pollution Impacts Urban Environments

Urban air pollution also has far-reaching socio-economic implications. Poor air quality discourages outdoor activities, impacting daily life, and leads to increased healthcare costs. In cities where pollution levels exceed international safety guidelines, governments face mounting pressure to implement stricter emissions controls and promote sustainable transportation options. However, for many urban areas, especially in developing nations, the rapid pace of industrial growth and urbanization has outstripped efforts to maintain air quality (5). This imbalance between development and environmental protection has led to acute pollution crises in cities like Cairo, Egypt, Beijing, China, and Delhi, India.

Cairo faces a persistent air pollution problem driven by its reliance on diesel-powered vehicles, unregulated industrial emissions, and the seasonal burning of agricultural waste (7). Road transport is the biggest sources of air pollution in this North Africa city, accounting for 33% of PM_2.5 pollutants in the air (7). Without an efficient and environmentally sustainable mass transit system, coupled with a 50% subsidy on gasoline and diesel, Egypt created an incentive for its residents to shift away from dirty old, diesel-powered vehicles (7). Agricultural slash and burn is the second-leading cause of PM_2.5 air pollution (20%). Poor access to sustainable farming technologies results in Egyptian farmers burning rice straw following the Egyptian harvest seasons, creating black clouds over the Nile Delta and in Cairo (7). Despite governmental initiatives like a partial ban on rice straw burning and plans to convert vehicles to natural gas, progress has been slow, leaving millions vulnerable to the effects of toxic air (7).

The skyline of Cairo, Egypt. | Image Credit: © Andrei Starostin - stock.adobe.com

Meanwhile, Beijing, China, can point to some similar reasons why smog and air pollution was an issue. Over the past two decades, Beijing has seen a rapid increase of vehicles on the road by 335% (8). Beijing also still struggles with episodes of severe air pollution in the winter when coal-burning spikes for heating, which adds toxic pollutants in the air. Although the average annual PM_2.5 levels in Beijing have dropped from 101.56 micrograms per cubic meter of air in 2013 to 31.74 micrograms per cubic meter of air in 2022, the city continues to battle high levels of PM_2.5, especially from sources beyond its control, such as regional pollution and dust storms coming from Mongolia (8).

An aerial bird view of the the famous Forbidden City in Beijing, China. The vast area of the architectural complex is covered with evening mist. | Image Credit: © romas_ph - stock.adobe.com

To the south, China’s neighbor, India, also deals with the consequences of urban air pollution, and many of the causes for this crisis both nations deal with are similar. Delhi, the capital of India, suffers too from an exponentially growing population and a rapid increase in vehicle traffic and factory emissions (9). However, unlike Beijing, Delhi has not made as much progress in combatting air pollution, and it is now impacting public life. Delhi officials have had to close schools and reduce outdoor activities for students when the air quality readings reach certain levels (9). The Indian government has also mandated their workers to work remotely 50% of the time to reduce carbon emission levels caused by commuting (9). The air quality levels have also made the celebration of the Hindu festival, Diwali, an even more dangerous activity, because the timing of this annual celebration coincides with the seasonal crop burning in neighboring states (9). Despite judicial interventions and the government's introduction of air quality monitoring systems and temporary bans on certain polluting activities, Delhi continues to grapple with the immense challenge of bringing its air pollution crisis under control.

Silhouette of triumphal arch architectural style war memorial during hazy morning. Pollution level rises and causes smog in autumn season due stagnant winds. | Image Credit: © anjali04 - stock.adobe.com

The Role of Chromatography in Combatting Air Pollution

As urbanization and industrial activities continue to expand, air quality has become a major concern, with pollutants like volatile organic compounds (VOCs), particulate matter (PM), nitrogen oxides (NO_x), sulfur dioxide (SO₂), and polycyclic aromatic hydrocarbons (PAHs) posing significant health risks (5,6). Chromatography, when used in conjunction with sampling techniques, can quantify and identify harmful substances in the atmosphere, helping to safeguard human health while improving the environment.

Techniques in Air Pollution Monitoring

Various chromatographic techniques, such as gas chromatography (GC), ion chromatography (IC), and combinations like GC-mass spectrometry (GC–MS), are commonly used for air quality analysis.

GC is one of the most widely used methods for analyzing VOCs and semi-volatile organic compounds (SVOCs) in air samples (10). It separates compounds based on their volatility and interaction with the stationary phase, making it ideal for detecting trace levels of pollutants. By coupling GC with MS, it’s possible to identify compounds with high sensitivity and specificity, even in complex mixtures (10).

GC–MS has also been combined with pyrolysis (Py-GC–MS). Pyrolysis–gas chromatography–mass spectrometry (Py-GC-MS) is a chemical analysis technique that uses heat to break down samples into smaller components through pyrolysis, allowing for both qualitative and quantitative analysis (11).

“We heat it up to the point of charring it,” said Gobet Advincula, a Governor’s Chair Professor at the University of Tennessee Knoxville and Oak Ridge National Laboratory. “The beauty of the technique is that you can use the sample without much sample preparation.”

Unlike traditional GC–MS, Py-GC-MS eliminates the need for extraction and dilution during sample preparation, making it suitable for analyzing a wide range of polymers and composite materials (11). Advincula added that Py-GC–MS can be used for monitoring captured gases in the form of aerosol or smoke.

Why Py-GC–MS Is Preferred Over Spectroscopic Techniques

As a technique, Py-GC–MS offers many benefits that other techniques used for this purpose, such as Fourier transform-infrared (FT-IR) spectroscopy and Raman spectroscopy, do not have, which come in handy when analyzing microplastics (MP) and contaminants in the air.

Ana Torres-Agullo and Silvia Lacorte from the Department of Environmental Chemistry at the Institute of Environmental Assessment and Water Research (Barcelona, Spain) from the Spanish Research Council have done work in this space. They’ve used Py-GC–Orbitrap-MS to monitor microplastics and pollutants in air samples (12).

Although the researchers acknowledge the popularity of FT-IR and Raman spectroscopies for monitoring air pollution (12), both believe that they may not be the best option for this type of analysis.

“The primary limitation of FT-IR is its size restriction,” said Ana Torres-Agullo and Silvia Lacorte from the Department of Environmental Chemistry at the Institute of Environmental Assessment and Water Research (Barcelona, Spain) from the Spanish Research Council (12). “Currently, particles below 20 μm cannot be effectively analyzed using this technique, potentially introducing a significant bias to the results. In addition, often it is necessary to transfer the MP from the collection filter used for sampling to the gold-coated or silicon filter used for FT-IR analysis, which is generally done manually using tweezers, and this can be a source of error as this procedure is dependent on the skills and ability of the analyst.”

Meanwhile, Raman spectroscopy, Torres-Agullo and Lacorte note, do not have the size restrictions that FT-IR spectroscopy has, but the technique has other challenges that make it difficult for it to be adopted widespread in monitoring air pollution.

“Raman spectroscopy does not have size limitations, but it is a time-consuming and an expensive technique, allowing only the analysis of a small portion of the filter used for sampling,” Torres-Agullo and Lacorte said (12). “Pyr-GC–Orbitrap-MS allows us to overcome the size limitations inherent in spectroscopic techniques. With this technique, it is possible to analyze all the MP present in a sample regardless of their size. Furthermore, sample manipulation is minimized, thus the risk of losing particles or suffering external contamination is avoided.”

People walking in the streets of Delhi amidst smog | Image Credit: © saurav005 - stock.adobe.com

Solutions Being Implemented

Mitigating air pollution requires a collaborative effort between governmental bodies and the scientific community. A look at the recent policy implementations in Cairo, Beijing, and Delhi suggest that this collaboration is happening in real time.

Cairo, for example, has taken into consideration insights gained from geospatial technologies in analyzing regional air pollution to inform urban air quality management and environmental planning (13). For example, the Egyptian government led by President Abd el-Fattah el-Sisi enacted “Egypt Vision 2030,” which states an objective to decrease PM10 concentration in their air by 50% by 2030 (14). They also have invested in sustainable transportation endeavors, reducing vehicle emissions through supporting the piloting of electric buses and investing in public transportation (14,15). As of the start of 2023, the number of licensed vehicles in Cairo decreased approximately 9%, suggesting that its current efforts at curbing vehicle emissions are moving in an encouraging direction (16).

In Beijing, researchers have made progress in identifying the primary sources of organic aerosols contributing to Beijing's haze, revealing that solid-fuel combustion and multiphase chemical processes, particularly involving aromatic compounds, have played a key role in both winter and summer pollution (17). The conclusion was that effective mitigation requires addressing regional emissions beyond Beijing, especially from the Beijing–Tianjin–Hebei Plain and areas across the Xi'an–Shanghai–Beijing corridor (17).

Meanwhile, in Delhi, government officials have introduced BS-VI compliant engines to enhance engine efficiency in vehicles (18). In addition, they have implemented an odd-even vehicle rationing scheme as an emergency measure in case of unhealthy air quality levels (19). They have also begun using drones to monitor areas where PM_2.5 concentration levels are radically high (19). These policy measures are part of Delhi’s Winter Action Plan, which takes into consideration the air quality data they have access to (19).

A Persistent Problem

Py-GC–MS plays an important role in monitoring air pollution. The ability of chromatography in providing detailed chemical profiles of airborne pollutants, combined with advanced sampling techniques, allows scientists, policymakers, and industries with the data needed to monitor air quality, assess pollution sources, and implement effective solutions.

These solutions are beginning to unfold in three of the world’s largest cities. However, many other cities globally experience this problem and face a growing air quality crisis. Beijing, Cairo, and Delhi are still combatting this issue, but their neighbors are also encountering difficulties controlling their air pollution as well. For example, Ulaanbaatar, the capital city of Mongolia and directly north of China, deals with poor air quality because of its nomadic residents outside the city burning raw coal for heat (20). Much of Mongolia’s population lives in gers, which are traditional Mongolian yurts, and they do not have access to the central electricity grid, resulting in them using less environmentally sustainable methods for their daily needs. India’s neighbor, Bangladesh, also faces an air pollution crisis in their capital city of Dhaka, which has the worst traffic congestion of any major city around the world (21,22). Already vulnerable to climate change, Dhaka’s large population, coupled with its infrastructure issues and its industrial pollution and smog issues, has contributed to the current crisis, which is now a public health emergency (22).

Spectacular panoramic view of Ulaanbaatar, capital of Mongolia, from the Zaisan Memorial that honors the Soviet and Mongolian friendship. | Image Credit: © sphraner - stock.adobe.com

A very strong toxic and unhealthy dust in the morning in Dhaka Bangladesh, a city with a population of 20 million peoples. | Image Credit: © Friemann - stock.adobe.com

PM_2.5 particles, along with other pollutants, cause respiratory and cardiovascular diseases, and they are contributing to one of the biggest health-related problems the world faces. Chromatography, and the information that these analytical techniques provide, allow governments and scientists to study changes in their city’s air quality and enact measures based on the data.

References

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