Planes, Trains and Automobiles: Dual Challenge of Greenhouse Gases and Air Pollutants

While there is no ambiguity that John Candy is a comedic legend, there remains confusion about what comes out of the smokestack or tailpipe of our planes, trains, automobiles, and other emitting sources. What are greenhouse gases, and how do they differ from air pollutants? What impacts do these have on society? Shedding more light on this issue will allow us to better understand the dual challenge of simultaneously reducing greenhouse gas emissions and air pollutants to avoid the worst impacts of climate change while also promoting public health.

Greenhouse Gas Emissions and Societal Impacts

Greenhouse gas (GHG) emissions from human activities are the fundamental driver of climate change. These gases stem from both energy and non-energy related activities and include carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O), and the fluorinated gases which are further categorized as hydrofluorocarbons (HFCs), perfluorocarbons (PFCs), sulfur hexafluoride (SF6), nitrogen trifluoride (NF3), and chlorofluorocarbons (CFCs, which are currently being phased out under the Montreal Protocol).

CO2 is the major driver of climate change and accounts for around three-fourths of total annual GHG emissions from human activities. Estimates suggest global annual emissions are 37 billion metric tonnes (gigatons) of CO2 and 50 gigatons of total GHGs. These emissions are dominated by energy-related activities from fossil fuel combustion, but also stem from land-use changes (e.g., agricultural activities related to food and fiber production), industrial process emissions (e.g., cement production), and waste (e.g., anaerobic digestion).

Earth’s distance from the sun and suitable atmospheric composition has created the conditions for a ‘Goldilocks Planet’ that has allowed life of all forms to flourish. However, GHG emissions from human activities have altered the chemical composition of our atmosphere and disrupted the natural greenhouse effect, meaning that the gaseous ‘blanket’ which envelopes our planet is getting thicker and trapping more heat near the Earth’s surface. This planetary energy imbalance leads to myriad negative consequences to the economy (e.g., climate disasters and associated economic damages), human society (e.g., human health impacts such as heat stress and shifting disease vectors), and the natural environment (e.g., greater wildfire risk). We must find ways to make this blanket less thick through both mitigation measures and carbon dioxide removal from the atmosphere.

As of February 2021, the atmospheric CO2 concentration as measured at Mauna Loa Observatory in Hawaii was 415 parts per million (ppm). In other words, for every random sampling of 1 million air molecules in our atmosphere globally, 415 of those are CO2 molecules. This stands in stark contrast to the atmospheric CO2 concentration during the pre-industrial era of approximately 280 ppm. This means that human activities have increased atmospheric CO2 by roughly 50 percent since the Industrial Revolution, threatening to push us into an entirely new geologic epoch dubbed the Anthropocene.

While the climate impacts of increased CO2 and other GHGs are global, the impacts on human society are not evenly distributed. Vulnerable populations in all countries are and will continue to be more exposed to adverse impacts from climate change, and suffer greater harm from them. This includes extreme heat and cold, flooding, food insecurity, sea-level rise, ocean acidification, and many other dimensions. The culmination of these and other climate-related stressors threaten to push more than 120 million additional people into poverty by 2030.

Air Pollutants and Societal Impacts

Air pollutants, also known as criteria air pollutants in the U.S., are also released during the combustion of fossil fuels at power plants, industrial facilities, and passenger vehicles. These air pollutants include ground-level ozone (O3), particulate matter (PM10 and PM2.5), carbon monoxide (CO), sulfur dioxide (SO2), and nitrogen dioxide (NO2).

Air pollutants can be detrimental to human health by exacerbating cardiorespiratory ailments and result in millions of deaths per year through increased mortality from events such as stroke, heart disease, chronic obstructive pulmonary disease, and lung cancer which are among the leading causes of death worldwide. Estimates suggest that household air pollution leads to 4.3 million deaths while ambient air pollution causes 3 million deaths globally each year. The World Health Organization has reported that 9 out of 10 people globally are exposed to unhealthy air quality with high levels of pollutants.

While most of these pollutants do not result in a planetary warming impact like that caused by GHG emissions, ozone is a GHG. When ozone remains in the upper atmosphere (stratosphere) it provides important protection from harmful ultraviolet light. However, when it is released in the lower atmosphere (troposphere) near ground level, it harms public health by contributing to smog formation. 

As with climate change, the burdens of air pollution fall disproportionately on vulnerable populations. Poor people and people of color have greater exposure to air pollutants, and are at higher risk of suffering adverse health effects from that exposure. Air pollution also harms children more than adults, with over 90 percent of children around the world exposed to high levels of air pollutants. Environmental health hazards such as air pollution are correlated with poverty, which exacerbates negative health effects through limited access to information and health care.

Role of Carbon Capture and Storage

The dual challenge of reducing both GHG emissions and air pollutants can be addressed in part through the use of carbon capture and storage (CCS) technologies on stationary GHG emissions sources, many of which are also major contributors to the release of air pollutants. (It should be noted that CCS will not be able to abate most transportation-related GHG emissions and air pollutants, particularly from passenger vehicles, which will require other solutions such as electrification.) Beyond its ability to capture CO2 from flue gas streams, CCS can also reduce air pollutants through two mechanisms:

  1. CCS retrofit systems require that the flue gas streams do not contain air pollutants such as particulate matter and SO2. Such pollutants can clog filter beds and poison the sorbent materials associated with the carbon capture process, so there is a need to eliminate these pollutants to enable effective system performance. It should be noted that other approaches such as cryogenic carbon capture would more directly remove these pollutants from system operations rather than indirectly requiring their removal prior to system performance.
  2. Major modifications to the original equipment of an emitting facility could subject the owner to more stringent pollution control measures that adhere to current regulatory requirements. For example, the U.S. Environmental Protection Agency regulates common air pollutants by establishing National Ambient Air Quality Standards through the Clean Air Act (CAA) to protect public health. In 1990, the CAA was amended to require technology-based standards for major sources of air pollutants defined as emitting (or having the potential to emit) 10 tons of a regulated air pollutant per year or 25 tons of a combination of hazardous air pollutants. Since the enactment of the CAA 50 years ago, total emissions of common air pollutants have decreased by 74 percent in aggregate across all types of air pollutants.

Given the compatibility of CCS to reduce both GHG emissions and air pollutants, the technology could also comport with environmental justice imperatives to help reduce the health burden of emissions sources in neighborhoods around the country and globe. Many frontline communities, particularly those with a range of socioeconomic inequities, have been unduly burdened by negative health impacts from nearby emissions sources, and there is an urgent need to ameliorate this situation. 

Any investments in and deployment of CCS technologies should thus be done through a lens of both mitigating climate change and promoting public health. Furthermore, there remains a need for greater public awareness, community dialogue, and information dissemination related to the intersection of CCS and environmental justice. Carbon Direct’s investments carefully analyze these issues, and we recognize that addressing both GHG emissions and air pollutants is central to a better future for all.

May planes, trains, automobiles, and energy infrastructure of the future be enabled in a manner that rises to meet this dual challenge. 


Tim Bushman, Science Analyst at Carbon Direct

Dr. Colin McCormick, Chief Innovation Officer at Carbon Direct