The start of spring in Los Angeles was quite different this year. Following California Governor Gavin Newsom’s March 19 “stay-at-home” order restricting travel across the state, the city’s famous 12-lane freeways emptied. And in their place, LA residents started seeing things that this urban area of 13 million people had not before: clear views of the snow-capped San Gabriel Mountains to the northeast and Santa Catalina Island off the southwestern coast.
But it was not only Los Angeles; clear skies and incredible views were reported in Bangkok, Bogota, Beijing, Nairobi, and Paris. Some of the most dramatic reports in early April were from Delhi, where the omnipresent brownish-gray haze was swept away, replaced by blue skies in the aftermath of a nationwide lockdown. Delhi city-dwellers reported magnificent views of the stars at night, and residents of cities in the north of India reported seeing the Himalayas for the first time.
Dirty skies and reduced visibility are a byproduct of our industrial civilization and were much worse in some places in the past. In the nineteenth century, the pollution from burning coal was so intense between London and Manchester that it blackened trees and even caused a species of moth to adapt by turning black. The effects on human health were just as bleak; as late as 1952 an episode of “killer fog” laced with pollutants from burning coal killed at least 4,000 residents of London.
Thankfully, those days are largely over. In many industrialized nations, levels of the most dangerous air pollutants have declined over the past few decades due to scrubbers in power plants and catalytic converters in automobiles, a switch from coal to gas, and other measures. But filthy air with persistent smog and haze are still a regular feature of cities, from England to California to India.
The return of commerce in the wake of the coronavirus begs the question: can we have clear skies in cities while still maintaining a thriving economy?
The Danger of PM2.5
We can start by looking at what it is that obscures our skies. In terms of visible air pollution, the main culprit is particulate matter, a mixture of solid particles and liquid droplets that can represent a range of chemical compositions.
This stuff that makes the air look dirty is, unsurprisingly, not good to breathe. Particulate matter can lead to decreased lung function, aggravate asthma, and cause premature death in people with heart and lung disease. Additionally, some airborne particles can serve as carriers for other toxic chemicals.
And while particulate matter comes in a variety of sizes, health professionals have become increasingly concerned about particles 2.5 micrometers and smaller (PM2.5), much smaller than a human hair or grain of sand. “From a respiratory disease perspective, PM2.5 is far more dangerous than larger particles,” notes Akshima Ghate, principal at RMI India.
These smaller particles can penetrate deeper into lungs and even enter the human bloodstream; one prominent study found that PM2.5 caused 4.1 million premature deaths around the world in 2016.
There are many different primary sources of PM2.5, and the composition varies greatly depending on where you are. The largest global sources identified in a recent study are industrial processes, fossil fuel-fired power plants, agriculture, and burning fuels in buildings. This would include cooking, and globally 3 billion people cook with solid fuels like wood or charcoal.
In US cities, the main sources are different. A 2019 study by the US Environmental Protection Agency (EPA) found varying compositions in individual urban areas, but vehicles made up a larger portion than shown in global numbers. Across different cities, industrial activities, motor vehicles, road dust, cooking, and fuel combustion combined to make up the majority of primary PM2.5.
Another significant form of particulate pollution in many areas is fire itself. In rural areas, burning crops is a significant primary source of PM2.5, and the wildfires that have ravaged the western United States and Australia in recent years have led to spikes in PM2.5 levels.
In a recent report, The American Lung Association (ALA) notes that wildfires are causing an increasing amount of people to experience unhealthy air due to short-term levels of particulate pollution. For the last two years, the organization moved Santa Barbara, California, from the list of the cities with the cleanest air to those with the worst short-term pollution, and for two days Los Angeles reported hazardous levels of particulate matter—both due to fires.
For ALA, the connection is clear. “In the western United States, climate change has made more likely the conditions of heat and drought that promote wildfire hazards,” the group’s State of the Air 2020 report notes.
SOx, NOx, and PM2.5
In addition to these primary sources, there are a number of other ways that PM2.5 can form in the atmosphere. Oxides of sulfur and nitrogen (called SOx and NOx), both of which are formed from burning fossil fuels, are dangerous pollutants in their own right. However, according to the EPA, sulfur dioxide (SO2) and NOx are also the “predominant” precursor gases in the formation of secondary PM2.5. This means that they react with other compounds in the atmosphere to make more particulate matter.
While the two share the property of making more PM2.5, they have important differences. SO2 in the atmosphere comes primarily from burning coal and oil, which contain sulfur. In the United States, two-thirds of the SO2 comes from fossil fuel-fired power plants, and this is a particular concern for areas that have a lot of coal-fired power plants or where coal is used in industrial processes like making steel.
Nitrogen dioxide (NO2) is the most commonly studied form of NOx, in part because nitric oxide (NO) quickly oxides to become NO2. Unlike SO2, the nitrogen in NO2 doesn’t have to come from fuel. Nitrogen is the most common element in the air we breathe and can be created in any high-temperature combustion process: in a power plant, in a jet engine, or in a car with an internal combustion engine.
In the United States, the majority is from “mobile” sources: cars, trucks, buses, and heavy equipment. And while coal plants and heavy industry are often located far from large cities, the NO2 from vehicle exhaust and from gas stoves and heaters is concentrated in cities.
Not only does NO2 concentrate in cities, but many cities also get a higher portion of their particulate matter from motor vehicles and their use. In Los Angeles, the EPA found that mobile sources, including cars, trucks, and heavy equipment, made up 22 percent of the PM2.5. Road dust was another 7 percent.
In India it is even more dramatic. A 2018 study by the Ministry of Heavy Industries and Public Enterprises in Delhi, India, found that vehicles were the source of 39 percent of its particulate matter; road dust added another 18 percent. It also found that 80 percent of the NO2 in that city of 13 million came from motor vehicles.
It’s important to note that while this article focuses on visible air pollution, what you can’t see can shorten your life, give you asthma, and harm your children—including before they are born. A good place to look is the EPA’s criteria pollutants list, which lists six major pollutants. SO2, NO2, and particulate matter make up half of the list. Of the other three, carbon monoxide and ozone are also produced by motor vehicle exhaust; and lead was a component of gasoline until it was banned in the United States for on-road vehicles in 1996.
What Can Be Done?
Not every source of PM2.5 is easily actionable, such as naturally occurring PM2.5 from dust or pollen. Additionally, steps to deal with the PM2.5 from agriculture and waste disposal are beyond the scope of this article. But there are many sources that we can act on.
Globally, an important step to reduce PM2.5 and its SO2 precursor is to stop burning coal to make electricity, to heat homes, and for other purposes. As explored in a previous article (“Zombie Coal,” April 2020), coal is not only a public health disaster, but coal-fired power plants are increasingly uncompetitive with wind and solar.
In the developing world, emissions of particulate matter can be eliminated by moving from traditional biomass such as wood or dung to more efficient cookstoves that use other fuels. The United Nations has identified access to clean and safe cooking fuel as an important step in Sustainable Development Goal 7, and this would address the estimated 12 percent of global PM2.5 that comes from the burning of wood and solid fuels for cooking.
In the affluent world, fossil fuels burned in buildings are a major source of precursor gases, with coal producing NO2 and SO2 and natural gas producing NO2. Moving heating and cooking to cleaner electric options would eliminate these precursors for PM2.5, particularly in cities.
By contrast, many of the solutions to reduce pollution from heavy industry are still in earlier phases of development, such as the hydrogen-based direct reduction steelmaking process that the Hybrit consortium is piloting in Sweden (see “Decarbonizing Steel and Cement,” November 2019). However, if scaled these could cut down on industrial emissions as well.
These solutions have the additional benefit of reducing CO2, and less warming means less increased risk of the sort of catastrophic fires which have been seen in recent years.
The Vehicle in the Room
In cities around the world—whether LA or Delhi—a main source of both PM2.5 and its precursor NO2 is internal combustion engine vehicles. Fortunately, there are a number of things that have been done and are being done to make what comes out of the tailpipes of automobiles cleaner.
It’s important to note that as bad as LA’s air pollution is now, it was even worse in the past. In LA and across the United States, the introduction of three-way catalytic converters has considerably cut down on emissions of NO2 from vehicles. Likewise, India introduced its Bharat stage emission standards for vehicles in 2000. The nation is currently progressing to Bharat Stage VI, which was rolled out in Delhi in 2018 and which limits PM2.5 and NOx emissions.
RMI India’s Akshima Ghate says that these standards have been important for limiting pollution in Delhi, and that now the problem is largely with older vehicles that are still on the road. “The real polluting vehicles in our fleet are less than 5 percent of the total stock,” notes Ghate. She estimates that it will take between seven and ten years to turn over the entire fleet to the new standards.
For both India and the United States, a more complete step to clean the air is to go electric. Electrifying cars, trucks, buses, and heavy equipment (hydrogen fuel cells are another option for heavy-duty vehicles) will reduce visible air pollution by cutting down on both NO2 and particulate pollution from gasoline and diesel engines. Additionally, less use of oil refineries to make fuel means less industrial pollution.
But while electrification can reduce the majority of the pollution from vehicles, there are limitations to this approach. First, the average automobile remains on the road for many years after it is first sold, so there is a substantial lag between increasing the portion of EVs in new vehicles sold and the portion of EVs on the road. And for nations like India, the EV market for electric automobiles is nascent; although it is far more well developed in the larger two-wheeler market.
Finally, EVs still produce dust from tire and brake wear, as well as raising dust from roads, all of which contribute to PM2.5 pollution.
Thinking Beyond the Car
There is a way to more fully eliminate automobile-based pollution in cities: by getting people out of their cars and getting cars out of cities. Even before the pandemic, cities such as Barcelona, London, New York, Oslo, Paris, and Vancouver (Canada) were experimenting with giving more space in the city center to outdoor activities and non-automotive forms of transit, such as walking and biking. These cities followed the lead of Amsterdam and Copenhagen, where bicycles have been a leading form of transit for decades.
In some ways, the changes in society driven by responses to COVID-19 have accelerated this process. The shutdown of workplaces deemed “non-essential” in many nations means more white-collar workers are working from home, eliminating commutes entirely. And as cities start to re-open and seek ways for their citizens to spend time outside and for restaurants to function under social distancing rules, one popular tactic has been to reclaim areas previously dedicated to cars for such activities.
Ultimately, the most effective way to reduce air pollution could be a combination of multiple tactics. “Work from home, staggering of shifts, better public transportation, and the switch to EVs give you that hope and promise of keeping some of these gains,” states Akshima Ghate.
A Future of Clear Skies
While streets are being closed now, many of the solutions to the pollution from motor vehicles will take years to decades to scale. This is true of both the zoning and land-use changes to make it easier to get around cities without a car, and also of vehicle electrification. Even the most aggressive forecasts have EVs as a minority of autos on the road globally in 2030; however, bans on internal combustion engine cars such as are underway in Norway (2025), the Netherlands (2030), and Iceland (2030) could accelerate progress.
In the interim, as commerce, industry, and traffic return we are already seeing skies become polluted again. For Delhi, the sparkling blue skies in April were the result not only of the lockdown, but also of heavy rains that washed away lingering particulate matter. “There were some days of really good air, but still we are getting satisfactory air quality,” notes Akshima Ghate.
For Los Angeles, as the state’s stay-at-home order relaxes, the freeways are filling up again and the mountains are disappearing into the haze, as California braces for another wildfire season.
But with more people experiencing what it is like to have cities temporarily free of pollution, and more people are using this time to experience being outside without their cars, we can set a vision of a cleaner future. As we replace coal and gas-fired power plants with wind, solar, and batteries, electrify buildings and transportation, and redesign our cities for people and not cars, we can clean up most of the filthy air that obscures the sky and shortens our lives.
We can have a civilization without widespread use of fossil fuels and we can have clear skies in cities again, but it is going to take both vision and work to make this a reality.