High-intensity emitters disproportionately pollute the air we breathe. Understanding where sources contribute the most potent emissions can help drive smarter clean air solutions.

Cutting the most damaging emissions from the air can be a bit like picking which foods to limit in your diet. You know the concept—fruits, vegetables, whole grains and lean proteins contribute far less to obesity than chocolate cake, cheesy pizza or greasy burgers. Healthy eating means paying attention not only to how much we consume but also the composition of each item.

The same can be true for controlling emissions of harmful pollutants like nitrogen oxide (NO), nitrogen dioxide (NO2) and small particles. Some “high-intensity” sources—like ships, diesel generators and heavy-duty trucks—produce more potent pollution than new, gasoline-fueled passenger vehicles.  In addition, conditions like stop-and-go traffic, larger cargo loads, and driving up hill can increase emission intensity, compared to freely flowing, lighter-duty traffic. Pollution varies from block to block and city to city, so understanding where sources contribute the most potent emissions can help us tailor more effective, local solutions. Our recent paper maps London’s air pollution and hotspots of emission intensity on an unprecedented street-by-street scale.

How to spot high-intensity emissions

In London, our teams used Carbon Dioxide (CO2), a key indicator of combustion, to determine the intensity of NO and NO2 pollution (NOx, in combination). Taking on-road air pollution measurements every second using mobile instruments, we identified local peaks in CO2, signaling recent emissions.  Then we calculated the emission intensity for these events as the ratio of NOx to CO2 concentrations.

Why emission intensity matters

Our measurements coincided with the implementation of Central London’s Ultra-Low Emissions Zone (ULEZ), where highly-polluting vehicles must pay a fee to enter the city center. This policy led to a cleaner vehicle fleet in and around the ULEZ and 35% lower total NOx emissions in the first year, even as overall traffic volume stayed about the same, by effectively reducing the emission intensity of individual vehicles. In fact, the ULEZ has been so successful that the Greater London Authority expanded it to an even larger area.

Emission intensity mapped in Central London. For more information on the image or to read the article, visit the journal Atmospheric Environment.

While the Central London ULEZ and its recent expansion are effective, air quality remains poor throughout London, and hot spots remain. By measuring emission intensity, we understand more about the overall causes of pollution than if we had relied solely on total concentration measurements. By digging deeper, we can show where higher-intensity sources, like heavy-duty diesel, are having a disproportionate impact on air quality. For example, we saw higher-intensity pollution along the Thames river near shipping piers, heavy construction sites and poorly-timed lights that caused traffic jams.

Crafting smart policies to combat air pollution

Equipped with local, street-scale emission intensity data, in addition to more typical total pollution measurements, policymakers in London and beyond can craft tailored solutions to cut air pollution and improve health. Some changes are easy, actionable and don’t require legislation—like fixing poorly-timed traffic lights or enacting anti-idling rules at passenger bus terminals. Other fixes—like limiting the number of warehouses that can be sited in one area to reduce truck traffic, staggering the timing and location of construction projects in order to reduce emissions from heavy equipment, electrifying buses or reducing the number of used, dirty vehicles in operation—would require more political will.

While we need to reduce all combustion-related emissions to achieve air quality and climate goals, using new methods to identify emissions intensity allows leaders to see where the dirtiest sources are, so they can focus initial efforts where tangible impacts are possible.

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Lower-cost air quality sensors can be a game changer for cities looking to understand and improve air quality at the neighborhood level. However, issues with accuracy have been a key barrier to their adoption. Our new paper shows how users can make the most of their data by evaluating sensor performance on a continuous basis.

Collocating sensors to track performance

As part of the Breathe London consortium, we installed 100 sensor devices across the city  to measure key pollutants including nitrogen dioxide (NO2) and particulate matter for more than two years. Lower-cost sensors like the ones we installed are more sensitive than reference-grade instruments to environmental factors like temperature, relative humidity, or even levels of other pollutants. That can make their measurements less reliable in some environments, or even in certain seasons of the year.

To make sure our data was both accurate and useful, the Breathe London consortium developed rigorous quality assurance procedures. For our NO2 dataset, the procedures included multiple methods to calibrate the sensors, as well as applying an algorithm to correct for sensitivity to ozone, which the sensor can mistake for NO2.

While most of our sensors were collecting measurements at new locations across Greater London, we also installed two “test” sensors alongside London reference-grade monitors for most of the project. By tracking when data from these “test” sensors deviated from the more expensive reference instruments, we had an indication of how sensors across our network were performing at different times.

In the left panel, the “test” sensor measurements show a large deviation from the collocated reference monitor (right), indicating a period when the sensor was not performing well.

This approach provided a reality check for our pollution data. If the sensor network reported high NO2 values but the “test” sensors were completely off track from the reference at that time, we could infer that the network result may have been affected by poor sensor performance and adjust accordingly. This kind of ongoing sensor evaluation is important. Without it, users could mistake erroneous sensor data as evidence of major pollution events or local hotspots.

Why performance matters

Our NO2 sensors performed well most of the time, producing data that revealed a variety of actionable insights, including:

• Times of day and days of week with the highest pollution levels
• Regional pollution episodes (for example, a multi-day period with high pollution caused by weather conditions)
• Hotspot detection
• Impacts of sources on pollution patterns at different locations
• Long-term trends (for example, seasonal changes or year-over-year improvements)

Improving our understanding of air pollution in cities around the world

While the uncertainties associated with lower-cost sensors may make them unsuitable for some applications, our project demonstrates a way to generate actionable insights from sensors. The Breathe London network’s NO2 data shows that with rigorous quality assurance and ongoing evaluation of sensor performance, cities can utilize lower-cost sensors to better understand local air pollution. That can allow more communities to take advantage of this relatively new technology, even if they do not have the resources to purchase a network of more costly  reference-grade monitors.

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Asthma changes the physical, emotional and academic trajectory of a child’s life. More than 5 million children in the United States have asthma, and every year there are over 750,000 emergency room visits and over 74,000 hospitalizations for asthma among children. Asthma is the leading cause of missed school days each year, and it has been linked to diminished school performance. Although ambient air pollution exposure has long been associated with the worsening of asthma symptoms, mounting evidence indicates that it also leads to the development of asthma among children. 

A recent study found that annually nearly 2 million children worldwide develop asthma due to exposure to nitrogen dioxide (NO2), a traffic-related air pollutant. Transportation is a key driver of this pollution. Freight trucks and buses make up less than 10% of the vehicles on U.S. roads, but they are responsible for half of the transportation sector’s nitrogen oxide emissions. In some urban areas, 1 in 5 new childhood asthma cases are due to exposure to nitrogen dioxide; in particular neighborhoods, this risk can be twice as high. 

How NO2 causes asthma

Studies exploring how NO2 affects the lungs indicate that repeated or long term exposure results in activation of biological pathways that contribute to the development of asthma: secretion of inflammatory cytokines, altered cellular structure, oxidative stress, allergic sensitization, increased mucus formation, airway remodeling and airway hyperresponsiveness. Studies of NO2 exposure to human bronchial epithelial cells find an increase in pro-inflammatory mediators and inflammation involved in the pathology of asthma.

A growing body of evidence describes the impact of NO2 on new cases of childhood asthma. It shows consistent and reproducible effects across different cities and populations in North America. Below are a few:

• In studies of Latino and African American children across Chicago, Bronx, Houston, San Francisco and Puerto Rico, a higher average NO2 exposure during the first year of life was associated with higher odds of being diagnosed with asthma. This was also seen in another study of children in East Boston, Massachusetts.
• Among 1.2 million children in Quebec, scientists found that higher childhood exposures to NO2 levels at their residential address were linked to increased risk of asthma development.
• A recent study of 4,140 elementary school children (with no history of asthma) in southern California provides particularly strong evidence.  Scientists found that a drop in nitrogen dioxide, over a period of air pollution decline, was associated with a reduction in asthma incidence. This finding was reinforced when using cutting edge causal methods, which found that “childhood asthma incidence rates would have been significantly higher had the observed reduction in ambient NO2 in southern California not occurred in the 1990s and early 2000s, and asthma incidence rates would have been significantly lower had NO2 been lower than what it was observed to be.”

These are just a few of the studies that have been done in North America. A systematic review and meta-analysis of 41 studies from around the world investigated the impact of different air pollutants on asthma incidence among children. Of these, 20 studies directly assessed the impact of NO2, and found that a small increase (4 µg/m3) in NO2 exposure led to a 5% increase in the risk of developing childhood asthma.

Across these studies, scientists took pains to ensure the findings were not due to other factors like age, sex, race-ethnicity, poverty or smoking in the household. 

We have an opportunity to protect our children’s health

We have an opportunity to protect our children by identifying communities overburdened by NO2 pollution and its sources. 

First, we need to end the blindspots on NO2. We must make the true cost of diesel clear through investments in transparency and accountability. New satellite data and community monitoring can help identify pollution hotspots. Robust funding for NO2 monitoring, analysis and enforcement will enhance existing data to support protective action. The US EPA’s $20 million in grant funding for increased air quality monitoring in communities overburdened by pollution is an important step in this direction.

Better emissions inventories–especially around areas of high truck traffic like ports and warehouses–are important to further illuminate sources and target solutions.

Finally, eliminating harmful pollution from diesel vehicles is crucial. Transitioning to electric school buses, cars and trucks is feasible. New research from EDF finds that by 2027, electric freight trucks and buses will be cheaper to purchase and operate than their combustion engine counterparts. EPA recently proposed stronger pollution standards for medium- and heavy-duty freight trucks and buses, but it needs to go much further in leveraging zero-emitting solutions. Bold clean energy investments by Congress would provide credits to people who purchase electric vehicles, support development of additional charging infrastructure and increase air quality monitoring to ensure that NO2 doesn’t linger in frontline communities.

We must act now to reduce NO2 pollution and prevent more asthma every year. The status quo is clearly unacceptable for our children.

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Originally Appeared Here