Feature: Why emission intensity matters

Lauren Padilla, Data Scientist at the Environmental Defence Fund (EDF) explains why understanding which air pollution 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 uphill 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 NO_x 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.

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.