180,000 Vehicles, One Verdict: Diesel Still Dirtier Than You Think
A summer-long remote-sensing campaign in Paris measured exhaust from more than 180,000 vehicles and uncovered stark gaps between lab promises and street reality. Petrol cars’ NOx fell steadily with each Euro standard, but diesel cars showed little improvement from Euro 2–5 and only modest gains at Euro 6—still averaging 4.8× the NOx of Euro 6 petrol and roughly 6× the lab limit. Heat mattered: Euro 5–6 diesel NOx rose 20–30% when ambient temperatures topped 30°C. Euro 5–6 diesels, though classed as Crit’Air 2, accounted for an estimated 63% of passenger-car NOx at the sites. Euro VI buses cut NOx sharply vs. Euro V, and early Euro 6d-TEMP diesels lowered NOx about 70% vs. earlier Euro 6 but still trailed petrol.
Paris set out to make real-world exhaust transparent—and the numbers are sobering. Using overhead laser remote sensing at three inner-city corridors, researchers captured 965 hours of traffic and more than 180,000 usable records across cars, vans, buses, and two-wheelers. The verdict for nitrogen oxides is clear: petrol cars get cleaner with each Euro step; diesels largely did not until Euro 6—and even then, the gap remains yawning.
On a distance basis, Euro 6 diesel passenger cars in Paris averaged about 0.48 g/km NOx, roughly six times the lab limit and 4.8 times higher than Euro 6 petrol cars. Compared with the oldest petrol cohort still on the road, Euro 6 diesels were only 18% lower. Conditions shifted the picture, too. When temperatures exceeded 30°C, Euro 5 and Euro 6 diesel NOx climbed roughly 20–30% compared with 20–30°C operation, suggesting calibration choices that trim aftertreatment effectiveness in heat outside the type-approval window.
Because they are numerous and high-emitting, Euro 5 and Euro 6 diesels dominated the on-road NOx budget: about 63% of passenger-car NOx in the measurements, despite qualifying as Crit’Air 2 vehicles allowed into the low-emission zone until 2024. In contrast, particulate emissions told a different story. Once diesel particulate filters arrived with Euro 5, diesel PM fell to petrol-like levels, and diesel CO remained low relative to petrol across standards. Petrol CO dropped steeply with modern standards but rises with age, hinting at catalyst deterioration in older Euro 4 vehicles that will still circulate for years without tighter policy.
Two-wheelers deserve attention. Mopeds and motorcycles (L-category) improved under newer standards, but their fuel-specific CO and NOx remained well above petrol cars—often resembling Euro 2–3 petrol performance even when certified to Euro 4. Yet Euro 4 L-category vehicles qualify for Crit’Air 1 and can circulate freely until 2030, raising the risk that their share of urban emissions grows as other segments clean up.
Buses showed how standards can work. Real-world NOx from Euro VI city buses was about 59% lower than Euro V, and Euro VI coaches about 84% lower—so effective that average Euro VI transit-bus NOx per fuel burned under these conditions beat Euro 6 diesel cars. Paris’s ongoing shift to electric and biomethane-fueled fleets should compound these gains.
Finally, early Euro 6d-TEMP diesel cars—those subjected to on-road RDE limits—cut average NOx roughly 70% versus earlier Euro 6 diesels. But their NOx still exceeded Euro 6 petrol levels, and one observed diesel family hinted at rising NOx under higher engine loads that stretch beyond typical RDE dynamics. With vehicles measured at under one year old, durability remains an open question.
Bottom line: the city’s remote-sensing campaign confirms that on Paris streets, diesel NOx remains the stubborn pollutant to beat—especially in hot weather—and that policy timelines for LEZ access and Crit’Air labels should reflect real-world, not laboratory, performance.