Unraveling the Science Behind Secondary Aerosol Formation

Imagine sunlight streaming over a city skyline. While the air appears calm, invisible chemical reactions are transforming vehicle exhaust into new, more dangerous pollutants—secondary aerosols. These tiny particles not only degrade air quality but also pose significant risks to human health. But how exactly does this "emission alchemy" occur?

A groundbreaking study has investigated the role of photochemical transformations in creating secondary aerosols. Conducted at the ILMARI Combustion Laboratory of the University of Eastern Finland, the research focused on two Euro 6-compliant passenger vehicles:

• A gasoline-powered SEAT Arona (Euro 6b) equipped with a three-way catalytic converter
• A diesel-powered SEAT Ateca (Euro 6d-temp) featuring an oxidation catalyst, diesel particulate filter (DPF), and selective catalytic reduction (SCR) system

Using a chassis dynamometer (Rototest VPA-RX3 2WD), researchers simulated four distinct driving scenarios to replicate real-world conditions and analyze their impact on secondary aerosol formation.

The study carefully reconstructed four driving scenarios to understand emission patterns under different conditions:

• Cold Start and 70 km/h Cruise (CSC70): Simulated engine startup after prolonged inactivity (minimum 12 hours), with sampling beginning immediately at ignition and reaching stable speed within 15 seconds.
• 120 km/h Highway Driving (D120): Recreated sustained high-speed travel to assess emissions during typical freeway conditions.
• High Engine Load (3000 rpm, ~40 kW wheel power): Mimicked demanding situations like hill climbing or acceleration for overtaking.
• Extreme Engine Load (5000 rpm, ~50 kW wheel power): Represented maximum performance scenarios to evaluate emission limits.

For non-cold-start tests, researchers preconditioned engines by running at 3000 rpm with 50 Nm load for five minutes before adjusting to test parameters, ensuring stable engine temperatures and emission concentrations.

The study incorporated diverse fuel formulations to evaluate their environmental impact:

• Diesel Vehicles: Tested with standard B7 biodiesel (7% renewable content) and 100% hydrotreated vegetable oil (HVO), a cleaner-burning renewable alternative.
• Gasoline Vehicles: Evaluated using commercial ethanol blends (E5, E10) and reformulated gasoline (RFG) containing approximately 20% alcohol content.

All fuel changes occurred in certified service centers with thorough tank cleaning between tests to prevent cross-contamination.

This research provides critical insights into how vehicle emissions evolve in sunlight, particularly regarding nitrogen oxides (NOx) and volatile organic compounds (VOCs)—key precursors for ozone and secondary aerosols. The findings suggest:

• High-load conditions generate elevated NOx and VOC emissions, accelerating photochemical reactions
• Ethanol-blended gasoline may increase aldehyde emissions, potentially raising secondary aerosol production
• Advanced aftertreatment systems (DPF, SCR) demonstrate varying effectiveness depending on operating conditions

These results will inform more accurate air quality modeling and help policymakers develop targeted emission reduction strategies. As vehicle technology evolves with increasing electrification, future studies may examine how hybrid and electric vehicles influence secondary aerosol formation through non-exhaust emissions and energy production pathways.