DIFUSO: Diesel fuel and soot - fuel formulation and its atmospheric implications

• Research unit: EU RFP
• Project number: 98.0215

Key words (English)

Photosmog; HONO formation on diesel soot; wet effluent diffusion denuder

Alternative project number (English)

EU project number: ENV4-CT97-0390

Research programs (English)

EU-programme: 4. Frame Research Programme - 3.1 Environment

Short description (English)

See abstract

Partners and International Organizations (English)

Coordinator: Universität- Gesamthochschule, Wuppertal (D)

Abstract (English)

The influence of diesel exhaust on photochemistry needs to be assessed for various diesel fuel compositions in order to define photosmog abatement strategies. Nitrous acid (HONO) may play an important role in initiating daytime photochemistry by its rapid photolysis yielding OH radicals. The source of HONO in the atmosphere has not yet been conclusively identified, but several studies have recently identified soot as a possible reactive surface for reaction with NO2.
For the Diesel Fuel and Soot project DIFUSO an engine test rig was installed in the machine room of the European Photoreactor (EUPHORE) chamber B in March 1999. The EUPHORE is an outdoor photoreactor of almost 200 m3 volume in which photochemistry can be simulated under natural sunlight condition. The engine test rig consists of a 1.8 l commercial Diesel engine and an air-cooled eddy current brake to apply operation at constant load. The exhaust gas is guided in a heated tube (150°C) to a three-way valve by which the exhaust is dosed into the smog chamber.
During the first phase of the project we tested the hypothesis that HONO is formed on soot particles, although it was difficult to increase the soot to NOx ratio. Using a wet effluent diffusion denuder (WEDD) coupled to an ion chromatography system we observed an instant increase of the HONO signal after exhaust injection into the smog chamber. However, no additional HONO formation was observed over time as expected from the reaction of NOx with soot particles, either because the introduced soot surface was too small or the particles were rapidly passivated. On the other hand, HONO formation was observed when water was added into the chamber even when no NOx was present; probably some species from previous experiments still adsorbed on the chamber wall react and evaporate as soon as water is added. In fact, several laboratory experiments revealed that polar organics contained in the exhaust gas may dissolve into adsorbed water (on the WEDD or chamber walls) and reduce gaseous NO2 to nitrite, the aqueous precursor of HONO. We were able to show that HONO is formed in the reaction of NO2 with an organic substance contained in the exhaust. While the reaction on soot alone is not able to account for the observed atmospheric HONO levels, this new reaction is able to explain the HONO to NOx ratios of 0.02 observed in the atmosphere.
During the last campaign, HONO emission indices were measured for all the five diesel fuels (standard, 5%, 15%, 25% aromatics and biodiesel) using four different detection methods: DOAS, WEDD, HPLC and a long path absorption photometer. The HONO emission indices were found to be almost independent of fuel formulation and engine operating conditions and amounted to 0.11 g/kg in agreement with literature data.

References in databases (English)

Swiss Database: Euro-DB of the
State Secretariat for Education and Research
Hallwylstrasse 4
CH-3003 Berne, Switzerland
Tel. [phone]
Swiss Project-Number: 98.0215