First online measurement of gaseous sulfuric acid in diesel exhaust helps clarify role in formation of nucleation particles

8 October 2012

Researchers in Germany and Finland report the first online measurement of gaseous sulfuric acid (GSA) in the exhaust gas of a modern heavy-duty diesel engine in an effort to quantify the role of low-vapor-pressure gases in the formation of nucleation particles (NUP). Their findings are published in the ACS journal Environmental Science & Technology.

...the oxidative exhaust after-treatment [for diesel engines] may also generate undesired oxidation products. A striking example is SO₃, which is formed by oxidation of engine-generated SO₂ and reacts with water vapor, leading to gaseous sulfuric acid (GSA). GSA has a very low saturation vapor pressure, and therefore, it may condense and even nucleate in the cooling dilution process of the exhaust. Thus, the existence of GSA can lead to formation and growth of sulfuric acid–water particles, a particular form of nucleation particles (NUP)....However, although the oxidative exhaust after-treatment can cause the increases of certain undesired emissions like GSA and thus increases of NUP emissions, it should be keep in mind that typically the exhaust after-treatment decreases the total emissions (e.g., of hydrocarbons) significantly.

—Arnold et al.

Using a novel mass spectrometric method, the team performed online measurements of low-vapor-pressure gases from exhaust of a modern heavy-duty diesel engine operated with a modern aftertreatment system and combusting low and ultra-low sulfur fuels and also biofuel.

They observed that the gaseous sulfuric acid (GSA) concentration varied strongly, although engine operation was stable; exhaust GSA was observed to be affected by fuel sulfur level, exhaust after-treatment, and driving conditions.

Significant GSA concentrations were also measured when biofuel was used, indicating that GSA can be originated also from lubricant oil sulfur.

They found that the exhaust GSA promotes NUP formation, but also that organic (acidic) precursor gases can have a role. The model results indicated that that the measured GSA concentration alone is not high enough to grow the particles to the detected sizes.

Resources

• F. Arnold, L. Pirjola, T. Rönkkö, U. Reichl, H. Schlager, T. Lähde, J. Heikkilä, and J. Keskinen (2012) First Online Measurements of Sulfuric Acid Gas in Modern Heavy-Duty Diesel Engine Exhaust: Implications for Nanoparticle Formation. Environmental Science & Technology doi: 10.1021/es302432s