Measurements of the size distribution of particles emitted from a modern heavy duty diesel engine using fuel with a sulfur content of between 0.03 and 0.05 by mass have been made under constant engine operating conditions, but with variations inthe humidity of dilution air and dilution ratio prior to particle size measurement. The results show clearly that the measured size distribution is crucially dependent upon the conditions of dilution, hence creating real difficulties for comparison ofdata between different investigators. Conditions of high dilution ratio and high relative humidity both tend to favor the production of nanoparticles, especially within the range below 50 nm diameter. Application of homogeneous nucleation theory showsthat nanoparticle production during dilution is qualitatively consistent with the production of sulfuric acid, but the predicted nucleation rates are lower than those measured, in common with studies of nucleation in the atmosphere. Chemical analysis ofsize-fractionated particles shows enhancement of sulfate concentrations in humid dilution conditions and at high dilution ratios consistent with the above mechanism. The possible role of semivolatile organic compounds in these processes has not beeninvestigated.
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