Mass spectrometric measurements of size and composition of diesel exhaust particles have been performed under various conditions: chassis dynamometer tests, field measurements near a German motorway, and individual car chasing. Nucleation particles consisting of volatile sulfate and organic material could be detected both at the chassis dynamometer test facility and during individual car chasing. We found evidence that if nucleation occurs, sulfuric acid/water is the nucleating agent. Low-volatile organics species condense only on the preexisting sulfuric acid/water clusters. Nucleation was found to depend strongly on various parameters such as exhaust dilution conditions, fuel sulfur content, and engine load. The latter determines the fraction of the fuel sulfur that is converted to sulfuric acid. The organic compounds (volatile and low-volatile) condense only on preexisting particles, such as both sulfuric acid nucleation particles and larger accumulation mode soot particles. On the latter, sulfuric acid also condenses, if the conditions for nucleation are not given. The overall ratio of sulfate to organic (volatile and low-volatile) is also strongly dependent on the engine load. It was found that the production of nucleation particles even at high engine load can be suppressed by using low-sulfur fuel.

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Abstract. Detailed investigations of the chemical and microphysical properties of rural continental aerosols were performed during the HAZE2002 experiment, which was conducted in May 2002 at the Meteorological Observatory Hohenpeissenberg (DWD) in Southern Germany.Online measurements included: Size-resolved chemical composition of submicron particles; total particle number concentrations and size distributions over the diameter range of 3 nm to 9 µm; gas-phase concentration of monoterpenes, CO, O 3 , OH, and H 2 SO 4 . Filter sampling and offline analytical techniques were used to determine: Fine particle mass (PM2.5), organic, elemental and total carbon in PM2.5 (OC2.5, EC2.5, TC2.5), and selected organic compounds (dicarboxylic acids, polycyclic aromatic hydrocarbons, proteins).Overall, the non-refractory components of submicron particles detected by aerosol mass spectrometry (PM1, 6.6±5.4 µg m −3 , arithmetic mean and standard deviation) accounted for ∼62% of PM2.5 determined by filter gravimetry (10.6±4.7 µg m −3 ). The relative proportions of nonrefractory submicron particle components were: (23±39)% ammonium nitrate, (27±23)% ammonium sulfate, and (50±40)% organics (OM1). OM1 was closely correlated with PM1 (r 2 =0.9) indicating a near-constant ratio of nonrefractory organics and inorganics.Correspondence to: J. Schneider (schneider@mpch-mainz.mpg.de)The average ratio of OM1 to OC2.5 was 2.1±1.4, indicating a high proportion of heteroelements in the organic fraction of the sampled rural aerosol. This is consistent with the high ratio of oxygenated organic aerosol (OOA) over hydrocarbon-like organic aerosol (HOA) inferred from the AMS results (4:1), and also with the high abundance of proteins (∼3%) indicating a high proportion of primary biological material (∼30%) in PM2.5. This finding was confirmed by low abundance of PAHs (<1 ng m −3 ) and EC (<1 µg m −3 ) in PM2.5 and detection of several secondary organic aerosol compounds (dicarboxylic acids) and their precursors (monoterpenes).New particle formation was observed almost every day with particle number concentrations exceeding 10 4 cm −3 (nighttime background level 1000-2000 cm −3 ). Closer inspection of two major events indicated that the observed nucleation agrees with ternary H 2 SO 4 /H 2 O/NH 3 nucleation and that condensation of both organic and inorganic species contributed to particle growth.