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The reaction of alumina as a model substance for mineral aerosols with NO2 or HNO3 was studied using diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). The formation of nitrate on the Al2O3 surface was observed in both cases. In addition, during the initial phase of the NO2 reaction, intermediate nitrite formation was observed. The DRIFTS data provide insight into the reaction mechanism, which involves reaction of surface OH groups, the formation of a {AlOOH···NO2} adduct, and the formation of acidic OH groups. The reaction order in NO2 of 1.86 ± 0.1 was determined from a quantitative kinetic evaluation of a series of experiments with NO2 concentrations in the range of 1013 to 1015 molecules cm-3. The reactive uptake coefficient, γ, was determined from the infrared absorbance, which was calibrated by ion chromatography, and from the Al2O3 Brunauer−Emmett-Teller (BET) surface area. γ depended linearly on the NO2 concentration and varied from γ = 7.3 × 10-10 to 1.3 × 10-8 for [NO2] = 2.5 × 1013 to 8.5 × 1014 molecules cm-3. Estimations of the atmospheric impact showed that at these above conditions (γ = 10-9) nitrate formation on mineral aerosol from the NO2 reaction would be negligible.

Soot samples from a spark generator, a flame, and a diesel passenger car were either collected on a Teflon filter and transferred to an IR-transparent window or deposited directly from a flame onto the window and investigated by Fourier transform infrared (FTIR) spectroscopy. The soot-covered windows were mounted in a 10 cm vacuum cell connected to a standard flow system with He as carrier gas. Reactive gases, such as NO2 and HNO3, were added to the carrier gas flow at a concentration of (0.016 to 2.5) × 1014 molecule cm-3. FTIR spectra of soot samples before and after exposure to HNO3, NO2, and O3 are presented. Formation of IR absorption bands was analyzed as a function of exposure time. IR bands attributable to soot surface oxidation products and nitrogen containing species, e.g. −CO, R−NO2, R−ONO2, and R−ONO were observed. The observed time dependence of the absorption bands of the spark generator soot can be fitted by two parallel reactions, a slow and a fast process. Both processes have a reaction order of n ≈ 0.2 (±0.3) for the NO2 + soot reaction and n ≈ 0.5 (±0.6) for the HNO3 + soot reaction. The number of active sites, N max = 2.2 × 1014 molecules cm-2 soot surface, has been estimated from saturation experiments. Surface reaction probabilities depend on reactant concentration and reaction time and were in the range of γ ≈ 10-6 to 10-8 for the slow, and γ ≈ 10-3 to 10-6 for the fast processes. The reaction probability on diesel engine soot was nearly 1 order of magnitude slower. It is concluded that the reaction of NO2 with soot cannot account for the HONO levels observed in urban air.

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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