Karen Marie Adams

Lean NOx catalysis for diesel passenger cars: Investigating effects of sulfur dioxide and space velocity

Abstract European Stage III emissions requirements will be difficult to meet for diesel passenger cars if lean NOx catalysts are not available. Current prototype lean NOx technology for diesels consists of Pt based and Cu zeolite catalysts. Both types have been examined for this study. The former is most active at low temperatures, approximately 190–250°C. The latter has optimum activity at higher temperatures, usually above 350°C. Maximum flow reactor activity of 40–55% NOx conversion (25 000–50 000/h space velocity) has been measured for Pt catalysts using a synthetic feedgas. During the MVEuro2 driving cycle, 35–40% of mass NOx has been emitted at inlet catalyst temperatures from 120 to 200°C. These temperatures fall below optimum temperatures for current Pt based lean NOx catalysts. For temperatures above 350°C, where Cu zeolite catalysts are active, one vehicle has emitted ca. 30% of mass NOx during MVEuro2. These high temperatures are achieved during high speed, hard acceleration driving; although attained briefly during the MVEuro2 cycle, these high temperature emissions could be a critical contribution under customer driving conditions. Effects of sulfur dioxide (SO2) and space velocity (SV) have been investigated as part of a strategy to optimize NOx removal with lean NOx catalysts. Elimination of feedgas SO2 can lower NOx light off temperature for both Pt and Cu zeolite. Some Pt catalysts do not show this behavior. Additional evaluation of a Cu zeolite catalyst demonstrates that poisoning by feedgas SO2 is reversible during evaluation or aging. This result suggests that if sulfur could be removed from diesel fuel, aged Cu zeolite catalysts could be practical. Decreasing space velocity will help NOx removal over Pt by (i) lowering NOx light off temperature, (ii) lowering the temperature at which peak NOx conversion occurs, (iii) increasing the level of peak NOx conversion, and (iv) widening the temperature window for NOx reduction. For Cu zeolite, decreasing space velocity can help mainly by lowering NOx light off temperature and temperature where maximum NOx conversion starts. Both increased catalyst volume and sulfur removal provide Pt catalysts with a NOx temperature window that coincides better with low temperatures where most NOx mass is emitted. SV effects on lean NOx reduction are explained by discussion of possible mechanistic features.

Real-time in situ measurements of atmospheric optical absorption in the visible via photoacoustic spectroscopy. 1: Evaluation of photoacoustic cells

Four configurations of resonant photoacoustic cells were evaluated for maximum signal sensitivity to light absorption by the sample, with minimal noise and background. Theoretical and experimental data are discussed. Azimuthal and radial resonant modes were compared for one cell. Of the four, the best cell was a brass cylinder, 2.5-cm radius and 9.5-cm length, which was operated in the azimuthal mode. An argon-ion laser (lambda = 514.5 nm) was the light source. A continuous sample flow through the cell, required for real-time in situ atmospheric measurements, gave an acceptable noise level and time for signal response at ~500 cc/min. Linearity of the photoacoustic signal was checked in the range applicable to atmospheric absorption. At a signal-to-noise ratio (SNR) equal to 1, a light absorption detection limit of 4.7 x 10(-6) m(-1) could be achieved.

Intercomparison of photoacoustic and thermal-optical methods for the measurement of atmospheric elemental carbon

Two atmospheric elemental carbon measurement methods based on different analytical principles have been compared using data collected during the summer of 1987 at the Claremont, California site of the Southern California Air Quality Study (SCAQS). An optical absorption method, photoacoustic spectroscopy, measured the visible light absorption (γ = 514.5 nm) of atmospheric elemental carbon in its natural aerosol-state in real time. Elemental carbon concentrations were obtained by applying the appropriate value of the absorption cross-section for elemental carbon to the optical absorption data. The other method was a thermal-optical technique, which measures elemental and organic carbon concentrations on a filte--collected sample by combustion and corrects for the pyrolytic conversion of organic to elemental carbon by measuring the transmittance of laser light through the sample. Aerosol was collected on a filter mounted inside the carbon analyzer and analyzed in place. A 2-h collection and analysis cycle was used. The real-time photoacoustic data integrated over ∼100 min were compared with thermal-optical data. The two methods compare quite well. The linear least squares fit gave a correlation coefficient of R=0.905, and no significant difference was seen between the two data sets at the 95% confidence level.

Measurement of atmospheric elemental carbon: Real-time data for Los Angeles during summer 1987

Two fundamentally different techniques for measuring atmospheric elemental carbon (EC) aerosol were compared to validate the methods. One technique, photoacoustic spectroscopy, was used to measure the optical absorption (2-514.5 nm) of in situ atmospheric aerosol in real time. This optical absorption can be converted to EC concentration using the appropriate value of the absorption cross- section for C, so that a comparison could be made with the second technique, thermal-optical analysis of filter-collected samples, which measures the collected EC by combustion. Solvent extraction of the filter samples prior to the thermal analysis procedure was required to minimize errors due to pyrolysis of organic carbon. Excellent 1 : 1 correlation of atmospheric EC concentrations resulted for measurements by the photoacoustic method vs the thermal method over coincident sampling times. The linear regression gave y = 1.006 ( + 0.056) x + 0.27 ( _+ 0.56) with r = 0.945 ( n = 41 ), where y is the photoacoustic EC concentra- tion and x is the thermal elemental carbon concentration, both in #g m- 3. This data set was collected in Los Angeles as part of the Southern California Air Quality Study (SCAQS) during the summer 1987, and supplements the results of an earlier, more limited data set taken in Dearborn, MI. The diurnal variability of EC aerosol in Los Angeles during SCAQS, as determined by photoacoustic spectroscopy, is discussed. Key word index: Atmospheric carbon, black carbon, elemental carbon, particulate carbon, photoacoustic, spectrophone, atmospheric optics, visibility, light absorption, optical absorption.

Real-time, in situ measurements of atmospheric optical absorption in the visible via photoacoustic spectroscopy—IV. Visibility degradation and aerosol optical properties in Los Angeles☆

Abstract Aerosol light absorption ( b abs ) has been measured in real-time in Los Angeles with a validated photoacoustic technique, and its impact on visibility degradation has been examined. These measurements were collected during ten days in the summer of 1987 for the Southern California Air Quality Study (SCAQS). Aerosol b abs ( λ = 514.5 nm) varied from an hourly average value of 7 × 10 −6 m −1 in the 3–4 and 4–5 a.m. periods of 13 July to 9 × 10 −5 m −1 in the 7–8 a.m. period of both 28 August and 3 September. This b abs , which is due solely to elemental carbon (EC) showed a distinct diurnal pattern with low values at night, increasing around sunrise to higher values through mid-afternoon. Comparison of these data with aerosol light scattering data clearly illustrates that the contribution of aerosol light absorption to visibility degradation increases in importance under less polluted conditions. Other urban and rural studies show similar results.

Effects of Combustion Chamber Deposit Location and Composition

Combustion chamber deposits were analyzed for chemical composition in an attempt to explain the octane requirement increase (ORI) and exhaust hydrocarbon (HC) increase observed during mileage accumulation in two vehicle test fleets. Comparisons are made of deposit composition differences within different areas of a combustion chamber, between cylinders, and between engines. Deposits accumulating in the end gas areas, on exhaust valves, and in cylinders with high oil consumption can be distinguished by comparison of H/C atom ratios, carbon content and inorganic compound content. Differences in deposit composition can be observed between engine families and between driving cycles with the same engine family. Composition differences between deposits from two engine families suggest a possible explanation for the ORI trends observed in the fleet test. Chemical composition alone is not sufficient to identify a unique relation between deposits and ORI. The flame end gas region was located with ionization gaps and was compared with deposit mass distribution to examine the influence of chamber geometry on ORI. Similarly, the HC increase mechanism requires a more complete definition than available from chemical characterization alone. A vehicle was tested with fuel containing MMT followed by operation and tests with clear fuel. The results suggest that HC are controlled by the deposits on the exposed surface only and that deposit equilibrium takes place primarily on the exposed surface rather than by massive deposit removal and replacement.