摘要:

The greater Los Angeles metropolitan area (the South Coast Air Basin) historically has had a severe air pollution problem whose visible symptom is a dense smoke-like haze mixed with fog (smog). More than 25 years ago, Friedlander and his associates began attempts to characterize the detailed chemistry of smog particles, and to interpret these results in terms of source emissions. Source apportionment was formalized using a “model”, called the chemical mass balance (CMB). The method has been applied periodically since 1968, with major attempts reported in 1975 and 1994, based on large field studies. Despite improvements in particulate sampling and chemical characterization, data availability and major emissions changes, the CMB results qualitatively remain similar. Only broad source categories are identifiable, but they generally make up more than 75% of the total mass concentration. These include primary motor vehicle emissions, suspended (earth) crustal material, and secondary species of ammonium nitrate and sulfate salts. The decoupling of source components in the ambient aerosols suggests that extreme PM events are significantly influenced by factors other than simple mixing and dispersion. Carbonaceous material from combustion emissions and secondary atmospheric production also represent a significant fraction of southern California airborne particles. The carbon fraction includes a complex mixture of soot or black carbon, oxygenated compounds, as well as traces of polycyclic aroma tics. The last group has been of particular interest because of its apparent common origins and its carcinogenic activity. The major contributor to primary carbonaceous material is motor vehicles, The observations also indicate that a significant fraction of carbon derives from contemporary sources such as vegetation or cooking of meat. The CMB has been used to guide regulatory strategies for particulate emissions management. The method could be improved substantially with more extensive source profile development, as well as improved knowledge about the origins of the organic fraction.

ISSN:0098-6445    

DOI:10.1080/00986449608936548

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

摘要:

A model for aerosol-phase densification of particles during their synthesis by spray pyrolysis is presented. The model was used to describe the evolution of the specific surface area of PdO powders synthesized at temperatures between 400 to 800 °C (residence times 27.9 to 22.7 s). Surface areas and grain sizes ranged between 56 m2/g and 4 nm at 400 °C to 3.2 m2/g and 40 nm at 800 °C The characteristic coalescence lime was determined as: τ= 1.7 x 1013T d3exp(1.3 x 105/RT) [for lattice diffusion] and t = 2.7 x 1019Td4exp (1.6 x 105/RT) [for grain boundary diffusion] (τ is in seconds,Tin degrees Kelvin, R is 8.314J/(mol.K)and d is in meters), but the data and model did not allow identification of the specific mechanism. The model provides a general approach for correlating changes in particle surface area with reactor operating conditions that is applicable to a wide variety of materials.

ISSN:0098-6445    

DOI:10.1080/00986449608936549

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

摘要:

Particles and gases can deposit from the atmosphere to polar snow by several mechanisms. Dry deposition can be considered to consist of three steps: aerodynamic transport from the free atmosphere to the viscous sublayer near the surface, boundary layer transport across the sublayer, and interactions with the surface. The particle dry deposition mass flux is dominated by the largest particles present in a size distribution. Wet deposition includes in-cloud and below-cloud scavenging, where the former refers to uptake of particles during nucleation of cloudwater as well as scavenging of particles and gases by existing droplets and ice crystals. Of all the wet deposition mechanisms, nucleation scavenging is often the most important mechanism for particles in the polar regions. Finally, incorporation of particles and gases into fog droplets and subsequent settling of the fog to the snow surface can be an important removal process in regions of frequent fog. For Summit, Greenland, the total deposition of MSA, SO42-, Na+, K+, and Ca2+during May 24-July 13, 1993 was dominated by wet deposition: this mechanism accounted for an average of 62% of the total deposition for these species. Fog and dry deposition accounted for 21% and 17% of the total, respectively. These results suggest that all three mechanisms may need to be considered when estimating total deposition of certain chemical species to polar snow.

ISSN:0098-6445    

DOI:10.1080/00986449608936550

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

摘要:

The photocatalytic destruction of phenol and salicylic acid in aerated aqueous suspensions of titania powders made in flame reactors was studied. These powders were made in five hydrocarbon diffusion flames by hydrolysis and oxidation of TiCI4that resulted in powders of high specific surface area and high anatase content. The photoactivity of the flame-made titania powders was comparable and slightly better to that of commercial Degussa P25. Doping the titania with SiO2was detrimental to the photoactivity of the powders in contrast with what was seen in non-aerated suspensions. The photodegradation of phenol followed a first-order law while the Langmuir-Hinshelwood model was found to most accurately represent the photodegradation of salicylic acid.

ISSN:0098-6445    

DOI:10.1080/00986449608936551

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor