Atmospheric Environment. Part B. Urban Atmosphere最新文献

A simple numerical model that attempts to simulate the energy budget of an urban canyon is assessed by comparing its predictions against measurements made at an urban field site. A street is chosen that satisfieds many of the model's requirements and a measurement scheme is devised to measure variables which are also simulated. The model is run using input information collected from the field and the data produced are compared with the measured data. The results indicate that the model is capable of simulating many of the climate attributes (such as surface temperatures and net radiation at the canyon top) accurately, but does not simulate the dominant mechanism of sensible heat exchange between the canyon and its environment. Nevertheless, the model predicts a small and largely invariant exchange, similar to that measured. The results of the field study, when compared with other published work, indicates that as street canyons become narrower they become increasingly isolated in terms of heat exchange from the overlying atmosphere. The results of the computer runs appear to follow this pattern.

The time dependence of the emission of organic compounds from a polyamide floor covering with styrene-butadiene-rubber (SBR) backing was studied in three climate chambers (0.03, 1.0 and 38 m³) at 23°C 5nd 45% RH. While volatile compounds such as toluene reach a maximum concentration in the gas phase within 1 h and decrease in concentration to less than 2% within 60 h, the concentration of less volatile compounds, such as 4-phenylcyclohexene, decreases slowly over a period of months.

If the chamber is well mixed and a defined chamber loading is maintained the observed concentrations do not depend on the chamber size, the wall material and air velocity. The concentration of the observed emissions is roughly proportional to the chamber loading. Surprisingly it is not inversely proportional to the air exchange rate. Rather, at high air exchange rates mass transfer from the carpet to the gas phase is enhanced.

The “decreasing source models” of Dunn and Tichenor (Atmospheric Environment22, 885–894, 1988) have been applied to the data. They allow the extrapolation of experimental data beyond the time available for measurement.

The model calculations reveal the presence of sink effects. The role of the chamber walls as sinks can be determined more reliably if constant sources of an organic compound are placed into the chamber and their increase in concentration with time is compared with the theoretical predictions neglecting sink effects.

Atmospheric concentrations of lead have been measured at two sites in each of two cities in the U.K. (London and Manchester). The results have been used to evaluate the effect of the increasing use of unleaded petrol (gasoline) in motor vehicles. Atmospheric concentrations of lead measured in London were found to be correlated to the U.K. lead in petrol consumption figures. However, a similar correlation was not evident for Manchester indicating the importance of other sources of atmospheric lead. Lead concentrations were correlated between the two cities and a large variation in measured levels illustrates the importance of meteorological effects in determining atmospheric concentration.

Atmospheric deposition of trace metals like Pb, Cd, Cu and Zn has been studied at Deonar, Bombay during 1988 and 1989. The atmospheric concentrations of these metals at Deonar as well as the levels of total suspended particulate matter (TSPM) at different locations in Bombay were also measured. The bulk deposition flux for these metals was found to vary from 0.3 to 102.1 kg km⁻²yr^–1 while the dry deposition flux varied from 0.2 to 104.6 kg km⁻² yr⁻¹. Deposition velocities for these metals have also been estimated which varied from 0.05 to 2.5 cm s⁻¹. The TSPM levels ranged from 17 to 588 μg m⁻³. The levels of TSPM were found to be at a minimum at Deonar and, as such, this study is believed to indicate representative minimum deposition values for Bombay.

Rainwater samples were collected in Los Angeles, during 1985–1991 to determine concentration levels, sources and deposition rates of atmospheric H₂O₂, aldehydes and organic acids, in addition to major cations, anions and pH. Volume-weighted mean concentrations of H₂O₂, aldehydes (formaldehyde + acetaldehyde + glyoxal + methylglyoxal) and organic acids (formic acid + acetic acid) in rain collected at Westwood were 4.4., 3.9 and 16.5 μM, respectively, during the 6-year study period. Monocarboxylic organic acids were estimated to account for 27% (2–80%) of total free acidity (as on overall average) in rain collected at Westwood, whereas sulfuric acid and nitric acid accounted for 39% and 34% of the total acidity, respectively. Concentrations of aldehydes were strongly dependent on precipitation volume and decreased with increasing precipitation volume, whereas H₂O₂ and organic acids were only weakly dependent on precipitation volume. These results indicate that concentrations of aldehydes in rain are mainly controlled by dilution, whereas H₂O₂ and organic acid concentrations are controlled by other factors, such as decomposition of H₂O₂ by reacting with S(IV) and continuous aqueous formation/decomposition of organic acids by reactions involving aldehydes, dissolved OH radicals and H₂O₂. Principal component analyses indicate that aldehydes in rainwater mainly originate from gases and aerosols derived from anthropogenic sources, whereas the sources of H₂O₂ and organic acids in rain do not correlate with anthropogenic sources or marine and continental sources. There is good agreement between reported gas-phase concentrations of H₂O₂, aldehydes and organic acids in Los Angeles and calculated equilibrium concentrations of these chemical species from their rainwater concentrations and Henry's law constants. Temporal variations of concentrations of chemical species indicate that H₂O₂, aldehydes and organic acids were highest in the early afternoon. Summer rains contained the highest concentration of these chemical species, suggesting the photochemical activities during rain storms significantly affect their concentration levels. Estimation of annual rate of wet and dry depositions of H₂O₂, aldehydes and organic acids for the period studied, indicates that 84% of H₂O₂, 97% of aldehydes and 94% of organic acids, respectively, are annually scavenged from the atmosphere, by dry deposition, which is the dominant process for removal of these atmospheric pollutants in Los Angeles.

Differential optical absorption spectrometry (DOAS) has been used by a number of investigators over the past 10 years to measure a wide range of gaseous air pollutants. Recently OPSIS AB, Lund, Sweden has developed and made commercially available a DOAS instrument that has a number of features which make the unit attractive for field monitoring studies in both remote and urban areas. The DOAS is composed of a broad band light source (emission between 200 and 1000 nm) and a receiver-spectrometer assembly. The spectral data from the spectrometer are processed in real time using a PC computer to calculate the concentrations of the pollutants programmed to be monitored by the system. The distance between the light source and receiver can range from 100 to 2000 m, depending on the pollutant to be monitored and species concentrations. In September and October of 1989 an OPSIS AB DOAS was operated in the Research Triangle Park, NC on the roofs of the two main Environmental Protection Agency laboratories. The distance between the light source and receiver was 557 m and the pollutants monitored were SO₂, NO₂, O₃, HCHO and HNO₂. Comparisons between the federal reference and equivalent fixedpoint methods measuring SO₂, O₃ and NO₂ and simultaneous data derived from the DOAS had correlations typically greater than 0.90 during the evaluation.

Particle size distributions of Fe, Pb, Mz, Zn, Cu and Ni were measured at two locations in the Los Angeles area during the winter and summer of 1989 using a cascade impactor. Measures were taken to minimize particle bounce and elemental analysis was performed by X-ray fluorescence. Measured Fe size distributions were consistent, with suspended dust being the major source. Pb distributions were observed to be multimodal with modes due to automotive emissions, particle growth and suspended dust. The importance of suspended dust was investigated using Fe as tracer and suspended dust ratios of Fe to Pb and other metals. Mn distributions were observed to be similar to those of Pb with automotive emissions and suspended dust being important sources. Particle size distributions of Zn, Cu and Ni showed significant amounts of these metals to be persent in particles less than 1μm in diameter, indicating anthropogenic sources. Suspended dust was found to be an important source of Zn, but not of Cu or Ni, in particles with diameters greater than 2 μm.

A gas chromatographic (GC)-mass spectrometric (MS) study of the large (> 7.2 μm) and small (< 0.5 μm) particle fractions of aerosols collected in Barcelona (Catalonia, Spain) has allowed the identification of the major sources of airborne organic matter in urban environments. The extracts have been column chromatography separated in three fractions encompassing aliphatic hydrocarbons, aromatic hydrocarbons+semi-polar compounds, and polar products. The whole analytical procedure has allowed the identification of 186 molecular species that have been grouped according to their precursors. The study has also afforded the characterization of organic source-particle size associations as well as the effects of seasonality on the occurrence of the compounds identified.