Atmospheric Environment. Part B. Urban Atmosphere最新文献

Results of a field observation programme to investigate the surface energy balance of Mexico City, D.F., are presented. Direct measurements of net all-wave radiation and the turbulent sensible heat flux were conducted from a 28 m tower located about 4 km from the city centre. Heat storage in the urban fabric was parameterized and the turbulent latent heat flux was found by the residual in the energy balance equation. Most of the data were gathered in the dry season from February to the end of March 1985. The results from this tropical highland city resemble those from temperate cities in respect of the magnitude of the Bowen ratio and the diurnal trend in the amount of the net radiation partitioned into turbulent sensible heat. On the other hand, the role of heat storage is much larger than in the temperate case so that the partitioning of the sensible heat between turbulent and conductive transfer seems to be more nearly equal. Nevertheless evaporation appears to be an important term in both the surface water and energy balances. The nature of the energy balance may help explain some of the previously observed features of the heat island and humidity regimes of Mexico City.

In the present study a wet/dry precipitation collector was used (Canadian MIC) to collect wet and dust deposition samples in one station in the center of the Athens basin, Greece, for the period September 1987–August 1988. Concentrations of the main cations (H⁺, NH₄⁺, Na⁺, K⁺, Ca²⁺ and Mg²⁺) and main anions (Cl⁻, NO₃⁻ and SO₄²⁻) were determined. The analytical chemical methods were evaluated with NBS standard reference material and with rainwater samples from EMEP (European Monitoring Evaluation Programme).

Statistical analyses of correlation between the concentration of chemical species and correlation diagrams between chemical species concentrations and wind directions were made in order to estimate the origin of the determined ions both in wet and dust deposition.

In all samples non-marine molar sulfate concentrations exceed nitrate concentrations despite the dominance of low sulfur oil burning in the region (0.3% for domestic heating, 0.7% for industrial uses).

For 37% of the samples the pH was lower than 4.5 and for 69% of samples was lower than 5.6. The wet flux of sulfur was calculated to be 0.67 gm⁻²yr⁻¹.

The results support the conclusion that chemical constituents in rain and dust deposition are a combination of emissions from natural and anthropogenic sources.

Nairobi city has a population of over 1.5 million and is growing at a rate of about 70 persons per day. Various activities in the city such as construction work, industrial processes, use of unroadworthy cars, and dust blown off unpaved roads contribute enormously to suspended particulate matter in the air. In this paper, analysis by gravimetric and energy dispersive X-ray fluorescence (EDXRF) of the suspended particulate matter in the air in the city centre, an industrial area and one residential area was carried out. The total suspended particulate matter (TSP) mean levels ranged from 69.983 to 397.903 μg m⁻³. The following components were measured from the TSP, mean values in μg m⁻³: iron 6.014–7.547, potassium 1.252–6.432, titanium 0.286–1.698, manganese 0.158–1.683, lead 0.395–1.321, bromine 0.122–0.707, zinc 0.159–0.678 and zirconium 0.017–0.245. The values of lead obtained (0.395–1.321 μg m⁻³) fall within the WHO recommendations, but compared to the values reported in some European countries, they are high. Most of the elements had low enrichment factors except for lead (104–353), bromine (429–1533) and zinc (14–79). Bromine and lead were highly correlated to the number of light vehicles (p=0.874 and 0.942, respectively). In addition the ratio of Br:Pb by weight was in the range 0.309–0.535, while the correlation factor for Br:Pb was 0.951, leading to the conclusion that both elements came from leaded gasoline.

Outdoor smog chamber experiments were used to study the sensitivity of the yields of two important nitrogen-containing pollutants, nitric acid (HNO₃) and peroxyacetyl nitrate (PAN) to changes in nonmethane hydrocarbon (HC) and nitrogen oxide (NO_x) concentrations in Los Angeles. The experiments were conducted at two sites in the Los Angeles Basin using eight chambers filled with morning Los Angeles air on 33 days. At least one chamber was unchanged and served as a control, while the initial HC and/or NO_x concentrations were changed by 25–50% in up to seven chambers to simulate O₃ control strategies and to broaden the range of HC - NO_x conditions studied. Empirical models that predict the maximum yields of HNO₃ and PAN were used to determine the response of these pollutants to three possible ozone control strategies. All three strategies (reductions in HC, NO_x or both HC and NO_x) reduced PAN while only NO_x reductions decreased HNO₃. However, reducing NO_x increased the HC reductions required to attain lower O₃ levels. Thus, there is a conflict between the O₃ and HNO₃ control strategies.

Receptor modeling on ambient aerosol and air quality data collected at Duarte, CA (a residential site near Los Angeles), in 1983 and 1987–1988 was carried out. A significant change in the ambient concentrations of SO₄²⁻, Al, Si, Mn, Fe, Pb, Br, volatile and organic carbon and fine particle (FP) mass took place from 1983 to 1987–1988. A drastic reduction (∼80%) of the ambient Pb and Br concentrations took place as the lead content and the usage of leaded gasoline decreased in the Los Angeles Basin during that period. A day-of-the-week analysis indicated that both crustal (Si, Ca, Fe) and transportation (Pb, CO, organic carbon and black carbon) related pollutants exhibit significantly different concentrations between weekdays and weekends of 1987–1988. this indicates that loadings of suspended soil dust are more affected by anthropogenic activities than meteorological patterns. In contrast, sulfate and volatile carbon concentrations seem to be insensitive to that cycle indicating that other sources/processes can be responsible for the ambient levels of these pollutants. Principal component analysis of aerosol and air quality data showed that the major contributions to the variance of the ambient aerosol loadings come from soil, motor vehicles and sulfates.

Canyon geometry is an important determinant characteristic airflow regimes observed within urban canyons. Three principal flow regimes are: “skimming” flow, “wake interference” flow and “isolated roughness” flow, following the nomenclature of Oke (1987, Boundary Layer Climates, 2nd edn, Methuen, London). The transition between flows is determined by canyon geometry and can be described in terms of threshold height/width (H/W) ratios for an arbitrary length/height (L/H) ratio.

The determination of threshold H/W ratios has previously relied on repeated wind tunnel experiments or repeated runs of a numerical model, with canyon geometry altered until the observed flow regime changes. The present numerical investigation of typical three-dimensional flows within urban canyons identifies the key parameters which mark transition between flow regimes. On this basis it is possible to establish the geometric thresholds between regimes with analysis of a horizontal cross-section of a few simulated flows.

Measurements of aerosol concentrations, chemical species and meteorological quantities in the air above Mexico City were obtained from an instrumented research aircraft. Concentrations of particles in the size range between 0.12 and 3.12 μm were nearly invariant with height, and typical values were of the order of 5000 cm⁻³. However, particles smaller than 0.12 μm were confined to the lowest few hundred meters of the atmosphere until the morning temperature inversion dissipated, after which time those particles, together with newly formed particles created by secondary processes, mixed to a greater height above the city. Total particle concentrations near the surface attained values in excess of 60,000 cm⁻³. An examination of the corresponding profiles of SO₂ suggests that combustion processes are likely sources for the additional small particles.

Odor perception of approximately 200 subjects was measured to determine whether visual contact with an odor source affects sensory responses and to estimate the magnitude of such an effect. Environmental tobacco smoke (ETS) odors were generated in a chamber either by a smoke machine or by an investigator who smoked. Several levels of odor intensity were generated. Odor intensity, odor hedonics and odor characters were the parameters measured before and after visual contact with the odor source. Visual contact increased the perceived odor intensity, the hedonic nature of the odor changed directionally toward unpleasant and the number of subjects perceiving tobacco odor increased. The change caused by visual contact led to differences that were statistically significant.

The inventory of aluminum in the atmospheric air from human activities was discussed using data from the Japanese National Air Surveillance Network. The source of atmospheric Al is considered to be mainly soil-derived particles. However, an hropogenically generated Al could also be added to airborne particulates less than 10 μm in aerodynamic diameter. Scandium, which is also found in soil dust, has a value lower than unity in enrichment factor normalized by Al. The intercept, A of the AlSc regression curve, (Al) = A + B(Sc), was much larger in industrial cities such as Kawasaki, Amagasaki, etc. This could be explained by the difference of elemental composition of various emission sources and the existence of excess Al added to airborne particulate samples.

The magnitude of the urban effect on temperature in the tropical city of Guadalajara is examined. Parallel to the city's growth, air temperature shows an increasing trend; over a 40-year period (1931–1970) this rate was of the order of 0.03°C yr⁻¹. As would be expected, this rate of temperature increase has been uneven over the period. When population increase per decade was large (90%), as in the 1940s the corresponding rate was significant (0.4°C per decade). The largest warming rate (0.7°C per decade) occurred during the 1960s when population growth was 73%. These results suggest that other factors (on a regional/global scale) may have been at play. Results show that estimates of the intensity of the heat island in a tropical city are likely to be dependent (besides the physical factors implicit in the city size), on the land-use characteristics in the rural/suburban control. Since the suburban/rural substrates undergo a significant annual variation in their physical properties in Guadalajara, heat-island intensity is highest during the dry season and declining in the wet season when contrasts in urban/rural thermal admittance are likely to be minimal. The presence of lower temperatures in the city during the afternoon hours suggests that the city (especially in the dry season) acts as a moisture source. This “cool” island has also been observed in other cities with similar regional climate.