Atmospheric Environment. Part B. Urban Atmosphere最新文献

This paper presents the formulation and results of a simple numerical model designed to simulate the climate of an urban canyon. The model is two-dimensional in nature and is based on the observation that ambient airflow which tranverses the long-axis of a symmetrical canyon drives a circulating vortex within the canyon air space which results in the exchange of heat, mass and momentum at the canyon top. A simple model is described which consists of two components: a semi-empirical model to relate within-canyon airflow to ambient wind velocity, and an energy budget model for canyon surfaces. The model is capable of simulating many aspects of the canyon climate, including the canyon surface and top energy budget and surface and air temperatures. Sensitivity tests with the model indicate that the canyon top energy budget is remarkably stable for many changes in canyon parameters. Canyon geometry (expressed as a height/width ratio) caused the greatest changes with increased narrowness being associated with less heat exchange across the canyon top. The model's predictions appear intuitively reasonable and compare well with existing measurement data. The results suggest that the coupling between the urban boundary and canopy layers in terms of heat exchange may be a function of canyon geometry.

An observational and modeling study of the microclimate of a suburban area, as related to the physical and biological nature of the site, is presented. The measurements and calculations are made in comparison with a nearby open agricultural location or “control site”. The measurement program was conducted during the summer of 1981 in Davis, CA, and consisted of a series of paired observations in which simultaneous measurements were made at a control site, located in a nearby rural area, and one suburban site at a time. Results indicate that this methodology was generally successful. The various suburban sites were as often cooler than the rural site as they were warmer. In one case, a suburban site was found to be substantially cooler, averaging 7.38°C over a 5 day period, than the rural site. This special case was observed to be associated with unusually dry environmental air resulting in large evaporative cooling in the plant canopy. Comparison between the suburban temperature deficit and the physical nature of the various sites reveals that canopy height explained most of the variance of this data set. Two generally important mechanisms are hypothesized to be operating in this system: the effect of canopy size on turbulent mixing and on site shading, especially of paved areas. A simple energy balance model was applied to study the processes that control the daytime suburban temperature deficit. In particular, it is found that suburban sites can be as cool as observed when the following conditions are obtained: low-canopy humidity, large canopy size, low wind speed and high radiation load. The model simulated the overall average suburban temperature deficit of all sites for physically reasonable choices of model parameters. It is pointed out that the fact that suburban areas may be cooler than surrounding rural areas may have significance to the problem of assessing the role of the urban heat island in relation to possible global warming.

A new method for short-term air pollution prediction is described, based on the neural network. It was developed for prediction for SO₂ pollution around the biggest Slovenian thermal power plant at Sostanj. Because of the high SO₂ emissions, there is a need for a reliable air pollution prediction method that would enable lowering the peaks of pollutant concentrations in critical meteorological situations. In complex topography, classical methods for air pollution modelling are not reliable enough. The results obtained by this new method are very promising.

The method can also be used, with slight modifications, for other important air pollutants, the concentrations of which can be measured continuously.

The deposition velocity of SO₂ on marble and dolomite stone surfaces in a humid atmosphere was measured as a function of time in the laboratory using continuous monitoring techniques. The deposition velocity of SO₂ on marble varied between 0.02 and 0.23 cm s⁻¹, and was generally observed to decrease with time. The deposition velocity of SO₂ on dolomite varied between 0.02 and 0.10 cm s⁻¹, and gradually increased over the first 2000 ppm-h of exposure. For both types of stones, the deposition velocity increased significantly when condensed moisture was observed on the stone surface. Chemical analysis of the stone samples indicated that the SO₂ deposited reacted with the stone materials to form gypsum (CaSO₄·2H₂O) on the marble surfaces and gypsum and epsomite (MgSO₄·7H₂O) on the dolomite surfaces.

Brightness temperatures were derived from the Advanced Very High Resolution Radiometer (AVHRR) at channel 4 (10.5–11.5 μm) on the NOAA-9 and NOAA-10 satellites to examine the applicability of the AVHRR thermal data to the study of urban heat islands. Air and ground surface temperatures measured at meteorological stations in large cities (population over 300,000) in South Korea were compared with in situ brightness temperature data.

The correlation coefficient between air temperatures and brightness temperatures is 0.85 and the relationship may be expressed by the regression: AT=0.59 BT + 2.54. This equation explains 73% of variances at the 0.02% significance level. The best-fit line, however, underestimates air temperatures in such heat-processing industrial cities as Ulsan and Pohang, where smoke puffs up from the high stacks of industrial plants, and overestimates them in the Seoul metropolitan area. The regression equation of ground surface temperatures on brightness temperatures explains 72% of variances. Assuming that optimal meteorological conditions can be selected, the regression equation can be used as a tool to assess air temperature fields in cities.

Urban land-use, such as built-up, residential and industrial areas, was clearly identified from the AVHRR thermal data, while small-scale land-use, like parks, were not distinguishable. Brightness temperatures for the intensity of heat islands were related to the population size of cities. The areal magnitude of heat islands in the Seoul metropolitan area expanded considerably, reflecting the conurbation trend for the period 1986–1989. Urban temperatures in the area increased during the period, while temperature gradients declined.

Aerosol sampling was carried out during December 1989 in Khartoum, Sudan, using Nuclepore membrane filters. Twenty-four aerosol samples were collected and analysed by X-ray fluorescence (XRF) spectrometry and particle-induced X-ray emission (PIXE). In addition, individual particle analysis was also performed on 19 samples using electron probe X-ray microanalysis (EPXMA). Good agreement between XRF and PIXE results was obtained for most of the elements. Enrichment factor calculations indicated that soil dispersion is the dominant source for most elements in the aerosol. However, certain elements showed high enrichment factors indicating the presence of anthropogenic sources. From a comparison with available literature data it appeared that the enrichment factors for the enriched elements in the Khartoum aerosol are among the lowest recorded values for urban aerosol. Absolute principal components analysis (APCA) was performed on the data and confirmed the findings from the enrichment factor calculations, i.e. a dominant soil dispersion source and an anthropogenic source for some of the elements. Because of the very limited number of impotant aerosol sources, the data set was reporduced by the APCA model with a reasonable degree of success. Single particle analysis also showed that most of the particles were soil dust. These particles could further be differentiated into alumino-silicates, quartz and CaCO₃ particles. Some of the particles were found to originate from combustion sources. EPXMA gave clues to the process of formation for some of the particles from combustion sources.

Twenty-four-hour precipitation samples from four sites: Dayalbagh (DB), Hari Parvat (HP), Taj Mahal (TM) and Udyog Kendra (UK) in Agra city, during the monsoon season (July–September) of 1991, were analysed for formate and acetate. Each site was representative of a different level of anthropogenic activity. The formate/acetate ratio observed appeared to be characteristic of the dominant activity at the site; the geometric means of the formate/acetate ratios calculated for individual samples were 0.99, 0.17, 0.83 and 0.21 for DB, HP, TM and UK, respectively. These corresponded to the level of pollution at the site. Direct acetate inputs from extensive combustion and automobile exhaust could contribute to elevated levels of the species at two of the four sites. Another possible indirect input could be from the alkaline hydrolysis of PAN, aided by relatively high pH values of rain water (volume-weighted averages = 6.79, 6.69, 7.22, 7.15) at the four sites.

The black crusts coating the surfaces of building materials located in urban (polluted) environments contain all kinds of organic compounds present in aerosols and particulate matter. Wet and dry deposition processes combined with gypsum crystal growth result in dirty, grey-to-black crust formation, in which aerosols, spores, pollen, dust and every class of particulate matter are entrapped in the mineral matrix. Analysis of the organic compounds extracted from black crusts demonstrate them to be mainly composed of molecular markers that are characteristic of petroleum derivatives. The composition of each crust is governed by the composition of the particular airborne pollutants in the area.

Analysis of hourly O₃ and NO_x data for nine stations for the period 1984–1990 indicates that urban Montreal forms a net sink for O₃ in a regional regime characterized by the long-range transport of O₃ and its precursors northeastward along the Windsor-Quebec corridor. At central city sites and those adjacent to major transport routes, scavenging of O₃ by NO reduces maximum daily O₃ concentrations by ∼50% on average. This effect is strongest in winter, when increased stability and reduced mixing leads to high NO_x concentrations. The combination of scavenging and channeling of flow along the St Lawrence Valley produces distinct and persistent spatial patterns in the O₃ field. The 82 ppb hourly O₃ standard is exceeded simultaneously at two or more stations in Montreal on about 10 days per year. These episodes tend to be associated with persistent, slow moving anticyclonic systems. The considerable interannual variability in the frequency of high ozone episodes is related to interannual variability in the synoptic meteorological conditions conducive to ozone formation and long-range transport. VOC/NO_x ratios of ∼5 ppbc/ppb at central city locations suggest that local NO_x reduction strategies would likely increase ozone concentrations in urban Montreal. Consequently, effective VOC/NO_x control initiatives need to be regional and international in scope.

This paper describes the Urban Airshed Model (UAM) and provides an overview of the history of the model's conception and development, a list of the applications to date, a summary of the model's technical formulation and a description of model inputs and outputs.

The Urban Airshed Model is the U.S. Environmental Protection Agency's (EPA) preferred tool for developing ozone air quality plans for urban areas.