Urban scale variation

In the atmosphere peroxy radicals react with NO, NO2, HO2 radicals and other peroxy radicals (R 02). The importance of these reactions is dictated by the abundances of NO, NO2, and HO2 radicals and by the rates of the reactions of RO2 radicals with these species. In the troposphere the concentrations of NO, NO2, and HO2 vary widely, however, for the present purposes reasonable average concentrations are approximately (2.5—10) x 10s cm-3. Under atmospheric conditions, typical rate constants for the reactions of RO2 radicals with NO, NO2, and HO2 radicals lie in the ranges (8-20)xlO-12, (5-10) xlO 12, and (5-15)xl0 12 cm3molecule 1 s, respectively [4]. Hence, on average these reactions are of comparable importance in the atmospheric fate of RO2 radicals. On a local scale one reaction may dominate because of variation in the concentrations of NO (NO and NO2) and HO2 radicals. Thus, in remote marine locations with low NO levels, reaction of RO2 radicals with HO2 will be much more important than in urban air masses with high NO concentrations. 

The use of local theories, incorporating parameters such as the eddy viscosity Km and eddy thermal conductivity Ke, has given reasonable descriptions of numerous important flow phenomena, notably large scale atmospheric circulations with small variations in topography and slowly varying surface temperatures. The main reason for this success is that the system dynamics are dominated primarily by inertial effects. In these circumstances it is not necessary that the model precisely describe the role of turbulent momentum and heat transport. By comparison, problems concerned with urban meso-meteorology will be much more sensitive to the assumed mode of the turbulent transport mechanism. The main features of interest for mesoscale calculations involve abrupt... 

Over most canopies/urban areas, with horizontal scale Lc, as shown in Figure 2.1, there are considerable variations in the types, sizes and layout of obstacles, buildings and streets. Over larger canopies/urban areas there are usually also significant variations in the natural topography around the canopy and ground level within the canopy. 

We conclude that over the continuum scale the determining parameters are the wind speed Uh and turbulence initial parameters of the cloud/plume when it reaches the top of the canopy or, equivalently, the virtual source at the level of the canopy. Using suitable fast approximate models for the flow field over urban areas (e.g. RIMPUFF, FLOWSTAR), the variation of the mean velocity and turbulence above the canopy can be calculated. The FLOWSTAR code (Carruthers et al., 1988 [105]) has been extended to predict how (Uc) varies within the canopy. Dispersion downwind of the canopy can also be estimated using cloud/plume profiles, denoted by Gc,w,GA,w which are shown in Figures 2.20 and 2.22. 

For dispersion in flows with significant variation in direction and speed at different heights and different times, the only reliable modelling method is to track individual fluid particles or track many clouds of particles from the source. The former method is now used for regional and synoptic scale dispersion prediction from localised sources, such as nuclear accidents and volcanoes, e.g. Maryon and Buckland, 1995 [396], Assumptions have to be made about how atmospheric turbulence on scales less than 3Ax diffuses particles as they are advected by the resolved flow field on scale Ax. This method requires large computer resources and then can be computed in minutes. For studying critical events in UK urban areas this method should be considered. 

Minor differences in the HPAC computations are found between experiments using the 3-km mesh and 9-km mesh MM5 data. This is likely due to the relatively smooth terrain of West Texas, such that its effects on the surface flow of the two scales are not very distinguishable. It appears that a 10-km grid mesh weather model would be adequate for an operational warning system over rural areas of relatively consistent terrain. However, this would probably not be the case in a rough urban setting, in which detailed urban terrain with substantial variations within a distance of 10 km must be considered. 

To date, most definitions deal with spatial boundaries set within the Earth system (generally as a closed system, with two forms of openness absorption of energy -predominantly solar - and dissipation of heat into space). Within the spatial scale of Earth, all variations are permitted, from the most local (from rural, village to urban) to national, regional, sub- and full continental to global. 

To provide a more detailed overview of temporal variation patterns in fixed-point monitoring, we will present analyses of several time series of urban air pollution concentration measurements. Hourly concentration data on NO, NO2, and O3, recorded at a SLAMS in central Oklahoma City between October 1998 and June 1999, were obtained from the Oklahoma Department of Environmental Quality. The station, which was designed to be representative of population exposure on the neighborhood scale, was located on a sprawling campus not immediately adjacent to high-traffic streets. The probe was at the unusually high elevation of 15.5 meters above street level. 

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