Urban and Regional Atmosphere

The urban and regional atmosphere is characterized by anthropogenic emissions of NO and non-methane organic compounds. Also, biogenic 

Warneck (1988) has estimated the annual global emission rates of non-methane organic compounds, expressed in units of 1012 g yr-1  

Biomass burning Biogenic sources Foliage Grasslands Soils 

830 (isoprene, monoterpenes) 47 (light alkanes and higher) 3 (ethene) 

We now discuss briefly the essential features of the atmospheric chemistry of the different organic compound classes. 

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J.H. Seinfeld, Chemistry of ozone in the urban and regional atmosphere, in J.R. Barker (Ed.), Progress and Problems in Atmospheric Chemistry, World Scientific, Singapore, 1995, pp. 34-57. 

If, on the other hand, [NO2]0 = [O3]0 = O.thcn [Oj] = 0. This is clear since in the absence of N02 there is no means to produce atomic oxygen and therefore ozone. Thus the maximum steady-state ozone concentration would be achieved with an initial charge of pure NO2- The mixing ratios of ozone attained in urban and regional atmospheres are often greater than those in the sample calculation. Since most of the NO emitted is in the form of NO and not N02, the concentration of ozone reached, if governed solely by reactions 1-3, cannot account for the actual observed concentrations. It must be concluded that reactions other than 1-3 are important in tropospheric air in which relatively high ozone concentrations occur,... 

T Trban airshed models are mathematical representations of atmospheric transport, dispersion, and chemical reaction processes which when combined with a source emissions model and inventory and pertinent meteorological data may be used to predict pollutant concentrations at any point in the airshed. Models capable of accurate prediction will be important aids in urban and regional planning. These models will be used for ... 

Jef ies, H.E. M. W. Gery, and W. P. L. Carter Protocol for evaluating oxidant mechanisms for urban and regional models. Report for U.S. Environmental Protection Agency Cooperative Agreement No. 815779, Atmospheric Research and Exposure Assessment Laboratoiy, Research Triangle Park, NC. (1992). 

Over the past 15 years, the atmospheric science community has developed a series of mobile platforms with highly accurate and specific fast response instrumentation that have revolutionized atmospheric chemistry field measurements. These include high-altitude aircraft, such as NASA s ER-2 and WB-57, and lower-altitude aircraft like the NASA DC-8, the National Oceanic and Atmospheric Administration (NOAA) and Center for Interdisciplinary Remotely-Piloted Aircraft Studies (CIRPAS) (Naval Postgraduate School) Twin Otters, the National Center for Atmospheric Research (NCAR) C-130, and the DOE Gl. In addition, mobile surface laboratories are now being used for a wide variety of urban and regional air quality and emission source characterization studies.4 Typical configurations for the ER-2 and the mobile laboratory are shown in Figures 1 and 2. 

Table II gives typical ozone and oxides of nitrogen levels in these four regions. Urban- and regional-scale atmospheric chemistry is characterized by the definitive influence of anthropogenic emissions. The goals of a study of urban- and regional-scale atmospheric chemistry are to understand the atmospheric transformations of emitted species to be able to predict the formation of ozone and other pollutants, and to predict the pathways of removal of emitted species and their transformation products from the atmosphere.

The organic peracids and peroxides are important oxidants in the atmosphere. These are responsible for the formation of H2SO4 in the aqueous phase and are thought to have toxic effects on plants. They function as reservoirs for the peroxy radicals and reflect the radical levels of the atmosphere. The organic peracids and peroxides should be given serious attention, especially as we begin to control NOx emission levels in an attempt to reduce urban and regional O3 levels. 

Spatial scales characteristic of various atmospheric chemical phenomena are given in Table 1.1. Many of the phenomena in Table 1.1 overlap for example, there is more or less of a continuum between (1) urban and regional air pollution, (2) the aerosol haze associated with regional air pollution and aerosol-climate interactions, (3) greenhouse gas increases and stratospheric ozone depletion, and (4) tropospheric oxidative capacity and stratospheric ozone depletion. The lifetime of a species is the average time that a molecule of that species resides in the atmosphere before removal (chemical transformation to another species counts as removal). Atmospheric lifetimes vary from less than a second for... 

Because of the expanded scale and need to describe additional physical and chemical processes, the development of acid deposition and regional oxidant models has lagged behind that of urban-scale photochemical models. An additional step up in scale and complexity, the development of analytical models of pollutant dynamics in the stratosphere is also behind that of ground-level oxidant models, in part because of the central role of heterogeneous chemistry in the stratospheric ozone depletion problem. In general, atmospheric Hquid-phase chemistry and especially heterogeneous chemistry are less well understood than gas-phase reactions such as those that dorninate the formation of ozone in urban areas. Development of three-dimensional models that treat both the dynamics and chemistry of the stratosphere in detail is an ongoing research problem. 

Area sources of either a selected chemical or a precursor present a common problem for modeling. In particular, the rich and complex patterns of hydrocarbon emissions from general urban and industrial sources either include or might produce through atmospheric photochemical reactions some of the species on the analysis list. The treatment of such species in photochemical airshed modeling is difficult (8, 9). The effort required for any one such exercise is substantial, and the effort required for a comprehensive analysis of all urban regions relevant to this program would be prohibitive. 

While the focus in terms of acid deposition has been on sulfuric and nitric acids, it has been increasingly recognized that organic acids can also contribute significantly to the acidity of both the gas and aqueous phases in both urban and remote regions. A review of carboxylic acids in the atmosphere is given by Chebbi and Carlier (1996). 

The nitrogen species enter the atmosphere from a variety of natural and anthropogenic sources (7). The largest sources are concentrated in urban and industrialized areas. The levels of the species in the atmosphere vary from hundreds of parts per billion by volume (ppbv, that is, 10 9 mole fraction) in these source regions to below one part per trillion by volume (pptrv, 10"12 mole fraction) in remote areas. Even at the pptrv level, these species can play significant roles in atmospheric chemistry, and measurements of species at the sub-pptrv level can yield useful information concerning atmospheric photochemistry. 

Particle deposition velocities depend on a number of factors, including wind speed, atmospheric stability, relative humidity, particle characteristics (diameter, shape, and density), and receptor surface characteristics. Recent studies on dry particle deposition to surrogate surfaces and derived from atmospheric particle size distributions and micrometeorology suggest that a V equal to about 0.5 cm s 1 is applicable to urban/industrial regions [116-120]. 

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