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218             15. Air Pollution Monitoring and Surveillance

       to stagnant meteorological conditions and warm temperatures. Seasonal
       climatological patterns of prevailing winds and frequency of inversions will
       influence the location of sampling stations. Various types of maps are useful
       for establishing topography, population density, and location of sources
       of various pollutants. Wind roses overlaid with emission sources and popu-
       lation densities help to locate the general areas for location of samplers.
         Various types of diffusion models are available which can use as input
       emission patterns, climatological data, and population data to rank sam-
        pling locations by concentration threshold, resolution of peak concentra-
       tions, and frequency of exposure (4) or to rank sampling locations for
        maximum sensitivity to source emission changes, to provide coverage of
        as many sources or to cover as large a geographic area as possible (5).
         The last step in selecting specific sites is based on the following: availabil-
       ity of land and electrical power, security from vandalism, absence of nearby
        structures such as large buildings, probe height (inlet >3 m), and cost.
         An example of matching scale and objective is the determination of CO
       exposure of pedestrians on sidewalks in urban street canyons. The location
        of a station to meet this objective would be an elevation of —3 m on a street
        with heavy vehicular traffic and large numbers of pedestrians.
          Figure 15-1 shows the Los Angeles, California, basin stationary air moni-
        toring network, one of the most extensive in the United States (6). At
        most of these locations, automated instruments collect air quality data
        continuously. Five pollutant gases are monitored, and particulate matter
        filter samples are collected periodically.



                 II. MOBILE MONITORING AND SURVEILLANCE

         Mobile monitoring is accomplished from a movable platform, i.e., an
       aircraft or vehicle. Emissions are measured by source monitoring techniques
        (see Chapter 31). Atmospheric transport and chemical transformation pro-
       cesses occur in the region between the source and the receptor. By using
       mobile platforms containing air pollution instrumentation, one can obtain
       data to help understand the formation and transport of photochemical
        smog, acidic deposition, and the dispersion of air pollutants from sources.
       Mobile monitoring platforms may also be moved to hot spots, areas sus-
       pected of having high concentrations of specific air pollutants. These areas
       may be nearby locations downwind of a large source or a particular location
       that is an unfavorable receptor due to meteorological conditions. Vehicular
       and aircraft monitoring systems can also be moved to locations where
       hazardous chemical spills, nuclear and chemical plant accidents, or volca-
       noes or earthquakes have occurred.
         The major advantage of a mobile monitoring system is its ability to obtain
       air quality information in the intermediate region between source monitors
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