Urban plume

Despite these limitations, mobile monitoring systems have been used to obtain useful information, such as the verification and tracking of the St. Louis, Missouri, urban plume. The measurement of a well-defined urban plume spreading northeastward from St. Louis is shown in Fig. 15-2 (7). These data were collected by a combination of instrumented aircraft and mobile vans. Cross-sectional paths were flown by the aircraft at increasing distances downwind. Meteorological conditions of low wind speed in the same direction helped to maintain this urban plume in a well-defined... 

Fig. 15-2. The St. Louis, Missouri, urban plume. Ozone and profiles at four distances downwind of St. Louis track a detectable urban plume for 150 km. Source Wilson, W. E., Jr., Atmos. Environ. 12, 537-547 (1978).

Describe the chemical behavior of the and ozone concentration profiles of the St. Louis urban plume in Fig. 15-2. What is the reason for the sharp increase of b t and the sharp decrease of ozone in the vicinity of power plants ... 

When the regional wind allows the outflow to take place in primarily one direction and the rising warm urban air moves off with this regional flow, the circulation is termed the urban plume (Fig. 17-22). Under this circumstance, the inflow to the urban center near the surface may also be asymmetric, although it is more likely to be symmetric than the outflow at higher altitudes. 

Some attempts have been made to use reactive hydrocarbons in conjunction with inert chemical tracers to deduce HO concentrations in urban plumes (139,140,141). Difficulties in deducing [HO ] from these experiments have been studied by McKeen et al (142), who conclude that such experiments can underpredict HO concentrations by a factor of 2 when more reactive hydrocarbons are used and parameterization of transport processes is not properly accounted for. 

In addition to observations in Los Angeles, Blumenthal and White have reported measurements of a power-plant plume and an urban plume 35 and 46 km downwind from St. Louis, Nfissouri. Bgute 4-25 shows the evidence of extensive ozone buildup in the urban plume. Simultaneous measurements of scattering coefficient, 6>cat, trace the spread and dilution of suspended particulate material. It is interesting that in the urban plume, which spreads to 20 km in width, the ozone increases while the particulate matter decreases this suggests considerable photochemical production at an altitude of 750 m. Contrary to the statements of Davis and co-workers reported above, the power-plant plume causes a decrease, rather than an increase, in ozone. Nitric oxide in the plume reacts with the ozone as it mixes. This is clearly indicated by the distribution of particulate matter, which acts as a tracer. 

The transport of injurious concentrations of ozone and other oxidants to rural areas downwind from urban centers at numerous locations in the United States appears to be on the increase. Blumenthal et al. conservatively estimated that the urban plume from the Los Angeles area could cause ozone concentrations to exceed the Federal standard of 0.08 ppm at locations as far as 260 km. Other areas where significant rural concentrations of oxidant have been observed are Salt Lake City, Denver, and the Blue Ridge Mountains. 

Pickering, K. E., A. M. Thompson, J. R. Scala, W.-K. Tao, R. R. Dickerson, and J. Simpson, Free Tropospheric Ozone Production Following Entrainment of Urban Plumes into Deep Convection, J. Geophys. Res., 97, 17985-18000 (1992). 

Blake, N. J., S. A. Penkett, K. C. Clemitshaw, P. Anwyl, P. Lightman, A. R. W. Marsh, and G. Butcher, Estimates of Atmospheric Hydroxyl Radical Concentrations from the Observed Decay of Many Reactive Hydrocarbons in Well-Defined Urban Plumes, J. Geophys. Res., 98, 2851-2864 (1993). 

A slightly modified approach has also been used in a number of studies in which the sum of (03 + N02) is plotted against NOz (e.g., St. John et al., 1998). This minimizes the effects of short-term variations in 03 caused by its rapid reaction with NO. Thus, when 03 is titrated by the NO reaction, the measured 03 concentrations will be small however, the N02 generated is a source of 03 through its subsequent photolysis. Flence the sum of (03 + N02) is sometimes chosen as a measure of the ultimate formation of ozone. The ozone production efficiency determined from slopes of plots of (03 + N02) against NOz in the Nashville, Tennessee, area was measured to be typically 5-6 if it was assumed that NOr is not removed by other processes. The production efficiency appeared to be about the same for the general urban plume and for an air mass in which a plume from a power plant was also embedded. Including other losses for NOy such as deposition lowers the estimated production efficiency by about a factor of two (St. John et al., 1998 Nunnermacker et al., 1998). 

Previous studies using l C and 13c/12c measurements in connection with other source apportionment techniques have demonstrated their value in assessing the importance of various sources (3). In winter studies in Portland, OR, at a site in which the impact from wood-burning is large, the fraction of contemporary carbon was near 100% urban plume values of l C for Denver samples were substantially lower (10-5A% contemporary) (8). Previous values reported for Long Island, NY, and Barrow, AK, sites were in the range of 30% contemporary carbon (5), with 13c/12c ratios consistent with a combination of fossil fuel and C-3 plant biogenic sources ( 13c ---26). 

Peak ozone in urban plumes is found most commonly 50-100 km downwind of the city center. Once formed, ozone in urban plumes has an effective lifetime of approximately three days. For this reason, urban plumes with high ozone concentrations can travel great distances. Transport of ozone can be even longer in the middle and upper troposphere, where the lifetime of ozone extends to three months. 

Williams PI, Gallagher MW, Choularton TW, et al. 2000. Aerosol development and interaction in an urban plume. Aerosol Sci Technol 32(2) 120-126. 

The mesoscale transport of urban plumes carrying ozone precursor substances into rural and clean air regions has become a well-studied issue (Coffey and Stassink, 1975 Cleveland et al, 1976 Dimitriades and Altshuller, 1976 Robinson, 1977 Wolff et al., 1977). The evidence indicates... 

Fig. 7-3. Average volume size distributions for continental aerosols. [Adapted from Whitby and Sverdrup (1980).] The measurement data were smoothed and idealized by fitting to them additive log-normal distributions. (1) Background aerosol, very clean (2) normal background aerosol (3) background aerosol disturbed by an urban plume (these data from measurements at Goldstone, California). (4) Average urban aerosol (from data taken at Minneapolis, Minnesota, Denver, Colorado, and various locations in California). The dashed curve gives the volume distribution resulting from the number density distribution for the rural continental aerosol shown in Fig. 7-1. The integrated volumina, given by the area underneath each curve, are shown in the insert.

In a number of studies, field measurements have been utilized to derive N02 lifetimes. For this purpose it is necessary to compare the behavior of N02 with that of a tracer of lower reactivity. Chang et al. (1979) examined data from Los Angeles and from St. Louis in this manner and found N02 lifetimes of 1-2 days by two independent methods. Spicer (1980, 1982) made aircraft measurements in the urban plumes of Phoenix, Arizona, Boston, Massachusetts, and Philadelphia, Pennsylvania. He found daylight lifetimes of more than 1 day in the first study and of 4-8 h in the other two studies. His measurements confirmed that N02 is converted to HN03, particulate nitrate, and peroxyacetyl nitrate. The field studies thus support the estimate given above. 

Field studies of S02 oxidation have been performed mostly in the downwind regions of sufficiently isolated sources. The favored objects were stacks of electric power plants, whose plumes often are identifiable at distances up to 300 km from the source, and larger cities. S02 and particulate sulfate were measured and the data were analyzed in terms of the concentration ratio (S04-)/(S02) + (S04 ) as a function of time or a combination of wind speed and distance. Sampling was done mostly by aircraft, although urban plumes have also been studied by ground-based measurements. Table 10-11 presents results from a variety of such investigations. 

Measurements in urban plumes have in many cases produced conversion rates considerably in excess of those deduced from studies of power plant plumes. Because of their greater spatial extent, urban plumes come into contact with the ground surface more readily than power-plant plumes, so... 

Alkezweeny (1978) 8-11.5 9.8 0.4 Urban plume of St. Louis, Missouri aircraft observations on three days in August 1975. 

Forrest el al. (1979) 0-4 2 2 Urban plume of St. Louis, Missouri, followed by manned balloon on a day in June 1976. 

Miller and Alkezweeny (1980) 1-9 4 1 Urban plume of Milwaukee, Wisconsin, over Lake Michigan, on 2 days in August 1976, 1 day in July 1977. 

McMurry and Wilson (1983), dry McMurry and Wilson (1983), r.h. >75% 0-5 ssl 2 — — Urban plume of Columbus, Ohio, ground-based measurements in July and August 1980. 

Spicer, C. W. (1982). Nitrogen oxide reactions in the urban plume of Boston. Science 215, 1095-1092. 

N03 mixing ratios at night in urban plumes have been observed to reach values of a few hundred ppt, and values up to 40 ppt are common in more remote regimes. N205 mixing ratios of up to 3 ppb have been observed near Boulder, Colorado (Brown et al. 2003a) and up to 200 ppt in the area of San Francisco Bay (Wood et al. 2004). 

In the immediate vicinity of Vorkuta and Inta there were elevated concentrations in suspended solids in snow-melt of the major combustion ash constiments Al and Sr (Fig. 6). Urban plumes for Pb are evident from analysis of snow-melt particulates around Vorkuta and Inta. In addition, elevated concentrations of Ba, Ca and K in snow were found around Vorkuta, which may suggest that combustion ash was the source (Reimann et al. 1996). The entire transect through Usinsk generally reflected background concentrations in suspended solids in snow-melt similar to those observed in pristine areas at both ends of the Inta and Vorkuta transects. 

TABLE I Maximal Ozone Concentrations in Urban Plumes... 

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