V. METEOROLOGY AND ATMOSPHERIC DYNAMICS OF OZONE and PM FORMATION AND ACCUMULATION (MD)
The SOS program on ozone- and PM-related meteorology and atmospheric dynamics consisted mainly of ambient monitoring studies to define meteorological conditions and scenarios associated with accumulation of high ozone concentrations. Given the extremely complex meteorology in various parts of the SOS region, the SOS studies were not merely a routine application of standard meteorological measurement methods. Substantial evaluation, adaptation, and further development of existing meteorological and atmospheric dynamics measurement methods were included in the SOS program. Furthermore, the findings listed below regarding meteorological conditions and scenarios within the SOS region, in themselves, have primarily local applicability and utility. Comparison of such conditions/scenarios with those in other US regions can only serve the purpose of explaining differences in pollutant climatology among regions. Nevertheless, there is one important implication that has general utility (see below).
MD1. Small-scale features, such as urban plumes, point sources, and convective pumping, played important roles in determining the locations and magnitudes of the maximum ozone concentration in the SOS region (McNider et al., 1993). Because of this mesoscale variability, fine-scale grid resolutions probably will be required in regional models to adequately simulate accumulation of ozone in the SOS region (McNider, 1994).
MD2. Temperature was confirmed to be a very important meteorological factor determining the rate of formation of ozone in many parts of the SOS region (Olszyna et al., 1997; Sillman and Samson, 1995).
MD3. Vertical profiles of isoprene and other ozone precursor VOC with maximum concentrations at heights of 100-300 meters above ground level were observed in the SOS region and were associated with as yet uncertain transient meteorological phenomena. Because of this vertical variability of VOC concentrations in the boundary layer, a measurement strategy involving sampling at 30-60 meters above ground level provides a more robust measure of average hydrocarbon abundance in the mixed layer than a surface sampling strategy (Andronache et al., 1994; Lawrimore et al., 1995).
MD4. A significant portion of the mass exchange between forest canopies and the atmosphere was caused by large, intermittent eddies. The importance of these large eddies in transporting biogenic VOC mass from the canopy into the atmosphere was greatest during periods of enhanced stability. These large intermittent eddies during stagnation conditions may be at least part of the cause of the complex vertical profiles with maximum precursor concentrations observed at 100-300 meters above ground level during rural and urban field experiments (see also finding MD3) (Marsik and Samson, 1994; Marsik et al., 1995).
MD5. Latitude was found to be an important parameter in determining the rapidity of dispersion of power plant and urban plumes in the SOS region (McNider et al., 1993).
MD6. The height of the mixed layer in the Nashville/Middle Tennessee region was strongly dependent on land-use patterns, with the lowest mixing-layer heights being observed over forested areas in this region and much higher mixing heights being observed over agricultural and urban areas in this region. This discovery of substantial land-use-controlled variation in mixing height was made using an airborne ozone LIDAR (White et al., 1999; Angevine et al., 2003).
MD7. During the Nashville/Middle Tennessee Ozone Study in 1995, the typical regional ozone events were dominated by light and variable daytime and nocturnal winds, which were punctuated on many days by short-term, relatively localized convective storms, sometimes with lightning. Regional background concentrations of ozone varied from 40 to 80 ppbv; peak day-time concentrations of ozone in both rural and urban areas varied from about 75-90 ppbv, with some rural and urban areas reaching day-time peaks of 80 to more than 110 ppbv. Emissions leading to these moderately high regional ozone event days were derived from both distant and local sources - biogenic and anthropogenic - point, area, mobile, biogenic, and other natural sources (McNider et al., 1998; Banta et al., 1998).
MD8. The episode that generated the highest ozone concentrations during the 1995 Nashville/Middle Tennessee Ozone Study (138 ppbv) occurred from July 11-15. This episode was dominated by an extraordinarily intense air stagnation event in which the Nashville urban plume was confined directly over the urban area of approximately 600 km2. The regional background concentration during this period was only about 45-63 ppbv. Thus the 75-90 ppbv increase in ozone concentration over the regional background was the result of photochemical smog reactions that occurred within the immediate vicinity of the Nashville metropolitan area. On July 12, 1995 the plume from one of the power plants near Nashville merged with the urban plume (Valente et al., 1998).
MD9.During stagnation conditions in the Nashville/Middle Tennessee Ozone study, nighttime winds tended to dominate pollutant transfer processes. These nighttime winds appeared to be the major mechanism for transporting ozone and its precursors into rural areas under these stagnant weather conditions (McNider et al., 1998, Banta et al., 1998).
MD10. The average daytime ozone deposition velocity over rural areas unperturbed by urban plumes or power plant plumes, as measured by aircraft during the Nashville/Middle Tennessee Ozone Study, was uniformly about 0.5 cm/sec. This measured deposition velocity agrees well with the deposition velocity estimates used in many air quality models. By contrast, ozone deposition velocities measured from aircraft flying above urban areas were highly variable. This high variability in observed deposition velocity over urban areas suggests that current air quality models must be used with great caution in such areas (Meyers et al., 1998).
MD11. The larger the radius, the more horizontally homogeneous the field of a specified wind variable became. The magnitude of the radius for a specified amount of change in a wind variable depends on the wind variable chosen. The radius computed for wind speed for a given location will, in general, not be the same as the radius computed for the u- or v-component. Each variable has its individual degree of degradation with distance from the base location (Norris, 2003).
MD12. Under light wind conditions during the Nashville/Middle Tennessee Ozone Study, substantial (~40%) horizontal variations in daytime mixing height due to the urban-rural contrast in the surface energy balance (an "urban dome of ozone") was observed over the city of Nashville. This dome allowed venting of urban emissions aloft, making them available for horizontal transport but unavailable for vertical mixing downwind of the dome during the day (Banta et al., 1998).
MD13. Cumulus clouds vented pollutants from the boundary layer and reduced the sunlight available for photochemistry during the Nashville-Middle Tennessee Ozone Study. Because direct measurements of cumulus venting are difficult to obtain experimentally, this process was not quantified during SOS' Nashville '95 and Nashville '99 studies or during TexAQS 2000. Deep vertical mixing associated with convective storms may have resulted in some stratosphere/troposphere exchange (Banta et al., 1998).
MD14. Synoptic-scale subsidence associated with high pressure strengthened the boundary-layer capping inversion during the Nashville/Middle Tennessee Ozone Study. This inhibited vertical transport of momentum and pollutants and cumulus convection. This behavior, combined with the stagnant conditions resulting from relaxation of the synoptic-scale pressure gradient, allowed pollutants to accumulate locally during the day (Banta et al., 1998).
MD15. The morning transition caused photochemically aged NOx and VOC and their reaction products from the residual layer to interact with pollutants emitted at night into the shallow nocturnal boundary layer. During the Nashville/Middle Tennessee Ozone Study, the breakup of the nocturnal inversion occurred at an urban site 1 to 2 hours earlier than at three rural sites. In the humid environment in the SOS field studies, surface water vapor mixing ratio was often an excellent meteorological tracer for the timing of the morning transition (White et al., 2002).
MD16. At night during the Nashville/Middle Tennessee Ozone Study, the winds above a shallow (tens of meters) layer at the surface accelerated as the atmosphere decoupled from the surface (White et al., 2002).
MD17. During the Nashville/Middle Tennessee Ozone Study, the nocturnal winds rotated in time in accordance with the principles of the inertial oscillation. McNider et al. (1998) demonstrated the persistent nature of this phenomenon using wind spectra obtained from deployed wind profilers. Under sufficiently weak synoptic forcing, the low-level jet and inertial oscillation dominated nocturnal transport. Trajectories derived from the wind profiler network deployed during this study demonstrated the combined effect of these important features.
MD18. Vertical transport was suppressed at night during the Nashville/Middle Tennessee Ozone Study. In the absence of convective storms, the atmosphere stabilized at night and suppressed any significant vertical transport. In many cases, intermittent turbulence was observed in the nocturnal boundary layer, which may be linked to wind shear associated with a low-elevation jet. The effect of intermittent turbulence on pollutant concentrations at ground level is an important topic of SOS research (Banta et al., 1998; McNider et al., 1998).
MD19. During the Nashville/Middle Tennessee Ozone Study, remote sensors provided reliable measurements of mixing height. A comparison of mixed-layer depth estimates deduced from wind profiler and airborne lidar data showed very good agreement under clear or partly cloudy conditions (White et al., 1999).
This result confirmed that radar wind profilers and lidars are well suited to measure the depth of the mixed layer and its variability.
MD20. During the Nashville/Middle Tennessee Ozone Study, variations in mixing height were found to be related to differences in such ground-surface characteristics as soil and vegetation type as well as surface moisture. These different surface characteristics were reflected in varying energy, ozone, and carbon fluxes at ground level (White et al., 1999).
MD21. During the Nashville/Middle Tennessee Ozone Study, differences in mixing height were most pronounced under light wind conditions. Under stagnant conditions, air parcels tended to dwell over regions of one surface type, which allows surface heating differences to express themselves as variations in mixing height. Stronger flow moves air parcels over many surface types, thus producing a more uniform mixing height (White et al., 1999).
MD22. During the Nashville/Middle Tennessee Ozone Study, the strong differences observed in surface heating between the Nashville urban area and the surrounding agricultural and forested areas resulted in significantly deeper mixed layers over the city, especially under stagnant conditions. Urban mixing heights of 2 km or more were frequently observed; this was as much as 800 m deeper than the mixing heights over adjacent rural areas. (Angevine et al., 2003)
MD23. During the Nashville/Middle Tennessee Ozone Study, mixing processes due to convective or mechanical turbulence acted to smooth out vertical inhomogeneities in the height of the daytime boundary layer (Banta et al., 1998).
MD24. During the Nashville/Middle Tennessee Ozone Study, ozone tended to form horizontal layers or patches that often persisted throughout the night until they were mixed out by the growing boundary layer the next morning. This horizontal layering and patchiness of ozone was caused in large part by minimal vertical mixing under stagnant air conditions during nighttime hours (Banta et al., 1998).
MD25. During the Nashville/Middle Tennessee Ozone Study in 1995, and again during SOS' Nashville '99 ozone study, ozone concentrations in the free troposphere were shown to be affected by long-range transport or stratosphere-troposphere exchange processes. Ozone sonde and aircraft measurements showed that concentrations of ozone and other pollutants in the free troposphere were highly variable and were primarily affected by regional to continental-scale advection of clean or polluted air masses. Another significant process contributing to high tropospheric ozone concentrations is the intrusion of stratospheric air. Through entrainment processes, pollutant concentrations in the lower free troposphere can impact the air quality in the atmospheric boundary layer and at ground level (Banta et al., 1998).
MD26. During TexAQS 2000, synoptically driven winds were found to be the dominant daytime horizontal transport mechanism. Mesoscale circulations caused by topography or land use differences also contributed to daytime transport. Synoptic flow exported the Houston/Ship Channel and Texas City pollution plumes to rural, source-free areas, resulting in ozone concentrations well above the ozone standard far downwind of the Houston metropolitan area. Many of these ozone exceedances were not detected by the surface monitoring network because of the sparse distribution of monitoring sites in rural areas (Senff et al., 2002).
MD26. During TexAQS 2000, peak ozone concentrations downwind of the Houston/Ship Channel were anti-correlated with mixing height. In the Houston area, mixing depth typically increases with distance away from the coast. Thus, transport of the pollution plume from the Houston Ship Channel to coastal areas tended to produce higher peak ozone concentrations than transport to inland areas (Senff et al., 2003).
MD27. During TexAQS 2000, off-shore to on-shore flow reversal was observed very frequently in conjunction with high ozone concentration events. Severe ozone exceedances on flow-reversal days were linked to a combination of two meteorological factors: 1) Light wind conditions that facilitated the buildup of ozone plumes over strong VOC and NOx emissions source areas during the middle of the day, followed by 2) Afternoon sea breeze phenomena that transported aged air masses back over source areas, thus increasing still further the already high ozone concentrations. The distribution of precursors and the severity of the ozone event depended on the morning offshore flow regime, the timing of the sea breeze onset, and the strength of the sea breeze flow reversal (Banta et al., 2005; Senff et al., 2002; Nielsen-Gammon, 2001).
MD28. During TexAQS 2000, other high ozone events were associated with a coupling of the sea breeze flow reversal and the inertial oscillation. These two phenomena are nearly congruent at the latitude of Houston, where they produce a few hours of nearly calm winds during late morning or early afternoon when large-scale winds are light from the south or southeast. Large-scale mean winds must be lighter than a threshold value of about 3 meters per second for flow reversal to occur. On these occasions, flow reversal takes place almost simultaneously throughout the metropolitan area, not only in association with the sea breeze front. When winds are sufficiently light, the likelihood of an ozone exceedance is greater than 50%. Exceedances are also relatively likely when synoptic winds flow from northeast to southwest (Senff et al., 2002; Nielsen-Gammon, 2001; see also McNider et al, 1998).
MD29. During TexAQS 2000, measurements made by the Baylor aircraft downwind of industrial sources in the fall of 2001 suggested that while some industrial plumes are well mixed, other plumes are spatially heterogeneous. The spatially heterogeneous plumes can contain regions with high concentrations of VOC, regions with high concentrations of NOx and regions with high concentrations of both VOC and NOx. Whether a plume is well mixed or heterogeneous is likely to depend on the distance from the source and atmospheric stability conditions (Daum et al., 2002).
MD30. Observations made during TexAQS 2000 indicated that improper treatment of aerosols in mesoscale numerical weather prediction models (PSU-NCAR MM5 and NCEP Eta Models) contributed to forecast errors regarding the amount of solar radiation reaching the surface. Aerosol absorption and scattering decrease the amount of sunlight that reaches the Earth's surface. Solar irradiance estimates were in good agreement with observations for smaller aerosol optical depths (Zamora et al., 2005).
The important implication of scientific findings MD1-MD30, which has general applicability and utility both inside and outside the ten state SOS region, is that a very long list of meteorological variables need to be measured for the purpose of understanding the meteorology and atmospheric dynamics of ozone formation, accumulation, and both vertical and horizontal distribution in the atmosphere. Such variables include: 1) mixing height and its spatial variation,2) atmospheric stability conditions (stagnation vs. advection), 3) variables related to day-time and night-time pollutant transport processes, 4) "heat island" phenomena as they are influenced by the structure and species composition of urban vegetation, land use patterns, and the structure and distribution of buildings, pavement, playgrounds, parks, etc., 5) synoptic scale subsidence,6) transport of biogenic emissions from canopy to atmosphere, 7) pollutant effects on solar radiation (i.e., aerosols and their optical properties), and 8) off-shore and on-shore flow-reversal patterns in coastal areas.