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  • Author or Editor: S. Trivikrama Rao x
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Jhumoor Biswas and S. Trivikrama Rao

Abstract

This paper examines the uncertainty associated with photochemical modeling using the Variable-Grid Urban Airshed Model (UAM-V) with two different prognostic meteorological models. The meteorological fields for ozone episodes that occurred during 17–20 June, 12–15 July, and 30 July–2 August in the summer of 1995 were derived from two meteorological models, the Regional Atmospheric Modeling System (RAMS) and the Fifth-Generation Pennsylvania State University–National Center for Atmospheric Research Mesoscale Model (MM5). The simulated ozone concentrations from the two photochemical modeling systems, namely, RAMS/UAM-V and MM5/UAM-V, are compared with each other and with ozone observations from several monitoring sites in the eastern United States. The overall results indicate that neither modeling system performs significantly better than the other in reproducing the observed ozone concentrations. The results reveal that there is a significant variability, about 20% at the 95% level of confidence, in the modeled 1-h ozone concentration maxima from one modeling system to the other for a given episode. The model-to-model variability in the simulated ozone levels is for most part attributable to the unsystematic type of errors. The directionality for emission controls (i.e., NOx versus VOC sensitivity) is also evaluated with UAM-V using hypothetical emission reductions. The results reveal that not only the improvement in ozone but also the VOC-sensitive and NOx-sensitive regimes are influenced by the differences in the meteorological fields. Both modeling systems indicate that a large portion of the eastern United States is NOx limited, but there are model-to-model and episode-to-episode differences at individual grid cells regarding the efficacy of emission reductions.

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Jian Zhang and S. Trivikrama Rao

Abstract

Aircraft measurements taken during the North American Research Strategy for Tropospheric Ozone-Northeast field study reveal the presence of ozone concentration levels in excess of 80 ppb on a regional scale in the nocturnal residual layer during ozone episodes. The air mass containing increased concentrations of ozone commonly is found on a horizontal spatial scale of about 600 km over the eastern United States. The diurnal variation in ozone concentrations at different altitudes, ozone flux measurements, and vertical profiles of ozone suggest that ozone and its precursors trapped aloft in the nocturnal residual layer can influence the ground-level ozone concentrations on the following day as the surface-based inversion starts to break up. A simple one-dimensional model, treating both meteorological and chemical processes, has been applied to investigate the relative contributions of vertical mixing and photochemical reactions to the temporal evolution of the ground-level ozone concentration during the daytime. The results demonstrate that the vertical mixing process contributes significantly to the ozone buildup at ground level in the morning as the mixing layer starts to grow rapidly. When the top of the mixing layer reaches the ozone-rich layer aloft, high ozone concentrations are brought down into the mixing layer, rapidly increasing the ground-level ozone concentration because of fumigation. As the mixing layer grows further, it contributes to dilution while the chemical processes continue to contribute to ozone production. Model simulations also were performed for an urban site with different amounts of reduction in the ground-level emissions as well as a 50% reduction in the concentration levels of ozone and its precursors aloft. The results reveal that a greater reduction in the ground-level ozone concentration can be achieved by decreasing the concentrations of ozone and precursors aloft than can be achieved from a reduction of local emissions. Given the regional extent of the polluted dome aloft during a typical ozone episode in the northeastern United States, these results demonstrate the necessity and importance of implementing emission reduction strategies on the regional scale; such regionwide emission controls would reduce effectively the long-range transport of pollutants in the Northeast.

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Robert E. Eskridge and S. Trivikrama Rao

Abstract

The primary objectives of this investigation are to determine the temporal and spacial resolution needed to adequately measure vehicle wake turbulence and the characteristics of turbulence near roadways using the knowledge gained in the General Motors (GM) Sulfate Dispersion Experiment, the Long Island (LI) Expressway Diffusion Experiments and wind tunnel experiments.

Observed wind velocity fluctuations at a fixed point near a roadway are due to three distinct causes: wake turbulence, ambient turbulence and the time variation in the wind velocity as a vehicle's wake passes the observation point, hereafter referred to as wake-passing effect. The wake-passing effect can be separated in the data from the ambient and vehicle wake turbulence because of the special spacing and timing of vehicles used in the GM experiment. The measured wake-passing effect is then compared with vehicle wake model predictions. The wake-passing effect, which is shown to constitute a significant portion of the measurable velocity variance near the roadway, does not diffuse pollutants.

In the Long Island Expressway experiment it was shown that most of the velocity variance associated with the vehicle traffic occurred at frequencies greater than 0.5 Hz. It is shown that the GM velocity data, which were recorded once per second, underestimated the velocity variance in short wavelengths and the magnitude of the wind velocity changes due to the vehicle wake.

Recommendations are made, based on wind tunnel and modeling results, as to the time resolution and vertical spacing that are necessary to resolve vehicle wake turbulence and the role of pseudoturbulence in modeling pollutant diffusion near roadways is discussed.

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Robert E. Eskridge and S. Trivikrama Rao

Abstract

No abstract available

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S. Trivikrama Rao and Zon-Hwa Feng

Abstract

A parameterization scheme is developed to predict the supersaturation and the first two moments of predominantly maritime nuclei distributions during the condensation phase. The supersaturation and mean droplet growth predicted through this parameterization agree very well with those computed from the explicit microphysical equations using 55 spectral intervals.

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Stephen Berman, Jia-Yeong Ku, and S. Trivikrama Rao

Abstract

A study of the temporal and spatial variations of mixing layer height over the Ozone Transport Region of the northeastern United States for the summer of 1995 is presented using meteorological data obtained from the North American Research Strategy for Tropospheric Ozone-Northeast (NARSTO-NE) 1995 field program. Rawinsonde balloon soundings made every 4 h during 13 ozone episode days during NARSTO-NE provided the principal source of upper-air data, supplemented by virtual temperature profiles from five radio acoustic sounder system sites. Forty-four weather stations provided surface data. Daytime mixing depths were estimated using a profile-intersection technique. The height of the surface inversion was used as a measure of the depth of the turbulent boundary layer at night.

For the 13 ozone episode days, the average maximum mixing depth ranged from less than 500 m offshore to greater than 2000 m inland, with most of the increase occurring within the first 100 km of the coastline. The coefficient of variation of maximum mixing depths averaged over the 13 episode days varied from 0.65 at coastal stations to 0.19 at inland locations. Greater variability at the coast may be caused by the interplay of sea-breeze circulations with synoptic wind patterns there. The rate of growth of the mixing depth between 0600 and 1000 EST (UTC − 5 h) averaged 165 m h−1 for all stations, ranging from 20–60 m h−1 at coastal sites to more than 350 m h−1 at inland stations. Ventilation coefficients were about 50% lower on ozone episode days than on nonepisode days from 0700–0900 EST.

For the ozone episode of 13–15 July a comparison was made of mixing depth estimates from three different methods: rawinsonde virtual potential temperature profiles, C2n (the atmosphere’s refractive index structure parameter), and output from running the Fifth-Generation Pennsylvania State University–National Center for Atmospheric Research Mesoscale Model (MM5) version 1, a widely used nonhydrostatic mesoscale model. Estimates obtained from the three methods varied by as much as 200 m at night and by up to 500 m during the daytime. Mixing depths obtained from running MM5 were in good agreement with estimates from the other methods at Gettysburg, Pennsylvania, an inland station, but were 10%–20% too low at New Brunswick, New Jersey, a location within 30 km of the Atlantic coast. The discrepancy may be caused by the model’s 12-km grid spacing being too coarse to locate the marine–continental airmass boundary with high precision.

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S. Trivikrama Rao, Leon Sedefian, and Ulrich H. Czapski

Abstract

The primary objective of this study is to assess the effect of traffic on the turbulence structure and to infer the time and space scales of the eddies generated by the traffic. To this end, time series of wind and temperature were obtained by a three-component sonic anemometer and by copper-constantan thermo-couples adjacent to the Long Island Expressway in New York State. Eddy fluxes of heat and momentum were computed under different atmospheric conditions. Spectral distributions of these parameters were obtained using the fast Fourier transform technique. The flow characteristics in the surface layer are inferred from the wind profiles adjacent to the highway.

Results show a distinct bulge in the high-frequency range of the wind spectrum. This bulge appears only during moderate to heavy traffic conditions and with wind across the highway. This traffic-induced turbulent energy appears to be dominant at mean frequencies to 0.1–1.0 Hz corresponding to eddy sizes of the order of a few meters. Even under quite stable atmospheric conditions, no organized convection due to vehicle exhaust heat can be distinguished in the spectral structure. The aerodynamic drag due to the moving vehicles on the highway is manifested by a pronounced acceleration of wind in the lowest 8 m, especially in the cases of wind directions nearly parallel to the highway. The impact of traffic-induced turbulence on the near-roadway dispersion of air pollutants is also discussed.

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Edith Gégo, P. Steven Porter, Alice Gilliland, and S. Trivikrama Rao

Abstract

Ozone is produced by chemical interactions involving nitrogen oxides (NOx) and volatile organic compounds in the presence of sunlight. At high concentrations, ground-level ozone has been shown to be harmful to human health and to the environment. It has been recognized that ozone is a regional-scale problem and that regionwide control strategies would be needed to improve ozone air quality in the eastern United States. To mitigate interstate transport of ozone and its precursors, the U.S. Environmental Protection Agency issued a regional rule in 1998 known as the “NOx State Implementation Plan (SIP) Call,” requiring 21 states in the eastern United States to reduce their summertime NOx emissions by 30 May 2004. In this paper, the effectiveness of the new emission control measures mandated by the NOx SIP Call is assessed by quantifying the changes that occurred in the daily maximum 8-h ozone concentrations measured at nearly 50 locations, most of which are rural (33 sites of the Clean Air Status and Trend Network and 16 sites of the Air Quality System), over the eastern United States. Given the strong dependence of ozone formation and accumulation on meteorological conditions, the incidence of the latter is first mitigated, and meteorologically adjusted ozone concentrations are extracted using a multiple regression technique. By examining the differences between the cumulative distribution functions of the meteorologically adjusted ozone concentrations, it is shown that ozone concentrations in the eastern United States are now on average 13% less than those prior to the NOx SIP Call. Using back-trajectory analyses, it is also shown that emission controls on the electricity-generating units located in the Ohio River Valley have contributed toward the improvement of ozone air quality in downwind regions, especially east and northeast of the Ohio River Valley.

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