S. Trivikrama Rao

A modeling study of the impact of urban trees on ozone

Modeling the e!ects of increased urban tree cover on ozone concentrations (July 13}15, 1995) from Washington, DC, to central Massachusetts reveals that urban trees generally reduce ozone concentrations in cities, but tend to increase average ozone concentrations in the overall modeling domain. During the daytime, average ozone reductions in urban areas (1 ppb) were greater than the average ozone increase (0.26 ppb) for the model domain. Interactions of the e!ects of trees on meteorology, dry deposition, volatile organic compound (VOC) emissions, and anthropogenic emissions demonstrate that trees can cause changes in dry deposition and meteorology, particularly air temperatures, wind elds, and boundary layer heights, which, in turn, a!ect ozone concentrations. Changes in urban tree species composition had no detectable e!ect on ozone concentrations. Increasing urban tree cover from 20 to 40% led to an average decrease in hourly ozone concentrations in urban areas during daylight hours of 1 ppb (2.4%) with a peak decrease of 2.4 ppb (4.1%). However, nighttime (20:00}1:00 EST) ozone concentrations increased due to reduced wind speeds and loss of NO x scavenging of ozone from increased deposition of NO x . Overall, 8-hour average ozone concentration in urban areas dropped by 0.5 ppb (1%) throughout the day. ( 2000 Elsevier Science Ltd. All rights reserved.

TURBULENT DIFFUSION BEHIND VEHICLES: EXPERIMENTALLY DETERMINED TURBULENCE MIXING PARAMETERS

Abstract The wake of a moving vehicle was stimulated using a specially constructed wind tunnel with a moving floor. A ‘block-shaped’ model vehicle was fixed in position over the test-section floor while the floor moved at the freestream air speed to produce a uniform, shear-free, approach flow. This simulates an automobile traveling along a straight highway under calm atmospheric conditions. Vertical and lateral profiles of tracer gas concentration were obtained in the wake. Profiles were taken at distances of 30, 45 and 60 model heights downwind. The equations describing the wake theory were solved numerically to determine the ‘best’ turbulence scale lengths by using wind tunnel data taken at 30 mode heights downwind as the inflow boundary condition and comparing the numerical computation made at 60 heights downwind to the wind tunnel data taken at this location. It was found that the ‘best’ scale lengths were the vehicle width along and across the wake, and height above the surface in the vertical directions, respectively. The ROADWAY model, in which the computer code incorporates the wake theory to predict air pollution concentrations along highways, was modified with these new results and found to better predict the General Motors data than the initial version of the model.

Characteristics of Turbulence and Dispersion of Pollutants Near Major Highways

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...

On the Comparative Assessment of the Performance of Air Quality Models

The reliability and accuracy of various atmospheric dispersion models currently being used by regulatory agencies have not been well documented due to a lack of sufficient field data to verify thes...

Effects of traffic-generated turbulence on near-field dispersion

Abstract Data from the General Motors dispersion experiment are used to assess the characteristics of the traffic-generated turbulence and its effects on the dispersion process near roadways. The evaluation of the bulk statistics and spectra allowed the magnitude and extent of wake turbulence in the horizontal and vertical directions to be identified. Differences are noted in the dependence of the traffic influence on the wind speed for cases of parallel and perpendicular orientations to the roadway. The increase of spectral energy in the high frequency range due to the presence of traffic-generated eddies is used to determine the length scales of the eddies. These, in turn, are used to estimate the-traffic components of the eddy diffusivities and dispersion parameters which can be incorporated into the diffusion models.

A study of pollutant dispersion near highways

Abstract As a part of a major roadway dispersion project undertaken by the New York State Department of Environmental Conservation, several tracer release experiments were conducted on an open, at-grade highway site. The observed concentration profiles indicate that the tracer concentration decreases with increasing distance from the roadway more rapidly for parallel wind-road orientation than for perpendicular wind-road orientation. However, the decrease of concentration with height on the tower at the nearest, downwind roadside receptor is slow for parallel case as compared to perpendicular case. Four Gaussian dispersion models (HIWAY, GM, CALINE-2, AIRPOL-4) and four numerical models (DANARD, MROAD 2, RAGLAND, ROADS) are used to predict the tracer gas concentrations. Of the models tested, GM and HIWAY perform well compared to other Gaussian models. The numerical models performed about the same as the above two Gaussian models. The GM model provides a better simulation by far for parallel wind cases than any of the other models tested. The computed wind flow patterns indicate upward motions over the roadway in parallel wind cases which may be due to the mechanical turbulence generated by the traffic flow. The dispersion parameter values computed from the concentration measurements agree very closely to those prescribed for neutral conditions in the GM model and unstable conditions in the HIWAY model. It is suggested that the stability adjacent to the roadway may be determined either through bulk Richardson number, or wind fluctuation data obtained at the site. Further, a better formulation of the dispersion parameters may be σ z ~ σ σ x , and σ y ~ σ θ x , rather than the power law relationship ( σ z ~ α 1 x b 1 and σ y ~ α 2 x b 2 ) commonly used in the existing highway dispersion models. In this regard, wind fluctuation statistics sampled for a duration of one hour are appropriate for describing the dispersion mechanism near roadways.

Numerical simulation of air pollution in urban areas: Model development

Abstract A three-dimensional, grid-based numerical air pollution model for the estimation of air pollutant concentrations in an urban area is developed. Based on the continuity equation, the modeling system incorporates the combined influences of advective transport, turbulent diffusion, chemical transformation, source emissions and surface removal of air contaminants. Recent developments in plume rise and plume penetration processes, objective wind field analysis procedures and numerical solution techniques incorporated into the model are described.

An Evaluation of Some Commonly Used Highway Dispersion Models

This paper presents an evaluation of four gaussian (GM, HIWAY, AIRPOL-4, CALINE-2), and three numerical (DANARD, MROAD 2, ROADS) models with the tracer gas data collected in the General Motors experiment. Various statistical techniques are employed to quantify the predictive capability of each of the above models. In general, the three numerical models performed rather poorly compared to the gaussian models. For this data set, the model with the best performance in accurately predicting the measured concentrations was the GM model followed in order by AIRPOL-4, HIWAY, CALINE-2, DANARD, MR0AD2, and ROADS. Although the GM model provides by far a better simulation than any of the models tested here, it is skewed toward underprediction. As a screening tool for regulatory purposes, however, HIWAY model would be useful since this model has the highest percentage in the category of overprediction if the concentration data in the range of 50th percentile through 100th percentile are included in the analysis. The ...

Numerical investigation of the effects of boundary-layer evolution on the predictions of ozone and the efficacy of emission control options in the northeastern United States

This paper examines the effects of two different planetary boundary-layer (PBL) parameterization schemes – Blackadar and Gayno–Seaman – on the predicted ozone (O3) concentration fields using the MM5 (Versionxa03.3) meteorological model and the MODELS-3 photochemical model. The meteorological fields obtained from the two boundary-layer schemes have been used to drive the photochemical model to simulate O3 concentrations in the northeastern United States for a three-day O3 episodic period. In addition to large differences in the predicted O3 levels at individual grid cells, the simulated daily maximum 1-h O3 concentrations appear at different regions of the modeling domain in these simulations, due to the differences in the vertical exchange formulations in these two PBL schemes. Using process analysis, we compared the differences between the different simulations in terms of the relative importance of chemical and physical processes to O3 formation and destruction over the diurnal cycle. Finally, examination of the photochemical models response to reductions in emissions reveals that the choice of equally valid boundary-layer parameterizations can significantly influence the efficacy of emission control strategies.

Space–time analysis of precipitation-weighted sulfate concentrations over the eastern US

Abstract This paper illustrates a simple technique of performing space–time analysis of precipitation-weighted SO 4 2− concentration data across the eastern US that were collected by the National atmospheric deposition program. Using a moving average filter and two-dimensional spatial data filtering algorithm on the time series of precipitation-weighted SO 4 2− concentrations, we show that decreases of about 50% have occurred in SO 4 2− concentrations in Minnesota, Wisconsin, and over the northeastern US between 1985 and 1998, generally consistent with SO 2 emissions’ reductions over this period. The decreases in SO 4 2− concentrations tended to be smaller in the midwest and south.