S. Pal Arya

Modeling and Parameterization of Near-Source Diffusion in Weak Winds

Abstract A critical assessment is made of several different approaches that can be used for modeling near-source diffusion in weak winds, including the calm condition. For the convective boundary layer, the statistical theory approach is used in conjunction with the mixed-layer similarity scaling to compute mean concentrations in the near field. The maximum ground-level concentration and its location relative to a continuous point source are given as functions of the source height and wind speed, all made dimensionless through the use of the mixed-layer similarity scaling. The modal and the suggested parameterizations for turbulence and diffusion are only valid for the convective boundary layer over a flat and homogeneous terrain. For near-neutral and stable conditions, the gradient-transport approach is used. Some analytical models for the near-field concentration under the calm condition are discussed. Recommendations are made for the formulation of more sophisticated numerical models of near-source dif...

A Laboratory Study of the Urban Heat Island in a Calm and Stably Stratified Environment. Part II: Velocity Field

Abstract A fully turbulent, low-aspect-ratio buoyant plume with no initial momentum under calm and stably stratified conditions is produced in a convection tank. The plume is generated by a circular heat island at the bottom of the tank. Two analytical models, a bulk convection model and a hydrostatic model, are developed to formulate similarity relations for the low-aspect-ratio plume. The convective velocity scale wD, suggested by the analytical models, is used as the similarity parameter for both the mean velocity and standard deviations of velocity fluctuations. The normalized standard deviations of horizontal and vertical velocities agree with each other for two heating rates, as well as with field observations in the center of Sapporo, Japan. The suggested scaling and empirical relations based on our experimental results may be applied to the velocity fields of other low-aspect-ratio plumes in calm and stably stratified environments. Further investigations are recommended to confirm the results of t...

Characterizing ammonia emissions from swine farms in eastern North Carolina: part 1--conventional lagoon and spray technology for waste treatment.

Abstract Ammonia (NH3) fluxes from waste treatment lagoons and barns at two conventional swine farms in eastern North Carolina were measured. The waste treatment lagoon data were analyzed to elucidate the temporal (seasonal and diurnal) variability and to derive regression relationships between NH3 flux and lagoon temperature, pH and ammonium content of the lagoon, and the most relevant meteorological parameters. NH3 fluxes were measured at various sampling locations on the lagoons by a flow-through dynamic chamber system interfaced to an environmentally controlled mobile laboratory. Two sets of open-path Fourier transform infrared (FTIR) spectrometers were also used to measure NH3 concentrations for estimating NH3 emissions from the animal housing units (barns) at the lagoon and spray technology (LST) sites.Two different types of ventilation systems were used at the two farms. Moore farm used fan ventilation, and Stokes farm used natural ventilation. The early fall and winter season intensive measurement campaigns were conducted during September 9 to October 11, 2002 (lagoon temperature ranged from 21.2 to 33.6 °C) and January 6 to February 2, 2003 (lagoon temperature ranged from 1.7 to 12 °C), respectively. Significant differences in seasonal NH3 fluxes from the waste treatment lagoons were found at both farms. Typical diurnal variation of NH3 flux with its maximum value in the afternoon was observed during both experimental periods. Exponentially increasing flux with increasing surface lagoon temperature was observed, and a linear regression relationship between logarithm of NH3 flux and lagoon surface temperature (T l) was obtained. Correlations between lagoon NH3 flux and chemical parameters, such as pH, total Kjeldahl nitrogen (TKN), and total ammoniacal nitrogen (TAN) were found to be statistically insignificant or weak. In addition to lagoon surface temperature, the difference (D) between air temperature and the lagoon surface temperature was also found to influence the NH3 flux, especially when D > 0 (i.e., air hotter than lagoon). This hot-air effect is included in the statistical-observational model obtained in this study, which was used further in the companion study (Part II), to compare the emissions from potential environmental superior technologies to evaluate the effectiveness of each technology.

Vertical dispersion from surface and elevated releases : an investigation of a non-Gaussian plume model

Abstract The vertical diffusion of a passive tracer released from surface and elevated sources in a neutrally stratified boundary layer has been studied by comparing field and laboratory experiments with a non-Gaussian K-theory model that assumes power-law profiles for the mean velocity and vertical eddy diffusivity. Several important differences between model predictions and experimental data were discovered: 1) the model overestimated ground-level concentrations from surface and elevated releases at distances beyond the peak concentration; 2) the model overpredicted vertical mixing near elevated sources, especially in the upward direction; 3) the model-predicted exponent α in the exponential vertical concentration profile for a surface release [C(z) exp(−zα)] was smaller than the experimentally measured exponent. Model closure assumptions and experimental shortcomings are discussed in relation to their probable effect on model predictions and experimental measurements.

Finite-Difference Errors in Estimation of Gradients in the Atmospheric Surface Layer

Abstract Errors associated with the linear and the logarithmic finite-difference approximations of gradients in the surface layer are computed using the well-known Monin-Obukhov flux-profile relations. It is shown that while the logarithmic approximation is much superior to the linear one in near-neutral and unstable conditions, the latter is equally good or better for stable conditions. The computed ratios of the estimated to the “exact” gradients are given as functions of stability and the ratio between the two heights used for finite-difference approximation. The results can be used for correcting the estimated gradients using either linear or logarithmic approximation over wide ranges of stability and height ratio.

Plume descriptors derived from a non-Gaussian concentration model

Abstract Equations for point-source releases are derived from the non-Gaussian solution to the diffusion equation for (l) the first four moments of the vertical concentration distribution (centroid, variance, skew-ness, and kurtosis), (2) the magnitude and downwind location of the maximum ground-level concentration, and (3) the plume advection velocity. Equations are obtained for both ground-level and elevated sources. Where applicable, results are compared to wind-tunnel and field measurements and/or to other less generalized equations.

A numerical model of the transport and diffusion of Peronospora tabacina spores in the evolving atmospheric boundary layer

Abstract Numerical solutions of the diffusion equation of Peronospora tabacina spores from a finite-area source over flat terrain in the evolving convective boundary layer are presented. Temporal variations in the release of spores, atmospheric stability, wind speed, and eddy diffusivity are considered. The model also includes the vertical variations of wind and eddy diffusivity. The model results indicate that ground level concentrations decrease with time as wind speed and eddy diffusivity increase in the evolving convective boundary layer. The loss of P. tabacina spores due to deposition at the surface also decrease with increasing instability and wind speed. Deposition is found to be particularly important close to the source area.

Ozone in the urban southeastern United States

Ozone measurements (daily maximum values) from the Aerometric Information Retrieval System database are analyzed for selected sites, during 1980 to 1988, in southeastern USA. Frequency distributions, for most sites during most years, show a typical bell-shaped curve with the higher frequency around the yearly daily maximum ozone mean of about 100 to about 110 microg m(-3) (50-55 ppbv). Abnormal years in ozone concentration may skew the distribution as the mean shifts. A correlation of daily maximum ozone concentrations above 140 microg m(-3) (70 ppbv) between sites shows a division between the sites in the northern protion of the region and those in the southern portion of the region. Variations in ozone levels are well correlated over distances of several hundred kilometers, suggesting that high values are associated with synoptic scale episodes. An ozone exposure analysis also shows higher ozone exposures (250-300 ppm days) in the northerly sites as compared to the southerly sites (150-170 ppm days).

Modelling and analysis of the atmospheric nitrogen deposition in North Carolina

The United States Environmental Protection Agencys Community Multiscale Air Quality (CMAQ) regional-scale model is used to study concentrations and dry deposition of nitrogen species in North Carolina (NC) during the summer season. Each modelled and measured species featured a similar diurnal trend. A process budget analysis (production and removal evaluation) of NO, NO2, and NOY depicted the models capability to evaluate various process contributions. Dry deposition of NH3 contributed 34.2 ± 57.9 µg N m-2 hr-1; whereas HNO3 contributed slightly larger dry deposition of nitrogen, 35.2 ± 16.0 µg N m-2 hr-1, in NC. NH4+ and NO3- hourly-averaged wet deposition fluxes were calculated as 37.3 ± 19.7 µg N -2 hr-1 and 40.6 ± 11.8 µg N m-2 hr-1, respectively. Examination of total nitrogen deposition during the summer season in NC found that NH3 contributes approximately 50% of the dry deposition and NO3- contributes approximately 50% of the wet deposition.

Atmospheric Boundary Layer and Its Parameterization

Mean flow and turbulence structure of atmospheric boundary layers over homogeneous surfaces is discussed. The basic concepts of static and dynamic stability, together with the relevant stability criteria and parameters are described. Fundamentals of turbulence are introduced with simpler gradient transport and similarity theories. Finally, methods of estimating or parameterizing surface fluxes of momentum and heat, mixing height, wind distribution and turbulence in the atmospheric boundary layer for applications to atmospheric circulation and dispersion modeling are discussed. These are largely based on similarity theory relations with input from field experiments and observations.