Dispersion climatology in a coastal zone

A crosswind integrated K-model with wind- and K-profiles described by Monin-Obukhov similarity expressions is solved for a continuous surface release to yield the vertical spread of the plume as a function of the surface roughness z₀ and the Monin-Obukhov length L for a given downwind distance. The vertical spread of the plume is translated into σ_z, and lines were traced in a (z₀, L⁻) plane for which the σ_z of the K-model matched the corresponding σ_z of Pasquill's system. By this technique a new classification scheme is constructed. Knowing z₀ and L, the scheme tells which σ_z curve in the Pasquill system should be used to describe the dispersion.This dispersion classification scheme is used to organize 3 years of data from two meteorological masts, one placed directly at a shoreline and the other roughly 1 km inland. Differences in the dispersion climatology over land and water are studied by averaging the data selectively. The large differences for water and land surfaces between the seasonal and diurnal variation of the dispersion class distributions are illustrated. It is found that the water surface influences the dispersion climatology as far as 20 km inland.     

Selection of an appropriate model to predict plume dispersion in coastal areas

In order to suggest a new methodology for selecting an appropriate dispersion model, various statistical measures having respective characteristics and recommended value ranges were integrated to produce a new single index by using fuzzy inference where eight statistical measures for various model results, including fractional bias (FB), normalized mean square error (NMSE), geometric bias mean (MG), geometric bias variance (VG), within a factor of two (FAC2), index of agreement (IOA), unpaired accuracy of the peak concentration (UAPC), and mean relative error (MRE), were taken as premise part variables. The new methodology using a single index was applied to the prediction of ground-level SO₂ concentration of 1-h average in coastal areas, where eight modeling combinations were organized with fumigation models, σ_y schemes for pre-fumigation, and modification schemes for σ_y during fumigation. As a result, the fumigation model of Lyons and Cole was found to have better predictability than the modified Gaussian model assuming that whole plume is immerged into the Thermal Internal Boundary Layer (TIBL). Again, a better scheme of σ_y (fumigation) was discerned. This approach, which employed the new integrated index, appears to be applicable to model evaluation or selection in various areas including complex coastal areas.     

Spectra of concentration fluctuations: The two time scales of a meandering plume

The integral time scale, T_C, of the concentration fluctuations in a narrow, meandering (intermittent) plume can be much less than the time scale, T_E, of the turbulence in which the plume is immersed, and is generally of the same order of magnitude as the time required for the plume to pass once over a receptor. However the total concentration fluctuation variance is equally strongly influenced by the larger time scale, T_E, associated with meandering caused by ambient turbulence. It is shown that observed energy spectra of concentration fluctuations can be fitted by a linear combination of two Markov spectra, one with the time scale of the meandering motions and another with the time scale of the small scale plume motions. The two components are weighted by I and 1 − I, respectively, where I is the intermittency or fraction of time that non-zero concentrations occur.     

Observations of plume trajectories in the initial momentum influenced phase and parameterization of entrainment

Observations of plume trajectories from a 500 MW power station are reported for that phase of the rise influenced by the initial momentum flux. The mass-radius (a) growth law combined with Briggs' (1975) concept of an effective transverse momentum radius of 1.5a are shown to be suitable empirical concepts for modelling the entrainment of mass and momentum into the integral conservation equations of a wet-plume. Cross-wind pressure effects are shown to be small and related to R₀ but further trials at R₀ > 5 are required to confirm their dependence on R₀. Bifurcating plumes are as well described as conventional plumes with no evidence of stack wake effects at R₀ = 1.8. (S is the distance along the plume centre line which is at an angle φ to the vertical, U_p is the integral plume speed, U_e the wind speed and R_o = U_p/U_e at the outlet.)     

An experimental investigation of some methods of estimating turbulence parameters for use in dispersion models

The standard deviations of wind fluctuations in the horizontal and vertical directions, σ_θ and σ_φ, are now used in some pollution dispersion models to estimate the plume spread parameters σ_y and σ_z. Methods exist for estimating σ_θ and σ_φ when direct measurements are unavailable, using routine weather observations or wind measurements and temperature profiles from meteorological towers. In this paper such estimates are compared with direct measurements made at a height of 56 m, for a sampling time of 1 h, for a range of meteorological conditions. The work was carried out at a site in relatively irregular terrain. This was flat to rolling with a mixed surface cover within 1 km of the tower, with hills rising beyond that distance. Profile measurements were made with robust instruments rather than research grade sensors.Estimates of σ_φ made during the daytime agreed well with measurements, with a bias in the estimates of less than 0.4°. The r.m.s. differences between estimates and measurements were 1.1° (profile method) and less than 2° (routine weather observations method). Daytime σ_θ estimates were generally too low (bias 5–6°), although they were positively correlated with the measurements. At night σ_θ, was severely underestimated, and σ_φ was also underestimated.     

Evaluation of the turbulent parameters of the unstable surface boundary layer outside Businger's range

The dispersion coefficient, σ_z, involves two important turbulent parameters, the friction velocity and the Monin-Obukhov length. We have used Businger's universal functions to compute those parameters which were developed within the range, − 2 ⩽ z / L ⩽ 0. Our purpose is to show that Businger's universal functions perform well outside that range. Correlation coefficients are satisfactory when the roughness length is used as a key to detect good runs. A wider range can be considered and given as follows: − 8 ⩽ z / L ⩽ 0. Two different ranges are defined and given by 3.3 ⩾ z₀ ⩾ 0.4 cmand 6.1 ⩾ z₀ ⩾ 0.2 cm. The iteration method used in this paper can be considered to be quickly convergent and computationally efficient, except for free convection.     

A wind tunnel study of dense gas dispersion in a stable boundary layer over a rough surface

Measurements of the vertical entrainment velocity into two-dimensional dense gas plumes over fully rough surfaces were carried out as part of a co-operative research programme with wind tunnel facilities in the USA. This paper presents results obtained for stable boundary layer conditions in the EnFlo wind tunnel at the University of Surrey; a companion paper treats the neutral boundary layer case. Mean velocity and temperature, turbulent normal and shear tresses, temperature fluctuations and heat fluxes were measured and used to demonstrate that a moderately stable atmospheric boundary layer had been successfully simulated in the tunnel. Entrainment velocities, W_E, were then deduced from the streamwise development of the concentration field, non-dimensionalised with respect to the friction velocity in the undisturbed flow, u^*, and correlated with the plume Richardson number, Ri^*. Higher non-dimensional entrainment speeds, W_E/u^*, were observed for Ri^*>5 in the stable boundary layer than in the neutral boundary layer, the difference growing with increasing Richardson number. Emission velocity ratios, W₀/u^*, were however larger in the stable experiments, and exceeded one at about Ri^*=18. Entrainment in the stable boundary layer appeared therefore to be more sensitive to emission velocity ratio than in the neutral case. Entrainment behaviour for Ri^*⩽5 followed that found in the neutral boundary layer. In this regime, use of the neutral boundary layer entrainment speed correlation is unlikely to lead to the over-prediction of plume dilution rates in moderately stable boundary layers.     

Visibility degradation across Hong Kong: its components and their relative contributions

Daily average levels of nitrogen dioxide (NO₂) and the time-matched, relative humidity-corrected airborne respirable suspended particulate (RSP^*) data from four air quality monitoring stations of the Environmental Protection Department (EPD) Network were subjected to analysis employing a linear multivariate model for visibility in Hong Kong. The respective NO₂ and RSP^* contributions to the extinction coefficient for Hong Kong Observatory, σ_ext(HKO) are estimated to be between 6–31% (mean: 17%) and 58–68% (mean: 62.5%), with a remainder of 11–30% (mean: 21%). The latter includes a small contribution due to Rayleigh scatter. The observed σ_ext agrees reasonably well with model prediction. During the July–October 1999 period, the particle scattering coefficient, σ_sp was determined at City University (CityU) in Kowloon with the aid of an integrated nepthelometer employing 3, 1-h sampling periods. This daily average value of σ_sp(CityU) is well correlated with the corresponding σ_ext(HKO). It is provisionally estimated that the fractional contribution by particulate absorption to σ_ext(HKO) is likely to be less than that by particulate scattering. A reasonable apportionment for σ_ext(HKO) is ∼39% particulate scattering, 19–29% particulate absorption, 17% gaseous absorption, and an estimated mean Rayleigh scatter of ∼4.3%. The unaccounted 16% is due to a negative bias assigned to deficiencies in sampling and in the various assumptions made. Results of continuous, 8-h daytime nephelometric measurements of σ_sp at the CityU, and the water-soluble ambient RSP constituents sampled in tandem at the same site during the February–April 2000 period suggest that SO₄^*, NH₄^* and K are apparently associated with light scattering at CityU. However, only the SO₄^* at this sampling site is related to time-matched σ_ext(HKO). Based on these and other results from a previous study, it is concluded that (i) σ_ext(HKO) is proportional to σ_sp, measured elsewhere in Hong Kong at distances between several to ∼10 km from the HKO headquarters and (ii) most of the fine SO₄ aerosol in Hong Kong is apparently advected into the territory.     

The evolution of photochemical smog in a power plant plume

The evolution of photochemical smog in a plant plume was investigated with the aid of an instrumented helicopter. Air samples were taken in the plume of the Cumberland Power Plant, located in central Tennessee, during the afternoon of 16 July 1995 as part of the Southern Oxidants Study – Nashville Middle Tennessee Ozone Study. Twelve cross-wind air sampling traverses were made at six distance groups from 35 to 116 km from the source. During the sampling period the winds were from the west–northwest and the plume drifted towards the city of Nashville TN. Ten of the traverses were made upwind of the city, where the power plant plume was isolated, and two traverses downwind of the city when the plumes were possibly mixed. The results revealed that even six hours after the release, excess ozone production was limited to the edges of the plume. Only when the plume was sufficiently dispersed, but still upwind of Nashville, was excess ozone (up to 109 ppbv, 50–60 ppbv above background levels) produced in the center of the plume. The concentrations image of the plume and a Lagrangian particle model suggests that portions of the power plant plume mixed with the urban plume. The mixed urban power plant plume began to regenerate O₃ that peaked at 120 ppbv at a short distance (15–25 km) downwind of Nashville. Ozone productivity (the ratio of excess O₃ to NO_y and NO_z) in the isolated plume was significantly lower compared with that found in the city plume. The production of nitrate, a chain termination product, was significantly higher in the power plant plume compared to the mixed plume, indicating shorter chain length of the photochemical smog chain reaction mechanism.     

The scale-free network behavior of ambient volatile organic compounds

A scale-free network model with surface and vertical field measurements was used to identify the connectivity distribution of the scale-free network behavior of ambient volatile organic compounds (VOCs). The results show that the carbon number (C _n ) with the total amount of C _n compounds (P(C _n )) possesses an explicit relationship with the scale-free network behavior. The proportionate coefficient (α) and exponent ( γ) of the scale-free network model with spatial and temporal variations are estimated and discussed. The analytical results demonstrate that although photochemical reactions cause the VOCs fraction variation, they do not alter the fraction of C _n compounds observably. Therefore, the values of α and of γ did not vary with time, but with local regional characteristics. The results indicate that the influence of local VOCs emissions occurs at a height of 100 m, but becomes insufficient at a height of 300 m. Air mass mixing increases with greater height; thus, the influence of regional characteristics at a height of 700 m is low. Finally, a successful empirical model was established to evaluate the distribution of surface VOCs in various regions.     

Octanol/water partition coefficients for environmentally important organic compounds

Partition coefficients P_O,w ⁽ⁱ⁾ describing the distribution of a solute i onto coexisting phases of 1-octanol and water are needed in a large variety of applications. They can be measured directly by HPLC as long as log P_O,w ⁽ⁱ⁾≧ 3.5. For more hydrophobic substances, several experimental procedures have been proposed in the literature. The reliability of those methods is questionable. Therefore, in the present work, P_O,w ⁽ⁱ⁾ is determined experimentally by three HPLC methods using reversed-phase HPLC [1]. Results from different procedures are compared critically. The method of Braumann [2] proved to be superior over the OECD-guidelines [3]. It was therefore applied to determine octanol/water partition coefficients for 23 substances at 25 °C. For eight of those substances (4-methylindole; 9-(hydroxymethyl)anthracene; N-ethylcarbazol; ethylcyclohexane; trans-2-octene; l,l-dimethyl-(ethy])cyclohexane; heptylbenzene; 4-dodecyl-l,3-benzenediol) no experimentally determined number for P_O,w ⁽ⁱ⁾ has been published before.     

Influence of temperature on the chemical removal of 3-methylbutanal,trans-2-methyl-2-butenal,and 3-methyl-2-butenal by OH radicals in the troposphere

Absolute rate coefficients for the gas-phase reactions of OH radical with 3-methylbutanal (k₁), trans-2-methyl-2-butenal (k₂), and 3-methyl-2-butenal (k₃) have been obtained with the pulsed laser photolysis/laser-induced fluorescence technique. Gas-phase concentration of aldehydes was measured by UV absorption spectroscopy at 185 nm. Experiments were performed over the temperature range of 263–353 K at total pressures of helium between 46.2 and 100 Torr. No pressure dependence of all k_i (i = 1–3) was observed at all temperatures. In contrast, a negative temperature dependence of k_i (i.e., k_i increases when temperature decreases) was observed in that T range. The resulting Arrhenius expressions (±2σ) are: k₁(T) = (5.8 ± 1.7)×10^?12 exp{(499 ± 94)/T} cm³ molecule^?1 s^?1, k₂(T)=(6.9 ± 0.9)×10^?12 exp{(526 ± 42)/T} cm³ molecule^?1 s^?1, k₃(T)=(5.6 ± 1.2)×10^?12 exp{(666 ± 54)/T} cm³ molecule^?1 s^?1.The tropospheric lifetimes derived from the above OH-reactivity trend are estimated to be higher for 3-methylbutanal than those for the unsaturated aldehydes. A comparison of the tropospheric removal of these aldehydes by OH radicals with other homogeneous degradation routes leads to the conclusion that this reaction can be the main homogeneous removal pathway. However, photolysis of these aldehydes in the actinic region (λ > 290 nm) could play an important role along the troposphere, particularly for 3-methyl-2-butenal. This process could compete with the OH reaction for 3-methylbutanal or be negligible for trans-2-methyl-2-butenal in the troposphere.     

Short range dispersion experiments in an urban area

We present results from a series of nine tracer experiments studying urban dispersion at distances under 600 m from a ground level point source in Worcester, Massachusetts. Gaussian forms provide a good fit to the lateral concentration distributions. The measured parameters were compared with models based on stability classification and wind direction fluctuations and to other urban and rural experiments. The comparison showed general consistencies with urban models and data, with some evidence for short range enhancement of the plume. The growth of the cross wind dispersion coefficient (σ_y) and of the standard deviation of wind direction fluctuation (σ_θ) with sampling time were investigated and showed considerably slower growth than is commonly assumed.     

A wind tunnel study of dense gas dispersion in a neutral boundary layer over a rough surface

Measurements of the vertical entrainment velocity into two-dimensional dense gas plumes over fully rough surfaces were carried out as part of a co-operative research programme with wind tunnel facilities in the USA. This paper presents results obtained for neutral boundary layer conditions in the EnFlo wind tunnel at the University of Surrey; a companion paper treats the stable boundary layer case. Entrainment velocities, W_E, were deduced from the streamwise development of the concentration field, non-dimensionalised with respect to the friction velocity in the undisturbed flow, u^*, and correlated with the plume Richardson number, Ri^*. Results for Richardson numbers in the range Ri^*<15 were found to be well fitted by the empirical expression: W_E/u^*=0.65/(1+0.2Ri^*). Flow visualisation studies showed layered plume structures with a sharp upper interface at higher Richardson numbers and in this regime turbulent motion below the interface became progressively more intermittent as Ri^* increased. Measured turbulence levels collapsed within such high Richardson number plumes and flow and dispersion were significantly affected by molecular processes. Up-welling above the source was observed when the emission speed exceeded the approach flow friction velocity, though there was no clear evidence that this affected plume behaviour away from the immediate vicinity of the source.     

A simple urban dispersion model tested with tracer data from Oklahoma City and Manhattan

A simple urban dispersion model is tested that is based on the Gaussian plume model and modifications to the Briggs urban dispersion curves. An initial dispersion coefficient (σ_o) of 40 m is assumed to apply in built-up downtown areas, and the stability is assumed to be slightly unstable during the day and slightly stable during the night. Observations from tracer experiments during the Joint Urban 2003 (JU2003) field study in Oklahoma City and the Madison Square Garden 2005 (MSG05) field study in Manhattan are used for model testing. The tracer SF₆ was released during JU2003 near ground level in the downtown area and concentrations were observed at over 100 locations within 4 km from the source. Six perfluorocarbon tracer (PFT) gases were released near ground level during MSG05 and sampled by about 20 samplers at the surface and on building roofs. The evaluations compare concentrations normalized by source release rate, C/Q, for each sampler location and each tracer release, where data were used only if both the observed and predicted concentrations exceeded threshold levels. At JU2003, for all samplers and release times, the fractional mean bias (FB) is about 0.2 during the day (20% mean underprediction) and 0.0 during the night. About 45 –50% of the predictions are within a factor of two (FAC2) of the observations day and night at JU2003. The maximum observed C/Q is about two times the maximum predicted C/Q both day and night. At MSG05, for all PFTs, surface samplers, and release times, FB is 0.14 and FAC2 is about 45%. The overall 60 min-averaged maximum C/Q is underpredicted by about 40% for the surface samplers and is overpredicted by about 25% for the building-roof samplers.     

Deriving sorption indices for the prediction of potential phosphorus loss from calcareous soils

The aim of this study was to develop techniques to evaluate soil phosphorus (P) sorption capacity (PSC) and determine critical soil P levels to predict P loss potential for calcareous soils. Seventy-five soils mostly from Northern China were analyzed for soil P using four extraction methods (water, P_w; carbonate, P_Ols; ammonium oxalate, P_ox; and Mehlich 3, P_M3) as well as PSC derived from single-point (PSC₁₅₀) and multipoint sorption (S _t) isotherms. Strong correlation was found between PSC₁₅₀ and S _t (r ²=0.89, p<0.001). The sum of αCa_M3 and βMg_M3 as an index of PSC (PSC_{(CaM3 + MgM3)}) was most closely related to the maximum amount of P sorbed (S _max) as given by the sum of S _t and soil initial P setting α=0.039 and β=0.462 (r ²=0.80, p<0.001). The degree of P saturation (DPS) was thereafter calculated from PSC_{(CaM3 + MgM3)} (DPS_{(CaM3 + MgM3)}), to which Olsen P (P_Ols) was significantly correlated (r ²=0.82, p<0.001). In a split-line regression from P_w against DPS_{(CaM3 + MgM3)} (r ²=0.87, p<0.05), a change point was identified at 28.1% DPS_{(CaM3 + MgM3)}, which was equivalent to 49.2 mg kg^?1 P_Ols and corresponded to a P_w concentration of 8.8 mg kg^?1. After the change point, a sharp increase in P_w was observed. Our results reveal a new approach to approximating DPS from Ca_M3 and Mg_M3 for calcareous soils without the need to generate a S _max. We conclude that in the absence of an environmental soil test criteria for P, the DPS_{(CaM3 + MgM3)} and P_Ols could be used to predict P loss potential from calcareous soils.