Mei-Kao Liu

On the treatment of point source emissions in urban air quality modeling

Abstract This paper presents the development and evaluation of an urban air quality model, the Plume-Airshed Reactive-Interacting System (PARIS), that is capable of providing a detailed treatment of large point source emissions. The PARIS model treats these large point sources by embedding one or more reactive plume models into the Systems Applications urban airshed model, which is a three-dimensional gridded model governed by the atmospheric diffusion equation. These embedded reactive plume models are used to decribe the chemistry and dynamics of large point source plumes and their interaction with the ambient urban environment. When the plume size becomes comparable to the airshed model grid cell size, a subgrid scale description is no longer necessary and the plume material is mixed into the airshed model grid cells. For purposes of evaluation and comparison, the Systems Applications urban airshed model and the PARIS model were applied to the St. Louis urban area for a one-day simulation and to the South Coast Air Basin (Los Angeles area) for a two-day simulation. Overall absolute errors between predictions and observations for the urban airshed and PARIS model simulations are on the order of 40–50% for O 3 and NO 2 concentrations. The overall differences in absolute errors between airshed and PARIS model predictions are between 1 and 5 %; the difference in overall model performance resulting from the PARIS model subgrid scale treatment of point sources is well within measurement uncertainties for both O 3 and NO 2 concentrations. These results indicate that for these two applications, detailed treatment of large point sources with the PARIS model has little effect on overall model performance. However, the model can provide information necessary for the study of the impact of individual point sources located in an urban environment.

On the validity of grid and trajectory models of urban air pollution

Abstract Of the two types of dynamic urban air pollution models commonly used, grid models emply a coordinate system fixed with respect to the ground, whereas trajectory models have a coordinate system centered on a fictitious vertical air column which moves horizontally with the advective wind. Both models are based on the atmospheric diffusion equation as the essential description of turbulent advection and diffusion. The primary errors committed in the grid model are those attributed to finite-difference approximations, whereas the errors in the trajectory model are mainly a result of the assumptions inherent in the model formulation, such as neglect of horizontal turbulent diffusion across the parcel boundaries and neglect of wind shear. Quantitative conditions based on comparison of exact solutions of the atmospheric diffusion equation with those of the grid and trajectory models are derived to indicate when these two classes of models may be accurately employed for various regimes of stability, wind shear and source configuration.

Development and application of a reactive plume model

Abstract A reactive plume model has been developed to simulate pollutant concentration distributions across a plume emanating from a major point source. The model, based on a simple mass balance, is composed of a fixed number of cells that can expand in a prescribed fashion as the plume travels downwind. For an inert pollutant species, there is no net mass flux across the expanding cell boundaries. For reactive or background pollutants, an equivalent diffusion coefficient is used to characterize the across-the-cell mass fluxes. An up-to-date kinetic module, the carbon-bond mechanism, has been incorporated into this model to treat photochemical reactions involving hydrocarbons and oxides of nitrogen. This model has been applied to the Widows Creek Power Plant in Alabama and the Oak Creek Power Plant in Wisconsin. Comparisons between the observed distributions of several key pollutant concentrations and the corresponding model predictions seem to indicate that the model has been able to simulate the pertinent transport, diffusion and photochemical processes that take place in a reactive plume.

Methodology for designing air quality monitoring networks: I. Theoretical aspects.

An objective methodology is presented for determining the number and disposition of ambient air quality stations in a monitoring network for the primary purpose of compliance with air quality standards. The methodolgy utilizes a data base with real or simulated data from an air quality dispersion model for application with a two-step process for ascertaining the optimal monitoring network. In the first step, the air quality patterns in the data base are collapsed into a single composite pattern through a figure-of-merit (FOM) concept. The most desirable locations are ranked and identified using the resultant FOM fields. In the second step the network configuration is determined on the basis of the concept of spheres of influence (SOI) developed from cutoff values of spatial correlation coefficients between potential monitoring sites and adjacent locations. The minimum number of required stations is then determined by deletion of lower-ranked stations whose SOIs overlap. The criteria can be set to provide coverage of less than some fixed, user-provided percentage of the coverage of tha SOIs of the higher ranked stations and for some desired level of minimum detection capability of concentration fluctuations.The methodology is applied in a companion paper (McElroy et al., 1986) to the Las Vegas, Nevada, metropolitan area for the pollutant carbon monoxide.

Development of a regional oxidant model and application to the Northeastern United States

Abstract Elevated oxidant concentrations due to the long-range transport of ozone and its precursors have been observed in many rural areas in the continental United States. The oxidation processes associated with ozone formation in the atmosphere have many important implications for regional air quality problems, such as regional haze and acid deposition. This paper describes the development and evaluation of a mesoscale photochemical air quality simulation model (RTM-III) covering the northeastern United States. The model considers an area 2080 km in the E-W direction by 1840 km in the N-S direction, with a spatial resolution on the order of 80 x 80 km, and a temporal resolution on the order of one hour. Data collected during an episode in July 1978 by the EPRI Sulfate Regional Experiment is used for testing and evaluating the model. In a comparison of hourly predictions with observations, the model predictions, with a few exceptions where local influences are suspected, generally track the measured spatial pattern and diurnal variation of ozone concentrations quite well. The correlation coefficient matched by time and location over more than 1500 pairs of hourly predicted and observed ozone concentrations is 0.7.

Methodology for designing air quality monitoring networks: II. Application to Las Vegas, Nevada, for carbon monoxide.

An objective methodology presented in a companion paper (Liu et al., 1986) for determining the optimum number and disposition of ambient air quality stations in a monitoring network for carbon monoxide is applied to the Las Vegas, Nevada, area. The methodology utilizes an air quality simulation model to produce temporally-varying air quality patterns for each of a limited number of meteorological scenarios representative of the region of interest. These air quality patterns in turn serve as the data base in a two-step procedure for the identification and ranking of the most desirable monitoring locations (step 1) and the removal of redundancies in spatial coverage among the desired locations (step 2.)The performance of the air quality simulation model, a key element in the design methodology, was evaluated in the Las Vegas area in a special field measurement program. In the Las Vegas demonstration for carbon monoxide, 19 stations covering concentration maxima and 3 stations covering background concentrations in rural areas were identified and ranked. A 10-station network, for example, consisting of 7 stations for peak average concentrations and 3 stations for background concentrations, was selected for a desired minimum detection capability of 50% of concentration variations. Networks with fewer stations would be selected if smaller minimum detection capabilities of concentration variations are acceptable, and vice versa. Background stations could, of course, be deleted for networks with the sole purpose of discerning peak concentrations.

A Mathematical Model for the Analysis of Acid Deposition

Abstract This paper describes the use of a regional-scale air quality model as a diagnostic tool for analyzing problems associated with acid rain. The model, which is hybrid in nature, consists of a puff module and a grid module. The puff module computes the evolution of individual puffs, such as the horizontal and vertical standard deviations of the puff spreads and the location of the center of mass, emitted continuously from each major point source. It also determines the location at which the puff will be released to the grid module and the amount of oxidation and deposition along the trajectory. The grid module then follows the transport, diffusion, and chemical reactions of these aged puffs, as well as emissions from a variety of diffuse sources. Elaborate schemes for both dry and wet deposition have also been incorporated into the model. This model has been exercised for two real-time meteorological scenarios—a dry case and a two-day rainstorm episode in the Northern Great Plains. On the basis of m...

Plume behaviors observed using lidar and SF6 tracer at a flat and hilly site

Two field experiments, one at Kincaid, IL, in flat terrain, the other at Bull Run, TN, in rolling terrain, were conducted under the auspices of the Electric Power Research Institutes (EPRI) Plume Model Validation and Development program. Simultaneous observations were made of ground-level SF6 concentrations; plume cross-sections using light detection and ranging (lidar); turbulence; and routine meteorology at the surface and aloft. Due to terrain influences, surface wind-speeds at the Bull Run site were significantly lower than those at the Kincaid site, whereas thermal winds at Kincaid were generally larger than at Bull Run. At both sites, a reduction in turbulent intensity and an increase in atmospheric stability with height correlate with a substantial decrease in the rate of vertical plume dispersion. SF6 ground-level concentration (GLC) patterns over distances of 1–50 km from the source were categorized by shape. The GLC patterns at Bull Run were frequently ‘blobby’, when significant GLCs occurred over an azimuth angle exceeding 90°, whereas patterns at Kincaid were generally coherent and nearly elliptical. Plume behavior was examined for 154 h during which both GLCs of SF6 tracer and lidar cross-sections of the plume were of good quality. Results show that plume looping was rare at Kincaid, but occurred substantially more often at Bull Run (3%: 14%), with the reverse true for meandering (11%: 14%). Inversions that trapped plume material occurred much more often at Kincaid that at Bull Run (11%: <1%). Correlation of cross-wind concentration distributions of the plume aloft with those cross-wind SF6 concentrations distributions at the ground were poor at both sites.

Evaluation of an episodic regional transport model for a multi-day sulfate episode

Abstract An episodic regional transport model (RTM-II) is applied to a sulfate episode that occurred over the eastern third of the United States and southeastern Canada during the latter part of July 1978. An evaluation of the model performance is presented. Measurements for regional SO2 and sulfate concentrations obtained from up to 54 monitoring stations geographically distributed throughout the modeling region are used. Simulation results are presented in a semi-quantitative manner through region-wide isopleth diagrams and time histories at individual stations. An analysis of the residuals over all measurements, the highest 50 per cent, and highest 10 per cent of the measurements is also included. The model has a tendency to overpredict low concentrations and to underpredict high concentrations for both SO2 and sulfate, though the effect is less pronounced for sulfate. Isopleths and time histories of sulfate predictions match observed concentrations very well though the spatial variability is not as well simulated. Correlation coefficients matched by time and location over all measurements are 0.80 for sulfate and 0.42 for SO2.

The effect of meteorology on the atmospheric dispersion of toxic chemicals in a river valley

Air pollution meteorology related to the dispersion of toxic chemicals in the southeast Ohio River valley was analyzed for a 32-mo period. Attendant air quality measurements of benzene, benzo(a)pyrene, As, Cd, and Cr concentrations were also studied for a 24-mo period. Analysis of the meteorological data showed that wind flow in the valley is significantly affected by the synoptic wind pattern above the hilltop. Sampling at multiple stations showed that daily average concentrations were on the order of 7 μg m−3 for benzene, 2 ng m−3 for benzo(a)pyrene and As, 7 ng m−3 for Cd, and 20 ng m−3 for Cr. Correlations between different species concentrations ranged from 0.25 to 0.6. Based on multi-variable regression analysis, the average benzene concentrations were shown to correlate with the average wind speed and the morning and evening potential temperature gradients at the airport, with a correlation coefficient on the order of 0.5.