Evaluating the effects of elevated levels of atmospheric trace gases on herbs and shrubs: a prototype dual array field exposure system

In the context of global climate change, an understanding of the long-term effects of increasing concentrations of atmospheric trace gases (carbon dioxide, CO(2), ozone, O(3), oxides of nitrogen, NO(x) etc.) on both cultivated and native vegetation is of utmost importance. Over the years, under field conditions, various trace gas-vegetation exposure methodologies with differing advantages and disadvantages have been used. Because of these variable criteria, with elevated O(3) or CO(2) levels, at the present time the approach of free-air experimental-release of the gas into study plots is attracting much attention. However, in the case of CO(2), this approach (using 15 m diameter study plot with a single circular array of vent pipes) has proven to be cost prohibitive (about 59000-98000 dollars/year/replicate) due to the consumption of significant quantities of the gas to perform the experiment (CO(2) level elevated to 400 ppm above the ambient). Therefore, in this paper, we present a new approach consisting of a dual, concentric exposure array of vertical risers or vent pipes. The purpose of the outer array (17 m diameter) is to vent ambient air outward and toward the incoming wind, thus providing an air curtain to reduce the velocity of that incoming wind to simulate the mode or the most frequently occurring wind speed at the study site. The inner array (15 m diameter) vents the required elevated levels of trace gases (CO(2), O(3), etc.) into the study plot. This dual array system is designed to provide spatial homogeneity (shown through diffusion modeling) of the desired trace-gas levels within the study plot and to also reduce its consumption. As an example, while in the single-array free-air CO(2)-release system the consumption of CO(2) to elevate its ambient concentration by 400 ppm is calculated to be about 980 tons/year/replicate, it is estimated that in the dual array system it would be approximately 590 tons/year/replicate. Thus, the dual array system may provide substantial cost savings (24000-39000 dollars/year/replicate) in the CO(2) consumption (60-100 dollars/ton of CO(2)) alone. Similarly, benefits in the requirements of other trace gases (O(3), NO(x), etc.) are expected, in future multivariate studies on global climate change.     

Wind tunnel study of air entrainment into two-dimensional dense gas plumes at the Chemical Hazards Research Center

Experiments in a neutrally stable wind tunnel boundary layer were made for two-dimensional (quasi-line) sources of carbon dioxide dispersing over two types of uniformly spaced (billboard) surface roughness elements. Velocity and concentration measurements were made with each surface roughness over a wide range of source Richardson number by varying carbon dioxide release rate and wind speed. Concentration measurements were made with a FID gas analyzer using an ethane tracer in the source gas, and velocity measurements were made with independent LDV and HWA systems. For each surface roughness, this paper describes the wind tunnel boundary layer and presents alongwind and vertical concentration profiles in the gas plume. Vertical velocity and concentration profiles were measured at selected downwind distances, and the profiles were integrated to confirm the consistency of the measurements with the mass of carbon dioxide released. The data are intended for development of improved vertical turbulent entrainment correlations for use in dense gas dispersion models applied to hazardous chemical consequence analyses.     

Population Exposure to Toxic Substances: Use of 1970 Census Data

A particulate sampling train has been constructed which satisfies the conflicting requirements of isokinetic sample extraction and constant flow rate through an inertial sizing device. Its design allows a variable fraction of the filtered emission gas to be added to the sample upstream of the inertial sizing device in order to maintain a preselected flow rate through this device while the gas flow rate into the sampling nozzle is adjusted to remain isokinetic with the local duct velocity. The prototype emission gas recycle (EGR) train was constructed by relatively simple modifications of a commercially available Method 5 sampling system, and much of the sampling technique is identical with Method 5 protocol. The train was tested by comparison of parallel runs in the EPA/SRI wind tunnel using redispersed fly ash. In one series of runs performed without inertial sizing devices, the EGR train and a colocated non- EGR train sampled 3-point traverses. The differences of the average mass concentrations measured by the two trains was 3.3 percent, with a standard deviation of 4.7 percent. In the next series of runs, identical cyclone inertial sizing devices were added to each train. The difference of the average mass concentration for these runs was 1.2 percent, with a standard deviation of 5.7 percent.     

Comparison of line– and point–source releases of tracer gases

Field measurements were made of greenhouse gas emissions from a wastewater treatment system using open path monitoring with detection by FTIR spectroscopy. Emission rates were determined by the ratio technique using a sulfur hexafluoride tracer gas released from a line source. As a quality control check, a second tracer gas – ethylene – was released from various single point locations. This paper presents a comparison of the line-source and point-source tracer releases for approximating emissions from the area source. The two types of tracer release showed excellent agreement when both release points were two hundred meters from the FTIR beam path. Data for other release points also were comparable, once differences in vertical dispersion as a function of distance are taken into account.     

Characterizing the Occurrence,Sources, and Variability of Radon in Pacific Northwest Homes

A compilation of data from earlier studies of 172 homes in the Pacific Northwest indicated that approximately 65 percent of the 46 homes tested in the Spokane River Valley/Rathdrum Prairie region of eastern Washington/northern Idaho had heating season indoor radon (²²²Rn) concentrations above the U. S. EPA guideline of 148 Bq m^?3 (4 pCi L^?1). A subset of 35 homes was selected for additional study. The primary source of indoor radon in the Spokane River Valley/Rathdrum Prairie was pressure-driven flow of soil gas containing moderate radon concentrations (geometric mean concentration of 16,000 Bq m^?3) from the highly permeable soils (geometric mean permeability of 5 × 10^?11 m²) surrounding the house substructures. Estimated soil gas entry rates ranged from 0.4 to 39 m³h^?1 and 1 percent to 21 percent of total building air infiltration. Radon from other sources, including domestic water supplies and building materials was negligible. In high radon homes, winter indoor levels averaged 13 times higher than summer concentrations, while in low radon homes winter levels averaged only 2.5 times higher. Short-term variations in indoor radon were observed to be dependent upon indoor-outdoor temperature differences, wind speed, and operation of forced-air furnace fans. Forced-air furnace operation, along with leaky return ducts and plenums, and openings between the substructure and upper floors enhanced mixing of radon-laden substructure air throughout the rest of the building.     

Atmospheric Tracer Techniques and Gas Transport in the Primary Aluminum Industry

The results of 35 Individual SF₆ tracer tests conducted in Norway during 1978 demonstrate the applicability of tracer techniques to the study of a wide variety of pollutant transport problems found in the primary aluminum industry. Tracer methods were employed to determine the efficiency of the pollutant control system over a single reduction cell under a variety of operating conditions. Two tests conducted during normal operation gave efficiencies equal to 100 ±19% and 79 ± 12%, while a test performed during the occurrence of an anode effect yielded an efficiency equal to 66 ± 22%.

Tracer investigations of flow in the wake of a smelter hall indicated that between 1 % and 11 % of secondary, roof-top emissions can become entrained in the recirculation cavity and reenter the hall through the ventilation fresh air supply. These reentry rates were observed for release heights as high as 8 m above the existing roof exhaust duct. Tracer dispersion data collected within 20 building heights of the smelter agreed very well with extrapolations of McEIroy- Pooler dispersion curves for an urban area. Dispersion curves determined from a previous wind tunnel study of flow downwind of an isolated building underestimated dispersion downwind of the vs.melter complex.

The total fluoride mass flow rate measured downwind of a smelter during wet, foggy conditions indicated that wet removal rates of fluorides are in the range 3.2 × 10^?4/s to 6.4 × 10^?4/s. Simulation of the source with several tracer point releases and simultaneous measurement of fluoride and tracer ground-level concentrations downwind of the smelter eliminated the need for measurements of vertical profiles of wind speed and fluoride concentration during the experiment.     

Concentrations of carbon monoxide and methane at two heights above a grass field and their deposition onto the field

To investigate whether wind is a significant driving force in the diffusion of CO and CH₄ from the atmosphere into soil, we measured the concentrations of these two gases at two heights above a temperate grass field in Japan and estimated their deposition velocities using micrometeorological techniques. The concentrations were inversely correlated with wind speed, indicating that the local concentrations were influenced by ground sources. The CO and CH₄ concentrations at 0.33 m were usually lower than those at 1.3 m. Although nocturnal data are suspected to be non-stationary, by selecting several periods when the changes of the concentrations were small but larger than analytical precision, we obtained a CO velocity of 2.9 and 3.9×10⁻² cms⁻¹, agreeing with a CO deposition velocity, 3.4×10⁻² cms⁻¹, obtained by applying a method using CO₂ as a tracer. The CH₄ influx obtained by the method using CO₂ as a tracer was 13 ngm⁻² s⁻¹. The ranges of the CO deposition velocity and CH₄ influx were similar to those obtained in previous studies in grassfields and in a nearby arable field using a closed-chamber technique. This shows that light winds do not greatly accelerate CO and CH₄ uptake by soil.     

挥发性污染物苯在水气界面耦合扩散的浓度分布

为研究苯水体泄漏后水气污染的浓度时空分布，利用挥发性污染物水气耦合扩散模型进行预测计算，预测结果与水槽实验相结合，准确地描述了苯在水气界面耦合扩散的浓度分布规律：沿水流方向（x轴）浓度随时间向前推移，距离投放点越远，浓度峰值越小，出现时刻越晚；沿水流深度方向（y轴）浓度分布关于投放点z=0.20 m纵断面对称，在水气界面和空间变化界面（水槽边缘）浓度出现极小值，随着挥发量的增加，浓度峰值出现的位置由水中推移至大气中；沿水流宽度方向（z轴）浓度由投放点向两侧推移。分析了亨利常数、水流速度及风速对苯耦合扩散浓度分布的影响。     

Energy Consumption Reduction Using Two-Stage Venturi Scrubbers

Performance of a three-component monostatic Doppler sodar was compared with performance of collocated meteorological tower instruments and evaluated for electric utility applications. The sensors were components of an air pollution monitoring network that was upgraded to assess the impact of coal conversion on ambient air quality at Northeast Utilities' Mt. Tom Generating Station situated in rugged terrain in the Connecticut River valley of western Massachusetts. To determine how well sodar and tower measurements agree, and how reliable sodar is for this type of application, meteorological data and system performance statistics obtained from August 1982 through July 1983 were examined. A statistical comparison of wind speed and direction measurements at 50 m indicated good agreement between the two monitoring techniques. High correlations also were observed for data collected during various weather conditions. System reliability was demonstrated by data recovery rates and sodar vertical range capabilities. Monthly data recovery rates exceeded 90 percent and occasionally approached 98 percent after initial break-in problems were resolved. Sodar range was 100 m more than 95 percent of the time and 200 m more than 50 percent of the time. Under optimum conditions, the vertical range extended beyond 200 m.     

Simulation of boundary layer trajectory dispersion sensitivity to soil moisture conditions: MM5 and Noah-based investigation

In this study, the sensitivity of trajectory paths to anomalous soil moisture was analyzed during three different synoptic episodes in June 2006. The MM5 and Noah land surface models were used to simulate the response of the planetary boundary layer. The HYSPLIT model was used for trajectory analysis. It was found that the response in horizontal lower-level wind field was larger at regions where vertical wind velocity changes were also large. In addition, the sensitivity to soil moisture changes was significant and localized where convective activity was well developed and synoptic effects did not dominate. A non-local effect was felt over the rest of the domain where convection was not present since the model atmosphere reacted as a whole to compensate for induced changes in vertical velocity. This finding was supported by the fact that domain averaged vertical velocities changes were of the order of 0.2 cm s^?1 or less at about 650 hPa and about 200 times smaller than modeled local vertical velocity changes. The largest change in horizontal wind field near the surface was found for weak synoptic events on June 11–12 and June 22–23 while the stronger synoptic event of June 17–18 showed smaller differences. These changes in wind field conditions impacted the transport and dispersion of pollutants. To quantify the sensitivity of air quality estimates to soil moisture uncertainty, we have used three well known measures of trajectory differences: the absolute horizontal transport deviation (AHTD), the relative horizontal transport deviation (RHTD) and the absolute vertical transport deviation (AVTD) for an ensemble of 98 trajectories departing from a region well within the computational domain. For the June 11–12 event it was found that for wet and dry soil moisture experiments, AHTD, RHTD, and AVGTD can reach values in the range 60–100 km, 10–20% and 500–900 m at 24 h run time, respectively. For the June 17–18 and June 22–23 events these values of trajectory differences were reduced more than half. These differences in behavior between time periods are largely attributed to the combined effects of synoptic forcing and the sensitivity of planetary boundary layer to soil moisture changes during well developed convection. The implication for air quality studies is that the soil moisture anomaly and related uncertainty in planetary boundary layer response needs to be incorporated in order to construct an ensemble of the most probable scenarios in which pollutants are released and transported throughout a given target region.     

Simulation of the evolution of particle size distributions containing coarse particulate in the atmospheric surface layer with a simple convection-diffusion-sedimentation model

The Fugitive Dust Model (FDM) and Industrial Source Complex (ISC), widely used coarse particulate dispersion models, have been shown inaccurate due to the neglect of vertical variations in atmospheric wind speed and turbulent diffusivity (Vesovic et al., 2001), omission of the gravitational advection velocity, and an underestimation of the ground deposition velocity (Kim and Larson, 2001). A simple, transient two-dimensional convection-diffusion-sedimentation model is proposed to simulate the evolution in particle size distribution of an aerosol ‘puff’ containing coarse particulate in the atmospheric surface layer. Monin-Okhubov similarity theory, accompanied by empirical observations made by Businger et al. (1971), is adopted to characterize the surface layer wind speed and turbulent diffusivity profiles over a wide range of atmospheric conditions. A first order analysis of the crossing trajectories effect suggests simulation data presented here are not significantly affected by particle inertia. The model is validated against Suffield experimental data in which coarse particulate deposition was measured out to a distance of 800 m from the source (Walker, 1965). Good agreement is found for the decay in ground deposits with distance from the source for stable atmospheres. Deposition data was also simulated for unstable atmospheric stratification and the current model was determined to modestly underestimate the peak concentration with increasing accuracy further downwind of the release. The current model's effective deposition velocity was compared to that suggested by Kim et al. (2000) and shows improvement with respect to FDM. Lastly, the model was used to simulate the dispersion of nine lognormal aerosol puffs in the lowest 50 m of the atmospheric surface layer for four classes of atmospheric stability. The simulated mass median aerodynamic diameters (MMAD) at multiple downwind sampling locations were calculated and plotted with distance from the source. The first 50 m from the source was found to have a substantial impact on the evolution of MMAD for stable atmospheric conditions. Away from the source, it was observed that particle size distributions were truncated by removal of all particles larger than about 60 μm. A particle Peclet number was also defined to quantify the relative importance of turbulent dispersion and sedimentation on particle motion in the vertical direction.     

Characterization of the incipient smoke point for steam-/air-assisted and nonassisted flares

Flares are important safety devices for pressure relief; at the same time, flares are a significant point source for soot and highly reactive volatile organic compounds (HRVOCs). Currently, simple guidelines for flare operations to maintain high combustion efficiency (CE) remain elusive. This paper fills the gap by investigating the characteristics of the incipient smoke point (ISP), which is widely recognized as the condition for good combustion. This study characterizes the ISP in terms of 100–% combustion inefficiency (CE), percent opacity, absorbance, air assist, steam assist, air equivalence ratio, steam equivalence ratio, exit velocity, vent gas net heating value, and combustion zone net heating value. Flame lengths were calculated for buoyant and momentum-dominated plumes under calm and windy conditions at stable and neutral atmosphere. Opacity was calculated using the Beer–Lambert law based on soot concentration, flame diameter, and mass-specific extinction cross section of soot. The calculated opacity and absorbance were found to be lognormally distributed. Linear relations were established for soot yield versus absorptivity with R² > 0.99 and power-law relations for opacity versus soot emission rate with R² ≥ 0.97 for steam-assisted, air-assisted, and nonassisted flares. The characterized steam/air assists, combustion zone/vent gas heating values, exit velocity, steam, and air equivalence ratios for the incipient smoke point will serve as a useful guideline for efficient flare operations.

Implications: A Recent EPA rule requires an evaluation of visible emissions in terms of opacity in compliance with the standards. In this paper, visible emissions such as soot particles are characterized in terms of opacity at ISP. Since ISP is widely recognized as most efficient flare operation for high combustion efficiency (CE)/destruction efficiency (DE) with initial soot particles formed in the flame, this characterization provides a useful guideline for flare operators in the refinery, oil and gas, and chemical industries to sustain smokeless and high combustion efficiency flaring in compliance with recent EPA regulations, in addition to protecting the environment.     

Characterizing the occurrence, sources, and variability of radon in Pacific Northwest homes

A compilation of data from earlier studies of 172 homes in the Pacific Northwest indicated that approximately 65 percent of the 46 homes tested in the Spokane River Valley/Rathdrum Prairie region of eastern Washington/northern Idaho had heating season indoor radon (222Rn) concentrations above the U. S. EPA guideline of 148 Bq m-3 (4 pCi L-1). A subset of 35 homes was selected for additional study. The primary source of indoor radon in the Spokane River Valley/Rathdrum Prairie was pressure-driven flow of soil gas containing moderate radon concentrations (geometric mean concentration of 16,000 Bq m-3) from the highly permeable soils (geometric mean permeability of 5 x 10(-11) m2) surrounding the house substructures. Estimated soil gas entry rates ranged from 0.4 to 39 m3h-1 and 1 percent to 21 percent of total building air infiltration. Radon from other sources, including domestic water supplies and building materials was negligible. In high radon homes, winter indoor levels averaged 13 times higher than summer concentrations, while in low radon homes winter levels averaged only 2.5 times higher. Short-term variations in indoor radon were observed to be dependent upon indoor-outdoor temperature differences, wind speed, and operation of forced-air furnace fans. Forced-air furnace operation, along with leaky return ducts and plenums, and openings between the substructure and upper floors enhanced mixing of radon-laden substructure air throughout the rest of the building.     

Nocturnal boundary layer characteristics and land breeze development in Houston,Texas during TexAQS II

The nocturnal boundary layer in Houston, Texas was studied using a high temporal and vertical resolution tethersonde system on four nights during the Texas Air Quality Study II (TexAQS II) in August and September 2006. The launch site was on the University of Houston campus located approximately 4 km from downtown Houston. Of particular interest was the evolution of the nocturnal surface inversion and the wind flows within the boundary layer. The land–sea breeze oscillation in Houston has important implications for air quality as the cycle can impact ozone concentrations through pollutant advection and recirculation. The results showed that a weakly stable surface inversion averaging in depth between 145 and 200 m AGL formed on each of the experiment nights, typically within 2–3 h after sunset. Tethersonde vertical winds were compared with two other Houston data sets (High Resolution Doppler Lidar and radar wind profiler) from locations near the coastline and good agreement was found, albeit with a temporal lag at the tethersonde site. This comparison revealed development of a land breeze on three nights which began near the coastline and propagated inland both horizontally and vertically with time. The vertical temperature structure was significantly modified on one night at the tethersonde site after the land breeze wind shift, exhibiting near-adiabatic profiles below 100 m AGL.     

Performance of Copper Chloride-Impregnated Sorbents on Mercury Vapor Control in an Entrained-Flow Reactor System

Abstract

An entrained-flow system has been designed and constructed to simulate in-flight mercury (Hg) capture by sorbent injection in ducts of coal-fired utility plants. The test conditions of 1.2-sec residence time, 140 °C gas temperature, 6.7 m/sec (22 ft/sec) gas velocity, and 0–0.24 g/m³ (0–15 lbs of sorbent per 1 million actual cubic feet of flue gas [lb/MMacf]) sorbent injection rates were chosen to simulate conditions in the ducts. Four kinds of sorbents were used in this study. Darco Hg-LH served as a benchmark sorbent with which Hg control capability of other sorbents could be compared. Also, Darco-FGD was used as a representative raw activated carbon sorbent. Two different copper chloride-impregnated sorbents were developed in our laboratory and tested in the entrained-flow system to examine the possibility of using these sorbents at coal-fired power plants. The test results showed that one of the copper chloride sorbents has remarkable elemental mercury (Hg⁰) oxidation capability, and the other sorbent demonstrated a better performance in Hg removal than Darco Hg-LH.     

Moving Bed Adsorption System for Control of VOCs from an Aircraft Painting Facility

An activated carbon moving bed system (10 to 100 acf m air flow) was tested for controlling VOC emissions from a commercial aircraft painting facility. The cross-flow moving adsorbent bed showed a VOC collection efficiency in the 77.1 to 99.6 percent range over a superficial gas velocity range of 27 to 185 ft/min (0.14-0.94 m/sec). The collection efficiencies were neither affected by a change in carbon flow rates from 5 to 8 Ib/hr (2.3 to 3.6 kg/hr) nor by a change in the gas superficial velocity from 27 to 185 ft/min. The VOC concentration in the emission stream from the painting hangar was found to vary by at least a factor of 20 (from 0.18 to 15 ppm) both over the five month period (during which the 15 system tests of about three hours each were conducted) and within a single eight hour work shift.     

Simulation of large particle transport near the surface under stable conditions: comparison with the Hanford tracer experiments

A plume model is presented describing the downwind transport of large particles (1–100 μm) under stable conditions. The model includes both vertical variations in wind speed and turbulence intensity as well as an algorithm for particle deposition at the surface. Model predictions compare favorably with the Hanford single and dual tracer experiments of crosswind integrated concentration (for particles: relative bias=−0.02 and 0.16, normalized mean square error=0.61 and 0.14, for the single and dual tracer experiments, respectively), whereas the US EPA's fugitive dust model consistently overestimates the observed concentrations at downwind distances beyond several hundred meters (for particles: relative bias=0.31 and 2.26, mean square error=0.42 and 1.71, respectively). For either plume model, the measured ratio of particle to gas concentration is consistently overestimated when using the deposition velocity algorithm of Sehmel and Hodgson (1978. DOE Report PNL-SA-6721, Pacific Northwest Laboratories, Richland, WA). In contrast, these same ratios are predicted with relatively little bias when using the algorithm of Kim et al. (2000. Atmospheric Environment 34 (15), 2387–2397).     

Predicting and Managing the Health Risks of Sour-Gas Wells

The development of sour-gas resources in Canada and the United States has prompted concerns about the public health risks of accidental releases of gas contaminated with hydrogen sulfide (H₂S) from wells. This paper focuses on methods for improving the prediction and management of those risks. Data associated with the health effects of hydrogen sulfide are examined, and it is suggested that sublethal effects should be addressed in risk assessments of sour-gas wells along with the life-threatening effects normally considered. The demarcation of hazard zones around wells can be improved by using a statistical approach for estimating an upper-bound H₂S release rate; this rate can then be used in an atmospheric dispersion model to estimate maximum distances to downwind concentrations for lethal (300 ppmv) and sublethal (50 ppmv) effects resulting from an accidental release. A vertical release is found to have little impact, especially under stable atmospheric conditions; horizontal releases, on the other hand, result in the greatest downwind distances for health impacts. Management of health risks depends on a mix of safety technologies and contingency actions, such as well-ignition options and provision for post-release monitoring and assessment of ambient H₂S concentrations.     

Source apportionment of PAHs and n-alkanes in respirable particles in Tehran,Iran by wind sector and vertical profile

The vertical concentration profiles and source contributions of polycyclic aromatic hydrocarbons (PAHs) and n-alkanes in respirable particle samples (PM₄) collected at 10, 100, 200 and 300-m altitude from the Milad Tower of Tehran, Iran during fall and winter were investigated. The average concentrations of total PAHs and total n-alkanes were 16.7 and 591 ng/m³, respectively. The positive matrix factorization (PMF) model was applied to the chemical composition and wind data to apportion the contributing sources. The five PAH source factors identified were: ‘diesel’ (56.3 % of total PAHs on average), ‘gasoline’ (15.5 %), ‘wood combustion, and incineration’ (13 %), ‘industry’ (9.2 %), and ‘road soil particle’ (6.0 %). The four n-alkane source factors identified were: ‘petrogenic’ (65 % of total n-alkanes on average), ‘mixture of petrogenic and biomass burning’ (15 %), ‘mixture of biogenic and fossil fuel’ (11.5 %), and ‘biogenic’ (8.5 %). Source contributions by wind sector were also estimated based on the wind sector factor loadings from PMF analysis. Directional dependence of sources was investigated using the conditional probability function (CPF) and directional relative strength (DRS) methods. The calm wind period was found to contribute to 4.4 % of total PAHs and 5.0 % of total n-alkanes on average. Highest average concentrations of PAHs and n-alkanes were found in the 10 and 100 m samples, reflecting the importance of contributions from local sources. Higher average concentrations in the 300 m samples compared to those in the 200 m samples may indicate contributions from long-range transport. The vertical profiles of source factors indicate the gasoline and road soil particle-associated PAHs, and the mixture from biogenic and fossil fuel source-associated n-alkanes were mostly from local emissions. The smaller average contribution of diesel-associated PAHs in the lower altitude samples also indicates that the restriction of diesel-fueled vehicle use in the central area of Tehran has been effective in reducing the PAHs concentration.     

Analysis of air quality data near roadways using a dispersion model

We used a dispersion model to analyze measurements made during a field study conducted by the U.S. EPA in July–August 2006, to estimate the impact of traffic emissions on air quality at distances of tens of meters from an eight-lane highway located in Raleigh, NC. The air quality measurements consisted of long path optical measurements of NO at distances of 7 and 17 m from the edge of the highway. Sonic anemometers were used to measure wind speed and turbulent velocities at 6 and 20 m from the highway. Traffic flow rates were monitored using traffic surveillance cameras. The dispersion model [Venkatram, A., 2004. On estimating emissions through horizontal fluxes. Atmospheric Environment 38, 2439–2446] explained over 60% of the variance of the observed path averaged NO concentrations, and over 90% of the observed concentrations were within a factor of two of the model estimates.Sensitivity tests conducted with the model indicated that the traffic flow rate made the largest contribution to the variance of the observed NO concentrations. The meteorological variable that had the largest impact on the near road NO concentrations was the standard deviation of the vertical velocity fluctuations, σ_w. Wind speed had a relatively minor effect on concentrations. Furthermore, as long as the wind direction was within ±45° from the normal to the road, wind direction had little impact on near road concentrations. The measurements did not allow us to draw conclusions on the impact of traffic-induced turbulence on dispersion. The analysis of air quality and meteorological observations resulted in plausible estimates of on-road emission factors for NO.