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Atmospheric Environment (v.43, #20)

Editorial board (pp. i).

Outdoor air pollution in close proximity to a continuous point source by Neil E. Klepeis; Etienne B. Gabel; Wayne R. Ott; Paul Switzer (pp. 3155-3167).

Data are lacking on human exposure to air pollutants occurring in ground-level outdoor environments within a few meters of point sources. To better understand outdoor exposure to tobacco smoke from cigarettes or cigars, and exposure to other types of outdoor point sources, we performed more than 100 controlled outdoor monitoring experiments on a backyard residential patio in which we released pure carbon monoxide (CO) as a tracer gas for continuous time periods lasting 0.5–2h. The CO was emitted from a single outlet at a fixed per-experiment rate of 120–400ccmin⁻¹ (∼140–450mgmin⁻¹). We measured CO concentrations every 15s at up to 36 points around the source along orthogonal axes. The CO sensors were positioned at standing or sitting breathing heights of 2–5ft (up to 1.5ft above and below the source) and at horizontal distances of 0.25–2m. We simultaneously measured real-time air speed, wind direction, relative humidity, and temperature at single points on the patio. The ground-level air speeds on the patio were similar to those we measured during a survey of 26 outdoor patio locations in 5 nearby towns. The CO data exhibited a well-defined proximity effect similar to the indoor proximity effect reported in the literature. Average concentrations were approximately inversely proportional to distance. Average CO levels were approximately proportional to source strength, supporting generalization of our results to different source strengths. For example, we predict a cigarette smoker would cause average fine particle levels of approximately 70–110μgm⁻³ at horizontal distances of 0.25–0.5m. We also found that average CO concentrations rose significantly as average air speed decreased. We fit a multiplicative regression model to the empirical data that predicts outdoor concentrations as a function of source emission rate, source–receptor distance, air speed and wind direction. The model described the data reasonably well, accounting for ∼50% of the log-CO variability in 5-min CO concentrations.

Keywords: Secondhand tobacco smoke; Exposure; Carbon monoxide tracer gas; Outdoor proximity effect; Wind; Controlled source; Real-time monitoring; Regression modeling

Photocatalytic degradation of C₅–C₇ alkanes in the gas–phase by Aikaterini K. Boulamanti; Constantine J. Philippopoulos (pp. 3168-3174).

The gas-phase photocatalytic oxidation (PCO) of pentane, i-pentane, hexane, i-hexane and heptane over illuminated titanium at ambient temperatures was studied in a continuous stirring-tank reactor and for different values of VOC feed concentrations and relative humidity levels. Conversions achieved were over 90% for residence times from 50 to 85 s and the only products formed were CO₂ and H₂O, while no catalyst deactivation was observed. The obtained results indicate that the molecular and stereochemical structures of the compounds play an important role in the reaction, as the rate was increasing with higher molecular weight, and the presence of a tertiary carbon atom enhanced the reactivity. It was also observed that the increase of the carbon chain by a methyl group had the same influence in the reaction rate in the case of both pentane and i-pentane, while the ratio of the rates for the linear and branched structure was the same for both C₅ and C₆ isomers. The presence of water in the system had an inhibitory effect in all cases. The PCO kinetics was well fit by a Langmuir–Hinshelwood model, modified so as to take into consideration the influence of water vapour. The rate constants ranged from 1.87 × 10⁻⁷ mol m⁻² s⁻¹ for pentane to 3.03 × 10⁻⁷ mol m⁻² s⁻¹ for heptane, and the VOC adsorption constants from 1.14 10⁴ to 2.83 10⁴ m³ mol⁻¹, while the water adsorption constant was 11.2 m³ mol⁻¹.

Keywords: Alkanes; Humidity; Langmuir–Hinshelwood kinetics; Molecular structure; Stereochemical structure

Comparison of emission rate values for odour and odorous chemicals derived from two sampling devices by N. Hudson; G.A. Ayoko (pp. 3175-3181).

Field and laboratory measurements identified a complex relationship between odour emission rates provided by the US EPA dynamic emission chamber and the University of New South Wales wind tunnel. Using a range of model compounds in an aqueous odour source, we demonstrate that emission rates derived from the wind tunnel and flux chamber are a function of the solubility of the materials being emitted, the concentrations of the materials within the liquid; and the aerodynamic conditions within the device – either velocity in the wind tunnel, or flushing rate for the flux chamber. The ratio of wind tunnel to flux chamber odour emission rates (OU m⁻² s) ranged from about 60:1 to 112:1. The emission rates of the model odorants varied from about 40:1 to over 600:1.These results may provide, for the first time, a basis for the development of a model allowing an odour emission rate derived from either device to be used for odour dispersion modelling.

Keywords: Odour; Wind tunnel; Flux chamber; Emission; Rate; Flux

Gas-phase reaction of ( E)-β-farnesene with ozone: Rate coefficient and carbonyl products by Ivan Kourtchev; Iustinian Bejan; John R. Sodeau; John C. Wenger (pp. 3182-3190).

The gas-phase ozonolysis of ( E)-β-farnesene was investigated in a 3.91m³ atmospheric simulation chamber at 296±2K and relative humidity of around 0.1%. The relative rate method was used to determine the reaction rate coefficient of (4.01±0.17)×10⁻¹⁶cm³molecule⁻¹s⁻¹, where the indicated errors are two least-squares standard deviations and do not include uncertainties in the rate coefficients for the reference compounds (γ-terpinene, cis-cyclooctene and 1,5-cyclooctadiene). Gas phase carbonyl products were collected using a denuder sampling technique and analyzed with GC/MS following derivatization with O-(2,3,4,5,6-pentafluorobenzyl) hydroxylamine (PFBHA). The reaction products detected were acetone, 4-oxopentanal, methylglyoxal, 4-methylenehex-5-enal, 6-methylhept-5-en-2-one, and ( E)-4-methyl-8-methylenedeca-4,9-dienal. A detailed mechanism for the gas-phase ozonolysis of ( E)-β-farnesene is proposed, which accounts for all of the products observed in this study. The results of this work indicate that the atmospheric reaction of ( E)-β-farnesene with ozone has a lifetime of around 1h and is another possible source of the ubiquitous carbonyls, acetone, 4-oxopentanal and 6-methylhept-5-en-2-one in the atmosphere.

Keywords: Sesquiterpenes; (; E; )-β-farnesene; 4-Oxopentanal; Ozonolysis; Rate coefficient; Kinetics

Modeling the impacts of traffic emissions on air toxics concentrations near roadways by Akula Venkatram; Vlad Isakov; Robert Seila; Richard Baldauf (pp. 3191-3199).

The dispersion formulation incorporated in the U.S. Environmental Protection Agency's AERMOD regulatory dispersion model is used to estimate the contribution of traffic-generated emissions of select VOCs – benzene, 1,3-butadiene, toluene – to ambient air concentrations at downwind receptors ranging from 10-m to 100-m from the edge of a major highway in Raleigh, North Carolina. The contributions are computed using the following steps: 1) Evaluate dispersion model estimates with 10-min averaged NO data measured at 7 m and 17 m from the edge of the road during a field study conducted in August, 2006; this step determines the uncertainty in model estimates. 2) Use dispersion model estimates and their uncertainties, determined in step 1, to construct pseudo-observations. 3) Fit pseudo-observations to actual observations of VOC concentrations measured during five periods of the field study. This provides estimates of the contributions of traffic emissions to the VOC concentrations at the receptors located from 10 m to 100 m from the road. In addition, it provides estimates of emission factors and background concentrations of the VOCs, which are supported by independent estimates from motor vehicle emissions models and regional air quality measurements. The results presented in the paper demonstrate the suitability of the formulation in AERMOD for estimating concentrations associated with mobile source emissions near roadways. This paper also presents an evaluation of the key emissions and dispersion modeling inputs necessary for conducting assessments of local-scale impacts from traffic emissions.

Keywords: Air quality; Roadways; Air toxics; Dispersion modeling; Mobile sources; Benzene; Toluene; 1,3-Butadiene

Numerical simulation of dispersion around an isolated cubic building: Comparison of various types of k– ɛ models by Yoshihide Tominaga; Ted Stathopoulos (pp. 3200-3210).

Prediction accuracy of flow and dispersion around a cubic building with a flush vent located on its roof was examined using various k– ɛ models, and numerical results were compared with wind-tunnel data. Four types of turbulence models, i.e., the standard k– ɛ model, the RNG k– ɛ model, the k– ɛ model with Launder and Kato modification and the Realizable k– ɛ model were compared in this study. The standard k– ɛ model provided inadequate results for the concentration field, because it could not reproduce the basic flow structure, such as the reverse flow on the roof. However, revised k– ɛ models provided concentrations in better agreement with the experimental data. The effect of an oblique wind angle and vent locations on the prediction accuracy was also investigated. It was confirmed that the prediction accuracy of the velocity field strongly affected that of the concentration field. The RNG model showed general agreement with the experiment, and was the best of the turbulence models tested. However, it becomes clear that the results for all CFD models show poor prediction accuracy of concentration distribution at the side and leeward surfaces of the building since they all underestimate the concentration diffusion on these regions. The concentrations predicted by all CFD models were less diffusive than those of the experiment.

Keywords: CFD; Dispersion; Building; Turbulence model; Vent

Uptake rate constants and partition coefficients for vapor phase organic chemicals using semipermeable membrane devices (SPMDs) by Walter L. Cranor; David A. Alvarez; James N. Huckins; Jimmie D. Petty (pp. 3211-3219).

To fully utilize semipermeable membrane devices (SPMDs) as passive samplers in air monitoring, data are required to accurately estimate airborne concentrations of environmental contaminants. Limited uptake rate constants ( k_ua) and no SPMD air partitioning coefficient ( K_sa) existed for vapor-phase contaminants. This research was conducted to expand the existing body of kinetic data for SPMD air sampling by determining k_ua and K_sa for a number of airborne contaminants including the chemical classes: polycyclic aromatic hydrocarbons, organochlorine pesticides, brominated diphenyl ethers, phthalate esters, synthetic pyrethroids, and organophosphate/organosulfur pesticides. The k_uas were obtained for 48 of 50 chemicals investigated and ranged from 0.03 to 3.07 m³ g⁻¹ d⁻¹. In cases where uptake was approaching equilibrium, K_sas were approximated. K_sa values (no units) were determined or estimated for 48 of the chemicals investigated and ranging from 3.84E+5 to 7.34E+7. This research utilized a test system (United States Patent 6,877,724 B1) which afforded the capability to generate and maintain constant concentrations of vapor-phase chemical mixtures. The test system and experimental design employed gave reproducible results during experimental runs spanning more than two years. This reproducibility was shown by obtaining mean k_ua values ( n = 3) of anthracene and p, p′-DDE at 0.96 and 1.57 m³ g⁻¹ d⁻¹ with relative standard deviations of 8.4% and 8.6% respectively.

Keywords: SPMD; Air sampling; Uptake; Calibration

Night-time radical chemistry during the TORCH campaign by K.M. Emmerson; N. Carslaw (pp. 3220-3226).

We present one of the most comprehensive studies of night-time radical chemistry to date, from the Tropospheric ORganic CHemistry experiment (TORCH) in the summer of 2003. TORCH provided a wealth of measurements with which to study the oxidizing capacity of the atmosphere. The measurements provided input to a zero-dimensional box model which has been used to study night-time radical chemistry during the campaign. Average night-time predicted concentrations of OH (2.6 × 10⁵ molecule cm⁻³), HO₂ (2.9 × 10⁷ molecule cm⁻³) and [HO₂+ΣRO₂] radicals (2.2 × 10⁸ molecule cm⁻³) were an order of magnitude smaller than those predicted during the daytime. The model under-predicted the night-time measurements of OH, HO₂ and [HO₂+ΣRO₂] radicals, on average by 41%, 16% and 8% respectively. Whilst the model captured the broad features of night-time radical behaviour, some of the specific features that were observed are hard to explain. A rate of radical production assessment was carried out for the whole campaign between the hours of 00:00 and 04:00. Whilst radical production was limited owing to the absence of photolytic reactions, production routes via the reactions of alkenes with O₃ provided an effective night-time radical source. Nitrate radical concentrations were predicted to be 0.6 ppt on average with a peak of 18 ppt on August 9th during a polluted heat wave period. Overall, the nitrate radical contributes about a third of the total initiation via RO₂, mostly through reaction with alkenes.

Keywords: Hydroxyl radical; Nitrate radical; Night-time chemistry; Tropospheric modelling

Sensitivity of modelled sulphate and nitrate aerosol to cloud, pH and ammonia emissions by A.L. Redington; R.G. Derwent; C.S. Witham; A.J. Manning (pp. 3227-3234).

A Lagrangian dispersion model has been used to predict daily sulphate aerosol in 2006 at five UK rural measurement sites and hourly nitrate aerosol in April 2003 at Harwell (UK). The sensitivity of aqueous phase sulphate production to the meteorological input has been investigated. Large differences were found between cloud fraction and cloud liquid water output from the regional and mesoscale Met Office Unified Model. The impact on the sulphate aerosol was relatively small, with the mesoscale meteorology giving better results.Sulphate aerosol production in the aqueous phase was found to be very sensitive to modelled cloud pH. As the cloud becomes acidic sulphate production is greatly limited, conversely if the cloud is basic large amounts of sulphate aerosol are produced. A fixed model pH of 5.8 was found to produce better results than allowing the model to calculate pH which resulted in large over-predictions of measured sulphate aerosol in some episodes.Nitrate aerosol was not sensitive to cloud variables or pH, but showed a slight increase with 30% more ammonia emissions, and a slight decrease with 30% less ammonia.Sulphate production in model runs with fixed pH was not sensitive to changing ammonia emissions, however the sulphate production with modelled pH was very sensitive to plus or minus 30% ammonia. This work suggests that good modelling of ammonia is essential to correct estimation of aqueous phase sulphate aerosol if cloud pH is modelled. It is concluded that modelling to estimate the effect of reduced ammonia emission scenarios on future ambient aerosol levels should also take into account the neutralising effect of ammonia in cloud and hence the effect on aqueous phase production of sulphate.

Keywords: Sulphate; Nitrate; Modelling; pH; Particulates

Particle emission factors during cooking activities by G. Buonanno; L. Morawska; L. Stabile (pp. 3235-3242).

Exposure to particles emitted by cooking activities may be responsible for a variety of respiratory health effects. However, the relationship between these exposures and their subsequent effects on health cannot be evaluated without understanding the properties of the emitted aerosol or the main parameters that influence particle emissions during cooking. Whilst traffic-related emissions, stack emissions and concentrations of ultrafine particles (UFPs, diameter < 100 nm) in urban ambient air have been widely investigated for many years, indoor exposure to UFPs is a relatively new field and in order to evaluate indoor UFP emissions accurately, it is vital to improve scientific understanding of the main parameters that influence particle number, surface area and mass emissions. The main purpose of this study was to characterise the particle emissions produced during grilling and frying as a function of the food, source, cooking temperature and type of oil. Emission factors, along with particle number concentrations and size distributions were determined in the size range 0.006–20 μm using a Scanning Mobility Particle Sizer (SMPS) and an Aerodynamic Particle Sizer (APS). An infrared camera was used to measure the temperature field. Overall, increased emission factors were observed to be a function of increased cooking temperatures. Cooking fatty foods also produced higher particle emission factors than vegetables, mainly in terms of mass concentration, and particle emission factors also varied significantly according to the type of oil used.

Keywords: Ultrafine particle concentration; Indoor emissions; Aggregate; Size distribution; Cooking activities

Modeling Pareto efficient PM10 control policies in Northern Italy to reduce health effects by Enrico Pisoni; Marialuisa Volta (pp. 3243-3248).

High PM10 concentrations can cause human health problems, both related to short-term and long-term exposure to particles. In this work the impact of efficient PM10 control problems in Northern Italy is assessed by means of a two-stage methodology. In the first stage a multi-objective optimization approach is applied. The multi-objective problem defines two control objectives (the emission reduction costs and the air quality index) to be minimized varying the decision variables (precursor emission reductions). The solution of the multi-objective problem is the Pareto efficient PM10 control policies. In the second stage, the ExternE methodology is applied to estimate health impacts and external costs for the efficient emission reduction scenarios computed in the first stage. The methodology has been applied over Lombardia region, one of the most polluted areas in Europe.

Keywords: External costs; Health impacts; Integrated assessment modeling; Pareto efficiency; PM10 air quality policies

Algorithms and analytical solutions for rapidly approximating long-term dispersion from line and area sources by Steven R.H. Barrett; Rex E. Britter (pp. 3249-3258).

Predicting long-term mean pollutant concentrations in the vicinity of airports, roads and other industrial sources are frequently of concern in regulatory and public health contexts. Many emissions are represented geometrically as ground-level line or area sources. Well developed modelling tools such as AERMOD and ADMS are able to model dispersion from finite (i.e. non-point) sources with considerable accuracy, drawing upon an up-to-date understanding of boundary layer behaviour. Due to mathematical difficulties associated with line and area sources, computationally expensive numerical integration schemes have been developed. For example, some models decompose area sources into a large number of line sources orthogonal to the mean wind direction, for which an analytical (Gaussian) solution exists. Models also employ a time-series approach, which involves computing mean pollutant concentrations for every hour over one or more years of meteorological data. This can give rise to computer runtimes of several days for assessment of a site. While this may be acceptable for assessment of a single industrial complex, airport, etc., this level of computational cost precludes national or international policy assessments at the level of detail available with dispersion modelling. In this paper, we extend previous work [S.R.H. Barrett, R.E. Britter, 2008. Development of algorithms and approximations for rapid operational air quality modelling. Atmospheric Environment 42 (2008) 8105–8111] to line and area sources. We introduce approximations which allow for the development of new analytical solutions for long-term mean dispersion from line and area sources, based on hypergeometric functions. We describe how these solutions can be parameterized from a single point source run from an existing advanced dispersion model, thereby accounting for all processes modelled in the more costly algorithms. The parameterization method combined with the analytical solutions for long-term mean dispersion are shown to produce results several orders of magnitude more efficiently with a loss of accuracy small compared to the absolute accuracy of advanced dispersion models near sources. The method can be readily incorporated into existing dispersion models, and may allow for additional computation time to be expended on modelling dispersion processes more accurately in future, rather than on accounting for source geometry.

Keywords: Dispersion modelling; Long-term average concentrations; Area sources; Line sources

Detecting improvement in ambient air toxics: An application to ambient benzene measurements in Houston, Texas by Loren H. Raun; Elena M. Marks; Katherine B. Ensor (pp. 3259-3266).

Traditional regulatory methods for evaluating air toxics have several limitations. Two common methods rely either on self-reported industrial emissions from the Toxics Release Inventory or a single summary statistic such as the average or arithmetic mean. A novel statistical approach for detecting overall long term improvement in ambient air quality is demonstrated using measurements of the air toxic benzene evaluated over five years in Houston, Texas. Through trends of seven key statistical measures, long term improvements were detected at more monitors than would have been found using traditional methods while lack of improvement is highlighted at other monitors. This new approach includes analysis of high and low end concentrations, as well as central tendency, evaluated at specific air toxic human health risk thresholds.

Keywords: Benzene; Houston; Statistical trend; HAP; Human health risk

Characteristics of atmospheric speciated mercury concentrations (TGM, Hg(II) and Hg(p)) in Seoul, Korea by Seung-Hee Kim; Young-Ji Han; Thomas M. Holsen; Seung-Muk Yi (pp. 3267-3274).

Ambient speciated mercury concentrations including total gaseous mercury (TGM), gaseous divalent mercury (Hg(II)), and particulate mercury (Hg(p)) were measured on the roof of the Graduate School of Public Health building in Seoul, Korea from February 2005 to February 2006. The average concentrations were 3.22 ± 2.10 ng m⁻³, 27.2 ± 19.3 pg m⁻³, and 23.9 ± 19.6 pg m⁻³ for TGM, Hg(II), and Hg(p), respectively. Hg(II) and Hg(p) concentrations were higher during the daytime than during the nighttime, probably because of high photochemical activity. Hg⁰ concentrations were not significantly correlated with ozone however a positive correlation between ozone and Hg(II) was found during periods of high humidity. Eighteen days were characterized as pollution events with 24 h average PM_2.5 concentrations >65 μg m⁻³. The average concentrations of TGM and Hg(p) during these events were 1.4–2 times higher than those during non-pollution events. In order to identify the contribution of long-range transported mercury to the enhanced mercury concentrations in Korea, an episode was defined as a period with hourly average TGM and CO concentrations higher than the monthly average TGM and CO concentrations and with significant enhancement of both TGM and CO concentrations for at least 10 h. A total of 70 episodes were identified during the sampling period: 36 local episodes and 34 long-range transport episodes. The mean ΔTGM/ΔCO slope for all episodes was 0.0063 ng m⁻³ ppbv⁻¹ which agreed well with the slope (0.0036–0.0074 ng m⁻³ ppbv⁻¹) found in previous studies that identified long-range transport of TGM from China. The mean slope during non-events was 0.0011 ng m⁻³ ppbv⁻¹. Back-trajectory analysis showed that during episodes, air parcels arrived mostly from the major industrial areas in China ( n = 25, 73%), followed by Japan ( n = 4, 12%), Yellow Sea ( n = 3, 9%), and Russia ( n = 2, 6%).

Keywords: Speciated mercury; Long-range transport; CO; Pollution event; Episode

Lagrangian sampling of 3-D air quality model results for regional transport contributions to sulfate aerosol concentrations at Baltimore, MD, in summer 2004 by T. Duncan Fairlie; James Szykman; Alice Gilliland; R. Bradley Pierce; Chieko Kittaka; Stephanie Weber; Jill Engel-Cox; Raymond R. Rogers; Joe Tikvart; Rich Scheffe; Fred Dimmick (pp. 3275-3288).

We use ensemble-mean Lagrangian sampling of a 3-D Eulerian air quality model, CMAQ, together with ground-based ambient monitors data from several air monitoring networks and satellite (MODIS) observations to provide source apportionment and regional transport vs. local contributions to sulfate aerosol and PM_2.5 concentrations at Baltimore, MD, for summer 2004. The Lagrangian method provides estimates of the chemical and physical evolution of air arriving in the daytime boundary layer at Baltimore. Study results indicate a dominant role for regional transport contributions on those days when sulfate air pollution is highest in Baltimore, with a principal transport pathway from the Ohio River Valley (ORV) through southern Pennsylvania and Maryland, consistent with earlier studies. Thus, reductions in sulfur emissions from the ORV under the EPA's Clean Air Interstate Rule may be expected to improve particulate air quality in Baltimore during summer. The Lagrangian sampling of CMAQ offers an inexpensive and complimentary approach to traditional methods of source apportionment based on multivariate observational data analysis, and air quality model emissions separation. This study serves as a prototype for the method applied to Baltimore. EPA is establishing a system to allow air quality planners to readily produce and access equivalent results for locations of their choice.

Keywords: Air quality; Source apportionment modeling; CAIR; CMAQ; Satellite; MODIS; Sulfate; Particulate matter

Neural modelling of the spatial distribution of air pollutants by H. Pfeiffer; G. Baumbach; L. Sarachaga-Ruiz; S. Kleanthous; O. Poulida; E. Beyaz (pp. 3289-3297).

In this paper, a new method to calculate the average spatial distribution of air pollutants based on diffusive sampling measurements and artificial neural networks evaluation is presented. Most established methods like interpolation algorithms are inflexible or limited in considering important distribution parameters such as emission sources or land use. Of special interest are air quality measurements since they provide a direct view on the actual pollutant level. With diffusive samplers, the average concentration of many gaseous species over a large area can be determined simultaneously. During a project in Cyprus, NO₂ diffusive samplers were exposed at 270 sites in six month-long campaigns throughout one year providing the database for the model described in this paper. A multilayer perceptron was trained with the NO₂ measurement data and distribution parameters like population density and meteorological parameters using a 1×1km grid covering Cyprus. The best fit could be achieved with an emissions inventory including previously simulated concentration plumes and population density data as input nodes for the neural network, resulting in realistic maps of the annual average distribution of NO₂ in Cyprus.

Keywords: Artificial neural networks; Spatial approximation; Air quality modelling; Nitrogen oxides; Diffusive sampling evaluation

Erratum to “Opposing behaviour of organic compounds” at different interfaces. [Atmos. Environ. 42 (2008) 7520–7529] by Galal Elmanfe; Abdelhaq Acharid; Mireille Privat (pp. 3298-3299).

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Atmospheric Environment (v.43, #20)