Atmospheric Environment (v.35, #20)

The entrapment of particles by windbreaks by M.R. Raupach; N. Woods; G. Dorr; J.F. Leys; H.A. Cleugh (3373-3383).
A theory is developed for calculating the entrapment of particles by a windbreak, with four results. (1) The fraction of particles in the oncoming flow which pass through the windbreak, or transmittance of the windbreak for particles (σ), is related to the optical porosity (τ). The very simple approximation σ=τ works well for most applications involving the interception of spray droplets by windbreaks. Results from a field experiment agree with the theoretical predictions. (2) A new equation for the bulk drag coefficient of a windbreak is tested against numerical, wind tunnel and field experiments. This enables the bleed velocity for the flow through the windbreak to be predicted in terms of the screen pressure coefficient (k) of the barrier. (3) The relationship between k and τ is different for a vegetative barrier than for a screen across a confined duct, implying a lower k for given τ. (4) The total deposition of particles to a windbreak is determined by a trade-off between particle absorption and throughflow, implying an optimum value of τ for maximum total deposition. For particles larger than 30 μm and vegetation elements smaller than 30 mm, this occurs near τ=0.2.
Keywords: Particle deposition; Spray deposition; Vegetation; Shelter; Porosity;

Investigations of emissions and heterogeneous formation of HONO in a road traffic tunnel by R. Kurtenbach; K.H. Becker; J.A.G. Gomes; J. Kleffmann; J.C. Lörzer; M. Spittler; P. Wiesen; R. Ackermann; A. Geyer; U. Platt (3385-3394).
Simultaneous measurements of nitrous acid (HONO) and nitrogen dioxide (NO2) using a differential optical absorption spectroscopy system, nitrogen oxide (NO) by an in situ chemiluminescence analyser and carbon dioxide (CO2) by a gas chromatographic technique were carried out in the Wuppertal Kiesbergtunnel. At high traffic density HONO concentrations of up to 45 ppbV were observed. However, at low traffic density unexpectedly high HONO concentrations of up to 10 ppbV were measured caused by heterogeneous HONO formation on the tunnel walls. In addition to the tunnel campaigns, emission measurements of HONO, NO2, NO and CO2 from different single vehicles (a truck, a diesel and a gasoline passenger car) were also performed. For the correction of the HONO emission data, the heterogeneous HONO formation on the tunnel walls was quantified by two different approaches (a) in different NO2 emission experiments in the tunnel without traffic and (b) on tunnel wall residue in the laboratory. The HONO concentration corrected for heterogeneous formation on the tunnel walls, in relation to the CO2 concentration can be used to estimate the amount of HONO, which is directly emitted from the vehicle fleet. From the measured data, emission ratios (e.g. HONO/NO x ) and emission indices (e.g. mg HONO kg−1 fuel) were calculated. The calculated emission index of 88±18 mg HONO kg−1 fuel allows an estimation of the HONO emission rates from traffic into the atmosphere. Furthermore, the heterogeneous formation of HONO from NO2 on freshly emitted exhaust particles is discussed.
Keywords: Nitrous acid; Road traffic tunnel; Road traffic emissions; Emission ratios; Emission indices; Heterogeneous reactions;

Urban street-canyon flows with bottom heating by Jae-Jin Kim; Jong-Jin Baik (3395-3404).
Urban street-canyon flows in the presence of street-bottom heating are investigated using a two-dimensional numerical model with a kε turbulence closure scheme. The street aspect ratio (H/D, where H and D are the building height and the width between two buildings, respectively) varies from 0.6 to 3.6 (in 0.2 interval) and the initial potential temperature difference between the street-canyon bottom and the air (ΔΘ) ranges from 0 to 16 K (in 2 K interval). Five flow regimes are identified. Regime I is observed when the aspect ratio is very small but the bottom heating is very strong (H/D=0.6 and ΔΘ⩾10 K). In regime I, as the heating intensity increases, the thermally induced vortex expands but the mechanically induced vortex contracts. Regime II is mainly observed when the aspect ratio is relatively small or the bottom heating is weak. In regime II, the vortex intensity increases with increasing heating intensity. Regime III is observed when the bottom heating is relatively significant (ΔΘ⩾4 K) and the aspect ratio lies in the range of 1.2–2. This regime differs from regime II in that the vortex induced by temperature gradient on the upwind side of the upper layer has meaningful intensity and size and the maximum horizontal velocity decreases with increasing heating intensity. When the bottom heating is relatively significant, regime IV is most commonly observed. This consists of two corotating vortices in the vertical within combined streamlines. Regime V exhibits two counterrotating vortices induced on both sides of the maximum temperature axis in the lower layer. The upper vortex is little influenced by the bottom heating. The numerical model result is shown to be reasonably in good agreement with the wind-tunnel data.
Keywords: Urban street-canyon flows; Flow regimes; Street aspect ratio; Bottom heating;

Personal exposures to NO2 in the EXPOLIS-study: relation to residential indoor, outdoor and workplace concentrations in Basel, Helsinki and Prague by Anu Kousa; Christian Monn; Tuulia Rotko; Sari Alm; Lucy Oglesby; Matti J Jantunen (3405-3412).
Personal exposures, residential indoor, outdoor and workplace levels of nitrogen dioxide (NO2) were measured for 262 urban adult (25–55 years) participants in three EXPOLIS centres (Basel; Switzerland, Helsinki; Finland, and Prague; Czech Republic) using passive samplers for 48-h sampling periods during 1996–1997. The average residential outdoor and indoor NO2 levels were lowest in Helsinki (24±12 and 18±11 μg m−3, respectively), highest in Prague (61±20 and 43±23 μg m−3), with Basel in between (36±13 and 27±13 μg m−3). Average workplace NO2 levels, however, were highest in Basel (36±24 μg m−3), lowest in Helsinki (27±15 μg m−3), with Prague in between (30±18 μg m−3). A time-weighted microenvironmental exposure model explained 74% of the personal NO2 exposure variation in all centres and in average 88% of the exposures. Log-linear regression models, using residential outdoor measurements (fixed site monitoring) combined with residential and work characteristics (i.e. work location, using gas appliances and keeping windows open), explained 48% (37%) of the personal NO2 exposure variation. Regression models based on ambient fixed site concentrations alone explained only 11–19% of personal NO2 exposure variation. Thus, ambient fixed site monitoring alone was a poor predictor for personal NO2 exposure variation, but adding personal questionnaire information can significantly improve the predicting power.
Keywords: Nitrogen dioxide; Exposure; EXPOLIS; Regression model; Time-activity diary;

The electric mechanism of dry deposition is well known in the case of unattached radon daughter clusters that are unipolar charged and of high mobility. The problematic role of the electric forces in deposition of aerosol particles is theoretically examined by comparing the fluxes of particles carried by different deposition mechanisms in a model situation. The electric mechanism of deposition appears essential for particles of diameter 10–200 nm in conditions of low wind speed. The electric flux of fine particles can be dominant on the tips of leaves and needles even in a moderate atmospheric electric field of a few hundred V m−1 measured over the plane ground surface. The electric deposition is enhanced under thunderclouds and high voltage power lines. Strong wind suppresses the relative role of the electric deposition when compared with aerodynamic deposition. When compared with diffusion deposition the electric deposition appears less uniform: the precipitation particulate matter on the tips of leaves and especially on needles of top branches of conifer trees is much more intensive than on the ground surface and electrically shielded surfaces of plants. The knowledge of deposition geometry could improve our understanding of air pollution damage to plants.
Keywords: Aerosol deposition; Atmospheric electricity; Coniferous needles;

Origin and variability of upper tropospheric nitrogen oxides and ozone at northern mid-latitudes by V. Grewe; D. Brunner; M. Dameris; J.L. Grenfell; R. Hein; D. Shindell; J. Staehelin (3421-3433).
Year-long measurements of NO x and ozone performed during the NOXAR project are compared to results from the ECHAM4.L39(DLR)/CHEM (E39/C) and GISS coupled chemistry–climate models. The measurements were taken on flights between Europe and the eastern United States and between Europe and the Far East in the latitude range 40–65°N. Our comparison concentrates on the upper troposphere and reveals strong longitudinal variations in seasonal mean NO x of more than 200 pptv, which both models are able to reproduce qualitatively. Vertical profiles show maximum NO x values 2–3 km below the tropopause (“E-shape”) with a strong seasonal cycle. E39/C simulates a maximum located at the tropopause and with a reasonable seasonal cycle. The GISS model reproduces the seasonal cycle but not the profile's shape due to its coarser vertical resolution. A comparison of NO x frequency distributions reveals that both models are capable of reproducing the observed variability, except that E39/C shows no very high NO x mixing ratios.Both models show that lightning and surface NO x emissions contribute the most to the seasonal cycle of NO x at tropopause altitudes. The impact of lightning in the upper troposphere does not vary strongly with altitude, whereas the impact of surface emissions decreases with altitude. Among all sources, lightning contributes the most to the variability of NO x in the upper troposphere in northern mid-latitudes during summer.
Keywords: NOXAR; GCM; Upper troposphere NO x and ozone; Lightning NO x ;

A series of backscatter Lidar measurements were made around a bio-waste power station at Eye in Suffolk over a period of 10 days in May 1999. These measurements were supplemented with bag samples of SF6 tracer, analysed on site using gas chromatography with an electron capture detector. Despite problems with contamination, a detection limit of 20 ppt was eventually achieved and this permitted useful plume measurements from a release rate of 1–2 l min−1. Concentration/flux ratios were estimated from the Lidar measurements using an integral technique. Of the Lidar runs obtained, 24 were coincident with a tracer release. After allowing for the background of both aerosol and tracer, it was apparent that the independent calibrations of concentration/flux ratio from Lidar or tracer agreed with each other to within 20–60%. This permits the Lidar scans to be used to estimate peak near-ground concentrations, though because of various technical difficulties (poor alignment, background sources of aerosol, or an inconvenient wind direction) this cannot always be achieved. Besides giving confidence in the Lidar calibration, the tracer measurements were valuable in permitting aerosol from the stack to be distinguished from aerosol from fugitive sources. Meteorological parameters were logged simultaneously with the dispersion measurements. These parameters included conventional means of wind speed and direction, temperature, humidity and insolation, and also micrometeorological measurements of turbulence and of turbulent fluxes. The Lidar was used to estimate wind speed and direction at plume height and the boundary layer depth and cloud-base where possible. Source emission characteristics were also logged.
Keywords: Lidar; Dispersion; Backscatter; Ground-level concentrations; Tracer; SF6; Calibration;

Surface emission of landfill gas from solid waste landfill by Jin-Won Park; Ho-Chul Shin (3445-3451).
The surface emission of landfill gas (LFG) was studied to estimate the amount of LFG efflux from solid waste landfills using an air flux chamber. LFG efflux increased as atmospheric temperature increased during the day, and the same pattern for the surface emission was observed for the change of seasons. LFG efflux rate decreased from summer through winter. The average LFG efflux rates of winter, spring and summer were 0.1584, 0.3013 and 0.8597 m3 m−2 h−1 respectively. The total amount of surface emission was calculated based on the seasonal LFG efflux rate and the landfill surface area. From the estimates of LFG generation, it is expected that about 30% of the generated LFG may be released through the surface without extraction process. As forced extraction with a blower proceeded, the extraction well pressure decreased from 1100 to –100 mm H2O, and the LFG surface efflux decreased markedly above 80%. Thus, the utilization of LFG by forced extraction would be the good solution for global warming and air pollution by LFG.
Keywords: Landfill; Landfill gas; LFG efflux; Surface efflux rate; Flux chamber; Forced extraction; Air pollution;

The estimated annual throughfall deposition flux of Hg in a northern mixed-hardwood forest in the Lake Huron Watershed was 10.5±1.0 μg m−2 compared to an annual precipitation Hg flux of 8.7±0.5 μg m−2 (June 1996–June 1997). The source of this additional Hg in throughfall is often attributed to wash-off of dry deposition, but foliar leaching of Hg may also be important. To determine the influence of both dry deposition and foliar leaching of Hg and other elements in throughfall, we measured a suite of trace elements (Hg, Al, Mg, V, Mn, Cu, Zn, As, Rb, Sr, Cd, Ba, La, Ce, and Pb) in throughfall, precipitation, and ambient air samples from a northern mixed-hardwood forest. Based on a multiple linear regression model, dry deposition had the most important influence on Hg, Al, La, Ce, V, As, Cu, Zn, Cd, and Pb fluxes while foliar leaching strongly influenced Mg, Mn, Rb, Sr, and Ba fluxes in net throughfall. The Hg dry deposition flux was estimated using gaseous and aerosol Hg measurements and modeled deposition velocities. The calculated dry deposition flux (∼12–14 μg m−2) of Hg to the canopy indicated that atmospheric deposition of Hg could easily account for all of the Hg deposited in net throughfall (1.9±0.1 μg m−2). Although there is a large uncertainty associated with these techniques, the modeling estimates indicate that atmospheric Hg may account for all of the Hg deposited in litterfall (11.4±2.8 μg m−2).
Keywords: Biogeochemical cycling; Atmospheric deposition; Foliar leaching; Throughfall; Mercury; Trace elements;

The relationship between indoor and outdoor airborne particles in the residential environment by Lidia Morawska; Congrong He; Jane Hitchins; Dale Gilbert; Sandhya Parappukkaran (3463-3473).
The relationship between indoor and outdoor airborne particles was investigated for 16 residential houses located in a suburban area of Brisbane, Australia. The submicrometer particle numbers were measured using the Scanning Mobility Particle Sizer, the larger particle numbers using the Aerodynamic Particle Sizer and an approximation of PM2.5 was also measured using a DustTrak. The measurements were conducted for normal and minimum ventilation conditions using simultaneous and non-simultaneous measurement methods designed for the purpose of the study. Comparison of the ratios of indoor to outdoor particle concentrations revealed that while temporary values of the ratio vary in a broad range from 0.2 to 2.5 for both lower and higher ventilation conditions, average values of the ratios were very close to one regardless of ventilation conditions and of particle size range. The ratios were in the range from 0.78 to 1.07 for submicrometer particles, from 0.95 to 1.0 for supermicrometer particles and from 1.01 to 1.08 for PM2.5 fraction. Comparison of the time series of indoor to outdoor particle concentrations shows a clear positive relationship existing for many houses under normal ventilation conditions (estimated to be about and above 2 h−1), but not under minimum ventilation conditions (estimated to be about and below 1 h−1). These results suggest that for normal ventilation conditions, outdoor particle concentrations could be used to predict instantaneous indoor particle concentrations but not for minimum ventilation, unless air exchange rate is known, thus allowing for estimation of the “delay constant”.
Keywords: Air pollution; Indoor air quality; Submicrometer particle; Supermicrometer particles; PM2.5; Ventilation;

The cycle of mercury (Hg) from a gigantic landfill area (area ∼2.72 km2) was investigated by conducting micrometeorological measurements of its exchange rates across soil–air boundary during the spring season of 2000. Based on this field campaign, we attempted to provide various insights into the Hg exchange processes, especially with respect to the decoupling of the mixed signatures of complex source processes. According to our analysis, the cycle of Hg in the study site appeared to be affected significantly by the vent processes; excessive amount of Hg was expected to be released via ventpipes penetrating up to 60 m depths of the deep landfilled waste layer. The influence of these vent source processes was reflected very sensitively by the windrose pattern. The data collected during the non-easterly winds were representing the typical pattern for a strong source area in which upward emission is predominant in both strength and frequency. On the other hand, the data collected from the easterly winds were characterized by excessive deposition of Hg which we suspect is due mostly to the nearest vent located easterly from our measurement spot. The unique characteristics of each data group, divided by windrose pattern, were consistent from apparent difference in: (1) the absolute magnitude of gradient/flux data sets, (2) frequency of exchange for each of two vertical directions, and (3) E/D (emission/deposition) ratios for most relevant parameters. The analysis of the short-term variability of exchange patterns over a 24-h scale, also exhibited that the patterns for two different conditions were quite contrasting as a function of time. The magnitude of bidirectional fluxes in the present study is significantly high with values of 254±224 (N=71 emissions out of 79 fluxes quantified during non-easterly winds) and −1164±1276 ng m−2  h−1 (N=14 depositions out of 16 fluxes during easterly winds), respectively. If the computed emission rate is extrapolated, we estimate that annual emission of Hg from the study area can amount to approximately 6 kg which is comparable with the estimates for other areas around the globe under strong Hg-pollution.
Keywords: Mercury; Exchange; Emission; Dry deposition; Ozone; Open dumping landfill area;

The fate of PAHs in the carbon black manufacturing process by Perng-Jy Tsai; Hong-Yong Shieh; Lien-Te Hsieh; Wen-Jhy Lee (3495-3501).
This study measured PAHs contained in the feedstock oil, carbon black products, and stack flue gas, then the fate of PAHs was assessed from the mass balance point of view for a carbon black manufacturing process. Results show the carbon black manufacturing process would result in the depletion of total-PAHs and the summation of top three carcinogenic PAH species (i.e., BbF+BaP+DBA) up to 98.15% and 99.83%, respectively. The above results suggest that the carbon black manufacturing process would result in not only the decrease of the amount of total-PAHs, but also the carcinogenic potencies of PAHs originally contained in the feedstock oil. Regarding PAHs contained in the carbon black products and stack flue gas, this study suggest they might be resulted mostly from high-temperature pyrolytic process, rather than the PAHs originally contained in the feedstock oil. For the carbon black manufacturing industry, since the soot (i.e., the carbon black) was completely collected as its final product, therefore most of carbon black-bearing PAHs did not directly release to atmosphere. On the other hand, PAHs contained in the stack flue gas were directly exhausted to the atmosphere and thus were assessed in this study. The results show the emission rates for total-PAHs and BbF+BaP+DBA for the stack flue gas were 2.18 kg/day and 1.50 g/day, respectively, which were approximately 25% and 40% of those exhausted from a municipal incinerator with a treatment capacity of 300 metric tons/day. It is concluded that the carbon black manufacturing process might not be a significant PAHs emission source, as compared to the municipal incinerator.
Keywords: PAHs; Carbon black; Feedstock oil; Stack flue gas; Emission rate;

Although stack downwash is not a widespread problem at modern fossil-fuel power plants and larger industrial stacks, it is a very important problem in simulating buoyant plumes in wind tunnels and towing tanks. Scaling criteria for avoiding downwash in ten subcritical model experiments have been reviewed. A comparison was made between data on the occurrence of downwash from 10 modeling studies to the theory proposed by Tatom (1986). In general, there was good agreement of Tatom's theory with the occurrence of downwash.
Keywords: Stack downwash; Plume rise; Buoyancy; Modeling 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).
Keywords: Monte Carlo Model; Fugitive Dust Model (FDM); Particle deposition; Plume model; Hanford tracer experiments;

The robustness problem is considered for mathematical indices that describe the adverse effects of vegetation exposure to ozone. It is shown that some of them may demonstrate infinitely high sensitivity of the exposure estimate to small variations of ozone concentrations or internal parameters of specific functional. In particular, for the accumulated exposure over a threshold of 40 ppb (AOT40) index such conditions are not extraordinary, but rather describe quite often situations in remote areas in Northern Europe. Taking into account inevitable uncertainties in both calculated and observed ozone concentrations, a high sensitivity of ozone impact indices results in an instability of the exposure estimates and creates problems in their use. Theoretical consideration of the problem enabled to formulate the necessary and sufficient conditions for the limited sensitivity of a time-integrating index. An example of a modified AOT formulation fulfilling the obtained criteria and hence not experiencing the sensitivity problem is considered.
Keywords: Tropospheric ozone; Exposure index; Sensitivity analysis; Ozone flux; Critical levels;

Factors influencing aerosol solubility during cloud processes by K.V. Desboeufs; R. Losno; J.L. Colin (3529-3537).
The water-soluble fraction of an aerosol determines its chemical and physical properties and also its behaviour. The origin of the aerosol and its atmospheric transport influence its solubility. Cloud process simulations have been conducted on both Saharan and anthropogenic aerosols. The rate of solubilisation was followed for native and processed aerosol particles; it is controlled by the pH variations due to release of acids or bases. It appears that one condensation/evaporation cycle increases the solubility of aerosol particles. Increasing the number of cloud process simulations does not affect the solubility profile. The solubility depends only on the conditions of the last cloud cycle and, in particular, on the factor controlling pH during this process.
Keywords: Evaporation/condensation cycles; Weathering; Aerosol composition; pH; Heterogeneous chemistry;

Inhalation of quartz and cristobalite dusts is commonly linked with health effects although the mechanisms involved are poorly understood. Grinding of these piezoelectric silica polymorphs produces particulates with transient piezoelectric charges. This is likely to cause vigorous reaction with atmospheric gases and, through interaction with surface charges and ‘dangling bonds’, may lead to the formation of highly deleterious ozonide, superoxide and hydroxyl radicals. It is hoped that this study will encourage experimental work to quantify piezoelectric effects in silica dusts and to develop a method for their neutralisation during cutting and grinding processes.
Keywords: Silica dusts; Piezoelectricity; Surface radicals; Airborne particulates; Silicosis;

Total suspended particulate matter and heavy metal (Pb, Mn, Cd and Hg) concentrations were measured at the location in the vicinity of the waste dump to determine the air pollution level of these pollutants prior to the operation of the Mobile Thermal Treatment Plant. Samples were collected over one year period. Seasonal differences, and the influence of meteorological parameters (temperature, relative humidity, pressure and wind direction) on the air pollution levels were studied. Results show relatively low concentrations of TSP, Pb, Mn and Cd, while Hg levels were higher compared to the guideline values. Good weather conditions are connected to long range transport of particulate matter, while higher temperatures result in elevated mercury concentrations. Because of the predominant northeast wind direction, the contribution of air pollution from the direction of the waste dump at the measuring site is significant, but that does not necessarily mean that the pollutants originated from that source.
Keywords: Lead; Manganese; Cadmium; Mercury; Meteorological parameters;