On the errors associated with the use of hourly data in wind-driven rain calculations on building facades

Wind-driven rain (WDR) is the one of the main moisture sources for building facades. It is an important factor in the dry and wet deposition of pollutants, facade surface soiling and facade erosion. WDR calculations require data records of wind speed, wind direction and horizontal rainfall intensity as input. Most meteorological datasets contain at best arithmetically averaged hourly wind and rain data. Their use is common practice in WDR calculations. As an example, existing WDR standards request at best hourly data. This paper however demonstrates that the use of such data can yield (very) large errors in the calculated WDR amounts and intensities. The reason is that arithmetic averaging on an hourly basis generally causes an important loss of information about the co-occurrence of wind and rain. An improved data averaging technique for wind and rain data is proposed that respects this co-occurrence by applying appropriate weighting factors in the averaging procedure. The performance of this technique is evaluated by WDR calculations on buildings in three cities with different climates. While arithmetically averaged hourly data yield large underestimation errors (Eindhoven, The Netherlands: 11%, Bloomington, USA: 45%, Grahamstown, South Africa: 31%), the improved averaging technique provides very good results (errors: 0%, 4%, 3%, respectively). In conclusion, WDR calculations should not be performed with arithmetically averaged hourly data. Instead, either high-resolution data (e.g. 10-min data) or hourly data that have been obtained with the proposed weighted averaging technique should be used.     

Influence of meteorological factors and polluting environment on rain chemistry and wet deposition in a rural area near Chimay,Belgium

This paper focuses on a detailed analysis of the effects of meteorological factors explaining the variability of rain composition.Inorganic composition of 113 individual rain events was measured from May 2002 to October 2005 at a rural site near Chimay, in the western part of the Belgian Ardennes. Original models were fitted for each studied ion (H⁺, Mg²⁺, Ca²⁺, K⁺, NH₄⁺, Na⁺, Cl⁻, NO₃⁻ and SO₄²⁻) to relate rain event concentration or wet deposition to the rainfall volume (R), the length of the antecedent dry period (ADP), the volume of the previous event (R_prev) as well as to the mean wind speed and the prevailing wind direction during both the dry and the rainy periods. These variables explained from 32% (H⁺) to 69% (NO₃⁻) of rain concentration variability. Concentrations decreased logarithmically with increasing R values except in case of H⁺ for which a positive effect of rain volume on rain concentration was observed. ADP affected positively rain concentrations of all ions excluding K⁺ and H⁺ for which, respectively, a nonsignificant and a negative effect of this variable was observed. Increasing R_prev strengthened the effect of the variable R on H⁺, Mg²⁺, Ca²⁺, Na⁺, NH₄⁺ and SO₄²⁻ concentrations while it softened the effect of ADP on NO₃⁻ concentrations. Wind speed and direction during dry and rainy periods explained together from 8% (K⁺) to 38% (Na⁺) of rain concentration total variability. R² coefficients of the wet deposition models ranged from 0.51 (K⁺) to 0.79 (SO₄²⁻). For all ions, wet deposition increased significantly with increasing R values while the effects of the other variables were similar to those on concentrations. Wind conditions during dry and rainy periods explained from 4% (H⁺) to 24% (Na⁺) of wet deposition total variability. On an annual scale, the total dry period duration, the total rainfall volume as well as the shape of the distributions of the length of the antecedent dry periods and of the rain event volume are important parameters that influence annual wet deposition.     

Natural chloroform emissions from the blanket peat bogs in the vicinity of Mace Head,Ireland over a 14-year period

Simultaneous chloroform (CHCl₃) emission and ozone (O₃) deposition are regularly observed under nocturnal inversions during the summer months from and to the peat bogs in the vicinity of the Mace Head Atmospheric Research Station, Connemara, Co Galway, Ireland. Emissions were estimated using the nocturnal box model applied to routine atmospheric observations collected over a 14-year period from 1995 to 2008. Strict criteria were applied in the selection of events of low wind speed, under a stable night-time inversion layer in baseline air conditions, with no transport from Europe. The mean peatland CHCl₃ flux was 2.91 μg m^?2 h^?1 with highly variable fluxes ranging from 0.44 to 12.94 μg m^?2 h^?1. These fluxes are generally larger than those reported previously for similar biomes and if representative would make a significant contribution to the global estimated source of CHCl₃. Fluxes were not strongly correlated with either atmospheric temperature or the level of precipitation. Over the 14-year period there appears to have been a small increase in overall CHCl₃ emissions, although we stress that the nocturnal box model has a number of limitations and assumptions which should be taken into account.     

Application of the nested grid STEM to an early summer acid rain in South Korea

The nested grid Sulfur Transport Eulerian Model (STEM) was developed and used to simulate the acid rain in Korea that occurred on 10 June 1996. The present nested grid system consists of three-grid systems. The coarsest grid system includes China, Korean Peninsula and Japan with the horizontal grid size of 80 km and the finest grid system includes only Korea with the horizontal grid size of 8.9 km. The calculated gas-phase SO₂ and O₃ concentrations agree relatively well with the field measurements. In addition, the model successfully reproduces the measured sulfate and nitrate concentrations in the rain water and futhermore identified the high concentration regions of liquid-phase sulfate and nitrate. In the present simulation conditions, most of the gas-phase of SO₂ and HNO₃ were washed out. A close relationship between wet deposition fluxes and precipitation rates were found for sulfate and nitrate. Finally, the model results also showed that a fine grid size is required to accurately calculate gas-phase concentrations as well as acid deposition fluxes.     

Air ventilation impacts of the “wall effect” resulting from the alignment of high-rise buildings

The objective of this study is to investigate the air ventilation impacts of the so called “wall effect” caused by the alignment of high-rise buildings in complex building clusters. The research method employs the numerical algorithm of computational fluid dynamics (CFD – FLUENT) to simulate the steady-state wind field in a typical Hong Kong urban setting and investigate pollutant dispersion inside the street canyon utilizing a pollutant transport model. The model settings of validation study were accomplished by comparing the simulation wind field around a single building block to wind tunnel data. The results revealed that our model simulation is fairly close to the wind tunnel measurements. In this paper, a typical dense building distribution in Hong Kong with 2 incident wind directions (0° and 22.5°) is studied. Two performance indicators are used to quantify the air ventilation impacts, namely the velocity ratio (VR) and the retention time (T_r) of pollutants at the street level. The results indicated that the velocity ratio at 2 m above ground was reduced 40% and retention time of pollutants increased 80% inside the street canyon when high-rise buildings with 4 times height of the street canyon were aligned as a “wall” upstream. While this reduction of air ventilation was anticipated, the magnitude is significant and this result clearly has important implications for building and urban planning.     

Dry deposition to a uniform canopy: Evaluation of a first-order-closure mathematical model

A mathematical model is developed for the transport of momentum and matter within a canopy consisting of identical elements protruding vertically from a smooth substrate. Turbulent flux is modelled using a mixing-length approach. The loss of momentum (or matter) to individual elements is related to the mean wind speed, and the element-element interaction via the turbulent wind field is represented by a sheltering factor. Careful consideration is given to the formulation of lower boundary conditions. The model assumptions are compared with those of other models.The model predictions are compared with measurements on a vertically- and horizontally-uniform artificial canopy in a wind-tunnel. The model reproduces well the observed relationship between the parameters of the logarithmic wind speed profile above the canopy and the observed deposition velocities of thorium-B (ThB) atoms and particles in the diameter range 0.08–32 μm, using a sheltering factor which is little dependent on wind speed and has the same magnitude for momentum, gas and particles. The predicted dependences of deposition velocity on friction velocity and, for particles, on diameter shed light on the performance of semi-empirical correlations proposed in the literature. For ThB atoms, the calculated deposition velocities are compared with those of other mathematical canopy models: a comparable degree of agreement is obtained here with fewer free parameters.The fraction of deposit on the substrate is underpredicted by an order of magnitude in some cases, pointing to the limitations of the modelling of conditions near the substrate in terms of quasi-shear flow.     

In-stack emissions of heavy metals estimated by moss biomonitoring method and snow-pack analysis

Data from the chemical analysis of moss growing close to a thermal power station and snowpack have been used for the estimation of heavy metal deposition close to the point pollution sources. A semi-empirical model was proposed to describe atmospheric trace metal deposition close to the point pollution source. Model parameters were derived from experimental data, and nickel and vanadium quantities, washed out with snow and rain, were calculated. Using long-term meteorological observation data of rain and snow duration and metal uptake efficiencies in moss, the average emission rates of vanadium and nickel from the stack were calculated. The coincidence between data from emission inventory and model results was within 25%. It was estimated that in the vicinity of pollution source (within 30 km) about 15% out of total emitted metals were washed out by rain and snow events. Metal concentrations in the environment become indistinguishable from the background at a distance of about 20 km from the stack.     

Assessment of pollutant fluxes across the frontiers of the Federal Republic of Germany on the basis of aircraft measurements

Forty operational measurements by aircraft are described, which were performed during 1985–1987. The purpose was to obtain an overall picture of transboundary transports of SO₂, NO_x sulfate, nitrate and O₃ between the F.R.G. and its neighbours. Transboundary fluxes were calculated by means of measurement results with respect to the concentrations of the considered pollutants, depth of the planetary boundary layer, wind speed and wind direction. The largest SO₂ fluxes have been observed across the eastern border of the F.R.G., during periods characterized by easterly winds (300–2500 ton SO₂ h⁻¹ across a 500–600 km border section). The major fraction of this SO₂ appeared to have originated from the easterly neighbours of the F.R.G. and to a lesser extent also from countries further eastward. During easterly winds it has been observed that SO₂, which originated from countries east of the F.R.G., contributed considerably to SO₂ fluxes across the western border to countries west of the F.R.G. The results of the measurement flights as presented have provided valuable indications about transboundary transports of pollutants despite considerable uncertainties in the fluxes due to inaccuracies in the concentration profiles, wind data and depth of the planetary boundary layer.     

A validated 2-D diffusion–advection model for prediction of drift from ground boom sprayers

Correct field drift prediction is a key element in environmental risk assessment of spraying applications. A reduced order drift prediction model based on the diffusion–advection equation is presented. It allows fast assessment of the drift potential of specific ground boom applications under specific environmental wind conditions that obey the logarithmic wind profile. The model was calibrated based on simulations with a validated Computational Fluid Dynamics (CFD) model. Validation of both models against 38 carefully conducted field experiments is successfully performed for distances up to 20 m from the field edge, for spraying on flat pasture land. The reduced order model succeeded in correct drift predictions for different nozzle types, wind velocities, boom heights and spray pressures. It used 4 parameters representing the physical aspects of the drift cloud; the height of the cloud at the field edge, the mass flux crossing the field edge, the settling velocity of the droplets and the turbulence. For the parameter set and range considered, it is demonstrated for the first time that the effect of the droplet diameter distribution of the different nozzle types on the amount of deposition spray drift can be evaluated by a single parameter, i.e., the volume fraction of droplets with a diameter smaller than 191 μm. The reduced order model can be solved more than 4 orders of magnitude faster than the comprehensive CFD model.     

Atmospheric deposition of organochlorine pesticides by precipitation in a coastal area

Wet deposition fluxes of organochlorine pesticides (OCPs) were determined for rain samples collected in a coastal area of Turkey. Seventeen precipitation samples were collected over a 1-year period from 2008 to 2009. Rainwater was accumulated at the beginning of rain events using real time monitoring. Atmospheric concentrations were also measured in parallel with deposition samples. Both atmospheric concentrations and deposition fluxes were determined as particle and gas phases. The particle phase and dissolved phase deposition fluxes were 794.26?±?756.70 ngm^?2 day^?1 and 800.77?±?672.63 ngm^?2 day^?1, respectively. The washout ratios for OCP compounds were calculated separately for the particle and dissolved phases using the atmospheric concentrations and rain concentrations. The minimum washout ratio for the particle phase was 2339.47 for Endrin aldehyde, whereas the maximum washout ratio was 497593.34 for Methoxychlor. The maximum washout ratio for the dissolved phase was 247523.89 for Endosulfan beta, whereas the minimum washout ratio was 10169.69 for p,p′-DDT. The dry deposition velocities ranged from 0.01 to 1.67 cms^?1. The partitioning of wet deposition between the particle and dissolved phases was 50 % in terms of total OCP deposition.     

Seasonal variability in gaseous mercury fluxes measured in a high-elevation meadow

Seasonal patterns of atmospheric mercury (Hg) fluxes measured over vegetated terrestrial systems can provide insight into the underlying process controlling emission and deposition of Hg to vegetated surfaces. Gaseous elemental Hg fluxes were measured for week-long periods in each season (spring, summer, fall, and winter) over an uncontaminated high-elevation wetland meadow in Shenandoah National Park, Virginia using micrometeorological methods. Mean net deposition was observed in the spring (?4.8 ng m^?2 h^?1), emission in the summer (2.5 ng m^?2 h^?1), near zero flux in the fall (0.3 ng m^?2 h^?1), and emission in the winter (4.1 ng m^?2 h^?1). Nighttime deposition (when stomata are closed) and the poor correlation between Hg fluxes and canopy conductance during periods of active vegetation growth suggest that stomatal processes are not the dominant mechanism for ecosystem-level GEM exchange at this site. The strong springtime deposition relative to summer implies that young vegetation is better at scavenging Hg, with the highest deposition occurring at night possibly via a cuticular pathway. These results suggest that spring is a period of GEM deposition while other seasons exhibit net emission, emphasizing the importance of capturing GEM flux seasonality when determining total Hg budgets.     

Approximation and spatial regionalization of rainfall erosivity based on sparse data in a mountainous catchment of the Yangtze River in Central China

In densely populated countries like China, clean water is one of the most challenging issues of prospective politics and environmental planning. Water pollution and eutrophication by excessive input of nitrogen and phosphorous from nonpoint sources is mostly linked to soil erosion from agricultural land. In order to prevent such water pollution by diffuse matter fluxes, knowledge about the extent of soil loss and the spatial distribution of hot spots of soil erosion is essential. In remote areas such as the mountainous regions of the upper and middle reaches of the Yangtze River, rainfall data are scarce. Since rainfall erosivity is one of the key factors in soil erosion modeling, e.g., expressed as R factor in the Revised Universal Soil Loss Equation model, a methodology is needed to spatially determine rainfall erosivity. Our study aims at the approximation and spatial regionalization of rainfall erosivity from sparse data in the large (3,200 km²) and strongly mountainous catchment of the Xiangxi River, a first order tributary to the Yangtze River close to the Three Gorges Dam. As data on rainfall were only obtainable in daily records for one climate station in the central part of the catchment and five stations in its surrounding area, we approximated rainfall erosivity as R factors using regression analysis combined with elevation bands derived from a digital elevation model. The mean annual R factor (R _a) amounts for approximately 5,222 MJ?mm?ha^?1?h^?1?a^?1. With increasing altitudes, R _a rises up to maximum 7,547 MJ?mm ha^?1?h^?1 a^?1 at an altitude of 3,078 m a.s.l. At the outlet of the Xiangxi catchment erosivity is at minimum with approximate R _a?=?1,986 MJ?mm?ha^?1?h^?1?a^?1. The comparison of our results with R factors from high-resolution measurements at comparable study sites close to the Xiangxi catchment shows good consistance and allows us to calculate grid-based R _a as input for a spatially high-resolution and area-specific assessment of soil erosion risk.     

A theoretical estimate of O3 and H2O2 dry deposition over the northeast United States

Estimates of short-term, regional-scale spatial distributions of ozone (O₃) and hydrogen peroxide (H₂O₂) dry deposition over the northeast U.S. are presented. Dry deposition fluxes to surfaces are computed using a regional tropospheric chemistry model with deposition velocities which vary with local meteorology, land type, insolation, seasonal factors and surface wetness. A compilation of O₃ surface resistances is presented based on a survey of O₃ dry deposition measurements. The surface resistance for H₂O₂ is assumed to be small under most conditions, causing H₂O₂ to dry deposit at a rate which is frequently limited by surface-layer turbulence. Regional patterns of dry deposition velocities for these oxidants over the northeast U.S. are computed using landuse data and meteorological information predicted using a mesoscale meteorology model. Domain-averaged O₃ deposition velocities during a spring period reach a mid-day peak of 0.7–0.8 cm s⁻¹ and drop to 0.1–0.2 cm s⁻¹ at night. Domain-averaged H₂O₂ deposition velocities at a height of approximately 80 m are predicted to reach a mid-day peak of 1.6–2.0cm s⁻¹, and fall to 0.6–0.9 cm s⁻¹ at night. Time-averaged surface-layer H₂O₂ concentrations show a latitude dependence, with higher concentrations in the south. H₂O₂ concentrations are significantly reduced due to efficient wet removal and chemical destruction during the passage of a cyclonic frontal system. In contrast, O₃ concentrations are predicted to rise during the passage of a frontal system due to efficient vertical exchange of midtropospheric air into the boundary layer during convective conditions, followed by synoptic-scale subsidence occurring in the high pressure airmass following a cyclone. Maximum O₃ deposition during this 3-day springtime period occurs in polluted agricultural areas. In contrast, H₂O₂ dry deposition exhibits a latitude dependence with maximum 3-day accumulations occurring in the south. Domain-averaged mid-day deposition rates for O₃ and H₂O₂ were 45–50 μmol m⁻² h⁻¹ and 4–5 μmol m⁻² h⁻¹. At night, deposition rates were approximately 5–10 μmol m⁻² h⁻¹ and 1.5–2.5 μmol m⁻² h⁻¹ for O₃ and H₂O₂. These model results show that regional patterns of oxidant dry deposition are strongly influenced by oxidant concentrations, atmospheric stability, surface roughness and numerous other surface and meteorological factors. Each of these factors must be well-characterized before regional patterns of biological damage associated with oxidant dry deposition can be quantified.     

A modeling assessment of association between East Asian summer monsoon and fate/outflow of α-HCH in Northeast Asia

Using a dynamic numerical atmospheric transport model for organochlorine pesticides (OCPs), the relationship between the East Asian summer monsoon and the fate of α-hexachlorocyclohexane (α-HCH), a banned OCP, in the atmosphere over Northeast Asia was investigated and assessed. The modeled temporal and spatial patterns and variability of α-HCH air concentrations during the summer months of 2005 revealed a strong link between this chemical in the atmosphere over Northeast Asia and the East Asian summer monsoon. At lower atmospheric levels, easterly and southeasterly winds blowing from relatively cold ocean surface convey α-HCH air concentration from southeast China to northeast China. A monsoon front extending from southeast China to Japan, characterized by a strong wind convergence, carried the air concentration to a high elevation of the atmosphere where it was delivered by southerly monsoon flow to northern China and North Pacific Ocean. This summer monsoon associated northward atmospheric transport caused a reversal of the soil/air exchange from outgassing to net deposition during spring–summer period. The modeled wet deposition fluxes of α-HCH agreed well with the changes in the typical summer monsoon rain bands, designated as Meiyu in China, Changma in Korea, and Baiu in Japan. The major wet deposition flux paralleled with the monsoon front as well as the monsoon rain bands. The temporal change in the fluxes exhibits abrupt northward advances, which is associated with a stepwise northward and northeastward advance of the East Asian summer monsoon. The modeled α-HCH outflow in the atmosphere from China occurs mostly in the summer months and through northeast China, featured strongly by the evolution of the summer month. This study suggests that the East Asian summer monsoon provides a major atmospheric pathway and summer outflows to α-HCH over East Asia.     

Atmospheric deposition of phthalate esters in a subtropical city

In Chinese cities, air pollution has become a serious and aggravating environmental problem undermining the sustainability of urban ecosystems and the quality of urban life. Bulk atmospheric deposition samples were collected two-weekly, from February 2007 to January 2008, at three representative areas, one suburban and two urbanized, in the subtropical city, Guangzhou, China, to assess the deposition fluxes and seasonal variations of phthalate esters (PAEs). Sixteen PAE congeners in bulk deposition samples were measured and the depositional fluxes of ∑₁₆PAEs ranged from 3.41 to 190 μg m^?2 day^?1, and were highly affected by local anthropogenic activities. The significant relationship between PAEs and particulate depositional fluxes (correlation coefficient R² = 0.72, P < 0.001) showed PAEs are associated primarily with particles. Temporal flux variations of PAEs were influenced by seasonal changes in meteorological parameters, and the deposition fluxes of PAEs were obviously higher in wet season than in dry season. Diisobutyl phthalate (DiBP), Di-n-butyl phthalate (DnBP), and Di(2-ethylhexyl) phthalate (DEHP) dominated the PAE pattern in bulk depositions, which is consistent with a high consumption of the plasticizer market in China. PAE profiles in bulk deposition showed similarities exhibited in both time and space, and a weak increase of high molecular weight PAE (HMW PAE) contribution in the wet season compared to those in the dry season. Average atmospheric deposition fluxes of PAEs in the present study were significantly higher than those from other studies, reflecting strong anthropogenic inputs as a consequence of rapid industrial and urban development in the region.     

The use of inferential models for estimating nitric acid vapor deposition to semi-arid coniferous forests

Urban areas emit significant amounts of pollutants that impact forest ecosystems. One of the most important of these is nitric acid vapor (HNO₃), a nitrogen-containing gas that deposits efficiently to forest canopies. Since measuring HNO₃ fluxes directly is often impractical and costly in remote forest locales, inferential techniques are most often used to estimate HNO₃ flux. Given the highly efficient deposition of HNO₃, many of these inferential models assume that leaf surfaces are a ‘perfect sink’ for HNO₃ (i.e., that resistance to HNO₃ deposition is negligibly small or zero). This study tests the ‘perfect sink’ assumption in an open gas exchange system by exposing Abies magnifica, Abies concolor, and Pinus jeffreyi seedlings to concentrations of 1–13 ppb at 4–20% relative humidity. We find that, at these humidities and concentrations, cuticles are not perfect sinks for HNO₃, with cuticular resistance values ranging from 20 to 184 s m⁻¹. In addition, our results indicate that accumulating HNO₃ on leaf cuticles at these concentrations leads to higher cuticular resistance over 8–12 h exposure periods. Based on this laboratory data, we then parameterized cuticular resistance using a single-layer inferential model for semi-arid forests in the Lake Tahoe Basin. Modeled fluxes using this modification were 33% lower during well-mixed daytime conditions than the fluxes from an identical model run using the perfect sink assumption. Since HNO₃ can often account for more than half of atmospheric deposition, we conclude that inferential models that assume foliage to be perfect HNO₃ sinks are inaccurate, especially in semi-arid forests where significant amounts of HNO₃ can accumulate on leaf surfaces during dry periods.     

Control measure implications of COVID-19 infection in healthcare facilities reconsidered from human physiological and engineering aspects

The Wells-Riley model invokes human physiological and engineering parameters to successfully treat airborne transmission of infectious diseases. Applications of this model would have high potentiality on evaluating policy actions and interventions intended to improve public safety efforts on preventing the spread of COVID-19 in an enclosed space. Here, we constructed the interaction relationships among basic reproduction number (R₀) − exposure time − indoor population number by using the Wells-Riley model to provide a robust means to assist in planning containment efforts. We quantified SARS-CoV-2 changes in a case study of two Wuhan (Fangcang and Renmin) hospitals. We conducted similar approach to develop control measures in various hospital functional units by taking all accountable factors. We showed that inhalation rates of individuals proved crucial for influencing the transmissibility of SARS-CoV-2, followed by air supply rate and exposure time. We suggest a minimum air change per hour (ACH) of 7 h⁻¹ would be at least appropriate with current room volume requirements in healthcare buildings when indoor population number is < 10 and exposure time is < 1 h with one infector and low activity levels being considered. However, higher ACH (> 16 h⁻¹) with optimal arranged-exposure time/people and high-efficiency air filters would be suggested if more infectors or higher activity levels are presented. Our models lay out a practical metric for evaluating the efficacy of control measures on COVID-19 infection in built environments. Our case studies further indicate that the Wells-Riley model provides a predictive and mechanistic basis for empirical COVID-19 impact reduction planning and gives a framework to treat highly transmissible but mechanically heterogeneous airborne SARS-CoV-2.

     

Measurement and modeling of the dry deposition of peroxides

easurements of the dry deposition velocity (V_d) of hydrogen peroxide (H₂O₂) and total organic peroxides (ROOH) were made during four experiments at three forested sites. Details and uncertainties associated with the measurement of peroxide fluxes by the flux-gradient method are discussed. The results are compared to those predicted using a bulk-resistance model of the type commonly used in regional photochemical models. Good agreement between the H₂O₂ V_d measurements and a bulk resistance model is obtained when the model contains a zero surface resistance (R_c) and a common form for the laminar leaf-layer resistance (R_b) based on Schmidt and Prandtl numbers. In this case, a near-zero (<5 s m^-1) surface resistance is confirmed for H₂O₂ within experimental uncertainties. Surface resistances for ROOH were determined to be about 10–15 s m^-1 over a coniferous forest and 20–40 s m^-1 over a deciduous forest. Higher uncertainties for ROOH prevent a detailed analysis of the differences in R_c among forest types. However, the ratio of deposition velocities (ROOH/H₂O₂), computed from normalized concentration gradients, ranged from 0.28 to 0.61 (geometric mean) at the three sites. Differences in molecular diffusivities between H₂O₂ and ROOH can only account for an estimated 16% difference in V_d. Thus, the major constituent of ROOH must also be less soluble and/or less reactive than H₂O₂, which is consistent with the characteristics of methylhydroperoxide (MHP).     

A methodology to urban air quality assessment during large time periods of winter using computational fluid dynamic models

The representativeness of point measurements in urban areas is limited due to the strong heterogeneity of the atmospheric flows in cities. To get information on air quality in the gaps between measurement points, and have a 3D field of pollutant concentration, Computational Fluid Dynamic (CFD) models can be used. However, unsteady simulations during time periods of the order of months, often required for regulatory purposes, are not possible for computational reasons. The main objective of this study is to develop a methodology to evaluate the air quality in a real urban area during large time periods by means of steady CFD simulations. One steady simulation for each inlet wind direction was performed and factors like the number of cars inside each street, the length of streets and the wind speed and direction were taken into account to compute the pollutant concentration. This approach is only valid in winter time when the pollutant concentrations are less affected by atmospheric chemistry. A model based on the steady-state Reynolds-Averaged Navier–Stokes equations (RANS) and standard k-? turbulence model was used to simulate a set of 16 different inlet wind directions over a real urban area (downtown Pamplona, Spain). The temporal series of NO_x and PM₁₀ and the spatial differences in pollutant concentration of NO₂ and BTEX obtained were in agreement with experimental data. Inside urban canopy, an important influence of urban boundary layer dynamics on the pollutant concentration patterns was observed. Large concentration differences between different zones of the same square were found. This showed that concentration levels measured by an automatic monitoring station depend on its location in the street or square, and a modelling methodology like this is useful to complement the experimental information. On the other hand, this methodology can also be applied to evaluate abatement strategies by redistributing traffic emissions.     

Dry deposition of particles and canopy exchange: Comparison of wet,bulk and throughfall deposition at five forest sites in Italy

The contribution of dry deposition to the total atmospheric input of acidifying compounds and base cations is of overwhelming importance. Throughfall measurements provide an estimate of the total deposition to forest soils, including dry deposition, but some uncertainties, related to the canopy interaction processes, affect this approach. We compared the concentrations and the fluxes of the main ions determined in wet-only, bulk and throughfall samples collected at five forest sites in Italy. The contribution of coarse particles deposited onto the bulk samplers was of prime importance for base cations, representing on average from 16% to 46% of the bulk deposition. The extent of this dry deposition depended on some geographical features of the sites, such as the distance from the sea and the annual rainfall. The possibility of applying specific bulk/wet ratios to estimate the wet deposition proved to be limited by the temporal variability of these ratios, which must be considered together with the spatial variability. A direct comparison of the dry contribution deriving from the bulk–wet and the throughfall–wet demonstrated that an extensive natural surface (forest canopy) performs better than a small synthetic surface (funnel of the bulk sampler) in collecting dry deposition of SO₄²⁻, NO₃⁻ and Na⁺. The canopy exchange model was applied to both bulk and wet data to estimate the contribution of dry deposition to the total input of base cations, and the uncertainty associated to the model discussed. The exclusive use of bulk data led to a considerable underestimation of base cation dry deposition, which varies among the study sites.