Validation study of WindTrax reverse dispersion model coupled with a sensitivity analysis of model-specific settings.

In last years, atmospheric dispersion models have reached considerable popularity in environmental research field. In this regard, given the difficulties associated to the estimation of emission rate for some kind of sources, and due to the importance of this parameter for the reliability of the results, Backward dispersion models may represent promising tools. In particular, by knowing a measured downwind concentration in ambient air, they provide a numerical value for the emission rate. This paper discusses a critical validation of the WindTrax Backward model: the investigation does not only deal with the strict reliability of the model but also assesses under which conditions (i.e. stability class, number, and location of the sensors) the model shows the greatest accuracy. For this purpose, WindTrax results have been compared to observed values obtained from available experimental datasets. In addition, a sensitivity study regarding model-specific parameters required by WindTrax to replicate the physics and the random nature of atmospheric dispersion processes is discussed. This is a crucial point, since, for these settings, indications on the numerical values to be adopted are not available. From this study, it turns out that the investigated model specific settings do not lead to a significant output variation. Concerning the validation study, a general tendency of the model to predict the observed values with a good level of accuracy has been observed, especially under neutral atmospheric conditions. In addition, it seems that WindTrax underestimates the emission rate during unstable stratification and overestimates during stable conditions. Finally, by the definition of alternative scenarios, in which only a portion of the concentration sensors was considered, WindTrax performance appears better than acceptable even with a small number of concentration sensors, as long as the positioning is in the middle of the plume and not in the strict vicinity of the source.

Toxicological assessment method for evaluating the occupational risk of dynamic olfactometry assessors.

The paper aims to propose a new method to evaluate the occupational exposure risk for examiners involved in dynamic olfactometry. Indeed, examiners are possibly exposed to hazardous pollutants potentially present in odorous samples. A standardized method to evaluate the examiners' occupational safety is not yet available and the existing models present some critical aspect if applied to real odorous samples (no uniform reference concentrations applied and presence of compounds for which no toxicity threshold is available). A deepening of assessment procedure to evaluate the occupation exposure risk for olfactometric assessors is necessary. This paper proposes a standardized approach for risk assessment in dynamic olfactometry. The proposed approach allows the quantification synthetic and conservative risk indices. In this model, the use of the hazard index for the odorous mixture was proposed to assess the non-carcinogenic risk; the calculation of the inhalation risk was applied to estimate the carcinogenic risk. Different databases can be used to retrieve proper occupational exposure limits, according to the proposed hierarchical basis. These implementations allow obtaining the complete characterization of real samples which can be used to calculate the minimum dilution factor for protecting the panellists' health.

Proposal of Odor Nuisance Index as Urban Planning Tool.

This article analyzes the state of the art of the methods and models used for the characterization of odor annoyance and it preliminary advances some proposals for the evaluation of the olfactory nuisance. The use of a sensorial technique, such as dynamic olfactometry, is proposed for the analysis of odor concentrations, odor emission rates, and odor dispersions. A simple model for the quantification of environmental odor nuisance, based on the use of FIDOL factors, that are, frequency, intensity, duration, hedonic tone, and location, is proposed.

Chemical and olfactometric inventory of gaseous spot vents in Mt. Amiata volcanic-geothermal area (Italy).

Geothermal areas are typically characterised by the presence of gases and odours in the background atmosphere, stemming from natural emissions and possible mining exploitation of the area. This study presents the first olfactometric investigation of endogenous gas emissions from natural and archaeo-industrial vents in a geothermal area. Mt. Amiata is known for its complex geology and historical cinnabar mining. This study offers an inventory of spot gas emissions, not only in terms of odour and chemical concentration but also including flux data, a ground-breaking achievement in this field. The primary challenge of this investigation was estimating the emitted flow from ground holes or mine entrances, posing the risk of hazardous anoxic conditions. To address this challenge, an innovative and adaptive approach was adopted. The main breakthrough method involved the adaptation of a balometer, typically employed for indoor ventilation systems, to measure the flow of endogenous gases. Field surveys revealed odour concentrations that can exceed 106 of ouE/m3, surpassing industrial emission level considerably. Chemical concentrations, primarily consisted of CO2 (80/90 %v/v) and CH4 (∼10%v/v), providing critical insights into the global warming potential (GWP) associated with natural emissions. Moreover, these spots, often located at ground level and lacking a substantial atmospheric dilution, pose potential risks to nearby individuals, with concentrations of gases such as H2S surpassing safety thresholds. Total emissive flux of the investigated spot vents in the Mt. Amiata area, showed that the emission rate of H2S is notably substantial (55 kg/h), roughly equivalent to emissions from approximately four 20 MW geothermal plants, as along with odour emission rates in the order of 107 ouE/s. Considering the GWP derived from emitted gases, the total inventory assessment of the spot vents resulted in 36 t/h or 23 t/h CO2eq, depending on the time horizon considered (GWP20 or GWP100 respectively).

Experimental evaluation on liquid area sources: Influence of wind velocity and temperature on the wind tunnel sampling of VOCs emissions from wastewater treatment plants.

The investigation of Volatile Organic Compounds (VOCs) emission from wastewater basins is a challenging issue. In particular, the quantification of an accurate emission rate appears quite tricky, since the release of VOC compounds from this type of source, and the subsequent dispersion into the atmosphere, is ruled by different complex phenomena, potentially affected by a variety of external chemical and physical parameters. In this regard, the wind velocity and the liquid temperature represent variables that are worth investigating. Given this, the present paper discusses an experimental study aimed at evaluating the influence of these variables on the emission rate of VOCs (i.e. acetone, toluene and butanol) in solution with water at low concentrations (0.5 mL/L and 5 mL/L). The experimental trials are conducted using a wind tunnel system, changing the sweep air flow from 0.02 m/s to about 0.06 m/s and the liquid temperature from 20 °C to 35 °C. This study reveals that while the wind velocity seems to slightly influence the emission rate of VOCs estimated by wind tunnel sampling, the effect of the temperature appears much more significant. This behaviour is also confirmed by experimental trials conducted on real-case industrial wastewater, coming from an equalization tank. In view of this, the approach commonly applied to evaluate the influence of wind velocity (i.e. a dependence of the odour emission rate on the square root of the wind velocity) appears not fully consistent with the experimental results obtained at low concentrations by wind tunnel sampling. Also, the influence of temperature seems more pronounced in the case of butanol, in accordance with the theoretical trend of Henry constant as a function of temperature.

Micrometeorological Methods for the Indirect Estimation of Odorous Emissions.

Odors are typically released into the atmosphere as diffuse emissions from area and volume sources, whose detailed quantification in terms of odor emission rate is often hardly achievable by direct source sampling. Indirect methods, involving the use of micrometeorological methods in order to correlate downwind concentrations to the emission rates, are already mentioned in literature, but rarely found in real applications for the quantification of odor emissions. The instrumentation needed for the development of micrometeorological methods has nowadays become accessible in terms of prices and reliability, thus making the implementation of such methods to industrial applications more and more interesting. For this reason, this work aims to provide an overview of micrometeorological methods and investigate their effective applicability to odors, thereby providing a short description of the physics related to such methods and analyzing the relevant scientific literature. The theoretical basis of these methods is presented, and their advantages and disadvantages are discussed. Moreover, their applicability to the estimation of odor emissions is discussed by providing some suggestions about the suitable ways to evaluate the most critical parameters needed for the calculation of the odor emission rate.

Application of different packing media for the biofiltration of gaseous effluents from waste composting.

A pilot-scale experiment was implemented in a waste bioreactor with an inner capacity of 1 m3 in order to simulate a real-scale composting process. The waste underwent composting conditions that are typical of the initial bio-oxidation phase, characterised by a high production of volatile organic compounds (VOCs), hydrogen sulphide (H2S) and odorants. The waste bioreactor was fed with an intermittent airflow rate of 6 Nm3/h. The target of this study was to investigate the air treatment performance of three biofilters with the same size, but filled with different filtering media: (1) wood chips, (2) a two-layer combination of lava rock (50%) and peat (50%), and (3) peat only. The analyses on air samples taken upstream and downstream of the biofilters showed that the combination of lava rock and peat presents the best performance in terms of mean removal efficiency of odour (96%), total VOCs (95%) and H2S (77%) concentrations. Wood chips showed the worst abatement performance, with respective mean removal efficiencies of 90%, 88% and 62%. From the results obtained, it is possible to conclude that the combination of lava rock and peat can be considered as a promising choice for air pollution control in waste composting facilities.

Dynamic Olfactometry and Oil Refinery Odour Samples: Application of a New Method for Occupational Risk Assessment.

Refineries are characterized by relevant odour impacts, and the control and monitoring of this pollutant have become increasingly important. Dynamic olfactometry, a sensorial analysis that involves human examiners, is currently the most common technique to obtain odour quantification. However, due to the potential presence of hazardous pollutants, the conduction of occupational risk assessment is necessary to guarantee examiners' safety. Nevertheless, the occupational risk for olfactometric examiners, specifically correlated with oil refineries emissions, has not been investigated yet. Therefore, this paper applies a new methodology of risk assessment for workers involved in dynamic olfactometry, focusing on odorous refineries emissions. The chemical characterization of refinery emissions was obtained by TD-GC-MS, analysing odorous samples collected at different refinery odour sources. A database of chemical pollutants emitted from a refinery plant was built up, and the minimum dilution values to be adopted during the analysis of refinery odorous samples was calculated. In particular, this evaluation highlighted that, in this scenario, a non-negligible carcinogenic risk may exist for panellists exposed to refineries' samples, and the carcinogenic risk is sometimes higher than what is acceptable. Therefore, a minimum dilution value between 1.01 and 5, according to the specific sample, must be set to guarantee the examiners' safety.

Simulating Pollutant Dispersion from Accidental Fires with a Focus on Source Characterization.

BACKGROUND: Storage tanks in oil and gas processing facilities contain large volumes of flammable compounds. Once the fuel-air mixture is ignited, it may break out into a large fire or explosion. The growing interest in monitoring air quality and assessing health risks makes the evaluation of the consequences of a fire an important issue. Atmospheric dispersion models, which allow for simulation of the spatial distribution of pollutants, represent an increasingly widespread tool for this type of evaluations. OBJECTIVES: The present study discusses the set up and results of a modeling study relevant to a hypothesized fire in an oil refinery. METHODS: After choosing the most suitable dispersion models, i.e. the Lagrangian model SPRAY and the puff model CALPUFF, estimation of the required input data is discussed, focusing on the source variables, which represent the most uncertain input data. The results of the simulations were compared to regulatory limits to effectively evaluate the environmental consequences. Finally, a sensitivity analysis was employed to identify the most influential variables. RESULTS: The simulation results revealed that ground concentration values were far below the cited long-term limits. However, the most interesting outcome is that depending on the dispersion model and the source type modeled, different results may be obtained. In addition, the sensitivity study indicates that the source area is the most critical variable, since it determines a significantly different behavior depending on the modeled source types, producing, in some cases, variability in the pollutant ground concentrations on selected receptors up to +/- 60%. CONCLUSIONS: Depending on the selected model and the algorithms available to describe the physics of emission, the results showed a different sensitivity to the input variables. Although this can be explained from a mathematical point of view, the problem remains of choosing case by case the option that best approximates the real behavior of the incidental source under investigation. COMPETING INTERESTS: The authors declare no competing financial interests.

A model for the evaluation of organic compounds emission from aerated liquid surfaces.

The purpose of this study is to deepen the knowledge of the various emission phenomena present in aerated tanks, widely used systems for municipal and industrial wastewater treatment. In order to investigate the emission mechanism, a specific model was developed. The theoretical model proposes to consider three different contributions to the emission of organic compounds from aerated wastewater tanks: the convection due to the sweep air flow rate, the rising bubbles stripping and the aerosol formation and successive evaporation. To compare the modeled results, an experimental campaign was conducted with two different solutes, acetone and butanol. The sampling was carried out with a Wind Tunnel system and the outflow gas samples were analysed with gas chromatographic technique. Moreover, this study investigates the dependence of the concentration in the gaseous phase from the speed of the air on the surface (1-5 cm/s) and from the flow of air diffused inside the liquid body (50-200 L/h). The empirical data were compared with theoretical curves. The results confirm two facts: the gas solute concentration decreases as the air velocity increases and, instead, increases with the air flow diffused through the tank.

Odour impact assessment by considering short-term ambient concentrations: A multi-model and two-site comparison.

Short-term events are one of the specific aspects that differentiate odour nuisance problems from conventional air quality pollutants. Atmospheric dispersion modelling has been considered the gold standard to realise odour impact assessments and to calculate separation distances. Most of these models provide predictions of concentrations of a pollutant in ambient air on an hourly basis. Even when the hourly mean odour concentration is lower than the perception threshold, concentration peaks above the threshold may occur during this period. The constant peak-to-mean factor is nowadays the most widespread method for evaluating short-term concentrations from the long-term ones. Different approaches have been proposed in the scientific literature to consider non-constant peak-to-mean factors. Two prominent approaches to do so are the i) variable peak-to-mean factor which considers the distance from the source and atmospheric stability and the ii) concentration-variance transport. In this sense, the aim of this work is to compare the results of three different freely available dispersion models (namely, CALPUFF, LAPMOD and GRAL), which implement three distinct ways to evaluate the short-term concentration values. Two sites, one in Austria and the other in Italy, were selected for the investigation. Dispersion model results were compared and discussed both in terms of long-term (hourly) concentrations and short-term. An important outcome of this work is that the dispersion models provided more equivalent results for hourly mean concentrations, in particular in the far-field. On the contrary, the method to evaluate short-term concentrations can deliver disparate results, thereby revealing a potential risk of poor assessment conclusions. The utilistion of a multiangle methodological approach (dispersion models, study site locations, algorithms to incorporate short-term concentrations) allowed providing useful information for future studies and policymaking in this field. Accordingly, our findings call for awareness on how the use of a particular dispersion model and its sub-hourly peak calculation method can affect odour impact assessment conclusions and compliance demonstrations.

Assessment of the chemical-physical variables affecting the evaporation of organic compounds from aqueous solutions in a sampling wind tunnel.

The aim of this work is the evaluation and the analysis of the different chemical-physical variables that affect the emission of volatile organic compounds (VOC) and odours from passive liquid area sources inside a wind tunnel, which is typically used for emission sampling. Three different compounds (acetone, butanol and ethanol), having different volatilization properties (e.g., boiling point, solubility), were studied in solution with water at different concentrations. The following physical parameters affecting the VOC volatilization in the Wind Tunnel system were evaluated: the velocity of the air flowing through the device, in a range from 0.01 to about 0.05 m/s, and the temperature of both the liquid source and the sweep air flow, in a range from 12 °C to 42 °C. The experimental results were compared with the existing volatilization models available in literature. In most cases the proposed theoretical model predicts well the experimentally measured concentration. Some discrepancies were observed for lower velocities and also by moving from the room temperature (20 °C); and those were discussed by making some considerations about the volatilization phenomenon. Moreover, the study clearly shows that it is not the gas phase temperature that controls the emission, but the temperature of the liquid phase, due to the effect of the latter on the vapour pressure of the compound, which is the main driving force of the phenomenon.