Locating industrial VOC sources with aircraft observations

https://doi.org/10.1016/j.envpol.2011.02.013Get rights and content

Abstract

Observation and characterization of environmental pollution, focussing on Volatile Organic Compounds (VOCs), in a high-risk industrial area, are particularly important in order to provide indications on a safe level of exposure, indicate eventual priorities and advise on policy interventions. The aim of this study is to use the Solid Phase Micro Extraction (SPME) method to measure VOCs, directly coupled with atmospheric measurements taken on a small aircraft environmental platform, to evaluate and locate the presence of VOC emission sources in the Marghera industrial area. Lab analysis of collected SPME fibres and subsequent analysis of mass spectrum and chromatograms in Scan Mode allowed the detection of a wide range of VOCs. The combination of this information during the monitoring campaign allowed a model (Gaussian Plume) to be implemented that estimates the localization of emission sources on the ground.

Highlights

► Flight plan aimed at sampling industrial area at various altitudes and locations. ► SPME sampling strategy was based on plume detection by means of CO2. ► Concentrations obtained were lower than the limit values or below the detection limit. ► Scan mode highlighted presence of γ-butyrolactone (GBL) compound. ► Gaussian dispersion modelling was used to estimate GBL source location and strength.

Introduction

Investigating relations between environmental air pollution and public health in urban and industrial areas requires the capability to properly sample and model air properties, concentrations and dispersion of the compound of interest, at appropriate spatial and temporal scales. Pollutants do not remain in the medium where they originate but move across environmental phase boundaries, becoming distributed throughout the various environmental compartments as the result of complex physical, chemical, and biological processes. The resulting environmental and human health risks depend upon the degree of exposure of human and ecological receptors, via multiple pathways, to pollutant chemicals (Cohen, 1996). Volatile organic compounds (VOCs), in particular, are highly mobile in the environment; VOCs that are initially present in soil or water can readily volatilize to the atmosphere where they can be transported over significant distances from the source location (Cohen, 1996). Considerable quantities of VOCs are produced in industrialized nations, they are contained in many manufactured products, including paints, adhesives, gasoline, plastics and many of them are mobile, persistent and toxic. In the atmosphere, many VOCs can have a relatively short half-life of a few hours due to degradation, whereas in other media they can be very persistent and display little degradation over a period of years (Squillace et al., 1999). In most urban areas, VOCs can contribute substantially to the total cancer risk associated with toxic air pollutants (Cohen, 1996, Kampa and Castanas, 2008). The potential health risks associated with exposure to VOCs and their role in the formation of photochemical smog have led to increasing public concern about the presence of VOCs in the environment (Dunovant et al., 1986, Loehr and Ward, 1987, Govind et al., 1991, Finlayson-Pitts and Pitts, 2000, Molina and Molina, 2002, Volkamer et al., 2005).

Several methods are available for the sampling of VOCs in the air. The most frequently used are adsorption techniques (Pompe et al., 2000, Helmig and Vierling, 1995, Zielinska et al., 1996). The trapped VOCs are subsequently thermally desorbed and analyzed by GC–FID or GC–MS. These measurements are highly sensitive and give very detailed information on the atmospheric composition (Taipale et al., 2008). Another technique for online measurement of atmospheric volume mixing ratios of VOCs is based on Proton Transfer Reaction Mass Spectrometry (PTR-MS) (Lindinger et al., 1998). In the PTR-MS instrument, ambient air is continuously pumped through a drift tube reactor and the VOCs in the sample are ionized in proton transfer reactions with hydronium ions (H3O+) (Taipale et al., 2008). Measurements of VOCs in the earth’s atmosphere using PTR-MS have recently been reviewed by De Gouw and Warneke (2007) and various approaches have been reported to improve the selectivity by combining the PTR-ionization with more powerful separation techniques. The GC–PTR-MS approach, i.e., coupling a chromatographic column to a PTR-MS instrument (Fall et al., 2001, Karl et al., 2001, Warneke et al., 2003) allows for greater selectivity than PTR-MS alone, but sample pretreatment steps are necessary and the sampling frequency is strongly reduced to only one per 30 min. Recently, several groups have reported the coupling of a chemical ionization unit to a time of flight (TOF) mass spectrometer (Blake et al., 2004, Ennis et al., 2005, Tanimoto et al., 2007, Wyche et al., 2007) increasing mass resolution (Grauss et al., 2010).

All such techniques require instruments located in a controlled environment, with continuous on-site monitoring or mobile monitoring that is expensive and difficult to calibrate (Tumbiolo et al., 2005).

By exposing a media to air that is capable of containing or adsorbing certain types of molecules, and then analyzing the media in the Lab with one of the above-cited techniques, it becomes possible to monitor in a wide range of conditions. Examples are canisters, flasks and SPME.

Solid Phase Micro Extraction (SPME) is a sample preparation technique used both on-site and in the laboratory based on a simple and inexpensive technique that can be thought of as a very short gas chromatography column turned inside out. SPME involves the use of a fibre coated with an extracting phase, that can be a liquid (polymer) or a solid (sorbent), which extracts different kinds of analytes (including both volatile and non-volatile) from different kinds of media that can be in liquid or gas phase. After extraction, the SPME fibre is transferred to the injection port of separating instruments, such as a Gas Chromatography (GC), where desorption of the analyte takes place and analysis is carried out. The attraction of SPME is that the extraction is fast and simple and can be done without solvents, and detection limits can reach parts per trillion levels for certain compounds.

SPME has been widely used for many years in various applications, such as environmental and water samples, food and fragrance analysis, or biological fluids (Khaled and Pawliszyn, 2000, Isetun et al., 2004, Martos and Pawliszyn, 1999, Pacenti et al., 2010).

By measuring pollutants concentrations in the atmosphere with one of the above-cited techniques, the level of exposure can be assessed, but it is not generally possible to estimate the location and strength of source areas (Kume et al., 2008), due to turbulent transport and mixing. On the other hand, when air and turbulence properties are sampled at an appropriate temporal and spatial resolution, this combined information can be used to investigate and resolve atmospheric transport and diffusion processes.

Aircraft measurements, simultaneously providing the measurement of the wind vector and the compound of interest, have been used to estimate the CO2 source strength of relatively large industrial areas by applying mass budgeting techniques (Alfieri et al., 2010). Such techniques are particularly suitable for diffuse emissions over large areas, while in the presence of strong emissions from industrial plants, behaving like point sources, the strength of the source can be estimated by measuring both turbulent transport and concentration inside the plume downwind of the emission point and applying inverse dispersion modelling techniques (Lushi and Stockie, 2010, Flesch et al., 2005).

While continuous measurement of gases like CO2 or pollutants like NOx and O3 is now relatively easy, the determination of VOC compounds remains much more difficult. When dealing with aircraft observations, in-flight real time determination of VOCs is an even a bigger challenge, due to installation and operational constraints. Only a few applications exist implementing a PTR-MS system on aircraft (Hansel et al., 1999, Murphy et al., 2010), but they are confined to the use of large and expensive aircraft facilities. The SPME method can instead provide a simple and easily deployable solution.

The aim of this study is to use the SPME method to measure VOCs, directly coupled with atmospheric measurements taken on a small aircraft environmental platform, to evaluate and locate the presence of VOC emission sources on the ground in industrial areas. The aircraft is the Sky Arrow ERA, originally equipped to measure wind speed and direction at high frequency, CO2 and H2O concentrations, temperature, and other variables, and capable of flying at low altitude and reduced ground speed (Gioli et al., 2004).

Our experimental flight strategy was to fly repeated transects downwind of an intensive industrial area, at different altitudes and over different horizontal tracks. During these transects fibres were exposed to high CO2 concentrations (i.e., within the plume of an emission area), given that CO2 patterns were available in real time during flights. A Gaussian dispersion model was then run on the flight spatial domain with an unknown emission source strength, and an optimization algorithm was adopted to potentially derive both location and strength of emission sources.

The challenges of this method are related to the nature of the SPME technique, which is conditional sampling, i.e., fibres are exposed to air for given time intervals during the flight, in a non-continuous mode, and to the spatial resolution of the measurements, since the aircraft covers distances of approximately 3.7–4.4 km and the fibres are exposed for 120 s during the flight, so the measurements result as spatially integrated, limiting the detection capability of the method.

Section snippets

Sky Arrow ERA platform

Aircraft measurements were made by a Sky Arrow ERA (Fig. 6), equipped with a Mobile Flux Platform (MFP) which consists of a set of sensors for atmospheric measurements. The platform is described in detail in other works (Dumas et al., 2001, Gioli et al., 2006). In brief, the aircraft uses the Best Atmospheric Turbulence (BAT) probe to measure the velocity of air with respect to the aircraft using a hemispheric nine-hole pressure sphere that records static and dynamic pressures by means of four

Results and discussion

Weather conditions for the measurement period showed a high pressure system centred over North-western Europe in EAWR regime (East Atlantic/West Russia high pressure pattern), with higher than average stability conditions and temperature for the season. Vertical profiles were measured by the Sky Arrow aircraft between 100 m and 1200 m in the morning flight and between 100 m and 2000 m in the afternoon flight. Fig. 2 shows profiles of potential temperature, relative humidity and wind speed and

Conclusions

This paper presents the first reported instance of sampling and high-throughput automated analysis of airborne pollutants using the SAS/MFX Fast GC–MS robotic system, able to process great quantities of samples in a very short time, so reducing the costs of the monitoring campaigns. The attained sensitivity permits the evaluation of airborne concentrations with extremely reduced sampling periods, producing an instantaneous measurement of air pollution levels. An additional advantage of SPME

Acknowledgements

Google, Google Earth, Google, Map are trademarks of Google. Google Earth Toolbox, Ether Skawtus, 10 Nov 2006 (Updated 30 Jun 2009). The authors acknowledge Prof. Luigi Di Prinzio (IUAV University of Venice and UNISKY srl) for providing financial support for flight operations, Emanuele Menna for his valuable help during the campaign, the Sky Arrow pilots Ottavio Fratini and Alessandro Sestili. This work was supported by a grant ofRegione Toscana in the frame of CASPA project.

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