Development of DOAS System for Hazardous Methane Detection in the Near-Infrared Region

Development and application of UV-visible and mid-IR differential absorption spectroscopy techniques for pollutant trace gas monitoring

A differential optical absorption spectroscopy (DOAS) system for measurement of atmospheric NO2 was developed. The system uses a battery-operated, high luminance LED and a fiber-coupled spectrometer, and is portable. Laboratory experiments using a gas cell of length 0.22 m with varying NO2 concentrations were performed to evaluate the sensitivity of the DOAS system. The DOAS measurement results are in agreement with NO2 concentrations obtained simultaneously by a FT-IR (Fourier Transform Infrared) system for NO2 concentrations down to 20 ppm. Experiments with an optical path length of 93 m were also performed, and NO2 concentrations down to 0.20 ppm were measured. Since measurement of atmospheric NO2, which is in the order of several tens of ppb, requires optical path lengths of several hundred m, system improvements to improve the signal detection are necessary.

The real-time measurement of atmospheric ammonia at municipal solid waste (MSW) landfills and adjacent areas is necessary for landfill management and the health of nearby residence. Continuous, fast, and real-time monitoring of landfill odor gases is a challenge, especially for ammonia. To our knowledge, this was the first study for the characteristics and seasonal variabilities of atmospheric ammonia at a whole landfill using a Mobile White cell Differential Optical Absorption Spectroscopy (MW-DOAS) system, which also simultaneously offers high sensitivity and fast response. Results show that atmospheric ammonia levels at various landfill areas were significantly dependent on the characteristics of areas, such as municipal solid waste-related areas, leachate-related areas, sludge-related areas, and fly ash-related area, the atmospheric ammonia peak or average level at the active leachate pool of the active MSW site was the highest among all areas of the whole landfill, and the ammonia concentrations at the closed MSW landfill sites were low and dependent on the ages. Moreover, it was found that the seasonal variabilities of ammonia concentrations at most of those areas were significantly dependent on the ambient temperature, and ambient temperature variation caused the atmospheric ammonia level at the active leachate pool and active MSW landfill site in the summer survey to raise 3.5 times and 5.58 times than in the winter survey, respectively. Implications: Continuous, fast, and real-time monitoring ambient ammonia at or nearby a landfill is critical for landfill operators and local EPAs. This study demonstrates that the mobile White cell Differential Optical Absorption Spectroscopy (MW-DOAS) system is an effective tool for real-time monitoring ambient ammonia of a whole landfill. The results in this article provided a guideline to the characteristics and seasonal changes of ambient ammonia at various types of areas of a whole landfill as well as the impact of age to ambient ammonia at the closed landfill areas.

A high-resolution differential optical absorption spectroscopy (DOAS) system for long-path atmospheric pollution monitoring is described. The system, consisting of a broadband lamp and a dispersive, fast-scanning optical receiver, separated by a few kilometers, was used in measurements of different pollutants, highlighted by the monitoring of the local concentration of atomic mercury. Mercury levels in the ppt (1:10(12)) range were assessed by comparisons with laboratory measurements.

Boundary layer trace gas and particulate extinction measurements in the wavelength region from 320 nm to 680 nm have been performed by means of a differential optical absorption spectroscopy (DOAS) system at Cape Arkona (Island Rügen/Germany). The wavelength‐dependent extinction coefficient σe(λ) is determined from the attenuation of radiation along two paths of different lengths. Nearly simultaneous recording of the concentrations of O3 and NO2 within the same air mass allows a correction of their contribution to σe(λ). The extinction coefficients are compared with volume scattering coefficients measured by an open integrating nephelometer. Differences between the measurements can be attributed to atmospheric inhomogeneities. A simplified model involving monomodal log‐normal distributions is applied to approximate the total particle surface area concentration and an upper limit of the accumulation mode radius. This allows a rough estimate of the particle number concentration. During the summer of 1995 the mean (median) surface area concentration was about 350 (250) µm²/cm³, and the particle number concentration ranged from about 100 cm−3 to some 1000 cm−3.

A differential optical absorption spectroscopy (DOAS) system was designed and operated in Christchurch, New Zealand, during the winter of 1997. We report the detection of an unidentified absorber in the 279 to 289 nm spectral region, which is used by many commercial DOAS systems for the measurement of atmospheric ozone on short absorption paths. Tests were performed to ensure that this absorber is not an artifact of the analysis procedure. The presence of the unidentified absorber can have serious consequences on ozone concentrations obtained with the DOAS technique when primary pollutant concentrations are high, while it is hardly noticeable under clean conditions. It is argued that either the absorber may be specific to New Zealand emissions, possibly due to the widespread use of wood fires for domestic heating, or that its influence is generally overlooked because interest in ambient ozone usually concentrates on summer smog conditions, where primary pollutant concentrations are generally lower than during winter.

The objective of this study is to analyze the concentrations of SO2, NO2, and O3 measured by a Differential Optical Absorption Spectroscopy (DOAS) system that was operating at the campus of Technological Education Institute of Piraeus during 2008 and 2009 warm periods (July to September) in relation to the prevailing meteorological conditions. The DOAS system was operating in a particularly polluted area of the West part of Attica basin on a continuous basis, measuring the concentration levels of the main pollutants (O3, NO2, and SO2) as well as aromatic hydrocarbon substances (benzene, toluene, and xylene). According to the analysis, the SO2 concentration levels at this measuring site are rather high and this may be attributed to the characteristics of this measuring site. Proximity of roadways and local circulation are just some of the factors that can affect the concentration levels of monitoring of pollutant concentrations such as NO2 and surface ozone. The results provide evidence for the occurrence of an atmospheric phenomenon that produces higher ozone concentrations during weekends despite lower concentrations of ozone precursors. This phenomenon is known as the weekend effect.

We describe an instrument for measuring the particle extinction coefficient at ambient conditions in the spectral range from 270 to 1000 nm. It is based on a differential optical absorption spectroscopy (DOAS) system, which was originally used for measuring trace-gas concentrations of atmospheric absorbers in the ultraviolet-visible wavelength range. One obtains the particle extinction spectrum by measuring the total atmospheric extinction and subtracting trace-gas absorption and Rayleigh scattering. The instrument consists of two nested Newton-type telescopes, which are simultaneously used for emitting and detecting light, and two arrays of retroreflectors at the ends of the two light paths. The design of this new instrument solves crucial problems usually encountered in the design of such instruments. The telescope is actively repositioned during the measurement cycle. Particle extinction is simultaneously measured at several wavelengths by the use of two grating spectrometers. Optical turbulence causes lateral movement of the spot of light in the receiver telescope. Monitoring of the return signals with a diode permits correction for this effect. Phase-sensitive detection efficiently suppresses background signals from the atmosphere as well as from the instrument itself. The performance of the instrument was tested during a measurement period of 3 months from January to March 2000. The instrument ran without significant interruption during that period. A mean accuracy of 0.032 km(-1) was found for the extinction coefficient for an 11-day period in March.

The development of an Ultra Violet (UV) Differential Optical Absorption Spectroscopy (DOAS) fibre-optic sensor for the monitoring of nitric oxide gases is described in this paper. Experimental results describing the operation of this sensor with cylinder gases are presented. These experimental results are compared with existing published spectroscopic absorption measurements. The sensor was developed to operate within an exhaust environment and demonstrate a low susceptibility to interferences from other gases present. A LabVIEW program was created to interrogate the highest absorbing wavelength for nitric oxide and calculate the concentrations present before outputting them to the user. The lower limit of detection for the sensor was found to be 5ppm with response times of 3.4 seconds.

The Institute for Environmental Research and Sustainable Development of the National Observatory of Athens in collaboration with the Technological Educational Institute of Piraeus (TEIP) has installed a Differential Optical Absorption Spectroscopy (DOAS) system at the TEIP campus in order to study the atmospheric pollution impacts in the western Athens area, aiming at the representation of the air pollution levels in a particularly polluted urban area. This system, operating on a continuous basis, measures the concentration of the main pollutants (O3, NO2, SO2) as well as BTX (benzene, toluene, xylene). The main objective of this study is to analyse the mean hourly benzene concentrations measured by the DOAS system for the whole of 2009 in order to study both the diurnal and the seasonal variation. The major finding is that the mean seasonal variation of the examined air pollutant presents a minimum during the warm period of the year and a maximum during the cold period. The mean diurnal variation of benzene concentrations is characterized by a double peak structure. The mean values of benzene concentration for the whole year were found equal to 5.97 μg/m3, which is in accordance with the EU upper limit value of 6 μg/m3 for the year 2009.

The ambient concentration of SO2, NOx and Ox in the atmosphere of Hiroshima, Fukuyama and Fuchu city which were monitored by the prefectural monitoring stations, are examined to give a picture of the typical air pollution at these sites. Results show that the yearly concentrations of SO2 in these areas are significantly fall from 20 to 6 ppb during 1978 –1996 when the NOx concentrations having no such significant change which varies from 40 to 30 ppb. The Photochemical Oxidant (Ox) increases annually at the rate of 0.3 ppb to 0.6 ppb in Hiroshima city only. To know the present situation of air pollution the Differential Optical Absorption Spectroscopy (DOAS) system is used in the city of Higashi Hiroshima. The daily average concentrations of SO2 NO2, O3 and HONO measured during the period of August 1999 to March 2000 ranged from 1.4 ppb to 2.8 ppb, 13 ppb to 26.9 ppb, 21 ppb to 53. 6 ppb and 1 ppb to 4.3 ppb respectively. The patterns of concentrations of NO2 and O3 measured by DOAS look similar to the seasonal patterns of NOx and Ox by the conventional system.

Ten-year measurements of gaseous pollutants in urban air by an open-path analyzer

Aspects of year-long differential optical absorption spectroscopy and ground station measurements in an urban street canyon near industrial pollution sources

SO2 and NO2 are the most important pollution in atmosphere. An optimized long path (LP) differential optical absorption spectroscopy (DOAS) system of high light intensity at an ultraviolet (UV) wavelength is proposed and used to measure the concentration of SO2 and NO2 simultaneously. In contrast to the traditional DOAS, the system adopted a Y-type optical fiber structure instead of a combination of mirrors in the telescope. The UV light intensity test shows that the light intensity of UV can arrive to above 80% of the max measuring range when the light path reaches 135 m, and the integral time of the spectrograph is only 15 ms. The system is proved to be efficacious through laboratory calibration. The maximum error of SO2 calibration is 4.19%, and is 5.22% for NO2. The error of the SO2 and NO2 mixture calibration is within 10%. Field measurement is implemented in a wastewater treatment plant in winter. The measurement light path is 738 m. The concentration of SO2 varies from 6 μg/m3 (2.26 ppb) to 20 μg/m3 (7.52 ppb), and the concentration of NO2 varies from 100 μg/m3 (53.2 ppb) to 200 μg/m3 (106.4 ppb) approximately. The results are in accordance with the data from a monitoring station nearby in magnitude order and variation tendency mostly.

Up to now emission source strengths of diffuse and heterogenous emission of important VOCs are not well known especially from gas stations and gasoline tank farms. To estimate the total emission of these sources non-intrusive measurements were performed by a differential optical absorption spectroscopy (DOAS) system to determine the path- integrated concentrations of exhaust compounds downwind of the source through the whole exhaust plume. Simultaneously, the meteorological parameters were measured for modeling the dispersion of the plume inversely to obtain the emission source strengths of these compounds. The emissions by road traffic were determined by an additional open-path DOAS measurement. Measurement campaigns were performed during different wether conditions and at different sources which were characterized by well defined and easy air flow conditions. The emission source strengths were calculated with the Gaussian model PAL. The determined total emission of gas stations with gasoline vapor recovery system are about 20 mg benzene per kg refueled gasoline and the emission from refueling activities vary between 1 and 9 benzene per kg refueled gasoline depending on the technical behavior of the gasoline vapor recovery system. These values which were found from measurements during times with a and without refueling activities show a high amount of diffuse emissions. The emission rates from a gasoline taken farm were measured on an open path through the middle of that area and a maximum of 8 (mu) g/(m<SUP>2</SUP>s) was determined.