Open-path Fourier Transform Infrared Research Articles

Design and simulation of control and sampling circuits for an open-path Fourier transform infrared spectrometer based on a rotating mirror

Abstract We designed a laser pulse sampling circuit to achieve equal optical path difference sampling for an open-path Fourier transform infrared (OP-FTIR) spectrometer based on a rotating mirror. The trigger signals for the equal optical path difference sampling were successfully obtained through circuit simulations, and an intelligent control circuit based on a 52 single-chip microcomputer (SCM) was used to accurately determine the sampling starting point and range for the spectrometer. By processing the grating scale signal and the grating reference signal, the sampling starting point and range of the spectrometer were determined precisely. Finally, the sampling trigger signals required to accomplish equal optical path difference sampling of the infrared spectrum were successfully outputted within the defined sampling range.

Quantification of gas, ash, and sulphate aerosols in volcanic plumes from open path Fourier transform infrared (OP-FTIR) emission measurements at Stromboli volcano, Italy

Field-portable Open Path Fourier Transform Infrared (OP-FTIR) spectrometers can be used to remotely measure the composition of volcanic plumes using absorption spectroscopy, providing invaluable data on total gas emissions. Quantifying the temporal evolution of gas compositions during an eruption helps develop models of volcanic processes and aids in eruption forecasting. Absorption measurements require a viewing geometry which aligns infrared source, plume, and instrument, which can be challenging. Here, we present a fast retrieval algorithm to estimate quantities of gas, ash and sulphate aerosols from thermal emission OP-FTIR measurements, and the results from two pilot campaigns on Stromboli volcano in Italy in 2019 and 2021. We validate the method by comparing time series of SO2 slant column densities retrieved using our method with those obtained from a conventional UV spectrometer, demonstrating that the two methods generally agree to within a factor of 2. The algorithm correctly identifies ash-rich plumes and gas bursts associated with explosions and quantifies the mass column densities and particle sizes of ash and sulphate aerosols (SA) in the plume. We compare the ash sizes retrieved using our method with the particle size distribution (PSD) of an ash sample collected during the period of measurements in 2019 by flying a Remotely Piloted Aircraft System into the path of a drifting ash plume and find that both modes of the bimodal PSD (a fine fraction with diameter around 5–10 μm and a coarse fraction around 65 μm) are identified within our datasets at different times. We measure a decrease in the retrieved ash particle size with distance downwind, consistent with settling of larger particles, which we also observed visually. We measure a decrease in the SO2/SA ratio as the plume travels downwind, coupled with an increase in measured SA particle size (range 2–6 μm), suggesting rapid hygroscopic particle growth and/or SO2 oxidation. We propose that infrared emission spectroscopy can be used to examine physical and chemical changes during plume transport and opens the possibility of remote night-time monitoring of volcanic plume emissions. These ground-based analyses may also aid the refinement of satellite-based aerosol retrievals.

Real-World Application of Open-Path UV-DOAS, TDL, and FT-IR Spectroscopy for Air Quality Monitoring at Industrial Facilities

Open-path spectroscopy is known for its ability to provide real-time measurements of dozens of compounds over sampling paths of up to 1000 meters in length. Advances in open-path monitoring technology and data processing techniques, coupled with new regulatory requirements, have greatly increased the acceptance and widespread application of spectroscopy-based open-path measurements. Large industrial facilities adjacent to residential communities are a particular application of interest, because traditional fixed-point analyzers lack the spatial coverage of the open-path instruments. This work discusses technical and practical considerations for the installation and operation of more than 120 open-path analyzers that are currently providing continuous data at several oil refineries in California. Open-path analyzers include ultraviolet differential optical absorbance spectroscopy (UV-DOAS), Fourier transform infrared (FT-IR), and tunable diode laser (TDL) technologies. We will discuss lessons learned from these projects, including fundamental approaches to compound identification, target species detectability, interferences, and data management.

Increasing the accuracy of atmospheric ammonia concentrations calculated from Open-Path Fourier transform infrared spectra using partial least squares model by scanning and removing interference spectral data

Monitoring atmospheric pollutants has raised worldwide concerns in last decades, due to deterioration of air quality. Multivariate regression methods have gained extensive applications in calculating and monitoring concentrations of air pollutants, for example ammonia (NH 3 ), using Open-Path Fourier Transform Infrared (OP/FT-IR) spectra data. However, the prediction accuracy of multivariate regression models is interfered heavily by the dominant and omnipresent absorption bands of atmospheric H 2 O vapor and CO 2 in OP/FT-IR spectra. Hence, a new method of variable selection, referred to as window of scanning and removing interference information (WSRII), is developed to remove interference data of OP/FT-IR spectra. The key of WSRII is to confirm informative variables according to the change of root mean square error of calibration in the partial least squares regression (PLSR) model after removing a spectral window. If the change is greater than 0, the spectral window is reserved as an informative information window. Then, the new matrixes of spectral data are reconstructed by using the informative information windows, which are selected by changing position of the spectral window in full wave number range. Based on this, a PLSR model is rebuilt to predict accurately NH 3 concentrations. The results showed that the proposed method was able to eliminate uninformative information and improved prediction accuracy of PLSR models. Moreover, this process of variable selection is significantly potential to improve prediction accuracy of PLSR model for monitoring atmospheric NH 3 concentrations in real time.

Performance of open-path lasers and Fourier transform infrared spectroscopic systems in agriculture emissions research

Abstract. The accumulation of gases into our atmosphere is a growing global concern that requires considerable quantification of the emission rates to mitigate the accumulation of gases in the atmosphere, especially the greenhouse gases (GHGs). In agriculture there are many sources of GHGs that require attention in order to develop practical mitigation strategies. Measuring these GHG sources often relies on highly technical instrumentation originally designed for applications outside of the emissions research in agriculture. Although the open-path laser (OPL) and open-path Fourier transform infrared (OP-FTIR) spectroscopic techniques are used in agricultural research currently, insight into their contributing error to emissions research has not been the focus of these studies. The objective of this study was to assess the applicability and performance (accuracy and precision) of OPL and OP-FTIR spectroscopic techniques for measuring gas mole fractions from agricultural sources. We measured the mole fractions of trace gases methane (CH4), nitrous oxide (N2O), and ammonia (NH3), downwind of point and area sources with a known release rate. The mole fractions measured by OP-FTIR and OPL were also input into models of atmospheric dispersion (WindTrax) allowing the calculation of fluxes. Trace gas release recoveries with WindTrax were examined by comparing the ratio of estimated and known fluxes. The OP-FTIR provided the best performance regarding stability of drift in stable conditions. The CH4 OPL accurately detected the low background (free-air) level of CH4; however, the NH3 OPL was unable to detect the background values <10 ppbv. The dispersion modelling using WindTrax coupled with open-path measurements can be a useful tool to calculate trace gas fluxes from the well-defined source area.

Quantifying the Impact of the COVID-19 Pandemic Restrictions on CO, CO2, and CH4 in Downtown Toronto Using Open-Path Fourier Transform Spectroscopy

During the global COVID-19 pandemic, anthropogenic emissions of air pollutants and greenhouse gases (GHGs), especially traffic emissions in urban areas, have declined. Long-term measurements of trace gas concentrations in urban areas can be used to quantify the impact of emission reductions on GHG mole fractions. Open-path Fourier transform infrared (OP-FTIR) spectroscopy is a non-intrusive technique that can be used to simultaneously measure multiple atmospheric trace gases in the boundary layer. This study investigates the reduction of mole fractions and mole fraction enhancements above background for surface CO, CO2, and CH4 in downtown Toronto, Canada (the fourth largest city in North America) during the 2020 and 2021 COVID-19 stay-at-home periods. Mean values obtained from these periods were compared with mean values from a reference period prior to the 2020 restrictions. Mean CO mole fraction enhancement declined by 51 ± 23% and 42 ± 24% during the 2020 and 2021 stay-at-home periods, respectively. The mean afternoon CO2 mole fraction enhancement declined by 3.9 ± 2.6 ppm (36 ± 24%) and 3.5 ± 2.8 ppm (33 ± 26%) during the stay-at-home periods in 2020 and 2021. In contrast, CH4 mole fraction enhancement did not show any significant decrease. Diurnal variation in CO during the stay-at-home period in 2020 was also significantly reduced relative to the reference period in 2020. These reductions in trace gas mole fraction enhancements coincide with the decline of local traffic during the stay-at-home periods, with an estimated reduction in CO and CO2 enhancements of 0.74 ± 0.15 ppb and 0.18 ± 0.05 ppm per percentage decrease in traffic, respectively.

Ammonia, methane and nitrous oxide emissions from furrow irrigated cotton crops from two nitrogen fertilisers and application methods

• Urea broadcast and fertigation were applied at a commercial irrigated cotton farm. • Gas emissions were measured by micrometeorological technique and OP-FTIR technique. • NH 3 flux from anhydrous NH 3 was 24% higher than broadcasting urea with irrigation. • Daily fluxes of CH 4 and N 2 O were both negative following both fertilisation events. Abstract Agricultural systems use nitrogen (N) fertilizers to improve crop nutrition and yield, but applied N can negatively impact the environment and human health due to the gases emitted during and after N application. There are few direct measurements of multiple gaseous emissions from N fertiliser applied to irrigated cotton farms. This study was conducted in a commercial cotton farm in NSW, Australia to measure gaseous emissions from two N fertilisation strategies (urea broadcast onto dry soil followed by irrigation, anhydrous ammonia injected into water at the main irrigation channel), with the N fertilizer rate of 27 kg N ha −1 for each application. Gas fluxes of ammonia (NH 3 ), methane (CH 4 ), and nitrous oxide (N 2 O) were measured simultaneously using an inverse-dispersion technique coupled with open-path Fourier transform infrared (OP-FTIR) spectroscopic technique, located 150 and 800 m from the input source of the irrigation water. We found that anhydrous ammonia injected into the irrigation water contributed to higher peak NH 3 emissions (0.79 ± 0.09 kg N ha −1 day −1 ) than broadcasting urea onto dry soil (-0.06 ± 0.02 kg N ha −1 day −1 ). When daily NH 3 fluxes were accumulated over 8 − 10 days, 24% of the applied N was lost as NH 3 during anhydrous ammonia application, but no N loss was detected over the course of measurement period from the broadcast urea followed by irrigation. The daily fluxes of CH 4 and N 2 O were both negative following both fertilisation events. Our study suggests that cotton farm managers should avoid the use of anhydrous NH 3 in irrigation water to mitigate the environmental impact, as well as low efficiency of N fertiliser use.

Improving Quantitative Accuracy of Ammonia from Open-Path Fourier Transform Infrared Spectroscopy by Incorporating Actual Spectra into Synthetic Calibration Set of Partial Least Squares Regression

Partial least squares (PLS) regression is often employed for quantification of ammonia (NH3) from open-path Fourier transform infrared (OP/FT-IR) spectra. In this work, PLS models built with both synthetic and actual calibration sets are evaluated, and it is found that the root mean square error of prediction (RMSEP) of the actual model is 86% lower than that of the synthetic model. Although the quantitative accuracy is mediocre, synthetic calibration set is readily constructed, and still required in the beginning when there are insufficient actual OP/FT-IR spectra. However, the RMSEP of the synthetic model is significantly reduced by incorporating some actual spectra into the synthetic calibration set, i.e. by constructing a mixed calibration set. Results show that a mixed calibration set composed of three quarters of synthetic and one quarter of actual spectra reduces the RMSEP by 80%, which performs similarly to the actual calibration set. Synthetic, mixed, and actual calibration sets all have pros and cons, and should be utilized according to respective advantages, which results in a novel strategy for NH3 quantification. The strategy starts with a synthetic model, later builds mixed models by incorporating actual spectra into the synthetic calibration set, and eventually builds an actual model after a large number of OP/FT-IR spectra are collected. This strategy is more accurate than a synthetic calibration set, and more efficient than an actual calibration set, the preparation of which is time-consuming.

Monitoring Urban Greenhouse Gases Using Open-Path Fourier Transform Spectroscopy

ABSTRACTUrban areas are large sources of greenhouse gases (GHGs) to the atmosphere. Measurements of atmospheric GHGs in urban areas provide information on these emissions, which can complement bottom-up estimates. Here, we present an Open-Path Fourier Transform Infrared (OP-FTIR) spectroscopy system for GHG monitoring in Toronto, Canada. We describe the installation of the OP-FTIR and retrieval of CO2, CO, CH4, N2O, and H2O dry-air mole fractions and δD over a two-way atmospheric open path of approximately 320 m using non-linear least squares fitting. The OP-FTIR measurements of CO2, CO, and CH4 are then calibrated using measurements from two Picarro GHG Cavity Ringdown Spectrometers deployed at both ends of the system. Our results show that retrieved dry-air mole fractions of CO2, CO, CH4, and N2O are sensitive to urban emissions from Toronto. In addition, CH4 measurements are influenced by a localized source southwest of the observing system, presumably a natural gas leak, and N2O measurements are influenced by an undetermined source to the northeast of the OP-FTIR. By performing comparisons with measurements from an in situ detector 5.4 km south of the OP-FTIR system, it is demonstrated that the diurnal gradients in CO2 and CO between these sites are enhanced for weekdays relative to weekends, consistent with bottom-up emission inventories. Emissions of CO2 and CO are then calculated from the gradients between the sites. The emissions are found to be consistent with bottom-up estimates but are too imprecise to further refine the bottom-up inventories.

Gas emissions during cattle manure composting and stockpiling.

Manure composting is a common management practice for cattle feedlots, but gaseous emissions from composting are poorly understood. The objective of this study was to quantify ammonia (NH3 ), nitrous oxide (N2 O), carbon dioxide (CO2 ), and methane (CH4 ) emissions from windrow composting (turning) and static stockpiling (nonturning) of manure at a commercial feedlot in Australia. An inverse-dispersion technique using an open-path Fourier transform infrared (OP-FTIR) spectrometer gas sensor was deployed to measure emissions of NH3 , N2 O, CO2 , and CH4 over a 165-d study period, and 29 and 15% of the total data intervals were actually used to calculate the fluxes for the windrow and stockpile, respectively. The nitrogen (N) lost as NH3 and N2 O emissions represented 26.4 and 3.8% of the initial N in windrow, and 5.3 and 0.8% of that in the stockpile, respectively. The carbon (C) lost as CO2 and CH4 emissions represented 44 and 0.3% of the initial C in windrow, and 54.8 and 0.7% of that in the stockpile, respectively. Total greenhouse gas (GHG) emissions from the manure windrow were 2.7 times higher than those of the stockpiled manure. This work highlights the value that could be accrued if one could reduce emissions of NH3 -N and N2 O-N from composting, which would retain manure N content while reducing GHG emissions.

Application of factor and cluster analyses to determine source–receptor relationships of industrial volatile organic odor species in a dual-optical sensing system

Abstract. Most odor nuisance investigations rely on either human olfactory examination or on-site sampling and analytical techniques, but these methods are often subject to spatial and temporal limitations and thus impractical for locating odor emission sources. This study developed an alternative approach with a dual-optical sensing system, a meteorological station, and the combination of factor and cluster analyses to identify and characterize emission sources of multiple air contaminants. Factor and cluster analyses were employed to establish the emission profile of multiple odorous substances from each emission source. Both receptor and source monitoring data were collected to characterize the emission sources of various odorous substances. Open-path Fourier transform infrared (OP-FTIR) as a receptor path detected concurrent trends of several organic solvents with concentrations higher than the reference odor threshold values, indicating that these compounds were potential causes of odor nuisance. Qualitative source apportionment by factor and cluster analyses suggested that these odorous substances were used as organic solvents in surface coating or painting processes. Closed-cell Fourier transform infrared (CC-FTIR) at two nearby surface-coating companies indicated that only one company's stack exhibited the same odorous substance profile found by the OP-FTIR receptor path. The major odor emission source was thus identified in this study. This study demonstrated the feasibility of using the alternative investigative framework to successfully identify emission sources from an industrial odor nuisance site. The major emission sources were identified, and future enforcement plans can be conducted to enhance odor investigation efficiency and improve overall air quality.

Vehicle Ammonia Emissions Measured in An Urban Environment in Sydney, Australia, Using Open Path Fourier Transform Infra-Red Spectroscopy

Airborne particulate matter (PM) is a major health risk in urban settings. Ammonia (NH3) from vehicle exhaust is an under-recognised ingredient in the formation of inorganic PM and there remains a shortage of data to properly quantify the role of NH3 from vehicles in PM formation. An Open-path Fourier transform infra-red (OP-FTIR) spectrometer measured atmospheric NH3, carbon monoxide (CO) and carbon dioxide (CO2) at high temporal resolution (5 min) in Western Sydney over 11 months. The oxides of nitrogen (NO2 and NO; NOx) and sulphur dioxide (SO2) were measured at an adjacent air quality monitoring station. NH3 levels were maxima in the morning and evening coincident with peak traffic. During peak traffic NH3:CO ratio ranged from 0.018 to 0.022 ppbv:ppbv. Results were compared with the Greater Metropolitan Region 2008 (GMR2008) emissions inventory. Measured NH3:CO was higher during peak traffic times than the GMR2008 emissions estimates, indicating an underestimation of vehicle NH3 emissions in the inventory. Measurements also indicated the urban atmosphere was NH3 rich for the formation of ammonium sulphate ((NH4)2SO4) particulate was SO2 limited while the formation of ammonium nitrate (NH4NO3) was NH3 limited. Any reduction in NOx emissions with improved catalytic converter efficiency will be accompanied by an increase in NH3 production and potentially with an increase in NH4NO3 particulate.

Comparison of slant open-path flux gradient and static closed chamber techniques to measure soil N<sub>2</sub>O emissions

Abstract. Improving direct field measurement techniques to quantify gas emissions from cropped agricultural fields is challenging. We compared nitrous oxide (N2O) emissions measured with static closed chambers to those from a newly developed aerodynamic flux gradient (FG) approach. Measurements were made at a vegetable farm following chicken manure application. The FG calculations were made with a single open-path Fourier transform infrared (OP-FTIR) spectrometer (height of 1.45 m) deployed in a slant-path configuration, sequentially aimed at retro reflectors at heights of 0.8 and 1.8 m above ground. Hourly emissions were measured with the FG technique, but once a day between 10:00 and 13:00 with chambers. We compared the concurrent emission ratios (FG∕chamber) of these two techniques and found N2O emission rates from a celery crop farm measured at midday by FG were statistically higher (1.22–1.40 times) than those from the chambers measured at the same time. Our results suggest the OP-FTIR slant-path FG configuration worked well in this study: it was sufficiently sensitive to detect the N2O gradients over our site, giving high temporal resolution N2O emissions corresponding to a large measurement footprint.

Emissions of trace gases from Australian temperate forest fires: emission factors and dependence on modified combustion efficiency

Abstract. We characterised trace gas emissions from Australian temperate forest fires through a mixture of open-path Fourier transform infrared (OP-FTIR) measurements and selective ion flow tube mass spectrometry (SIFT-MS) and White cell FTIR analysis of grab samples. We report emission factors for a total of 25 trace gas species measured in smoke from nine prescribed fires. We find significant dependence on modified combustion efficiency (MCE) for some species, although regional differences indicate that the use of MCE as a proxy may be limited. We also find that the fire-integrated MCE values derived from our in situ on-the-ground open-path measurements are not significantly different from those reported for airborne measurements of smoke from fires in the same ecosystem. We then compare our average emission factors to those measured for temperate forest fires elsewhere (North America) and for fires in another dominant Australian ecosystem (savanna) and find significant differences in both cases. Indeed, we find that although the emission factors of some species agree within 20 %, including those of hydrogen cyanide, ethene, methanol, formaldehyde and 1,3-butadiene, others, such as acetic acid, ethanol, monoterpenes, ammonia, acetonitrile and pyrrole, differ by a factor of 2 or more. This indicates that the use of ecosystem-specific emission factors is warranted for applications involving emissions from Australian forest fires.

Gaseous emissions from an intensive vegetable farm measured with slant-path FTIR technique

A recently developed slant-path flux gradient (FG) technique, combined with open-path Fourier transform infrared (OP-FTIR) spectroscopy, was deployed to concurrently measure gas emissions of ammonia (NH3), nitrous oxide (N2O), carbon dioxide (CO2), and methane (CH4) from an intensive vegetable farm in Australia. Gas fluxes were continuously measured for three weeks following chicken manure application to a celery crop, followed by intermittent measurements for three days a week for another three weeks. The average flux measured over the 41 day measurement period for NH3 and N2O, was 5.2 and 2.9 mg N m−2 h−1, respectively, CH4 was 1.1 mg C m−2 h−1, and CO2 was 0.7 C g m−2 h−1. Manure and fertilizer application substantially increased the emissions of these gases, by providing carbon (C) and nitrogen (N) substrates to the soil. The cumulative N losses as NH3 and N2O following fertilizer application were 6.7% and 3.7% of total N applied, respectively. Using this FG/OP-FTIR technique, we demonstrated that the N2O emission factor for this vegetable farm is much higher than the IPCC default emission factor for manure applied to managed lands. These results highlight the need for large-scale measurements to quantify multiple gas losses from intensive agricultural systems.

In-situ open path FTIR measurements of the vertical profile of spray drift from air-assisted sprayers

Estimating pesticide spray drift, which is a part of total drift loss, is complex as airborne pesticide concentrations are low and depend on multiple factors. The aim was to measure and compare vertical profiles of spray drift generated by different sprayers using Open Path Fourier-Transform-Infra-Red (OP-FTIR) spectrometer. Field tests included three types of commercial agricultural sprayers. The OP-FTIR was placed at the edge of an apple orchard with the line of sight parallel to tree rows. The OP-FTIR and its reflector were mounted on platform lifts to allow measurements at 4 heights: 3 (canopy height), 4, 5, and 6 m above ground. The sprayers sprayed water within the three tree rows closest to the OP-FTIR as well as outside each tree row in order to estimate the spray drift as function of distance with and without tree interference. The results of the experiments showed that, under the meteorological conditions prevailing, there were substantial differences between the sprayers in terms of spray drift of droplets with diameter > 5 μm. Additionally, the results showed that spray drift can be reduced substantially (by up to 50%) by using a tree-line barrier or a buffer zone.

Large-scale search method for locating and identifying fugitive emission sources in petrochemical processing areas

BackgroundFugitive emission sources generated from leaking components are often difficult to identify and locate, especially in petrochemical processing areas, which have concentrated facilities and equipment. MethodsA large-scale search method for locating and identifying fugitive emission sources in a petrochemical processing area by using multiple intersecting open-path Fourier transform infrared (OP-FTIR) beam paths and multivariate statistical methods is proposed in this study. Multivariate statistical methods, namely principal component analysis (PCA) and correspondence analysis (CA), were applied to the measured data set and the characteristics of emission sources were summarized. ResultsStyrene, 1,3-butadiene, cyclohexane, ammonia, ethylene, propylene and methanol were identified in most beam paths with mean concentrations up to 75.3±11.4ppbv, 355.2±34.6ppbv, 188.1±17.82ppbv, 255.57±19.28ppbv, 194.3±18.2ppbv, and 94.0±16.7ppbv, respectively. PCA extracted at least three source categories in the plant. CA provided additional information about the approximate locations of each source, and thus, future emission reduction plans can be developed accordingly. ConclusionThis study established a dynamic approach for locating fugitive emission sources in a complex petrochemical plant by applying an OP-FTIR matrix-path approach combined with PCA and CA. Future emission reduction plans can be developed according to the findings of PCA and CA.

Estimating drift of airborne pesticides during orchard spraying using active Open Path FTIR

The use of pesticides is important to ensure food security around the world. Unfortunately, exposure to pesticides is harmful to human health and the environment. This study suggests using active Open Path Fourier Transform Infra-Red (OP-FTIR) spectroscopy for monitoring and characterizing pesticide spray drift, which is one of the transfer mechanisms that lead to inhalation exposure to pesticides. Experiments were conducted in a research farm with two fungicides (Impulse and Bogiron), which were sprayed in the recommended concentration of ∼0.1%w in water, using a tractor-mounted air-assisted sprayer. The ability to detect and characterize the pesticide spray drift was tested in three types of environments: fallow field, young orchard, and mature orchard. During all spraying experiments the spectral signature of the organic phase of the pesticide solution was identified. Additionally, after estimating the droplets' size distribution using water sensitive papers, the OP-FTIR measurements enabled the estimation of the droplets load in the line of sight.

Challenges associated with the atmospheric monitoring of areal emission sources and the need for optical remote sensing techniques—an open-path Fourier transform infrared (OP-FTIR) spectroscopy experience report

In recent years, OP-FTIR spectrometry has been successfully used to monitor hazardous air pollutants, greenhouse gases and other emission products. This ground-based remote sensing method has proven itself to be a flexible long-path technique for the characterization of large atmospheric volumes, enabling simultaneous detection of various volatile atmospheric compounds relevant for environmental assessment via a single rapid measurement. Within our research, we applied both active and passive ground-based OP-FTIR spectroscopy as a screening tool for the detection of fugitive greenhouse gas emissions. The method was proven as offering a suitable ‘compliance toolbox’ for continuous monitoring of the complex systems at the ground surface–atmosphere boundary, allowing large-scale identification and quantification of atmospheric composition over large areas to be achieved, in terms of identifying zones of higher leakage vulnerability. In this paper, we compiled our research results and highlight the adaption options of the field technology, illustrated measuring options and case studies at urban, natural and industrial sites. Furthermore, this technology was validated regarding its applicability in environmental atmospheric monitoring. The results show that passive OP-FTIR measurements offer the chance to achieve robust and reliable surveys in various arbitrary measurement directions to gain a comprehensive overview. However, to improve quantitative analysis in the case of weak sources, the application of an active open-path spectrometer is recommended. It should be noted that our investigations demonstrate that site-specific parameters such as prevailing wind directions, meteorological conditions, topographic influences, infrastructure, other artificial emission sources, and biological background need to be measured both prior to and during atmospheric monitoring and should be taken into account for comprehensive interpretation.

Detection and quantification of water-based aerosols using active open-path FTIR.

Aerosols have a leading role in many eco-systems and knowledge of their properties is critical for many applications. This study suggests using active Open-Path Fourier Transform Infra-Red (OP-FTIR) spectroscopy for quantifying water droplets and solutes load in the atmosphere. The OP-FTIR was used to measure water droplets, with and without solutes, in a 20 m spray tunnel. Three sets of spraying experiments generated different hydrosols clouds: (1) tap water only, (2) aqueous ammonium sulfate (0.25–3.6%wt) and (3) aqueous ethylene glycol (0.47–2.38%wt). Experiment (1) yielded a linear relationship between the shift of the extinction spectrum baseline and the water load in the line-of-sight (LOS) (R2 = 0.984). Experiment (2) also yielded a linear relationship between the integrated extinction in the range of 880–1150 cm−1 and the ammonium sulfate load in the LOS (R2 = 0.972). For the semi-volatile ethylene glycol (experiment 3), present in the gas and condense phases, quantification was much more complex and two spectral approaches were developed: (1) according to the linear relationship from the first experiment (determination error of 8%), and (2) inverse modeling (determination error of 57%). This work demonstrates the potential of the OP-FTIR for detecting clouds of water-based aerosols and for quantifying water droplets and solutes at relatively low concentrations.