Open-path Tunable Diode Laser Research Articles

Highly Responsive, Miniaturized Methane Telemetry Sensor Based on Open-Path TDLAS

This paper proposes the design of a highly responsive, compact, non-contact methane telemetry sensor, employing the open-path tunable diode laser absorption spectroscopy (OP-TDLAS) technology. The sensor uses the dual-core structure of FPGA and ARM to achieve high-speed methane telemetry at 100 KHz repeated modulation frequency for the first time with a non-cooperate target, and a higher gas responsive time of 1.8 ms was achieved than previously reported. Moreover, the optical system (L × W × H: 68.8 × 52 × 62.7 mm) and the electronic system (L × W: 70 × 50 mm) make the sensor more compact. Methane gas samples of varying integral concentrations were examined at a distance of 20 m. The amplitude of the absorption peaks was subjected to a linear fit with the standard concentration values, resulting in a robust linear correlation coefficient (R2 = 0.998). Notably, despite the compact form factor of the methane sensor, it demonstrated commendable stability in gas concentration detection, offering a minimum detection limit of 43.14 ppm·m. Consequently, this highly responsive and compact methane sensor, with its open-path feature, is apt for integration into a variety of applications requiring such attributes. These include handheld telemetry devices, Unmanned Aerial Vehicle (UAV) gas detection systems, vehicle mounted gas detection, and laser gas radar.

Contactless optical hygrometry in LACIS-T

Abstract. The Fast Infrared Hygrometer (FIRH), employing open-path tunable diode laser absorption spectroscopy at the wavelengths near the 1364.6896 nm line, was adapted to perform contactless humidity measurements at the Turbulent Leipzig Aerosol Cloud Interaction Simulator (LACIS-T), a unique turbulent moist-air wind tunnel. The configuration of the setup allows for scanning from outside the walls of the wind tunnel and at various positions without the need for repeated optics adjustments. We identified three factors which significantly influence the measurement – self-broadening of the absorption line, interference in the glass windows and parasitic absorption in the ambient air outside the wind tunnel – and developed correction methods which satisfactorily account for these effects. The comparison between FIRH and a reference hygrometer (dew-point mirror MBW 973) indicated a good agreement within the expected errors across the wide range of water vapour concentration 1.0–6.1×1017 cm−3 (equivalent to dew-point temperature of −5.4 to +21 ∘C at the temperature of 23 ∘C). High temporal resolution (∼2 kHz) allowed for studying turbulent fluctuations in the course of intensive mixing of two air streams which had the same mean velocity but differed in temperature and humidity, also including the settings for which the mixture can be supersaturated. The obtained results contribute to improved understanding and interpretation of cloud formation studies conducted in LACIS-T by complementing the previous characterizations of turbulent velocity and temperature fields inside the wind tunnel.

A simulation process of tunable open path diode laser spectrometer to detect a carbon monoxide gas in NIR region

By modifying the wavelength of the open path tunable diode laser spectrometer (TDLS) in the near infrared region, theoretical research was implemented to improve the detection limit of carbon monoxide gas. To adjust the correct wavelength in the NIR area, MatLab code was created. Following that, frequency domain measurements were performed in order to extract the second harmonic as an indicator of gas presence. According to the results, the correct wave length in the NIR area is (1584.877 nm), and the lowest limit of CO gas concentration is (0.012 ppb).

Assessment of the inverse dispersion method for the determination of methane emissions from a dairy housing

Methane (CH4) emissions from dairy housings, mainly originating from enteric fermentation of ruminating animals, are a significant source of greenhouse gases. The quantification of emissions from naturally ventilated dairy housings is challenging due to the spatial distribution of sources (animals, housing areas) and variable air exchange. The inverse dispersion method (IDM) is a promising option, which is increasingly used to determine gaseous emissions from stationary sources, as it offers high flexibility in the application at reasonable costs. We used a backward Lagrangian stochastic model combined with concentration measurements by open-path tunable diode laser spectrometers placed up- and downwind of a naturally ventilated housing with 40 dairy cows to determine the CH4 emissions. The average emissions per livestock unit (LU) were 317 (±44) g LU−1 d−1 and 267 (±43) g LU−1 d−1 for the first and second campaign, in September – October and November – December, respectively. For each campaign, inhouse tracer ratio measurements (iTRM) were conducted in parallel during two subperiods. For simultaneous measurements, IDM showed average emissions which were lower by 8% and 1% than that of iTRM, respectively, for the two campaigns. The differences are within the uncertainty range of any of the two methods. The IDM CH4 emissions were further analysed by wind direction and atmospheric stability and no differences in emissions were found. Overall, IDM showed its aptitude to accurately determine CH4 emissions from dairy housings or other stationary sources if the site allows adequate placement of sensors up- and downwind in the prevailing wind direction. To acquire reliable emission data, depending on the data loss during measurements due to quality filtering or instrument failure, a measuring time of at least 10 days is required.

Research on Laser Absorption Spectrum Detection Technology for CO2 and NH3 Regional Emission

High-sensitivity emission monitoring of CO2 and NH3 in farmland region is very important for analyzing environment and climate change. The on-line detection technology was studied based on open-path tunable diode laser absorption spectroscopy technology (OP-TDLAS). The detection spectrum of open path was extracted to realize accurate concentration inversion by designing the multi-line fitting algorithm. The system detection stability was evaluated by Allan variance that the detection limit of NH3 and CO2 were about 0.048 L/L and 4.31 L/L respectively. The detection experiment was carried out in farmland of north Anhui to prove that both of the fertilization and straw returning to the field were the emission source of CO2 and NH3 from soil. The growth effect of the former was slower but higher than that of fertilization. This stable spectrum detection method can obtain large-scale concentration results and clear emission rules, which provide technical support for the environment-friendly agriculture

High temporal resolution measurements of ammonia emissions following different nitrogen application rates from a rice field in the Taihu Lake Region of China

Ammonia emission is one of the dominant pathways of nitrogen fertilizer loss from rice fields in China. It is difficult to measure ammonia emissions by high-frequency sampling with the chamber methods widely used in China, which is of great significance for investigating the environmental effects on the ammonia emissions. The chamber methods also can not accurately determine the ammonia emissions. In this study, the backward Lagrangian stochastic dispersion model, with ammonia concentrations continuously measured by the open-path tunable diode laser absorption spectroscopy technique, was used to determine ammonia emissions from a rice field after fertilizer application at excessive (270 kg N ha−1) and appropriate (210 kg N ha−1) rates in the Taihu Lake Region of China. High temporal resolution measurements of ammonia emissions revealed that high intraday fluctuations of ammonia emissions were significantly affected by the meteorological conditions. Multiple regression analysis showed a dominant solar radiation dependence of intraday ammonia emission cycles, especially during the rice panicle formation stage. The NH4+-N concentrations of the surface water of the rice field were found to be the decisive factor that influenced interday dynamics of ammonia emissions. Accurate quantifications of ammonia emissions indicated that the total ammonia losses under appropriate nitrogen application rate were 27.4 kg N ha−1 during the rice tillering stage and 11.2 kg N ha−1 during the panicle formation stage, which were 29.4% and 17.0% less than those under traditional excessive nitrogen application rate used by the local farmers, respectively. The ammonia loss proportions during the rice panicle formation stage were significantly lower than those of the tillering stage, which might be due to different nitrogen application rates and environmental effects during the two stages. This study indicated that the open-path tunable diode laser absorption spectroscopy technique could facilitate the investigation of high temporal resolution dynamic of ammonia emissions from farmland and the environmental influence on the ammonia emissions.

Comparison of backward Lagrangian stochastic model with micrometeorological mass balance method for measuring ammonia emissions from rice field

A field experiment was conducted to assess the accuracy of the backward Lagrangian stochastic (BLS) model combined with an open-path tunable diode laser (OPTDL) system for estimating ammonia emissions from rice field by comparing with the micrometeorological mass balance (MMB) method. During the experimental period, the average height of rice was 0.65 m, and the average height of laser path was 1.3 m. The results showed that there was no significant difference between the ammonia emission rates estimated by the two methods, indicating no significant effect of tall rice on the accuracy of the BLS model. The high temporal resolution data showed that ammonia emissions had a dramatic diurnal cycle: rise at daytime and fall at night. Although the OPTDL technique was not able to measure ammonia concentrations at low emission rates due to relatively high detection limit, the total ammonia loss estimated by the BLS model was not significant different from that of the MMB method. The results demonstrate the effectiveness of the BLS model with the OPTDL technique for estimating ammonia emissions from farmland covered with tall vegetation. Within the acceptable accuracy, the relatively low laser path height is recommended for farmland with tall vegetation due to relatively high detection limit of the OPTDL technique and low ammonia concentration of farmland.

High-Rate Solid-Liquid Separation Coupled With Nitrogen and Phosphorous Treatment of Swine Manure: Effect on Ammonia Emission

A new treatment system was developed to meet multiple environmental performance standards including to substantially reduce ammonia emissions. It was tested full-scale for 2-years in a 5,145-head finishing swine farm with two anaerobic lagoons. The system combined high-rate solid-liquid separation with nitrogen and phosphorus removal processes. Both vertical radial plum mapping (VRPM) and floating static chamber techniques were used to measure NH3 emission fluxes from anaerobic storage lagoons and the total farm-level NH3 emission rates. The VRPM used an open-path tunable diode laser absorption spectroscopy (TDL) and the flux chamber used a photoacoustic gas analyzer to accurately measure NH3 concentration. After the treatment system started, one of the two lagoons became inactive without receiving anymore flushed manure. The ammonia emission flux from the other lagoon with the treated effluent decreased from 43.9 to 6.8 kg-N ha-1 d-1 1.5 years after implementation of the new treatment system. The NH3 emission flux from the inactive lagoon also decreased similarly because the already stored old manure of the lagoon prior to inactivation was diluted with rainfalls and lost some NH3 via volatilization. The total farm-level NH3 emission rates decreased from 1.72 g s-1 to below detection level of the VRPM technique. Using the minimum detection level of the TDL with R2 > 90% (i.e., 8.1 8.1 µL L-1-m), the total farm-level NH3 emission rates in the second year were less than 0.04 – 0.15 g s-1. These results suggested that the impact of the new treatment system on NH3 emission reduction was equivalent to closing conventional swine lagoons while actively growing 5,145 pigs with minimal ammonia emissions from the farm.

An open-path tunable diode laser absorption spectrometer for detection of carbon dioxide at the Bonanza Creek Long-Term Ecological Research Site near Fairbanks, Alaska

We have developed a low-power, open-path, near-infrared (NIR) tunable diode laser sensor for the measurement of near ground-level concentrations of greenhouse gases. Here, we report on instrument design, characterization, and initial measurements of carbon dioxide concentrations during deployment to a thermokarst collapse scar bog near Fairbanks, AK (USA). The optics “launch-box” portion of the instrument couples radiation from an NIR, distributed feedback diode laser operating near 1572 nm with a visible laser for alignment purposes. The outgoing beam is directed through a 3.2-mm hole in a parabolic mirror and the launch-box is oriented using a two axis, altitude-azimuth telescope mount such that the beam strikes a retroreflector target at a set distance downfield. The beam then retraces the path back to the launch-box where the light is collected on the surface of the parabolic mirror and focused onto a multimode fiber that transfers the radiation to an InGaAs detector. Sweeps over a ~1.6 cm−1 spectral region were collected at a rate of 500 scans per second and were typically stored as 10 s sweep averages. These averaged sweeps could be individually spectrally fit for CO2 concentration or averaged into a single spectrum for fitting (after correction for slight frequency drift). Field data reported here was averaged for 2.5 min and was found to follow trends in diurnal cycles of CO2 concentration cycles reported by sensors located nearby in the field site.

Research on atmospheric CO2 remote sensing with open-path tunable diode laser absorption spectroscopy and comparison methods

An open-path atmospheric CO2 measurement system was built based on tunable diode laser absorption spectroscopy (TDLAS). The CO2 absorption line near 2 μm was selected, measuring the atmospheric CO2 with direct absorption spectroscopy and carrying on the comparative experiment with multipoint measuring instruments of the open-path. The detection limit of the TDLAS system is 1.94×10−6. The calibration experiment of three AZ-7752 handheld CO2 measuring instruments was carried out with the Los Gatos Research gas analyzer. The consistency of the results was good, and the handheld instrument could be used in the TDLAS system after numerical calibration. With the contrast of three AZ-7752 and their averages, the correlation coefficients are 0.8828, 0.9004, 0.9079, and 0.9393 respectively, which shows that the open-path TDLAS has the best correlation with the average of three AZ-7752 and measures the concentration of atmospheric CO2 accurately. Multipoint measurement provides a convenient comparative method for open-path TDLAS.

Evaluation of a backward Lagrangian stochastic model for determining surface ammonia emissions

The rate of ammonia emissions from a small circular plot of maize was estimated by three procedures: (i) by inverse dispersion (based on upwind and downwind laser concentration-detectors interpreted using a backward Lagrangian stochastic model); (ii) by the height-integrated horizontal flux measured by point flux-detectors arrayed along the axis of the plot (i.e., the integrated horizontal flux method, IHF); and (iii) by extrapolation from static flux chambers (SC). The results indicated that the estimates made by the open-path tunable diode laser (OPTDL) system combined with the backward Lagrangian stochastic (BLS) model were statistically equivalent to those made by the IHF method. The ammonia fluxes estimated by the OPTDL-BLS technique were only 2.3% higher than those from the IHF method. Although the OPTDL technique failed to monitor concentration differences at low ammonia fluxes due to its detection limit, the OPTDL-BLS technique estimated the total ammonia loss to be only 10.9% less than the IHF results. The SC method was found to underestimate ammonia emissions and cumulative ammonia loss significantly compared with both the OPTDL-BLS and IHF methods Although the ammonia emissions estimated by the OPTDL-BLS technique showed a similar emission pattern to those estimated by the IHF method, the former provided an opportunity to estimate the diurnal pattern of ammonia emissions and to understanding the primary driving factors. A clear diurnal cycle and a dominant net solar radiation dependence in ammonia emissions were found.

Quantification of ammonia emissions from dairy and beef feedlots in the Jing-Jin-Ji district, China

It is necessary to obtain accurate ammonia (NH3) emission estimates from concentrated cattle operations to evaluate the contribution of emissions from dairy and beef feedlots in the Beijing-Tianjin-Hebei (or the “Jing-Jin-Ji”) district of China to local dust-haze pollution such as fine particulate matter (PM2.5) and the national animal NH3 inventory. This study examines NH3 emission rates from two dairy and two beef feedlots in four different seasons, which were measured using an inverse dispersion technique in combination with an open-path tunable diode laser. Ammonia emissions from both dairy and beef feedlots are characterized by a distinctive diel pattern with high emission rates during the middle of the day and low rates during the night, which is in good agreement with the diel variation in air temperature and wind speed. In addition, apparent seasonal difference in NH3 emissions from dairy and beef feedlots were also observed, where the greatest emission occurred in summer followed by spring/fall and winter. Annual emission factors (EFs) for beef and dairy feedlots in the Jing-Jin-Ji district were estimated to be approximately 19.8kg NH3 head−1 yr−1 and 47.8kg NH3 head−1 yr−1, respectively, accounting for 25% and 30% of the feed N inputs. The NH3 emissions from beef and dairy cattle in this district were estimated to be 45.73 and 105.52 kt, respectively. In future, more field measurements on animal feedlots with various feed/manure management practices, stocking densities and weather conditions are highly requested to reduce NH3 emission uncertainty and characterize the N cycling within animal feedlots.

Assessing the backward Lagrangian stochastic model for determining ammonia emissions using a synthetic source

The backward Lagrangian stochastic (BLS) model for estimating ammonia emissions from agricultural sources was assessed in an ammonia recovery experiment. The open-path tunable diode laser spectrometer was used to measure atmospheric ammonia concentrations. The results indicated that the ratio of estimated to actual emission (QBLS/Q) decreased with increasing concentration measurement height, and the optimum measurement height increased as fetch (F) increased. The accuracy of QBLS was insensitive to fetch with a range of 15–60m. The optimum ratio of concentration measurement height to fetch decreased with increasing fetch. In this study, the optimum ratio of concentration measurement height to fetch was 0.083 and 0.045 for F=15m and F=30m, respectively. It is recommended that the laser sensor should be placed as close to the source as possible to reduce concentration measurement uncertainty for measurement of ammonia emission from farmland. The wheat canopy had a significant effect on QBLS/Q when the concentration measurement height above the wheat canopy was no more than 0.38m. The laser path should be placed at least 0.58m above the wheat canopy to minimize the effect of wheat canopy on the accuracy of the BLS model.

Evaluation of the MOZAIC Capacitive Hygrometer during the airborne field study CIRRUS-III

Abstract. The MOZAIC Capacitive Hygrometer (MCH) is usually operated aboard passenger aircraft in the framework of MOZAIC (Measurement of Ozone by Airbus In-Service Aircraft) for measuring atmospheric relative humidity (RH). In order to evaluate the performance of the MCH, the instrument was operated aboard a Learjet 35A research aircraft as part of the CIRRUS-III field study together with a closed-cell Lyman-α fluorescence hygrometer (Fast in situ Stratospheric Hygrometer, or FISH) and an open-path tunable diode laser system (Open-path Jülich Stratospheric TDL ExpeRiment, or OJSTER) for water vapour measurement. After reducing the CIRRUS-III data set to data corresponding to MOZAIC aircraft operation conditions, the 1 Hz RH data cross correlation between the MCH and reference instruments FISH (clear sky) and OJSTER (in-cirrus) yielded a remarkably good agreement of R2 = 0.92 and slope m = 1.02 and provided a MCH uncertainty of 5% RH. Probability distribution functions of RH deduced from the MCH and reference instruments agreed well between 10 and 70% RH with respect to liquid water in the ambient temperature range of ca. −70 to −40 °C. The use of MCH data is limited to sensor temperatures above the calibration limit of Tsensor = −40 °C (corresponds to ambient temperature of Tambient = −70 °C at typical cruising speed of long-haul passenger aircraft). Good performance of the MCH for clear sky as well as for in-cirrus conditions demonstrated the sensor robustness also for operation inside ice clouds.

Quality assessment of MOZAIC and IAGOS capacitive hygrometers: insights from airborne field studies

In 2011, the MOZAIC (Measurement of Ozone by AIRBUS In-Service Aircraft) successor programme IAGOS (In-service Aircraft for a Global Observing System) started to equip their long-haul passenger aircraft with the modified capacitive hygrometer Vaisala HUMICAP® of type H. The assurance of the data quality and the consistency of the data set during the transition from MOZAIC Capacitive Hygrometers to IAGOS Capacitive Hygrometers were evaluated within the CIRRUS-III and AIRTOSS-ICE field studies. During these performance tests, the capacitive hygrometers were operated aboard a Learjet 35A aircraft together with a closed-cell Lyman-α fluorescence hygrometer, an open-path tunable diode laser (TDL) system and a closed-cell, direct TDL absorption hygrometer for water vapour measurement. For MOZAIC-typical operation conditions, the comparison of relative humidity (RH) data from the capacitive hygrometers and reference instruments yielded remarkably good agreement with an uncertainty of 5% RH. The temperature dependence of the sensor's response time was derived from the cross-correlation of capacitive hygrometer data and smoothed data from the fast-responding reference instruments. The resulting exponential moving average function could explain the major part of the observed deviations between the capacitive hygrometers and the reference instruments.

Determination of methane emission rates on a biogas plant using data from laser absorption spectrometry

The aim of the work was to establish a method for emission control of biogas plants especially the observation of fugitive methane emissions. The used method is in a developmental stage but the topic is crucial to environmental and economic issues.A remote sensing measurement method was adopted to determine methane emission rates of a biogas plant in Rhineland-Palatinate, Germany. An inverse dispersion model was used to deduce emission rates. This technique required one concentration measurement with an open path tunable diode laser absorption spectrometer (TDLAS) downwind and upwind the source and basic wind information, like wind speed and direction.Different operating conditions of the biogas plant occurring on the measuring day (December 2013) could be represented roughly in the results. During undisturbed operational modes the methane emission rate averaged 2.8g/s, which corresponds to 4% of the methane gas production rate of the biogas plant.

Use of the open-path TDL analyzer to monitor ammonia emissions from winter wheat in the North China Plain

The backward Lagrangian stochastic (BLS) model and open-path tunable diode laser (OPTDL) analyzer were used to monitor ammonia (NH3) emissions from urea applied to winter wheat in the North China Plain. The high-temporal resolution measurements of ammonia concentrations provided an opportunity for estimating the diel patterns of ammonia emissions, as well as valuable information about the factors that influence NH3 emissions. The results showed both large diel variability and daily variability in NH3 volatilization, with NH3 emissions highest during the daytime. The diel pattern of ammonia volatilization depended mainly on the diel variation of wind speed and soil temperature, while the overall pattern of NH3 loss was strongly affected by soil moisture content, soil NH4 +-N concentration, wind speed and soil temperature. At the end of the measurement period, the cumulative NH3 loss was 12.21–16.43 kg N ha−1, calculated based on different time scale average Q BLS. Due to sensitivity of the OPTDL analyzer, the estimated total ammonia loss was still doubtful in this study.

Multisource emission retrieval within a biogas plant based on inverse dispersion calculations--a real-life example.

Open digestate storage tanks were identified as one of the main methane (CH4) emitters of a biogas plant. The main purpose of this paper is to determine these emission rates using an inverse dispersion technique in conjunction with open-path tunable diode laser spectroscopy (OP-TDLS) concentration measurements for multisource reconstruction. Since the condition number, a measure of "ill-conditioned" matrices, strongly influences the accuracy of source reconstruction, it is used as a diagnostic of error sensitivity. The investigations demonstrate that the condition number for a given source-sensor configuration in the highly disturbed flow field within the plant significantly depends on the meteorological conditions (e.g., wind speed, stratification, wind direction, etc.). The CH₄ emissions are retrieved by removing unrepresentative periods with high condition numbers, which indicate uncertainty in recovering the individual sources. In a final step, the CH₄ emissions are compared with the maximum biological methane potential (BMP) in the digestate analyzed under laboratory conditions. The retrieved methane emission rates represent an average of 50% of the maximum BMP of the stored digestate in the winter months, while they comprised an average of 85% during the measurement campaigns in the summer months. The results indicate that the open tanks have the potential to represent a substantial emission source even during colder periods.

Comparative study to evaluate three ground-based optical remote sensing techniques under field conditions by a gas tracer experiment

Today, ground-based optical remote sensing (ORS) has become an intensively used method for quantifying pollutant or greenhouse gas emissions from point or area sources, and for the validation of airborne or satellite remote sensing data. In this study, we present the results of a release experiment using acetylene (C2H2) as a tracer gas, where three ORS techniques were simultaneously tested for two main purposes: (1) the detection of emission sources and (2) the quantification of release rates. Therefore, passive and active open-path Fourier transform infrared spectroscopy (OP-FTIR) and open-path tunable diode laser absorption spectroscopy (TDLAS) were applied and evaluated. The concentration results of the active ORS methods are compared to those estimated by a Lagrangian stochastic atmospheric dispersion model. Our results reveal that passive OP-FTIR is a valuable tool for the rapid detection and imaging of emission sources and the spatial tracer gas distribution; while with active OP-FTIR and TDLAS, C2H2 concentrations in the sub-ppm range could be quantified that correlated well with the concentration data obtained by our modeling approach.

Comparison of a backward-Lagrangian stochastic and vertical radial plume mapping methods for estimating animal waste lagoon emissions

The long-term measurement of emissions from defined open area sources is needed to properly inventories annual emissions and assess regulatory compliance. Comparisons between the NH3 emissions calculated using the Vertical Radial Plume Mapping (VRPM) method and a backward Lagrangian Stochastic (bLS) method were made using NH3 concentration and wind measurements from eight livestock farms between 2006 and 2009. Concentration measurements were made along twenty optical paths using open-path tunable diode laser absorption spectroscopy. Wind measurements were made at three heights using three-dimensional sonic anemometers. Two concentration measurement configurations were evaluated: when the area source was surrounded by concentration measurements (‘fenced’) and when the concentration measurements were made perpendicular to the area source. The mean ammonia (NH3) emissions from the eight farms ranged from 0.3gs−1 to 2gs−1 with the bLS-calculated NH3 emissions on average 0.06gs−1 (8%) lower than the VRPM-calculated NH3 emissions in the perpendicular configuration and 0.04gs−1 (5%) lower than the VRPM-calculated emissions in the ‘fenced’ configuration. There was great variability in the comparison farm to farm due to the wide range of atmospheric, measurement, and farm configurations. Variability in the mean difference between the two methods from farm to farm was less than the difference in calculated bLS emissions resulting from the measured turbulence from the source or the surrounding landscape. The overall mean difference between the bLS and VRPM emissions methods under optimal conditions was −0.05g NH3s−1 (−5%). The variability in the VRPM NH3 emissions (1.4g2s−2) was significantly different (F=1.2) from the bLS NH3 emissions (1.6g2s−2).