Broadband Cavity Enhanced Absorption Spectrometer Research Articles

A mini broadband cavity enhanced absorption spectrometer for nitrogen dioxide measurement on the unmanned aerial vehicle platform

We developed a small size and light weight analyzer based on the broadband cavity enhanced absorption spectroscopy for measuring nitrogen dioxide (NO2) with high spatial-temporal resolution and high accuracy. The instrument utilizes a LED centered at 450 nm, a short optical cavity, and a spectrometer to determine the extinction coefficients span 435–465 nm, allowing us to retrieve the mixing ratio of NO2 by absorption cross-section. The measurement precision (1σ) for NO2 is 98 ppt in 2 s under laboratory conditions. The uncertainty is estimated at 6% mainly attributed to cross-section and the mirror reflectivity. Good consistency was achieved by comparing with commercial chemiluminescence detector and cavity attenuation phase shift spectrometer (CAPS) (both R2 = 0.99). We further mounted the instrument on an unmanned aerial vehicle (UAV), and maintained the UAV flight at the height of 20 m, the comparative measurement with a CAPS set in a neighbor building with the same sampling height showed a good correlation (R2 = 0.97), which confirmed the instrumental feasibility and stability on the UAV platform. At last, we present a successful NO2 vertical measurement test, the good performance indicates this portable technique has great potential for vertical measurement. In particular, the mini-CEAS technique can further extend to measure the vertical profiles of other trace gas species (such as nitrous acid) with strong optical structural absorptions, which would largely promote the understanding of atmospheric chemistry on a vertical scale.

A pptv Level Incoherent Broadband Cavity-Enhanced Absorption Spectrometer for the Measurement of Atmospheric NO3

NO3 radicals are one of the very important trace gases in the atmosphere. Accurate measurements of NO3 can provide data support for atmospheric chemistry research. Due to the extremely low content of NO3 radicals in the atmosphere, it is a challenge to accurately detect it. In this paper, an incoherent broadband cavity-enhanced absorption spectrometer (IBBCEAS) with high sensitivity is developed for measuring atmospheric NO3. The IBBCEAS absorption spectra of NO3 in the range of 648–674 nm are measured. The concentration of NO3 is retrieved by fitting the absorption cross-section of NO3 to the measured absorption coefficient using the least square method. The interference absorption of water vapor is effectively removed by an iterative calculation of its absorption cross-section. The detect limit of the spectrometer is analyzed using the Allan variance and the standard variance. The NO3 detection limit (1σ) of the spectrometer is 1.99 pptv for 1 s integration time, and improves to be 0.69 pptv and 0.21 pptv for 10 s and 162 s integration time, respectively. The developed spectrometer with pptv level sensitivity is applied to the measurements of the real atmospheric NO3 for verifying the effectiveness.

Development of a broadband cavity-enhanced absorption spectrometer for simultaneous measurements of ambient NO3, NO2, and H2O

Abstract. We describe the characteristics and performances of our newly built broadband cavity-enhanced absorption spectrometer for measurements of nitrate radical (NO3), nitrogen dioxide (NO2), and water vapor (H2O). A customized vibration-resistance cavity layout incorporated with N2 purging on high-reflection mirror surfaces was implemented with a red light-emitting diode (LED) as a light source. In general, this system achieved over 40 km (up to 101.5 km) of effective light path length at 662 nm from a 0.52 m long cavity. For accurate NO3 measurement, the measured absorption spectrum of H2O was used for simultaneous concentration retrievals with the other species instead of being treated as interferences to be removed or corrected prior to NO3 detection. Synthesized N2O5 crystals under atmospheric pressure were used for performance tests of linear response and transmission efficiency. From the standard injection experiments of NO3, NO2, and H2O, high linearities were observed (R2≥0.9918). The total NO3 transmission efficiency through the system was determined to be 81.2 % (±2.9, 1σ) within the residence time of 2.59 s. The precisions (1σ) of NO3, NO2, and H2O in 1 Hz measurement from a single pixel on the charge-coupled device (CCD) were 1.41 pptv, 6.92 ppbv, and 35.0 ppmv with uncertainties of 10.8 %, 5.2 %, and ≥20.5 %, respectively, mainly from the errors in the literature absorption cross-section. The instrument was successfully deployed aboard the Korean icebreaker R/V Araon for an expedition conducted in the remote marine boundary layer in the Arctic Ocean during the summer of 2021.

Car-Borne Measurements of Atmospheric NO2 by a Compact Broadband Cavity Enhanced Absorption Spectrometer

Car-Borne Measurements of Atmospheric NO2 by a Compact Broadband Cavity Enhanced Absorption Spectrometer

A dual-channel incoherent broadband cavity-enhanced absorption spectrometer for sensitive atmospheric NOx measurements.

We describe and characterize a dual-channel incoherent broadband cavity-enhanced absorption spectrometer (DC-IBBCEAS) for the sensitive measurements of NOx: the sum of nitrogen monoxide (NO) and nitrogen dioxide (NO2) in the atmosphere. The instrument employs two 1 m long optical cavities, with air being extracted from a common sampling line. The first channel (cavity-1) spans 340-380 nm and the second channel (cavity-2) spans the 405-460 nm spectral range, both measuring NO2 simultaneously. High absorption cross-sections of NO2 in both channels are effectively utilized for its sensitive quantification. NO is quantified by titrating it with ozone to NO2 in channel-2, where the difference of NO2 measured from that in channel-1 corresponds to the NO concentration in the sampled air. The instrument offers 1-ppb detection sensitivity for both NO and NO2 with a maximum possible uncertainty of ∼9%. The use of close yet different spectral regions in the two channels readily extended measurements to a broader range without compromising its sensitivity to NOx quantification. This would extend the DC-IBBCEAS applicability to simultaneously monitor interfering species with significant absorption cross-sections in the region in either channel, such as glyoxal (CHOCHO), methylglyoxal (CH3COCHO), and nitrous acid (HONO).

A compact incoherent broadband cavity-enhanced absorption spectrometer for trace detection of nitrogen oxides, iodine oxide and glyoxal at levels below parts per billion for field applications

Abstract. We present a compact, affordable and robust instrument based on incoherent broadband cavity-enhanced absorption spectroscopy (IBBCEAS) for simultaneous detection of NOx, IO, CHOCHO and O3 in the 400–475 nm wavelength region. The instrument relies on the injection of a high-power LED source in a high-finesse cavity (F∼33 100), with the transmission signal being detected by a compact spectrometer based on a high-order diffraction grating and a charge-coupled device (CCD) camera. A minimum detectable absorption of 2.0×10-10 cm−1 was achieved within ∼22 min of total acquisition, corresponding to a figure of merit of 1.8×10-10 cm−1 Hz-1/2 per spectral element. Due to the multiplexing broadband feature of the setup, multi-species detection can be performed with simultaneous detection of NO2, IO, CHOCHO and O3 achieving detection limits of 11, 0.3, 10 ppt (parts per trillion) and 47 ppb (parts per billion) (1σ) within 22 min of measurement, respectively (half of the time is spent on the acquisition of the reference spectrum in the absence of the absorber, and the other half is spent on the absorption spectrum). The implementation on the inlet gas line of a compact ozone generator based on electrolysis of water allows for the measurement of NOx (NO+NO2) and therefore an indirect detection of NO with detection limits for NOx and NO of 10 and 21 ppt (1σ), respectively. The device has been designed to fit in a 19 in., 3U (5.25 in.) rack-mount case; weighs 15 kg; and has a total electrical power consumption of <300 W. The instrument can be employed to address different scientific objectives such as better constraining the oxidative capacity of the atmosphere, studying the chemistry of highly reactive species in atmospheric chambers as well as in the field and looking at the sources of glyoxal in the marine boundary layer to study possible implications on the formation of secondary aerosol particles.

Incoherent broadband cavity enhanced absorption spectroscopy (IBBCEAS)-based strategy for direct measurement of aerosol extinction in a lidar blind zone.

In this Letter, the development of a custom-designed incoherent broadband cavity enhanced absorption spectrometer (IBBCEAS) and its application to in situ measurement of aerosol extinction near the ground surface are described in an effort to address the issue of missing data in the light detection and ranging (lidar) blind zone in the first hundreds of meters of the observation range. Combined measurements of aerosol extinction at the same location using lidar remote sensing at 355 nm and in situ IBBCEAS operating in the UV spectral region around 370 nm showed results with a good correlation (${{\rm R}^2} = {0.90}$R2=0.90) between the two measurement techniques. This Letter highlights a new strategy for near-end lidar calibration, using a ground-based compact and robust IBBCEAS located at the lidar measurement site to determine the vertical profile of the aerosol extinction coefficient with a higher accuracy.

Open path incoherent broadband cavity-enhanced measurements of NO3 radical and aerosol extinction in the North China Plain

We describe the observation of the NO3 radical using an incoherent broadband cavity-enhanced absorption spectrometer in an open-path configuration (OP-IBBCEAS) in a polluted summer environment in continental China. The instrument was installed 17 m above the ground at the top of a residential complex near the CAREBeijing-NCP 2014 site in Wangdu, Hebei province, about 200 km southwest of Beijing over the period 28 to 30 June 2014. The separation between the transmitter and receiver components of the instrument was 335 cm and the effective pathlength in clean reference air was ~3.4 km. NO3 was detected above the detection limit on all three nights when the instrument was operational. The maximum mixing ratio measured was ~175 pptv with a detection sensitivity of ~36 pptv for measurements with an average acquisition time of 10 min. While most extractive instruments try to avoid interferences arising from aerosol extinction, the open path configuration has advantages owing to its ability to detect trace gases even in the presence of aerosol loading. Moreover, concurrent retrieval of aerosol optical extinction is possible from analysis of the absorption magnitude of the oxygen B-band at 687 nm. The experimental setup, its calibration, data acquisition, and analysis procedure are discussed, and the results presented here demonstrate the sensitivity and specificity that can be achieved at high spatial and temporal resolution using the novel configuration of IBBCEAS in the open path.

Ground and Airborne U.K. Measurements of Nitryl Chloride: An Investigation of the Role of Cl Atom Oxidation at Weybourne Atmospheric Observatory

AbstractNitryl chloride (ClNO2) measurements from the Weybourne Atmospheric Observatory (WAO) are reported from March to April 2013 using a quadruple chemical ionization mass spectrometer with the I− ionization scheme. WAO is a rural coastal site with generally low NOx concentrations, a type of location poorly studied for ClNO2 production. Concentrations of ClNO2 exceeded that of the limit of detection (0.8 ppt) on each night of the campaign, as did concentrations of N2O5, which was also measured simultaneously with the Cambridge Broadband Cavity Enhanced Absorption Spectrometer. A peak concentration of 65 ppt of ClNO2 is reported here. Vertical profiles of ClNO2 from early‐ to middle‐morning flights in close proximity to WAO are also reported, showing elevated concentrations at low altitude. The photolysis of observed ClNO2 and a box model utilizing the Master Chemical Mechanism modified to include chlorine chemistry was used to calculate Cl atom concentrations. This model utilized numerous VOCs from the second Tropospheric Organic Chemistry project in 2004, at the same location and time of year. From this the relative importance of the oxidation of three groups of measured VOCs (alkanes, alkenes, and alkynes) by OH radicals, Cl atoms, and O3 is compared. Cl atom oxidation was deemed generally insignificant at this time and location for total oxidation due to the much lower concentration of ClNO2 observed, even following the night of greatest ClNO2 production.

Development of a portable cavity-enhanced absorption spectrometer for the measurement of ambient NO<sub>3</sub> and N<sub>2</sub>O<sub>5</sub>: experimental setup, lab characterizations, and field applications in a polluted urban environment

Abstract. A small and portable incoherent broadband cavity-enhanced absorption spectrometer (IBBCEAS) for NO3 and N2O5 measurement has been developed. The instrument features a mechanically aligned non-adjustable optical mounting system, and the novel design of the optical mounting system enables a fast setup and stable operation in field applications. To remove the influence of the strong nonlinear absorption by water vapour, a dynamic reference spectrum through NO titration is used for the spectrum analysis. The wall loss effects of the sample system were extensively studied, and the total transmission efficiencies were determined to be 85 and 55 % for N2O5 and NO3, respectively, for our experimental setup. The limit of detection (LOD) was estimated to be 2.4 pptv (1σ) and 2.7 pptv (1σ) at 1 s intervals for NO3 and N2O5, respectively. The associated uncertainty of the field measurement was estimated to be 19 % for NO3 and 22–36 % for N2O5 measurements from the uncertainties of transmission efficiency, absorption cross section, effective cavity length, and mirror reflectivity. The instrument was successfully deployed in two comprehensive field campaigns conducted in the winter and summer of 2016 in Beijing. Up to 1.0 ppb NO3+N2O5 was observed with the presence of high aerosol loadings, which indicates an active night-time chemistry in Beijing.

Supercontinuum high-speed cavity-enhanced absorption spectroscopy for sensitive multispecies detection.

Cavity-enhanced absorption spectroscopy is promising for many applications requiring a very high concentration sensitivity but often accompanied by low temporal resolution. In this Letter, we demonstrate a broadband cavity-enhanced absorption spectrometer capable of detection rates of up to 50 kHz, based on a spatially coherent supercontinuum (SC) light source and an in-house-built, high-speed near-infrared spectrograph. The SC spectrometer allows for the simultaneous quantitative detection of CO2, C2H2, and H2O within a spectral range from 1420 to 1570 nm. Using cavity mirrors with a specified reflectivity of R=98.0±0.3% a minimal spectrally averaged absorption coefficient of αmin=1·10-5 cm-1 can be detected at a repetition rate of 50 kHz.

A broadband cavity enhanced absorption spectrometer for aircraft measurements of glyoxal, methylglyoxal, nitrous acid, nitrogen dioxide, and water vapor

Abstract. We describe a two-channel broadband cavity enhanced absorption spectrometer (BBCEAS) for aircraft measurements of glyoxal (CHOCHO), methylglyoxal (CH3COCHO), nitrous acid (HONO), nitrogen dioxide (NO2), and water (H2O). The instrument spans 361–389 and 438–468 nm, using two light-emitting diodes (LEDs) and a single grating spectrometer with a charge-coupled device (CCD) detector. Robust performance is achieved using a custom optical mounting system, high-power LEDs with electronic on/off modulation, high-reflectivity cavity mirrors, and materials that minimize analyte surface losses. We have successfully deployed this instrument during two aircraft and two ground-based field campaigns to date. The demonstrated precision (2σ) for retrievals of CHOCHO, HONO and NO2 are 34, 350, and 80 parts per trillion (pptv) in 5 s. The accuracy is 5.8, 9.0, and 5.0 %, limited mainly by the available absorption cross sections.

Near infrared cavity enhanced absorption spectra of atmospherically relevant ether-1, 4-Dioxane

1, 4-Dioxane (DX) is a commonly found ether in industrially polluted atmosphere. The near infrared absorption spectra of this compound has been recorded in the region 5900–8230cm−1 with a resolution of 0.08cm−1 using a novel Fourier transform incoherent broadband cavity-enhanced absorption spectrometer (FT-IBBCEAS). All recorded spectra were found to contain regions that are only weakly perturbed. The possible combinations of fundamental modes and their overtone bands corresponding to selected regions in the measured spectra are tabulated. Two interesting spectral regions were identified as 5900–6400cm−1 and 8100–8230cm−1. No significant spectral interference due to presence of water vapor was observed suggesting the suitability of these spectral signatures for spectroscopic in situ detection of DX. The technique employed here is much more sensitive than standard Fourier transform spectrometer measurements on account of long effective path length achieved. Hence significant enhancement of weaker absorption lines above the noise level was observed as demonstrated by comparison with an available measurement from database.

Light extinction by secondary organic aerosol: an intercomparison of three broadband cavity spectrometers

Abstract. Broadband optical cavity spectrometers are maturing as a technology for trace-gas detection, but only recently have they been used to retrieve the extinction coefficient of aerosols. Sensitive broadband extinction measurements allow explicit separation of gas and particle phase spectral contributions, as well as continuous spectral measurements of aerosol extinction in favourable cases. In this work, we report an intercomparison study of the aerosol extinction coefficients measured by three such instruments: a broadband cavity ring-down spectrometer (BBCRDS), a cavity-enhanced differential optical absorption spectrometer (CE-DOAS), and an incoherent broadband cavity-enhanced absorption spectrometer (IBBCEAS). Experiments were carried out in the SAPHIR atmospheric simulation chamber as part of the NO3Comp campaign to compare the measurement capabilities of NO3 and N2O5 instrumentation. Aerosol extinction coefficients between 655 and 690 nm are reported for secondary organic aerosols (SOA) formed by the NO3 oxidation of β-pinene under dry and humid conditions. Despite different measurement approaches and spectral analysis procedures, the three instruments retrieved aerosol extinction coefficients that were in close agreement. The refractive index of SOA formed from the β-pinene + NO3 reaction was 1.61, and was not measurably affected by the chamber humidity or by aging of the aerosol over several hours. This refractive index is significantly larger than SOA refractive indices observed in other studies of OH and ozone-initiated terpene oxidations, and may be caused by the large proportion of organic nitrates in the particle phase. In an experiment involving ammonium sulfate particles, the aerosol extinction coefficients as measured by IBBCEAS were found to be in reasonable agreement with those calculated using the Mie theory. The results of the study demonstrate the potential of broadband cavity spectrometers for determining the optical properties of aerosols.

Aircraft based four-channel thermal dissociation laser induced fluorescence instrument for simultaneous measurements of NO<sub>2</sub>, total peroxy nitrate, total alkyl nitrate, and HNO<sub>3</sub>

Abstract. A four-channel thermal dissociation laser induced fluorescence (TD-LIF) instrument has been developed for simultaneous measurements of nitrogen dioxide (NO2), total peroxy nitrate (∑PNs), total alkyl nitrate (∑ANs) and nitric acid (HNO3). NO2 is measured directly by LIF at 532 nm, whereas organic nitrates and nitric acid are thermally dissociated at distinct temperatures in the inlet to form NO2, which is then measured by LIF. The concentrations of each dissociated species are derived by the differences in measured NO2 relative to the reference colder inlet channel. The TD-LIF was adapted to fly on board the UK Facility for Airborne Atmospheric Measurements (FAAM) BAe 146-301 atmospheric research aircraft in summer 2010, and to date has successfully flown in five field campaigns. This paper reports novel improvements in the TD-LIF instrumentations, including (1) the use of a single wavelength laser, which makes the system compact and relatively cheap; (2) the use of a single beam laser that allows easy alignment and optical stability against the vibrational aircraft environment; and (3) the optical assembly of four detection cells that allow simultaneous and fast (time resolution up to 0.1 s) measurements of NO2, ∑PNs, ∑ANs and HNO3. Laboratory-generated mixtures of PNs, ANs and HNO3 in zero air are converted into NO2 and used to fix the dissociation temperatures of each heated inlet to test the selectivity of the instrument and potential interferences due to recombination reactions of the dissociated products. The effectiveness of the TD-LIF was demonstrated during the RONOCO aircraft campaign (summer 2010). A chemiluminescence system that was measuring NO2 and a broadband cavity enhanced absorption spectrometer (BBCEAS) that was measuring one of the PNs (N2O5) were installed on the same aircraft during the campaign. The in-flight intercomparison of the new TD-LIF with the chemiluminescence system for NO2 measurements and the intercomparison between ∑PNs measured by the TD-LIF and N2O5 by the BBCEAS are used to assess the performance of the TD-LIF.

Absolute ozone absorption cross section in the Huggins Chappuis minimum (350–470 nm) at 296 K

Abstract. We report the ozone absolute absorption cross section between 350–470 nm, the minimum between the Huggins and Chappuis bands, where the ozone cross section is less than 10−22 cm2. Ozone spectra were acquired using an incoherent broadband cavity enhanced absorption spectrometer, with three channels centered at 365, 405, and 455 nm. The accuracy of the measured cross section is 4–30%, with the greatest uncertainty near the minimum absorption at 375–390 nm. Previous measurements vary by more than an order of magnitude in this spectral region. The measurements reported here provide much greater spectral coverage than the most recent measurements. The effect of O3 concentration and water vapor partial pressure were investigated, however there were no observable changes in the absorption spectrum most likely due to the low optical density of the complex.

Measurement of glyoxal using an incoherent broadband cavity enhanced absorption spectrometer

Abstract. We describe an instrument for simultaneous measurements of glyoxal (CHOCHO) and nitrogen dioxide (NO2) using cavity enhanced absorption spectroscopy with a broadband light source. The output of a Xenon arc lamp is coupled into a 1 m optical cavity, and the spectrum of light exiting the cavity is recorded by a grating spectrometer with a charge-coupled device (CCD) array detector. The mirror reflectivity and effective path lengths are determined from the known Rayleigh scattering of He and dry zero air (N2+O2). Least-squares fitting, using published reference spectra, allow the simultaneous retrieval of CHOCHO, NO2, O4, and H2O in the 441 to 469 nm spectral range. For a 1-min sampling time, the precision (±1σ) on signal for measurements of CHOCHO and NO2 is 29 pptv and 20 pptv, respectively. We directly compare measurements made with the incoherent broadband cavity enhanced absorption spectrometer with those from cavity ringdown instruments detecting CHOCHO and NO2 at 404 and 532 nm, respectively, and find linear agreement over a wide range of concentrations. The instrument has been tested in the laboratory with both synthetic and real air samples, and the demonstrated sensitivity and specificity suggest a strong potential for field measurements of both CHOCHO and NO2.

A compact broadband cavity enhanced absorption spectrometer for detection of atmospheric NO2 using light emitting diodes

A compact and low power detector has been developed for the in situ measurement of atmospheric NO(2) using broadband cavity enhanced absorption spectroscopy. Absorption by the O(2)-O(2) collisional pair was used to determine the cavity mirror reflectivity, thus enabling the retrieval of absolute absorber concentrations by differential spectral fitting techniques. Quantitative amounts of ambient NO(2) (between 3 and 32 parts per billion) were retrieved from spectra recorded in the presence of ambient aerosol with statistical uncertainties approaching 100 ppt for a 60 s averaging period. The instrument's response was compared to that of a commercial chemiluminescence detector and was found to agree to within 6%.