Search Results (10)

Formaldehyde is the most abundant carbonyl globally and the biggest driver of cancer risk in the United States among hazardous air pollutants. Ambient formaldehyde concentration measurements are generally sparse due to high measurement costs and limited measurement infrastructure. Recent studies have used low-cost air quality sensors to affordably improve spatial coverage and provide real-time measurements. Our previous research evaluated the laboratory performance of a low-cost electrochemical formaldehyde sensor (Sensirion SFA30) over formaldehyde concentrations ranging from 0 to 76 ppb. The sensors exhibited good linearity of response, a low limit of detection, and good accuracy in detecting formaldehyde. This study evaluated the cross-sensitivity of the SFA30 and the Gravity sensors (electrochemical formaldehyde sensors) over formaldehyde concentrations ranging from 0 to 326 ppb in a laboratory evaluation system, with broadband cavity-enhanced absorption spectroscopy used to obtain the reference measurements. We evaluated the sensors in a mixture of formaldehyde with five outdoor trace gases (CO, NO, NO₂, O₃, and isobutylene) and two indoor VOCs (methanol and isopropyl alcohol). The results suggest that the Gravity sensors may be useful for outdoor formaldehyde measurements when formaldehyde levels are well above background levels and that the SFA30 sensors may be useful screening tools for indoor environments, if properly calibrated. Full article

Terahertz (THz) molecular fingerprint spectroscopy provides a powerful label-free tool for detecting trace-amount analytes. Introducing extra microstructures such as metasurfaces to confine the field energy is essential to improve the sensitivity. However, the area of analyte film on conventional enhancing metasurfaces must be larger than the beam spot in a free-space measuring setup. Here, we propose a tunable defect cavity of one-dimensional photonic crystal in the combined coaxial waveguide (CCW) and enhance the broadband THz fingerprint of trace analytes on a much smaller area. The peaks of high Q resonances can form a wide absorption spectrum by changing the length of the rubber part of the coaxial waveguide. For the 0.2 µm α-lactose film sample in the frequency range of 0.48–0.58 THz, the absorption enhancement factor of 89.2 times based on the thickness can be achieved and the sample area is about 1/1700 of that in the free-space measurement with the 5 mm beam waist. We first introduce the coaxial waveguide in the terahertz absorption spectra enhancement. With our proposed structure the analyte volume is effectively reduced which is significant in the real application scenario. Full article

Glyoxal (CHOCHO) is a trace gas in the atmosphere, often used as an indicator of biogenic emissions. It is frequently compared to formaldehyde concentrations, which serve as indicators of anthropogenic emissions, to gain insights into the characteristics of the environmental source. This study employed broadband cavity-enhanced absorption spectroscopy to detect gaseous CHOCHO, methylglyoxal, and NO₂. Two different detection methods are compared. Spectrograph and CCD Detection: This approach involves coupling the system to a spectrograph with a charge-coupled device (CCD) detector. It achieved a 1 min 1-σ detection limit of 2.5 × 10⁸ molecules/cm³, or 10 parts per trillion (ppt). Methylglyoxal and NO₂ achieved 1 min 1-σ detection limits of 34 ppt and 22 ppt, respectively. Interferometer and PMT Detection: In this method, an interferometer is used in conjunction with a photomultiplier tube (PMT) detector. It resulted in a 2 min 1-σ detection limit of 1.5 × 10¹⁰ molecules/cm³, or 600 ppt. The NO₂ 2 min 1-σ detection limit was determined to be 900 ppt. Concentrations of methylglyoxal were difficult to determine using this method, as they appeared to be below the detection limit of the instrument. This study discusses the advantages and limitations of each of these detection methods. Full article

Formaldehyde is a known human carcinogen and an important indoor and outdoor air pollutant. However, current strategies for formaldehyde measurement, such as chromatographic and optical techniques, are expensive and labor intensive. Low-cost gas sensors have been emerging to provide effective measurement of air pollutants. In this study, we evaluated eight low-cost electrochemical formaldehyde sensors (SFA30, Sensirion^®, Staefa, Switzerland) in the laboratory with a broadband cavity-enhanced absorption spectroscopy as the reference instrument. As a group, the sensors exhibited good linearity of response (R² > 0.95), low limit of detection (11.3 ± 2.07 ppb), good accuracy (3.96 ± 0.33 ppb and 6.2 ± 0.3% N), acceptable repeatability (3.46% averaged coefficient of variation), reasonably fast response (131–439 s) and moderate inter-sensor variability (0.551 intraclass correlation coefficient) over the formaldehyde concentration range of 0–76 ppb. We also systematically investigated the effects of temperature and relative humidity on sensor response, and the results showed that formaldehyde concentration was the most important contributor to sensor response, followed by temperature, and relative humidity. The results suggest the feasibility of using this low-cost electrochemical sensor to measure formaldehyde concentrations at relevant concentration ranges in indoor and outdoor environments. Full article

We present a novel sensing approach for ambient ozone detection based on deep-ultraviolet (DUV) cavity-enhanced absorption spectroscopy (CEAS) using a laser driven light source (LDLS). The LDLS has broadband spectral output which, with filtering, provides illumination between ~230–280 nm. The lamp light is coupled to an optical cavity formed from a pair of high-reflectivity (R~0.99) mirrors to yield an effective path length of ~58 m. The CEAS signal is detected with a UV spectrometer at the cavity output and spectra are fitted to yield the ozone concentration. We find a good sensor accuracy of <~2% error and sensor precision of ~0.3 ppb (for measurement times of ~5 s). The small-volume (<~0.1 L) optical cavity is amenable to a fast response with a sensor (10–90%) response time of ~0.5 s. Demonstrative sampling of outdoor air is also shown with favorable agreement against a reference analyzer. The DUV-CEAS sensor compares favorably against other ozone detection instruments and may be particularly useful for ground-level sampling including that from mobile platforms. The sensor development work presented here can also inform of the possibilities of DUV-CEAS with LDLSs for the detection of other ambient species including volatile organic compounds. Full article

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. Full article

We report an open-path incoherent broadband cavity-enhanced absorption spectroscopy (OP-IBBCEAS) technique for in situ simultaneous optical monitoring of NO₂, NO₃, and H₂O in a reaction chamber. The measurement precision values (1σ) are 2.9 ppbv and 2.9 pptv for NO₂ and NO₃ in 2 s, respectively, and the measurement uncertainties are 6% for NO₂ and 14% for NO₃. Intercomparison of measured concentrations of NO₂ and NO₃ by open-path and extractive IBBCEAS was carried out in the SAES-ARC reaction chamber during the reaction of NO₂ with O₃. The measurement accuracy of OP-IBBCEAS is verified by an NO₂ intercomparison and the NO₃ transmission efficiency of the extractive IBBCEAS is determined by comparison against the in situ NO₃ measurement. The relationship between H₂O absorption cross section and its mixing ratio at 295 K and 1 atm was analysed. Due to the spectral resolution of IBBCEAS system, the strong and narrow absorption lines of H₂O are unresolved and exhibit non-Beer–Lambert Law behaviour. Therefore, a correction method is used to obtain the effective absorption cross section for fitting the H₂O structure. An inappropriate H₂O absorption cross section can cause an overestimation of NO₃ concentration of about 28% in a humid atmosphere (H₂O = 1.8%). This spectroscopic correction provides an approach to obtain accurate NO₃ concentrations for open-path optical configurations, for example in chamber experiments or field campaigns. The measurement precision values are improved by a factor of 3 to 4 after applying Kalam filtering, achieving sub-ppbv (0.8 ppbv) and sub-pptv (0.9 pptv) performance in 2 s for NO₂ and NO₃, respectively. Full article

Sulfur dioxide (SO₂) is an important precursor for the formation of atmospheric sulfate aerosol and acid rain. We present an instrument using Broadband Cavity-Enhanced Absorption Spectroscopy (BBCEAS) for the measurement of SO₂ with a minimum limit of detection of 0.75 ppbv (3-$\sigma$) using the spectral range 305.5–312 nm and an averaging time of 5 min. The instrument consists of high-reflectivity mirrors (0.9985 at 310 nm) and a deep UV light source (Light Emitting Diode). The effective absorption path length of the instrument is 610 m with a 0.966 m base length. Published reference absorption cross sections were used to fit and retrieve the SO₂ concentrations and were compared to fluorescence standard measurements for SO₂. The comparison was well correlated, R² = 0.9998 with a correlation slope of 1.04. Interferences for fluorescence measurements were tested and the BBCEAS showed no interference, while ambient measurements responded similarly to standard measurement techniques. Full article

Nitrous acid (HONO) is the major source of OH radicals in polluted regions and plays a key role in the nitrogen cycle of the atmosphere. Therefore, accurate measurements of HONO in the atmosphere is important. Long Path Absorption Photometer (LOPAP) is a common and highly sensitive method used for ambient HONO measurements. Incoherent Broadband Cavity Enhanced Absorption Spectroscopy (IBBCEAS) is a recent alternative for the detection of HONO with high temporal and spatial resolutions, which has shown a detection limit of 0.76 ppbv at a sampling average of 180 s. In this study, LOPAP and IBBCEAS-HONO instruments were deployed in a Shanghai Urban Site (Shanghai Academy of Environmental Sciences) and simultaneously recorded the data from both instruments for a quantitative intercomparison of the measured atmospheric HONO for four days from 30 December 2017–2 January 2018. The HONO concentration measured by IBBCEAS and LOPAP were well matched. The campaign average concentrations measured by IBBCEAS and LOPAP were 1.28 and 1.20 ppbv, respectively. The intercomparison results demonstrated that both the IBBCEAS-HONO instrument and LOPAP-HONO instrument are suitable for ambient monitoring of HONO in a polluted urban environment. Full article

Incoherent broadband cavity-enhanced absorption spectroscopy (IBBCEAS) is of importance for gas detection in environmental monitoring. This review summarizes the unique properties, development and recent progress of the IBBCEAS technique. Principle of IBBCEAS for gas sensing is described, and the development of IBBCEAS from the perspective of system structure is elaborated, including light source, cavity and detection scheme. Performances of the reported IBBCEAS sensor system in laboratory and field measurements are reported. Potential applications of this technique are discussed. Full article