Hongming Yi

Off-beam quartz-enhanced photoacoustic spectroscopy

An off-beam (OB) detection approach is suggested and experimentally investigated and optimized for quartz-enhanced photoacoustic spectroscopy (QEPAS). This OB-QEPAS configuration, very simple in assembly, not only allows for use of larger excitation optical beams and facilitating optical alignment but also provides higher enhancement of photoacoustic signals than previously published results based on the common on-beam QEPAS under the same experimental conditions. A normalized noise equivalent absorption coefficient (1sigma) of 5.9 x 10(-9) cm(-1)W/Hz(1/2) was obtained for water vapor detection at normal atmospheric pressure.

Trace gas detection based on off-beam quartz enhanced photoacoustic spectroscopy: Optimization and performance evaluation

A gas sensor based on off-beam quartz enhanced photoacoustic spectroscopy was developed and optimized. Specifically, the length and diameter of the microresonator tube were optimized, and the outer tube shape is modified for enhancing the trace gas detection sensitivity. The impact of the distance between the quartz tuning fork and an acoustic microresonator on the sensor performance was experimentally investigated. The sensor performance was evaluated by determining the detection sensitivity to H(2)O vapor in ambient air at normal atmospheric pressure. A normalized noise equivalent absorption coefficient (1σ) of 6.2×10(-9) cm(-1) W/Hz(1/2) was achieved.

Application of a broadband blue laser diode to trace NO2 detection using off-beam quartz-enhanced photoacoustic spectroscopy.

We applied for the first time, to our knowledge, broadband off-beam quartz-enhanced photoacoustic spectroscopy (BB-OB-QEPAS) to trace NO2 detection using a broadband blue laser diode centered at 450 nm. A detection limit of 18 ppbv (parts in 10(9) by volume) for NO2 in N2 at atmospheric pressure was achieved with an average laser power of 7 mW at a 1 s integration time, which corresponds to a 1 σ normalized noise equivalent absorption coefficient of 4.1×10(-9)  cm(-1) W=Hz(1=2). An Allan variance analysis was performed to investigate the long-term stability of the BB-OB-QEPAS-based NO2 sensor.

Short-lived species detection of nitrous acid by external-cavity quantum cascade laser based quartz-enhanced photoacoustic absorption spectroscopy

Spectroscopic detection of short-lived gaseous nitrous acid (HONO) at 1254.85 cm−1 was realized by off-beam coupled quartz-enhanced photoacoustic spectroscopy (QEPAS) in conjunction with an external cavity quantum cascade lasers (EC-QCL). High sensitivity monitoring of HONO was performed within a very small gas-sample volume (of ∼40 mm3) allowing a significant reduction (of about 4 orders of magnitude) of air sampling residence time which is highly desired for accurate quantification of chemically reactive short-lived species. Calibration of the developed QEPAS-based HONO sensor was carried out by means of lab-generated HONO samples whose concentrations were determined by direct absorption spectroscopy involving a ∼109.5 m multipass cell and a distributed feedback QCL. A minimum detection limit (MDL) of 66 ppbv (1 σ) HONO was achieved at 70 mbar using a laser output power of 50 mW and 1 s integration time, which corresponded to a normalized noise equivalent absorption coefficient of 3.6 × 10−8 cm−1 W/Hz1/...

Monitoring of nitrous acid (HONO) by external-cavity quantum cascade laser-based off-beam quartz-enhanced photoacoustic spectroscopy (QEPAS)

In this presentation, we report on the high-sensitivity and high-selectivity measurement of HONO by off-beam quartz-enhanced photoacoustic spectroscopy (QEPAS) in a very small gas sample volume (of ~ 40 mm3) resulting in a ultrashort residence time of less than 10 ms (compared to ~ 7 min for a conventional 210 m multipass cell or ~ 10-min. integration time for currently used chemical analytical instruments). A minimum detection limit of 66 ppbv (1σ) HONO was achieved at 70 mbar using a laser output power of 50 mW and 1 s integration time. This MDL was down to 7 ppbv at the optimal integration time of 150 s.

Filter-free measurements of black carbon absorption using photoacoustic spectroscopy

A photoacoustic (PA) spectrophone was developed for filter-free measurement of black carbon (BC) mass absorption coefficient (MAC) in the spectral region of ~ 442 nm using a blue diode laser. The PA sensor was characterized and calibrated using NO2 at standard calibrated concentrations, as well as at indoor and outdoor air concentrations via a side-by-side intercomparison with a commercial NOx analyzer. Black carbon (graphite) and volcanic ash samples were measured under laboratory condition. MAC of BC and volcanic ash sample were determined based on the measured particles optical absorption coefficient by the PA spectrophone, in combination with the mass concentration measured using a scanning mobility particle sizer (SMPS). A minimum detectable absorption coefficient (1σ) of ~ 1.2 Mm-1 for BC was achieved with a time solution of 1 second.

Monitoring of nitrous acid (HONO) by off-beam quartz-enhanced photoacoustic spectroscopy (QEPAS) using external-cavity quantum cascade laser

Spectroscopic monitoring of short-lived gaseous nitrous acid (HONO) near 8 µm was realized by off-beam coupled quartz-enhanced photoacoustic spectroscopy (QEPAS) using an external cavity quantum cascade lasers with an ultra-short residence time for air sampling.