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 similar to 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 similar to 175 pptv with a detection sensitivity of similar to 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. (C) 2018 The Authors. Published by Elsevier B.V.

The application of two gain switched optical frequency combs (OFCs) in dual comb gas phase spectroscopy is demonstrated. We report on the stability analysis of the wavelength and power of individual comb lines of the two OFCs. The examination reveals that a maximum wavelength fluctuation of <2.5 pm and a maximum peak power fluctuation of ∼0.3 dB is achievable for the OFCs. The radio frequency (RF) beat tone spectrum shows the standard deviation of the peak power of an individual beat tone from the mean is as low as ∼0.14 dB with negligible frequency fluctuations. In a proof-of-principle experiment the dual comb system is applied to the detection of hydrogen sulphide (H2S) with a detection sensitivity of (740 ± 160) ppmv, demonstrating its excellent frequency and power stability. The dual OFCs can in principle be monolithically integrated and thus enable the development of compact, cost-efficient dual comb devices, for the detection of multiple trace gas species or isotopologues.

A custom-designed gain-switched frequency comb (GSFC) source was passively coupled to a medium finesse (F ≈ 522) cavity in off-axis configuration for the detection of ammonia (¹⁴NH₃) in static dry air. The absorption of ammonia was detected in the near infrared spectral region between 6604 and 6607 cm⁻¹ using a Fourier transform detection scheme. More than 30 lines of the GSFC output (free spectral range 2.5 GHz) overlapped with the strongest ro-vibrational ammonia absorption features in that spectral region. With the cavity in off-axis configuration, an NH₃ detection limit of ∼3.7 ppmv in 20 s was accomplished in a laboratory environment. The experimental performance of the prototype spectrometer was characterized; advantages, drawbacks and the potential for future applications are discussed.