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A High-Precision Mid-Infrared Spectrometer for Ambient HNO3 Measurements

At the end of 2022 scientific group from the Empa–Swiss Federal Laboratories for Materials Science and Technology (Switzerland) published an article in Sensors where they report on a mid-IR laser spectrometer for high-precision HNO3 measurements using VIGO product.

Precise measurements of reactive nitrogen oxides are crucial for understanding various atmospheric processes and are needed to support modeling studies. These compounds are e.g. involved in ozone formation or the nitrate fertilization of soils via wet and dry deposition. Reactive nitrogens are mainly emitted due to the burning of fossil fuels. 

The authors presented a quantum cascade laser absorption spectrometer (QCLAS) for ambient HNO3 measurements for very low amount fractions (down to tens of ppt). The use of VIGO HgCdTe (MCT) photovoltaic detector (PVI-4TE-6) in combination with QCL (emits approx. 5.8 µm) and multipass cell with an optical path length of 110 m makes it possible to reach a precision of 140 ppt at a resolution of 1 s and a best precision of 20 ppt at a resolution of 2 min. Excellent linearity between 0 and 25 ppb HNO3 indicates the very good quality of the used PVI detector.  

The described system was used not only in the laboratory, but it has been working also during a three-week field campaign in September 2021 in central Switzerland. The measured time series of the HNO3 correlated well with the daily total nitrates measured using a filter trapping method. We hope that such systems containing the best quality VIGO detectors will be more popular and support modeling atmospheric studies.

Link to the article