Researchers at Anhui Institute of Optics and Fine Mechanics (AIOFM), Hefei Institutes of Physical Science recently designed and constructed a portable broadband cavity enhanced spectrometer (BBCES) for real-time in situ measurement of ambient glyoxal (CHOCHO), according to a paper newly published in the journal of Optics Express.
Glyoxal is the smallest (α-)dicarbonyl as well as present in the atmosphere as a first generation product from the photochemical oxidation of anthropogenic and biogenic volatile organic compounds (VOCs). Its lifetime of a few hours makes it a promising indicator molecule for VOC oxidation chemistry on local and global scales.
Field measurements of CHOCHO are important because comparing observations to model predictions is a key test to understand VOC photochemistry and their impact on air quality.
Both BBCES and differential optical absorption spectroscopy (DOAS) detect CHOCHO directly by measuring its unique and structured absorption in the visible spectral range (420-465 nm). Compared to DOAS, which has a very long physical pathlength through the atmosphere, BBCES achieves comparable or superior sensitivity in a compact and robust system that is suited to mobile, high spatial resolution observations.
In addition, it uses a relatively inexpensive incoherent broadband light as the probe light, and can simultaneously detect multiple species with good temporal resolution and high selectivity.
Prof. ZHANG Weijun’s group at AIOFM has long been devoted to the development of BBCES and its application in the detection of trace atmospheric components.
The new developed BBCES instrument, designed by Prof. ZHAO Weixiong, a member of ZHANG’s group, has been significantly improved by using a custom cage system and symmetrical transmitter and receiver units.
These changes have improved both the instrument’s physical specifications (reduced size and mass) and its performance characteristics (superior sensitivity and stability).
Moreover, the Kalman filtering algorithm, developed by Kalman in 1960, was for the first time applied in this new BBCES instrument, for real-time optimization of the detection sensitivity and precision.
Consequently, the achievable precision, 24 pptv (3σ, 21 s) for CHOCHO measurement, was close to the state-of-the-art performance, but with a 6-8 times shorter effective absorption pathlength.
Now, this newly developed instrument has been applied to the real-time monitoring of glyoxal in China’s Pearl River Delta (PRD) and Yangtze River Delta (YRD) regions. “This will play a very important role in our understanding of VOC degradation mechanisms and even the causes of air pollution in these regions”, said Prof. ZHAO Weixiong.
Link to the paper: Portable broadband cavity-enhanced spectrometer utilizing Kalman filtering: application to real-time, in situ monitoring of glyoxal and nitrogen dioxide
Figure 1 (a) Layout of the custom cage based optical system. The coupling system for transmitter (LED output) and receiver units (CCD spectrometer) were interchangeable. (b) Side view of the optical system with dimensions in mm. (Image by FANG Bo)
Figure 2 Performance evaluation of the BBCES instrument for NO2 (a-c) and CHOCHO (d-f) measurement with zero air. (Image by ZHAO Weixiong)
Figure 3 Simultaneous measurement of ambient NO2 (a) and CHOCHO (b) and the associated fit residual (c). (Image by ZHAO Weixiong)