Development of a portable Cavity Enhanced Absorption Spectrometer for the measurement of ambient N2O5: experimental setup, lab characterizations, and field applications under polluted urban environment

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Wang, Haichao ; Chen, Jun ; Lu, Keding (2016)

A small and portable incoherent broadband cavity enhanced absorption spectrometer (IBBCEAS) for the dinitrogen pentoxide (N<sub>2</sub>O<sub>5</sub>) measurement has been developed. The instrument is featured with a mechanically aligned nonadjustable optical mounting system. To minimize the influence of the aerosol extinction and strong nonlinear absorption by water vapor, a dynamic reference spectrum with NO titration is used for the spectrum analysis. The range of spectrum fitting is 640&ndash;680 nm. Moreover, the wall losses of NO<sub>3</sub> and N<sub>2</sub>O<sub>5</sub> on the surface of the inlet and cavity tubes were extensively characterized. We determined that the surface reactivity for the heated PFA materials toward NO<sub>3</sub> and N<sub>2</sub>O<sub>5</sub> is 0.16 s<sup>&minus;1</sup> ± 0.04 s<sup>&minus;1</sup> and 0.019 s<sup>&minus;1</sup> ± 0.004 s<sup>&minus;1</sup>, respectively. The N<sub>2</sub>O<sub>5</sub> transmission efficiencies over the filter is 93 % (± 3 %). For the typical field experimental set up we used, the total transmission efficiency is about 82.9 %, the optimal limit of the detection (LOD) is estimated as 1.9 ppt (1σ) at 50 s intervals, the total uncertainty of the N<sub>2</sub>O<sub>5</sub> measurement is 15 %, which is dominated by the uncertainty of the NO<sub>3</sub> cross section calculated for 353 K in this system. Our instrument has been successfully deployed in two comprehensive field campaigns conducted in northern rural areas of Beijing in 2016. In both campaigns, the new design of the optical mounting system enabled us a fast setup and stable running of the IBBCEAS system for the detection of N<sub>2</sub>O<sub>5</sub>. High concentrations of N<sub>2</sub>O<sub>5</sub>. up to 1 ppb were detected for the two campaigns indicating an active nighttime chemistry presented in rural Beijing.
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