Development of a portable cavity-enhanced absorption spectrometer for the measurement of ambient NO<sub>3</sub> and N<sub>2</sub>O<sub>5</sub>: experimental setup, lab characterizations, and field applications in a polluted urban environment

Abstract

Abstract. A small and portable incoherent broadband cavityenhanced absorption spectrometer (IBBCEAS) for NO 3 and N 2 O 5 measurement has been developed. The instrument features a mechanically aligned non-adjustable optical mounting system, and the novel design of the optical mounting system enables a fast setup and stable operation in field applications. To remove the influence of the strong nonlinear absorption by water vapour, a dynamic reference spectrum through NO titration is used for the spectrum analysis. The wall loss effects of the sample system were extensively studied, and the total transmission efficiencies were determined to be 85 and 55 % for N 2 O 5 and NO 3 , respectively, for our experimental setup. The limit of detection (LOD) was estimated to be 2.4 pptv (1σ ) and 2.7 pptv (1σ ) at 1 s intervals for NO 3 and N 2 O 5 , respectively. The associated uncertainty of the field measurement was estimated to be 19 % for NO 3 and 22-36 % for N 2 O 5 measurements from the uncertainties of transmission efficiency, absorption cross section, effective cavity length, and mirror reflectivity. The instrument was successfully deployed in two comprehensive field campaigns conducted in the winter and summer of 2016 in Beijing. Up to 1.0 ppb NO 3 +N 2 O 5 was observed with the presence of high aerosol loadings, which indicates an active night-time chemistry in Beijing.

Efficient N<sub>2</sub>O<sub>5</sub> uptake and NO<sub>3</sub> oxidation in the outflow of urban Beijing

“…A peak occurred near 20:00 CNST and decreased below the instrument detection limit at sunrise. The time at which N 2 O 5 maxima occurred is similar to our previous observation in urban Beijing (H. C. Wang et al, 2017b). However, the decrease rate of the observed N 2 O 5 after the peak time was much slower than that in urban Beijing, where the N 2 O 5 dropped to zero in 2-4 h, which suggests a relatively slow N 2 O 5 loss rate in suburban Beijing.…”

“…The N 2 O 5 concentration was elevated in Beijing (H. C. Wang et al, 2017a, b) but was moderate in other parts of the North China Plain (NCP), such as Wangdu, Jinan, and Mount Tai (Tham et al, 2016;. Recently, the N 2 O 5 uptake coefficients were determined to be very high, even up to 0.1 in the NCP, but the reason is still not well studied (H. C. Wang et al, 2017b;. Reactive N 2 O 5 chemistry was also reported in Hong Kong and showed the highest field-observed N 2 O 5 concentration to date Brown et al, 2016).…”

“…Reactive N 2 O 5 chemistry was also reported in Hong Kong and showed the highest field-observed N 2 O 5 concentration to date Brown et al, 2016). Observations and model simulations revealed that fast heterogeneous uptake of N 2 O 5 is an important pathway of pNO − 3 formation in China (H. C. Wang et al, 2017b;Su et al, 2017); the reaction also contributed significantly to removal (Z. Brown et al, 2016).…”

“…A comprehensive suite of trace gas compounds and aerosol properties was measured in the field study, and the details are listed in Table 1. N 2 O 5 was measured using a newly developed cavity-enhanced absorption spectrometer (CEAS; H. C. Wang et al, 2017a). In the CEAS, ambient N 2 O 5 was thermally decomposed to NO 3 in a perfluoroalkoxy alkane (PFA) tube (length: 35 cm, I.D.…”

A review of experimental techniques for aerosol hygroscopicity studies

“…Compared to CRDS, one major advantage of BBCEAS is that optical extinction can be measured as a function of wavelength. BBCEAS, as described in details elsewhere (Platt et al, 2009;Varma et al, 2013;Washenfelder et al, 2013;Zhao et al, 2014;Washenfelder et al, 2016;Wang et al, 2017a;Li et al, 2018b), has also been widely used in gas and aerosol measurements. Zhao et al (2014) utilized BBCEAS to measure aerosol optical extinction at 641 nm as a function of RH, and for 200 nm (NH4)2SO4, the measured optical growth factors agreed well with those calculated using the Mie theory.…”