Simultaneous detection of atmospheric HONO and NO<sub>2</sub> utilizing an IBBCEAS system based on an iterative algorithm

Abstract. We present an improved incoherent broadband cavity-enhanced absorption spectroscopy (IBBCEAS) system based on iterative retrieval algorithm for simultaneous detection of atmospheric nitrous acid (HONO) and nitrogen dioxide (NO2). The conventional IBBCEAS retrieval algorithm depends on the absolute change of light intensity, which requires high stability of light source and stable transmission of light intensity of all optical components. The new algorithm has an iterative module to obtain the effective absorption optical path length, and then the concentrations of HONO and NO2 are determined by differential optical absorption spectroscopy (DOAS) retrieval, so it is insensitive to the fluctuation of absolute light intensity. The robustness of the system is verified by simulating the influence of the relative change of light intensity on the spectral retrieval results. The effect of nitrogen purging in front of the cavity mirrors on shortening the actual cavity length was measured and corrected by using NO2 gas samples. Allan deviation analysis was conducted to determine the system stability, indicating the detection limits (2δ) of HONO and NO2 are 0.08 ppbv and 0.14 ppbv at an integration time of 60 s, respectively. Furthermore, Kalman filtering was used to improve the detection precision of the system. The detection sensitivity of the system at an integration time of 3 s can be improved by 4.5-fold with applying Kalman filtering, which is equivalent to the detection sensitivity at an integration time of 60 s without applying Kalman filtering. The atmospheric HONO and NO2 were observed by the IBBCEAS system based on iterative algorithm, and compared with those measured by conventional IBBCEAS.