Detection of iodine oxide by Broadband Cavity-Enhanced Absorption Spectroscopy (BBCEAS) based on an ICAD (iterative cavity-enhanced DOAS) algorithm

Correspondence: Min Qin (mqin@aiofm.ac.cn)

Abstract: Iodine oxide (IO) radicals, as a significant active halogen species in the iodine cycle, play a crucial role in the ozone depletion process. They affect atmospheric chemistry by altering the distribution of NO/NO₂ and OH/HO₂ radicals. In coastal regions, IO radical play an important role in the formation of ultrafine aerosol particles. The nucleation of these particles affects cloud properties, thereby impacting the climate.

Therefore, for the monitoring and understanding of its tropospheric chemical processes, it is important to explore and develop new techniques for the sensitive measurement of IO. Here, the quantitative method for detection atmospheric IO radicals using Broadband Cavity-Enhanced Absorption Spectroscopy (BBCEAS) technique was introduced. Considering the spectral absorption characteristics of IO radicals in the 435 - 465 nm wavelength region, as well as the actual atmospheric levels in the marine boundary layer, the parameters which affected the BBCEAS system performance were investigated. An iterative algorithm (ICAD) which actually models the light path reduction from the derived absorbers in the optical resonator applied to BBCEAS technique was established. Constant IO concentration used to evaluate the IO sampling loss was generated from the photolysis of molecular iodine (I₂) and then reacting with ozone (O₃). Nitrogen was bubbled through a solution of I₂ and potassium iodide (KI) to take out I₂ and then diluted before the reaction between iodine (I) and O₃ occurring. The loss of IO within a 10-meter sampling tube was estimated to be approximately 2%, which can be neglected. The system's performance was assessed using Allan variance. For an acquisition time of 60 s, 2σ detection limits for IO and NO₂ were about 1.9 pptv and 20 pptv, respectively. The dependency between IO radicals generated from the photolysis of iodine emitted by seaweed and seaweed activity was investigated in the laboratory. The peak of IO radicals reached a maximum value of 50 pptv with O₃ at ppmv level, and subsequently the IO radicals showed a declining trend with the reduction of seaweed activity.

Acknowledgements: This work was supported by the National Key Research and Development Program of China (Grant No. 2022YFC3700300) and the HFIPS Director’s Fund (Grant No. YZJJQY202205).