A newly designed dual-optical cell DOAS system for generating and measuring iodine monoxide radical and observation-constrained model development

As a key reactive iodine species, iodine monoxide (IO) plays a significant role in atmospheric oxidative capacity and particle formation. We developed a newly designed dual-optical cell Differential Optical Absorption Spectroscopy (DOAS) system to generate and measure IO radical. IO was produced through the photochemical interaction between molecular iodine (I₂) and ozone (O₃), allowing independent control of precursor and oxidant levels. By varying I₂ or O₃ concentrations under stable environmental conditions, we demonstrate that IO can be generated reproducibly and maintained at steady concentrations over experimental timescales. The measured IO concentrations were subsequently used to constrain a zero-dimensional box model incorporating state-of-the-art iodine chemistry. Model development focused on revising key reaction pathways governing I₂-O₃ interactions and subsequent IO formation, motivated by discrepancies between observed and simulated IO at high oxidant levels. Adjustments to branching ratios significantly improved model performance, with correlation coefficients (R) between observed and simulated values exceeding 0.9 and slope error below 23%. The dual-optical cell DOAS system is suitable to provide a stable and reproducible IO source for instrument calibration and chemical mechanism evaluation.