Iodine oxoacids and their roles in sub-3 nanometer particle growth in polluted urban environments

Abstract. New particle formation processes contribute significantly to the number concentration of ultrafine particles (UFP), and have great impacts on human health and global climate. Iodine oxoacids (HIO_x, including iodic acid, HIO₃ and iodous acid, HIO₂) have been observed in pristine regions and proved to dominate NPF events at some sites. However, the knowledge of HIO_x in polluted urban areas is rather limited. Here, we conducted a long-term comprehensive observation of gaseous iodine oxoacids and sulfuric acid in Beijing from January 2019 to October 2021 and also in Nanjing from March 2019 to February 2020, and investigated the contribution of HIO_x to UFP number concentration in urban environments. HIO₃ concentration is highest in summer, up to 2.85×10⁶ cm^-3 and 2.78×10⁶ cm^-3 in Beijing and Nanjing, respectively, and is lowest in winter, with a more prominent seasonal variation than H₂SO₄. HIO₃ concentration shows a clear diurnal pattern at both sites with a daily maximum at around noontime, similar to the atmospheric temperature, radiation and ozone (O₃) levels. HIO₂ concentration has the same diurnal and seasonal trend as HIO₃ but is overall about one order of magnitude lower than HIO₃ concentration. Back trajectory analysis suggests that the sources for inland iodine species could be a mix of marine and terrestrial origins, both having peak iodine emission in warm seasons. While the contribution of HIO₂ to particle growth is marginal in Beijing and Nanjing, our results demonstrate that HIO₃ enhances the particle survival probability of sub-3 nm particles by about 40 % (median) and occasionally by more than 100 % in NPF events, suggesting HIO_x are non-negligible contributor to UFPs in polluted urban areas. As the growth contribution from HIO₃ and H₂SO₄ is similar on a per-molecule basis, we propose that the sum of HIO₃ and H₂SO₄ could be used to estimate sub-3 nm particle growth of inorganic acid origin, in the polluted atmospheres with a significant amount of HIO_x.