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

Abstract

New particle formation contributes significantly to the number concentration of ultrafine particles (UFPs, d≤100nm) and has a great impact on human health and global climate. Iodine oxoacids (HIOx, including iodic acid, HIO3, and iodous acid, HIO2) have been observed in pristine regions and proved to dominate new particle formation (NPF) at some sites. However, the knowledge of HIOx in polluted urban areas is rather limited. Here, we conducted a long-term measurements 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 HIOx to UFP number concentration in both urban environments. HIO3 is highest in summer, up to 2.85×106 and 2.78×106cm-3 in Beijing and Nanjing, respectively, and is lowest in winter by 96% and 75%, respectively. HIO3 exhibits more prominent variation than H2SO4 in both urban sites. HIO3 concentration shows a clear diurnal pattern at both sites with a daily maximum at around noontime, similar to the atmospheric temperature, solar radiation, and ozone (O3) levels. HIO2 concentration has the same diurnal and seasonal trend as HIO3 but is overall about an 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, with 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-3nm particles by about 40% (median) and occasionally by more than 100% in NPF events, suggesting HIOx are significant 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-3nm particle growth of inorganic acid origin in polluted atmospheres with a significant amount of HIOx.