Publications

New particle formation contributes significantly to the number concentration of ultrafine particles (UFPs, d ≤ 100 nm) 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. 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 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-3 nm particle growth of inorganic acid origin in polluted atmospheres with a significant amount of HIOx.

Y. Zhang⁺, Duzitian Li⁺, X.-C. He*, W. Nie*, …, J. Jiang, A. Ding, M. Kulmala

Atmospheric Chemistry and Physics (2024)

Understanding atmospheric new particle formation is of paramount importance, since it is impacting the climate. Particle formation through iodine oxoacid nucleation, has rarely been incorporated into global simulations, despite observations at various coastal locations and increasing concentration of iodine, primarily driven by climate change. This study employs chamber measurements and a kinetic model to explore the influence of temperature, ionisation rate, and humidity on iodine oxoacid nucleation. Our findings indicate that ion-induced formation rates of iodine oxoacid are not significantly affected by temperature but the neutral formation rates increase with decreasing temperature. Furthermore, increases in ionisation rates decrease the ion induced nucleation rate under atmospherically relevant iodic acid concentrations and humidity has no significant effect on nucleation.

B. Rorup, X.-C. He*, J. Shen, …, R. Volkamer, D.R. Worsnop, K. Lehtipalo

Environmental Science Atmosphere (2024)

How are new particles formed in the air above the oceans, where ammonia, an important species in the process, is not very abundant? He et al. report that iodine oxoacids, which are plentiful in marine environments, can substantially increase the rate of new particle formation in the low-ammonia conditions commonly found in pristine marine and polar regions. This effect could be particularly important in low-level marine stratocumulus clouds, which reflect a large fraction of incident solar radiation back into space and have an important influence on global radiation balance and climate. —H. Jesse Smith

X.-C. He*, M. Simon, S. Iyer, H.-B. Xie*, …, J. Kirkby, N.M. Donahue, M. Sipila*, M. Kulmala*

Science (2023)

The multi-scheme chemical ionisation inlet 1 (MION1) enables rapid switching between the measurement of atmospheric ions without chemical ionisation and neutral molecules using various atmospheric pressure chemical ionisation methods. In this study, we introduce the upgraded version, the multi-scheme chemical ionisation inlet 2 (MION2). The new design incorporates enhanced ion optics, resulting in increased reagent ion concentration, ensuring a robust operation, and enabling the use of multiple chemical ionisation methods with the same ionisation time.

X.-C. He*, J. Shen*, S. Iyer, …, J. Mikkila, M. Sipila, J. Kangasluoma

Atmospheric Measurement Techniques (2023)

Iodic acid (IA) has recently been recognized as a key driver for new particle formation (NPF) in marine atmospheres. However, the knowledge of which atmospheric vapors can enhance IA-induced NPF remains limited. The unique halogen bond (XB)-forming capacity of IA makes it difficult to evaluate the enhancing potential (EP) of target compounds on IAinduced NPF based on widely studied sulfuric acid systems. Herein, we employed athree-step procedure to evaluate the EP of potential atmospheric nucleation precursors on IA-induced NPF. First, we evaluated the EP of 63 precursors by simulating the formation free energies (ΔG) of the IA-containing dimer clusters. Among all dimer clusters, 44 contained XBs, demonstrating that XBs are frequently formed. Based on the calculated ΔG values, aquantitative structure−activity relationship model was developed for evaluating the EP of other precursors.

F. Ma, H.-B. Xie^*, R. Zhang, …, M. Engsvang, J. Elm, X.-C. He*

Environmental Science and Technology (2023)

Iodine is a reactive trace element in atmospheric chemistry that destroys ozone and nucleates particles. Iodine emissions have tripled since 1950 and are projected to keep increasing with rising O₃ surface concentrations. Although iodic acid (HIO₃) is widespread and forms particles more efficiently than sulfuric acid, its gas-phase formation mechanism remains unresolved. Here, in CLOUD atmospheric simulation chamber experiments that generate iodine radicals at atmospherically relevant rates, we show that iodooxy hypoiodite, IOIO, is efficiently converted into HIO₃ via reactions (R1) IOIO + O₃ → IOIO₄ and (R2) IOIO₄ + H₂O → HIO₃ + HOI + (1)O₂. The laboratory-derived reaction rate coefficients are corroborated by theory and shown to explain field observations of daytime HIO₃ in the remote lower free troposphere. The mechanism provides a missing link between iodine sources and particle formation. Because particulate iodate is readily reduced, recycling iodine back into the gas phase, our results suggest a catalytic role of iodine in aerosol formation.

H. Finkenzeller⁺*, S. Iyer⁺, X.-C. He, …, T. Kurten, M.P. Rissanen, R. Volkamer*

Nature Chemistry (2022)

The precise role of HIO₂ in iodine oxoacid nucleation remains unclear. In this study, we probe such a role by investigating the cluster formation mechanisms and kinetics of (HIO₃)m(HIO₂)n (m = 0–4, n = 0–4) clusters with quantum chemical calculations and atmospheric cluster dynamics modeling. The fastest nucleation rate is predicted for mixed HIO₃–HIO₂ clusters rather than for pure HIO₃ or HIO₂ ones. Our calculations reveal that the strong binding results from HIO₂ exhibiting a base behavior (accepting a proton from HIO₃) and forming stronger halogen bonds. Our predicted cluster formation rates and dimer concentrations are acceptably consistent with those measured by the Cosmic Leaving Outdoor Droplets (CLOUD) experiment. This study suggests that HIO₂ could facilitate the nucleation of other acids beyond HIO3 in regions where base vapors such as ammonia or amines are scarce.

R. Zhang, H.-B. Xie*, F. Ma., …, M. Sipila, M. Kulmala, X.-C. He*

Environmental Science and Technology 56, 19 (2022)

Iodine species are one of only a handful of atmospheric vapors known to make new aerosol particles, which play a central role in controlling the radiative forcing of climate. He et al. report experimental evidence from the CERN Cosmics Leaving Outdoor Droplets, or CLOUD, chamber demonstrating that iodic acid and iodous acid rapidly form new particles and can compete with sulfuric acid in pristine regions.

X.-C. He*, Y.J. Tham, L. Dada, …, J. Kirkby*, D.R. Worsnop, M. Sipila*

Science 371, 6529 (2021)

See also Interview with Jasper

Ions enhance the formation rate of atmospheric aerosol particles, which play an important role in Earth’s radiative balance. Although theoretical frameworks exist to calculate the collision rate coefficients between neutral molecules and ions, they need to be experimentally confirmed, ideally under atmospherically relevant conditions of around 1000 ion pairs cm^-3$. Here, in experiments performed under atmospheric conditions in the CERN CLOUD chamber, we have measured the collision rate coefficients between neutral iodic acid (HIO₃) monomers and charged iodic acid molecular clusters containing up to 11 iodine atoms. Three methods were analytically derived to calculate ion-polar molecule collision rate coefficients.

X.-C. He*, S. Iyer, M. Sipila, …, J. Kirkby*, T. Kurten, M. Kulmala

Aerosol Science and Technology 55, 2 (2021)

Iodine species are important in the marine atmosphere for oxidation and new-particle formation. Here, we describe the application of a bromide chemical ionization mass spectrometer (Br-CIMS) to measure iodine species. We have measured gas-phase iodine species and sulfuric acid using two BrCIMS. From offline calibrations and intercomparisons with other instruments, we have quantified the sensitivities of the Br-MIONCIMS to HOI, I₂, and H₂SO₄ and obtained detection limits of 5.8 × 10⁶, 3.8 × 10⁵, and 2.0 × 10⁵ molec. cm⁻³, respectively, for a 2 min integration time.

M. Wang⁺, X.-C. He⁺*, H. Finkenzeller, …, Y.J. Tham*, N.M. Donahue, M. Sipila

Atmospheric Measurement Techniques 14, 6 (2021)

The postulation that heterogeneous cycling of reactive iodine on aerosols may significantly influence the lifetime of ozone in the troposphere not only remains poorly understood but also heretofore has never been observed or quantified in the field. Here, we report direct ambient observations of hypoiodous acid (HOI) and heterogeneous recycling of interhalogen product species (i.e., iodine monochloride [ICl] and iodine monobromide [IBr]) in a midlatitude coastal environment. Significant levels of ICl and IBr with mean daily maxima of 4.3 and 3.0 parts per trillion by volume (1-min average), respectively, have been observed throughout the campaign. We show that the heterogeneous reaction of HOI on marine aerosol and subsequent production of iodine interhalogens are much faster than previously thought.

Y.J. Tham, X.-C. He, Q. Li, …, M. Dal Maso, A. Saiz-Lopez*, M. Sipila*

Proceedings of the National Academy of Sciences 118, 4 (2021)