Heterogeneous iodine-organic chemistry fast-tracks marine new particle formation

Depletion of iodide in ageing aerosols and the role of humidity: A case study of mixed sodium iodide-malonic acid aerosol

Global sea to air iodine emissions, along with organic emissions and their oxidation products, have increased tremendously. This work presents a comprehensive analysis of the humidity mediated changes in ageing aerosols comprising iodide and water soluble dicarboxylic acid using aerosol micro-Raman spectroscopy. The studies in the model system, sodium iodide-malonic acid mixed aerosols, unveiled the depletion in iodide. Mechanistic insights gleaned through comparative studies conducted under inert (nitrogen) and oxidative (air) atmospheres reveal the iodide depletion occurs possibly via oxidation to molecular iodine. The reaction involves gaseous components, diffusion of which across the particles will be impacted by the physical state of the particles, such as viscosity, which in turn is intricately linked to ambient humidity levels. To this end, studies on the temporal evolution of the reaction at three distinct RHs covering 30-80% revealed the enhanced progression of the reaction with increasing humidity. Given that geographical locations serving as major sources for atmospheric iodine typically experience high humidity, these reactions could emerge as an additional process controlling iodine speciation in ageing aerosols.

Photochemical Mechanisms in Atmospherically Relevant Iodine Oxide Clusters

Atmospheric new particle formation events can be driven by iodine oxides or oxoacids via both neutral and ionic mechanisms. Photolysis of new particles likely plays a significant role in their growth mechanisms, but their spectra and photolysis mechanisms remain difficult to characterize. We recorded ultraviolet (UV) photodissociation spectra of (I2O5)0-3(IO3-) clusters, observing loss of an O atom, I2O4, and (I2O5)1,2 in the atmospherically relevant range of 300-340 nm. With increasing cluster size, the intensity of absorption red shifts and generally increases, suggesting particles photolyze more frequently as they grow. Estimates of the rates indicate that even relatively small clusters are likely to undergo photolysis under high-UV conditions. Vibrational spectra identify the covalent moiety I3O8- as the likely chromophore, not IO3-. The I2O5 loss pathway competes with particle growth, while the slower O loss pathway likely produces 3O + 3(cluster) products that could drive subsequent intraparticle chemistry, particularly with co-adsorbed organic or amine species.

HIO3-HIO2-Driven Three-Component Nucleation: Screening Model and Cluster Formation Mechanism

Iodine oxoacids (HIO3 and HIO2)-driven nucleation has been suggested to efficiently contribute to new particle formation (NPF) in marine atmospheres. Abundant atmospheric nucleation precursors may further enhance HIO3-HIO2-driven nucleation through various multicomponent nucleation mechanisms. However, the specific enhancing potential (EP) of different precursors remains largely unknown. Herein, the EP-based screening model of precursors and enhancing mechanism of the precursor with the highest EP on HIO3-HIO2 nucleation were investigated. The formation free energies (ΔG), as critical parameters for evaluating EP, were calculated for the dimers of 63 selected precursors with HIO2. Based on the ΔG values, (1) a quantitative structure-activity relationship model was developed for evaluating ΔG of other precursors and (2) atmospheric concentrations of 63 (precursor)1(HIO2)1 dimer clusters were assessed to identify the precursors with the highest EP for HIO3-HIO2-driven nucleation by combining with earlier results for the nucleation with HIO3 as the partner. Methanesulfonic acid (MSA) was found to be one of the precursors with the highest EP. Finally, we found that MSA can effectively enhance HIO3-HIO2 nucleation at atmospheric conditions by studying larger MSA-HIO3-HIO2 clusters. These results augment our current understanding of HIO3-HIO2 and MSA-driven nucleation and may suggest a larger impact of HIO2 in atmospheric aerosol nucleation.

Spontaneous Iodide Activation at the Air-Water Interface of Aqueous Droplets

We present experimental evidence that atomic and molecular iodine, I and I2, are produced spontaneously in the dark at the air-water interface of iodide-containing droplets without any added catalysts, oxidants, or irradiation. Specifically, we observe I3- formation within droplets, and I2 emission into the gas phase from NaI-containing droplets over a range of droplet sizes. The formation of both products is enhanced in the presence of electron scavengers, either in the gas phase or in solution, and it clearly follows a Langmuir-Hinshelwood mechanism, suggesting an interfacial process. These observations are consistent with iodide oxidation at the interface, possibly initiated by the strong intrinsic electric field present there, followed by well-known solution-phase reactions of the iodine atom. This interfacial chemistry could be important in many contexts, including atmospheric aerosols.

Recent Progress in Atmospheric Chemistry Research in China: Establishing a Theoretical Framework for the "Air Pollution Complex"

Atmospheric chemistry research has been growing rapidly in China in the last 25 years since the concept of the "air pollution complex" was first proposed by Professor Xiaoyan TANG in 1997. For papers published in 2021 on air pollution (only papers included in the Web of Science Core Collection database were considered), more than 24 000 papers were authored or co-authored by scientists working in China. In this paper, we review a limited number of representative and significant studies on atmospheric chemistry in China in the last few years, including studies on (1) sources and emission inventories, (2) atmospheric chemical processes, (3) interactions of air pollution with meteorology, weather and climate, (4) interactions between the biosphere and atmosphere, and (5) data assimilation. The intention was not to provide a complete review of all progress made in the last few years, but rather to serve as a starting point for learning more about atmospheric chemistry research in China. The advances reviewed in this paper have enabled a theoretical framework for the air pollution complex to be established, provided robust scientific support to highly successful air pollution control policies in China, and created great opportunities in education, training, and career development for many graduate students and young scientists. This paper further highlights that developing and low-income countries that are heavily affected by air pollution can benefit from these research advances, whilst at the same time acknowledging that many challenges and opportunities still remain in atmospheric chemistry research in China, to hopefully be addressed over the next few decades.