It is well recognized that secondary organic aerosols (SOA) represent a major fraction of tropospheric fine particles that contribute to serious air pollution, damage human health and affect Earth’s climate. SOA typically originate from complex atmospheric (photo)chemical oxidation processes of volatile organic compounds, which are emitted from natural and man-made sources. It has been suggested that organic peroxides (ROOR, where R denotes H or an organic group), a major class of SOA components, can significantly contribute to aerosol toxicity and related health effects. This is mainly due to their oxidizing properties, and thus peroxides are a compound class contributing to the so-called reactive oxygen species (ROS), which also includes oxygen-centred inorganic and organic radicals. Despite their atmospheric and health importance, the analytical identification and quantification of compound-specific organic peroxides in atmospheric aerosols is highly challenging, due to their labile properties, complex composition and limited availability of chemical standards.

As a major topic of my postdoc work at University of Basel, I am currently focusing on analytical method development for molecular-level identification and characterization of particle-phase organic peroxides, with elucidation of their formation chemistry, reactivity, and health impact in SOA.

Related publications:

Li K*, Resch J, Kalberer M*. Synthesis and Characterization of Organic Peroxides from Monoterpene-Derived Criegee Intermediates in Secondary Organic Aerosol. Environ Sci Technol 2024, 58 (7), 3322–3331. DOI: 10.1021/acs.est.3c07048 (highlight work)

Water is the most common liquid on Earth and is stable and inert under ambient conditions. Very recent studies suggest that reactions that do not usually occur in bulk solution can occur spontaneously in small water droplets. It has been reported that microdroplets could accelerate chemical reactions, and trigger spontaneous degradation of organic compounds and oxidant formation (i.e., H2O2). While the underlying mechanism of such high surface reactivity is still debated, it is likely driven by the localized high electric field (~10⁹ V m⁻¹) that forms at the air-water interface. The emerging microdroplet chemistry is profoundly influencing many areas including atmospheric chemistry, green synthesis, origins of life, etc.

As a major focus of my previous postdoc work at CNRS-IRCELYON, we explore the atmospheric significance of this process by demonstrating efficient spontaneous production of interfacial hydroxyl radicals (OH) from aqueous droplets under ambient conditions, possibly due to the strong electric field that forms at such interfaces. This interfacial OH production does not involve precursors or catalysts, such as light or heat, and is likely the largest aqueous OH source in atmospheric droplets at nighttime.

We further 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. This spontaneous iodide activation represents another example of redox chemistry that is mediated by the high potentials present at the air–water interface of aqueous droplets.

Related publications:

Li K, Guo Y, Nizkorodov S A, Rudich Y, Angelaki M, Wang X, An T, Perrier S, George C^*. Spontaneous dark formation of OH radicals at the interface of aqueous atmospheric droplets. Proc Natl Acad Sci U S A 2023, 120 (15), e2220228120. DOI: 10.1073/pnas.2220228120  (highlight work)

Guo Y^#, Li K^#,*, Perrier S, An T^*, Donaldson D J^*, George C^*. Spontaneous Iodide Activation at the Air-Water Interface of Aqueous Droplets. Environ Sci Technol 2023, 57 (41), 15580-15587. DOI: 10.1021/acs.est.3c05777  (highlight work)

The gas phase chemical mechanism is the core of the chemical transport models (CTMs), which representing chemical reactions for emitted pollutants to form secondary pollutants, i.e. ozone formation. For the past a few decades, a variety of chemical mechanisms are being developed, and they are typically classified as simplified mechanism (i.e., CB, SAPRC, RACM) and detailed mechanism (i.e., MCM). The development and evaluation of gas-phase chemical mechanism is mainly based on laboratory smog chamber experiments. The smog chamber (or simulation chamber) has the advantage to isolate atmospheric chemistry for a few selected compounds under well-controlled conditions, and the derived experimental data are especially useful when comparing with the mechanism modelling results and provide a direct evaluation on how reliable of the current chemical mechanism. Therefore, for the past several decades, a great number of smog chambers have been constructed worldwide and are being used for various purposes, in particular for gas-phase mechanism development and evaluation as well as secondary organic aerosol (SOA) formation.

We characterize two smog chamber facilities (i.e., 3 m³ at Zhejiang University and 24.7 m³ at CSIRO) and the data are used to evaluate existing gas-phase mechanisms. As a research highlight, we develop and evaluate a new chemical mechanism for one type of amine (2-amino-2-methyl-1-propanol, AMP) from CSIRO smog chamber experiments, where amines are considered as an emerging class of atmospheric pollutants that are of great importance to atmospheric chemistry and new particle formation.

We also vary experimental factors in chamber experiments and characterize physical-chemical properties of secondary aerosol formation under different conditions. These chamber experiments involve different reaction systems including photochemical aging of soot particles, addition of NH3 on aerosol formation from aromatic/NOx photooxidation, as well as interaction between anthropogenic and biogenic emissions.

Related publications:

Li K, Chen L^*, Han K, Lv B, Bao K, Wu X, Gao X, Cen K. Smog chamber study on aging of combustion soot in isoprene/SO2/NOx system: Changes of mass, size, effective density, morphology and mixing state. Atmospheric Research 2017, 184, 139-148. DOI: 10.1016/j.atmosres.2016.10.011.

Li K, Chen L^*, White S^*, Yu H, Wu X, Gao X, Azzi M, Cen K. Smog chamber study of the role of NH3 in new particle formation from photo-oxidation of aromatic hydrocarbons. Sci Total Environ 2018, 619-620, 927-937. DOI: 10.1016/j.scitotenv.2017.11.180 

White S^*, Angove D, Li K, Campbell I, Element A, Halliburton B, Lavrencic S, Cameron D, Jamie I, Azzi M. Development of a new smog chamber for studying the impact of different UV lamps on SAPRC chemical mechanism predictions and aerosol formation. Environmental Chemistry 2018, 15 (3). DOI: 10.1071/en18005.

Li K^*, Lin C, Geng C, White S, Chen L, Bao Z, Zhang X, Zhao Y, Han L, Yang W^*, Azzi M. Characterization of a new smog chamber for evaluating SAPRC gas-phase chemical mechanism. J Environ Sci (China) 2020, 95, 14-22. DOI: 10.1016/j.jes.2020.03.028 

Li K, White S^*, Zhao B, Geng C, Halliburton B, Wang Z, Zhao Y, Yu H, Yang W, Bai Z^*, Azzi M^*. Evaluation of a New Chemical Mechanism for 2-Amino-2-methyl-1-propanol in a Reactive Environment from CSIRO Smog Chamber Experiments. Environ Sci Technol 2020, 54 (16), 9844-9853. DOI: 10.1021/acs.est.9b07669   (highlight work)

Bao Z, Xu H, Li K^*, Chen L^*, Zhang X, Wu X, Gao X, Azzi M, Cen K. Effects of NH3 on secondary aerosol formation from toluene/NOx photo-oxidation in different O3 formation regimes. Atmospheric Environment 2021, 261. DOI: 10.1016/j.atmosenv.2021.118603.

Li K^*, Zhang X, Zhao B^*, Bloss W J, Lin C, White S, Yu H, Chen L, Geng C, Yang W, Azzi M, George C, Bai Z^*. Suppression of anthropogenic secondary organic aerosol formation by isoprene. npj Climate and Atmospheric Science 2022, 5 (1). DOI: 10.1038/s41612-022-00233-x.  (highlight work)

China has implemented national clean air actions since 2013, with the aim of reducing primary emissions and hence improving air quality (i.e., PM2.5 and ozone) at a national level. Therefore, source apportionment and causes of atmospheric particulate matter and ozone are needed for developing effective control measures in many local regions of China. We perform online and offline field measurements including aerosol composition and trace gases (i.e., volatile organic compounds, ozone, etc) in several cities of China (i.e., Hangzhou, Longyou, Zibo). We also apply random forest machine learning technique to evaluate the real changes of air quality due to clean air actions in a number of Chinese mega-cities. These projects often involve collaboration with colleagues from Hangzhou EPA, CSIRO, CRAES, and Tsinghua University.

One research highlight during my PhD is to perform a comprehensive field campaign (~50 days, in collaboration with Hangzhou EPA) at urban Hangzhou China during a unique period of 2016 summer, where strict emission controls were implemented by the local and regional administrators to ensure good air quality during the period of 2016 Hangzhou G20 Summit (2 weeks). We utilized this opportunity to understand the response of aerosol composition (measured by HR-ToF-AMS) to temporary regional emission reductions.

Related publications:

Li K, Chen L^*, Ying F, White S J, Jang C, Wu X, Gao X, Hong S, Shen J, Azzi M, Cen K. Meteorological and chemical impacts on ozone formation: A case study in Hangzhou, China. Atmospheric Research 2017, 196, 40-52. DOI: 10.1016/j.atmosres.2017.06.003.

Li K, Chen L^*, White S J, Zheng X, Lv B, Lin C, Bao Z, Wu X, Gao X, Ying F, Shen J, Azzi M, Cen K. Chemical characteristics and sources of PM1 during the 2016 summer in Hangzhou. Environ Pollut 2018, 232, 42-54. DOI: 10.1016/j.envpol.2017.09.016   (highlight work)

Bao Z, Chen L^*, Li K, Han L, Wu X, Gao X, Azzi M, Cen K. Meteorological and chemical impacts on PM2.5 during a haze episode in a heavily polluted basin city of eastern China. Environ Pollut 2019, 250, 520-529. DOI: 10.1016/j.envpol.2019.04.045 

Han L, Chen L^*, Li K, Bao Z, Zhao Y, Zhang X, Azzi M, Cen K. Source Apportionment of Volatile Organic Compounds (VOCs) during Ozone Polluted Days in Hangzhou, China. Atmosphere 2019, 10 (12). DOI: 10.3390/atmos10120780.

Li K^*, Shen J, Zhang X, Chen L, White S, Yan M, Han L, Yang W, Wang X^*, Azzi M. Variations and characteristics of particulate matter, black carbon and volatile organic compounds in primary school classrooms. Journal of Cleaner Production 2020, 252. DOI: 10.1016/j.jclepro.2019.119804.

Zhao Y, Chen L^*, Li K, Han L, Zhang X, Wu X, Gao X, Azzi M, Cen K. Atmospheric ozone chemistry and control strategies in Hangzhou, China: Application of a 0-D box model. Atmospheric Research 2020, 246. DOI: 10.1016/j.atmosres.2020.105109.

Li K^#, Wang X^#, Li L, Wang J, Liu Y, Cheng X, Xu B, Wang X, Yan P, Li S, Geng C^*, Yang W, Azzi M, Bai Z. Large variability of O3-precursor relationship during severe ozone polluted period in an industry-driven cluster city (Zibo) of North China Plain. Journal of Cleaner Production 2021, 316. DOI: 10.1016/j.jclepro.2021.128252.

Guo Y, Li K^*, Zhao B, Shen J, Bloss W J, Azzi M, Zhang Y. Evaluating the real changes of air quality due to clean air actions using a machine learning technique: Results from 12 Chinese mega-cities during 2013-2020. Chemosphere 2022, 300, 134608. DOI: 10.1016/j.chemosphere.2022.134608 

Li L, Zheng Z, Xu B, Wang X, Bai Z, Yang W, Geng C^*, Li K^*. Investigation of O3-precursor relationship nearby oil fields of Shandong, China. Atmospheric Environment 2023, 294. DOI: 10.1016/j.atmosenv.2022.119471.

Zheng Z, Li K^*, Xu B, Dou J, Li L, Zhang G, Li S, Geng C, Yang W, Azzi M, Bai Z^*. O3–precursor relationship over multiple patterns of timescale: a case study in Zibo, Shandong Province, China. Atmospheric Chemistry and Physics 2023, 23 (4), 2649-2665. DOI: 10.5194/acp-23-2649-2023. (highlight work)