Imaging of pH distribution inside individual microdroplet by stimulated Raman microscopy

Author:

Gong Kedong1234ORCID,Ao Jianpeng56ORCID,Li Kejian1234,Liu Le7,Liu Yangyang1234ORCID,Xu Guanjun1234,Wang Tao1234,Cheng Hanyun123,Wang Zimeng1234,Zhang Xiuhui8ORCID,Wei Haoran9,George Christian10ORCID,Mellouki Abdelwahid1112ORCID,Herrmann Hartmut13ORCID,Wang Lin123ORCID,Chen Jianmin123ORCID,Ji Minbiao56ORCID,Zhang Liwu1234ORCID,Francisco Joseph S.1415ORCID

Affiliation:

1. Shanghai Key Laboratory of Atmospheric Particle Pollution and Prevention, Department of Environmental Science & Engineering, Fudan University, Shanghai 200433, Peoples’ Republic of China

2. National Observations and Research Station for Wetland Ecosystems of the Yangtze Estuary, Fudan University, Shanghai 200433, People’s Republic of China

3. Integrated Research on Disaster Risk, and RDR International Center of Excellence on Risk Interconnectivity and Governance on Weather, Fudan University, Shanghai 200433, People’s Republic of China

4. Shanghai Institute of Pollution Control and Ecological Security, Shanghai 200092, People’s Republic of China

5. State Key Laboratory of Surface Physics and Department of Physics, Fudan University, Shanghai 200433, Peoples’ Republic of China

6. Academy for Engineering and Technology, Fudan University, Shanghai 200433, Peoples’ Republic of China

7. Department of Atmospheric and Oceanic Sciences, Fudan University, Shanghai 200433, Peoples’ Republic of China

8. Key Laboratory of Cluster Science, Ministry of Education of China, School of Chemistry and Chemical Engineering, Beijing Institute of Technology, Beijing 100081, Peoples’ Republic of China

9. Department of Civil and Environmental Engineering, University of Wisconsin-Madison, Madison, WI 53706

10. Université de Lyon, Université Claude Bernard Lyon 1, CNRS, IRCELYON, Villeurbanne 69626, France

11. Institut de Combustion, Réactivité et Environnement (ICARE), Centre National de la Recherche Scientifique/The Observatory of Sciences of the Universe in the Center (CNRS/OSUC), Orléans Cedex 2, 45071, France

12. Mohammed VI Polytechnic University, Lot 660, Hay Moulay Rachid, 43150 Benguerir, Morocco

13. Leibniz Institute for Tropospheric Research, Atmospheric Chemistry Department, Leipzig 04318, Germany

14. Department of Earth and Environmental, Sciences, University of Pennsylvania, Philadelphia, PA 19104

15. Department of Chemistry, University of Pennsylvania, Philadelphia, PA 19104

Abstract

Aerosol microdroplets as microreactors for many important atmospheric reactions are ubiquitous in the atmosphere. pH largely regulates the chemical processes within them; however, how pH and chemical species spatially distribute within an atmospheric microdroplet is still under intense debate. The challenge is to measure pH distribution within a tiny volume without affecting the chemical species distribution. We demonstrate a method based on stimulated Raman scattering microscopy to visualize the three-dimensional pH distribution inside single microdroplets of varying sizes. We find that the surface of all microdroplets is more acidic, and a monotonic trend of pH decreasing is observed in the 2.9-μm aerosol microdroplet from center to edge, which is well supported by molecular dynamics simulation. However, bigger cloud microdroplet differs from small aerosol for pH distribution. This size-dependent pH distribution in microdroplets can be related to the surface-to-volume ratio. This work presents noncontact measurement and chemical imaging of pH distribution in microdroplets, filling the gap in our understanding of spatial pH in atmospheric aerosol.

Funder

Foundation for Innovative Research Groups of the National Natural Science Foundation of China

Natural Science Foundation of Shanghai

MOST | National Key Research and Development Program of China

Innovative Research Group Project of the National Natural Science Foundation of China

Publisher

Proceedings of the National Academy of Sciences

Subject

Multidisciplinary

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