Photochemistry of single optically trapped organic aerosol droplets: The photodegradation of oleic acid by oxygen
Organic aerosols represent 20 to 90 % of the total terrestrial aerosols at continental mid-latitudes and therefore impact the climate, air quality and health [1]. During their lifetime, they undergo transformations that alter their physical properties and they are exposed to sunlight and chemical species which can lead to complex chemical evolution [2]. These processes are known as aerosol aging. Nevertheless, the temporal evolution of the aging process is not well understood.
We present a method to study the chemical aging of organic aerosols. Particles are optically trapped in air with a counter propagating tweezer with a laser wavelength of 532 nm [3]. The chemical aging of the particle can be induced by light or by a surrounding gas. The monitoring of the elastically scattered light gives information on the temporal size evolution of the trapped droplet [4]. The inelastic scattered light is monitored to give time dependent Raman spectra which provide in-situ information on the droplet’s chemical composition. With this approach, we are able to study the temporal evolution of size and chemical composition of the aging particle.
In this work, we apply our method to study the photodegradation of isolated oleic acid droplets induced by 532 nm light. The trapping laser wavelength matches the excitation wavelength of ground state oxygen to singlet oxygen (1O2) [5]. The produced 1O2 reacts with oleic acid leading to the production of volatile compounds resulting in a decrease of droplet size. The temporal Raman spectra are used to identify the degradation of chemical bonds and retrieve reaction rates of the photoreaction of oleic acid. We study the influence of the excitation light power and oxygen concentration on the reaction path and its kinetics. Furthermore, a kinetic-multilayer model was developed to qualitatively support the experimental results giving insight into the mass transport and production rates of the volatile compounds in the droplet.
[1] J.L. Jimenez et al, Science, 2009, 326(5959), 1525-1529.
[2] Paul J. Ziemann, Roger Atkinson, Chem. Soc. Rev., 2012, 41, 6582-6605.
[3] K. Esat, G. David, T. Poulkas, M. Shein, R. Signorell, Phys.Chem. Chem. Phys., 2018, 20, 11598-11607.
[4] M. E. Diveky, S. Roy, J.W. Cremer, G. David, R. Signorell, Phys.Chem. Chem. Phys., 2019, 21, 4721-4731.
[5] G. D. Greenblatt, J. J. Orlando, J. B. Burkholder, A. R. Ravishankara, Journal of Geophysical Research, 1990, 95(D11), 18577-18582.