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Measurements of diffusion coefficients of organic dyes in proxies of atmospheric particles Ullmann, Dagny Anna

Abstract

Atmospheric particles play a significant role in the atmosphere and climate. To be able to better predict their rate of growth and reactivity, information on diffusion coefficients of organic molecules in these particles is needed. Diffusion coefficients of organic tracer molecules were measured in sucrose water solutions, which served as proxies for atmospheric particles. Diffusion coefficients of rhodamine 6G (hydrodynamic radius = 5.89 Å), an organic tracer molecule, ranged from 1.50∙10-¹⁴ to 4.73∙10-⁹ cm²/s as the water activity ranged from 0.38 to 0.8, respectively, in sucrose water solutions. The measured diffusion coefficients were compared to diffusion coefficients calculated using the Stokes-Einstein equation, which has often been used in the past to estimate diffusion coefficients in atmospheric particles. A breakdown of the Stokes-Einstein equation was observed at a water activity of 0.38 or a viscosity of 3.3∙10⁶ Pa s or Tg/T of 0.94 (Tg being the glass transition temperature and T the temperature of the measurement). In addition, diffusion coefficients of intrinsic fluorophores were measured in brown limonene secondary organic material (SOM) which is thought to be important in the atmosphere. The SOM was generated by the oxidation of limonene followed by aging with NH₃ vapour. The diffusion coefficients of the intrinsic fluorophores in brown limonene SOM ranged from 3.82∙10-¹² to 8.32∙10-⁹ cm²/s as the water activity ranged from 0.38 to 0.9. From the diffusion coefficients mixing times of large organics in brown limonene SOM were calculated, which ranged from 0.001 to 2.6 s across water activities of 0.9 and 0.38. Hence, mixing times of large organics are short and atmospheric particles consisting of brown limonene SOM are well mixed in the atmosphere when water activities range from 0.38 to 0.9. Furthermore, the hydrodynamic radius of the intrinsic fluorophores within brown limonene SOM was determined to be 7.34 ± 1.47 Å. This hydrodynamic radius is consistent with suggested structures for the intrinsic fluorophores.

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