AN INVESTIGATION OF THE INFLUENCE OF THE CO-CONDENSATION OF SEMI-VOLATILE ORGANICS ON CLOUD DROPLET NUMBER FOR VARIOUS ENVIRONMENTS

UoM administered thesis: Master of Philosophy

  • Authors:
  • Yichen Jia

Abstract

Clouds play a critical role in the Earth's climate system by virtue of their ability to regulate the radiative budget. The brightness and albedo of clouds are highly determined by cloud droplet number concentration (Nd). Atmospheric aerosol particles can serve as cloud droplet seeds—cloud condensation nuclei (CCN) and become cloud droplets through hygroscopic growth and activation. In the atmosphere, a substantial proportion of organic fine particle material is semi-volatile (i.e. partitioning between particulate and vapour phases). During the ascent of an air parcel, this fraction will co-condense onto particles along with water vapour according to the prevailing saturation ratio. The addition of soluble organic material to growing particles will enhance the uptake of more water and suppress the critical supersaturation for the activation of a particle, by which it can result in an increase in cloud droplet number concentrations. A Python version of a parcel model with "bin-resolved microphysics" has been developed to examine and quantify this effect for various continental environments including pristine forest, background, rural, near-city, urban and kerbside. For each environment, the representative volatility distribution of organic material and the resulting condensable semi-volatile organic material were repartitioned using equilibrium partitioning theory to match the representative aerosol particle number size distributions and chemical composition. In this way, along with adjustment of other parameters, we systematically initialised the model, and thereby investigated the co-condensational effect under various environmental conditions and explored the potential role of the transformation of semi-volatile organic material in the atmosphere. It is shown that, across all model scenarios, a maximum of around 70% more seed particles become cloud droplets due to the enhancing effect of co-condensation of semi-volatile organics compared to the simulations without co-condensation. We show that, despite relatively straightforward initialisation conditions in pristine forest, the links between the enhancement by co-condensation and other parameters are complex and non-linear and the activated fraction of particles without co-condensation should be taken into consideration. In the other five environments, the general trends of the percentage enhancement by co-condensation can be concluded as: (1) the enhancement increases with updraft speeds; (2) in summer, the increase is insignificant; (3) in winter, the increase is the most significant in clean environments while very limited in the intermediate and polluted environments. However, the trend of the percentage enhancement is too intricate and complex to be accessible to the simple assumption of behaviour as a function of, for example, per surface area. We found that initial temperature, updraughts and initial volatility distribution are the biggest contributory parameters to the large enhancement by co-condensation. On the basis of this work, a future study could be undertaken to estimate the co-condensational effect on the regional radiative budget under the environmental condition proposed in this study.

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Original languageEnglish
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Award date1 Aug 2021