Chemical Reactions and Cloud Nucleation on Soot Aerosols

Abstract

Incomplete combustion causes enhanced levels of soot aerosols and polycyclic aromatic hydrocarbons in the urban atmosphere. In turn, the increased abundance of aerosols may facilitate increased cloudiness by acting as cloud condensation nuclei (CCN) and ice nuclei (IN). The goal of our study is to first identify and quantify chemical changes in atmospheric aerosols, such as the formation of surface hydrophilic functional groups upon exposed to ozone, and second, to assess the cloud nucleating ability of the oxidized aerosols. When first introduced into the atmosphere, soot is a hydrophobic substance and relatively poor CCN or IN. As soot encounters atmospheric oxidizing agents, such as ozone, its surface chemical properties may be altered. Thus, the soot particles become hydrophilic, and subsequently may be much more efficient cloud nuclei. In this study, soot samples are generated using various fuel sources including diesel and propane. Using a Fourier Transform Infrared Spectrometer equipped with a Horizontal Attenuated Total Reflectance cell, we observe the chemical changes at the soot surface as a function of ozone exposure. In addition, the ice nucleating ability of fresh and chemically aged soot is identified using a microscope cooling apparatus. Results will be presented and atmospheric implications discussed.