On the Source of the Submicrometer Droplet Mode of Urban and Regional Aerosols

While atmospheric aerosols are typically described as consisting of three modes, the nucleation mode (0.01-0.1 μm in diameter), the accumulation mode (0.1-1.0 μm in diameter), and the coarse mode (> 1 μm in diameter), ambient measurements have shown that two distinct modes can exist in the 0.1-1.0-μm diameter range. These modes are referred to as the condensation mode (approximate aerodynamic diameter of 0.2 μm) and the droplet mode (approximate aerodynamic diameter of 0.7 μm). It has been postulated that the droplet mode results from aqueous-phase chemistry (Hering and Friedlander, 1982; John et al., 1990). In this work we examine the mechanisms of formation of the droplet mode. It is shown that growth of condensation mode particles by accretion of water vapor or by gas-phase or aerosol-phase sulfate production cannot explain the existence of the droplet mode. Activation of condensation mode particles to form fog or cloud drops followed by aqueous-phase chemistry and fog evaporation is shown to be a plausible mechanism for formation of the droplet mode.