Models for Simulating Atmospheric Aerosols, Nicole Riemer, Day 5 Part 2

This lecture, part of the Princeton-CEFRC Combustion Summer School, focuses on the physical processes governing aerosol size distributions, particularly coagulation and condensation. The instructor begins by revisiting coagulation, illustrating how the size distribution evolves as particles collide and stick together, emphasizing that the total number concentration decreases and small particles are preferentially depleted. The discussion then shifts to condensation, using water vapor as an example, to explain how aerosol particles grow into cloud droplets. Key concepts introduced include the Clausius-Clapeyron equation for equilibrium vapor pressure, the Kelvin effect (curvature increases equilibrium vapor pressure), and the Raoult effect (solute decreases equilibrium vapor pressure). These are combined in the Köhler theory, which describes the equilibrium saturation ratio over a solution droplet as a function of wet diameter. The Köhler curve exhibits a maximum at the critical diameter and critical supersaturation, marking the transition from stable haze droplets to unstable cloud droplet activation. The lecture concludes by discussing how supersaturation is generated in the atmosphere through cooling, and how aerosol particles compete for water vapor, with larger or more hygroscopic particles activating first.