The Evaporation or Growth Rates of Dicarboxylic Acid Solution Droplets As a Function of Relative Humidity

Dicarboxylic acids are frequently observed in ambient aerosol. Beyond inorganic salts like sea salt and ammonium sulfate, oxygenated organic carbon species like dicarboxylic acids are a key to the growth of aerosols driven by increases in relative humidity (RH). The size of the aerosols or particles in the atmosphere largely determines the amount of radiative forcing they contribute to the Earth system. Radiative forcing is the difference between solar energy absorbed by the Earth versus the radiation reflected by the atmosphere and the Earth, and represents the impact of a single aerosol type or other climate forcer such as carbon dioxide where together all contributors yield the total impact on the Earth system. Evaporation of surface active dicarboxylic acids, such as malonic and glutaric acid, following water exposure are unexpected and represent a competition between size increases and decreases. Bulk scale mass measurements as a function of time are made using the Electrobalance with Variable Active Particle Humidity Control (EVAP-HC) a custom-built housing for a Cahn 2000 Electrobalance. Post evaporation composition analysis is completed with a thermogravimetric analyzer (TGA). Variables including initial solution composition and RH are controlled. The rate of mass loss is used to calculate binary diffusion coefficients using Maxwell’s equations. These coefficients equal the rate at which mass, in the form of surface area, moves from the solution to gas phase and are related to the molecular size and number density. Furthermore, EVAP-HC provides an alternative method for measuring the deliquescence relative humidity (DRH), the RH where a solid particle takes up water and grows into a solution droplet. The experimentally determined linear relationship between the weight percent change over time as a function of RH for solution droplets of a specific solute is manipulated so that the weight percent change (y value) is equal to zero yielding the RH where evaporation transitions to growth (DRH) . Understanding the behavior of these dicarboxylic acid solutions via their evaporation rates, diffusion coefficients and DRHs will lead to better understanding of oxygenated organic components of aerosol and refinements in climate models to better represent the interaction with atmospheric water.