Influence of relative humidity on the diffusion of dicarboxylic acid molecules to secondary organic material from alpha-pinene ozonolysis
Authors
Yuemei Han — Harvard University
Zhaoheng Gong — Harvard University
Pengfei Liu — Harvard University
Suzane Simoes De Sa — Harvard University
Karena April McKinney — Harvard University
Scot T. Martin — Harvard University
Category
Secondary organic aerosol
Description
Secondary organic material (SOM) from the condensation or nucleation of hydrocarbon oxidation products comprises a substantial portion of atmospheric organic aerosols. SOM particles may present in amorphous solid, semisolid, or liquid states under varied relative humidity (RH) conditions, which affects the dynamic exchange and reactivity between particles and gas-phase molecules. Dicarboxylic acids are ubiquitous in the atmosphere and their uptake may lead to substantial changes in hygroscopicity and absorption property of aerosol particles. In this study, the influence of RH on the diffusion of dicarboxylic acid molecules to SOM particles has been investigated in order to understand the potential significant role of particle phase state. SOM particles, generated from dark ozonolysis of alpha-pinene in the Harvard Environmental Chamber, were introduced into a second reactor for the uptake of gas-phase molecules of four 13C-labeled dicarboxylic acids at variable RH conditions. The changes in SOM chemical composition were characterized with aerosol mass spectrometry. For the probe molecules of 13C-labeled oxalic acid, malonic acid, and alpha-ketoglutaric acid, the fractions of their tracer ions in the total organic mass increased clearly as RH increased, suggesting that the diffusion of dicarboxylic acid molecules to alpha-pinene SOM was enhanced with the particle phase transition from nonliquid to liquid state. Higher uptake amounts were observed with the increase of gas-phase concentration and the decrease of volatility, suggesting that gas-phase concentration and volatility are important factors governing the diffusion of the dicarboxylic acid molecules. The total uptake amounts of individual dicarboxylic acids were larger than the estimated maximum amounts dissolved in the liquid water on the particles, suggesting that the changes of Henry’s law constant with RH did not contributed substantially to the uptake of dicarboxylic acid molecules by alpha-pinene SOM.
