Ammonia Uptake by Secondary Organic Material and Dependence on Relative Humidity

Description

Secondary organic material (SOM) is a major component in atmospheric particles. It was widely assumed that SOM exists as low-viscosity, high-diffusivity liquid. Instantaneous equilibrium was thus normally assumed for gas-particle interactions including water uptake, volatile partitioning and particle-phase reaction. This assumption may not be always valid, because SOM can be semi-solid or even glassy under certain circumstances, and thus can have low particle-phase diffusivities. Relative humidity (RH) variation, as commonly occurs in the atmosphere, is the most perceptible way to induce change in particle phase state and thus particle-phase diffusivity. The extent that low particle-phase diffusivities affects gas-particle interaction, including particle-phase reaction, is also anticipated to depend on the precursor from which the SOM is produced. Herein, we show experimental results of the kinetic effects for ammonia uptake by SOM produced from six representative precursors including three terpenoids and three aromatic compounds. Diffusivities are regulated by changing RH with a wide range from <5% to >90%, with intervals of approximately 10%. Ammonia uptake by isoprene SOM had little dependence on RH, while all other five precursors showed strong RH dependence. SOM from β-caryophyllene had smallest ammonia uptake in the whole RH range, presumably due to relatively high molecular weight and low degree of oxygenation (thus low water uptake ability) of the SOM produced. SOM from aromatic compounds showed strong ammonia uptake, which also varied depending on the degree of oxygenation of the aromatic SOMs. Differences in behaviors for terpenoids compared to aromatic SOMs are discussed in conjunction with the chemical properties of the SOM obtained from particle mass spectrometry. Kinetics of ammonia uptake with different ammonia exposures is also shown for four of the six types of SOMs.