Technical note: Real-time diagnosis of the hygroscopic growth micro-dynamics of nanoparticles with Fourier transform infrared spectroscopy

Nanoparticles can absorb water to grow, and this process will affect the light-scattering behavior, cloud condensation nuclei properties, lifetime, and chemical reactivity of these particles. Current techniques for calculation of aerosol liquid water content (ALWC) usually restrict the size of particles to be within a certain range, which may result in a large uncertainty when the particle size is beyond the specified range. Furthermore, current techniques are difficult to use to identify the intermolecular interactions of phase transition microdynamics during particles' hygroscopic growth process because their limited temporal resolutions are unable to capture complex intermediate states. In this study, the hygroscopic growth properties of nanoparticles with electrical mobility diameters (D-em) of similar to 100 nm and their phase transition micro-dynamics at the molecular level are characterized in real time by using the Fourier transform infrared (FTIR) spectroscopic technique. We develop a novel real-time method for ALWC calculation by reconstructing the absorption spectra of liquid water and realize real-time measurements of water content and dry nanoparticle mass to characterize hygroscopic growth factors (GFs). The calculated GFs are generally in good agreement with the Extended Aerosol Inorganics Model (E-AIM) predictions. We also explore the phenomenon that the deliquescence points of the ammonium sulfate / sodium nitrate (AS/SN) mixed nanoparticles and the AS/oxalic acid (AS/OA) mixed nanoparticles are lower than that of the pure AS. We further normalize the FTIR spectra of nanoparticles into 2D IR spectra and identify in real time the hydration interactions and the dynamic hygroscopic growth process of the functional groups for AS, AS/SN, and AS/OA nanoparticles. The results show that both SN and OA compounds can lower the deliquescence point of AS, but they affect AS differently. The SN can change but OA cannot change the hydrolysis reaction mechanism of AS during the hygroscopic growth process. Compared with previous studies, we captured more complex processes and the intermediate state of the hygroscopic growth of nanoparticles. This study not only can provide important information with respect to the difference in the phase transition point under different conditions but also can improve current understanding of the chemical interaction mechanism between nanoparticles (particularly for organic particles) and the surrounding medium, which is of great significance for investigation of haze formation in the atmosphere.