Low-Temperature Raman Imaging of Component Distribution in Micron-Size Droplets

Atmospheric droplets exhibit heterogeneity due to their distinct mixing states and phases. Ice nucleation is a common and stochastic atmospheric process that potentially enhances differences between individual droplets. In this study, we investigated the distribution of chemical moieties within micron-size droplets by recording spatial maps of sulfate, ice crystals, and gold nanoparticles (AuNPs) via Raman imaging at 293 and 223 K. We observed a spatially even distribution of sulfate in ammonium sulfate (AS) solutions and droplets at 293 K, and in supercooled AS droplets at 223 K. Spatially enriched sulfate, expelled from ice crystals, appeared in frozen droplets and bulk solution at 223 K. Interestingly, a fraction of the frozen droplets exhibited spatially enriched sulfate distributions, while others froze evenly. A higher percentage of more evenly distributed droplets were found for higher initial AS concentrations. We differentiated the droplets as supercooled or frozen according to the brightness of the collected optical images. Using the Raman images of the AS droplets, we determined that >93% of the supercooled droplets were evenly distributed, while >90% of the frozen droplets were spatially enriched due to ice nucleation. We suggest that localized ice nucleation, within supercooled droplets, and glass formation, within completely frozen droplets are the major factors contributing to the remainder of the droplets (similar to 10%). The different component distributions reflect the stochastic nature of ice nucleation. We also investigated the distribution of functionalized pH-sensing AuNPs within frozen AS droplets and observed an independent distribution of AuNPs that differed from either the AS distribution or the ice nucleation sites. The relative peak intensity of the Raman spectrum of AuNPs changed at 223 K compared to its room temperature spectrum, which suggests that the formation of ice crystals in the vicinity of AuNPs at 223 K altered the spectral behavior of the functionalized nanoparticles.