The particle phase state is a key factor for determining gas particle partitioning, particle reactive gas uptake, and multiphase chemical reactions, with associated links to secondary aerosol formation. In this study, the particle phase state was investigated by measuring particle rebound fraction f in the highly polluted atmosphere of Beijing, China. The particle phase state was sensitive to ambient relative humidity (RH). The particles changed from rebounding to adhering when the RH increased above 60%, suggesting a transition from the semisolid to liquid state. This transition RH was below the deliquescence RH of both (NH4)2SO4 and NH4NO3. Submicrometer particles were in the liquid state during heavy haze episodes. This might be because the elevated RH and inorganic fraction in particles resulted in an increase in aerosol liquid water content. The transition to a liquid phase state, marking the beginning of the haze episode, might kick off a positive feedback loop. The liquid particles might readily take up pollutants that then react to form inorganics, thereby further increasing the rate of water uptake. We propose that the liquid phase state facilitates the mass transfer and multiphase reactions of the particles, thereby accelerating secondary particle growth in haze over the North China Plain.

pH-sensitive amide proton transfer (APT) MRI provides a surrogate metabolic biomarker that complements the widely-used perfusion and diffusion imaging. However, the endogenous APT MRI is often calculated using the asymmetry analysis (MTRasym), which is susceptible to an inhomogeneous shift due to concomitant semisolid magnetization transfer (MT) and nuclear overhauser (NOE) effects. Although the intact brain tissue has little pH variation, white and gray matter appears distinct in the MTRasym image. Herein we showed that the heterogeneous MTRasym shift not related to pH highly correlates with MT ratio (MTR) and longitudinal relaxation rate (R1w), which can be reasonably corrected using the multiple regression analysis. Because there are relatively small MT and R1w changes during acute stroke, we postulate that magnetization transfer and relaxation-normalized APT (MRAPT) analysis increases MRI specificity to acidosis over the routine MTRasym image, hence facilitates ischemic lesion segmentation. We found significant differences in perfusion, pH and diffusion lesion volumes (P<0.001, ANOVA). Furthermore, MRAPT MRI depicted graded ischemic acidosis, with the most severe acidosis in the diffusion lesion (−1.05±0.29%/s), moderate acidification within the pH/diffusion mismatch (i.e., metabolic penumbra, −0.67±0.27%/s) and little pH change in the perfusion/pH mismatch (i.e., benign oligemia, −0.04±0.14%/s), providing refined stratification of ischemic tissue injury.