Chemically segregated optical and microphysical properties of ambient aerosols measured in a single‐particle mass spectrometer

[1] This paper describes results from the first direct measurements of the size-resolved optical properties as a function of chemical mixing states for atmospheric particle types sampled in Mexico City and Riverside, California. The coupled size, chemistry, and optical measurements were used to derive refractive indices and effective densities for chemically distinct particle mixing states. On the basis of the measured dependence of scattering intensity as a function of relative humidity and size, the Riverside particles contained detectable amounts of liquid water, whereas the Mexico City particles were relatively dry. Aerosol particles in Mexico City were observed to exhibit a wide range of densities (1.1–3.4 g/cm3), suggesting a dynamic, externally mixed aerosol population. Daily variations were observed as the particles in Mexico City underwent photochemical aging processes leading to a diurnal variation in particle morphology. In contrast, the optical properties of the Riverside aerosol, sampled during the fall season, were strongly impacted by the condensation of water and ammonium nitrate during periods of intense pollution buildup, resulting in microphysical properties that were similar across mixing states for a specific relative humidity.