[1] The Research at the Nexus of Air Quality and Climate Change (CalNex) field campaign was undertaken to obtain a better understanding of the regional impacts of different pollution sources in California. As part of this study, real-time shipboard measurements were made of the size-resolved, single-particle mixing state of submicron and supermicron particles (0.2–3.0 µm aerodynamic diameter) along the California coast where major differences were noted between Southern and Northern California. In Southern California, particles containing soot made up the largest fraction of submicron particles (~38% on average and up to ~89% by number), whereas organic carbon particles comprised the largest fraction of submicron number concentrations (~29% on average and up to ~78% by number) in Northern California including the Sacramento area. The mixing state of these carbonaceous particle types varied during the cruise with sulfate being more prevalent on soot-containing particles in Southern California due to the influence of fresh shipping and port emissions in addition to contributions from marine biogenic emissions. Contributions from secondary organic aerosol species, including amines, and nitrate were more prevalent in Northern California, as well as during time periods impacted by agricultural emissions (e.g., from the inland Riverside and Central Valley regions). These regional differences and changes in the mixing state and sources of particles have implications for heterogeneous reactivity, water uptake, and cloud-nucleating abilities for aerosols in California.