Height-Resolved Measurements of Cloud Water Chemical Composition in Marine Stratocumulus Clouds

This study examines how cloud water chemical composition varies with height within a cloud. A total of 385 cloud water samples were collected in the months of July and August between 2011 and 2016 off the coast of California. These samples were analyzed for water-soluble ionic and elemental composition. Three characteristic air-equivalent mass concentration profiles are identified: (i) a reduction in concentration with in-cloud height of species that are directly emitted from sources below cloud without in-cloud sources (e.g., Cl^-, Na⁺); (ii) an increase in concentration with in-cloud height of species with possible entrainment and photochemical sources (e.g., NO₂^-, formate); and (iii) a mid-cloud peak in concentration for species that are significantly formed by secondary production within cloud droplets (e.g., non-sea salt SO₄^2-, NO₃^-, organic acids). Vertical profiles of rainout parameters such as loss frequency, lifetime, and change in concentration with respect to time show that the scavenging efficiency throughout the cloud depth depends strongly on the thickness of the cloud. Thin clouds exhibit a greater scavenging loss frequency at cloud top, while thick clouds have a greater scavenging loss frequency at cloud base. Directly emitted species are shown to be most efficiently scavenged in the bottom half of clouds. Species with secondary production mechanisms in cloud droplets are also scavenged most efficiently in the bottom half of clouds, but they exhibit lower scavenging in the top half of clouds where secondary production mechanisms replenish their loss. Finally, species with predominantly free tropospheric sources exhibit the poorest scavenging efficiency along the depth of clouds owing to their lack of sources near cloud base. The implications of these results for treatment of wet scavenging in models is discussed.