Atmospheric aerosols are a complex micro-structured mixture of inorganic and organic components, where organics can represent more than 50% of the aerosol mass depending on location. Understanding and predicting the climate effects due to atmospheric aerosols requires quantitative knowledge of their hygroscopic and chemical properties. The ability of aerosols to absorb water influences their optical and cloud forming properties, ice nuclei formation and transformation, atmospheric lifetime, and chemical reactivity. The presence of organic carbon in aerosols has a complex effect on their physical-chemical and reactive properties that is poorly understood. In particular, there is a lack of robust quantitative measurements on how properties of organic components (such as particle size, morphology, chemical composition, concentration and type of mixing) influence the rate and amount of water uptake on aerosols. The proposed work focuses on microscopic analysis of atmospheric aerosols collected during the CalNex field study in California in 2010. The main goal of the proposed research is to conduct detailed microscopic characterization of the size-dependent elemental and chemical composition, density and hygroscopic properties of organic compounds in aerosol samples collected at the Caltech, Pasadena ground site. Scanning Transmission X-ray Microscopy and Atomic Force Microscopy form a unique combination that will be utilized in this work to measure and quantify these properties on a single particle basis. The proposed work will address the following questions: 1) What chemical/elemental composition and concentration of organic aerosols are representative of the specific location of the CalNex 2010 study? 2) What effect does organic carbon have on aerosol hygroscopic properties and how does it depend on particle size/shape and aerosol micro-structure? 3) What is the chemical nature of organic compounds that exhibit the most substantial effects on their hygroscopic properties? 4) How do the atmospheric processing and subsequent differences in organic aerosol composition and micro-structure affect these properties? The results of this study will provide currently lacking input for closure studies focused on (a) aerosol emission inventories of aerosols, (b) chemical transformation and atmospheric aging of aerosols, (c) correlated elemental/chemical compositions and hygroscopic/ice nucleation properties of aerosols, and (d) determination of the type and concentration of organic compounds. Additionally, we will collaborate with other research groups that deployed instruments for the optical, cloud condensation nuclei and ice nuclei measurements at the Caltech supersite. Combined sets of data will contribute to the CalNex 2010 mission focused on understanding of the chemical composition of organic aerosols, mechanisms of their formation, growth and atmospheric transformations, and their indirect climate effects in California. The results to be obtained from this work are expected to provide invaluable fundamental data for understanding climate and air quality related impacts of organic aerosols at local and regional area of the CalNex 2010 study.