Quantifying how atmospheric particles interact with water vapor is critical for 32 understanding the effects of aerosols on climate. We present a novel method to measure the 33 mass-based hygroscopicity of particles while characterizing their elemental and carbon 34 functional group compositions. Since mass-based hygroscopicity is insensitive to particle 35 geometry, it is advantageous for probing the hygroscopic behavior of atmospheric particles, 36 which can have irregular morphologies. Combining scanning electron microscopy with energy 37 dispersive X-ray analysis (SEM/EDX), scanning transmission X-ray microscopy (STXM) 38 analysis, and in situ STXM humidification experiments, this method was validated using 39 laboratory-generated, atmospherically relevant particles. Then, the hygroscopicity and elemental 40 composition of 15 complex atmospheric particles were analyzed by leveraging quantification of 41 C, N, and O from STXM, and complementary elemental quantification from SEM/EDX. We 42 found three types of hygroscopic responses, and correlated high hygroscopicity with Na and Cl 43 content. The mixing state of 158 other particles from the sample broadly agreed with those of the 44
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