Technical note: Quantified organic aerosol subsaturated hygroscopicity by a simple optical scatter monitor system through field measurements

The hygroscopicity of organic aerosol (κ.sub.OA) plays a crucial role in cloud droplet activation and aerosol-radiation interactions. This study investigated the viability of an optical scatter monitor system, featuring two nephelometric monitors (pDR-1500), to determine κ.sub.OA after knowing the aerosol chemical composition. This system was operated during a mobile lab deployment on Long Island in the summer of 2023, which was executed to coordinate with the Atmospheric Emissions and Reactions Observed from Megacities to Marine Areas (AEROMMA) field campaign. The derived κ.sub.OA under subsaturated high-humidity conditions (RH between 85 % and 95 %) were categorized based on different aerosol sources, including wildfire aerosol, urban aerosol, and aerosol from rural conditions. The κ.sub.OA and the OA O:C ratio exhibited linear positive relationships for the urban aerosol and the aerosol from rural conditions, with a much higher slope (0.50 vs. 0.24) for the latter. However, there was no clear relationship between κ.sub.OA and the OA O:C ratio observed during each period affected by wildfire plumes. The system proposed here could be widely applied alongside the current aerosol component measurement systems, providing valuable insights into the large-scale spatial and temporal variations in OA hygroscopicity.