Humidity Effects on Mixed Carbon and Dust Aerosols in Houston Measured to Evaluate Radiative Forcing and Deep Convection Modules in Climate Models

Humidity alters aerosol microphysical and optical properties and radiative forcing, clouds and deep convection. Humidity effects that depend on aerosol size, composition, hygroscopicity and refractive index are computed in models by assuming a mixing state (e. g. external, internal or core-shell) and aggregating the component properties. We report comprehensive measurements of particle resolved composition and mixing state (SMPS, 2 SP-AMS's and a SP2), hygroscopic growth (Humidified CAPS) and optical properties at multiple wavelengths (CRDS, CAPS, and photoacoustic) taken in Houston during DOE ARM's TRACER-CAT campaign in July 2022. We interrogated a heterogeneous mixture of aerosols from local urban, shipping and industrial emissions, regional fires and long-range transport of Sahara dust during this period. We demonstrate that these diverse regimes exhibit distinct optical properties and their dependence on humidity. Increase in aerosol scattering and extinction, and single scatter albedo with humidity are quantified and related to their composition. Changes in absorption with humidity are more complex and depend on the composition of the mixed aerosols. We will harness observations of size resolved composition, hygroscopicity, and mixing state, and Mie and Kohler theory, to perform closure studies on humidity dependence of the optical properties and cloud condensation nuclei activity. Our results will be used to evaluate and refine the treatment of water dependence of aerosol properties in the MAM4 and deep convective modules in DOE's E3SM climate model.