Aerosol hygroscopic growth, mixing state, and cloud condensation nuclei activity during Tracking Aerosol Convection Interactions ExpeRiment (TRACER)

Abstract

Convective clouds play a critical role in the Earth’s climate system. Recent research has shown that realistic representation of convective processes is critical to constraining climate sensitivity in global climate models. Theoretical and modeling studies showed that aerosols could have a strong dynamic feedback to convection in warm and humid environments through enhancing ice-related processes and condensational growth. A few observation-based studies also suggested influence of aerosols on convective cloud and precipitation properties. However, robust observational quantification of an aerosol effect on convective clouds isolated from other factors remains elusive.

Understanding the impact of aerosol on convective clouds requires the knowledge of cloud condensation nuclei spectrum, which represents the number of particles that uptake water and form cloud droplets as a function of supersaturation. The water uptake by aerosol is also of critical importance for the direct interaction of aerosol with radiation (i.e., aerosol direct effect) due to light scattering and absorption by aerosol. While both droplet activation under supersaturated conditions (i.e., relative humidity > 100%) and the hygroscopic growth under sub-saturated conditions (i.e., relative humidity < 100%) are strongly influenced by particle hygroscopicity, the thermodynamic regimes and measurement methods are very different. Aerosol particles, especially organic particles, can exhibit elevated hygroscopicity for droplet activation than that for hygroscopic growth. In the subsaturated regime, the hygroscopicity of organic particles can also vary strongly with relative humidity. However, global climate models usually treat organic species in aerosols with a constant hygroscopicity, potentially introducing substantial uncertainties in the quantification of aerosol radiative effects.

We propose to deploy two advanced instruments, a size-resolved cloud condensation nuclei system and a Relative Humidity controlled Fast Integrated Mobility Spectrometer, during the Intensive Operation Period of the Tracking Aerosol Convection interactions ExpeRiment (TRACER).  The TRACER campaign focuses on the measurements of detailed convective cloud properties and the environment, including thermodynamic and aerosol properties. This proposed project has two main objectives: (1) to provide high quality and comprehensive characterization of aerosol microphysics including hygroscopic growth, mixing state, and cloud condensation nuclei activity that are necessary to address key objectives of the TRACER campaign, and (2) to quantify and understand the difference in hygroscopicity between subsaturated and supersaturated regimes and the variation of hygroscopicity with the relative humidity for aerosols of representative compositions and sources. This project helps maximize the scientific impact of TRACER campaign through deployment of advanced instruments that complements and leverages observations provided by the Atmospheric Radiation Measurement Climate Research Facility. The proposed research also helps improve the understanding and representation of aerosol hygroscopicity in global climate models such as Department of Energy’s Energy Exascale Earth System Model.