Size-resolved aerosol hygroscopicities under both supersaturated and subsaturated conditions at a rural site during TRACER

Description

The interaction of aerosol particles with water is of critical importance for both the direct interaction of aerosol with radiation (i.e., aerosol direct effect) and the interaction of aerosols with clouds and precipitation (i.e., aerosol indirect effects). The aerosol direct effects depend on aerosol optical properties and thus particle size, which can grow dramatically when particles swell with water under high relative humidity. Aerosol indirect effects arise from the impact of aerosol on cloud properties. Cloud droplet number is controlled by the number of particles that activate as air rises through the condensation level and relative humidity (RH) exceeds 100%. Both subsaturated aerosol hygroscopic growth and supersaturated droplet activation are controlled by particle hygroscopicity, but the thermodynamic regimes and measurement methods are very different. Laboratory and field studies show that hygroscopicity under supersaturated condition (κ_CCN) can be substantially higher than that under subsaturated conditions (κ_GF). However, the prevalence of such difference, the dependence of the difference on particle composition, and their underlying mechanisms are still not well understood for ambient aerosols. During the Tracking Aerosol Convection interactions ExpeRiment (TRACER) campaign, we characterized size-resolved aerosol hygroscopicities from 75% RH to supersaturation at a rural site (i.e., ANC site) located in in Guy, Texas, a rural area 60 km southwest of Houston from July to August 2022. Aerosol hygroscopicities under subsaturated and supersaturated conditions were measured using a RH-controlled Fast Integrated Mobility Spectrometer (HFIMS) and a size-resolved cloud condensation nuclei counter ( CCNc ) system, respectively.  Aerosol hygroscopicity, composition, and airmass backward trajectories indicate a variety of aerosol sources, including marine, urban, and regional aerosols. The variations of aerosol hygroscopicity with RH and saturation regime are examined for aerosols of representative chemical compositions and as functions of particle size. The potential underlying mechanisms for the observed differences between κ_CCN and κ_GF and the variations of κ_GF with RH are investigated. The diurnal variations of aerosol hygroscopicity, mixing state, and chemical composition are examined to understand the processes that control the aerosol hygroscopicity at the rural site.