Trace gases - gases that make up only a tiny amount of the atmosphere - can affect how cloud droplets form. Stable cloud droplets form when enough water molecules stick to a particle, called a cloud condensation nucleus, in the atmosphere.
Scientists call the point at which an aerosol (a particle suspended in a gas) changes into a cloud droplet the critical supersaturation. This study is the first one to use field observations to investigate how semivolatile trace gases influence cloud droplet formation in the atmosphere. (Semivolatile trace gases exist in a dynamic balance between being in a gas phase or being in a condensed phase as liquid or solid particles.) The researchers compared the sizes at which particles reached critical supersaturation and formed droplets with and without trace gases. Without trace gases, the droplets required more water molecules to form when aerosols were wet. (The aerosols were wet at higher relative humidity.)
This may happen because the system may lose semivolatile trace gases via evaporation during the particle phase. When the scientists started with higher trace gas concentrations and then removed those gases, they found that the droplets required fewer water molecules to form. This effect may occur because the trace gases have attached to the surface and changed how the surfaces of the particle and water molecules interact with each other.
The Impact
Results from this study indicate that scientists do not yet fully understand the fundamental mechanism of how cloud droplets form in the presence of semivolatile trace gases. Better understanding this mechanism is important for improving computer simulations of cloud formation and precipitation location, timing, and intensity. This study also suggests ways to improve how scientists conduct atmospheric aerosol research. This research suggests that drying atmospheric particles to measure their composition can significantly alter some of their semivolatile compounds. If so, this process will affect researchers' ability to accurately quantify how these particles affect clouds.
Summary
This study investigated the effect of trace gas removal on size-resolved cloud condensation nuclei activity. The researchers performed measurements as part of the U.S. Department of Energy (DOE) Office of Science User Facility Atmospheric Radiation Measurement's (ARM) Eastern Pacific Cloud Aerosol Precipitation Experiment (EPCAPE) atop Mt. Soledad in La Jolla, CA. The measurements quantified the hygroscopicity parameter κ in the presence and absence of a denuder which removed volatile organic compounds with fewer than 10 carbon atoms from the gas phase. Comparing the hygroscopicity parameter κ with and without the denuder showed that hygroscopicity can increase or decrease.
This result showed that hygroscopicity can influence cloud condensation nuclei (CCN) activity through either volatilization or surface adsorption or both. The observed range of changes in κ is surprisingly large when compared to other processes that modify κ under atmospheric conditions. Such changes in κ can correspond to substantial variation in calculated cloud droplet number concentrations. However, the global influence on CCN and droplet number concentration will depend on the concentration and spatial distribution of the trace gas(es) responsible for the perturbation in CCN activity. This study did not address this distribution and it remains to be ascertained in future studies.
Funding
This research was supported by the U.S. National Science Foundation, the U.S. Department of Energy Biological and Environmental Research program, and the Air Pollution Program of Environment and Climate Change Canada. It also used the Atmospheric Radiation Measurement (ARM) User Facility, an Office of Science User Facility.