In-Cloud Supersaturation Derived from CCN Measurements

Atmospheric aerosols affect the Earth's energy balance by interacting directly with incoming solar radiation or indirectly by altering cloud properties by acting as cloud condensation nuclei (CCN). The interaction between aerosols and clouds remains the largest uncertainty in model predictions of future climate change in response to anthropogenic activity. The concentration of available CCN depends on aerosol size and chemical composition as well as ambient water vapor supersaturation. Therefore, supersaturation serves as the link between aerosol and cloud droplet number.

The Eastern Pacific Emitted Aerosol Cloud Experiment (E-PEACE) included a targeted aircraft campaign off the coast of Monterey, California, in July and August 2011. The Center for Interdisciplinary Remotely-Piloted Aircraft Studies (CIRPAS) Twin Otter aircraft made measurements of aerosol and cloud properties targeting three types of emissions in the stratocumulus-rich northeastern Pacific: smoke emissions from the R/V Point Sur, salt aerosol released by the Twin Otter, and emissions from cargo ships. In-cloud measurements sampled cloud residuals from behind a counterflow virtual impactor (CVI.) A cloud condensation nuclei (CCN) spectrometer (a Droplet Measurement Technologies Continuous Flow Streamwise Thermal Gradient Chamber operating in Scanning Flow CCN Analysis mode) aboard the Twin Otter provided CCN spectra once every forty seconds and are used to derive ambient supersaturation. We present the effects of aerosol perturbations on cloud droplet properties and in-cloud supersaturation for a variety of environments sampled during E-PEACE.