A Finnish Meteorological Institute-Aerosol Cloud Interaction Tube (FMI-ACIT): Experimental setup and tests of proper operation.

The Finnish Meteorological Institute-Aerosol Cloud Interaction Tube (FMI-ACIT) is a multi-purpose instrument for investigating atmospherically relevant interactions between aerosol particles and water vapor under defined laboratory conditions. This work introduces an experimental setup of FMI-ACIT for investigation of the aerosol activation and the droplet growth under supersaturated conditions. Several simulations and experimental tests were conducted to find out what the proper operational parameters are. To verify the ability of FMI-ACIT to perform as a cloud condensation nuclei (CCN) counter, activation experiments were executed using size selected ammonium sulfate [(NH4)2SO4] particles in the size range of 10-300 nm. Supersaturations from 0.18% to 1.25% were tested by experiments with different temperature gradients. Those showed that FMI-ACIT can effectively measure CCN in this range. Measured droplet size distributions at supersaturations 0.18% and 1.25% are in good agreement with those determined by a droplet growth model.

Worldwide data sets constrain the water vapor uptake coefficient in cloud formation.

Cloud droplet formation depends on the condensation of water vapor on ambient aerosols, the rate of which is strongly affected by the kinetics of water uptake as expressed by the condensation (or mass accommodation) coefficient, αc. Estimates of αc for droplet growth from activation of ambient particles vary considerably and represent a critical source of uncertainty in estimates of global cloud droplet distributions and the aerosol indirect forcing of climate. We present an analysis of 10 globally relevant data sets of cloud condensation nuclei to constrain the value of αc for ambient aerosol. We find that rapid activation kinetics (αc > 0.1) is uniformly prevalent. This finding resolves a long-standing issue in cloud physics, as the uncertainty in water vapor accommodation on droplets is considerably less than previously thought.

CCN spectra, hygroscopicity, and droplet activation kinetics of secondary organic aerosol resulting from the 2010 Deepwater Horizon oil spill.

Secondary organic aerosol (SOA) resulting from the oxidation of organic species emitted by the Deepwater Horizon oil spill were sampled during two survey flights conducted by a National Oceanic and Atmospheric Administration WP-3D aircraft in June 2010. A new technique for fast measurements of cloud condensation nuclei (CCN) supersaturation spectra called Scanning Flow CCN Analysis was deployed for the first time on an airborne platform. Retrieved CCN spectra show that most particles act as CCN above (0.3 ± 0.05)% supersaturation, which increased to (0.4 ± 0.1)% supersaturation for the most organic-rich aerosol sampled. The aerosol hygroscopicity parameter, κ, was inferred from both measurements of CCN activity and from humidified-particle light extinction, and varied from 0.05 to 0.10 within the emissions plumes. However, κ values were lower than expected from chemical composition measurements, indicating a degree of external mixing or size-dependent chemistry, which was reconciled assuming bimodal, size-dependent composition. The CCN droplet effective water uptake coefficient, γ(cond), was inferred from the data using a comprehensive instrument model, and no significant delay in droplet activation kinetics from the presence of organics was observed, despite a large fraction of hydrocarbon-like SOA present in the aerosol.