North American and Asian aerosols over the eastern Pacific Ocean and their role in regulating cloud condensation nuclei

Abstract

[1] Measurements of aerosol and cloud properties in the Eastern Pacific Ocean were taken during an airborne experiment on the University of Wyoming's King Air during April 2004 as part of the Cloud Indirect Forcing Experiment (CIFEX). We observed a wide variety of aerosols, including those of long-range transport from Asia, clean marine boundary layer, and North American emissions. These aerosols, classified by their size distribution and history, were found in stratified layers between 500 to 7500 m above sea level and thicknesses from 100 to 3000 m. A comparison of the aerosol size distributions to measurements of cloud condensation nuclei (CCN) provides insight to the CCN activity of the different aerosol types. The overall ratio of measured to predicted CCN concentration (N CCN ) is 0.56 ± 0.41 with a relationship of N CCN,measured = N CCN,predicted 0.846±0.002 for 23 research flights and 1884 comparisons. Such a relationship does not accurately describe a CCN closure; however, it is consistent with our measurements that high CCN concentrations are more influenced by anthropogenic sources, which are less CCN active. While other CCN closures have obtained results closer to the expected 1:1 relationship, the different aerosol types (and presumably differences in aerosol chemistry) are responsible for the discrepancy. The measured N CCN at 0.3% supersaturation (S c ) ranged from 20 cm À3 (pristine) to 350 cm À3 (anthropogenic) with an average of 106 ± 54 cm À3 over the experiment. The inferred supersaturation in the clouds sampled during this experiment is $0.3%. CCN concentrations of cloud-processed aerosol were well predicted using an ammonium sulfate approximation for S c 0.4%. Predicted N CCN for other aerosol types (i.e., Asian and North American aerosols) were high compared to measured values indicating a less CCN active aerosol. This study highlights the importance of chemical effects on CCN measurements and introduces a CCN activation index as a method of classifying the efficiency of an aerosol to serve as CCN relative to an ammonium sulfate particle. This index ranged from close to unity for cloud processed aerosols to as low as 0.31 for aged aerosols transported from Asia. We also compare the performance of two CCN instruments (static thermal diffusion chamber and streamwise continuous flow chamber) on a 45 minute level leg where we observe an aged layer and a nucleation event. More than 50% of the aged aerosol served as CCN at 0.2% S c , primarily owing to their large size, while CCN concentrations during the nucleation event were close to 0 cm À3 . CCN concentrations from both instruments agreed within instrument errors; however, the continuous flow chamber effectively captured the rapid transition in aerosol properties.

Simultaneous observations of aerosol–cloud–albedo interactions with three stacked unmanned aerial vehicles

“…2b; Table 1). Activation indices, the ratio of the observed critical diameter to that of a soluble salt (22), were close to unity at 0.1% S c in both periods, indicating that activated particles contain a large soluble fraction. The CCN spectra approach the soluble limit at lower supersaturations ( Fig.…”

Impact of Asian dust and continental pollutants on cloud chemistry observed in northern Taiwan during the experimental period of ABC/EAREX 2005

“…Over time, these aerosols become more hygroscopic. As cloud condensation nuclei (CCN), this effect may enhance cloud formation potential during transport [Roberts et al, 2006]. The formation of cloud droplets depends, primarily, on the size and chemical composition of the particles and the supersaturation of water vapor.…”

Influence of dust composition on cloud droplet formation

“…Okada and Kai (2004) detected halite that was believed to originate from salt flats in about 10% of dust particles collected at Qira in the Taklamakan Desert. Trace amounts of soluble material in dust particles can drastically reduce the saturation required for activation (Dusek et al, 2006), and a recent study by Roberts et al (2006) indicates that chemical composition may have an important influence on the 8/21/2006 activation of long-range transported Asian dust. Therefore the numerous minor components present in dust at emission and acquired during transport may strongly influence dust's ability to serve as CCN under some conditions.…”