7th Conf. on Atmospheric Chemistry


Physicochemical transition of aerosols and its effect on cloud condensation nuclei over the Eastern Pacific Ocean: A case study

Greg C Roberts, SIO/Univ. Of California, La Jolla, CA; and O. Lariviere, G. S. Mauger, and V. Ramanathan

The Cloud Indirect Forcing Experiment (CIFEX), which occurred in April 2004, focused on the measurement of aerosol and cloud properties to assess the impact of long-range transport on aerosol-cloud interactions. The ground station at Trinidad Head complimented the airborne data from the University of Wyoming’s King Air aircraft. Airborne measurements included total number concentration, two co-located cloud condensation nuclei (CCN) instruments, aerosol size distribution, scattering efficiency, and black carbon mass concentration. The CCN instruments were of two different designs: the streamwise continuous-flow device operated at 0.2% supersaturation at 1 sample per second, while the static thermal-gradient instrument cycled through 0.2%, 0.4%, and 0.6% supersaturation once a minute. A streamwise CCN instrument also operated at the site on Trinidad Head.

During this experiment, we observed variety of aerosol layers during vertical and horizontal profiles, ranging from aged aerosols from two major Asian dust storms to ultrafine particles from recent nucleation events. Aerosol layers observed during the flights were found in thin stratified layers ranging from altitudes of 1000 meters to 7000 meters and thicknesses from 100 to 1000 meters. Aerosol size distributions indicated cloud processed aerosol in some of the layers, while other layers were clearly due to secondary particle formation with over 90 percent of the number concentration smaller than 30 nm diameter. The results presented here highlight a rapid transition in the physicochemical aerosol properties and their effect on CCN activation during a straight, level flight leg at 3000 meters above sea level. Accumulation mode aerosol with a median diameter near 150 micron characterized the beginning of the leg: total number concentration (NCN) at 630 cm-3 and CCN concentration (NCCN) at 335 cm-3 at 0.2% supersaturation. This aerosol was probably long-range transport from Asia based on back trajectories. The relatively high activation ratio (NCCN/NCN = 0.51) suggests a large soluble component which may be expected for cloud-processed aerosols. The accumulation mode quickly yielded to a bi-modal distribution with a large ultrafine component (NCN = 1100 cm-3; NCCN = 32 cm-3). An increase in the critical diameter (based on integrating the size distribution to obtain NCCN) from 134 nm (accumulation mode) to 190 nm (bi-modal) indicates a change in the chemical composition of the aerosols as well. During the final section of the leg, ultrafine aerosol (diameter < 20 nm) accounted for most of the aerosol population (NCN = 2000 cm-3), and as expected, the aerosol did not activate into CCN (NCCN = 2.4 cm-3) due to the small size.

Poster Session 1, Poster Session - Seventh Conference on Atmospheric Chemistry
Monday, 10 January 2005, 1:30 PM-4:00 PM

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