CLOUDCOND

Principal Investigators: J. Ogren, C. Berkowitz, R. Halthore, A. Laskin, A. Strawa, J. Wang, A. Wexler

As part of the ARM Mobile Facility (AMF) deployment to Point Reyes, CA in the spring and summer of 2005, a suite of instrumentation was installed to measure the chemical, physical and optical properties of aerosol particles at the site. This deployment took part in conjunction with the MArine Stratus Radiation Aerosol and Drizzle (MASRAD) study. Point Reyes has been called one of the foggiest places on earth, and while that may be hyperbole, the foggy nature of the locale provided an excellent opportunity to study the inter-relationship between aerosol particle and cloud droplet properties using a surface-based observing platform. In addition to the cloud/aerosol research, several other complementary aerosol projects took place at the site including (a) an intercomparison of the Cadenza cavity ring down technique (funded by NASA) and PSAP for measuring light absorption (b) coordinated ultrafine and chemistry aerosol particle measurements and (c) measurement of aerosol optical, physical and chemical properties during cloud free conditions at a marine site. The measurements and specific scientific questions addressed are described below:

Several of the participants were funded by DOE’s Aerosol Science Program (ASP). A major focus of the ASP research is the interaction between clouds and aerosol particles. To study this, a counter-flow virtual impactor (CVI) was used to selectively sample cloud drops. The CVI takes advantage of the higher inertia of the cloud drops to draw them through a slight counter-flow into the system while smaller particles are unable to overcome the counter-flow. Downstream of the CVI the cloud drops were evaporated and the resulting cloud condensation nuclei (CCN) was fed to aerosol instrumentation to characterize their optical, chemical, physical (size, number and shape) and cloud activation properties. In parallel with the CVI system, a second aerosol inlet with an upper cut-off size of 5 micrometers sampled the interstitial aerosol – i.e., the particles which have not activated to cloud droplets. Instrumentation downstream of this interstitial inlet was similar to that downstream of the CVI and also included an SMPS for sizing the particles and a humidograph for assessing hygroscopic growth of the aerosol. By running these two inlet systems in tandem the following scientific questions were addressed: • What are the differences in chemistry between interstitial and activated aerosol? • What role do organics play in aerosol formation and activation? • How do clouds change the optical properties of the aerosol? • How do the CCN spectra differ between interstitial and activated aerosol? • Can CCN closure be obtained?

While the research described above focused on aerosol-cloud relationships, many of the deployed instruments sampled continuously, thus providing characterization of cloud-free summertime aerosol at Point Reyes. This data set was used to address the following scientific questions: • How do light absorption measurement methods compare under a variety of sampling conditions (e.g., what is the influence of particle size on instrument performance?) • What are the chemical, optical, and physical properties of the aerosol at a marine site during fog- free conditions? • What can be learned about the chemical species and processes controlling homogeneous aerosol nucleation processes in the coastal marine boundary layer?

A number of different types of models, covering processes ranging from radiative transfer to marine aerosol microphysics to cloud droplet formation were used to interpret the measurements. Results from this study are relevant to improving the parameterization of aerosol forcing (direct and indirect effect) in climate models.

Timeline

Campaign Data Sets

PYE Data Sources