Wednesday, June 29, 2011

Aerosol and Cloud condensation nuclei (CCN) measurement (Texas A&M)

by Peter Deng [Texas A&M University]

I am so happy I finally made it to the cruise after two months preparation with a strong support from my advisor Sarah Brooks and my research group members in atmospheric sciences department. After two days of struggling in the ship, I got my sea legs, which is much faster than I have expected during the period of suffering. I can now focus on my particle measurement and carry on data treatment instead of fighting with sleepiness.

Aerosol denotes to the solid particles or liquid droplets suspended in the atmosphere. It is also named particulate matter (PM) by US EPA (Environmental protection agency) as an air quality criterion. Aerosol has its importance of research in two aspects: air pollution sources (health effect) and a factor influencing global climate. Since we are purely measuring marine aerosols in the cruise, we will pay attention to the climate factor rather than the health effect, which is a key focus for continental aerosols. Due to the large coverage of earth surface by the ocean, marine aerosol is believed to be a very important factor of influencing climate. The scenario of the influence, which is complicated and to state in a simple way, is to interfere the light transfer from the solar inward radiation (short wave) and the earth outward radiation (long wave). The influence could be made by either the aerosol particle itself or the cloud droplet to which it grows in a water vapor-supersaturated environment.

Our goal in the cruise is to measure both the aerosol concentration and the CCN concentration under a specified supersaturation (SS) level. In a supersaturated environment, water vapor is able to condense onto the surface of a particle. Aerosol particles which are able to grow to a certain size (approx. 1 micron) are considered as CCN. We vary the SS levels by a preprogrammed table to control the instrument (CCN counter) to achieve the wanted level. The SS levels are cycling with values defined in the table continuously, so the CCN number concentration based on different SS levels is available sequentially. Meanwhile we use a different instrument called condensation particle counter (CPC) to measure total particle concentration within the size range of 3-1000 nm. The ratio of CCN concentration and total particle concentration is then the efficiency of cloud activation for a specified SS level. We will be able to get this information along the track of the cruise. We then want to correlate the efficiency with the phytoplankton levels to investigate the mechanism of marine aerosol activation as cloud nuclei.

I also collect field samples on filters for chemical analyses after the cruise using Raman microspectroscopy (Raman) in our lab at Texas A&M. Raman is capable of single particle measurement in terms of its chemical composition. We would like to know what kinds of source are there for secondary aerosol formation except those from DMS. Ocean surface borne surfactants has been our hypothesis and we will use the filter sample to verify that.

Everyone in the cruise is super friendly and the food is wonderful. I look forward to learn more from the science party and enjoy three weeks’ great life on the ocean.