Shipboard measurements of aerosol chemical, physical, optical and cloud nucleating properties during the DYNAMO field campaign

We propose to participate in the DYNAMO field campaign during the IOP aboard the US funded ship. Our objective is to improve the understanding of the effects of aerosol particles on clouds and radiation transfer over the equatorial Indian Ocean. Aerosols in this region have both natural (ocean-derived) and anthropogenic (continental) sources as the Inter-Tropical Convergence Zone (ITCZ) can extend well south of the equator during the dry winter monsoon season (Bates et al., 2002_ allowing for transport of air masses from the Indian subcontinent to the DYNAMO region. The Indo-Asian haze that spreads over the region north of the ITCZ has a major impact on the regional radiative forcing (Ramanathan et al., 2002). During INDOEX the low-level heating induced by the aerosol haze led to enhanced moist convection and strengthened the rainfall along the ITCZ (Ramanathan et al., 2001). Aerosol data are thus essential to understanding the radiation budget and evolution of the cloud population (DYNAMO hypothesis II) in this region.

The proposed aerosol physical measurements will include number-size distributions from 20nm to 10 µm diameter using mobility separation condensation particle counting and aerodynamic particle sizing. Chemical analysis will include 2-stage impactors (above and below 1 µm) that will be analyzed for major ions via ion chromatography (IC), trace metals via XRF, and organic and elemental carbon via optical/thermal techniques. Non-refractory chemical composition will be measured with an aerosol mass spectrometer (AMS). Cloud condensation nuclei (CCN) potential at supersaturations in the range 0.2% to 2% will be measured with a continuous flow, thermal diffusion CCN counter. Aerosol optical parameters, light scattering and absorption coefficient, will be determined by nephelometry and filter-based absorption, respectively, at three visible wavelengths for sub-10µm and sub 1-µm size ranges at 60% relative humidity. The size distribution, AMS, optical parameters and CCN will have a time resolution of 15 minutes or less. The impactor-based mass and chemistry will have 12 to 24 hour resolution.

The time series of these parameters will be analyzed in the context of the larger set of meteorological, oceanographic, and satellite data to investigate the processes and cause-effect relation shops between aerosols, radiative transfer, cloud physics and precipitation. Multivariate statistical analysis will be used to determine relationship between aerosol parameters (e.g., concentration, size distribution, composition) and cloud physics parameters, (e.g., thermodynamic profiles; cloud albedo and effective radius; vertical mixing; cloud base, top and precipitation rate). The products, time series of aerosol parameters and derived empirical relationships will be archived on the PMEL Atmospheric Chemistry data server (http://saga.pmel.noaa.gov/data/) as well as the NCAR DYNAMO data server.