Chemical and Physical Properties of Marine Aerosol during the RICO-PRACS Experiment: Evidence of a Clean Period, Saharan Dust, and Anthropogenic Pollution

Atmospheric particles alter the earth's energy balance directly by scattering solar radiation in the atmosphere and indirectly by modifying cloud condensation nuclei concentrations (CCN). A significant fraction of CCN in the tropics may be formed from natural and anthropogenic organic aerosols. We are interested in understanding how marine organic aerosols affect tropical clouds and climate. This requires improved knowledge of the chemical and physical properties of aerosols and forms the main goal of our project. Measurements were performed during the months of December 2004 and January 2005 as part of the Rain in Cumulus over the Ocean Experiment (RICO) and the Puerto Rico Aerosol and Cloud Study (PRACS) campaigns. Aerosol samples were collected at two ground-based stations: Dian Point (DP), Antigua and Cape San Juan (CSJ), Puerto Rico. The sampling was conducted with three different samplers: a 13-stage Dekati low-pressure impactor (DLPI), a 10-stages micro-orifice uniform deposit impactor (MOUDI) and stacked-filter units (SFU). For the chemical characterization we used ion chromatography, total organic carbon analysis, aerosol mass spectrometry and a volatility system with an ASASP-X on it. For the physical and optical properties CCN counters, CN counters, a PCASP, an SMPS, a nephelometer, and an aethalometer were used. The total aerosol mass concentrations collected on the substrates of the DLPI, MOUDI and SFU were determined by gravimetric analysis. The mass concentrations of the water-soluble ions Na⁺, NH₄⁺, K⁺, Mg²⁺, Ca²⁺, formate, acetate, Cl^-, NO₃^-, SO₄^2-, and oxalate were determined. Back trajectories calculated using the HYSPLIT (HYbrid Single-Particle Lagrangian Integrated Trajectory) model from NOAA showed two possible sources for the air masses that influence our aerosols samples: air masses coming from North America and coming from the North Atlantic Ocean (trade winds). These results together with the aerosol chemical composition allowed us to identify three case studies: the clean (C) period, the period with Saharan dust (SD) influence, and the period with anthropogenic influence (AI) from North America. Size-resolved mass concentrations of ions at DP during a dust event showed an increase in the concentrations of Ca²⁺, Mg²⁺, and K⁺ (these ions are characteristics of dust and suggest a Saharan dust incursion). Results showed that average nss-sulphate concentrations for DP and CSJ during the polluted period were 142.7 and 160.9 ng m^-3, respectively. Mean mass ratios of Cl^-/Na⁺ for DP and CSJ were 0.915 and 0.978, respectively and far from the ratio in sea water of 1.8, suggesting the presence of acid contributing to sea-spray acidification (chlorine depletion). Additional results evidencing the three proposed periods at both site (DP and CSJ) including the TC, OC and EC concentrations will be presented.