Variations in the physicochemical and optical properties of natural aerosols in Puerto Rico – Implications for climate

Abstract. Atmospheric aerosols stay a major cause of uncertainty in climate prediction. Hundreds of teragrams and the absorbing properties of aerosols such as African dust and volcanic ash affect radiative balance changing atmospheric temperature and thus, climate. Since 2005, we began to check the physicochemical and optical properties of aerosols at the Cape San Juan Atmospheric Observatory, Puerto Rico. Based on the Hybrid Single-Particle Lagrangian Integrated Trajectory backward trajectories and satellite imagery from the Volcanic Ash Advisory Center in Washington D.C., Moderate Resolution Imaging Spectroradiometer, and Saharan air layer images, we grouped natural aerosols in three air masses: marine, African dust and volcanic ash. A sun-sky radiometer from the NASA's AErosol RObotic NETwork assessed total aerosol optical depth and its fine fraction. A 3-wavelength nephelometer and particle soot absorption photometer assessed the scattering and absorption coefficients. Two impactors segregated the submicron (D_p < 1 µm) particles from the total (D_p < 10 µm) enabling us to calculate the sub-micron scattering and absorption fractions. The measured variables served to calculate the single scattering albedo and radiative forcing efficiency. All variables but the single scattering albedo making up the aerosol climatology for Puerto Rico had different means as function of the air mass category at p < 0.05. For the period 2005–2010, the largest means ± 95 % confidence interval of the scattering coefficient (53 ± 4 Mm⁻¹), absorption coefficient (1.8 ± 0.16 Mm⁻¹), and optical depth (0.29 ± 0.03), suggested African dust is the main contributor to the columnar and surface aerosol loading in summer. About two thirds (63 %) of the absorption in African dust was due to the coarse mode and about one third due to the fine mode. In volcanic ash, fine aerosols contributed 60 % of the absorption while coarse contributed 40 %. Overall, the coarse and fine modes accounted for ~ 80 % and 20 % of the total scattering. The African dust load was 3.5 times the load of clean marine, 1.9 times greater than clean with higher sea salt content and 1.7 times greater than volcanic ash. African dust caused 50 % more cooling that volcanic ash at the top of the atmosphere and 50 % more heating than that of volcanic ash within the marine boundary layer.