Measurement and Modeling Results on the Evolution of Aerosol Size Distributions in the Tropics

Aerosol particles in the upper troposphere-lower stratosphere (UTLS) affect local chemistry and radiation balance due to their role in heterogeneous reactions and contribution to light scattering. Tropical UTLS particles also act as a source of lower stratospheric aerosol populations in the mid-latitudes. Therefore, understanding the processes controlling evolution of the particles in the tropical UTLS is of great importance. We present measurements of aerosol size distributions (4-1000 nm) in the tropics during winter (Pre-AVE, 2004 and CRAVE, 2006) and summer (TC4, 2007), using NMASS (Nuclei Mode Aerosol Size Spectrometer) and FCAS (Focused Cavity Aerosol Spectrometer) instruments aboard the NASA WB-57 aircraft. At altitudes below the tropical tropopause layer (TTL), integrated number and volume distributions indicate a factor of 2-5 variability between 2004 and 2006, reflecting the influence of different air mass origins on the local aerosol population. However, above TTL, the distributions are unified, without a significant change between the two years. Furthermore, above the TTL, number fraction of nucleation mode particles decreases from up to 90% to <40% while total aerosol volume and the volume fraction of particles larger than 350 nm increase. We use an aerosol dynamic model (MAIA, Kazil et al. (2007), Weigel et al. (2011)), constrained by observations to account for the horizontal air mass mixing from mid-latitudes, to simulate aerosol evolution in the tropical UTLS. We will discuss the results of MAIA's sensitivity runs along with the available aerosol composition information to gain insight into the processes controlling the increase in aerosol volume above the TTL. We will also use 2007 observations and MAIA's model results to compare winter-summer aerosol growth processes in the tropical UTLS. Kazil, J., et al., Is aerosol formation in cirrus clouds possible?, Atmos. Chem. Phys., 7, 1407-1413, doi:10.5194/acp-7-1407-2007, 2007. Weigel et al., In situ observations of new particle formation in the tropical upper troposphere: the role of clouds and the nucleation mechanism, Atmos. Chem. Phys., 11, 9983-10010, doi:10.5194/acp-11-9983-2011, 2011