Atmospheric Chemistry and Physics Discussions www.atmos-chem-phys-discuss.net 1680-7367 1680-7375 4 6 2004 10.5194/acpd-4-7795-2004 http://www.atmos-chem-phys-discuss.net/4/7795/2004/ http://www.atmos-chem-phys-discuss.net/4/7795/2004/acpd-4-7795-2004.html http://www.atmos-chem-phys-discuss.net/4/7795/2004/acpd-4-7795-2004.pdf 7795 7818 2004-11-29 Aerosol particles in the mexican east pacific part I: processing and vertical redistribution by clouds J. C. Jiménez D. Baumgardner darrel@servidor.unam.mx G. B. Raga Universidad Nacional Autónoma de México, Mexico City, Mexico Airborne measurements of aerosol particle size distributions were made in the Mexican Intertropical Convergence Zone. The shape of the spectra at cloud base was compared with those at higher altitudes and near cloud boundaries to identify signatures of cloud processed particles. Of 78 cases analyzed, 71% showed enhancement in volume of super-micron particles, 49% had enhanced volume of sub-micron particles, 28% were homogeneous mixtures with boundary layer air, and 24% had super-micron particles removed by precipitation. Almost 100% of the cases with enhanced volume in sub-micron particles also occurred with enhanced super-micron volume. The enhanced volume in super-micron particles is approximately 10 times larger than the sub-micron enhancement. Cloud processed particles in marine air masses had twice as much enhancement of super-micron mass than found in particles processed by clouds formed from continental sources, likely a result of a more efficient coalescence process in clean, maritime clouds. These results are in qualitative agreement with previous observational and theoretical studies that relate enhancements in particle mass to the uptake by cloud droplets of SO₂ and subsequent growth by coalescence.