Analysis of number size distributions of tropical free tropospheric aerosol particles observed at Pico Espejo (4765 m a.s.l.), Venezuela

Abstract

Abstract. The first long-term measurements of aerosol number and size distributions in South-American tropical free troposphere (FT) were performed from March 2007 until March 2009. The measurements took place at the high altitude Atmospheric Research Station Alexander von Humboldt. The station is located on top of the Sierra Nevada mountain ridge at 4765 m a.s.l. nearby the city of Mérida, Venezuela. Aerosol size distribution and number concentration data was obtained with a custom-built Differential Mobility Particle Sizer (DMPS) system and a Condensational Particle Counter (CPC). The analysis of the annual and diurnal variability of the tropical FT aerosol focused mainly on possible links to the atmospheric general circulation in the tropics. Considerable annual and diurnal cycles of the particle number concentration were observed. Highest total particle number concentrations were measured during the dry season (January-March, 519 ± 613 cm −3 ), lowest during the wet season (July-September, 318 ± 194 cm −3 ). The more humid FT (relative humidity (RH) range 50-95 %) contained generally higher aerosol particle number concentrations (573 ± 768 cm −3 during dry season, 320 ± 195 cm −3 during wet season) than the dry FT (RH < 50 %, 454 ± 332 cm −3 during dry season, 275 ± 172 cm −3 during wet season), indicating the importance of convection for aerosol distributions in the tropical FT. The diurnal cycle in the variability of the particle number concentration was mainly driven by local orography.

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“…The early increase in ambient particle number requires additional sources next to the advection of existing particulate matter. The results of Schmeissner et al (2011) indicate the possible presence of new particle formation. New particle formation was also observed during prior studies at high altitude observation sites and aircraft studies within different climatic regions (e.g.…”

“…Schmeissner et al (2011) note that 13 % of the DMPS observations had to be excluded due to erroneous data. The particle volume concentration was calculated from the particle number size distributions assuming spherical particles.…”

“…The data inversion method of the DMPS measurements is described in Schmeissner et al (2011) and Wiedensohler et al (2012). Schmeissner et al (2011) note that 13 % of the DMPS observations had to be excluded due to erroneous data.…”

“…(Calderón et al, 2008;Schmeissner et al, 2011) close to Venezuela's highest mountain, Pico Bolívar (4981 m a.s.l.). The mountain is part of the north-eastern branch of the Andes.…”

“…The DMPS system was operated with one Condensation Particle Counter (CPC) model 3010, TSI Inc. The analysis of the observed ambient aerosol number concentrations at the Pico Espejo was realized by Schmeissner et al (2011) and can be found in the respective publication. A second CPC was used in combination with a Thermodenuder which heated the sampled aerosol to 300 • C. The volatile and semi-volatile aerosol compounds were evaporated before the sampled probe reached the second CPC.…”