AAAR 32nd Annual Conference
September 30 - October 4, 2013
Oregon Convention Center
Portland, Oregon, USA
Abstract View
Morphology and Mixing State of Atmospheric Aerosol in Mexico City
SWARUP CHINA, Claudio Mazzoleni, Manvendra Dubey, Rajan K. Chakrabarty, Hans Moosmuller, W. Patrick Arnott, Timothy Onasch, Scott Herndon, Michigan Technological University
Abstract Number: 388 Working Group: Urban Aerosols
Abstract Morphology, mixing state, and size distribution of atmospheric aerosols influence their chemical and mechanical interactions with the environment, their physical properties, and radiative effects. We investigated the morphological and optical characteristics of atmospheric aerosols collected onboard the Aerodyne mobile laboratory during the MILAGRO (Megacity Initiative: Local and Global Research Observations) campaign that took place in Mexico City in March 2006. Aerosol samples were collected on nuclepore filters and individual particle were analyzed by scanning electron microscopy. Samples were collected at six sites with various atmospheric and geographical characteristics including urban, sub-urban, mountaintop, and industrial areas. Aerosol light absorption and scattering coefficients at 781nm were measured with an integrated photoacoustic nephelometer.
We show positive association between the relative abundance of soot particles and the aerosol co-albedo with some anomalies that are investigated using particle size distribution, morphology, and mixing state. Fractal dimension of aged soot is found to be more variable at the mountaintop and suburban areas than of fresh soot at the T0 site, close to the city center. Soot particles were found in different but distinct mixing states. We classified them into four categories based on their morphology and a visual classification: 1) bare (or thinly coated) 2) partly coated, 3) embedded, and 4) soot-inclusions, where soot is mixed with, but not uniformly coated by other material. Temporal variations of optical properties at T0 show that the minimum aerosol co-albedo was observed in the afternoon when the number fraction of soot was the lowest and the coated fraction of soot was the highest. On average,~70% higher number of soot-inclusion particles were found at the sub-urban area than at T0, suggesting that complex mixing and processing of soot particles can occur over a relatively short time.