Microscopic characterization of carbonaceous aerosol particle aging in the outflow from Mexico City

Abstract

Abstract. This study was part of the Megacities Initiative: Local and Global Research Observations (MILAGRO) field campaign conducted in Mexico City metropolitan area during spring 2006. The physical and chemical transformations of particles aged in the outflow from Mexico City were investigated for the transport event of 22 March 2006. A detailed chemical analysis of individual particles was performed using a combination of complementary microscopy and micro-spectroscopy techniques. The applied techniques included scanning transmission X-ray microscopy (STXM) coupled with near edge X-ray absorption fine structure spectroscopy (NEXAFS) and computer controlled scanning electron microscopy with an energy dispersive X-ray analyzer (CCSEM/EDX). As the aerosol plume evolves from the city center, the organic mass per particle increases and the fraction of carbon-carbon double bonds (associated with elemental carbon) decreases. Organic functional groups enhanced with particle age include: carboxylic acids, alkyl groups, and oxygen bonded alkyl groups. At the city center (T0) the most prevalent aerosol type contained inorganic species (composed of sulfur, nitrogen, oxygen, and potassium) coated with organic material. At the T1 and T2 sites, located northeast of T0 (∼29 km and ∼65 km, respectively), the fraction of homogenously mixed organic particles increased in both Correspondence to: M. K. Gilles (mkgilles@lbl.gov) size and number. These observations illustrate the evolution of the physical mixing state and organic bonding in individual particles in a photochemically active environment.

An overview of the MILAGRO 2006 Campaign: Mexico City emissions and their transport and transformation

“…Wang et al (2010) performed a closure study between the measured CCN and those calculated using Kohler theory with the measured aerosol size distributions and composition. Their results suggest that the mixing of nonhygroscopic POA and BC with photochemically produced hygroscopic species takes place in a few hours during daytime, which is consistent with the results of Moffet et al (2009Moffet et al ( , 2010. This rapid process suggests that during daytime, a few tens of kilometers away for POA and BC sources, N CCN may be derived with sufficient accuracy by assuming an internal mixture, and using bulk chemical composition.…”

“…Tables 1-4 summarize the PM measurements and instruments deployed during the campaign. This study was the first time that organic aerosol (OA) functional group concentrations by FTIR Gilardoni et al, 2009;Baumgardner et al, 2009), particlephase organic molecular markers (Stone et al, , 2010a, Scanning Transmission X-Ray Microscopy (STXM) spectra of collected particles (Moffet et al, 2010), and 14 C content of carbon fractions (organic carbon, elemental carbon, water-soluble OC (WSOC), and water-insoluble OC (WIOC)) were measured in the MCMA. In addition to the new measurements applied to collected particles, several real-time measurements of PM were deployed to the MCMA for the first time.…”

Airborne soil organic particles generated by precipitation

“…Carbon, oxygen and nitrogen K-edge NEXAFS spectra were acquired in this study. Further details on the applications of this technique to atmospheric particles and additional information on the STXM instrument are published elsewhere [36][37][38] .…”

Automated Chemical Analysis of Internally Mixed Aerosol Particles Using X-ray Spectromicroscopy at the Carbon K-Edge

“…20 The number of particles examined was limited both by the experimental time available for data acquisition as well as development of appropriate analytical methods. Over the past decade, STXM has grown into an active area of research examining topics such as the graphitic nature of combustion produced aerosols, 21-23 organic particle speciation, 20 the evolution of aerosol mixing states, 19 and chemical speciation of metal containing aerosols. 24,25 Relating the detailed analysis of a limited number of particles to the larger, more encompassing statistically representative picture has been one of the challenges of this method.…”

“…Knowledge of spatially resolved chemical composition also aids in deciphering particle aging mechanisms and sources. 18,19 Therefore, developing methodologies that accurately map the internal heterogeneity of individual particles with appropriate statistical depth is important.…”