The T1-T2 study: evolution of aerosol properties downwind of Mexico City

Abstract

Abstract. As part of a major atmospheric chemistry and aerosol field program carried out in March of 2006, a study was conducted in the area to the north and northeast of Mexico City to investigate the evolution of aerosols and their associated optical properties in the first few hours after their emission. The focus of the T1-T2 aerosol study was to investigate changes in the specific absorption αABS (absorption per unit mass, with unit of m2 g−1) of black carbon as it aged and became coated with compounds such as sulfate and organic carbon, evolving from an external to an internal mixture. Such evolution has been reported in previous studies. The T1 site was located just to the north of the Mexico City metropolitan area; the T2 site was situated approximately 35 km farther to the northeast. Nephelometers, particle soot absorption photometers, photoacoustic absorption photometers, and organic and elemental carbon analyzers were used to measure the optical properties of the aerosols and the carbon concentrations at each of the sites. Radar wind profilers and radiosonde systems helped to characterize the meteorology and to identify periods when transport from Mexico City over T1 and T2 occurred. Organic and elemental carbon concentrations at T1 showed diurnal cycles reflecting the nocturnal and early morning buildup from nearby sources, while concentrations at T2 appeared to be more affected by transport from Mexico City. Specific absorption during transport periods was lower than during other times, consistent with the likelihood of fresher emissions being found when the winds blew from Mexico City over T1 and T2. The specific absorption at T2 was larger than at T1, which is also consistent with the expectation of more aged particles with encapsulated black carbon being found at the more distant location. In situ measurements of single scattering albedo with an aircraft and a ground station showed general agreement with column-averaged values derived from rotating shadowband radiometer data, although some differences were found that may be related to boundary-layer evolution.

Black carbon over Mexico: the effect of atmospheric transport on mixing state, mass absorption cross-section, and BC/CO ratios

“…This is higher than the median values of ∼9 m 2 /g @ 550 nm reported by Doran (2007) at T1 (using a thermal-optical OC/EC analyzer and a photoacoustic absorption spectrometer, PAS) and 2.6 times the MAC values calculated by Baumgardner et al (2007) near a bus terminal in Mexico City, using BC measurements with an SP2 and Mie theory. The Baumgardner et al data may be biased low as the optical particle diameter used to estimate MAC of coated BC in their Mie calculations did not account for volatilization of the non-refractory coating described by Gao et al (2007); there could be other reasons as well.…”

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

“…During the MILAGRO study, meteorological measurements and modeling data indicated several days when aerosols originating in Mexico City passed through T1 and T2 (Doran et al, 2007). The PIXE and SXRF data confirmed that particles from a regional plume were sampled and aided in selecting a specific transport day for detailed particle analyses.…”

“…Although tempting to attribute most of the organic aerosol burden to automotive sources, recent findings suggest that widespread biomass burning contributes significantly to organic mass (Marley et al, 2009b;Yokelson et al, 2007;Moffet et al, 2008a). In the most recent field study, the Megacities Initiative: Local and Global Research Observations (MILAGRO) 2006, intensive measurements characterized the increase in organic mass with plume age (Doran et al, 2007(Doran et al, , 2008Kleinman et al, 2008). However crucial, quantifying the bulk organic mass is but a single piece of a complex puzzle.…”

“…After an initial analysis of the field data (Doran et al, 2007;de Foy et al, 2008), a specific time period was chosen for a detailed analyses -22 March 2006. During that day, the northeastern airflow was from T0 to T2.…”

Emission and chemistry of organic carbon in the gas and aerosol phase at a sub-urban site near Mexico City in March 2006 during the MILAGRO study

“…The T1 site was located ∼30 km to the northeast of the center of Mexico City on the campus of the Technical University of Tecámac and was chosen as part of a T0-T1-T2 series of sites to capture outflow from Mexico City to the northeast after different transport times (Doran et al, 2007;Fast et al, 2007). The Mexico City urban area extends to T1: local emissions were not insignificant and likely dominated during the night and early morning.…”