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 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 α ABS (absorption per unit mass, with unit of m 2 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 spectrometers, 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 speCorrespondence to: J. C. Doran (christopher.doran@pnl.gov) cific 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.

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

“…Enhanced mixing and removal processes likely contributed to the observed decrease in background concentrations of organic carbon at T1 (Doran et al, 2007) after the strongest cold front passed over the region on 23 March. Marley et al (2009a) found that aerosol scattering decreased when rain was observed between 23 and 27 March since scattering aerosols are partially inorganic and hydrophilic and expected to be scavenged more readily.…”

“…As described by Doran et al (2007), radar wind profilers at each site obtained wind speed and direction profiles up to 4 km above ground level (a.g.l.). A radar wind profiler was also deployed at Veracruz, located on the Gulf of Mexico east of Mexico City to obtain wind information aloft farther downwind of Mexico City.…”

“…Meteorological stations were deployed at several sites in the vicinity of Mexico City in addition to T0, T1, and T2, including CENICA and Paso de Cortes (Altzomoni) , Tenango del Aire, Santa Ana, Pico de Tres Padres , which measured near-surface wind speed, wind direction, temperature, pressure, humidity, and precipitation. Radiation and cloudiness was measured at T1 and T2 by multi-band rotating shadowband radiometers (MFRSR) and Eppley broadband radiometers (Doran et al, 2007;Fast et al, 2007). Broadband radiometers were also deployed at Paso de Cortes.…”

“…For example, Doran et al (2007) employed trajectories based on radar wind profiler measurements to infer periods of transport between Mexico City, T1, and T2. Nocturnal and early morning buildup from local sources was inferred from organic carbon (OC) and elemental carbon (EC) at T1, while transport from the MCMA was seen at T2.…”

Using spectral methods to obtain particle size information from optical data: applications to measurements from CARES 2010

“…These properties are not static in time; instead, evolving as particles, they are transported through the atmosphere as a result D. B. Atkinson et al: Using spectral methods to obtain particle size information of chemical processing, scavenging and changes in the environmental conditions (e.g., relative humidity and temperature) (Doran et al, 2007;George and Abbatt, 2010;Lack and Cappa, 2010).…”