Abstract

As part of the MIRAGE (MILAGRO) study conducted 7-30 March 2006 in Mexico City and its Metropolitan Area (MCMA), fine particulate matter (PM(2.5)) was collected using two Tapered Element Oscillating Microbalance (TEOM) systems, and a Partisol instrument at the T1 super-site (Tecamac, State of Mex... Show moreAs part of the MIRAGE (MILAGRO) study conducted 7-30 March 2006 in Mexico City and its Metropolitan Area (MCMA), fine particulate matter (PM(2.5)) was collected using two Tapered Element Oscillating Microbalance (TEOM) systems, and a Partisol instrument at the T1 super-site (Tecamac, State of Mexico). Inorganic analysis was performed on filter-based (PM(1), PM(2.5)-URG) measurements also collected at this site. Measurements from the gravimetric (TEOMs, Partisol) and URG systems were inter-compared with chemical speciation measurements using a Particle Into Liquid Sampler (PILS) and Thermal Optical methods. Mass and chemical balances applied over the first part of the study (11-22 March) showed that a TEOM using a device (SES) which reduces particle-bound water and retains a fraction of semi-volatile compounds (SVM) gives readings similar to 30% larger than a conventional TEOM. In the second part of the study (26-30 March), the loss of SVM during TEOM-heated filter collection (both systems) represented a significant fraction of PM(2.5) mass due to changes in particle composition. Overall, when nonvolatile nitrate dominated (i.e., when associated with crustal species and not NH(4)(+)) and/or sulfate dominates (SO(4)(2-)/NO(3)(-) molar ratio is >1), PM(2.5) mass readings are in agreement with those reported for the T1 site if TEOM is using a SES device. However, when volatile nitrate dominates (i.e., NH(4)NO(3)) or SO(4)(2-)/NO(3)(-) molar ratio is <1, a larger fraction is lost from both TEOMs (with or without the SES device). Under the latter regime, uncertainties are large and gravimetric losses may reach 30%-50%. The gravimetric PARTISOL instrument recorded lower readings under all of the aforementioned conditions with differences versus TEOMs decreasing with increasing RH. These findings call for a careful characterization of such volatilization biases to improve current PM (PM(10). PM(2.5)) measurements/networks, especially in alkaline-rich environments that can favor such biases. With regards to PM(1) and PM(2.5) filter-based measurements, findings are: 1) crustal-related elements are important features in the PM(2.5-1) size fraction; 2) a factor of similar to 2 overestimation of SO(4)(2-) concentrations is recorded on substrates during PM collection and 3) main elements of a typical urban aerosol size distribution are concentrated in the 1 mu m (versus 2.5 mu m) size fraction. (C) 2010 Elsevier Ltd. All rights reserved. Show less