Identification of chemistry-dependent artifacts on gravimetric PM fine readings at the T1 site during the MILAGRO field campaign

Abstract

As part of the MIRAGE (MILAGRO) study conducted 7–30 March 2006 in Mexico City and its Metropolitan Area (MCMA), fine particulate matter (PM2.5) 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 (PM1, PM2.5-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 ∼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 PM2.5 mass due to changes in particle composition. Overall, when nonvolatile nitrate dominated (i.e., when associated with crustal species and not NH4+) and/or sulfate dominates (SO42−/NO3− molar ratio is >1), PM2.5 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., NH4NO3) or SO42−/NO3− 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 (PM10, PM2.5) measurements/networks, especially in alkaline-rich environments that can favor such biases. With regards to PM1 and PM2.5 filter-based measurements, findings are: 1) crustal-related elements are important features in the PM2.5–1 size fraction; 2) a factor of ∼2 overestimation of SO42− concentrations is recorded on substrates during PM collection and 3) main elements of a typical urban aerosol size distribution are concentrated in the 1 μm (versus 2.5 μm) size fraction.