Browsing by Author "Robinson, Allen L"

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Open Access
Effect of peak inert-mode temperature on elemental carbon measured using thermal-optical analysis
(Aerosol Science and Technology, 2006-10-01) Subramanian, Ramachandran; Khlystov, Andrey Y; Robinson, Allen L
Thermal-optical analysis is a conventional method for classifying carbonaceous aerosols as organic carbon (OC) and elemental carbon (EC). This article examines the effects of three different temperature protocols on the measured EC. For analyses of parallel punches from the same ambient sample, the protocol with the highest peak helium-mode temperature (870°C) gives the smallest amount of EC, while the protocol with the lowest peak helium-mode temperature (550°C) gives the largest amount of EC. These differences are observed when either sample transmission or reflectance is used to define the OC/EC split. An important issue is the effect of the peak helium-mode temperature on the relative rate at which different types of carbon with different optical properties evolve from the filter. Analyses of solvent-extracted samples are used to demonstrate that high temperatures (870°C) lead to premature EC evolution in the helium-mode. For samples collected in Pittsburgh, this causes the measured EC to be biased low because the attenuation coefficient of pyrolyzed carbon is consistently higher than that of EC. While this problem can be avoided by lowering the peak helium-mode temperature, analyses of wood smoke dominated ambient samples and levoglucosan-spiked filters indicate that too low helium-mode peak temperatures (550°C) allow non-light absorbing carbon to slip into the oxidizing mode of the analysis. If this carbon evolves after the OC/EC split, it biases the EC measurements high. Given the complexity of ambient aerosols, there is unlikely to be a single peak helium-mode temperature at which both of these biases can be avoided. Copyright © American Association for Aerosol Research.
Open Access
Mass balance closure and the federal reference method for PM2.5 in Pittsburgh, Pennsylvania
(ATMOSPHERIC ENVIRONMENT, 2004) Rees, Sarah L; Robinson, Allen L; Khlystov, Andrey; Stanier, Charles O; Pandis, Spyros N
Daily ambient aerosol samples were taken in Pittsburgh, Pennsylvania from the summer 2001 to the winter 2002 as part of the Pittsburgh Air Quality Study (PAQS). The study measured PM2.5 mass by the Federal Reference Method (FRM) and the PM2.5 chemical composition by a variety of filter-based and continuous instruments. This paper examines the mass balance between the FRM-measured mass and the sum of the aerosol chemical components. For the 7-month study period, the average FRM-measured mass is 11\% greater than the sum of the mass of the aerosol chemical components. This mass balance discrepancy varies seasonally, with the average FRM-measured mass 17\% greater than the sum of the chemical components for the summer months, with discrepancies as large as 30\% during certain periods. Meanwhile, the FRM-measured mass was at or slightly below the sum of the chemical components for the winter months. The mass balance discrepancy and its seasonal shift cannot be explained by measurement uncertainty; instead the discrepancy is due to combination of retained aerosol water on the conditioned FRM filters and volatilization losses. The relative importance of these different effects varies with aerosol composition and causes the observed seasonal variation in the mass balance. The contribution of the aerosol water to the FRM-measured mass is estimated using continuous measurements of aerosol water at the site; volatilization losses are estimated from other filter-based instruments. Water contributes 16\% of the FRM mass in the summer, and 8\% of the FRM mass in the winter; it also appears responsible for episodes where the FRM-measured mass is significantly greater than the sum of components. Retention of water is greatest during acidic conditions, which commonly occur during the summer months. Volatilization losses are estimated at 5\% of the FRM mass during the summer, and 9\% for the winter. Volatilization losses appear to be most significant on days dominated by organic aerosol, or winter days with relatively high nitrate concentration. Accounting for the effects of water and volatilization losses closes the mass balance between the FRM and the sum of the chemical components, providing insight into the FRM measurements. (C) 2004 Elsevier Ltd. All rights reserved.
Open Access
Mass size distributions and size resolved chemical composition of fine particulate matter at the Pittsburgh supersite
(ATMOSPHERIC ENVIRONMENT, 2004) Cabada, Juan C; Rees, Sarah; Takahama, Satoshi; Khlystov, Andrey; Pandis, Spyros N; Davidson, Cliff I; Robinson, Allen L
Size-resolved aerosol mass and chemical composition were measured during the Pittsburgh Air Quality Study. Daily samples were collected for 12 months from July 2001 to June 2002. Micro-orifice uniform deposit impactors (MOUDIs) were used to collect aerosol samples of fine particulate matter smaller than 10 mum. Measurements of PM0.056, PM0.10, PM0.18, PM0.32, PM0.56, PM1.0, PM1.8 and PM2.5 with the MOUDI are available for the full study period. Seasonal variations in the concentrations are observed for all size cuts. Higher concentrations are observed during the summer and lower during the winter. Comparison between the PM2.5 measurements by the MOUDI and other integrated PM samplers reveals good agreement. Good correlation is observed for PM10 between the MOUDI and an integrated sampler but the MOUDI underestimates PM10 by 20\%. Bouncing of particles from higher stages of the MOUDI ( > PM2.5) is not a major problem because of the low concentrations of coarse particles in the area. The main cause of coarse particle losses appears to be losses to the wall of the MOUDI. Samples were collected on aluminum foils for analysis of carbonaccous material and on Teflon filters for analysis of particle mass and inorganic anions and cations. Daily samples were analyzed during the summer (July 2001) and the winter intensives (January 2002). During the summer around 50\% of the organic material is lost from the aluminum foils as compared to a filter-based sampler. These losses are due to volatilization and bounce-off from the MOUDI stages. High nitrate losses from the MOUDI are also observed during the summer (above 70\%). Good agreement between the gravimetrically determined mass and the sum of the masses of the individual compounds is obtained, if the lost mass from organics and the aerosol water content are included for the summer. For the winter no significant losses of material are detected and there exists reasonable agreement between the gravimetrical mass and the sum of the concentrations of the individual compounds. Ultrafine particles (below 100 nm) account on average, for