Mass size distributions and size resolved chemical composition of fine particulate matter at the Pittsburgh supersite

Abstract

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 offine particulate matter smaller than 10 mm. Measurements ofPM 0.056 ,P M 0.10, PM0.18 ,P M 0.32 ,P M 0.56 ,P M 1.0 ,P M 1.8 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 ofparticles f rom higher stages ofthe MOUDI (>PM 2.5) is not a major problem because ofthe low concentrations ofcoarse particles in the area. The main cause ofcoarse particle losses appears to be losses to the wall ofthe MOUDI. Samples were collected on aluminum foils for analysis of carbonaceous 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% ofthe organic material is lost f 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 ofthe masses ofthe individual compounds is obtained, ifthe lost mass from organics and the aerosol water content are included for the summer. For the winter no significant losses ofmaterial are detected and there exists reasonable agreement between the gravimetrical mass and the sum ofthe concentrations ofthe individual compounds. Ultrafine particles (below 100 nm) account on average, for o 5% ofthe PM 2.5 mass, and show different composition for the summer and the winter. During the summer the ultrafine mass is 50% carbonaceous material (organic material and elemental carbon) and 50% inorganic (mainly sulfate and ammonium); during the winter these percentages are 70% and 30%, respectively. r 2004 Elsevier Ltd. All rights reserved.