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Mass balance closure and the federal reference method for PM2.5 in Pittsburgh, Pennsylvania
Abstract
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.
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https://hdl.handle.net/10161/6547Collections
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