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[1] Organic carbon (OC) and elemental carbon (EC) in fine particles (PM 2.5 ) were measured at the Baltimore Supersite at Ponca Street for 9.5 months in 2002 using a Sunset Laboratory carbon analyzer with 1-hour time resolution. Monthly EC and CO diurnal averages were characterized by pronounced peaks in the early morning commute hours. OC concentration profiles were similar to CO and EC during the morning and evening rush hours, except when high ozone episodes occurred, i.e., generally in summer. Primary and secondary OC contributions were estimated during ozone episodes exceeding 100 ppb (1-hour average). The largest 1-hour ozone concentrations for the entire study period occurred between 11 and 13 August, during which secondary OC contributed, on average, $60% of the hourly OC concentrations and a maximum of 82%. Additionally, the annual EC emission rate is estimated for Maryland using published CO emission inventory data and a mean EC/CO ratio (0.0023 ± 0.0008) derived by regressing selected EC and CO measurements for each of the 9.5 months. The result, 2.31 ± 0.80 Gg EC yr À1 , is similar to the estimate (2.95 Gg EC yr À1 ), determined on the basis of the PM 2.5 emission inventory for Maryland and the mean EC abundance in PM 2.5 measured with a U.S. Environmental Protection Agency speciation monitor in 2002. Finally, the Baltimore Supersite and much of the northeastern United States experienced severe smoke fumigation in early July 2002 owing to uncontrolled forest fires in Quebec, during which time, CO, EC, and OC were predominately associated with the Canadian smoke. During this episode, EC emissions are estimated to be 4.75 Gg EC from Canadian Quebec natural fires in July, and an annual release rate of 9.94 Gg EC yr À1 is estimated for all Canadian boreal forest fires.

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[1] A new multivariate pseudodeterministic receptor model (PDRM), combining mass balance and Gaussian plume dispersion equations, was developed to exploit highly timeresolved ambient measurements of SO 2 and particulate pollutants influencing air quality at a site in Sydney, Florida, during the Tampa Bay Regional Aerosol Chemistry Experiment (BRACE) in May 2002. The PDRM explicitly exploits knowledge of the number and locations of major stationary sources, source and transport wind directions, stack gas emission parameters, and meteorological plume dispersion parameters during sample collections to constrain solutions for individual sources. Model outputs include average emission rates and time-resolved ambient concentrations for each of the measured species and time-resolved meteorological dispersion factors for each of the sources. The model was applied to ambient Federal Reference Method SO 2 and 30-min elemental measurements during an 8.5-hour period when winds swept a 70°sector containing six large stationary sources. Agreement between predicted and observed ambient SO 2 concentrations was extraordinarily good: The correlation coefficient (R 2 ) was 0.97, their ratio was 1.00 ± 0.18, and predicted SO 2 emission rates for each of four large utility sources lie within 8% of their average continuous emission monitor values. Mean fractional bias, normalized mean square error, and the fractions of the predictions within a factor of 2 of the observed values are À2.7, 0.9, and 94%, respectively. For elemental markers of coal-fired (As and Se) and oil-fired (Ni) power plant emissions the average ratio of predicted and observed concentrations was 1.02 ± 0.18 for As, 0.96 ± 0.17 for Se, and 0.99 ± 0.41 for Ni, indicating that the six sources located in the wind sector between approximately 200°and 260°well accounted for background-corrected concentrations measured at the sampling site. Model results were relatively insensitive to the choice of upper bound used to constrain solutions.Citation: Park, S. S., J. P. Pancras, J. Ondov, and N. Poor (2005), A new pseudodeterministic multivariate receptor model for individual source apportionment using highly time-resolved ambient concentration measurements,

Highly time-resolved measurements of PM 2.5 , its major constituents, particle size distributions (9 nm to 20 µm), CO, NO/NO 2 , and O 3 , and meteorological parameters were made from February through November 2002, at the Baltimore Supersite at Ponca St. using commercial and prototype semi-continuous instruments. The average PM 2.5 mass concentration during the study period was 16.9 µg/m 3 and a total of 29 PM 2.5 pollution episodes, each in which 24-h averaged PM 2.5 mass concentrations exceeded 30.0 µg/m 3 for one or more days, were observed. Herein, 6 of the worst episodes are discussed. During these events, PM 2.5 excursions were often largely due to elevations in the concentration of one or two of the major species. In addition, numerous short-term excursions were observed and were generally attributable to local sources. Those in OC, EC, nitrate, CO, and NO x levels were often observed in the morning traffic hours, particularly before breakdown of nocturnal inversions. Moreover, fresh accumulation aerosols from local stationary combustion sources were observed on several occasions, as evidenced by elevations in elemental markers when winds were aligned with sources resulting in PM 2.5 increments of ∼17 µg/m 3 . Overall, the results described herein show that concentrations of PM 2.5 and its major constituents vary enormously on time scales ranging from <1 hr to several days, thus imposing a more highly complex pattern of pollutant exposure than can be captured by 24-hr integrated methods, alone. The data suggest that control of Received 29 April 2005; accepted 27 April 2006. Although the research described in this article has been funded wholly by the United States Environmental Protection Agency through grant/cooperative agreement (#BSS R82806301) to University of Maryland, College Park (UMCP), it has not been subjected to the Agency's required peer and policy review and therefore does not necessarily reflect the views of the Agency and no official endorsement should be inferred. Also authors thank personnel from Maryland Department of the Environment (MDE) for providing criteria gas data and for other instrument support.Address correspondence to John Ondov, Chemistry and Biochemistry, University of Maryland, College Park, MD 20742, USA. E-mail: jondov@wam.umd.edu a limited number of local sources might achieve compliance with daily and annual PM 2.5 standards.

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