Diagnostic Air Quality Model Evaluation of Source-Specific Primary and Secondary Fine Particulate Carbon Sergey L. Napelenok, †, * Heather Simon, † Prakash V. Bhave, † Havala O. T. Pye, † George A. Pouliot, † Rebecca J. Sheesley, ‡ and James J. Schauer § † US Environmental Protection Agency, Research Triangle Park, North Carolina 27711, United States ‡ Department of Environmental Science, Baylor University, Waco, Texas 76798, United States § Water Science and Engineering Laboratory, University of WisconsinMadison, Madison, Wisconsin 53706, United States * S Supporting Information ABSTRACT: Ambient measurements of 78 source-specific tracers of primary and secondary carbonaceous fine particulate matter collected at four midwestern United States locations over a full year (March 2004-February 2005) provided an unprecedented opportunity to diagnostically evaluate the results of a numerical air quality model. Previous analyses of these measurements demonstrated excellent mass closure for the variety of contributing sources. In this study, a carbon- apportionment version of the Community Multiscale Air Quality (CMAQ) model was used to track primary organic and elemental carbon emissions from 15 independent sources such as mobile sources and biomass burning in addition to four precursor-specific classes of secondary organic aerosol (SOA) originating from isoprene, terpenes, aromatics, and sesquiter- penes. Conversion of the source-resolved model output into organic tracer concentrations yielded a total of 2416 data pairs for comparison with observations. While emission source contributions to the total model bias varied by season and measurement location, the largest absolute bias of -0.55 μgC/m 3 was attributed to insufficient isoprene SOA in the summertime CMAQ simulation. Biomass combustion was responsible for the second largest summertime model bias (-0.46 μgC/m 3 on average). Several instances of compensating errors were also evident; model underpredictions in some sectors were masked by overpredictions in others. ■ INTRODUCTION Carbonaceous aerosol is a substantial portion of fine particulate matter (PM 2.5 ) in the United States and throughout the world. 1,2 Accurate predictions of particulate carbon concen- trations by air quality models are essential for efficiently designing control strategies and for understanding chemical and physical properties of the troposphere. Epidemiological and clinical studies show substantial associations specifically for particulate elemental carbon (EC) and organic carbon (OC) concentrations with various health end points. 3 Particulate carbon in the atmosphere both absorbs and scatters incoming solar radiation and affects the Earth’s energy balance, making it the second largest climate forcing agent after CO 2 . 4 Furthermore, controlling primary particulate carbon emissions may be an efficient way to delay and reduce the effects of global climate change. 5,6 Given its short lifetime in the atmosphere, due to removal by precipitation, controls on particulate carbon would have an immediate impact on both human health and climate forcing. Particulate carbon comes from a myriad of emissions sources and atmospheric processes. Although correctly simulating particulate carbon concentrations in the ambient atmosphere has been an active area of research, persistent biases remain in regional air quality models. 7,8 These model biases have been difficult to diagnose, due to the fact that routine observations are limited to bulk characterization with distinction provided only between OC and EC, oxygenated and hydrocarbon-like OC, or water-soluble and insoluble OC. Therefore, model evaluation has largely been possible only for these bulk quantities. 9,10 Some more detailed diagnostic air quality modeling studies have been conducted in the past on the basis of nonroutine measurements of source-specific organic tracer compounds. These studies initially focused only on primary sources in a single urban area such as Los Angeles, 11,12 they were later expanded to larger geographic regions for a single season, 13 and more recently, secondary carbon formation was analyzed in an urban area. 14 As a parallel effort, other Received: July 25, 2013 Revised: November 12, 2013 Accepted: November 18, 2013 Published: November 18, 2013 Article pubs.acs.org/est © 2013 American Chemical Society 464 dx.doi.org/10.1021/es403304w | Environ. Sci. Technol. 2014, 48, 464-473