MODELING SECONDARY ORGANIC AEROSOL FORMATION FROM SEMI-VOLATILE ORGANIC COMPOUNDS EMITTED FROM MOBILE SOURCES AND OTHER COMBUSTION SOURCES

Project No. A-72

Leaders:   J. C. Ball
                  A. J. Krol

Scope and Objective

The objective of this study is to investigate whether the inclusion of SOA formation from SVOC emitted from combustion sources, particularly including mobile sources, can reconcile observations and model simulations in the Atlanta region.

Current Status and Future Program

AER recently completed an analysis of carbonaceous PM2.5 data at two monitoring sites in Georgia (CRC Project A-60) and the major conclusions were: (1) Both emissions and secondary organic aerosol (SOA) formation were important contributors to high observed organic aerosol (OA) events, while modeled episodes of high OA in Atlanta were driven mostly by primary emissions, (2) Rural SOA concentrations were fairly well simulated by the model, but urban SOA concentrations were not, (3) The observed diurnal SOA concentration profile was poorly reproduced by the model, suggesting that the SOA vaporization enthalpies are overestimated in the model, and (4) Day-of-the-week variability of SOA concentrations was not well reproduced by the model, probably because of incorrect emission profiles. 

A possible hypothesis to explain several of these conclusions is that semi-volatile organic compounds (SVOC) emitted by mobile sources and other combustion sources are not treated in models.  Once emitted, SVOC partition between the gas and particulate phases depends on the ambient temperature.  The oxidation of SVOC can lead to rapid formation of SOA when photochemistry is favorable, especially in the vicinity of urban sources.  Such a pathway has been proposed in the recent literature to explain SOA concentrations in the Pittsburgh area. 

The following tasks are proposed for this new project:  1) Develop SVOC emission profiles for mobile source emissions; 2) Develop pathways for SOA formation from SVOC; 3) Incorporate the SVOC/SOA module into a 3-D air quality model; and 4) Make recommendations. 

The project deliverables for the study will include quarterly progress reports, computer code, and a manuscript suitable for publication.  The statement of work for the project has been prepared in draft and approved by the committee with plans for a project start later in 2008.

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