Estimating the secondary organic aerosol contribution to PM2.5 using the EC tracer method

The EC tracer method is applied to a series of measurements by different carbonaceous aerosol samplers in the Pittsburgh Air Quality Study (PAQS) in order to estimate the concentration of secondary organic aerosol. High-resolution measurements (2-6 h) and daily averaged concentrations were collected during the summer 2001 intensive (1 July to 4 August 2001) and are used for the analysis. The various samplers used during PAQS show differences in the measured concentrations of OC and EC due to the different sampling artifacts and sampling periods. A systematic approach for the separation of periods where SOA contributes significantly to the ambient OC levels from the periods where organic and elemental carbon concentrations are dominated by primary emissions is proposed. Ozone is used as an indicator of photochemical activity to identify periods of probable secondary organic aerosol production in the area. Gaseous tracers of combustion sources (CO, NO, and NOx) are used to identify periods where most of the OC is primary. Periods dominated by primary emissions are used to establish the relationship between primary OC and EC, a tracer for primary combustion-generated carbon for the different sets of measurements for July 2001. Around 35% of the organic carbon concentration in Western Pennsylvania during July of 2001 is estimated to be secondary in origin.