Modeling Atmospheric Age Distribution of Elemental Carbon Using a Regional Age-Resolved Particle Representation Framework.

The aging process of soot particles has significant implications when estimating their impacts on air quality and climate. In this study, the source-oriented University of California at Davis/California Institute of Technology model with externally mixed aerosol representation is expanded to track the age distribution of elemental carbon (EC) in Southeast Texas. EC with the age of 0-3 h (i.e., emitted less than 3 h ago) accounted for ∼70-90% of the total in urban Houston and 20-40% in rural areas of southeast Texas in August 2000. Significant diurnal variations in the mean age of EC are predicted, with higher contributions from fresh particles during the morning and early evening due to increased traffic emission and reduced atmospheric mixing. Spatially, the mean age of EC decreases with proximity to major sources. Ground-level EC with the age >6 h is less than 20% of the first age group over land, and background EC accounts for the majority over the Gulf of Mexico. Differences in EC spatial distribution indicate that age distribution could have regional impact on aerosol optical and hygroscopic properties, and thus potentially affect cloud formation and radiation balance. Appropriately accounting for the differential properties due to age distribution is needed to better evaluate aerosol direct and indirect effects.