September 2006
Concentrations and sources of organic carbon aerosols in the free troposphere over North America

Aircraft measurements of water-soluble organic carbon (WSOC) aerosol over NE North America from the summer 2004 ITCT-2K4 campaign (part of the ICARTT mission) were simulated using the GEOS-Chem model to test our understanding of the sources of organic carbon aerosol in the free troposphere. This work followed our earlier examination of the ACE-Asia aircraft observations which suggested that OC was underestimated by a factor of 10-100 in the free troposphere [Heald et al., 2005]. Concentrations of WSOC observed outside of biomass burning plumes were 4 times lower than OC concentrations reported during ACE-Asia. Simulated concentrations of OC aerosol were higher due to both biomass burning contributions to the background air over North America and enhanced SOA production in summertime conditions. Despite reproducing mean observed WSOC concentrations in the FT to within 25%, the model cannot account for the variance in the observations, suggesting an incomplete understanding of SOA formation.

The figure shows the mean vertical profiles of a) sulfur oxides (SOx = SO2 + sulfate) and b) water soluble organic carbon aerosol (WSOC) as observed (solid black) and simulated by the GEOS-Chem model (thick red) for the ITCT-2K4 data with biomass burning plumes removed. The simulated hydrophobic OC aerosol (not included in WSOC) is also shown in panel b as a thin red line (inverted triangles). Panel c) shows the contributions to total (red) simulated OC aerosol from biomass burning (orange), anthropogenic (blue), and biogenic (green) sources. The standard deviation of the observations in each altitude bin is shown as an error bar with the number of observations in each bin given on the right of the figure.

A full description is given in: Heald et al. [2006].