Observational Constraints on Ambient Brown Carbon with IMPROVE Network Observations

The light-absorbing component of organic aerosols, often referred to as brown carbon (BrC), represents a major uncertainty in current estimates of aerosol radiative forcing. This uncertainty is largely due to the lack of extensive evaluation of BrC with field observations. Recently, the Interagency Monitoring of Protected Visual Environments (IMPROVE) network starts to provide measurements of aerosol absorption at multiple wavelengths, allowing quantification of brown carbon in a regional scale. We use IMPROVE network observations, combined with a global chemical transport model (GEOS-Chem), to examine the magnitude, seasonality and spatial distribution of brown carbon in the United States. Our model shows reasonable agreement with observations on the magnitude and spatial distribution of organic carbon mass and BrC absorption aerosol optical depth (AAOD). In addition, both the model and observations show that BrC accounts for a large fraction of the total AAOD. We also find that the model run without photobleaching tends to overestimate BrC AAOD, suggesting an important role of photobleaching on the lifetime of brown carbon.