Trans‐Pacific transport of black carbon and fine aerosols (D < 2.5 μm) into North America

Abstract

This study presents estimates of long‐range transport of black carbon (BC) and aerosol fine mass (diameter less than 2.5 μm) across the Pacific Ocean into North America during April 2004. These transport estimates are based on simulations by the Chemical Weather Forecast System (CFORS) model and evaluated across 130°W, (30°N–60°N) from 26 March through 25 April 2004. CFORS calculates BC transport into North America at 25–32 Gg of which over 75% originates from Asia. Modeled fine aerosol mass transport is between 900 and 1100 Gg. The BC transport amounts to about 77% of the published estimates of North American BC emissions. Approximately 78% of the BC and 82% of the fine aerosol mass transport occur in the midtroposphere above 2 km. Given the relatively large magnitude of the estimated BC transport, we undertake a detailed validation of the model simulations of fine aerosol mass and BC over the west coast of North America. In situ aircraft data were available for the month of April 2004 to assess the accuracy of model simulations of aerosols in the lower troposphere. Aircraft data for aerosol mass collected in the eastern Pacific Ocean during April 2004 as part of the Cloud Indirect Forcing Experiment, as well as surface measurements of fine mass and BC at 30 west coast locations, are compared to CFORS predictions. These surface sites are part of the Interagency Monitoring of Protected Visual Environments (IMPROVE) network. Both the aircraft and the IMPROVE data sets reveal similar patterns of good agreement near and above the boundary layer accompanied by large overprediction within the boundary layer. The observational data validate the CFORS simulations of BC and fine aerosol mass above the boundary layer. The near‐surface overprediction does not impair the major conclusions of this study regarding long‐range aerosol and BC transport, as most of the long‐range transport occurs above 2 km. From this we conclude that the transport of BC from Asia and other regions west is a major source of BC at high elevations over North America. The simulated concentrations of BC between 1 and 3 km, as well as the measured BC concentrations over the elevated IMPROVE sites, range from 0.1 to 0.3 μg/m3. Direct radiative forcing over North America due to the modeled BC concentration between 1 and 15 km is estimated at an additional 2.04–2.55 W/m2 absorbed in the atmosphere and a dimming of −1.45 to −1.47 W/m2 at the surface. The impact of transported BC on the regional radiation budget through direct and indirect effects of the transported BC and other aerosols warrants further study.