Influence of air mass history on black carbon concentrations and regional climate forcing in southeastern United States

Atmospheric black carbon (BC) mass concentrations in the southeastern United States have been measured at a regionally representative site near Mount Mitchell, North Carolina (35°44′05°N, 82°17′15″W, 2038 m elevation), the highest peak in the eastern United States, during a 9 month period from June to October 1996 and March to June 1997. BC concentrations are measured by an aethalometer, which operates by measuring the attenuation of light through a sample. All measured BC concentrations are reported in terms of air mass histories determined from back trajectory analysis using the Hybrid Single-Particle Lagrangian Integrated Trajectory (HY-SPLIT) model. Air masses influencing the site have been classified as polluted, marine, and continental according to SO_x, and NO_x. emission inventories. The average BC mass concentrations for each sector are 216.6 ± 47.8 ng m^-3 for polluted air masses, 65.6 ± 23.5 ng m^-3 for marine air masses, and 169.9 ± 50.6 ng m^-3 for continental air masses. A positive relationship between cloud condensation nuclei and BC concentrations suggest at times the BC measured at the site may be internally mixed. The average BC concentration found, in cloud water is 74.2 μg kg^-1. Derived BC to sulfate mass ratios ranged from 0.01 to 0.06.