A Three-dimensional Characterization of Arctic Aerosols from Airborne Sun Photometer Observations: PAM-ARCMIP, April 2009

The arctic climate is modulated by atmospheric aerosols that affect the surface-atmosphere radiation balance through interactions with solar and terrestrial radiation. During April 2009, the Pan-Arctic Measurements and Arctic Regional Climate Model Inter-comparison Project was conducted by Alfred Wegener Institute of Germany, utilizing their research aircraft, Polar-5. Characterizing aerosols was one objective of the campaign. Sun photometric procedures were adopted to quantify aerosol optical depth AOD along the flight track. Independent measurements of particle size distribution and light extinction are used to corroborate inferences made using the AOD results. During April 2009, the Arctic atmosphere was variably hazy with total column AOD at 500 nm ranging from ~ 0.12 to > 0.35. The haze was concentrated within and just above the surface-based temperature inversion layer, where light extinction was greatest. Black carbon BC was observed at all levels sampled, but at moderately low concentrations compared with historical records. BC was highest near North Pole suggesting there had been an accumulation of soot within the Arctic vortex. Few, distinct elevated aerosol layers were observed, although lidar observations revealed evidence of volcanic aerosol that contributed to abnormally high values of AOD above 4 km. Enhanced opacity at higher altitudes is attributed to an accumulation of industrial pollutants in combination with volcanic aerosol from the March/April 2009 eruptions of Mount Redoubt in Alaska. The presence of Arctic haze during April 2009 is estimated to have reduced the net shortwave irradiance by ~ 2-5 W m-2, resulting in a slight cooling of the surface.