Source Identification and Airborne Chemical Characterisation of Aerosol Pollution from Long-range Transport over Greenland during POLARCAT Summer campaign 2008
Résumé
We present results from the deployment of a unit mass resolution Aerodyne aerosol mass spectrometer (AMS) during the IPY POLARCAT-France (Polar Study using Aircraft, Remote Sensing, Surface Measurements and Models, of Climate, Chemistry, Aerosols, and Transport) campaign. Free tropospheric measurements were taken from aboard the French research aircraft ATR-42 which operated out of Kangerlussuaq, Greenland, between 30 June and 14 July 2008. The primary purpose of the AMS deployment in this field campaign was to chemically characterise high latitude tropospheric background as well as long-range transport aerosol from biomass burning (BB) and anthropogenic activities, i.e. fossil fuel combustion (FF), originating from North America, Eurasia and East Asia.
Online size resolved chemical composition data of non-refractory submicron aerosol were collected up to 7.6 km altitude in the region 60 to 71°N and 40 to 60°W over the North Atlantic and the Greenland ice sheet. Altogether, 48 BB and FF plumes were sampled. Transport pathways of detected events included advection below 700 hPa, air mass uplifting in warm conveyor belts, and high altitude transport in the upper troposphere. By means of the Lagrangian particle dispersion model FLEXPART, trace gas analysis of O3 and CO, particle size distributions and aerosol chemical composition the pollution events were identified and classified into five chemically distinct categories. Aerosol from North American BB with almost no anthropogenic emission influence consisted of 22 % particulate sulphate, while with increasing FF and Asian contributions the fraction of particulate sulphate increased. Aerosol from Asian FF dominated plumes was composed of up to 37 % sulphate on average. Overall, it was found that the organic matter fraction was larger (85 %) in pollution plumes than for background conditions (71 %). Despite different source regions and emission types the particle oxygen to carbon ratio of all plume classes was around 1 indicating low-volatility highly oxygenated, i.e. aged, aerosol.
In general, during this field experiment, the long-range transported aerosol plumes experienced strong wash-out. While the median CO mixing ratios increased in the free troposphere between 3 and 5.5 km from 100 to roughly 110 nmol mol-1 indicating advection of polluted air masses, the submicron particle mass median concentration remained below 0.5 μg m-3 which was lower than the concentration above 5.5 km. However, using precipitation data from HYSPLIT back trajectories for all 48 plumes it was found that aerosol mass remained constant near 0.4 μg m-3 for accumulated rainfall > 30 mm h-1 during transport meaning that even heavy precipitation events did not remove all of the particulate matter.
Also, an aerosol lifetime, including all processes from emission to detection, was derived for North American plumes which was in the range between 7 and 11 days.
These measurements show that the free and upper troposphere above Greenland was strongly perturbed in June/July 2008 due to the long-range transport of BB emission from North America and Siberia and anthropogenic emissions from North America and East Asia. Strong signatures of particulate sulphate were observed in association with East Asian air masses.