The Arctic Thread for Lerwick

    The Arctic thread for the Lerwick aerosol is shown in red italics in the table below. The figures begin with step 3, the first new part of the thread.

Lerwick, Shetland Islands, 15 elements
  Shetland Islands Thread UK/Europe Thread Arctic Thread
Step 0 Nothing chosen
Step 2 Sea and crust (FEC) chosen
Step 3 Collafirth added, fit 0.22 Chilton added, fit 0.77 Bear Island added, fit 0.75
Step 4   Styrrup added, fit 0.54 Ny-lesund added, fit 0.67
Step 5     Leiston added, BI/Ny disappear, fit 0.27

    Arctic step 3 adds the highest of the potential Arctic sources, Bear Island, and provides a respectable fit of 0. 75. It does not remove the high F-ratios in Collafirth and other sites in the UK and Europe, however.

    Arctic step 4 adds Ny-lesund, the Arctic site with the next-highest F-ratio. It improves the fit slightly, from 0.75 to 0.67, but at the cost of making the F-ratios for Collafirth and the other UK/Europe sites exceed those of Bear Island and Ny-lesund. In other words, this Arctic thread is destroying itself, and the self-correcting nature of the apportionment process is reasserting itself.

    The apportionment process could stop here, with a good Arctic fit for Lerwick (0.67) but a Collafirth/UK gorilla in the closet. The unacceptability of this solution is revealed by comparing it to the UK/Europe solution, repeated below for comparison. 

The UK/Europe solution is far "cleaner" than the Arctic solution.
    The Arctic solution cries out for one or more of the UK/Europe sources to be chosen. That sends the Arctic solution right back to the UK/Europe solution, as illustrated by Arctic step 5, which adds Leiston, the highest of the UK/Europe sources other than Collafirth.

    Adding Leiston improves the fit from 0.67 to the very good 0.27, but markedly decreases the F-ratios of Bear Island and Ny-┼lesund to the point where they don't matter much. It also opens the door to adding another UK/Europe source, which would likely return the solution to the pure UK/Europe version. Thus we can state that the solution for Lerwick is UK plus perhaps a touch of Arctic. Meteorologically, this corresponds to transport of aerosol to the Shetland Islands mostly by the prevailing Atlantic winds and secondarily by the large-scale pulses of "return flow" from the Arctic observed for Scandinavia during throughout the winter.

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