Keith Bein - Extratropical waves transport boreal wildfire emissions and drive regional air quality dynamics

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  Publication Details (including relevant citation   information):

  Bein, K.J., Zhao, Y., Johnston, M.V., Evans, G.J., and A.S.   Wexler (2008), Extratropical waves

       transport boreal wildfire emissions and   drive regional air quality dynamics, Journal of   Geophysical

       Research - Atmospheres, 113,   D23213, doi:10.1029/2008JD010169.

  Abstract:

       A synthesis of data and analyses   identified eight separate wildfire events – five in boreal Canada   and three in the western U.S. – that impacted the Pittsburgh   Supersite, as well as Toronto, during June and July, 2002.    These data also revealed a larger structure in the nature of   pollution episodes in Pittsburgh characterized by alternating   periods of stagnation and cleansing.  Stagnation resulted in   significant and sustained increases in pollutant concentrations   that were eventually flushed by clean air transported from   Canada.  Stagnation events were further characterized by   decreases in wind speed, increases in ambient temperature and   enhanced atmospheric processing.  Reverse trends were   observed during the “clean” periods.  Wildfire emissions   consistently arrived immediately prior to the stagnation events   and tended to be trapped and re-circulated by the associated   surface high pressure system, prolonging their impact.

       These receptor site dynamics were   correlated to the structure and propagation of transient   extratropical synoptic-scale waves, which were characterized by   eastward propagating transcontinental trough-ridge-trough   configurations.  Temporal patterns observed in the   Pittsburgh data – a repeating sequence of long-range transport   followed by stagnation and then flushing – were in phase with   alternating synoptic structures of the propagating waves.    Northwesterly subsiding flow between the ridge and trough   transported wildfire emissions from the injection height above   the fires down into the Pittsburgh and Toronto air sheds.    As the wave propagated, the surface high associated with the   upper-level ridge passed Pittsburgh, initiating a stagnation   event.  The cold front associated with the approaching   trough flushed the stagnant air but reset the sequence of events   as emissions from new wildfires were again transported behind the   front.  The sequence consistently repeated throughout the   study period with a frequency directly correlated to the   frequency of extratropical wave formation.  Stagnation   severity was related to the speed and maturity of the synoptic   disturbances while wildfire impact severity corresponded to fire   size.

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