Coastal influence on vertical pollutant partitioning
Abstract
Coastal outflow describes the horizontal ventilation of pollutants from the continental boundary layer (CBL) by advection above the shallower marine boundary layer. Passive tracers incorporated in the Met Office Unified Model (MetUM) are used to simulate coastal outflow over the eastern United States for the summer of 2004. The results are interpreted with a simple box-model containing a diurnally varying CBL height and uniform wind flow. Ventilation by coastal outflow from the US occurs at the same magnitude as vertical ventilation into the free troposphere for the eastern half of the US. The diurnal variability in coastal outflow is determined by the lifetime of the tracer and the cycle of CBL height. Over the diurnal cycle, pollutants with lifetimes less than 24 hours experience a maximum in the evening and a minimum in the mid-morning. Pollutants with lifetimes greater than 24 hours undergo continuous coastal outflow with little diurnal variability. The dominant parameters in this study are tracer lifetime α, wind-speed U and width of emissions (i.e. land) L. The box-model indicates the presence of a critical threshold, αU/L ≈ 10, above which increasing the cross-coastal wind speed decreases coastal out-flow by reducing the availability of tracer in the residual layer to undergo coastal outflow at night. Of tracer advected across the coast in the MetUM simulation, 65% and 67% of tracer with 3- and 24-hour lifetimes undergo coastal outflow respectively. For the 24-hour tracer, the box-model has the highest correlation with the MetUM simulation when the advection rate U/L≈ 2 x 10-5s-1. Given the observed wind speed the distance near the coast over which emissions significantly contribute to coastal outflow is ≈ 100-110km.
- Publication:
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Ph.D. Thesis
- Pub Date:
- 2012
- Bibcode:
- 2012PhDT.......261P
- Keywords:
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- Atmospheric Sciences