Quantification of topographic venting of boundary layer air to the free troposphere

[摘要] Net vertical air mass export by thermally driven flows from theatmospheric boundary layer (ABL) to the free troposphere (FT)above deep Alpine valleys was investigated. The vertical export ofpollutants above mountainous terrain is presently poorlyrepresented in global chemistry transport models (GCTMs) and needsto be quantified. Air mass budgets were calculated usingaircraft observations obtained in deep Alpine valleys. The resultsshow that on average 3 times the valley air mass is exportedvertically per day under fair weather conditions. During daytimethe type of valleys investigated in this study can act as anefficient "air pump" that transports pollutants upward. Theslope wind system within the valley plays an important role inredistributing pollutants. Nitrogen oxide emissions in mountainousregions are efficiently injected into the FT. This could enhancetheir ozone (O₃) production efficiency and thusinfluences tropospheric pollution budgets. Once lifted to the FTabove the Alps pollutants are transported horizontally by thesynoptic flow and are subject to European pollution export.Forward trajectory studies show that under fair weather conditionstwo major pathways for air masses above the Alps dominate. Airmasses moving north are mixed throughout the whole troposphericcolumn and further transported eastward towards Asia. Air massesmoving south descend within the subtropical high pressure systemabove the Mediterranean.

[学科分类] 大气科学