The pictures below were taken from a Cessna-172 aircraft over over the Washington-Baltimore metropolitan area during field missions performed by the Atmospheric Chemistry Group of the University of Maryland, Department of Meteorology in 1992 from elevations ranging from 3000-7000 ft.  For more information on aircraft flights over the Washington-Baltimore Metropolitan Area check out click on the aircraft to the right.   
 

 
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Figure 1. Looking out the windshield of the Cessna -172 at approximately 4000 ft.; this altitude is just below the top of the temperature inversion on this day (Aug. 1992). This  type inversion is known as a nocturnal inversion or radiational inversion because it forms after sunset, as the earth's surface cools (by releasing energy into the air above it). The visibility is very restricted because of the build up oxides of nitrogen, sulfates, hydrocarbons and ozone.  The origin and consequence of  these pollutants is the source of much scientific debate.  The trace gases causing the poor visibility were either carried aloft by thermal updrafts and or transported in from points to the west (i.e. the Ohio river valley).   
               An air parcel will tend to rise until it is surrounded by air that is similar in temperature. If winds from the updrafts (convective winds) are are strong enough they can burst through the inversion and erode the Planetary Boundary Layer (PBL). Once the boundary layer decays, ozone readings at the earths surface can drop dramatically as the plume of pollution is carried into the "upper-free-troposphere" were it encounters different atmospheric conditions.
 
 
 
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Figure 2. Cumulus clouds breaking through the inversion can be seen just above the PBL (~5000 ft.). This phenomenon is commonly referred to as "cumulus venting". Determining when, or if,  venting occurs is very important to understanding the extent of urban ozone production and observation on any giving day. Some of the factors which can help establish when and if the boundary layer will vent are: the amount of heating of the earths surface, the relative humidity in the PBL, the temperature of the air entering the region above the PBL and the strength of the radiational inversion.
 
 
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Figure 3. Much clearer skies (even if the picture does not look clear!) are evident in the lower free troposphere (~7000 ft.) as the available moisture typically drops off rapidly once free of the PBL. Ozone and ozone precursors have longer lifetimes in this regime and can travel much greater distances from their source region because of higher wind speeds. The eventual fate of the "urban plume" is in large part, determined by the boundary layer dynamics from which it came. If the boundary layer is very stable (little vertical motion) for an extended period of time (i.e. a few days), significant amounts of pollution can build.  Once the boundary layer does vent (imagine an afternoon thunderstorm in the summertime) much of the pollution is rapidly transported the lower troposphere.  To find more information on this topic and other important atmospheric chemistry topics a very nice place to start would be the home page of Professor Russell Dickerson.   

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