ELF operated for approximately 36 hours to monitor the atmospheric boundary layer during today and yesterday's Air Quality Action Event. The PM2.5 ground monitors in the Washington DC-Baltimore metro area (left panel) show that during the time that ELF operated the PM2.5 AQI levels were Moderate-Unhealthy for Sensitive groups. From the lidar timeseries we can see how the boundary layer rose on September 3rd up to 1.75 km. During the night of the 3rd the winds shifted from a northern to a southerly flow (right panel) recirculating pollutants and bringing moisture/water vapor into the Mid-Atlantic region. This humidity rose the PM2.5 concentrations and extended the boundary layer close to 3 km. Water vapor mixing ratio timeseries from ALEX,the Raman lidar, confirm the presence of H2O vapor up to 3km (plot not shown).
The increase in the PM2.5 concentrations is associated to the increase of photons scattered back to the lidar receiver (figure below). Also, related to the change of direction and speed of last night winds was the formation of a low level jet, as shown in the wind profiler timeseries. This event was weaker that the one we experienced on June 13th. We observed a region of convergence, around 6:00 UTC, characterized by low wind speeds (~ 5 m/s) associated with a southerly wind shift, that results in aerosol lifting. After 6:00 UTC, a thick residual layer above the core of the low level jet is coincidental with directional shear.Posted by Ruben Delgado at September 4, 2008 11:02 PM