However, we see in figure 2 lidar measurments made by the CCNY lidar on April 19. clearly illustrate a complex vertical structure of the observed atmosphere over CCNY with the majority of the optical depth carried in aloft plumes. In addition, we obseve a well mixed PBL layer with a height of approximately 1 km increasing to about 2 km during the afternoon allowing for an interaction between the PBL and the aloft plume.
Shortly after 14:00 EDT, the dust cloud separated into an aloft plume, and a downward moving plume which advected into the boundary layer.
The incursion of the transported aerosol plume into the boundary layer was clearly a cause for air quality concerns. Interestingly, when we examined the PM2.5 levels, a very small increase was seen in the PM2.5 mass but as seen in figure 3, the small PM2.5 loading occurred jointly with a large increase in the PM10 mass during the incursion consistent with the PBL. This finding is somewhat confusing until it is realized that the plume was not due tyo bio-mass burning or other smoke related event but was due to large
dust plumes undergoing large scale transport. To confirm, we have also taken coincident data at Princeton using the CCNY Mobil Lidar which shows the same transport event observed over both locations. Further analysis based on extended HYSPLIT trajectories (13 days) confirm that the observed dust plume seen by CCNY lidar on April 19 was transported from dust storms originating in the Gobi Desert.
These observations show a clear need for accurate measurements of optical data within the PBL but often the PBL is not well mixed but undergoes significant dynamic flux as seen in figure 4. Unfortunately, conventional lidar has problems see near the surface; consequently, not much structure is seen in the PBL of the CCNY lidar vertical structure. To compensate for such drawbacks, we have in operation, a Continuous Eye Safe Ceilometer (single wavelength ~905nm) which acts as a lidar for near surface distances, and its precision makes it ideal for vertical information on near surface (low altitude) aerosols. Using near field ceilometer backscatter measurements, with range confined between 20-80m window, we show very good near surface backscatter- PM2.5 correlation (R2=0.92). However, shifting this window to a higher altitude within the boundary layer (220-280m), there was a 20% degradation in the backscatter-PM2.5 correlation, particularly for high PM2.5 . This result provides an understanding of the vertical level and resolution to assess PM2.5 from space.
We show that the vertical structure of aerosols is very important in assessing transport events and how plumes can advect down to the PBL. These plumes were shown to dust from their influence of the PM10 mass in comparison to the lack of change in the PM2.5 mass and verified by HYSPLIT trajectories as Gobi-Desert Dust
The need for vertical information and plume detection has led to the launch of the CALIPSO space based Lidar system but the relationship between the optical backscatter and the PM2.5 mass is not simple. To investigate this, we show using near field ceilometer measurements that, unlike column aerosol measurements, near surface backscatter measurements within 100 meters of the surface accurately correlate very well to PM2.5 but by 300 meters, a significant degradation in the correlations is observed. This result gives practical guidance on the vertical resolution needed to process CALIPSO data in support of surface aerosol loading.