87th AMS Annual Meeting

Wednesday, 17 January 2007
Application of CCNY Lidar and ceilometers to the study of Aerosol Transport and PM2.5 monitoring
Exhibit Hall C (Henry B. Gonzalez Convention Center)
Leona Charles, The City College of New York, New York, NY; and S. Chaw, F. Moshary, B. Gross, S. Gedzelman, S. Ahmed, and V. Vladutescu
Poster PDF (2.9 MB)
1. INTRODUCTION PM2.5 has been shown to have health effects, not only to human health, but also to the entire global environment. In an effort to improve Air Quality Forecasting, the EPA together with NASA and NOAA, implemented an ambient air quality monitoring program to determine the composition of airborne PM2.5 in urban air in an effort to assimilate satellite measurements of aerosol column optical depth into air quality transport models.  However, current satellite based aerosol measurements cannot assess the distribution of aerosols in the atmosphere so transport models based on vertical layering are not suitable. However, with the advent of calipso, the possibility of assessing and correcting the errors made in transport due to vertical variability of the aerosols and the existence of plumes which may advect down to the PBL layer can begin. Finally, to complete the loop and connect the optical scattering data to PM2.5, it is important to assess the correlation between near surface optical backscatter and PM2.5. In particular, we see that for optical backscatter data obtained with heights under 100 meters, an excellen t correlation is seen but for heights above 200 meters, a significant degradation in the regression is seen for high PM2.5 concentrations. These results provide us with the needed spatial resolution needed to track PM2.5 from space using calipso. 2. Results To assess the need for lidar measurments to predict PM2.5, we first examine the case where large plumes were predicted to affect air quality in New York City. In support of this prediction, the CCNY lidar system observed some heavy transport activity over its location, which contributed to large aerosol loadings in the troposphere which was captured by MODIS derived AOD.  On April 19, a high-pressure system with AOD > 0.4 advected from the north (Quebec, Canada) towards NYC, as indicated in the IDEA forecast trajectory and the MODIS derived AOD

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.  


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