ACKNOWLEDGEMENTS: We gratefully acknowledge funding from the NOAA Climate Program, and thank the Dr. Brent Holben for establishing and maintaining the Bondville Aeronet site. For more information on AAO, see The NOAA/ESRL Airborne Aerosol Observatory: The First 2 Years of Operation P. Sheridan 1, J. Ogren 1, E. Andrews 1,2, and T. Anderson 3 1 NOAA Earth System Research Laboratory, Boulder, Colorado, USA 2 Cooperative Institute for Research in Environmental Sciences, Boulder, Colorado, USA 3 Department of Atmos. Sciences, University of Washington, Seattle, Washington, USA In order to estimate global aerosol radiative forcing and the effects of aerosols on the global climate, measurements of the aerosols are being made by the NOAA Earth System Research Laboratory (ESRL) Global Monitoring Division (GMD) at many locations around the world. Many major surface regions remain undersampled, however, and very few long term measurement efforts have been made above the surface. In an effort to characterize when, how often, and under what conditions the surface measurements are representative of the lower column, ESRL has undertaken regular aircraft measurements over two heavily instrumented surface sites: The DOE/ARM Southern Great Plains Central Facility near Lamont, Oklahoma, and the NOAA Surface Aerosol Monitoring Station near Bondville, Illinois. Unfortunately, lack of funding has caused the Oklahoma program to terminate at the end of 2007 after almost 8 years of successful operation. The Illinois aircraft has been flying regularly since June Here we report on the first 2 years of operations of the NOAA/ESRL Airborne Aerosol Observatory (AAO). Three-dimensional schematic of the NOAA/ESRL Airborne Aerosol Observatory, showing orientation of inlet and major measurement systems. The AAO was conceived, designed, and built by the Global Monitoring Division Aerosol Group to conduct regular long-term aircraft measurements over the central US. Aerosol optical, microphysical, and chemical properties are measured on this Cessna T206H aircraft, as well as atmospheric gases. The airplane is based at Champaign-Urbana Willard Airport, so that regular vertical profiles over the Bondville surface station (BND) can be performed. Specific aerosol measurements on the AAO aircraft include particle number concentration (using a condensation particle counter, CPC), size distribution (scanning electrical mobility sizer, SEMS), aerosol light scattering and absorption coefficients at 3 visible wavelengths (nephelometer + PSAP), aerosol hygroscopic growth factor, f(RH) (four nephelometers, measurements at different relative humidity (RH) levels), and aerosol ionic composition (particle into liquid sampler, PILS). In addition, ozone is monitored continuously, trace gases (carbon cycle, N 2 O, CFC’s, halons, HFC’s, HCFC’s, etc.) are collected in flasks, and ambient temperature and relative humidity are also measured. The above plots show aerosol data from all AAO profile flights conducted near the Bondville site through June The yellow boxes show segment-averaged data (no size cut) from the individual AAO flight segments. The vertical lines through the boxes represent the 25 th, 50 th and 75 th percentiles of the distributions, while the ends of the horizontal lines show the 5 th and 95 th percentiles. The lowest flight segment data are shown outlined in red, and represent low-level fly-bys (at ~200m above ground) of the BND station. The plot on the left shows total (Dp < 10  m diam) aerosol data from BND (purple box), while the one on the right displays submicrometer surface aerosol data from BND. The extinction data show the expected decrease in aerosol amount with altitude, while the single-scattering albedo (SSA) data show little variation in the vertical. The low-level fly-by SSA data agree more closely with the other airborne data than with the surface data, indicating a real difference in aerosol optical properties between the surface and the lowest flight level. The difference appears to be more related to absorption than scattering, suggesting the possibility of cloud processing and selective removal of hygroscopic scattering aerosols at altitude. Cessna Aerosol Inlet Particle Transmission Efficiency of Inlet Tubing The AAO Cessna has an inlet system featuring a sheathed inlet and large diameter stainless steel tubing. The flow rates were chosen to optimize the transmission of the optically important fraction of the size distribution. The above plot shows the transmission efficiency of particles through the inlet tubing (from the rear of the inlet to the optics instruments). Our calculations indicate efficient transmission of accumulation mode and supermicrometer particles. We are currently conducting modeling studies of the inlet itself and expect to have a similar plot of transmission efficiency for the entire inlet system available soon. Typical time series of aerosol light scattering (550 nm) and GPS altitude during a recent flight. Flight levels are 10 minutes duration at higher altitudes (15 kft, 12 kft, 10 kft, 8 kft, and 6 kft asl) and 5 minutes at lower altitudes (5 kft, 4 kft, 3 kft, 2 kft, and 1.5 kft asl). Automated instrument background checks occur just after takeoff and just prior to landing. This profile shows  sp = Mm -1 at the three highest levels, and a well mixed layer with  sp = Mm -1 below about 5 kft. The plots above show AAO flights conducted over the Bondville (BND) site through end of June Total scattering data (no size cut) from lowest leg of AAO profile is compared with BND surface scattering data. Top and bottom plots show comparisons with BND total (Dp 1  m are being less efficiently collected by the aircraft system, or c) both of the above. The long term average submicron scattering fraction at BND is ~ 0.87 (Delene and Ogren, 2002), in reasonable agreement with the slope of 0.84 shown in the upper plot. One of the goals of this program is to perform flight profiles coincident with remote sensing measurements to provide verification and validation of the surface and satellite sensors and retrieval algorithms. These include overpasses of the A-Train (AQUA and CALIPSO) and TERRA satellites and aerosol optical thickness measurements made by the Aeronet Cimel sunphotometer at BND. The top plot at right shows a comparison of aerosol optical depths from the BND Aeronet sunphotometer and those derived from AAO profile flights. The AAO data have been adjusted to 500 nm to match the Aeronet wavelength, and to ambient RH according to the long-term mean submicrometer f(RH) measurement at BND. The AAO AOD is typically lower than the corresponding Aeronet AOD because of several reasons. These include 1) the AAO inlet has a decreased sampling efficiency for large (i.e., supermicrometer) particles; 2) the AAO does not fly above 15 kft, and aerosols are there at times; 3) the AAO may miss aerosol layers between the sampling altitudes; and 4) the possibility that the submicrometer f(RH) adjustment is not appropriate in all cases. The bottom plot at right shows an example of the comparisons between AAO in situ aerosol and CALIPSO lidar data. The red line represents the lidar profile of attenuated backscatter (532 nm) of an 80-km mean, cloud-cleared viewing area around the BND site, and the dashed line shows the estimated profile of molecular backscatter. Aerosol layers encountered by aircraft at ~3650 m and ~1000 m asl with mean aerosol scattering coefficients (  sp, 550 nm) as shown are included for comparison. Our qualitative results for many profiles suggest that the lidar is able to detect these aerosol layers above background when the  sp is roughly 15 Mm -1 or higher. To assess how the aerosols over the BND site change with time, we have plotted segment- average aerosol properties as a function of time. A contour plot of the aerosol light scattering coefficient (550 nm) taken from the segment data is shown above left. This plot shows higher scattering coefficients extending to high altitudes in the summertime, consistent with the notion of more emissions and secondary aerosol formation in a deeper mixed layer. Measurements during wintertime flights show generally lower scattering coefficients, and they are confined to altitudes below about 1.5 km. The plot above right shows the single-scattering albedo (SSA, 550nm) plotted in the same way. Lower SSA values are observed at low altitudes during the late summer, fall and early winter over the BND site. Extinction (Mm -1, 550 nm)Single-scattering albedo (550 nm) Altitude (m asl) Extinction (Mm -1, 550 nm) Single-scattering albedo (550 nm) Altitude (m asl) In situ Aerosol Scattering (Mm -1, 550 nm) In situ  sp : 15 Mm -1 In situ  sp : 52 Mm -1