sunset

Aerosol Measurements for BOREAS



Robert Wrigley (retired), Michael Spanner, Philip Russell, John Livingston, Brad Lobitz

Automated tracking sun photometers were deployed by NASA Ames Research Center aboard the NASA C-130 aircraft and at a ground site for all three intensive field campaigns (IFCs) of the Boreal Ecosystem-Atmosphere Study (BOREAS) in central Saskatchewan, Canada, during the summer of 1994. BOREAS is a large multi-national, multi-disciplinary study of the interaction of boreal landscape with the atmosphere in the context of global change. The sun photometer data were used to derive aerosol optical depths for the total atmospheric column above each instrument. The airborne tracking sun photometer obtained data in both the southern and northern study areas at the surface prior to takeoff, along low altitude runs near the ground tracking sun photometer, during ascents to 6-8 km msl. along remote sensing flightlines at altitude, during descents to the surface, and at the surface after landing. The ground sun photometer obtained data from the shore of Candle Lake in the southern areas for all cloud-free times.

During the first IFC in May-June ascents and descents of the airborne tracking sun photometer indicated the aerosol optical depths decreased steadily from the surface to 3.5 km where they leveled out at ~0.05 (at 525 nm), well below levels caused by the eruption of Mt. Pinatubo for the last couple of years. On a very clear day, May 31st, surface optical depths measured by either the airborne or ground sun photometers approached those levels (0.06-0.08 at 525 nm), but surface optical depths were often several times higher. On June 4th they increased from 0.12 in the morning to 0.20 in the afternoon with some evidence of brief episodes of pollen bursts. During the second IFC, surface aerosol optical depths were variable in the extreme due to smoke from large western forest fires. On July 20th the aerosol optical depth at 525 nm decreased from 0.5 in the morning to 0.2 in the afternoon; they decreased still further the next day to 0.05 and remained consistently low throughout the day to provide excellent conditions for several remote sensing missions flown that day. Smoke was heavy for the early morning of July 24th but cleared partially by 10:30 local time and cleared fully by 11:30. Heavy smoke characterized the rest of the IFC in both study areas.

These and other aerosol optical depth measurements will be used to provide quantitative corrections for atmospheric effects in satellite and aircraft image data acquired at the BOREAS study areas.

The following graph shows the ground level aerosol optical depths for 21 July 1994 at the field sunphotometer site, Sandy Bay, Candle Lake, SSA. The field sunphotometer has ten channels ranging from 380 to 1027 nm. Only the 522, 667, and 866 nm channels are shown below. The spike before 17:25 is probably due to a small cloud or jet contrail across the sun.

AG 940721 AOD


Responsible NASA official: Dr. Philip Russell
Headquarters program office: OSSA


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Brad Lobitz (blobitz(at)mail.arc.nasa.gov)