TITLE: Twin Otter Particle Soot Photometer Data AUTHOR(S): Brian Mader Department of Environmental Science and Engineering Caltech MS 210-41 Pasadena CA 91125 626-395-4410 bmader@cheme.caltech.edu Richard Flagan Department Chemical Engineering Caltech MS 210-41 Pasadena CA 91125 626-395-4383 flagan@caltech.edu John Seinfeld Department Chemical Engineering Caltech MS 210-41 Pasadena CA 91125 626-395-4635 seinfeld@caltech.edu 1.0 DATA SET OVERVIEW Abstract Airborne measurements of particle light absorption coefficient were measured using the Twin Otter aircraft during ACE-Asia. Measurements were made using a Particle Soot Absorption Photometer (Radiance Research, Seattle WA) located onboard a modified De Havilland DHC-6 Twin Otter aircraft operated by the Center for Interdisciplinary Remotely Piloted Aircraft Studies (CIRPAS). A total of 19 Research Flights were conducted between March 31 and May 1, 2001. The center of aircraft operations was located at the Marine Corps Air Station (MCAS) Iwakuni, Japan and the sampling area included portions of the Sea of Japan south and east of the Korean Peninsula, the East China Sea between China, Japan and Korea, and the Philippine Sea south of Japan. 2.0 INSTRUMENT DESCRIPTION Measurements were made using a Particle Soot Absorption Photometer (Radiance Research, Seattle WA). Further information is available from Bond et al. 1999. 3.0 DATA COLLECTION AND PROCESSING Measured values of particle light absorption coefficient at 567 nm were saved on an onboard computer at a rate of 1 Hertz. The aircraft latitude, longitude, altitude and atmospheric pressure at the time that absorption measurements were recorded are also included. The value of the particle light absorption coefficient (Bap) was measured using a Particle Soot Absorption Photometer (PSAP) (Radiance Research, Seattle WA); a description of this instrument is provided by Bond et al. [1999]. Values of Bap at 567nm were saved on an onboard computer at a rate of 1 Hertz. The aircraft latitude, longitude, altitude and atmospheric pressure at the time that absorption measurements were recorded are also included. Briefly, ambient air is drawn through a filter, and the transmission of light ( wavelength = 567 nm) through the filter is continuously monitored. With knowledge of the change in filter transmission over some period of time, the flowrate of air through the filter, and the cross-sectional area of the filter, it is possible to calculate Bap. Light scattering by filter bound particles will also cause a reduction in filter transmission, resulting in a positive artifact in the value of Bap. Bond et al. [1999] provide a method to correct for such an artifact, as well as correcting errors in the manufacturer's calibration. 4.0 DATA FORMAT: Data file structure is comma delimited ASCII PI/DATA CONTACT = Mader Brian (Caltech); Flagan Richard (Caltech); Seinfeld John (Caltech) DATA COVERAGE = START: 20010402; STOP: 20010501 PLATFORM/SITE = Twin Otter SAMPLE COLLECTION INSTRUMENT = Radiance Research Particle Soot Absorption Photometer LOCATION = mobile DATA VERSION = 1.0 (11 March 2002) Final REMARKS = California Institute of Technology ACE-Asia Date = yyyymmdd Mission time = seconds UTC time = hhmmss (UTC) Lat= Latitude Long = Longitude Alt = Altitude (m) PSAP Ave Period = period of time over which light transmission is averaged for calculation of Bap (seconds) PSAP Bap = particle absorption coefficient for light of wavelength 550 nm (mega meters^-1) PSAP Filtr Tran = transmission efficiency of 550 nm light through PSAP glass filter (fraction) PSAP Q = flow rate of air through PSAP filter (liters min^-1) PSAP signal path = intensity of light transmitted through filter PSAP ref path = intensity of light incident to filter P(s) = pressure (millibars) 5.0 DATA REMARKS When operating a PSAP onboard an aircraft is that the PSAP response appears to be affected by changes in altitude, ostensibly due to physical or chemical changes in the filter unrelated to the levels of light absorbing species. The PSAP signal, specifically light transmission through the filter, was erratic during ascents and descents, but became reliable after operating a few minutes at a constant altitude. It may be possible that in some situations changes in the water content of the filter could have affected the transmission of light; quartz filters have polar surfaces that can absorb water. When determining the average value of Bap for a particular sampling period, data should be averaged over time periods during which the PSAP response is reliable. Errant data is labeled as -99999 The flowrate (PSAP Q) is measured using a mass flow controller and assuming that the atmospheric pressure is 1 bar. Therefore measured values of Bap are at 1 bar, we have included pressure data so that Bap can be corrected for the effect of pressure so that "in-situ" Bap values can be calculated at the altitude of interest. Values of Bap should be averaged over a period of time of at least a few minutes since the PSAP integration time was 5 seconds The effect of particle light scattering on the value of absorption coefficient is NOT corrected in this data set. Those using these data are referred to the reference by Bond et al. so that such corrections and others can be made. Nephelometer data is provided in a separate data product, these data are necessary for scattering corrections. 6.0 REFERENCES: Bond, T. C., T. L. Anderson and D. Campbell, Calibration and intercomparison of filter-based measurements of visible light absorption by aerosols, Aerosol Sci. Technol., 30, 582-600, 1999.