TITLE: C-130 Dimethyl Sulfide AUTHORS: Alan Bandy Department of Chemistry Drexel University Philadelphia, PA 19104 *Byron Blomquist Department of Oceanography University of Hawaii Honolulu, HI 96822 * contact for questions about the data 1.0 DATA SET OVERVIEW: Introduction: This README file contains information about dimethyl sulfide measurements made on the NCAR C-130 during the DYCOMS II field project and a description of the data files submitted to the DYCOMS II data archive. DMS data are contained in files named "DREX_RF##_DMS&.txt" where: ## = research flight number & = release version of data (versions a,b,c...) version "a" is the first preliminary data set released to the public. Time period covered: July 10 to July 28, 2001 Physical location: National Center for Atmospheric Research C-130 aircraft based out of North island NAS, Coronado, California. 2.0 INSTRUMENT DESCRIPTION: Dimethyl Sulfide measurements were made by an atmospheric pressure ionization mass spectrometer (APIMS). A detailed description of the instrument is presented in Glenn Mitchell's PhD thesis (Mitchell, 2001). Other publications are currently in submitted and in preparation. Briefly, the APIMS measures the intensity of protonated DMS positive ions (@ masses 63 and 66) produced in a Ni-63 APIMS source. It is necessary to remove excess water vapor with a multitube Nafion drier and then heat the incoming air to approximately 400¡C prior to ionization. Otherwise production of protonated water clusters prevents efficient ionization of DMS. The reactions producing protonated water clusters and protonated DMS are complex but the essential step is as follows... H3O+(H2O)n + DMS --> DMSH+ + (n+1)H2O where n is small An internal standard of DMS containing an isotopic label (d-3 DMS, 65 amu) is continuously added to the incoming air stream. The standard signal appears at mass 66. The ambient signal is at mass 63. All gas flow rates are controlled and measured by electronic mass flow controllers. 3.0 DATA COLLECTION AND PROCESSING From measurements of the incoming air flow rate and the flow of the standard addition, the standard concentration in the incoming airstream can be accurately calculated. Ambient DMS concentration is derived from the known standard concentration and the ratio of ion count rates at masses 63 and 66. The calculation must account for the small amount of undeuterated h-6 DMS in the standard. Ambient isotopomer abundances are derived from well established natural abundances of S, C and H in nature. The isotopic composition of the standard is measured in our lab. The concentrations are expressed as parts per trillion by volume (pptv). [[K1 * R] - K2] C(amb) = C(std) ----------------- K3 where: K1 = fraction of mass 65 DMS in the standard (0.9290) K2 = fraction of mass 62 DMS in the standard (0.02529) K3 = fraction of mass 62 DMS in ambient air (0.9280) R = measured ratio of mass 63 signal to mass 66 signal C(amb) = ambient concentration of DMS in pptv C(std) = deuterated standard concentration in pptv The intensity of the standard signal (mass 66) is more or less constant, changing slowly as flow rates change in the manifold. Therefore, the time series of the mass 66 signal was smoothed with a gaussian filter prior to computation of the 63/66 ratio. There is a small background signal at mass 63 which generally decreases throughout the flight as the vacuum system and air drier stabilize, but it can vary slowly as the air drier reequilibrates to changes in humidity. In DYCOMS we use zero air blanks to monitor this background (1-2 min out of every 30-60 min). An estimated background count rate is subtracted from the raw mass 63 data prior to computing the 63/66 ratio. The estimated background correction is the major source of error in the DMS calculation (approx. +/- 5 pptv during DYCOMS). The reported DMS concentration above the boundary layer inversion varies between +/- 5 pptv for this reason. Periods of blank measurement are replaced by NaN's in the final data files. Mass flow controllers/meters were calibrated during the project and were generally found to be within +/- 3%. Given this error in the gas flow rates, a comparable error in the cylinder concentration of the isotopic standard, and an error in the mass data equal to the square root of the count rate, the propagated random error in the final DMS concentration is about 5% in addition to the systematic error arising from the background correction. The instrument operates at a sample rate of approximately 25 Hz, but the exact sampling frequency is not identical to the C-130 data system. Over a 7 hour flight period the C-130 records a few more data points than the APIMS. Care should be exercised in aligning the two data records before attempting spectral analysis. In addition, the DMS signal is time lagged by approximately 0.75 seconds with respect to the lyman alpha water vapor measurement on the C-130 (MRLA1). 4.0 DATA FORMAT Data files are tab delimited ASCII, containing 3 columns. The first column is time stamp data in the form of a seconds past midnight (GMT) @ approx. 25 Hz. The second and third columns are raw and smoothed DMS concentration in pptv. Column 3 DMS was smoothed with a gaussian filter to facilitate plotting/visualization and should not be used for flux calculations. File Format: Rows 1-9: header information and remarks Row 10: Variable names Row 11: Variable Units Rows 12 to end: Data FOR EXAMPLE: PI = Alan Bandy, Dept. of Chemistry, Drexel Univ., Philadelphia, PA 19104, bandyar@drexel.edu CONTACT = Byron Blomquist, Dept. of Oceanography, Univ. of Hawaii, Honolulu, HI 96822, byronb... DATA COVERAGE = START: 20010710074447 STOP: 20010710135558 UTC PLATFORM = C130 INSTRUMENT = Atmospheric Pressure Ionization Mass Spectrometer analysis of dimethyl sulfide LOCATION = mobile DATA VERSION = Aug 2001 REMARKS = Missing value code is "NaN". REMARKS = This submission is the first public distribution and as such is considered... DMS_Time DMS DMS_smoothed UTC pptv pptv 27887.841 -13.718 -1.992 27887.881 -0.316 -1.981 27887.921 -0.309 -1.948 27887.961 6.932 -1.889 27888.001 -6.484 -1.803 5.0 DATA REMARKS Because the APIMS starts sometime after take off and shuts down before landing, the beginning and end of each data file does not correspond to the beginning and ending of NCAR C-130 parameter datafiles. 6.0 REFERENCES Mitchell, Glenn N., Determination of vertical fluxes of sulfur dioxide and dimethyl sulfide in the remote marine atmosphere by eddy correlation and an airborne isotopic dilution atmospheric pressure ionization mass spectrometer, PhD Dissertation, Drexel University, Philadelphia, PA, May, 2001.