TITLE: C-130 Sulfur Dioxide 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 sulfur dioxide measurements made on the NCAR C-130 during the ACE Asia field project and a description of the data files submitted to the ACE Asia data archive. SO2 data are contained in files named "DREX_RF##_SO2&.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: March 30 through May 4, 2001 Physical location: National Center for Atmospheric Research C-130 aircraft based out of the Marine Corps Air Station in Iwakuni, Japan. 2.0 INSTRUMENT DESCRIPTION: Sulfur dioxide 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 preparation. Briefly, the APIMS measures the intensity of SO5 negative ions (@ masses 112 and 114) produced by the following reactions in the Ni-63 APIMS source. Ozone and carbon dioxide (at concentrations typically orders of magnitude larger than SO2) form negative ions in the presence of the radioactive source, leading to production of the SO5(-) product ion. O3(-) + SO2 --> SO3(-) + O2 CO3(-) + SO2 --> SO3(-) + CO2 O2 + SO3(-) + M --> SO5(-) + M Usually, ambient levels of CO2 and O3 are sufficient. Occasionally, extra O3 is added from an ozone generator, supplementing the ambient concentration. An internal standard of SO2 containing an isotopic label (99 atom % S-34) is continuously added to the incoming air stream. The standard signal appears at mass 114. The ambient signal is predominantly at mass 112. 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 that of the standard addition, the standard concentration in the incoming airstream can be accurately calculated. Ambient SO2 concentration is calculated from the known standard concentration and the ratio of ion count rates at masses 112 and 114. The calculation must account for the small amount of S-34 in ambient SO2 and a very small amount of S-32 in the standard. Ambient isotopomer abundances are derived from well established natural abundances of S and O 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 - [K4 * R]] where: K1 = fraction of mass 114 SO5 from the standard (0.9850) K2 = fraction of mass 112 SO5 from the standard (0.0014) K3 = fraction of mass 112 SO5 from ambient air (0.9388) K4 = fraction of mass 114 SO5 from ambient air (0.0512) R = measured ratio of mass 112 signal to mass 114 signal C(amb) = ambient concentration of SO2 in pptv C(std) = standard concentration in airstream in pptv The instrument can operate at sample rates up to 25 Hz. For ACE Asia, mass data and all flow rates were logged at a bit more than 2 Hz. Final data were averaged to 1 Hz. The intensity of the standard signal (mass 114) is more or less constant, changing slowly as flow rates change in the manifold. Therefore, the time series of the mass 114 signal was smoothed with a gaussian filter prior to computation of the 112/114 ratio. There is a very small background signal at mass 112, presumably due to the negative ion CO4.2H2O(-). In ACE Asia we use zero air blanks to monitor this background level (1-2 min out of every 30-60 min). The background count rate is subtracted from the raw mass 112 data prior to computing the 112/114 ratio. Because the aircraft represents a challenging environment (changing pressures and flows), this background varies during the course of a flight and the correction is not always perfect. For this reason, when the ambient SO2 concentration is very low (10 pptv or less), the data will occasionally indicate concentrations between + 10 pptv and -10 pptv. The final data have not been filtered in any way to remove these negative values and their existence is a good indication of when the measurement is approaching the detection limit. Periods of blank measurement are replaced by NaN's in the final data files. Uncertainty: Mass flow controllers/meters were calibrated on 3 occasions 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 error in the final SO2 concentration is about 5%. 4.0 DATA FORMAT Data files are tab delimited ASCII, containing 3 columns. The first two columns are time stamp data in the form of a text string (UTC_Time) and a decimal Julian date (JulianTime). The third column is the SO2 concentration in pptv. Our data have the same start time, stop time, frequency, and array length as the NCAR C-130 parameter files. (Indeed, the UTC_Time variable was simply copied from the NCAR 1 second "subset" ASCII data files.) Thus, the time variables in our data files are identical to the time stamp in the NCAR files. If you are using the NCAR 1 second data, you don't need to use the time stamp data in our files. Simply append the SO2 array to the NCAR file. File Format: Rows 1-14: header information and remarks Row 15: Variable names Row 16: Variable Units Rows 17 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: 20010330234000 STOP: 20010331063401 UTC PLATFORM = C130 INSTRUMENT = Atmospheric Pressure Ionization Mass Spectrometer analysis of sulfur dioxide LOCATION = mobile DATA VERSION = 22-Oct_2001 REMARKS = Data are contiguous, 1-sec resolution, with the same start time, stop time, and... REMARKS = Thus, the SO2 time variables (UTC_Time, JulianTime) are identical to those in the... REMARKS = Simply append the SO2 data column to the NCAR file if you wish to plot SO2 vs... REMARKS = Missing value code is "NaN". REMARKS = This submission is the first public distribution and as such is considered... UTC_Time JulianTime SO2_1_sec_pptv UTC UTC pptv Fri Mar 30 23:40:00 2001 89.986111111 NaN Fri Mar 30 23:40:01 2001 89.986122685 NaN Fri Mar 30 23:40:02 2001 89.986134259 NaN Fri Mar 30 23:40:03 2001 89.986145833 NaN 5.0 DATA REMARKS Because the APIMS only starts sometime after take off and shuts down before landing, the beginning and end of each data file consists of NaNs. During RF01 and RF02 we conducted an intercomparison between the APIMS on the C-130 and a similar instrument flying on the NASA P-3. The preliminary results indicate very close agreement between the two measurements. More detailed examination of these data is underway. 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.