DATA SET DOCUMENTATION FOR ACE-ASIA TOTAL GASEOUS MERCURY (TGM) DATA SET


TITLE: ACE-Asia Total Gaseous Mercury (Research Flights 1-16) 


AUTHOR(S): 

  Hans Friedli (NCAR)
  NCAR/ASP
  1850 Table Mesa Dr.
  PO Box 3000
  Boulder, CO. 80303
  (303)497-1395
  (303)497-1646 FAX
  friedli@asp.ucar.edu  


DATA SET OVERVIEW: 

  Data Coverage:  Research Flights 1 - 16 (data collected between 03/30/2001 and 04/30/2001)
  Platform: C130
  Location: Mobile (LAT/LON/ALT orientation provided in data set)


INSTRUMENT DESCRIPTION: 

  The Tekran instrument is based on collecting mercury from sample air by amalgamation 
  on two gold cartridges (designated A and B) and thermally desorbing elemental mercury 
  from them and assaying it by cold vapor atomic fluorescence spectroscopy (CVAFS). The 
  output is designated as total gaseous mercury (TGM), which includes elemental mercury 
  (EM) and the fraction of reactive gaseous mercury (RGM), which is not scavenged on 
  inlet surfaces. Since RGM is small in most (Slemr, see His 3) but not all (Lindberg 
  arctic) cases and total particulate mercury (TPM) is removed by filters, TGM essentially
  represents EM. The response of the CVAFS detector is affected by pressure, i.e. flight 
  altitude in unpressurized aircraft or cabin pressure in controlled cabin pressure flights. 
  Ebinghaus and Slemr (2000) have demonstrated that at ambient pressures above 500 mb the 
  Tekran instrument is capable of collecting EM quantitatively but that the response of the
  CVAFS detector must be corrected for pressure differences. The effect of poisons present
  in polluted air is more complex. Poisons block mercury adsorption sites so that not all 
  mercury is adsorbed on the cartridges resulting in a lower detector response and a need 
  for frequent recalibration. Since most poisoning is reversible, making successive 
  measurements in areas of different degrees of pollution will change the instantaneous 
  response thus making quantitation less accurate. If poisoning is as severe as observed 
  in some of the ACE-ASIA flights, regeneration by oxidative treatment of the cartridges 
  and frequent manual calibration between flights and before and after regenerations is 
  required.



DATA COLLECTION/PROCESSING AND REMARKS: 

  	Air samples were collected and discharged in ¼” Teflon tubing through backward facing inlets 
  and outlets and aspirated continuously by the pump contained in the Tekran analyzer. The 
  inlets were located in the free airstream 25 cm outside the skin at the bottom center of 
  the C-130 aircraft aft of the wheel wells. Particulates in the sample air were removed by 
  a 50 mm 5 um Teflon filter in a Teflon housing located about 60 cm from the inlet point and 
  by a 47 mm 0.2 um Teflon filter installed at the back of theTekran analyzer. The distance 
  between inlet and Tekran was 3.5 m. Ultrahigh purity Argon (Scott Specialty Gases) and zero
  air (Scott Specialty Gases) for the Tekran operation were provided from gas cylinders; zero 
  air was additionally purified by a Resisorb canister (Tekran Inc.) containing activated 
  carbon. The Tekran instrument was operated on 300 s adsorption/desorption-assay cycles and 
  7.5 liter (STP) sample volume. 
	Calibrations with the internal permeation cell were performed at the beginning and end 
  of the field deployment where 24 hour conditioning was possible. Calibrations by manual 
  injections using the 2505 Mercury Vapor Calibration Unit (Tekran Inc., Toronto, Ontario) 
  were conducted before and after deployment and in the field. They agreed within 10% (n> 5 for A 
  and B) except when contamination of the syringe needle or of the septa at the Tekran and in the 
  calibrator had occurred. The two problems were remedied by septum replacement and heating of 
  the injection needle and the barrel of the syringe. The integrity of the sampling system was 
  confirmed by manual injections at the inlet and at the Tekran injection port; they ranged from 
  95 - 105% of the values measured at the Tekran indicating that mercury was not substantially 
  lost in the tubing or on the two filters. Periodic regeneration of the cartridges by treatment 
  with zero air was carried out according to the instructions of the Tekran manual. 
  	The integrated area counts for each cycle were adjusted for the corresponding background 
  counts and corrected for the pressure dependence using the recorded cabin pressure for each
  data point. The pressure dependence was calculated from calibrations at the home base at 823 
  mb before the ferry flights and at the deployment location at Iwakuni, Japan, at 1020 mb. Since 
  the transit flight was in clean air in the free troposphere (about 6.6 km) deactivation was 
  assumed to be negligible. The calculated pressure correction for both cartridges was 5.162 
  cts/pg/mb or .106 %/mb. This compares with the values of Ebinghaus and Slemr (2000) of .122 
  and .120%/mb for their A and B cartridges. The pressure corrected area values were then 
  converted to concentrations by applying the calibrations in counts/pg and the corresponding 
  sample volumes analyzed. Since the sample pressure was >500 mb for all research flights 
  (but could drop to >400 mb during ferry and transfer flights) the assumption was made that 
  all mercury was captured by the cartridges. Responses for individual flights were obtained 
  by linear interpolation between the calibration responses of the six on-site calibrations. 
  The resulting TGM concentration is calculated by

  ng/m3 = [pressure corrected area] /extrapolated calibration response/flow 

  The permeation cell and manual calibration at the beginning and end of the field experiment agree
  within 10% of each other, but the overall response had declined by 36.0 and 32.8% for A and B, 
  respectively. After some initial preferential poisoning of B, the decay rate of B remained larger
  during the experiment.  The decay was proportional to the total air sampled, i.e. the number of 
  flights between calibrations, suggesting that poisoning was an ongoing phenomenon.   Regeneration
  effectiveness was about the same for A and B and more the highly contaminated cartridge recovered
  to a higher extent.
  	When corrections and calibrations were applied to the TGM measurements, final A and B 
  concentrations were within about 10% with A being larger. This applies to all flights except 
  for flights RF 8-11 where there was an as yet not understood larger discrepancy. We elected to 
  arbitrarily adjust the concentrations to bring the A and B relationship into the same range as 
  all other measurements.
  	The source of deactivation is not known. Suspicious poisons are contaminants from the airbase,
  which include hydrocarbons and possibly sulfur compounds from a close-by pulp plant.  After 
  deactivation had been noted, the sampling of ambient air near the base was avoided and the Tekran
  was operated on zero air as sample or shut down during take off and landing. However, poisoning 
  occurred during most or all research flights, but partial regeneration during the mission was 
  noticed during at least some of the flights as indicated by systematic changes in the A/B 
  relationship at different portions of the same flights.  (e.g. RF # 09) 



DATA FORMAT: 

  Files: ACE-AsiaTGM.txt - A column delimited ASCII text file containing data for research flights 1-16. 
  Parameters: Time (UTC), Latitude (DEG), Longitude (DEG), Altitude (METERS), TGM (NG/M^3)
  Sampling frequency: 300 seconds
  Data version: 1.0 (October 5 2001) Final