SOS99NASH_WP3D_NMHC.txt read me file
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Principal Investigator and Contact Information:
Paul Goldan
NOAA Aeronomy Laboratory
Mail: R/AL7 325 Broadway, Boulder, CO 80307
Email: pgoldan@al.noaa.gov
Phone: 303-497-3814
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1) Most of the samples were acguired in a "grab sample" mode with a few samples acquired in a "dynamic sampling" mode. For the "grab" samples,a stainless steel canister was used for the C2->C5 alkanes, C2->C3 alkenes and acetylene. A Teflon container was used for all the other compounds. For the dynamically acquired samples, no sampling container was used for any compound; rather, the sample was accumulated for a 5 minute period from the Teflon sample line continuously under flush with ambient air.
The stop times in all files are:a) the actual stop times of dynamically acquired samples and b) the container shut-off times for grab samples. The start times are the actual acquisition start times for the dynamically acquired samples. For the grab samples, however, they are the stop times minus one canister flushing time in the grab sample containers. The time window for these grab samples varies as a function of aircraft altitude since the sample pump flow rate, and thus the canister flushing time, is a function of ambient pressure. These flushing times have been calculated for each grab sample and vary from approximately 18 sec in the boundary layer to about 1 min at 7.3 Km pressure altitude. Sample transfer from the "grab" containers to the analytical system takes 5 minutes ( the same time as for the dynamically acquired samples) so the mean sample residence time in the containers varies from roughly 3 minutes at in the boundary layer to 4.5 minutes at 7.3 Km altitude.
2) Data values
All positive values given are mixing ratios in ppbv. Precision for all listed compounds is estimated as being equal to +/-0.002 ppbv+2.5% of the reported value. Accuracy for all compounds is estimated as 10% or better.
All -999s imply a response below the system detection limit which is defined as a signal to noise ratio of 2. The detection limit varies with  compound being in the range 3pptv--->8pptv for most compounds with the detection limit of 8 pptv applying to acetylene.
Occasional -555s indicate no measurement for that compound due to a temporary system problem (ie noise glitch or water freezeup).
3)	For the following compounds, a significant system background was a problem:
		methanol
		acetaldehyde
		ethanol
		acetone
		mek
		toluene
This background precluded making measurements sufficiently accurate for posting at the present time for:
		acetaldehyde
		ethanol
		mek
		toluene
Measurements corrected for this background and deemed to have an accuracy of approximately +/-10% are posted for:
		methanol
		acetone
4) No propene measurements have been posted due to artifact problems.
5) 	For the ferry flight of 26 June 1999 from Tampa Fla. To Nashville Tn., technical difficulties made the second column of the 2 column GC inoperative. As a result there are no measurements for the following compounds for that flight:
isoprene, methanol, acetone, methacrolein,n-hexane, MVK and benzene.
The 2 column GC was taken off of the NOAA P3 for repairs for the flight of 30 June 1999 and replaced by the old 1 column GC with its more restricted suite of measurable compounds. As a result there are no measurements for the following compounds for that flight:
methanol, acetone, methacrolein, MVK and benzene.
Paul D. Goldan    02 December 1999
TACOH (Tropospheric Airborn Chromatograph for Oxy-hydrocarbons and Hydrocarbons)
2 channel Sampling System Description
1)Sample acquisition line. Heated (50C) teflon 1/4 inch line fed through rear facing SS  tube extended approximately 30 cm. outside the aircraft. Sample line extended only after takeoff and withdrawn prior to landing for each flight.
2)Sample pump. Internal to the analytical system, the sample lines are pressurized 2 minutes prior to each sample with a KNF Neuberger diaphragm pump (SS body, Teflon coated neoprene diaphragm) to approximately 7 psi above the plane's cabin pressure using a back pressure regulator. The flow capacity of the pump in this pressurized mode varies from ~8 SLPM at sea level to ~2 SLPM at 7.3 Km pressure altitude.
3)Sample capture. Two  samples are captured simultaneously in separate 1 L containers for subsequent loading into two parallel analytical streams; one in a SS can for loading into the first channel using an Al2O3 analytical column and one in a Teflon bottle for loading into the second channel using a DB-624 analytical column. The sample pump capacity implies that the residence time of the air being flushed through these two containers in parallel prior to sample capture varies from approximately 18 sec. at sea level to about 1 min. at 7.3 Km. The volume and pressure in the two containers provides for sufficient sample to load 350 cc STP into each sample processing stream.
4)Sample preparation and loading. Each separate sample stream is passed through a 20 cm. section of 1/4 inch glass tubing filled with approximately 15 cm of Ascarite to remove CO2. The Ascarite is held in place with glass wool plugs at each end. Next, each sample stream passes through a 20 cm section of Teflon tubing held at -50 C that removes much of the water by diffusion to the walls. Finally each sample stream passes through a 20 cm section of quartz capillary tubing held at -165 C to cyrogenically accumulate all remaining condensables with the exception of methane. Each sample stream flow is controlled at 70 cc/min STP by separate mass flow controllers located down stream of the complete sample preparation and cryogenic regions. A 5 min. loading time provides a 350 cc STP sample for analysis in each channel.
5)Between-sample operations. Immediately after each 5 min sample loading period both water traps are heated to 50 C and they and the Ascarite CO2 traps are back flushed with "zero" air for purging in preparation for the next sample acquisition cycle. At the same time, the sample back pressure regulator is bypassed lowering the pressure in the sample capture containers and all sample flow lines to near aircraft cabin pressure and thus considerably increasing the flow of air through the sample pump and sample capture containers above the 8 to 2 SLPM stated above. This allows a more rapid equilibration of the two sample capture containers with the air sample being forced through them and the purging of any liquid water that might have been accumulated during the last pressurization cycle under high humidity conditions.
6)Cryogenic sample traps. The cryogenic sample trap for the channel using the Al2O3 analytical column is a section of the same column material; an approximately 20 cm section of 0.53 mm inside diameter AL2O3 wall coated quartz capillary. The cryogenic sample trap for the channel using the DB-624 analytical column is an approximately 20 cm section of 0.32 mm inside diameter passivated quartz capillary. The two sample traps are suspended in separate holes in a copper block maintained at less than -175 C by high thermal conductivity contact with a liquid nitrogen reservior. Prior to sample acquisition, each sample trap is allowed to cool down to  -165 C through the loose thermal contact to the cold head provided by the air gap between the sample trap and the cold head (~3 min) and held at that temperature during sample acquisition by electrical heaters wound on each sample trap. At the end of each sample acquisition cycle, both traps are rapidly (~6 sec) heated to 100 C and the samples are transfered under purified hydrogen flow to separate "cold spots" at the head of each analytical column.
7)Cold spots. A short (~5 cm) section at the inlet end of each analytical column is passed through a separate cold head in good thermal contact with the same liquid nitrogen reservior used for the cryogenic traps described above. Prior to sample transfer, these two "cold spots" are allowed to cool down to the cold head temperature of less than -175 C. After the two separate cryogenically accumulated samples have been transferred to the "cold spots" (~1.5 min), the "cold spot" at the head of the Al2O3 column is flash heated to 100 C in approximately 200 msec to start the analytical processing of that sample. The second "cold spot" is kept cold to immobilize that sample until after the first sample analysis is completed. The flame ionization detector (FID) is then switched from the Al2O3 column exit end to the DB-624 column exit end and the second "cold spot" is flash heated to 100 C (~200 msec.) to start its analytical processing. This allows the same FID to be used for both analyses which are combined together in a single two part chromatogram.
8) Analytical columns. The Al2O3 column is temperature programmed from 50 C to 175 C at 60 C/min and  provides for the quantitation of C2->C5 alkanes,C2->C4 alkenes , acetylene(ethyne) and propyne. The fast chromatography provided by the 200 msec wide injection peaks allows these compounds to be all baseline resolved in about120 sec. Peaks eluting later than n-pentane are backflushed out of the column prior to the next analysis.
   The DB-624 column is temperature programmed from 35 C to 125 C at 30 C/min and provides for the redundant quantitation of the C4 and C5 alkanes, the C6 through C8 alkanes, C1->C4 alcohols, C2 and C3 aldehydes, C3->C5 ketones,isoprene, benzene and toluene in about 150 sec. Peaks elution later than n-octane are backflushed out of the column prior to the next analysis.The combined chromatogram is, thus, approximately 4.5 min long.
9) Cycle time. The combined time required for primary sample acquisition (5 min), sample transfer to the "cold spots" (~2 min), sample analysis (4.5 min), column back flush and all temperature equilibration cycles (cryogenic sample traps, "cold spots" and analytical columns) allows a system cycle time of 15 min to be achieved. That is, a sample acquisition and chromatographic analysis cycle can be completed each 15 min. Some of the operations (column back flush, column temperature equilibration and "cold spot" cool down) are overlapped with primary sample acquisition to achieve this cycle time.
10)System control and data storage. An onboard lap-top computer provides the timing  control for all the system operating functions. Column temperatures are controlled by separate precision temperature controllers. Completed chromatograms are stored onboard in the computer and are further processed post flight for species identification and quantitation.
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