ACATS-IV is a four-channel gas chromatograph that measures CCl3F (chlorofluorocarbon (CFC) -11), CCl2FCClF2 (CFC-113), CH3CCl3 (methyl chloroform), CCl4 (carbon tetrachloride), CH4 (methane), H2 (hydrogen) and CHCl3 (chloroform) every 140 s. N2O (nitrous oxide), CCl2F2 (CFC-12), CBrClF2 (halon-1211) and SF6 (sulfur hexafluoride) are measured every 70 s. ACATS-IV operates as follows.
All samples throughout a flight are injected at the same sample loop pressure. To achieve this, the loops prior to the injection of a sample are isolated from the pump (using solenoid valve (SV) 6, Figure 2) and allowed to depressurize through the exhaust, lightweight, proprietary pressure controller (PC1, Figure 2) to 87 kPa. All pressure controllers of ACATS-IV are identical (PC1 through 5, Figure 2), rely upon a pressure gradient across them, and will not function properly unless the exhaust pressure is sufficiently lower than the inlet
About 9 s after SV6 is closed, the desired pressure is reached in the sample loops and they are isolated from the exhaust using SV5 (Figure 2). Sample injection pressure (measured by the pressure controller) and temperature are recorded for later normalization of detector responses. Then, samples are injected onto each chromatography column using a twelve-port, two-position gas sampling valve (GSV, Valco model EWC12TGA).
Two packed chromatography columns are employed per channel: a pre-column and main column, selected to rapidly separate compounds of interest. Channel # 1 also employs a short, 0.15 m post-column, operating at higher temperature than pre- and main columns (Table 1, Figure 2) to separate N2O and SF6. Detectors and columns for each chromatography channel are enclosed in individual, thin-walled, insulated aluminum housings (ovens) and are heated using 50-100W electric heaters.Table 1 and channel configurations are shown in Table 2.
Foldback requires precise timing of GSV switches so that late-eluting molecules can pass completely through the pre-column before it is backflushed by switching a GSV into the 'load' position (Figure 2, GSV 1). Backflushing, the passing of carrier gas through pre-columns in the direction opposite to the normal flow, is necessary to expel from the pre-column all molecules eluting later than peaks of interest. The next sample can then be injected onto a clean pre-column. The fact that pre-columns are in use until the last eluting peaks are transferred into main columns, leaves a very short time (30 s) to backflush them. Sufficient backflushing is achieved if the volume of flushing carrier gas is at least 1.5 times the volume of carrier gas passed through a pre-column in the 'forward' direction. Therefore, backflush flow rates are maintained as high as 90-100 sccm. Although providing adequate pre-column cleansing, the high backflush flows create strong inverted pressure gradients in the pre-columns that hinder the 'forward' carrier gas flow through the pre-column / main column series when the GSV is switched into the 'inject' position. The resulting drop in the 'forward' flow through columns and ECD produces an increase of the baseline of the chromatogram, which can be detrimental to the detection of overlaying foldback chromatographic peaks. To accelerate the recovery of 'forward' flow rates, the 'forward' carrier gas flow at injection time is increased up to 200 sccm for 4-7 s, and then decreased back to normal in a series of steps. This carrier gas flow programming helps remove baseline aberrations that can affect the foldback peaks. Flow programming is performed using proprietary low-weight mass flow controllers controlled by the instrument computer.
The serial arrangement of sample loops (Figure 2) made it possible for P-5 carrier gas (pressurized at 620 kPa, compared to 150 kPa sample pressure) from the #1 pre-column to backstream into the #4 sample loop (which is used to measure methane), once the GSV #1 switches into the 'load' position. The 5% CH4 concentration in P-5 causes methane contamination of the sample on channel #4. To prevent this, a solenoid valve (SV7 on Figure 2) is used to isolate sample loops #1 and #2 from loops #3 and #4. The valve, SV7, is programmed to close shortly before the switching of GSV to 'load' on channel #1, and remains closed for 12 s until the P-5 carrier gas bleeds out of sample loops. After that SV7 is opened and all loops are filled and flushed with sample air.
ACATS-IV measures 84 x 49 x 33 cm, weighs 48 kg and is installed on the ER-2 aircraft next to the NOy instrument inside a common frame. The frame also supports a compressed gas cylinder rack with pressurized, 3 and 1.5-liter, AcuLife-IV (Scott Specialty Gases, Plumsteadville, PA) treated, Kevlar-reinforced, aluminum bottles that contain ACATS-IV and NOy carrier gases (Figure 1). An additional 1.5-liter external gas bottle was added to the underside of the ACATS-IV instrument in 2000 due to increased demand on the nitrogen carrier gas caused by faster chromatography. The entire ACATS-IV-NOy assembly is mounted to three support eyelets in the instrument bay (Q-bay) in the bottom forward section of the ER-2 aircraft fuselage (Figure 1 and Figure 2, inset).
The instrument operation is pre-programmed to allow automatic in-flight analyses of ambient air from the outside the aircraft, a standard gas and zero air from the gas rack. Instrument control and data acquisition are handled by an on-board Intel-486 based computer.
Further details on the operation, calibration, data processing and error analyses for ACATS-IVare available in the article "In Situ Measurements of Long-Lived Trace Gases in the Lower Stratosphere by Gas Chromatography" .