Figure 1. This gas-phase, dual chamber
is one of four chamber facilities at the UNC Ambient Air Research Facility,
and it is located in rural Chatham County about 32 kilometers south of UNC. This
chamber has been in continuous operation since 1971 (two rebuilds).
(click photo for bigger view)
(People in photo)
The UNC Ambient Air Research Facility or "site" is off-campus, approximately 32 kilometers from the University of North Carolina at Chapel Hill (see map). It is in the adjacent county, Chatham County, and is approximately 10 kilometers from the small town of Pittsboro in an isolated rural area (latitude 35.7083 N, longitude 79.1085 W). Chatham County is one of the most rural, least industrialized counties in North Carolina and is heavily wooded. These conditions contribute to low background in the air used to fill the chambers. The background concentrations of NOx and nonmethane hydrocarbons at this site are usually less than 5 ppb and less than 80 ppbC. More importantly, the air exhibits very low reactivity in the chamber.
The chamber surfaces are Fluorinated Ethylene Propylene (FEP) Teflon. The filmís transmission in the UV and visible regions of the solar spectrum is excellent, and it has only a few absorption bands in the IR, a property necessary to reduce the ìgreenhouse effectî inside the chamber. It has a very low permeability for most chemical species and can be heat-sealed to form large durable panels. For this application its worst property is its ability to hold a static charge for long periods of time. Type A film, 0.13 mm thick is used.
Inlet and outlet doors, stirring fans, manifolds and other fittings come in through a solid floor. The sides are free for light entry. The floor of the chamber is elevated approximately 1.2 m to allow for easy access under the chamber. The design is an A-frame 9.14 m wide, 12.10 m long and 6.10 m high at the peak on a plywood floor 1.22 m above the ground. Wooden beams, 5.08 cm by 20.32 cm, located on edge at 99.1 cm centers form an exterior framework on top of the plywood floor. Continuous 16.46 m lengths of film are attached to the inside by aluminum u-channels, screwed firmly to the wooden beams, thus compressing the film against the external support..
One piece, heat sealed Teflon film panels are used in such places as the triangular end panels and floors. The floor-to-side seals are achieved by a 0.61 m overlap of side panel film over floor panel film. A rubber strip under the film and an aluminum strip over the top of the film complete the seal. All other seals are Teflon-to-Teflon under pressure of the aluminum u channel. A single unsupported heat sealed Teflon panel similar to the end panels is used to separate the chamber into two halves of equal volume. It is sealed to the floor and side panels in the same manner as described above. Aluminum foil is placed under the film on the floor to reflect the light and heat back up through the chamber. This is necessary to reduce solar heating of the air to a value that is within normal urban environments and to compensate for transmission losses through the Teflon film. The chamber was rebuilt in 1994-95.
Photos from the reconstruction process.The chamber is oriented with its long axis approximately north to south. The actual long axis orientation is along a 27 degree true heading. The orientation with respect to the sunrise and sunset at different times of the year is illustrated in Figure 2.
There are four air handling systems in each half
of the chamber, one for exhausting, one for sampling, one for recalculating
through dehumidifiers, and one for mixing. The exhaustÝ system consists of two
intake stacks, 0.61 m x 0.91 m intake doors, 0.61 m x 0.61 m exhaust doors,
and an exhaust blower. See Figure 3.
The exhaust
blower is a dual blower on a single shaft driven by a 1.5 horsepower motor.
Air enters the system through the two 5.49 m high by 30.5 cm diameter stacks.
This system is designed to permit rapid exhausting of chamber contents and replacement
with ambient air. The filling rate is 7190 l/min. The chamber can be flushed
with a 99.6% decay of initial contents in 2 hours. The second air handling or
manifold system is for sampling and injection of pollutant materials into the
chamber halves. To insure representative sampling, a 3.17 cm I.D. glass manifold
more about sampling...
runs from a point 1.83 m above the floor in the center of each chamber half
down through the floor and over to a sampling laboratory. The sampling volume
required by all the instruments does not exceed 5 lpm but to reduce losses due
to long resident times it is necessary to have high flows in the manifold. The
flow rate in the manifold is 60 lpm. The manifold system is wrapped with a controlled
heating tape to maintain the sample slightly warmer than the chamber temperature.
To avoid the necessity of makeup air, the sampling manifold is a closed loop.
Squirrel cage blowers with housings and fans that are Teflon coated are used
to circulate air through the manifolds. The unused sampler air is then returned
through a 3.17 cm I.D. glass manifold to the chamber. These return manifolds
provide a convenient method for injecting the initial reactants. There is also
a special heated Teflon manifold for formaldehyde sampling.
Inside each chamber half are two mixing fans located in opposite corners. TheseÝ provide circulation and mixing of the chamber contents. The fans are 50.8 cm diameter cast aluminum units that are FEP Teflon coated. They operate in a horizontal position, 0.76 m above the floor on 2.54 cm diameter Teflon coated steel shafts that extend through the chamber floor. Under the floor, 1/4 horsepower, 1750 rpm motors provide power through a belt and pulley system to each fan. Each fan operates at approximately 31.15 m 3 /min.
The laboratory is adjacent to the chamber. It is
a 3.66 m W by 15.24 m L by 3.05 m H temperature-controlled wooden structure
oriented perpendicular to the chamber and 3.66 m away from it to avoid any shadowing.
more about
lab set-up...
The first 5.49 m nearest the chamber contain the instrumentation, manifolds,
and calibration systems. At the sample inlet of each gas instrument is a three
way Teflon AC solenoid valve. Since there are two intake manifolds (one for
each chamber half), air from either manifold can be drawn through the three-way
valve and into the instrument. In this manner timesharing of one instrument
between the two chamber halves is possible. The next 5.49 m contain the data
acquisition computer system (described in the Data Acquisition System section)
and the operations area. The last 4.27 m of the laboratory is a utility area
with running water and storage facilities. Gas tanks necessary to operate the
instruments and perform calibration are located in a 1.22 m x 1.83 m room completely
closed off from, but attached to, the laboratory. The injection system gas tanks
and valves are housed in a second 1.52 m x 1.52 m well-insulated heated room
adjacent to the end of the laboratory nearest the chamber.
Pollutants are injected into the chamber sides
via the return side of the sampling manifolds. The return manifolds enter the
chamber sides under one of the mixing fans. The injection process uses gas cylinders
more about injection system...
containing pollutants at high concentration (1000ñ10,000 ppm range), two-stage
stainless steel diaphragm regulators, on-off solenoid valves, and a precision
needle valve. The flow rate of injected material into the manifolds is established
by a mass flow meter with a 5 millisecond response time. The total injection
volume is accurately controlled as a function of the time the solenoid valve
is open. The open time of each solenoid valve is controlled by the computer
system on command. Conditions can be varied sufficiently to have the injection
time range from a few seconds for each component to 1ñ2 hours for a programmed
injection used to simulate the buildup of pollutants in urban areas.
A computer based data acquisition and control system (DAS) is used to acquire, process and record data for the chamber instrument system.
Output signals from each instrument are wired to a Hewlett Packard 3497A Data Acquisition System. The HP3497 has 48 solid state switches that form a micro volt-to-volt level signal multiplexer to connect any of the inputs to the built-in 5ñ1/2 digit digital voltmeter, which has a maximum sensitivity range of 0.1 volt. In addition, the HP3497 has a internal microprocessor control unit, data memory, time-of-day clock, and communications interface. The HP3497A is connected by a serial RS232 communications line to an Intel PC microcomputer equipped with 640 kilobytes of memory and a 100-megabyte fixed disk.
Under control of programs in the PC computer, the HP3497 is programmed to select the range of input channels to scan, store, and transmit. Whenever the PC signals the HP3497 that it is time to take data, the HP3497 scanner connects the selected signal leads to the input of the digital voltmeter. The digital voltmeter, which has excellent noise and spurious signal suppression and can measure a 0.1 volt signal with a resolution of 1 micro volts, is triggered to acquire a reading and to store the 5ñ1/2 digit ASCII coded decimal number in the HP3497ís memory. When all channels have been scanned, the HP3497 sends the entire data set to the PC computer. The PC computer processes the information and then stores the data in a designated file on the computer's fixed disk. The PCís programs provide the operator with data listings and graphs. The PC is equipped with a battery-back-up system that, in case of a power failure, allows time for the PCís volatile RAM memory contents to be written to the disk and for an orderly shutdown. When power is restored, the boot program on the PC restores the PCís RAM memory from the disk and operations continue as if nothing happened.
Also connected to the IBM PC are an A-bus control interface system and a single-board Zilog Z8 microcomputer, programmed as a master timer for time-of-day, 4-minute Red/Blue side clock, and instrument timer. The A-bus system is connected to AC solid state switches, to reed relays, and TTL outputs. The timing of the data acquisition and control processes is under the control of a battery backed-up single-board Z8 microcomputer with a Read Only Memory program that implements a digital date and time-of-day clock which signals event times to the computer. this Z8 microcomputer also provides control signals to operate the Teflon solenoid valve on the inlet of each instrument to connect the instruments to one chamber half during a given cycle of data acquisition. This multicomputer system provides fully automatic acquisitions and processing of data during a run and provides the operator with immediate, full data in physical units. Given this information about what is happening, the operator can then concentrate on what he wants to do. It also allows the massive amount of information generated during a run to be processed in a more effective manner after the run is over. All instruments are also connected to strip chart recorders. For the chromatographic systems, the recorders are the primary means of data collection. These data are later entered into the computer by use of a high-resolution (0.001 inches) digitizer pad, with which an operator can use a stylus or puck to ìpickî the coordinates of the peaks. The digitizer hardware sends the coordinate data to a computer where it is processed into time profile data.
