Measuring system for the vertical profile of the CO mixing ratio

Measurements of the vertical profiles of CO mixing ratio, temperature, humidity and wind began at the end of September 1994.

Mixing ratios of CO are measured at 10, 48, 82 and 115 m above the ground (Fig. ). Air is pumped through 9.5 mm (outer) diameter tubes to a CO analyzer located in the TV transmitter building. A 47 mm diameter particle filter is located at the inlet of each tube. The set-up for CO analysis is very similar to that described by Zhao et al. (1997a), which was used for the measurements reported in Bakwin et al. (1995, 1998). Diaphragm pumps are used to draw air continuously through each of the tubes from the four monitoring levels at a flow rate of about 2 l min. After the pump, the air at 40 kPa overpressure enters a glass trap for liquid water which is cooled in a regular household refrigerator, to dry the air to a dew point of 3-4C. Liquid water is forced out through an orifice at the bottom of each trap.

The four inlet tubes and the standard gases are connected to a computer controlled, 16-position valve, that selects which monitoring level or standard gas is sampled by the analyzer. The valve head is protected by 7 mm in-line filters. Ambient air flows continuously through the multiport valve so that the system is constantly flushed. The (expensive) standard gases are shut off when not in use by means of computer-controlled solenoid valves. The air leaving the multiport valve through its common outlet is further dried to a dew point of about -25C by passage through a 182 cm long Nafion drier, so that the water vapor interference and dilution effect are less than 0.1 ppm equivalent CO (Zhao et al., 1997a). The Nafion drier is purged in a counter-flow arrangement using waste sample air that has been further dried by passage through anhydrous CaSO.

Analysis for CO is carried out using an infrared gas analyzer (IRGA) (LI-COR model LI-6251). A constant sample flow rate of 100 cm min is maintained by a mass flow controller. The reference cell of the CO analyzer is continuously flushed at a flow rate of 5-10 cm min with a compressed reference gas of 330-340 ppm CO in air (Messer Hungarogáz). The analyzer is calibrated by four standards covering 330-420 ppm CO, that were prepared by NOAA/CMDL (Kitzis and Zhao, 1999).

The basic measuring cycle is two minutes, consisting of one minute flushing and one minute signal integration. Each one minute average and standard deviation is based on 6-7 measurements. The multiport valve steps through the four monitoring levels in eight minutes. Every 32 minutes, after four 8 minutes measuring cycles, the standard gas with the lowest CO mixing ratio is selected and analyzed for 2 minutes, and we term this measurement a zero. After every sixth cycle (every 202 minutes, that is 4x8 min sampling and 2 min zero five times, and one 4x8 min sampling without the zero) a full four-point calibration is carried out. The reference and sample cells of the CO analyzer are not pressure or temperature controlled. The zero measurements are used to account for any short-term drift of the analyzer due to changes in ambient pressure or temperature. A quadratic response function is fit to each set of calibration gas measurements. The zero offset and response function are linearly interpolated in time to obtain values appropriate to calculate CO mixing ratio from the instrument response.

The off-line postprocessing of the profile data consists of the calculation of the response functions for the CO analyzer and the conversion of the voltage data into physical units. If the change in the response function causes more than 2 ppm change between two consecutive calibrations, the data for the period is rejected. Such periods are rare, and almost always caused by significant change in room temperature. The usual change of the response function is below 0.3 ppm. It should be noted that the drift equally influences all monitoring levels, therefore the relative mixing ratio profile is correct even if the absolute accuracy is temporarily lower than usual. As this type of error is random, the long-term accuracy of the values is close to that of the standards (about 0.1 ppm, Zhao et al., 1997b).

For data acquisition and system control for the CO profiles and meteorological data we use a 286 PC with 1 MByte RAM and a 40 MByte hard disk. The analog signals of the CO analyzer and mass flow controller are read by a multiplexer-A/D converter (PCL-711B). The data acquisition and system control software is written in Turbo Pascal (Borland) and runs under MS DOS 5.0. During the data integration period the computer consecutively reads data from the meteorological sensors at each monitoring level through its serial port (RS232), then it reads the CO analyzer and the mass flow controller through the PCL-711B card. The profile system generates 5 MByte/month of data which is stored on a floppy diskette after compression. The data are mailed or carried to our laboratory in Budapest, where data corrections and data processing are performed.