Customer

The Environmental Engineering and Science Program at the University of Illinois, Urbana-Champaign (UIUC).

Introduction

The effect of human activities on global climate has become a topic of major concern in recent years: scientifically, economically, and politically. The increase in anthropogenic (human-generated) emissions in the atmosphere has the potential to significantly alter the radiative energy balance of the earth. Results could include global warming (or cooling), with attendant effects on weather, sea levels, even food production.

Situation

Recent atmospheric studies have shown that the net radiative effect of aerosols (particles) in earth's atmosphere is roughly equal to, but opposite, that of greenhouse gases so that it promotes cooling. Whereas greenhouse gases trap infrared energy inside the earth's atmosphere and cause atmospheric warming, aerosols have been found to have a cooling effect due to backscatter of solar radiation.

Problem

The effect of aerosols has more uncertainty and variability than greenhouse gases, due to the balance between absorption (e.g., dark soot particles) and backscatter (e.g., fine particles of less than 1 um). Aerosols still remain the largest unknown in estimating human influence on global climate. Hence, more accurate measurements are needed over an extended period of time to understand their effects.

Implications

If aerosol effects cannot be accurately measured, they will remain an unknown cause in global climate change. They must be measured regularly, over an extended period of time, and at various locations. Without this information, scientists will be unable to create comprehensive atmospheric models or make accurate, long-range predictions of climate change.

Solution

In designing a system for measuring aerosol concentrations and their effects, the University of Illinois at Urbana-Champaign (UIUC) decided that it needed a standardized measurement platform that would allow accurate, global, unattended measurements.

After investigating alternatives, UIUC chose a Agilent Technologies B-size VXI system as the basis of its test stations. The systems are coupled with a variety of sensors, including humidographs, thermocouples, nephelometers (for measuring optical properties of aerosols), and impactors (for measuring particle size). Output from the sensors is fed through multiplexers and measured by a VXI multimeter, with data then converted to scientific units. For situations where control is required, such as valves and heaters, a VXI Form-C switch is used.

HP VEE was chosen as the development software because it is an easy-to-use, point-and-click environment for creating a test program, avoiding the labor of manual code. HP VEE s "initial settings" screen allows parameters to be changed without reprogramming, making it easy to do quick adjustments and fine-tuning to the system. HP VEE was also noted for its excellent data display capabilities.

Description of Test

Air is sampled through a complex path of measurements and conditioners (see diagram). After intake, air is passed through a particle counter, then along one of four paths: (1) a combined (1 and 10 um impactor, thence to a humidity-controlled nephelometer, (2) a 1 um impactor, thence to a sampler filter, (3) a 10 um impactor, thence to a continuous light-absorption photometer, and (4) a seven-stage impactor. Air is then vented to the outside. A spare line is reserved for intensive sampling.

Each of these internal paths is complex. Path (1), for example, contains a thermal air-drying chamber with a dedicated controller, a combined impactor, a low relative humidity reference nephelometer, a pre-heater and humidifier (both connected to a dedicated controller), a cooler, a dew point hygrometer/capacitive relative humidity sensor (both connected to a dedicated controller), a main PC, a VXI mainframe, a second relative humidity scanning nephelometer, and a mass flow controller.

The system can sample nearly 20 detectors and control relative humidity and temperature at five points in the humidograph at a rate of 0.5 Hertz.

Results

Long-term air sampling is not yet complete, so results are not available. However, one early project was to take air measurements at two unpolluted sites to create a "baseline" or reference: Cape Grim, Tasmania, a Southern Hemisphere clean marine site; and Sagres, Portugal, a Northern Hemisphere site which receives air masses from Europe and Africa. A total of 200,000 measurements was made of each parameter during a month-long period. With this baseline established, it will soon be possible to make comparisons with other measurements when they become available. Bondville, Illinois is also being sampled as a continental polluted site.

The use of HP VEE made data display a simple, intuitive task. Upgrading the system to add new measurement capability is simplified by the modular plug-in card architecture of the VXI mainframe.

Configuration

PC	486-based
VXI	E1301B B-Size VXI Mainframe, 9-Slot E1326B 5.5 Digit Multimeter, B-Size E1364A 16-Channel Form C Switch E1347A 16-Channel T/C Low-Offset Relay Multiplexer E1328A 4-Channel D/A Converter
Other instruments	Humidographs, nephelometers, impactors, dew-point hygrometer, photometer, flow meter, particle counter
Software	HP VEE