DESCRIPTION OF THE TEOM AMBIENT PARTICULATE MATTER

The TEOM ambient particulate mat- ter monitor consists of two major system components:the TEOM sensor unit and the TEOM control unit (Figure 1). The control unit contains the data processing hardware, display, flow control components, and control electronics for the system. The sensor unit contains the sample inlet and TEOM microbalance.

Figure 1 TEOM® ambient particulate monitor, with control unit on left and sensor unit on right.

At the heart of the system is the TEOM microbalance, which provides the sensitive particulate mass mea-

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111

surement capabilities. The filter and flow path for the microbaiance are shown in Figure 2. The feedback electronics maintains the tapered tube in oscillation. A precision frequency counter in the control unit measures the frequency of the oscillation accu- rately, and provides its output to a microporcessor for evaluation. The frequency of the system changes as , .

»-—-jBfflfllr?—»-

Figure 2 Schematic diagram of the TEOM ambient particulate monitor.

Mass Added TEOM Frequency f

-2

5m (mg) f (Hz) (sec

2

)

Figure 3: Relation between mass load and tapered element frequency.

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challenging the validity of this expres- sion. Setting

(12) (13) (14) (15) equation 11 can be rewritten in the linear form

y = a + bx (16)

If the TEOM system is not a harmonic oscillator, the linear rela- tionship as expressed in equation 16 will not hold. The data in Figure 3 are plotted in this form in Figure 4.

Figure 4: Validity of the harmonic approxima- tion for the TEOM sensor.

A linear relationship is apparent. How linear this relationship is can be assessed by a least square fit of the data through the correlation coeffi- cient, r. K

^o

results as the inverse slope, and m

^o

/K

^o

as the 'y' intercept. The results of the least square fit of the data in Figure 3.

r = 0.99999 K

^o

= 1747(g/sec

²

) m

^o

= 20.1 (mg).

There is absolutely no doubt that the harmonic oscillator approximation is perfectly adequate for the data eva- luation of a tapered element oscillat- ing microbalance. This evaluation is valid to mass ranges significantly in excess of the mass load for the TEOM abmient monitor. Additionally, although the system is calibrated with gravimetrically determined masses which are larger than the particulate mass to be measured, the calibration constant, K

o

, obtained is obviously valid for small masses, i.e. small frequency excursions, as well.

INSTRUMENT OPERATION

The TEOM ambient particulate mat- ter monitor determines the particle mass concentration in real time by continuously measuring the mass uptake on the collection filter and combining this measurement with the flow rate through the filter (or tapered element). The TEOM detector is capable of operating with flow rates from 0.5 to 5 1/min. The flow rate chosen depends upon the require- ments of the sampling head selected for a specific cut point and applica- tion. For indoor sampling, inlets designed by Marple, Spengler and Turner (MST) provide cut points of 2.5 and 10 µrn at a flow rate of 4 1/

min. For outdoor sampling, the Sierra-Andersen dichotomous PM-10 sampling head is used with a flow rate of 16.7 1/min. In this case, a flow splitter and second flow controller are added to the TEOM system to bypass 13.7 1/min and provide a 3.0 1/min flow through the TEOM filter, thus maintaining the flow within the operating range of the mass detector.

Accurate direct mass measurements on filters require filter and sample conditioning to standardised environ- mental conditions. In conventional manual sampling procedures with filters, weighings are conducted after conditioning both before and after sampling. Unfortunately the standard ambient particle sampling method does not require sampling at a stan- dardised temperature, allowing the filter and sample to change temper- ature with ambient conditions. In some situations this leads to an ill- defined treatment of volatile compo- nents, whose mass is also counted as part of the particulate matter sample.

Filter samples have been known to continue to change mass even after 24 hours of conditioning. In order to standardise both the sampling and weighing conditions in the TEOM ambient particulate matter system, the filter and the air stream passing through it are held at a constant 50° C.

Operating at this constant tempera- ture ensures that the filter is always above the dew point and maintains a low relative humidity, minimising water uptake for all ambient temper- atures. It also assures a secure temper- ature reference for both sampling and weighing, and in addition provides a standard condition for the treatment of volatiles. In this regard, the TEOM system is superior to the conventional filter sampling technique.

Since the TEOM monitor is a continuous sampling and measure-

ment system, differences in filter conditioning compared to the refer- ence samplers exist by necessity, as mentioned above. It is impossible to perform a 24-hour pre-conditioning and post-conditioning of the filter in a real-time device. As a result of these differences, one would expect a slight offset when comparing results from the TEOM monitor and the reference samplers. For PM-10 measurements, the internally-calculated mass concen- tration in µ g / m

³

is adjusted upward using the formula

y= 3 . 0 + 1.03 x

(17)

These values were determined empir- ically at a number of sites in the US and overseas. This adjustment is introduced because the reference technique results in a 'wetter' measure- ment than the TEOM system oper- ating at a constant 50°C.

The TEOM ambient particulate matter monitor provides a choice of averaging times from 10 minutes to 24 hours. Shorter averaging times can be used for measurements involving higher concentrations. Filter lifetime is determined by a pressure transducer that measures the pressure drop. To maintain the pre-set flow through the system, this pressure differential must remain within the range accommo- dated by the flow controller. Typical filter lifetimes are 2 to 4 weeks.

For PM-10 monitoring the instru- ment can be installed in either a weatherproof enclosure or a measure- ment station with the sample inlet poisitioned above the roof. Figure 5 shows a typical PM-10 installation.

INSTRUMENT RESULTS

The instrument has undergone field evaluations throughout the world in both indoor and outdoor monitoring applications. In addition, the TEOM ambient particulate matter monitor is a candidate instrument for EPA certification as an equivalent method for PM-10 sampling. Tests have been conducted at numerous sites to dem- onstrate the correlation between the TEOM continuous monitor and the EPA designated reference samplers.

The first set of field data shown in Figures 6 and 7 are from an EPA test site in Birmingham, Alabama, oper- ated through Acurex Corp in May 1989. Figure 6 shows a comparison between 24-hour averages from the TEOM m o n i t o r and the Sierra- Andersen d i c h o t o m o u s PM-10 sampler. The same type of comparison

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Figure 5: Typical PM-10 configuration

Figure 6: Comparison of 24-hour averages from TEOM monitor and Sierra-Andersen dkho- tomous PM-10 sampler in Birmingham, Ala- bama, in May 1989.

Figure 7: Comparison of 24-hour averages from TEOM monitor and Wedding high volume sampler in Birmingham, Alabama, in May 1989.

is shown in Figure 7 between the TEOM instrument and the Wedding high volume sampler. The correlation coefficients, slopes and intercepts that result from a regression analysis of the results are well within the EPA

requirements for certification as an equivalent method.

Testing in El Paso, Texas, per- formed by Acurex Corp between 3 TEOM monitors and 3 Wedding high volume samplers also shows excellent correlation, once again within the EPA standards for certification (Fig- ure 8). In addition, Figure 9 shows the results from all 3 TEOM monitors.

It is clear that the TEOM instruments show excellent agreement.

Figure 8: Comparison of 24-hour averages from 3 TEOM monitors and 3 Wedding high volume samplers in El Paso, Texas, in January 1990.

Figure 9: Plot of 24-hour averages from 3 TEOM monitors operated in El Paso, Texas, during January 1990.

Further testing by the Norwegian Institute for Air Research between the TEOM monitor and the Sierra- Andersen dichotomous PM-10 sampler once again demonstrates the equivalency of the TEOM instrument to the reference method (Figure 10).

Figure 10: Comparison of 24-hour averages from TEOM monitor and Sierra-Andersen dichotomous PM-10 sampler by the Norwegian Institute for Air Research in spring 1989.

The data presented above are 24- hour averages. The TEOM instru- ment, however, is designed as a real- time device. Data collected with shorter averaging times show the variability that occurs within daily periods. This is demonstrated in Figure 11, which shows 1-hour aver- ages collected by the National Swedish Environmental Protection Board in Studsvik, Sweden. This type of reso- lution capability can be useful in identifying the major contributing sources to ambient particulate levels.

Figure 11: 1-hour average concentration levels recorded by the National Swedish Environmen- tal Protection Board in May 1989.

A remarkable set of real-time data was gathered with TEOM instrumen- tation at the University of Hamburg, West Germany. The experimenters mounted the T E O M instrument, along with gas analysers, in a mea- surement van designed for mobile measurements. The data shown in Figure 12 represent an investigation of ambient air quality in tunnels under the Elbe River in Hamburg. Utilising a 30-second averaging time for the

Figure 12: Real-time particle and gas concen- tration data in Elbe River tunnels recorded by the University of Hamburg, West Germany, in a moving measurement van.

TEOM monitor, particulate matter levels are plotted along with gas concentration data. The shaded areas represent the time periods during which the measurement van travelled through the tunnels. During these periods, the particulate and gas concentrations clearly track. An interesting peak in the particulate level

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Figure 13: Real-time indoor particulate matter concentration values over a 6-day period in a light industrial assembly area.

occurs around minute 12 which does not have corresponding gas concen- tration peaks. This phenonemon is due to the fact that although the van was outside the tunnel, it was turning around under a superhighway bridge to make another pass through the tunnels. In this situation, particulate

matter from traffic on the superhigh- way produced a high particulate matter measurement, but since the location was outdoors, elevated gas concentrations did not appear.

A demonstration of indoor air measurements is shown in Figure 13.

A six-day period of data was obtained

in a light industrial assembly area. The

mass concentration peaks follow the

pattern of employee activity in the

sampling area. The first high peak that

occurred on day 3 corresponds to

soldering activity. The second peak

occurred after normal working hours

when a cleaning crew was performing

its duties in the building. The low

particulate matter levels on days 4 and

5 correspond to the weekend. It should

be noted that the TEOM ambient

particulate matter monitor has

received the EPA method designation

IP-10B for continuous indoor partic-

ulate matter monitoring.