Final Report: Measurements of Non-Methane Volatile Organic Compounds in the Lower Troposphere From Tethered Balloon and Kite Sampling Platforms by Internal Standard Calibration Using Ambient CFC Reference CompoundsEPA Grant Number: R825417
Title: Measurements of Non-Methane Volatile Organic Compounds in the Lower Troposphere From Tethered Balloon and Kite Sampling Platforms by Internal Standard Calibration Using Ambient CFC Reference Compounds
Investigators: Helmig, Detlev , Balsley, Ben , Birks, John , Karbiwnyk, Christine , Mills, Craig
Institution: University of Colorado at Boulder
EPA Project Officer: Hunt, Sherri
Project Period: October 1, 1996 through September 30, 1999
Project Amount: $436,172
RFA: Analytical and Monitoring Methods (1996) RFA Text | Recipients Lists
Research Category: Environmental Statistics , Air Quality and Air Toxics , Water , Land and Waste Management , Air , Ecological Indicators/Assessment/Restoration
Objective:Ozone is a powerful oxidant and as such can cause respiratory distress in individuals exposed to excessive amounts . The U.S. Environmental Protection Agency (EPA) has set the minimum exposure level at 0.08 ppm ozone over an 8-hour period. Tropospheric ozone is formed when hydrocarbons react with NO on sunny days . Non-methane hydrocarbon emissions originate from both anthropogenic sources (propane, toluene, benzene, etc.) and biogenic sources (isoprene, monoterpenes, etc.) and are collectively referred to as volatile organic compounds (VOCs). To minimize the formation of ozone (a secondary pollutant), efforts are made to control the emission of the primary pollutants responsible for its formation. Due to their high reactivity with atmospheric oxidant species, accurate and precise measurements of VOCs are key to our understanding of tropospheric photochemistry [1, 3-6].
Vertical profiles through the boundary layer can reveal inversion layers and also whether emissions originate close to the site or were transported from a source upwind of the location. Vertical profile data also can be used to determine landscape-scale emission and deposition rates of atmospheric gases . Vertical profile measurements can be obtained from airborne platforms such as small aircraft, balloons, kites, and remotely piloted vehicles [8-9]. Solid adsorbent sampling tubes are utilized for VOC sampling because they allow small, light sampling packages to be developed for airborne platforms. Solid adsorbent sampling tubes also offer selectivity for compounds of interest based on the chosen adsorbent material (molecular sieve, TenaxTM, activated carbon, etc.). However, unlike during canister sampling, the volume of air sampled through the solid adsorbent tube must be measured precisely. Pump speed irregularities usually are regulated with a mass flow controller (MFC). The sample volume is then determined by the flow rate and the time over which the sample was collected. Unfortunately, MFCs have considerable weight and often are not amenable to vertical profile packages. There are small commercial pumps available that are accurately calibrated for flow rate and will even record the volume sampled. However, changes in temperature, pressure, and humidity, as would be encountered during a vertical profile, can cause volume errors to be as high as ~20?30 percent.
To accurately measure the sample volume without adding any bulk or weight to the sampling package, it was proposed that ambient chlorofluorocarbons (CFCs) could serve as internal standard compounds. The combination of CFC-use phase-out (Montreal Protocol) and their long atmospheric lifetimes (~50-100 years) has resulted in nearly steady atmospheric concentrations on both spatial and temporal scales . Because the CFCs are part of the same sample of VOCs collected and analyzed, any sample volume irregularities would be accounted for by the CFCs in the sample. The sample volume then can be determined by the amount of CFC detected in that air sample.
The main goal of this project was to develop and characterize an analytical
method for the analysis of atmospheric VOCs using atmospheric CFCs as internal
standard compounds. This technique will allow the development of low-cost,
light-weight, and battery-powered sampling packages to analyze VOCs in
atmospheric samples from airborne sampling platforms such as tethered balloons
and light aircraft. Along the way to achieving these goals, solid adsorbent
sampling methods were investigated for their reproducibility in the analysis of
selected CFCs and VOCs.
Summary/Accomplishments (Outputs/Outcomes):Materials and Equipment. The following pure gases were purchased from Airgas Houston, Houston, TX: He (99.999%), H2 (99.999%), N2 (99.999%); air: ultra zero grade (THC < 0.1 ppm, CO < 1 ppm, CO2 < 1 ppm, H2O < 5 ppm); make-up gas: 5 percent methane (99.97%), 95 percent Ar (99.999%). The ambient air test standard was collected with an oil-less compressor on June 6, 1996, between 5:30 and 6:30 p.m. in Boulder, CO. Niwot Ridge ambient background air test standard was collected with an oil-less compressor on March 6, 1998, at Niwot Ridge. CO and CFC concentrations were determined by the Climate Monitoring and Diagnostic Laboratory (CMDL) of the National Oceanic and Atmospheric Administration (NOAA). Neohexane certified standard was provided by Scott Specialty Gases and verified by CMDL/NOAA. A DryCal (Bios International) flowmeter (NIST certified flow standard) was used as well as adsorbent tubes, AirToxicsTM, purchased from Supelco, Bellefonte, PA. The tubes are stainless steel, 88.9 mm in length with a 4.8 mm inner diameter. SupeltexTM ?2A Vespel ferrules are used with Swagelok fittings to seal the tubes. Sample analysis was performed using a Perkin Elmer automated thermodesorber (ATD-400) interfaced with a Perkin Elmer Autosystem Gas Chromatograph with FID and ECD. Vertical profile samples were collected from a Cesna airplane launched from the Boulder airport.
Sample Collection Parameters. These AirToxics adsorbent tubes contain a 35 mm bed of CarbopackTM B plus 10 mm of a proprietary molecular sieve separated by glass wool. Tubes were conditioned by purging with ~120 ml min-1 UHP N2 in the desorb direction at 350?C. They were then capped with Swagelock fittings and Supeltex ferrules and stored in glass Mason jars until use. The sampling tubes were capped with O-ring sealed Teflon caps for desorption on the ATD-400.
Laboratory standard samples and vertical profile samples from the airplane were collected using a custom-built sampler that maintains a constant flow rate by a mass flow controller (Tylan). The sampler accommodates 10 sampling tubes that can be temperature-controlled during sampling. All tubing upstream of the sampling tube is silica lined stainless steel (SilcosteelTM, Restek). Electrically actuated valves (4-port stream selection valve and 10-port sampling valve, VICI) control the sampling stream. The sampler can be controlled manually or the valves can be timed and switched by a computer running a BASIC program. Samples and blanks were typically collected at 500 ml min-1 for 10 minutes.
On the plane, samples were collected through a 1/8-inch silico-steel sample intake line positioned at the front edge of the left wing. A sodium thiosulfate-coated glass fiber ozone filter  was placed in the sampling stream prior to the sampler. Lead acid batteries (12V) were used to power the sampler through a power converter. Pressure, temperature, and relative humidity were measured simultaneously via a RS-80 radio sonde (Vaisala). Volumes were corrected for pressure and temperature deviations to standard conditions (760 torr, 25?C).
Samples collected from our tethered balloon platform required light sampling packages positioned at various heights along the tether line. A portable, battery-powered and flow-controlled personal sampling pump (?Genie', Buck Inc., Orlando, FL) was used to pull sample air first through an ozone filter, and then through the sampling tube. After passing through the pump the air was collected into a Teflon sampling bag. The bag/sample volume could then be determined by an independent method on the ground. The Genie pump utilizes a differential pressure sensor, an RPM sensor, temperature sensor, and flow sensor to calculate the sample volume collected. The pumps also may be programmed to turn on and off at specified times. This method of sample collection also allows for simultaneous sample collection by turning all the pumps on and off at the same time.
Sample Analysis. Samples undergo desorption on the ATD-400 at 300?C for 15 minutes in a He flow of 25 ml min-1. The He was passed through oxygen traps and a hydrocarbon trap. Compounds are focused onto a cold trap (AirToxics) at -25?C. When the GC oven was ready (0?C), the cold trap was quickly heated to 325?C for 5 minutes. The sample was then transferred to the GC column through a 0.53 mm ID deactivated fused silica transfer line maintained at 150?C. Analytes were separated on a 30 m x 0.32 mm 5 mm film DB-1 column (J & W Scientific). Liquid nitrogen was used for sub-ambient temperature programming (0?C for 5 minutes, then ramped at 6?C per minute to 180?C). A second ramp of 30?C per minute to 250?C is used to bake out the column. The oven was held at 250?C for 5 minutes before being cooled to 0?C for the next sample. Total sequence time was 42.33 minutes. The end of the GC column was split into two flows by a glass Y connector. A deactivated fused silica capillary (0.18 mm internal diameter, 100 cm in length) directs 5 percent of the effluent to the ECD. Another deactivated fused silica capillary (0.32 mm x 53 cm), directs 95 percent of the effluent to the FID. The ECD make-up gas flow was 40 ml min-1. The ECD temperature was 375?C and the FID was operated at 300?C with flows of 350 ml min-1 air and 45 ml min-1 H2.
CFC Reference Compound Method. Two methods were developed that utilize CFCs as internal standard compounds for VOC quantification:
1. The first method used the CFC/ECD chromatogram peak area to determine the sample volume. The 10-port sampler was used to collect samples with varying volumes (2-6 L) of the Niwot Ridge Air Standard. The CFC/ECD peak areas were then plotted against the known sample volumes. This calibration curve then was used to determine the sample volume from the CFC peak area in any sample. This method has the advantage that no calibrated CFC gas standards are needed to establish a calibration function.
2. The second method used individual CFC response factors to eliminate sample
volume from the VOC mixing ratio calculation. Calibrated CFC and VOC standards
were needed to establish a calibration function. The peak area of the VOC is a
function of the FID carbon response factor, the effective carbon number of the
VOC, the VOC mixing ratio and the sample volume. Similarly, the peak area of the
CFC is a function of the specific CFC/ECD response factor, the CFC mixing ratio
and the sample volume:
PAVOC = RFFID * #C * [VOC] * vol
PACFC = RFCFC,i * [CFC] * vol,
where PAVOC = peak area of the VOC; RFFID = response factor of the FID; #C = effective carbon number of the VOC; [VOC] = mixing ratio of the VOC (ppb); vol = sample volume (L); PACFC = peak area of the CFC; RFCFC,i = response factor of the CFCi on the ECD; [CFC] = mixing ratio of the CFC (ppb).
Response factors for both detectors and for each CFC were determined with calibration mixtures. Because the sample volume is the same for both equations, the ratio of VOC peak area over CFC peak area cancels the volume variable. Thus, sample volume, the largest source of error for vertical profile samples collected onto solid adsorbent tubes, was eliminated from the VOC mixing ratio calculations:
PAVOC/PACFC = (RFFID * #C * [VOC] )/(RFCFC * [CFC])
and the VOC mixing ratio in a sample can be determined according to:
[VOC] = (PAVOC * RFCFC * [CFC])/(PACFC * RFFID * #C)
The Niwot Ridge Standard was used to develop a calibration of relative response factor ratios RFFID/RFCFC. The relationship between the ratios (slope of the calibration line) along with the equation constants (RFFID, RFCFC, #C, [CFC] from global mean value) and the measured values for PAVOC and PACFC were used to determine the VOC mixing ratio in a sample.
Results and Discussion. Analytical interferences stemming from high relative humidity (RH) were investigated. Water uptake reaches > 20 mg for 5 L samples collected at ~RH > 80 percent. It was found that the co-adsorption of water vapor in the tubes reduces the retention of light hydrocarbons (C3?C5). The adsorbed water also poses a severe interference during GC analysis, which makes it necessary to dry the tubes prior to thermal desorption. Failing to do so can result in an ice plug forming at the head of the GC column (0?C at start), noise interference in the ECD chromatograms, and/or loss of the FID flame. Sample tubes were weighed before and after sampling to determine the water uptake from ambient humidity. Sample tubes that gained more than 3 mg of water were placed on the 10-cartridge sampler for a dry purge. A 1-liter dry purge (100 ml/min UHP N2 for 10 minutes at 40?C) was found to be sufficient to remove ~20 mg of water from the sampling tubes.
Using the 10-cartridge sampler it was found that maintaining the cartridges at ~7?C above ambient temperature during sampling prevented the excessive uptake of water vapor even under high RH. This slight temperature increase did not compromise the VOC collection efficiency even for the lighter (C3?C5) hydrocarbons.
The method was extensively tested for the analysis of biogenic compounds and isoprene oxidation products (methacrolein, methyl vinyl ketone, a-pinene, b-pinene, and limonene). A compressed gas standard was prepared gravimetrically and calibration samples in the 100 ppt to 2 ppb range were generated by dynamic dilution. Regression coefficients for calibration lines were 0.998, 0.969, 0.891, 0.808, and 0.971, respectively, in the above listed compound order. These experiments demonstrate that this method is suitable for analysis of the isoprene oxidation products, but it also confirms the previously reported problems in the analysis of ?-pinene .
In an experiment for evaluating the best achievable precision with this relative calibration method, a 150 L Teflon bag was filled with ambient air from the CU campus. The bag was well mixed and a series of 10 adsorbent tubes was loaded with 5 L sample aliquots. Table 1 summarizes the relative standard deviations of the mean peak area ratios of VOC/CFC pairs.
Table 1. Relative standard deviation (in %) of the peak area ratio of VOCs (FID signal) to CFCs (ECD signal) in an ambient air sample.
These data demonstrate that relative standard deviations on the order of 1.5 to 3 percent can be achieved in many cases using this dual-detection method under controlled, laboratory-sampling conditions. Both of the above detailed calibration methods were tested in the field using vertical profile data. Vertical profiles were collected from the plane outside of Boulder, CO, utilizing the 10-port sampler. Vertical profiles from the balloon platform were collected during a field experiment at a site near Pellston, MI, utilizing the small sampling packages attached to the tether line. Our data showed that method (1) yielded greater errors compared to method (2). Using the volume-controlled data as reference, method (1) yielded deviations of 6.5 percent, 10.4 percent, and 29.9 percent (using the CFCs F-12, F-113, and CCl4 as reference compounds). In contrast, method (2) yielded errors of 0.97 percent, 0.4 percent, and 4.3 percent, respectively.
Standard deviations achievable during boundary profiling experiments will exceed the values in Table 1 because of the added temporal and special variations in ambient CFC and VOC mixing ratios and other factors, such as varying RH. However, from our own observations and from CFC ambient literature data, we conclude that the added errors are well within 1-2 x the range of the data in Table 1. Many sources that normally contribute to a deterioration of analytical precision and accuracy do not have an as-strong influence in the VOC/CFC method, because both VOC and CFC are impacted in a similar manner and a significant portion of the errors are thus corrected for.
Additional findings from this study have been reported in the publication/presentations cited below. Furthermore, two publications for submission to peer-reviewed journals currently are in preparation.
Quality Assurance. Commercial pre-loaded standard tubes (Supelco Inc., Bellefonte, PA) were used for both standard and ambient samples. Blanks were analyzed with every sample set to monitor the system background levels and any contamination that might be present. The system was routinely calibrated with standards and checked against previous calibrations. The sampling tubes also were evaluated for breakthrough volume at different flow rates and sample volumes. No breakthrough occurred for the compounds of interest at flowrates up to 1000 ml/minute and volumes up to 7.5 liters.
Conclusions. The most important factors affecting the precision and accuracy in the VOC/CFC relative standard method are: choice of reference CFC, VOC to be analyzed; VOC volatility; sample humidity/water management approach; and the adsorbent choice. The best results were obtained using a calibration method relying on response factors determined from calibrated CFC/VOC standards, representative CFC ambient air mixing ratio data, and using the CFCs F-12 and F-113 as reference compounds. These CFCs exhibit low background concentrations and high precision of analysis. CCl4 is another possible candidate compound, however, we experienced relatively high temporal changes in the ECD response factor.
Under typical ambient conditions encountered during profiling work in the boundary layer, analytical precision and accuracy on the order of 4-8 percent can be achieved for VOCs in the C5?C10 volatility range (down to C3?C5 for dry air with RH < 60 percent) using the approaches developed in this study. This is approximately a 2-3-fold improvement over previously used methods for quantitative analysis by accurate sampling volume control or measurement. Furthermore, lighter, less technical, and cheaper sampling equipment can be used in this method resulting in an overall significant advantage for profiling experiments.
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Journal Articles on this Report : 2 Displayed | Download in RIS Format
|Other project views:||All 8 publications||2 publications in selected types||All 2 journal articles|
||Karbiwnyk CM, Mills CS, Helmig D, Birks JW. Minimization of water vapor interference in the analysis of non-methane volatile organic compounds by solid adsorbent sampling. Journal of Chromatography A 2002;958(1-2):219-229.||
||Karbiwnyk CM, Mills CS, Helmig D, Birks JW. Use of chlorofluorocarbons as internal standards for the measurement of atmospheric non-methane volatile organic compounds sampled onto solid adsorbent cartridges. Environmental Science & Technology 2003;37(5):1002-1007.||