September 11, 2007
The IMPROVE_A Temperature Protocol for Thermal/Optical Carbon Analysis: Maintaining Consistency With a Long-Term Database
By Chow, Judith C Watson, John G; Chen, L-W Antony; Chang, M C Oliver; Et al
ABSTRACT Thermally derived carbon fractions including organic carbon (OC) and elemental carbon (EC) have been reported for the U.S. Interagency Monitoring of PROtected Visual Environments (IMPROVE) network since 1987 and have been found useful in source apportionment studies and to evaluate quartz-fiber filter adsorption of organic vapors. The IMPROVE_A temperature protocol defines temperature plateaus for thermally derived carbon fractions of 140 [degrees]C for OC1, 280 [degrees]C for OC2, 480 [degrees]C for OC3, and 580 [degrees]C for OC4 in a helium (He) carrier gas and 580 [degrees]C for EC1, 740 [degrees]C for EC2, and 840 [degrees]C for EC3 in a 98% He/2% oxygen (O2) carrier gas. These temperatures differ from those used previously because new hardware used for the IMPROVE thermal/optical reflectance (IMPROVE_TOR) protocol better represents the sample temperature than did the old hardware. A newly developed temperature calibration method demonstrates that these temperatures better represent sample temperatures in the older units used to quantify IMPROVE carbon fractions from 1987 through 2004. Only the thermal fractions are affected by changes in temperature. The OC and EC by TOR are insensitive to the change in temperature protocol, and therefore the long-term consistency of the IMPROVE database is conserved. A method to detect small quantities of O2 in the pure He carrier gas shows that O2 levels above 100 ppmv also affect the comparability of thermal carbon fractions but have little effect on the IMPROVE_TOR split between OC and EC.(ProQuest: ... denotes formula omitted.)
The Interagency Monitoring of Protected Visual Environments (IMPROVE) network1 has acquired inhalable particulate matter (PM^sub 10^) and fine particulate matter (PM^sub 2.5^) mass,2 PM^sub 2.5^ elemental,3,4 ionic,5 and carbon,6-9 measurements at U.S. National Parks and Wilderness Areas since 1987.10 The sampling locations have been expanded to approximately 170 sites since 2000. IMPROVE measurements have been used to determine single source impacts on visibility in mandatory Class I areas,11-13 understand atmospheric processes,14-17 verify long-range transport and chemical conversion models,18-20 identify and quantify multiple source contributions using receptor models, 21-25 and track long-term trends in aerosol concentrations and regional haze.26-32 Chemical extinction budgets,33,34 constructed from measured PM^sub 2.5^ sulfate, nitrate, crustal, organic carbon (OC), and elemental carbon (EC) from 2000 through 2004 establish the baseline for tracking improvements toward natural conditions in 2065 required by the U.S. Regional Haze Rule.35
As a long-term trends network, IMPROVE has kept abreast of new findings in aerosol measurement technology and evaluated the consequences of new knowledge on its sampling, laboratory analysis, and data reporting methods. As hardware wears out and becomes obsolete, the effects of changes on the long-term database are evaluated, typically by redundant sampling and/or analysis with the old and new hardware.
Carbonaceous material that consists of OC, EC, and carbonate accounts for a substantial fraction of PM^sub 2.5^ mass in most atmospheric environments. EC is more thermally resistant and light absorbing than OC and they are often separated by thermal and optical methods. However, this separation is operationally, rather than fundamentally, defined. Watson et al.36 showed that different carbon methods report different EC abundances for the same samples by up to an order of magnitude. The IMPROVE network has adopted the thermal/optical reflectance (IMPROVE_TOR) method6 since its inception. The IMPROVE carbon analysis method has been adopted for aerosol studies in other countries (e.g., refs 37-39) and will be applied to samples from the U.S. Chemical Speciation Network (CSN, including the Speciation Trends Network [STN]) using modified IMPROVE carbon samplers (i.e., URG 3000N carbon sampler; URG Corp.) after 2007.40
Reported here are modifications to the IMPROVE_ TOR protocol that better reflect the actual, as opposed to measured, temperatures to which quartz-fiber filter samples are subjected during analysis for OC, EC, and thermal carbon fractions. This follows the development and application of a temperature calibration method41 that shows differences between the sample and sensor temperatures in the carbon analyzers used for IMPROVE, as well as other thermal/optical analysis hardware.
It is shown that the changes in the IMPROVE temperature program have no effect on the OC and EC concentrations taken since 1987 that are used for visibility extinction budgets. It is also demonstrated that several of the thermal carbon fractions used for source apportionment studies25 are sensitive to approximately 20 [degrees]C differences in their defining temperature plateaus, and that higher temperatures than those specified by Chow et al.6 are needed in a temperature-calibrated instrument to replicate those in an uncalibrated instrument. An alternate thermal protocol (IMPROVE_A) is defined that has been used for analyzing IMPROVE samples acquired after January 1, 2005. IMPROVE_A does not affect the OC and EC values, and post-2005 thermal carbon fraction measurements are compatible with pre-2005 measurements. The carbon measurement procedure described here may be applicable to many of the 20 or more carbon methods used worldwide that have participated in more than 40 published interlaboratory comparison studies.36
IMPROVE_TOR carbon analysis is based on the work of Johnson et al.42 as modified by Rau.43 A 0.5-cm2 circular segment is removed from a quartz-fiber filter aerosol sample obtained by drawing air through a PM^sub 2.5^ size-selective inlet. This segment is inserted into a heating zone where the temperature is increased in stepwise increments under nonoxidizing (100% He) and oxidizing (98% He/2% oxygen [O2]) atmospheres. Carbonaceous material in the sample is volatilized, pyrolyzed, and combusted to gasphase compounds that are converted to carbon dioxide (CO2) as they pass through an oxidizer (manganese dioxide [MnO2] at 912 [degrees]C) and reduced to methane (CH4) as they pass through a granulated firebrick supported nickel catalyst at approximately 420 [degrees]C. The CH4 is then quantified by a flame-ionization detector (FID).
A helium-neon (He-Ne) laser (633 nm, red light) is directed at the deposit side of the sample punch and reflectance (R) of this light is monitored throughout the analysis. During heating in the inert He atmosphere, some of the OC pyrolyzes (chars) to light- absorbing EC, as demonstrated by decreasing R. When O2 is added, this pyrolysis char combusts along with the original EC collected on the filter's surface. The amount of pyrolyzed OC (OP, for optical pyrolysis) is defined as the carbon measured after the introduction of O2 until R returns to its initial value at the commencement of analysis.
The nominal IMPROVE temperature plateaus in 100% He are 120 [degrees]C, 250 [degrees]C, 450 [degrees]C, and 550 [degrees]C, and the corresponding thermal carbon fractions are called OC1, OC2, OC3, and OC4, respectively. Temperature is ramped to the next step when the FID response returns to baseline or remains constant for more than 30 sec; the residence time at each plateau is longer for more heavily loaded samples. The nominal temperature plateaus in the 98% He/2% O2 atmosphere are 550 [degrees]C (EC1), 700 [degrees]C (EC2), and 800 [degrees]C (EC3). OC, EC, and total carbon (TC) are calculated from the eight carbon fractions as:
OC = OC1 + OC2 + OC3 + OC4 + OP (1)
EC = EC1 + EC2 + EC3 - OP (2)
TC = OC + EC (3)
Each carbon fraction reported to the IMPROVE networkdatabase consists of a value and a precision. The uncertainty (Unc) for each individual carbon fraction (e.g., C^sub i^ for sample i) can be defined as:
where CV is the coefficient of variation from replicate analysis and MDL is the minimum detection limit. At least 10% of the samples in each batch are submitted to replicate analysis for determining the average CV for that batch. MDL is separately derived from three times the standard deviation of at least 100 laboratory blank filter pre-fired analyses.6
Since 1987, the IMPROVE method was implemented on five Desert Research Institue/Oregon Graduate Center (DRI/OGC) TOR analyzers based on the design of Rau.43 Thermal/optical transmittance (TOT) carbon analyzers that use transmittance (T) to monitor OC charring44 are based on the design of Turpin.45 From January 1, 2005 forward, eight DRI Model 2001 thermal/optical carbon analyzers (Atmoslytic, Inc.)7,8 that measure R and T simultaneously are used for IMPROVE carbon analyses. In addition to the ability to implement both TOR and TOT protocols, the Model 2001 has more precise sample positioning, closer proximity of the temperature sensor to the sample, faster temperature response times, and more flexible data acquisition hardware and software.41
In the DRI/OGC analyzers, the sample punch is held 2-4 mm from the tip of the shielded temperature sensor that requires approximately 18 sec to equilibrate its response to a given temperature plateau. Because the samples are manually loaded, the distance between the sample and thermocouple as well as the precise location of the sample within the oven may vary from run to run. The temperature gradient across the sample oven was determined to range from approximately 20 [degrees]C/cm (at 1% power output) to approximately 50 [degrees]C/cm (at 75% power output; >600 [degrees]C). In Model 2001 analyses, the tip of the unshielded temperature sensor is approximately 1 mm under the edge of the filter punch with approximately a 1-sec time constant. The temperature sensor is always in the same location relative to the sample and the sample is positioned at the same location within the heating zone by an electric step-motor transport mechanism that ensures a repeatable sample position in the oven. The DRI/OGC and Model 2001 analyzers operate under positive pressure in the sample stream. However, the DRI/OGC analyzers contain several joints and connections sealed with pliable Teflon ferrules. The seal for the manual insertion rod experiences the most wear. The automatic sample loading system on the Model 2001 seals the opening with an O-ring under 20 psi of air pressure. Other joints and connections are sealed with high-temperature silicone rings suitable for vacuum systems. The Model 2001 contains a pressure meter that monitors the sample oven pressure continuously, facilitating routine leak checks. Even under positive pressure, the gradient between the He atmosphere inside and ambient O2 and nitrogen (N^sub 2^) outside may result in small amounts of outside air diffusing into the analysis atmosphere.
Sample temperatures are related to those measured by the temperature sensor using temperature-indicating materials (Tempilaq[degrees] G, Tempil Inc.).41 Tempilaq liquids contain chemicals that change their appearance at specified temperatures. A thin Tempilaq layer (25 [mu]L) is uniformly applied to a glass or quartz disk with a 0.1 mL Eppendorf Combitip (Brinkman Instruments Inc.) and covered with a sliced quartz-fiber filter punch. This temperature standard is inserted into the carbon analyzer and the temperature is slowly ramped (2 [degrees]C/min) across a 50 [degrees]C range spanning the specified Tempilaq melting point (mp) while R and T are monitored. When the specified temperature is reached, the appearance of the sample changes as evidenced by rapid changes in R and T. The maximum or minimum of the second derivative (change in the slope) of R or T, respectively, designates the inflection point that provides the best indication that the given temperature is attained. Only R is used to determine changes in appearance for the DRI/OGC analyzer. R and T measurements with the Model 2001 show equivalent calibration temperatures. Several tests are made with Tempilaq 121, 184, 253, 510, 704, and 816 [degrees]C indicators to obtain an average and standard deviation of the actual sample (target) and measured (thermocouple) temperatures.
Chow et al.41 applied this method to the five DRI/ OGC analyzers used for IMPROVE analyses. The thermocouple temperature at the R inflection point was lower than the rated temperature of Tempilaq[degrees] G for all of the temperatures and analyzers tested. For the Model 2001 units, the sample was less than 10 [degrees]C higher than the sensor reading at the lowest two temperatures (121 and 184 [degrees]C) and was 20-30 [degrees]C higher at the higher temperatures. The precision of the temperature from repeated measurement was 1-3 [degrees]C at the lower temperatures and 1-7 [degrees]C at the higher temperatures for the Model 2001. The temperature difference was more variable for the DRI/ OGC units.
This calibration generates a linear calibration curve for each analyzer:
T^sub sample(target)^ = b x T^sub thermocouple(measured)^ - a (5)
where variables a and b are the intercept and slope, respectively, and are determined from a least-squares minimization between the sample and sensor temperatures. Model 2001 software includes a temperature calibration mode that implements the slow ramping with Tempilaq standards, calculates the parameters in eq 5, and reconciles sample and sensor temperatures.
Figure 1 illustrates the temperature calibration for the five DRI/ OGC analyzers that were used for IMPROVE samples before 2005. Table 1 shows the correspondence between actual sample and thermocouple temperatures for the IMPROVE plateaus as determined from the Figure 1 calibration curve. This actual temperature program is referred to as the IMPROVE_A protocol, as defined in Table 1. These temperatures are rounded to the nearest 10 [degrees]C as this is within the uncertainty (+- 5 [degrees]C) of the calibration method.46 Samples can be analyzed by the Model 2001 using either the IMPROVE or IMPROVE_A protocol, with appropriate calibration factors for each instrument to ensure accurate sample temperatures.
The inert atmosphere within which the OC fractions evolve is achieved with ultrahigh purity He (UHP, 99.999%) as the carrier gas. An O2 scrubber removes trace O2 in the He stream, reaching an O2 mixing ratio of
The sample oven is exposed to ambient air when a sample is loaded. After loading, it is purged with the UHP He for at least 90 sec. Trace-level O2 may be present because of residual ambient air or ambient air diffusion through the oven or seals and joints. At high temperatures, O2 may increase the oxidation rate of OP and EC, thereby changing the thermal carbon fractions, but not the TOR OC/ EC split.8 To characterize the analysis atmosphere, a fraction (~10 mL/min) of the total flow is bypassed from the sample oven, upstream of the sample punch location, to a six-way Carle valve with a 1-mL storage loop. Once the storage loop is filled, the valve is manually switched to inject the sample air into a gas chromatograph/mass spectrometer (GC/MS), 5973N, Agilent Technologies). The GC separates different gases on the basis of their differential mobility through the capillary column. These gas molecules are electron ionized in the MS and detected by their mass-to-charge (m/z) ratios. An m/z ratio of 32 is the primary indicator for O2. The MS is calibrated with a standard gas of 100 ppmv O2 in He with an MDL of approximately 1 ppmv. The total amount of O2 in the 1 mL of sample air is then translated into the O2 mixing ratio in the oven on the basis of known purgeair pressure and temperature.
Figure 2 demonstrates the O2 test results for the two types of analyzers. For the Model 2001, the O2 mixing ratio in the sample oven is 15-27 ppmv, compared with 2-8 ppmv at the oven inlet. Although a small amount of O2 could diffuse into the transit lines connecting the UHP He cylinder and instruments, it is evident that most O2 does not enter the oven from these lines.
For the DRI/OGC analyzers, the O2 mixing ratio in the sample oven is an order of magnitude higher than that in the Model 2001 analyzers. It varies from 162 to 316 ppmv, with an average of approximately 240 ppmv. The N^sub 2^/O2 molar ratio is between 3 and 4, consistent with the composition of ambient air and indicative of ambient air diffusion into the system. This high and variable O2 level is attributed to the design of the DRI/OGC analyzer, which did not employ high-performance sealing technologies.
Remnants of 243 24-hr duration samples from the IMPROVE network that were originally analyzed by the DRI/ OGC analyzers and archived at less than 4 [degrees]C were reanalyzed on a temperature- calibrated Model 2001 with the IMPROVE_TOR temperature plateaus (column 2 of Table 1). These samples cover a wide range of locations, time periods, and source influences (see Chow et al.,46 for details). An additional 57 24-hr PM^sub 2.5^ samples acquired from the urban Fresno Supersite47 using large 20.32 x 25.40 cm quartz-fiber filters were also analyzed. The Fresno Supersite is affected by a variety of urban sources including gasoline and diesel exhaust, cooking, and wood burning,47-53 and therefore experiences a higher PM^sub 2.5^ concentration than most of the IMPROVE sites.
To evaluate the equivalence of data processing software, the raw FID and R data for these samples from the DRI/OGC analyzer were processed by both the DRI/OGC and Model 2001 software. The results were equivalent within less than 1% for OC, EC, TC, and eight thermal carbon fractions. The only difference is that the Model 2001 data processing reports negative OP when the reflected laser returns to its initial value before O2 is added to the carrier gas. The potential causes and implications of this "early split" will be discussed further. For the same situation, the DRI/OGC analyzer aligned the OC/EC split with the O2 introduction and reported zero OP.
Comparison test groupings and performance measures are summarized in Tables 2 and 3 for OC, EC, and TC measurements and in Table 4 for the carbon fractions. 46 Comparability measures are based on Mathai et al.54
OC, EC, and TC Measurements
For the 243 IMPROVE samples, averages of ratios and nonweighted linear regression slopes in Table 2 indicate an overall agreement within +-10%. The agreement tends to be better for lower loading samples (i.e., TC
IMPROVE samples (25-mm diameter) accommodate only three 0.5 cm2 circular punches, thereby limiting the number of replicate analyses. An additional 160 IMPROVE samples were analyzed by the Model 2001 following the IMPROVE_A protocol. The 57 Fresno filters were large enough to obtain additional punches for analysis. As shown in Table 2, OC and EC were also comparable for this revised temperature protocol. For the 57 Fresno samples, paired analyses did not yield differences exceeding +-3sigma. This also holds for the comparisons between Fresno OC, EC, and TC measurements using the Model 2001 with IMPROVE and IMPROVE_A protocols (Table 3). The OC, EC, and TC measurements are not sensitive to the difference between the DRI/ OGC and Model 2001 analyzers in the absence of early split. However, the more frequent occurrence of early splits for the DRI/OGC analyzer reflects the different temperatures and/or analysis atmospheres of the different systems. An early split occurred on 15- 20% of the IMPROVE samples using the DRI/OGC analyzers, thereby overestimating OC and underestimating EC. The original IMPROVE data from 2000 to 2004 has since been reprocessed to report OC and EC measurements including negative OP corrections (vista.cira.colostate.edu/views/).
Temperature-Resolved Carbon Fractions
The differences between the temperature plateaus, analysis atmosphere, and optical charring affect the eight thermal carbon fractions, even though OC and EC concentrations are the same. As shown in Table 4, the carbon fractions do not compare well between the DRI/OGC and Model 2001 analyzers following the same IMPROVE temperature protocol for the 243 IMPROVE samples. The comparability measures are mostly outside of acceptable ranges (e.g., 20% for fractions), but the measurements are well correlated. Although OC2 and OC4 are within 15-26% of each other, on average the Model 2001 reported approximately 55% lower OC3, approximately 60% higher OP, and approximately 40% higher EC1 than the DRI/OGC analyzers. Similar differences are found for the Fresno samples.
The IMPROVE_A temperature protocol improves the agreement in carbon fractions between the two analyzers. For the 160 IMPROVE samples, the IMPROVE_A protocol not only reproduced OC, EC, and TC measurements within 10% of the DRI/OGC results but also within 15% for OC3 (1.12), OC4 (1.09), and EC1 (0.95) (Table 4; numbers in parentheses are nonweighted zero-intercept slopes). The t test shows statistically significant differences mainly for low temperature OC (i.e., OC1 and OC2) and high temperature EC (i.e., EC3). The distribution of differences shows that 85% of the samples fall within +-2sigma, with the exception of OC2. Averages of ratios for OC1, OC2, and EC3 are sensitive to sampling artifacts (i.e., vapor adsorption),59 the presence of carbonate,60 or are near the MDL. The 57 Fresno samples (Table 4) also show better agreement for the IMPROVE_A protocol implemented on the Model 2001. The largest deviations are found for OP and EC1, consistent with the influence of oxidants increasing the OP and EC oxidation rates in the DRI/OGC analyzers. On average, the Fresno samples are approximately three times the IMPROVE samples in loading, which leads to a longer analysis time for each carbon fraction.
For lower loading samples, sample temperature differences between the two analyzers are the dominant cause of differences in carbon fractions. Differences are minimized by temperature calibration and implementation of the IMPROVE_A protocol on the Model 2001 analyzers.
Sensitivity of Carbon Fractions to Temperature and Analysis Atmosphere
Higher O2 levels in the pure He carrier gas can reduce OC charring and increase the EC oxidation rate. To assess these effects, multiple punches from the same Fresno sample were analyzed in the Model 2001 with the IMPROVE protocol while systematically varying the amount of O2 in the He carrier gas from 20 to approximately 900 ppmv for the OC fractions. For EC fractions, the O2 mixing ratio remained at 2%. This experiment was repeated for plus and minus 20 [degrees]C in the IMPROVE protocol (e.g., "IMPROVE+20" and "IMPROVE-20", respectively) to simulate the difference between the IMPROVE and IMPROVE_ A protocols.
Figure 3 shows the carbon fractions as percentages of TC. OC1 and OC2 are independent of the oxidant level, but they are sensitive to temperature. As the temperature decreases from 140 to 100 [degrees]C, OC1 is reduced from 11% of TC to a negligible amount. Meanwhile, OC2 increases from 19% to 27% of TC. The oxidation/ decomposition rate of carbonaceous material below 300 [degrees]C is so low that higher O2 levels do not increase the reaction rate. Appreciable changes in OC3, OP, and EC1 are not observed until O2 exceeds 100 ppmv in the He atmosphere. OC3 depends on both temperature and O2 levels being more than 100 ppmv, but OC4 appears relatively constant. OP and EC1 are more influenced by O2 levels greater than 100 ppmv than by temperature, but the opposite is true for EC2. EC3 was negligible in these experiments. EC remains the same regardless of temperature or O2 concentration. TOR optical correction compensates for the migration of carbon fractions. An early split is observed as O2 approaches 1000 ppmv (0.1%) for the IMPROVE and "IMPROVE +20 [degrees]C" protocols. A negative OP was reported in those situations.
The Model 2001 retains temperature plateaus within a few degrees Celsius, with negligible levels of O2 (much lower than 100 ppmv) in a pure He atmosphere. The O2 level in the DRI/OGC analyzers is higher and more variable, in the range where OC3, OP, and EC1 are sensitive to changes in temperature and carrier gas composition. With temperature calibration, the Model 2001 is expected to provide more reproducible carbon fraction measurements than the DRI/OGC analyzers.
The DRI/OGC analyzers used for IMPROVE carbon analysis since 1987 are obsolete: spare parts are no longer manufactured, and data acquisition and processing software is antiquated. The Model 2001 carbon analyzer allows negligible O2 diffusion from outside air into the sample heating zone during the pure He stage used to quantify OC fractions. It also allows more accurate and precise control and monitoring of the sample temperature. Procedures have been created to: (1) calibrate the sample temperature against the sensor (measured) temperature using melting-point standards, and (2) monitor the trace O2 levels in the carrier gas. These procedures have been part of the quality assurance for IMPROVE sample analysis.
The thermal carbon fractions OC1, OC2, and EC2 of the IMPROVE protocol are most affected by temperature differences of 20 [degrees]C. OC3, OP, and EC1 are most affected when O2 exceeds 100 ppmv in the pure He carrier gas. OC, EC, and TC by TOR are not sensitive to temperature protocols. Therefore the potential drift of analysis temperature and O2 in the DRI/OGC analyzers did not impact long-term trend analysis on the basis of IMPROVE OC, EC, and TC data.
Carbon analyzers yield comparable OC, EC, and TC using either the IMPROVE or IMPROVE_A temperature protocols with an optical reflectance correction for sample charring. They provide different, but highly correlated, results for thermally-resolved carbon fractions when using the IMPROVE temperature protocol as specified by Chow et al.6 The temperature calibration indicates appreciable underestimation of sample temperature in analyses performed by the DRI/OGC analyzer following the original IMPROVE temperature plateaus. IMPROVE_A temperature plateaus reflect the "actual" sample temperature. Implementing the IMPROVE_A protocol with the Model 2001 analyzer not only maintains the consistency in OC, EC, and TC measurements; it improves the interinstrumental agreement in thermal carbon fractions. Larger deviations (>20%) remain for low temperature OC fractions (i.e., OC1 and OC2) and high temperature EC fractions (i.e., EC3).
This work was sponsored in part by the National Park Service IMPROVE Carbon Analysis Contract No. C2350000894, the U.S. Environmental Protection Agency STAR Grant No. RD-83108601-0, and the Fresno Supersite U.S. Environmental Protection Agency Cooperative Agreement No. R-82805701. The conclusions are those of the authors and do not necessarily reflect the views of the sponsoring agencies. Any mention of commercially available products and supplies does not constitute an endorsement of these products and supplies.
Although different hardware is being used for IMPROVE sample analysis for 2005 and into the future, the OC and EC fractions measured by TOR are comparable to those obtained with the previous hardware. This prevents discontinuity of the long-term trends for ambient concentrations and regional haze chemical extinction tracking. The thermal carbon fractions are also comparable when the sample temperature plateaus from the older hardware are implemented on the new hardware using the IMPROVE_A temperature protocol. REFERENCES
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Judith C. Chow, John G. Watson, L.-W. Antony Chen, M.C. Oliver Chang, Norman F. Robinson, Dana Trimble, and Steven Kohl
Desert Research Institute, Reno, NV
About the Authors
Judith C. Chow is a research professor, John G. Watson is a research professor, L.-W. Antony Chen is an assistant research professor, M.-C. Oliver Chang is an assistant research professor, Norman F. Robinson is an associate research professor, Dana Trimble is an assistant research scientist, and Steven Kohl is an associate research scientist with the Desert Research Institute, Reno, NV. Oliver Chang is currently an air pollution specialist with the California Air Resources Board, Sacramento, CA. Please address correspondence to: Judith Chow, Desert Research Institute, Division of Atmospheric Sciences, 2215 Raggio Parkway, Reno, NV 89512; phone: +1-775-674-7050; fax: +1-775-674-7009; e-mail: [email protected]
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