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The Determination of Carbon Dioxide Concentration Using Atmospheric Pressure Ionization Mass Spectrometry/Isotopic Dilution and Errors in Concentration Measurements Caused by Dryers

^a Science Applications International Corporation, P.O. Box 68, Gunpowder Branch, Aberdeen Proving Ground, MD 21010-0068
^b Dep. of Chemistry, Drexel Univ., 3141 Chestnut St., Philadelphia, PA 19104

^* Corresponding author (brendan.delacy{at}us.army.mil).

Received for publication May 31, 2007.

	ABSTRACT

TOP NOTES ABSTRACT INTRODUCTION Materials and Methods Results Discussion Conclusions REFERENCES

 
An atmospheric pressure ionization mass spectrometry/isotopically labeled standard (APIMS/ILS) method has been developed for the determination of carbon dioxide (CO₂) concentration. Descriptions of the instrumental components, the ionization chemistry, and the statistics associated with the analytical method are provided. This method represents an alternative to the nondispersive infrared (NDIR) technique, which is currently used in the atmospheric community to determine atmospheric CO₂ concentrations. The APIMS/ILS and NDIR methods exhibit a decreased sensitivity for CO₂ in the presence of water vapor. Therefore, dryers such as a nafion dryer are used to remove water before detection. The APIMS/ILS method measures mixing ratios and demonstrates linearity and range in the presence or absence of a dryer. The NDIR technique, on the other hand, measures molar concentrations. The second half of this paper describes errors in molar concentration measurements that are caused by drying. An equation describing the errors was derived from the ideal gas law, the conservation of mass, and Dalton's Law. The purpose of this derivation was to quantify errors in the NDIR technique that are caused by drying. Laboratory experiments were conducted to verify the errors created solely by the dryer in CO₂ concentration measurements post-dryer. The laboratory experiments verified the theoretically predicted errors in the derived equations. There are numerous references in the literature that describe the use of a dryer in conjunction with the NDIR technique. However, these references do not address the errors that are caused by drying.

Abbreviations: APIMS, atmospheric pressure ionization mass spectrometry • APIMS/ILS, atmospheric pressure ionization mass spectrometry/isotopically labeled standard • NDIR, nondispersive infrared

	INTRODUCTION

TOP NOTES ABSTRACT INTRODUCTION Materials and Methods Results Discussion Conclusions REFERENCES

 
THIS paper describes a newly developed technique using atmospheric pressure ionization mass spectrometry and isotopic dilution (APIMS/ILS) for the determination of carbon dioxide (CO₂). Although the APIMS/ILS method is not new to the atmospheric community, the application of the APIMS/ILS method for the measurement of CO₂ is new. The APIMS/ILS method is a highly precise and accurate alternative to the nondispersive infrared (NDIR) technique, which is the most widely used method to measure CO₂.

The NDIR technique has been described extensively in the literature and has been used in the development of a worldwide integrated system of CO₂ measurements (Bakwin et al., 1998; Baldocchi et al., 2001; Ciais et al., 1995; Ciais et al., 1997; Conway et al., 1994; Denning et al., 1995; Fung et al., 1997; Helliker et al., 2004; Keeling et al., 1989). Many CO₂ measurements were made from tower and ground sites. A few CO₂ flux determinations have been made from aircraft observations (Chou et al., 2002). The NDIR method was also used extensively to study the air–sea CO₂ exchange (Dacey et al., 1999; Donelan and Drennan, 1995; McGillis et al., 2001a; McGillis et al., 2001b; McGillis et al., 2004; Ward et al., 2004; Zemmelink et al., 2004).

A brief description of the NDIR technique is provided herein, although more extensive descriptions of the instrument may be found in the literature. When using the NDIR technique, the concentration of CO₂ is determined by measuring its absorption at 4.26 µm. Quantitation is based on the differential absorption of infrared radiation between a reference cell and a sample cell. The reference cell contains a known CO₂ concentration, whereas the sample cell contains an unknown CO₂ concentration. Infrared radiation is alternately transmitted through each cell path. Differences in absorption between the two cells produce differences in heat and pressure. The difference in pressure is ultimately reflected in the movement of a flexible metal diaphragm between the two cells, which in turn relates to the concentration of CO₂ in the sample cell.

Interference from water vapor limits the reliability of the NDIR technique when making CO₂ concentration measurements. This is due to the formation of CO₂ hydrates, which decreases the amount of free CO₂ available for detection. Additionally, water vapor can produce interferences in the absorption bands of CO₂. In an attempt to circumvent this interference, dryers often are used to remove water vapor before analysis. Intuitively, drying should not cause a problem. However, it was serendipitously found that the errors caused by drying are quite large. Models of the NDIR sampling system and laboratory experiments confirmed differences between pre-dryer and post-dryer CO₂ concentration. There are numerous references in the literature that describe the use of a dryer in conjunction with the NDIR technique (Zhao et al., 1997; Bakwin et al., 1998). However, these references do not address the errors that are caused by drying.

	Materials and Methods

TOP NOTES ABSTRACT INTRODUCTION Materials and Methods Results Discussion Conclusions REFERENCES

 
APIMS/ILS Method
The atmospheric pressure ionization mass spectrometry/isotopically labeled standard method has been described previously by Bandy (Bandy et al., 2002). The instrumental design and operation has also been discussed extensively in the literature (DeLacy, 2006). A schematic of the APIMS is provided in Fig. 1 . A brief summary of the instrument is provided herein. The instrument may be broken down into three main sections. These include the source, the mass analyzer, and the detector. The source is where ionization of the gas molecules occurs. A detailed description of the ionization chemistry is provided in the subsequent paragraph. Depending on the molecular species, positive or negative ions may be produced. After ionization occurs, voltages are applied to a series of lenses that focus the beam of ions and accelerate them toward the mass analyzer. The mass analyzer, in the case of the APIMS instrument, is a quadrupole. The quadrupole consists of four cylindrical parallel rods that are spaced longitudinally. Rods that are diagonally opposite one another are connected together electrically to a variable DC source. One pair of rods is connected to a positive terminal, and the other pair is connected to a negative terminal. Additionally, variable AC potentials are applied to each pair of rods. For any given set of DC and RF voltages, only ions of a given m/z avoid collision with the rods and successfully travel through the quadrupole. Hence, the quadrupole acts as a mass filter, allowing only certain ions to reach the detector. The magnitude of the RF to DC ratio can be varied so that a range of m/z values may be transmitted. Once a particular m/z value is transmitted longitudinally through the quadrupole, the ion reaches a detector. The detector used in the APIMS is a channel electron multiplier. This is a device that, when struck by an ion, produces a short pulse of current at the output. This output is amplified and converted to a digital signal.

View larger version (18K): [in this window] [in a new window]   Fig. 1. Schematic illustration of the isotopic dilution atmospheric pressure ionization mass spectrometry. Label descriptions: (a) Atmospheric pressure ionization source region. (b) 63Ni ionization source. (c) Aperture skimmer. (d) Decluster skimmer. (e) Atmospheric pressure ionization ion energy skimmer. (f) Ion optics lens 1. (g) Ion optics lens 2. (h) Ion optics lens 3. (i) Entrance lens. (j) Prefilter. (k) Quadrupoles. (l) Exit lens. (m) Ion multiplier. (n) Decluster region rough pump. (o) Turbo pumps.

[1]

[2]

[3]

The CO₃^– ion (m/e 60) is the most dominant ion present in the spectrum when sufficient amounts of O₃ are added and the air is dry. The CO₃^– ion may be the terminal ion because the electron affinity for CO₃ is so large (2.69 eV). Because even a small amount of CO₂ can consume all the available O₃^– in producing CO₃^–, the 60 amu peak corresponding to CO₃^– ion may not change when the CO₂ concentration is changed. In essence, the detector is saturated, and the absolute value of this ion may not be useful for analytical chemistry. However, the CO₃^– ion can be made useful by adding ¹³CO₂ to the ambient air sample. The available negative charge is apportioned among the ions ¹²CO₃^– (60 amu) and ¹³CO₃^– (61 amu), in relation to the relative concentrations of ¹²CO₂ and ¹³CO₂ (Bandy, 2001).

The concentration of ¹²CO₂ can be computed from the relationship:

[4]

where [¹²CO₂] is the analyte concentration, [¹³CO₂] is the added internal standard concentration, R is the signal ratio of 60 amu to 61 amu signal, K_SS is the fraction of ¹³CO₂ in the labeled standard, K_AS is the fraction of ¹²CO₂ in the labeled standard, K_AA is the fraction of ¹²CO₂ in the sampled air, and K_SA is the fraction of ¹³CO₂ in the sampled air (Bandy et al., 1993). The isotopic composition in the standard is determined by the purity of the standard gas used. The ¹³CO₂ standard used for all experiments is traceable to NIST and was prepared by Scott-Marrin (Riverside, CA).

Figure 2 provides a plot of detected [¹²CO₂] as a function of added [¹²CO₂]. Inspection of Fig. 2 demonstrates that the APIMS/ILS method is linear and accurate over a wide range of CO₂ concentrations. Concentration resolutions of 0.5 µL L^{–1 12}CO₂ and 0.2 µL L^{–1 12}CO₂ were achieved for 1-s and 10-s integration times, respectively. Additionally, the linearity, range, and accuracy are maintained when a dryer is used in the presence of moisture. Table 1 displays the linearity results for the detection of CO₂ using APIMS/ILS at dew points of –12°C, +2°C, and +12°C. In each scenario, a nafion dryer was used to dry the sampled air. Inspection of Table 1 confirms that the determination of CO₂ using APIMS/ILS is linear and accurate regardless of water vapor content. This linearity and accuracy demonstrates that the drying process does not introduce errors in the determination of [¹²CO₂] using APIMS/ILS. This is due to the fact that the APIMS/ILS method measures mixing ratios rather than molar concentrations. The measurement of mixing ratio also avoids errors in atmospheric flux measurements that may be caused by the Webb effect (Webb et al., 1980). The Webb effect can be summarized as variability in density effects that are caused by heat and water vapor transfer.

View larger version (10K): [in this window] [in a new window]   Fig. 2. Atmospheric pressure ionization mass spectrometry/isotopically labeled standard linearity and range.

The goal here is to demonstrate that errors are introduced by a dryer. The CO₂ transmitted through the dryer is expressed by the equation:

[5]

Here n_CO2 represents moles of CO₂. The unprimed and primed notations represent variables before and after the dryer, respectively. A perfect dryer that removes all the water vapor and does not introduce a pressure drop is used. The dryer is short and large in diameter. Using the ideal gas law, Eq. [5] becomes

[6]

where P_CO2 is the partial pressure of CO₂, V is the volume, R is the ideal gas constant, and T is the temperature. Using the dot embellishment to denote a time derivative, Eq. [6] becomes

[7]

Using the conservation of mass,

[8]

Here n is the total moles of gas in the manifold before the dryer, n' is the total moles of gas after the dryer, and n_H2O is the number of water vapor moles that are removed by the dryer. Using the ideal gas law, Eq. [8] becomes

[9]

Rearrangement of Eq. [9] yields

[10]

Rearrangement of Eq. [7] yields

[11]

This after substitution into Eq. [10] yields:

[12]

The signal in a concentration measurement device, s(CO₂)', is proportional to the number density of CO₂:

[13]

Here, k is a calibration constant, and [CO₂]' is the CO₂ concentration after the dryer in units of mol L^–1.

Again, using the ideal gas law:

[14]

Rearrangement of Eq. [14] yields:

[15]

Substitution of Eq. [15] into Eq. [13] yields:

[16]

Substitution of Eq. [12] into Eq. [16] yields:

[17]

The atmospheric mixing ratio for CO₂, _CO2, is selected to be 380 µL L^–1 in this discussion. In the absence of sources and sinks, it is the mixing ratio that is constant. Using Dalton's Law,

[18]

which, after substitution into Eq. [17], yields:

[19]

Inspection of Eq. [19] reveals that the signal is dependent on the partial pressure of water, total pressure, and temperature of the sample before entering the dryer. Fluctuations in moisture and temperature produce a fluctuation in s(CO₂)' even though the mixing ratio of CO₂ is constant. Equation [19] predicts that an NDIR CO₂ signal will be artificially high when a dryer is used to remove water.

To demonstrate errors associated with drying, laboratory experiments were conducted using the APIMS/ILS method. The goal of this experiment was to verify that there were differences between pre-dryer and post-dryer CO₂ concentration. The APIMS/ILS method demonstrates linearity regardless of whether or not a dryer is used and regardless of how much water vapor is present. This fact holds true when the isotopically labeled standard is added pre-dryer. In this scenario, the standard acts as an internal standard and undergoes the same effects of drying that the sample undergoes. The pre-dryer CO₂ concentration is always measured when the isotopically labeled standard is added pre-dryer. This is the desired result when making atmospheric CO₂ measurements.

To determine the effects of drying on the unlabeled CO₂ post-dryer, the isotopically labeled standard was added post-dryer. Figure 3 displays the setup used during the experiments. The purpose of this experimental setup was to verify that the model in Eq. [19] could accurately predict the concentration of unlabeled CO₂ at point B, [¹²CO₂]_B. Addition of ¹²CO₂ pre-dryer subjects the unlabeled sample gas to the effects of drying. Addition of ¹³CO₂ post-dryer ensured that only the unlabeled analyte, ¹²CO₂, would be subjected to the effects of drying. In all experiments, the ¹²CO₂ flow rate (F3), the ¹³CO₂ flow rate (F5), and the flow rate entering mass flow meter B (F6) were held constant. This ensured that the concentrations of ¹²CO₂ and ¹³CO₂ remained constant, regardless of water vapor added. Variability in water content was achieved by altering flow 1 (F1) and flow 2 (F2). A cold trap was added post-dryer to remove water vapor that the nafion dryer may not have removed. The cold trap consisted of Teflon tubing submerged in dry ice. The water vapor pressure entering the APIMS was negligible, regardless of the amount of water vapor added before the dryer. The dew point of the dried or non-dried air was determined using an E&G Model 911 Dew Point Analyzer (Waltham, MA), which was cooled to allow for measurements over the range of –40°C to 20°C.

View larger version (18K): [in this window] [in a new window]   Fig. 3. Experimental manifold setup for determining CO2 concentration post-dryer. APIMS, atmospheric pressure ionization mass spectrometry; MFC, mass flow controller; MFM, mass flow meter.

	Results

TOP NOTES ABSTRACT INTRODUCTION Materials and Methods Results Discussion Conclusions REFERENCES

[20]

The concentration of ¹³CO₂ at point C, [¹³CO₂]_C, can be determined from:

[21]

Here, F5 and F6 correspond to flows 5 and 6, respectively. Further inspection of the experimental setup reveals that

[22]

which, on rearrangement, yields:

[23]

Substitution of Eq. [20] and [21] into Eq. [23] yields:

[24]

Compared with the errors associated with the flow rates, the errors in the term are negligible. The propagation of errors associated with the measurement of [¹²CO₂]_B is therefore dependent on the uncertainty in F5 and F6 through the factor:

[25]

The propagated uncertainty in [¹²CO₂]_B is:

[26]

During the experiments, typical standard deviations for F5 and F6 were 0.1 mL min^–1 and 0.005 L min^–1, respectively. Upon substitution into Eq. [26], these errors in flow rates produce an error of 0.14% in [¹²CO₂]_B.

Four dew point levels were produced by varying the air flow (F1) and the water flow (F2). These dew points were –40°C, +8.4°C, +14°C, and +18°C. The nafion dryer was equilibrated for 30 min at each dew point level before each experiment. Table 2 provides the dew point levels, the corresponding water partial pressures, the measured [¹²CO₂]_B using Eq. [24], and the calculated theoretical [¹²CO₂] using Eq. [19]. The post-dryer CO₂ concentration measurements at point B, [¹²CO₂]_B, are an average of 200 data points. The standard deviations for the post-dryer CO₂ concentration measurements ranged from 0.7 to 0.9 µL L^–1.

	Discussion

TOP NOTES ABSTRACT INTRODUCTION Materials and Methods Results Discussion Conclusions REFERENCES

	Conclusions

TOP NOTES ABSTRACT INTRODUCTION Materials and Methods Results Discussion Conclusions REFERENCES

 
The APIMS/ILS method is a highly precise technique for measuring CO₂ concentration. The APIMS/ILS method is linear over a wide range of CO₂ concentrations, even when using a dryer before analysis. Furthermore, the linearity and range of this method is demonstrated in the presence of varying levels of water vapor. This is due to the fact that the isotopically labeled standard is added pre-dryer.

The NDIR technique, on the other hand, is a concentration-based measurement that does not use an internal standard. This technique is therefore subject to errors that are caused by drying. There are numerous references in the literature that detail the use of a dryer in conjunction with the NDIR technique. However, the errors associated with drying are not addressed in these references. These errors are magnified in the presence of higher levels of water vapor (e.g., over oceans and lakes).

	NOTES

TOP NOTES ABSTRACT INTRODUCTION Materials and Methods Results Discussion Conclusions REFERENCES

 
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	REFERENCES

TOP NOTES ABSTRACT INTRODUCTION Materials and Methods Results Discussion Conclusions REFERENCES

 

• Baldocchi, D., E. Falge, L. Gu, R. Olson, D. Hollinger, S. Running, P. Anthoni, C. Bernhofer, K. Davis, R. Evans, J. Fuentes, A. Goldstein, G. Katul, B. Law, X. Lee, Y. Malhi, T. Meyers, W. Munger, W. Oechel, K.T. Paw U, K. Pilegaard, H.P. Schmid, R. Valentini, S. Verma, T. Vesala, K. Wilson, and S. Wofsy. 2001. FLUXNET: A new tool to study the temporal and spatial variability of ecosystem-scale carbon dioxide, water vapor and energy flux densities. Bull. Am. Meteorol. Soc. 82 :2415–2434.[CrossRef]
• Bakwin, P.S., P.P. Tans, D.F. Hurst, and C. Zhao. 1998. Measurements of carbon dioxide on very tall towers: Results of the NOAA/CMDL program. Tellus 50B :401–415.
• Bandy, A.R. 2001. Identification and determination of contamination in industrial gases using negative ion APIMS and isotope dilution. Gases Technol. 1 :18–24.
• Bandy, A.R., D.C. Thornton, and A.R. Driedger. 1993. Airborne measurements of sulfur dioxide, dimethyl sulfide, carbon disulfide, and carbonyl sulfide by isotope dilution gas chromatography/mass spectrometry. J. Geophys. Res. 98 :23423–23433.[CrossRef]
• Bandy, A.R., D.C. Thornton, H.T. Fang, B.W. Blomquist, W. Nadler, G.M. Mitchell, and D.H. Lenshchow. 2002. Fast airborne sulfur dioxide measurements by Atmospheric Pressure Ionization Mass Spectrometry (APIMS). J. Geophys. Res. 107, D22 : ACH13.1–ACH13.10.
• Chou, W.W., S.C. Wofsy, R.C. Harriss, J.C. Lin, C. Gerbig, and G.W. Sachse. 2002. Net fluxes of CO₂ in Amazonia derived from aircraft observations. J. Geophys. Res. 107, D22 :4614.
• Ciais, P., P. Tans, A. Denning, R. Francey, M. Trolier, H. Meijer, J. White, J. Berry, D. Randall, G. Collatz, P. Sellers, P. Monfray, and M. Heimann. 1997. A three-dimensional synthesis study of ¹⁸O in atmospheric CO₂: Simulations with the TM2 transport model. J. Geophys. Res. 102 :5873–5883.[CrossRef]
• Ciais, P., P.P. Tans, M. Trolier, J.W.C. White, and R.J. Francey. 1995. A large Northern Hemisphere terrestrial CO₂ sink indicated by the ¹³C/¹²C ratio of atmospheric O₂. Science 269 :1098–1102.[Abstract/Free Full Text]
• Conway, T.J., P.P. Tans, L.S. Waterman, K.W. Thoning, D.R. Kitzis, K.A. Masarie, and N. Zhang. 1994. Evidence for interannual variability of the carbon cycle from the National Oceanic and Atmospheric Administration/Climate Monitoring and Diagnostics Laboratory Global Air Sampling Network. J. Geophys. Res. 99(D11) :22831–22855.
• Dacey, J.W.H., J.B. Edson, P.H. Holland, and W.R. Mcgillis. 1999. In situ estimation of air-sea gas transfer using DMS. Presented at 13th Symp. on Boundary Layers and Turbulence. Am. Meteorol. Soc., Dallas, TX.
• DeLacy, B.G. 2006. The determination of carbon dioxide flux in the atmosphere using atmospheric pressure ionization mass spectrometry and isotopic dilution. Ph.D. diss. Drexel Univ., Philadelphia, PA.
• Denning, A.S., I.Y. Fung, and D. Randall. 1995. Latitudinal gradient of atmospheric CO₂ due to seasonal exchange with land biota. Nature 376 :240–243.[CrossRef]
• Donelan, M.A., and W.M. Drennan. 1995. Direct field measurements of the flux of carbon dioxide. p. 677–683. In B. Jahne and E.C. Monahan (ed.) Air-water gas transfer. Aeon, Hanau, Germany.
• Fung, I., C. Field, J. Berry, M. Thompson, J. Randerson, C. Malstrom, P. Vitousek, G. Collatz, P. Sellers, D. Randall, A. Denning, F. Badeck, and J. John. 1997. Carbon-13 exchanges between the atmosphere and biosphere. Global Biogeochem. Cycles 11 :507–533.[CrossRef][Web of Science]
• Helliker, B.R., J. Berry, A. Betts, P. Bakwin, K. Davis, A. Denning, J. Ehleringer, J. Miller, M. Butler, and D. Ricciuto. 2004. Estimates of net CO₂ flux by application of equilibrium boundary layer concepts to CO₂ and water vapor measurements from a tall tower. J. Geophys. Res. 109 :1–13.
• Horning, E.C., M.G. Horning, I. Dzidic, and R.N. Stilwell. 1973. New picogram detection system based on a mass spectrometer with an external ionization source at atmospheric pressure. Anal. Chem. 45 :936–943.
• Keeling, C.D., R.B. Bacastow, A.F. Carter, S.C. Piper, T.P. Whorf, M. Heimann, W.G. Mook, and H. Roeloffzen. 1989. A three-dimensional model of atmospheric CO₂ transport based on observed winds: I. Analysis of observational data. p. 277–303. In D.H. Peterson (ed.) Aspects of climate variability in the Pacific and the Western Americas. Geophys. Monogr. Ser. Vol. 55. AGU, Washington, DC.
• McGillis, W.R., J.B. Edson, J.E. Hare, and C.W. Fairall. 2001a. Direct covariance air-sea CO₂ fluxes. J. Geophys. Res. 106 :16729–16745.[CrossRef]
• McGillis, W.R., J.B. Edson, J.D. Ware, J.W.H. Dacey, J.E. Hare, C.W. Fairall, and R. Wanninkhof. 2001b. Carbon dioxide flux techniques performed during GasEx-98. Mar. Chem. 75 :267–280.[CrossRef][Web of Science]
• McGillis, W.R., J.B. Edson, C.J. Zappa, J.D. Ware, S.P. McKenna, E.A. Terray, J.E. Hare, C.W. Fairall, W. Drennan, M. Donelan, M.D. DeGrandpre, R. Wanninkhof, and R.A. Feely. 2004. Air-sea CO₂ exchange in the equatorial Pacific. J. Geophys. Res. 109 :C08S02.[CrossRef]
• Siegel, M.W., and W.L. Fite. 1976. Terminal ions in weak atmospheric pressure plasmas. Applications of atmospheric pressure ionization to trace impurity analysis in gases. J. Phys. Chem. 80 :2871.[CrossRef][Web of Science]
• Ward, B., R. Wanninkhof, W.R. McGillis, A.T. Jessup, M.D. DeGrandpre, J.E. Hare, and J.B. Edson. 2004. Biases in the air-sea flux of CO₂ resulting from ocean surface temperature gradients. J. Geophys. Res. 109(C8) :C08S08.
• Webb, E.K., G.I. Pearman, and R. Leuning. 1980. Correction of flux measurements for density effects due to heat and water vapor transfer. Q. J. R. Meteorol. Soc. 106 :85–100.[CrossRef]
• Zemmelink, H.J., J.W.H. Dacey, E.J. Hintsa, W.R. McGillis, W.W.C. Gieskes, W. Klaassen, H. W. de Groot, and H.J.W. de Baar. 2004. Fluxes and gas transfer rates of the biogenic trace gas DMS derived from atmospheric gradients. J. Geophys. Res. 109:C08S10.
• Zhao, C.L., P.S. Bakwin, and P.P. Tans. 1997. A design for unattended monitoring of carbon dioxide on a very tall tower. J. Atmos. Oceanic Technol. 14 :1139–1145.[CrossRef]

This Article Abstract Figures Only Full Text (PDF) Free Alert me when this article is cited Alert me if a correction is posted Services Similar articles in this journal Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via Google Scholar Google Scholar Articles by DeLacy, B. G. Articles by Bandy, A. R. Search for Related Content PubMed PubMed Citation Articles by DeLacy, B. G. Articles by Bandy, A. R. Agricola Articles by DeLacy, B. G. Articles by Bandy, A. R. Related Collections Other Geophysical Methods Global Change Field evaluation techniques Other Models Air Pollution