The Model 408 Nitric Oxide Calibration Source™ (2B Technologies, Boulder, Colorado) makes use of a low-pressure mercury (Hg) lamp to photolyze pure nitrous oxide (N2O) to produce NO. The vacuum UV emission lines of mercury near 184.9 nm are absorbed by N2O to produce electronically excited oxygen atoms, O(1D), R4N2O+hν→N2+O(1D), where hν symbolizes a photon of light. These highly energetic oxygen atoms react with N2O with a near-collisional reaction rate coefficient (k=1.3×10-10 cm3 molec-1 s-1) to form three different sets of products: R5aO(1D)+N2O→2NOϕ=0.61±0.03,R5b→N2+O2ϕ=0.39±0.03,R5c→N2O+O(3P)ϕ<0.01, with the branching ratios (ϕ) shown (Burkholder et al., 2015). Since Reaction (R5a) produces two NO molecules, the overall quantum yield for NO production is approximately 1.22. The NO calibration source is similar in design to our ozone calibrator (Birks et al., 2018b), as both make use of the 184.9 nm line of a low-pressure mercury lamp. An important fundamental difference is that the 184.9 nm absorption cross section for N2O is approximately 14 times larger than that of O2. The absorption cross section, σ, of N2O at the 184.9 nm mercury emission line is ∼1.4×10-19 cm2 molec-1 (Creasey et al., 2000; Cantrell et al., 1997) compared to ∼1×10-20 cm2 molec-1 for O2 (Yoshino et al., 1992, Creasey et al., 2000). Also, O2 is only 21 % of the air that passes through the photolysis chamber in the ozone calibrator, while N2O is supplied to the photolysis chamber by a source that is >99 % N2O. The result is that the N2O gas absorbs the 185 nm light ∼70 times stronger than does O2 in air. At 298 K and 1 atm, the molecular concentration, c, is 2.46×1019 molec cm-3; thus, the absorption of 184.9 nm light from the low-pressure mercury lamp becomes optically thick (1/e attenuation) at a path length, 1/(σc), of 0.3 cm, and 99 % of the light is absorbed for a path length of 1.35 cm. Under such conditions, the rate of production of NO (molec cm-3 s-1) depends almost entirely on the lamp intensity and is independent of flow rate (i.e., residence time in the photolysis cell). The NO/N2O stream exiting the photolysis chamber is diluted into NOx-scrubbed air to produce the desired output concentration of NO in air. At constant flow rates of N2O and the dilution air, the concentration of NO in the calibrator's output is varied by merely changing the lamp intensity.

We typically observe a small amount of NO2 produced from the NO photolytic generator (≤3 % of the NO produced). This is likely due to the formation of O(3P) atoms in the photolysis cell, which combine with NO via the reaction R6O(3P)+NO+M→NO2+M, where M is a third body, most likely N2O in this case. O(3P) atoms can arise from several possible sources. Nishida et al. (2004) report a quantum yield for O(3P) from N2O photolysis of 0.005±0.002 (i.e., NO2/NO=0.005/1.22∼0.4 %). Quenching of O(1D) to O(3P) by N2O (Reaction R5c) likely contributes up to another 0.8 % (Vranckx et al., 2008, report a limit of ϕ5c<0.01 at 298 K). Oxygen (O2), which is a typical N2O impurity, can also photolyze to produce two O(3P) atoms. Even NO itself could be photolyzed at 184.9 nm (σ∼3×10-18 cm2 molec-1, thermodynamic dissociation threshold of 189.7 nm; Iida et al., 1986 and Burkholder et al., 2015) to produce O(3P) atoms; however, this would be expected to be of lesser importance due to the relatively lower NO concentrations (ppm) within the photolysis cell. In all of these cases, the amount of NO2 formed relative to NO should be small and approximately constant over time.

A final issue pertains specifically to CL NO analyzers for which the presence of N2O can lead to collisional quenching of the chemiluminescence signal. Clyne et al. (1964) report that N2O quenches the NO/O3 chemiluminescence 2.6 and 2.9 times more efficiently than O2 and N2, respectively. Therefore, a mixture of 1.5 % N2O in air (typical conditions – see Sect. 3) would be expected to reduce the observed chemiluminescent signal by ∼2.7 % relative to pure air. In practice, this is typically slightly less (2.2 %–2.4 %) as the sample flow in a CL analyzer is diluted by the addition of a reagent O3/air flow (10 %–25 % of the total flow). This correction term can be explicitly calculated from the measured flows in the photolytic NO calibration source and the flow rates in the CL analyzers (as in the example given here) or it can be eliminated depending upon the analyzer used during the initial calibration of the relationship between lamp intensity and NO output (see Sect. 3.1). If a CL analyzer is used to determine this relationship, then quenching by N2O is intrinsically included in the calibration of the photolytic source.

The photolytic ozone calibration source has been described in detail previously (Birks et al., 2018b) and the following is only meant to briefly highlight the important points of this calibrator since it plays a key role in the NO2 calibration device described in the following section. In the photolytic ozone calibration source that is used in the Model 306 Ozone Calibration Source™ (2B Technologies, Boulder, Colorado), a low-pressure mercury lamp produces calibrated concentrations of ozone by the photolysis of oxygen in air. R7O2+hν→2O(3P)R8(O(3P)+O2+M→O3+M)×2 Net:3O2+hν→2O3 A key difference compared to the NO photolytic source described above (Sect. 2.1) is that for a 1 cm path length (and at 1 atm and 25 ∘C), the O2 absorption in air is nearly optically thin (∼5 % light absorbed). For an optically thin system, the mixing ratio of ozone produced depends linearly on the residence time within the photolysis chamber; thus, it varies with volumetric flow rate. In the Model 306 Ozone Calibration Source, the mass flow rate, temperature, and pressure are continuously measured to compute the volumetric flow rate (and therefore the residence time), and the lamp intensity is adjusted in a feedback loop to maintain a constant ozone output mixing ratio. A further key point in the ozone calibration source is that the photolysis cell must be maintained at a constant (slightly heated) temperature to ensure constant overlap between the Hg lamp emission lines and the O2 absorption lines and to maintain a constant ratio of lamp intensities between the 184.9 nm Hg line and the 253.7 nm Hg line that is monitored in the feedback loop to maintain a constant photolysis rate (Birks et al., 2018b). The main difference between the previously described Model 306 Ozone Calibration Source and the one used for the NO2 calibrator described below (Sect. 2.3) is that the flow rate through the photolysis cell is much lower (∼50 cm3 min-1 as opposed to 3000 cm3 min-1). This increases the cell residence time from 0.06 to ∼3.6 s and consequently results in the production of much higher ozone mixing ratios (up to 15 ppm) exiting the photolysis cell in the Model 714. However, control of the lamp intensity and volumetric flow rate (as described in Birks et al., 2018b) still allows for precise control of the output ozone mixing ratio that is independent of pressure and temperature. The longer residence time and higher mixing ratios in the O3 photolysis cell do lead to complications due to water vapor that were not found in the individual O3 calibration source (the Model 306). A solution to this potential problem will be discussed in Sect. 4.2.

Combining the above two calibrators into a single unit (the Model 714 NO2/NO/O3 Calibration Source™, 2B Technologies, Boulder, Colorado) makes it possible to calibrate not only for NO and O3, but also a third gas, NO2. The calibrated concentrations of NO and O3 are produced as described above. Calibration of NO2 is accomplished via the gas-phase titration (GPT) technique (Reaction R3), making use of the NO and O3 produced in the combined calibration source. Here, O3 is reacted with an excess of NO to produce known concentrations of NO2 under conditions such that [NO]consumed=[NO2]produced. A key difference in the Model 714 from the two individual calibrators is that Reaction (R3) must be carried out at high concentrations (parts per million level) to drive Reaction (R3) to completion; therefore, the NO and O3 reagents are mixed before subsequent dilution. Modeling the second-order kinetics of Reaction (R3) (see Fig. 1) shows that with [NO] = 2×[O3] (i.e., NO a factor of 2 in excess of O3), ∼5 ppm of NO is required to consume 99.6 % of the ozone for a reaction time of 4 s. Increasing the reaction time allows for lower [NO] to be used to obtain the same completeness of reaction. It should be noted that for NOx analyzers that measure both NO and NO2, it is not necessary for Reaction (R3) to go to completion because one measures the consumption of NO relative to the production of NO2. However, residual ozone complicates the calibration protocol as Reaction (R3) continues to alter the NO/NO2 ratio (at a reduced rate after dilution) as the gas mixture is transported to the analyzer to be calibrated. Thus, the NO/NO2 ratio would depend on the residence time of the connection tubing. For NO2-only analyzers, it is critical to have >99 % conversion as the NO2 signal produced may be correlated with the loss of ozone (which is assumed to be at the calibrated target concentration and typically not explicitly measured). The calculations described in Fig. 1 can be used as a guide to the required concentrations and residence times of a GPT reactor.

One more point concerning the GPT chemistry is that NO must be maintained in excess over ozone (Ellis, 1975; U.S. EPA, 2002). If ozone is used in excess, NO2 can react with the excess ozone to produce NO3, R9NO2+O3→NO3+O2, and NO3 can subsequently form N2O5 rapidly via (Bertram et al., 2012) R10f, R10rNO2+NO3⇌N2O5. Reaction (R9) is ∼600 times slower than Reaction (R3) (k9=3.22×10-17 cm3 molec-1 s-1 at 298 K; Burkholder et al., 2015) but can proceed to a small extent at parts per million levels of NO2 and O3. At room temperature and NO2 concentrations greater than about 25 ppb, Reaction (R10) favors N2O5 formation and proceeds relatively rapidly (k10∼1.4×10-12 cm3 molec-2 s-1; Burkholder et al., 2015), thus resulting in a net loss of two NO2 molecules. In typical CL analyzers that use heated molybdenum to convert NO2 to NO, N2O5 production is not observable, since the heated catalyst will thermally decompose N2O5 rapidly (Reaction R10r), followed by the reduction of both NO2 and NO3 to NO, thus not affecting the observed [NO]consumed=[NO2]produced. However, in the case of photolytic NO2 converters and the long-path FTP method mentioned in the previous section, the formation of N2O5 would cause an underestimate in the calibration (i.e., [NO]consumed>[NO2]produced). For photolytic converters, there would be no way to elucidate the error as the lower observed NO2 would likely be incorporated into an incorrect conversion efficiency.

Figure 2 is a schematic diagram of the NO calibration source. An air pump draws ambient air into the instrument through NOx and ozone scrubbers (a mixture of Carulite^® and activated carbon) to produce the diluent air stream. The airflow rate is measured by a mass flowmeter and is controlled by the use of restrictors (not shown) and a needle valve that vents part of the flow. The needle valve is adjusted to produce diluent air at a total output volumetric flow rate of ∼3 L min-1. In the most portable configuration of the instrument, nitrous oxide is supplied by a cartridge containing either 8 or 16 g of liquid N2O with a headspace pressure of ∼50 atm at 20 ∘C. A combined cracker–regulator punctures the cartridge as it is tightened and also drops the outlet delivery pressure to below 25 psig. A 25 psig pressure-relief valve is installed inside the instrument housing to prevent overpressurization. The valve on the cracker–regulator provides a coarse adjustment of the N2O flow rate. A voltage-sensitive orifice (VSO) valve is then used to provide fine control of the N2O flow rate to 60±1 cm3 min-1 in a feedback loop. Pressure within the gas stream is measured but not controlled. The N2O then passes through an aluminum photolysis cell (volume ∼6 cm3) where a small fraction of the N2O is converted to NO (and N2 + O2) by a low-pressure mercury discharge lamp. As discussed above, because the system is optically thick (essentially every photon is absorbed), the NO production rate (molec s-1) is independent of photolysis cell pressure and N2O flow rate, the production rate depending only on the lamp intensity. NO output is varied by changing the pulse-width-modulated duty cycle to the Hg lamp while monitoring its intensity with a photodiode. Since nearly all of the 184.9 nm light is absorbed by the N2O in the cell, the lamp intensity at the 253.7 nm mercury line is monitored by the photodiode and controlled by the microprocessor. As the monitoring wavelength (253.7 nm) and the photolysis wavelength (184.9 nm) are different, it is important to maintain a constant ratio of lamp emission at these two wavelengths. This is accomplished by regulating the photolysis cell (which houses the lamp) at a constant temperature of 40 ∘C. Even as the lamp duty cycle is varied, the temperature regulation maintains a constant lamp temperature, thus ensuring a stable 184.9/253.7 nm output. The NO/N2O stream exiting the photolysis chamber is diluted into the ∼3 L min-1 flow of NOx-scrubbed air to produce the desired output concentration of NO in air.

For the NO calibration source to be a valid transfer standard, the photolytic NO source must be validated against a NIST-traceable NO standard to provide a lamp intensity vs. NO output concentration working curve. Therefore, the photodiode voltage (i.e., a measurement of lamp intensity) must be calibrated against the output NO concentration as measured by an NO analyzer that has been recently calibrated using a NIST SRM NO gas standard dilution system.

Nitrous oxide can be supplied to the instrument either by means of N2O cartridges (commercially available and often used as whipped-cream chargers) as shown in Fig. 2 or by connection to a lecture bottle or gas cylinder containing N2O. The cartridge holder and cracker allow the use of either 8 or 16 g cartridges containing liquid N2O and will supply a gas flow of N2O of 60 cm3 min-1 for approximately 1.2 or 2.5 h, respectively. Alternatively, a lecture bottle or tank of N2O may be used, allowing for continuous operation of 1.5 d for a lecture bottle containing 227 g or 174 d for a typical containing 27 kg of N2O.

A portable calibrated source of NO2 can be achieved by combining the NO photolytic calibration source (described in Sects. 2.1 and 3.1) with the photolytic ozone calibration source (described in Sect. 2.2 and in Birks et al., 2018b). This is commercially available as the 2B Technologies Model 714 NO2/NO/O3 Calibration Source™, which is capable of providing calibrated mixing ratios of NO2, NO, or O3. A schematic diagram of the Model 714 NO2/NO/O3 Calibration Source is shown in Fig. 3. The instrument produces O3 by the photolysis of oxygen in air, NO by the photolysis of N2O, and NO2 by the gas-phase titration (GPT) of known concentrations of O3 in an excess of NO.

An air pump pushes ambient air through an O3/NOx scrubber; thereafter the flow is split using a restrictor to send a volumetric flow of ∼50 cm3 min-1 through an ozone photolysis chamber, with the bulk of the flow, ∼2.7 L min-1, serving as a diluent gas. This main flow combines with the effluent of the photolysis cells just prior to the outlet. A voltage-sensitive orifice (VSO) valve controls the flow, as measured by a mass flowmeter, through the photolysis chamber. For the production of NO, a pressurized source of N2O passes through a mass flow controller and into an NO photolysis chamber at a volumetric flow rate of ∼40 cm3 min-1. Flows through the O3 and NO photolysis chambers join at a tee prior to entering a reaction zone having a volume of 6.5 cm3 consisting of 20.3 cm of 6.4 mm i.d. Teflon tubing. For the generation of NO2, the outputs of the NO and O3 photolysis cells are mixed and allowed to react in the Teflon reaction volume. The total flow rate passing through this reactor is 90 cm3 min-1 (40 cm3 min-1 of NO/N2O and 50 cm3 min-1 of O3/air), giving a reaction time of 4.3 s. The O3 is quantitatively converted to NO2 during this time. The high-concentration O3, NO, or NO/NO2 mixture is diluted by a factor of ∼30–54 (the larger being in the absence of the N2O flow for outputting only O3) with O3/NO2-scrubbed ambient air at a tee just prior to the instrument outlet. Ozone or NO is produced at calibrated concentrations by turning the corresponding lamps on and adjusting their intensities as measured by photodiode measurements in the respective chambers. Typically, the N2O flow is turned off when only ozone is being output to conserve N2O usage. To produce known mixing ratios of NO2, calibrated amounts of O3 (corrected for the slight dilution by N2O) are produced in the range 0–500 ppb with the NO output set at least twice the output ozone (e.g., 1000 ppb of NO is required for 500 ppb of O3 to be converted to NO2). Note that these are the concentrations exiting the calibrator as opposed to the much higher concentrations found within the reaction zone. For example, a final output concentration of 200 ppb of NO would give a concentration of 6 ppm of NO within the reaction zone.

As described in Sect. 1, the U.S. EPA sets out procedural guidelines for the calibration of monitors used for the regulatory monitoring of ambient O3 and NO2 (U.S. EPA, 2002; U.S. EPA, 2013). Since most NO2 monitors actually monitor NO (CL analyzers), their guidelines also describe the calibration of NO as a matter of necessity even though NO is not a criteria pollutant. For NOx, the basis of these procedures ties NO and NO2 calibrations to a NIST-traceable SRM gas mixture of NO. However, the U.S. EPA does not provide guidance for transfer standards that do not include the direct use of a either a NIST SRM gas mixture or a gas mixture that is somehow traceable to a NIST SRM (as with the photolytic NO2 calibrator described here). In contrast, specific statistical requirements are established for the use of either photolytic generators or analyzers based on photometry for use as transfer standards in the calibration of ozone monitors (U.S. EPA, 2013; Birks et al., 2018b). In lieu of direct statistical requirements for a photolytic NO and NO2 transfer standard, we have applied the same requirements that are established for a Level 4 ozone transfer standard (U.S. EPA, 2013). Level 4 ozone transfer standards must undergo a “6×6” verification in which six calibration curves, each consisting of six approximately equally spaced concentrations in a range including 0 % and 90 % (±5 %) of the upper range of the reference standard, are obtained on six different days (U.S. EPA, 2013). The relative standard deviations of the six slopes of the calibration plots must not exceed 3.7 %, and the standard deviation of the six intercepts cannot exceed 1.5 ppb.

The 6×6 verification requires an analyzer whose calibration is traceable to a NIST standard. For the measurements presented here, a 2B Technologies Model 205 or 202 ozone monitor was used as a reference photometer for O3, and a 2B Technologies Model 400 (for NO only) or a Model 405 (NO and NO2) was used as a reference analyzer for NOx. The ozone monitors (Model 202 or 205) are each certified as an ozone Federal Equivalent Method (FEM) by the U.S. EPA (EQOA-0410-190) and are NIST-traceable through comparison with a currently accredited ozone calibrator (Thermo Scientific model 49i-PS). The Model 405 also is designated as an FEM for NO2 (EQNA-0217-243), and it and the Model 400 were calibrated using a Teledyne-API model 700 Dynamic Dilution NO/NO2 calibrator using a NIST-traceable NO gas mixture (Scott Specialty or Airgas). Furthermore, the ozone photometer (used to measure the ozone for the GPT reaction) within the model 700 was also calibrated against the NIST-traceable Thermo Scientific ozone standard.

Because a goal of the calibration sources is their use in the field calibrations of analyzers, one also needs to consider the effect of environmental factors such as temperature, pressure, and humidity on the output mixing ratios of the photolytically generated analytes. The factors of optical opacity and the photochemistry discussed above in Section 2 imply that environmental variables such as temperature, pressure, and relative humidity should have minimal effects on the performance of the ozone and NO calibrators. However, this assumes that the N2O is completely optically thick and that the sensors for temperature, pressure, and mass flow rate are perfectly linear and independent of one another. For example, as pointed out below, mass flowmeters depend on molecular composition and will not be perfectly accurate when the water vapor mixing ratio changes. Similar tests of the ozone calibrator have been described previously (Birks et al., 2018b), and here we carry out additional environmental tests of the NO calibration source. Furthermore, temperature certainly affects the GPT chemistry (Reaction R3) and can place limitations on the usable concentration ranges – typically at low concentrations for which Reaction (R3) may not go to full completion. The methods for varying the temperature, pressure, and humidity will be described as the results are presented in the following section.

The Model 408 Nitric Oxide Calibration Source was first introduced as a product by 2B Technologies in 2007 but has not been described in the scientific literature. Applications of this highly portable NO calibrator have been limited primarily because users need an instrument that also calibrates for NO2 measurements. The photolytic NO generator described here has since been used in the 2B Technologies Model 211 Scrubberless Ozone Monitor, wherein NO serves as a gas-phase scrubber, and more recently the Model 714 NO2/NO/O3 Calibration Source. The NO generator is identical in all three instruments – the only differences being the N2O flow rates used and the degree of dilution. Since, as discussed in Sect. 2.1, 1 atm of N2O is optically thick at 184.9 nm, the flow rate of N2O through the chamber is not critical. Because essentially every photon is absorbed, the production rate of NO is determined only by the lamp intensity – a low flow rate of N2O through the chamber produces the same number of molecules of NO per second as a high flow rate. The effect of changing the N2O flow rate is only to change the total flow into which the NO produced is diluted, which is small since the N2O flow rate is only 1 %–2 % of the total flow. Optical opacity was verified experimentally by using the NO calibration source in a Model 714, varying the N2O flow rate through the photolysis chamber from 5 to 50 cm3 min-1, and setting the lamp intensity to output a constant 400 ppb of NO. The resulting NO mixing ratios measured are given in Table 1. As is readily apparent, there is no observable dependence of the NO concentration produced on the flow rate of N2O within the measured uncertainties. Due to this invariance with N2O flow, this flow rate is often set based on the balance between N2O usage and the response time to a change in NO concentration. Faster flow rates result in a quicker flush time of the photolysis cell and lead to more rapid changes in NO concentration. For example, in the 2B Technologies Model 211 Scrubberless Ozone Monitor, only a constant amount of excess NO is required, and thus a small flow rate (10–15 cm3 min-1) serves to conserve N2O usage. However, in both the NO calibrators (the Model 408 and 714), higher flow rates (40–60 cm3 min-1) are used to allow for more rapid concentration changes (<1 min).

The NO calibration source is typically configured to deliver a calibration gas at a volumetric flow rate of 2.5 to 3.0 L min-1. A change in the flow rate of diluent air would be expected to change the concentration of NO produced. However, the instrument continuously measures the total mass flow rate and adjusts the lamp intensity to compensate for changes in dilution so as to produce a constant output mixing ratio of NO. In typical operation, these intensity adjustments are small as the total flow rate is usually rather constant (within ±5 %). However, the intensity vs. total flow rate feedback loop was tested by measuring the NO output (at 200 ppb) as the total flow rate was varied between 2.2 and 4.5 L min-1. There was no measurable difference in the NO mixing ratios (<±2 % or 4 ppb; data not shown).

The 2B Technologies Model 714 NO2/NO/O3 Calibration Source is a combination of a Model 306 Ozone Calibration Source and a Model 408 NO Calibration Source (Sect. 4.1) that allows for the generation of calibrated mixing ratios of either O3, NO, or NO2 (the latter via the GPT Reaction R3). The NO source is identical to the one described in Sects. 3.1 and 4.1, except the N2O flow rate is typically lowered to around 40 cm3 min-1 compared to the earlier Model 408 NO Calibration Source. As noted in Sect. 3.1, the NO output is not affected by the choice of N2O flow rate due to the optical opacity of the N2O. The ozone calibration source in the Model 714 differs from that described previously for the Model 306 (Birks et al., 2018b) in that only a small fraction of the airflow passes through the photolysis cell (described in Sect. 2.2). For the generation of NO2, the outputs of the NO and O3 photolysis cells are mixed and allowed to react within a Teflon reaction volume (described in Sect. 3.3) to quantitatively convert O3 (which is lower in concentration) to NO2 before final dilution to the prescribed set point concentration.