Environ. Sci. Technol. 1999, 33, 3672-3679
A Method for the Measurement of Atmospheric HONO Based on DNPH Derivatization and HPLC Analysis X I A N L I A N G Z H O U , * ,†,‡ HUANCHENG QIAO,† GUOHONG DENG,‡ AND KEVIN CIVEROLO† Wadsworth Center, New York State Department of Health, Empire State Plaza, Albany, New York 12201, and School of Public Health, State University of New York, Empire State Plaza, Albany, New York 12201
A simple measurement technique was developed for atmospheric HONO based on aqueous scrubbing using a coil sampler followed by 2,4-dinitrophenylhydrazine (DNPH) derivatization and high-performance liquid chromatographic (HPLC) analysis. Quantitative sampling efficiency was obtained using a 1 mM phosphate buffer, pH 7.0, as the scrubbing solution at a gas sampling flow rate of 2 L min-1 and a liquid flow rate of 0.24 mL min-1. Derivatization of the scrubbed nitrous acid by DNPH was fast and was completed within 5 min in a derivatization medium containing 300 µM DNPH and 8 mM HCl at 45 °C. The azide derivative was separated from DNPH reagent and carbonyl derivatives by reverse-phase HPLC and was detected with an UV detector at 309 nm. The detection limit is e5 pptv and may be lowered to 1 pptv with further DNPH purification. Interferences from NO, NO2, PAN, O3, HNO3, and HCHO were studied and found to be negligible. Ambient HONO concentration was measured simultaneously in downtown Albany, NY, by this method and by an ion chromatographic technique after sampling using a fritted bubbler. The results, from 70 pptv during the day to 1.7 ppbv in the early morning, were in very good agreement from the two techniques, within ( 20%.
Introduction Reactive nitrogen species, including NO, NO2, HNO3, HONO, peroxyacetyl nitrate (PAN), and organic nitrate, play critically important roles in the chemistry of the troposphere (1-3). As such, these species have been the focuses of investigation in atmospheric chemistry (3-5). Among these species, HONO is probably one of the least understood, especially in the clean environment where few measurements have been conducted due to the lack of sensitive techniques (6). One important role HONO may play in the atmosphere is as an OH source upon photolysis. Measurements in the urban environments (7-10) have suggested that HONO photolysis could be a dominant radical source in the early morning, when HONO concentration is high due to nighttime accumulation and when other radical sources such as photolyses of ozone and formaldehyde are small. This role is less * Corresponding author e-mail:
[email protected]; fax: (518)473-8117. † New York State Department of Health. ‡ State University of New York. 3672
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clear in the clean atmosphere for no distribution data are available. A number of techniques have been developed for the measurement of atmospheric HONO in the past 20 years. The first unambiguous identification of HONO in the troposphere was made by Perner and Platt (11), over Julich, Germany, using differential optical absorption spectroscopy (DOAS). Since then, this technique has been successfully employed by many researchers in the United States and in Europe in the urban and semiurban atmosphere, providing reliable and extensive concentration information for HONO in these environments (7, 10). However, the method has limited sensitivity with a detection limit of g0.1 ppbv with a 15-min averaging time and 1-km light path and suffers from significant performance degradation under conditions of poor visibility. In addition, the system is complex and relatively expensive to build and operate and is not readily adapted to mobile sampling platforms. Another popular technique involves trapping of atmospheric HONO by alkaline coated annular denuders (AD), followed by determination of trapped nitrous acid using ion chromatography (IC) with conductivity detection (CD) (1214) or spectrophotometry (SP) (15). This technique is simple and relatively inexpensive. However, the detection technique offers only limited sensitivity (∼100 nM) and thus requires long sample integration times, e.g., a detection limit of ∼0.4 ppbv for nitrous acid with a integration time of 6 h using IC/CD (13). The method also suffers from some inherent drawbacks such as being labor-intensive and being prone to interference from surface reactions of NOx and ozone within the apparatus (15). Some other techniques have also been developed for atmospheric HONO measurements. Rodgers and Davis (16) developed a novel technique involving HONO fragmentation by a laser light at 355 nm followed by detection of the OH radicals generated by laser-induced fluorescence spectroscopy (PF/LIF). This technique is relatively sensitive, with a detection limit in the low tens of ppt level for a 15-min integration time. However, the routine utility of this technique has been deterred by the complexity and cost of the instrument. Another sensitive technique has been developed by Vecera and Dasgupta (9) based on aqueous scrubbing of HONO using a diffusion denuder sampler followed by analysis of nitrite by IC with UV detection (DS-IC-UV). The technique offers advantages such as relatively high sensitivity (with a detection limit of ∼20 pptv and time resolution of 7-15 min), moderately low cost, and ease of automation and adaptation to a mobile platform. Some field data have been reported using this technique (9, 17, 18). In this paper, we will describe a highly sensitive technique for the measurement of atmospheric HONO based on aqueous scrubbing of this species using a coil sampler, followed by derivatization with 2,4-dinitrophenylhydrazine (DNPH) and high-performance liquid chromatography (HPLC).
Experimental Section Reagents and Materials. 2,4-Dinitrophenylhydrazine (DNPH) was 99+% purity from Radian. A DNPH stock solution was prepared by dissolving 1 g of DNPH in 1 L of acetonitrile (∼5 mM) and was stored in the dark and refrigerated. A DNPH working solution was prepared from the stock solution by dilution with 50 mM HCl. Reagent-grade chemicals, including NaH2PO4, K2HPO4, 36% HCl, NaOH, peracetic acid, HNO3, H2SO4, NaNO3, and NaNO2, were from Aldrich. HPLC grade acetonitrile was from Burdick & Jackson or J. T. Baker. All 10.1021/es981304c CCC: $18.00
1999 American Chemical Society Published on Web 09/03/1999
chemicals were used without further purification. Water was purified with a Millipore Mili-Q water system, with resistivity g18 MΩ. Sampling. Gaseous HONO was sampled by a coil sampler consisting of an inlet T for the sample gas and scrubbing solution, a 15-turn glass coil for scrubbing soluble analyte from gas into liquid, and an enlarged tube section for gasliquid separation (19). A 1 mM phosphate buffer scrubbing solution was pumped into the coil by a peristaltic pump (IPC, Ismatec) at a flow rate of 0.24 ( 0.01 mL min-1. Sample gas was pulled through the coil by a vacuum pump (model 7055-60, Vacuubrand) at 2.0 L min-1 controlled by a mass flow controller (0-5 SLM, Tylan). The return scrubbing solution from the coil sampler was pumped with the same peristaltic pump into a derivatization coil where the scrubbed nitrous acid was allowed to react with derivatizing reagents. Derivatization. On-line derivatization was achieved by mixing scrubbing solution with a DNPH working solution and passing the mixture through a derivatization coil of about 3 mL, which allowed a 6-min derivatization time. Derivatization kinetics of nitrite by DNPH were studied by mixing a NaNO2 standard solution with a DNPH solution in a 100-mL volumetric flask and analyzing the azide derivative by HPLC at time intervals after mixing. Factors affecting the derivatization rate were investigated, including pH (acidity), DNPH concentration, and temperature. HPLC Analysis. The HPLC system consisted of a pump (Hitachi model L-7100 gradient pump or Beckman model 110B isocratic pump), a 6- or 10-port electrically actuated autoinjection valve (Valco) with two preconcentration columns filled with C18 packing material (Upchurch) in the place of sample loop, a C18 reverse-phase column (C18, 3 µm, 5 cm × 4.6 mm, Rainin), and a UV-visible absorption detector (Thermo-Separation, model 100) with a 10-mm flow cell. Isocratic elution was accomplished with 35% acetonitrile in water. The detection wavelength was fixed at 309 nm. A Macintosh PC-based HPLC data system (MacIntegrator, Rainin) was used for data acquisition and reprocessing. Sample injection volume was in the range of 0.1-10 mL, depending on the purposes of the experiments. For ambient HONO measurement, a 5-min preconcentration time was used, equivalent to a 2.4-mL sample injection volume. Blank and Calibration. Three types of blank controls were compared during this study: (i) a reagent blank obtained by injecting the DNPH working solution into the HPLC; (ii) a system blank obtained by passing the scrubbing solutions through only the liquid plumbing without exposure to ambient air, i.e., with the coil sampler bypassed; and (iii) a system blank obtained by feeding UHP N2 gas or zero air (Praxair) into the coil sampler with the system operated under normal condition. Aqueous NaNO2 standard solutions were used to calibrate the measurement system. A 10 mM NaNO2 stock solution was prepared from a new bottle of AR-grade NaNO2 and was found to be stable for at least 3 months when stored in the dark and refrigerated. A series of NaNO2 working standard solutions were prepared fresh daily with a two-stage dilution of the stock solution. To conduct system calibration, NaNO2 standard solutions were fed into the sampling system in the place of the scrubbing solution with either the coil sampler bypassed or with the coil sampling UHP N2/zero air while the system operated under normal conditions. Interference. Experiments were carried out to study potential interference of NO, NO2, nitrate, and PAN with the measurement of HONO. The 10 ppmv NO and NO2 gas standards in N2 (Scott Specialty Gases) were diluted with UHP N2 to produce artificial samples containing NO and/or NO2 in a concentration range of 10-200 ppbv, using a dynamic mixer equipped with mass flow controllers (series 2000, Environics). The gas samples were fed to two coil
FIGURE 1. Schematic diagram of an automated measurement system for ambient HONO. Typical operation conditions: air sampling rate, 2 L min-1; scrubbing solution flow rate, 0.24 mL min-1; DNPH solution flow rate, 0.24 mL min-1; derivatization coil volume, 3 mL; derivatization temperature, 45 °C; sample integration time, 5 min; eluent composition, 35% acetonitrile in water; HPLC elution flow rate, 2 mL min-1; detection wavelength, 309 nm. samplers connected in series. The first coil sampler was used to strip HONO impurities in the NO or NO2 standard gas, and the second was used to study NO/NO2 interference via in situ HONO formation or NO2 dissolution in H2O in the coil during sampling. PAN was synthesized according to the published method (20, 21). Briefly, 2.5 mL of peracetic acid was added into 25 mL of cooled n-tridecane (ice bath), followed by 2 mL of H2SO4. After 5 min with constant stirring, 0.5 mL of concentrated HNO3 was added in 50-µL aliquots. The reaction mixture was stirred for another 5 min and poured into 25 mL of ice water in a separatory funnel. The tridecane layer containing PAN was separated and stored in the freezer until use. To generate a steady flow of PAN gas, a slow flow of N2 gas (∼2-5 cm3 min-1) was first passed through a 20-mL vial containing ∼2 mL of the PAN-tridecane solution kept at 0 °C and then diluted with 2-10 L/min UHP N2. The PAN content in the diluted gas was monitored by a NOx analyzer (model 2108, Dasibi). The conversion efficiency was greater than 90% for PAN for the Mo converter temperature of 290 °C. The diluted PAN gas was fed to coil samplers to study its interference with HONO measurements. Nitrate interference was examined by spiking DNPH working solutions with 10 µM and 1 mM NaNO3 and monitoring the formation of 2,4-dinitrophenyl azide (DNPA), the derivative formed from the analyte nitrite. System Automation. The HONO measurement system was automated based on that of Zhou et al. (22) and is illustrated in Figure 1. The coil sampler is placed in a desirable sampling spot. The air sample is being scrubbed continuously. The DNPH solution is added into the returning scrubbing solution right after it is separated from the sample air. The solution mixture goes through a 3-mL glass coil for online derivatization. The solution is then loaded onto one of the two online C18 preconcentration columns on a 10-port autoinjection valve. While a time-integrated sample is being injected into HPLC for analysis, another sample is being collected which is alternated every 5 min. The system was VOL. 33, NO. 20, 1999 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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automated by using Macintosh-based HPLC software that controlled sample injection and acquired chromatographic data. Field Sampling and Intercomparison. The technique was deployed for the measurement of ambient HONO in downtown Albany, NY. The sampling site was located at a large central courtyard that serves as the air intake for the Empire State Plaza. The courtyard is roughly a cube of 30 m in length, width, and height with about 15 300 m3 min-1 air drawn through the area, resulting in a 1.8-min residence time of air in the courtyard. Therefore, the air sample collected would be representative of ambient air in downtown Albany. As a part of the method validation procedure, a second and independent method was also deployed at the same time to measure ambient HONO concentration. A fritted glass bubbler containing phosphate buffer solution (1 mM at pH 7.0) was used for sample collection. Scrubbing solution volume was 10 mL, and sampling flow rate was 1 L min-1. The samples were stored in the refrigerator and were analyzed within 24 h using an ion chromatograph (Dionex, model 2000i). Due to the relatively low sensitivity of IC technique (∼0.3 µM), a long sample integration time (4 h) was required.
Results and Discussion The DNPH reagent, in combination with HPLC separation and UV detection, has been widely used for the measurement of carbonyl compounds in atmospheric and other environmental samples (19, 23-25). It was recently used by Kieber and Seaton (26) for the first time to measure nitrite in natural waters. The reaction of nitrous acid with DNPH produces a highly UV-absorbing derivative, 2,4-dinitrophenyl azide (DNPA):
FIGURE 2. HPLC chromatograms of (a) a standard with 20 nM NaNO2 spiked, (b) a reagent blank, containing ∼4 nM nitrite, and (c) a blank of DNPH reagent further purified by filtration through a DNPHsaturated C18 cartridge. Identification: 1, reagent; 2, nitrite derivative; 3, formaldehyde derivative. HPLC conditions: isocratic at 35% acetonitrile in water at an elution flow rate of 1 mL min-1; injection volume 2.4 mL; detection wavelength 309 nm. significantly deviate from the theoretical calculation for equilibrium may not be reached between the scrubbing solution and the sample gas. An earlier study (19) indicated that quantitative collection could be achieved using a coil sampler for a species with an apparent Henry’s law solubility of g5 × 104 M atm-1. Therefore, atmospheric HONO is expected be quantitatively scrubbed by the coil sampler using a scrubbing solution at pH g 7. Laboratory experiments have been carried out to determine the collection efficiency of HONO by the coil sampler under various conditions. Ambient air was sampled by two identical samplers connected in series so that the second one sampled the effluent air from the first one. The collection efficiency, β, was determined from the ratio of nitrous acid collected by the two coils, using the following equation (19):
β ) (1 - n2/n1) × 100% DPHA is then separated from reagents and hydrazones by C18 reverse-phase HPLC using acetonitrile and water as mobile phases. DNPA is detected at 309 nm by an UV-visible detector. Figure 2 shows typical chromatograms of a standard and reagent blanks with and without further purification (see discussions later). In this study, we combined this analytical technique with a sampling technique using a coil scrubber to become a highly sensitive method for the measurement of atmospheric nitrous acid (HONO). Collection Efficiency. A maximum collection efficiency, βmax, is achieved when the scrubbing solution is in equilibrium with the sampling gas and is expressed in
βmax ) FlH*RT/(Fg + FlH*RT)
(1)
where Fl and Fg are flow rates of scrubbing solution and sampling gas, H* is the apparent Henry’s law constant, R is the gas constant, and T is the absolute temperature. Nitrous acid is a weak acid, with a pKa of 3.25 at 25 °C (27). Although HONO is only marginally soluble in acidic solutions with an intrinsic Henry’s law solubility of 49 M atm-1 (28), it is highly soluble in solutions at neutral or higher pH, e.g., the apparent Henry’s law solubility of 2.6 × 105 M atm-1 in a solution of pH 7. The maximum collection efficiency is 99.9% using a pH 7 scrubbing solution. The actual efficiency may, however, 3674
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(2)
where n1 and n2 were the amounts of nitrous acid collected by the first and the second samplers, respectively. The measured collection efficiency as functions of the sample gas flow rate and of the pH of the scrubbing solution is shown in Figure 3a,b. A high sampling flow rate of 4 L min-1 was used in the pH-dependence study to enhance the effect of the studied variable. The β value increased with pH (Figure 3a), as a result of the higher apparent Henry’s law constant for HONO. At pH 6.3 or higher, quantitative collection (g95%) was achieved at an air sampling flow rate of 4 L min-1 and at a scrubbing solution flow rate of 0.24 mL min-1. At pH 7, the collection efficiency decreased slightly with air flow rate to about 90% at a sampling flow rate of 5.8 L min-1 (Figure 3b). Under the typical sampling conditions, i.e., air sampling flow rate of 2 L min-1, a scrubbing solution flow rate of 0.24 mL min-1, and a scrubbing solution of pH 7.0, the collection efficiency was 97%. Derivatization. To optimize derivatization conditions, the reaction equilibrium and kinetics of nitrite and DNPH were investigated. An apparent equilibrium constant, K ) [DNPA]/ ([DNPH][nitrite]), was estimated to be ∼2.8 × 105 M-1 at room temperature (23 ( 1 °C) and with [H+] ) 3 × 10-3 M. Nitrite was quantitatively converted to azide at DNPH concentrations of g100 µM at equilibrium. Although HONO conversion to azide is quantitative at equilibrium even at relatively low DNPH concentrations, the
FIGURE 3. (a) HONO collection efficiency by a 15-turn coil sampler as a function of sample flow rate. Conditions: scrubbing solution pH ) 7.0, scrubbing solution flow rate ) 0.24 mL min-1, ambient temperature ∼23 °C, and ambient pressure ∼1000 mbar. (b) HONO collection efficiency by a 15-turn coil sampler as a function of scrubbing solution pH. Conditions: air sampling flow rate 4 L min-1, scrubbing solution flow rate 0.24 mL min-1, ambient temperature ∼23 °C, and ambient pressure ∼1000 mbar. actual derivatization yield was also dependent upon the rate and time of derivatization. The reaction (R1) is acid catalyzed. The effect of pH is shown in Figure 4a. At low acidity, the reaction is quite slow, e.g., resulting in 10% yield at 90 min and 36% at 360 min in a reaction medium containing 200 µM DNPH and 0.3 mM HCl. At a higher acidity, e.g., in a 3 or 10 mM HCl solution, the yield was 100% at 90 min or longer. A detailed kinetic study is illustrated in Figure 4b. For 8 mM HCl and 300 mM DNPH, the typical derivatizing solution used in field measurement, the pseudo-first-order reaction rate constant is 0.98 min-1 at 45 ( 1 °C. That is, it takes only about 5 min to convert 99% nitrite into azide at this temperatures. In our measurement system, the combined flow rate for scrubbing solution and DNPH working solution was 0.5 mL min-1, and the derivatization coil was 3.0 mL. The resulting 6-min derivatization time would allow quantitative derivatization of nitrite in the scrubbing solution. Blank and Calibration. Among the three blanks, the reagent blank was the lowest, from below the instrumental detection limit of 0.1 nM for purified reagent to 3-5 nM for unpurified reagent (see discussions below). The method blank with the coil sampler bypassed was slightly higher than the reagent blank, by a fraction of nM, due to impurities in the phosphate buffer scrubbing solution, but it was about 2-5 nM lower than the N2/zero air blank. When the HONO impurity in the N2 or zero air was scrubbed by bubbling through an ice-water bath before feeding to the coil sampler, the two system blanks were identical within (5%. Figure 5 is a calibration curve for aqueous nitrite with concentrations ranging from 20 to 1000 nM. The circles represent data obtained with the coil sampler bypassed, and the two squares are for data with N2 gas flowing through the coil sampler. The two calibrations were almost identical, with
FIGURE 4. (a) Derivatization yield as a function of derivatization time at different acidities: open circles, 0.3 mM HCl; open triangles, 1 mM HCl; open diamonds, 3 mM HCl; crosses, 10 mM HCl. Conditions: [DNPH] ) 200 µM and T ) 23 °C. HPLC signals have been normalized to the highest signal. (b) Detailed derivatization kinetics of nitrous acid. Conditions: [DNPH] ) 300 µM, [HCl] ) 8 mM, and T ) 45 °C. Different symbols represent replicates of experiments. The curve fit gives the pseudo-first-order rate constant of 0.98 min-1.
FIGURE 5. Calibration curves for nitrous acid. Open circles represent the aqueous calibration data with coil sampler bypassed, and open squares represent the aqueous calibration data with coil sampling UHP N2. The correlation for the linear fit is 0.9998 (r 2). Injection volume was 1.3 mL. only a slightly larger intercept of the latter than the former, probably due to a contribution of HONO impurity in the N2. Under typical sampling conditions, i.e., Fg ) 2 L min-1 and Fl ) 0.24 mL min-1, 1 nM nitrite in the scrubbing solution corresponds to 2.7 pptv HONO in the gas phase. The VOL. 33, NO. 20, 1999 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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calibration curve (Figure 5) covers a corresponding gas-phase concentration range of 54 pptv-2.7 ppbv. Reagent Purification and Detection Limit. When DNPH was used as received without further purification, a reagent blank value of about 4 nM nitrite was observed. With a relative uncertainty of e10% for the same batch of reagent, a lower detection limit of 1.2 nM was estimated ((3 σ). Under our typical sampling and analysis conditions (Fg ) 2 L min-1, Fl ) 0.24 mL min-1, sample integration time ) 5 min), this corresponds to a gas-phase detection limit of e3.3 pptv. The low detection limit is sufficient for ambient HONO measurement in most atmospheric environments with a possible exception of the remote free troposphere during the daytime. To lower the detection limit, DNPH needs further purification. Although extraction with CCl4 is an effective way to remove hydrazone and azide impurities from DNPH aqueous solutions (25, 26), it is a tedious procedure. In this study, we connected a DNPH-saturated Sep-Pak C18 cartridge in the reagent line to selectively remove azide and hydrazones, which are more hydrophobic than the reagent itself. This treatment effectively eliminated all azide and almost all formaldehyde hydrazone impurities (Figure 2c in comparison with Figure 2b) and significantly lowered the detection limit of this method to 0.3 nM in the aqueous phase or e1 pptv in the gas phase. The aqueous detection limit of 0.3 nM, which is higher than the 0.1 nM reported by Kieber and Seaton (26), was due to the elevated blank with a contribution of nitrite impurity from the phosphate buffer solution (the scrubbing solution). Potential Interference from NOx. NOx may be present at high concentrations in the source regions, often in tens of ppbv and sometimes even up to hundreds of ppbv levels. It is thus important, as a part of method validation, to make sure that NO/NO2 does not cause any significant artifact during sampling and analysis under atmospheric conditions, especially in urban environments. Experimental results indicated that a small HONO signal was observed when NO2 was added to the sample gas (N2) (Figure 6a). The signal increased linearly with NO2 concentration in the range of 25-200 ppbv, with a slope of 10 pptv HONO/100 ppbv NO2. For urban environments, a high NO2 concentration of 100 ppbv would cause ∼10 pptv artifact in the HONO signal. Fortunately, the ambient HONO concentration observed in the polluted air masses was much higher than this artifact, in several hundreds of pptv during daytime and several ppbv in the nighttime (7, 9). In the remote clean atmosphere, NO2 is mostly below the 1 ppbv level. The expected artifact signal of e0.1 pptv will thus be below the method detection limit of 1 pptv. The artifact signal from NO2 may be caused by the combination of two processes: direct dissolution of NO2 in the scrubbing solution followed by DNPH derivatization of dissolved NO2 and in situ HONO formation via NO2-H2O reaction in the coil during sampling. NO2 is barely soluble in water, with a Henry’s law solubility of ∼10-2 M atm-1 (29), i.e., an aqueous concentration of 1 nM in equilibrium with a 100 ppbv NO2 gas. The dissolved NO2 reacts with DNPH to form a DNPA derivative. The signal of 1 nM in the aqueous phase corresponds to ∼3 pptv in the gas phase, which is only a fraction of ∼10 pptv observed in the artifact HONO signal. The second process involves NO2 reaction with water to form nitrous acid and nitric acid in the sampling coil:
2NO2 + H2O f HNO2 + NO3- + H+
(R2)
The aqueous reaction R2 is relatively slow, with a secondorder rate constant of 1 × 108 M-1 s-1 (30, 31). At a maximum concentration of 1 nM NO2 in equilibrium with 100 ppbv in gas phase, a maximum of 1 nM HNO2 (corresponding to 2.7 pptv in the gas-phase signal) may be produced in the 3676
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FIGURE 6. Artifact HONO signals as a function of NOx concentration added into N2 gas: (a) NO2, (b) NO, and (c) NO + NO2 with a ratio of 1:1. A 28-turn coil sampler was used in all the experiments except one in which a 15-turn coil sampler was used (diamond symbols in panel a). Sampling conditions: sampling flow rate 2 L min-1, scrubbing solution flow rate 0.24 mL min-1, sample integration time 5 min. sampling coil during a 10-s gas-liquid contact time, which is again only a fraction of ∼10 pptv observed in the artifact signal. In addition to the homogeneous reaction in the aqueous phase, reaction R2 may also proceed heterogeneously at the gas-liquid interface, which has been shown to be first-order to NO2 and at a higher rate at low NO2 concentrations (32, 33). Its rate, however, is difficult to estimate due to the lack of information on its reaction kinetics (33). NO was found to cause negligible interference, producing ∼2.5 pptv HONO signal for 100 ppbv NO in sample gas (Figure 6b). Later measurement using a NOx analyzer showed that ∼10% of NOx in the 2-year-old 10 ppmv NO cylinder was in the form of NO2. This leads us to conclude that the artifact was not caused by NO alone, but rather involved a NO2 impurity through reaction R2 and reaction R3:
NO + NO2 + H2O f 2HNO2
(R3)
To examine the importance of reaction R3 in producing the artifact HONO signal, equal concentrations of NO and NO2 were added simultaneously to the sample gas (N2). Experiments showed that about 30% more HONO signal was produced with NO present (resulting in twice the NOx concentration) than NO2 alone (Figure 6a,c). This suggests that reaction R3 may indeed play some role in forming HONO artifact signal when both NO and NO2 are present.
FIGURE 7. Artifact HONO signals as a function of PAN concentration added into zero air. Sampling conditions were the same as Figure 6.
FIGURE 8. HPLC response as a function of preconcentration volume. Sample contained 0.8 µM NaNO2. Derivatization conditions: 300 µM DNPH and 8 mM HCl.
It should be noted that the above discussions on NO/NO2 interference were based on experimental results using a 28turn coil sampler to enhance the interference effect. When a 15-turn coil sampler was used, the artifact HONO signal was reduced by almost half (Figure 6a), making the interference from NO and NO2 even less significant. Potential Interference from PAN. PAN is ubiquitous in the troposphere with its concentration reaching several ppbv in photochemically reactive high-NOx and high-VOC environments (34). It is therefore important to study the effect of PAN on the HONO measurement by this method. PAN is slightly soluble in water with a Henry’s law solubility of 3.6 M atm-1 (35). If equilibrium between gas and liquid phases were reached in the coil during sampling and if all the dissolved PAN was hydrolyzed to form equal moles of HNO2, the 1 ppbv PAN in the sample gas would produce a maximum of 3.6 nM HNO2, i.e., a maximum of ∼10 pptv HONO. Figure 7 shows experimental results of artifact HONO signal plotted against PAN concentration in the sample gas. At low PAN concentrations, a small amount of artifact HONO was produced, e.g., ∼6 pptv for 17 ppbv PAN. At higher PAN concentrations, significantly more HONO signal was observed, e.g., ∼ 30 pptv HONO for 36 ppbv PAN and ∼59 pptv HONO for 57 ppbv PAN. Extrapolations of the higher concentration data may thus provide the upper limit of PAN interference, i.e., 1 pptv HONO signal from 1 ppbv PAN. This is still far less than the maximum value of 10 pptv HONO signal per 1 ppbv PAN from the above calculation, suggesting that the scrubbing solution is far from reaching an equilibrium with respect to the gaseous PAN in the coil sampler and/or that hydrolysis of the scrubbed PAN is too slow to form equivalent moles of nitrite in the scrubbing solution. Since PAN concentration rarely exceeds a few ppbv even in the polluted environments (34) where HONO is also expected to be at relatively high concentrations, PAN is not expected to cause any significant interference. Potential Interference from Nitrate. Nitric acid is present at much higher concentrations than HONO in the atmosphere and is quantitatively scrubbed in the coil sampler. However, it is not expected to cause any interference in HONO measurements by the current method, for no mechanism exists for nitrate and DNPH to react and produce the DNPA derivative. Experimental results showed that no difference was observed between a DNPH reagent blank and a reagent solution spiked with 10 µM NaNO3, confirming the above conclusion. A 1-mM NaNO3 sample derivatized with DNPH produced HONO signals of a few nanomolars, which varied
depending on the sources of NaNO3. These signals were more likely induced as a HONO impurity in the nitrate salt. Potential Interference from Ozone. O3 is a major photooxidant in the troposphere with its concentration up to over 100 ppbv. Potential interference from O3 was therefore examined by calculating potential losses of the scrubbed nitrite and DNPH reagent via reactions with O3. During sampling, the contact time between sample air and scrubbing solution in the coil sampler was short, in the order of seconds. Loss of the scrubbed nitrite due to its aqueous reaction with O3 during sampling is calculated to be
