- Email: [email protected]
HNO3 analyzer by scrubber difference and the NO–ozone chemiluminescence method
Atmospheric Environment 35 (2001) 5339–5346
HNO3 analyzer by scrubber difference and the NO–ozone chemiluminescence method Masatoshi Yamamotoa,*, Motonori Tamakia, Hiroshi Bandowb, Yasuaki Maedab a
Environmental Science Institute of Hyogo Prefecture, 3-1-27 Yukihira-cho, Suma-ku, Kobe, 654-0037, Japan b Osaka Prefecture University, Graduate School of Engineering, 1-1 Gakuen-cho, Sakai 599-5831, Japan Received 5 February 2001; received in revised form 8 May 2001; accepted 16 May 2001
Abstract A fast response analyzer for HNO3 in highly polluted air is described. The time resolution attainable was 12 s. The method is based on the difference in a technique for HNO3-scrubbed and non-scrubbed air and the reduction of HNO3 to NO with the use of a line of catalytic converters and a method for the subsequent NO-ozone chemiluminescence. A sample air stream, in which particulates are removed with a Teflon filter, is divided into two channels. CH-1 is directly connected to the converter line, and CH-2 contains a HNO3 scrubber packed with a nylon fiber that goes to another converter line. Each converter line is composed of a hot quartz-bead converter (QBC) and a molybdenum converter (MC) in a series. A QBC reduces HNO3 to (NO+NO2), which is called NOx. The MC reduces the NOx to NO. For CH-1, the analyzer detects most compounds that typically comprise NOy (J. Geophys. Res. 91 (1986) 9781). These CH-1 compounds are called NO0y hereafter (NOy-particulate nitrate) because the particulates are removed by the filter. A difference in the detector signal for the two channels indicates HNO3. For a blank test, atmospheric air in which HNO3 was pre-scrubbed by an extra nylon fiber was introduced to the analyzer. Variations in the blank value were 0.3870.42 and 0.3470.55 ppb during the high readings (NO0y -HNO3 ) (called NO*y hereafter) (111712 ppb, N ¼ 180), and low NO*y readings (6278 ppb, N ¼ 180), respectively, indicating that the lowest detection limit of the analyzer is 1.1 ppb (2s). When the data obtained with the analyzer is compared to the data using the denuder method, a linear correlation with the regression of Y ¼ 0:973X þ 0:077 (r2 ¼ 0:916 (N ¼ 20)) in the range of 0–6.5 ppb HNO3 is obtained, which is an excellent agreement. Atmospheric monitoring was carried out at Kobe. Although the average concentration of HNO3 was 2.671.3 ppb, ca.10 ppb for a HNO3 concentration was occasionally observed when the NO*y concentration was high, i.e., more than 100 ppb. r 2001 Elsevier Science Ltd. All rights reserved. Keywords: Fast response HNO3 analyzer; Quartz-bead converter; Molybdenum converter; Nylon filter; NOy
1. Introduction As a component of acid aerosol, HNO3 is one of the most serious atmospheric pollutants and is the key species of nitrogen chemistry in the atmosphere. In order to understand the behavior of the nitrate species in the atmosphere, it will be important to learn more details about the temporal changes of gaseous HNO3. There*Corresponding author. Fax: +81-78-735-7817. E-mail address: [email protected] (M. Yamamoto).
fore, a method that would allow a fast analysis of HNO3 is needed. To speed up the analysis, we have examined the use of a scrubber for different techniques, using a catalytic converter to detect NO or NO2. Tanner et al. (1998) measured inorganic nitrate (HNO3+particulate nitrate) using a method involving a dual-channel converter. Each channel contained an Au–CO converter that reduced NOy (Fahey et al., 1986; Fehsenfeld et al., 1987; Williams et al., 1998): NO+NO2 +HNO3+HONO+HNO2NO2+NO3+2N2O5+PAN +Particulate nitrate(NO 3 )+? to NO; the other channel contained a nylon filter to remove the nitrate
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species. For HNO3 detection, we used a Teflon pre-filter to remove particles prior to splitting the two channels. Each channel contains a QBC and an MC that reduce NO0y to NO (CH-1), and one channel(CH-2) also contains a nylon filter that removes HNO3 prior to the converter. QBC was used for the HNO3 reducing catalyst in place of the Pyrex-bead converter (Kelly et al., 1979; Burkhardt et al., 1988; Harrison and Msibi, 1994). The NO0y detected by the QBC–MC system described here, is supposed to more closely resemble a total reactive nitrogen oxide (NOy). The NO*y (NOy-(HNO3+particulate nitrate)) described by Tanner et al. (1998) may contain the same NO*y compounds as those detected in the CH-2 in this report. In the highly polluted NOy atmosphere, the HNO3 mixing ratio to NOy is too small (only several percentage points) to measure when using the difference technique. In this report, each channel of the apparatus was carefully designed and operated identically to the other. A blank test was used under atmospheric conditions to examine the identity of each channel. In the preliminary experiment, we found that the Pyrex glass tube and beads for the HNO3 converter and the molybdenum oxides for the NO2 converter had deteriorated considerably during the several months that they were monitored at temperatures in excess of 3501C. The QBC is thermally more stable, and, therefore, the present system is expected to be more convenient for long-term monitoring.
2. Experimental 2.1. Instrumentation A schematic view of the measurement system is shown in Fig. 1. A sample air stream, in which particulates are
removed by a Teflon filter, is divided into two pathways. One contains a HNO3 scrubber packed with a nylon fiber sheet followed by the QBC–MC (CH-1, NO0y HNO3). The other is directly connected, i.e., without the HNO3 scrubber, to another QBC–MC (CH-2, NO0y ). CH-1 is connected to a NO-inlet and CH-2 to a NOxinlet on the analyzer (Monitor Lab. Inc., model 9841A). In this report, the part of the valve manifold and NO detection system of the analyzer is called an NO detector. The CH-2 output signal corresponds to NO0y ; and the signal that is electrically subtracted, (CH-1) (CH-2), corresponds to the HNO3 concentration. HNO3 is known to readily adsorb many materials (Goldan et al., 1983; Appel et al., 1988; Neuman et al., 1999). To lower the adsorption on the surface of the inlet line that transports sample air to the HNO3 converter, Teflon materials were used for the inlet parts, which consisted of the following: a particle filter (Sumitomo Electric Co., AF07P, F 47 mm), a filter 1 00 holder (F 47 mm), and a tube and connecting tee (16 00 o.d.). The HNO3 scrubber is a Teflon tube (14 i.d. and 12 cm long) packed with a nylon fiber sheet (Unitica, Spanbond-No. 4, 0.5 10 cm2). The detection system of the instrument provides an alternate measurement of the sample air in CH-1 and CH-2, respectively, by one detection cell. The sample air in each channel flows at a constant rate of 0.32 l/min without stopping during NO detection in one reaction cell by switching valves (as shown in Fig. 1) that are controlled by computer. One channel requires 6 s to detect NO, and both channels require 12 s for the completion of one cycle of measurement. In each cycle, in order to synchronize air samples from each channel, the sample air stream from CH-2 must be delayed by 6 s from CH-1. A Teflon tube 00 (14 o.d.) was inserted as a delay coil behind an HNO3 scrubber in CH-1. It was cut to the calculated size of 122 cm.
Fig. 1. A schematic view of the fast response HNO3 analyzer by the scrubber-difference method followed by NO–O3 chemiluminescence. V and O indicate an air valve and an orifice used to control the flow rate of sample air, respectively.
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In order to lower HNO3 adsorption on the inner surface of the inlet, all the inlet lines between the prefilter and the QB converter were placed in a temperature-controlled (501C) box, except for the two HNO3 scrubbers used to measure atmospheric HNO3 and blanks as shown in Fig. 1. In the summer, water vapor condensation was occasionally observed on the inner surface of the indoor part of the inlet tube when the room temperature where the analyzer was placed and cooled by an air conditioner was considerably lower than the outdoor temperature and the outdoor humidity was high. Under such conditions, heating the indoor part of the inlet effectively prevented the condensation. The HNO3 converter was made up of a quartz tube (7 mm i.d., 24 cm long) filled with quartz beads (F4 mm). Two QBCs, one for each channel, were set together in a heater block 360721C. Two MCs that had the same 00 geometry and material (stainless steel tube, 38 o.d. and 587 mm long) as commercially available ones were placed together in a heater block 315721C. The configuration of the two sets of QBCs and MCs were designed to be identical physically and chemically. The data of the NO0y and HNO3 concentrations were output from the analyzer every 12 s, which implied that the intrinsic time resolution attainable was 12 s. An acquisition of these original data was conducted by a personal computer. A 3-min running average of the original data was used for this paper. 2.2. Standard HNO3 gas
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Institute of Hyogo Prefecture. The building is located in a residential area about 10 km west of the center of Kobe and close to a highway about 150 m west of the building. The sampled air at the site is directly and frequently affected by the traffic exhaust.
3. Results and discussion 3.1. Calibration by the standard HNO3 The standard gases were generated, and the analyzer was calibrated using standard HNO3 gas of 0–88 ppb concentration, which was determined by IC. These tests were performed in dry clean air, which was purified by passing through four columns packed with silica gel, soda lime, charcoal, and molecular sieves. Impurities in the clean air were about 0.5 ppb NO. As can be seen in Fig. 2, linearity was demonstrated for HNO3 concentrations that were relatively high, above 20 ppb, showing a regression of the data which yielded the formula, Y ¼ ð1:0570:06ð1sÞÞX þ ð1:8572:69Þ; and the correlation coefficient of 0.994 (N ¼ 4) for the regression line. In the range between 0 and 20 ppb, no calibration could be performed because it was too difficult to generate stable standard HNO3 concentrations, even though HNO3 was typically measured in that range (Figs. 5 and 7). However, we assumed that linearity existed in that range because the extrapolated regression line nearly intercepted the origin.
To calibrate the response of the analyzer, standard HNO3 was generated by a diffusion tube (4 ml Pyrex glass tube containing 60% HNO3 solution). The concentration of the generated standard gas was determined by ion chromatography (IC) (Dionex DX100) after sampling by two fritted bubblers in a series with an IC eluent solution of 10 ml as an absorbent. 2.3. Annular denuder A denuder technique (Possanzini et al., 1983) was used for simultaneous measurement of HNO3 in ambient air. The annular denuder (URG2000F30 150F3css) was coated with NaCl/glycerol to collect gaseous HNO3. NaCl was prepared as a 1% solution in a 1 : 1 methanol/water ratio that contained 1% glycerol. After sampling, the collected HNO3 was extracted in deionized water and analyzed as nitrate ions using IC. 2.4. Atmospheric measurement Atmospheric measurements were made on the veranda of the second floor of the Environmental Science
Fig. 2. Calibration of the HNO3 analyzer to standard HNO3. Standard gases were generated with the use of a diffusion-tube method, and the concentration was determined by a frittedbubbler sampling and an ion-chromatography. These tests were performed in dry clean air.
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With 20-min sampling periods for each concentration, the precision of the measurement was near 3%, and overall accuracy was estimated to be 710 ppb for the standard HNO3 measurement with a fritted-bubbler-IC method and 75% for the HNO3 analyzer.
3.2. Temperature dependence of the QBC–MC system The response of the analyzer to the temperature of the QBC was examined. The temperature was controlled within a range of 310–3861C for the QBC and kept
Fig. 3. The temperature dependence of the efficiency of a quartz-bead converter for HNO3 conversion. The temperature of a molybdenum converter was constant at 3151C.
Fig. 4. The variation of the NO*y concentration and the blank value of the HNO3 analyzer response for HNO3-scrubbed ambient air. The scale of the blank ordinate is expanded by 50 times for clarity.
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constant at 315721C for the MC. The standard HNO3 of 3671 ppb was constantly generated during the 1-hplus experiment. As shown in Fig. 3, the relative response to the standard HNO3 concentration was obtained at 1.0 within a range of 340–3851C, and 3601C was used as the optimal operating temperature for the QBC. From the result, it can be concluded that HNO3 was completely converted to NO by the QBC– MC system. 3.3. Blank value and detection limit To verify the identity for the two channels under atmospheric conditions and to evaluate the detection limit, HNO3-free ambient air was examined. Two valves immediately after the pre-filter, which are shown in Fig. 1, were switched into a position requiring an extra HNO3 scrubber for the blank experiment. Since HNO3 in the sample air had already been scrubbed by the extra scrubber, the observed signals for the two streams should be equal, and, thus, the output for HNO3, i.e., the difference between the two streams must be 0 ppb. Fig. 4 shows an example of the result for the blank experiment. The scale of the blank is multiplied by 50 of
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the NO*y scale. During periods of high (111712 ppb), middle (9377 ppb), and low (6278 ppb) NO*y regimes, the average blank values for the HNO3 measurement corresponding to the period and the standard deviation associating with the average were (0.3870.42), (0.3370.57), and (0.3470.55) ppb, respectively. These results demonstrate that: (1) although all average blank values are positive, they suggest that the two measurement modes (the NO0y and the NO*y) may have a systematic difference and that the values are all within the standard deviations; (2) the blank values do not associate with the NO0y concentration levels, implying that the blank may not be a plausible artifact resulting from an improper subtraction between the NO0y and the NO*y and (3) although the cause of the positive deviation is not identified yet, it would be rational to regard the blank values as practically zero. From the results shown in Fig. 4, with the consideration made above, the detection limit of this method for HNO3 could be conservatively evaluated to be 1.1 ppb (2s) with a 3-min running average. Generally, HNO3 accounts for only several percentage points of the total nitrogen oxide species in urban
Fig. 5. A comparison of the HNO3 analyzer response with the denuder method. Gray line: The HNO3 concentration measured by the denuder method. The width of the lines corresponds to each denuder sampling period. Solid line: HNO3 concentration measured by the fast response HNO3 analyzer. 3-min-running mean and its average values for the periods corresponding to the denuder sampling.
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air. When measuring HNO3 in highly polluted air in which an NO0y concentration often changes drastically and extensively, a small discrepancy in the stream of both the NO0y and NO*y lines could cause a large variability or high blank values of the HNO3 concentration. To lower the variability and the blank values and to obtain a higher degree of accuracy, a precisely designed delay coil was successfully used. However, the small blank values and the standard deviations could no longer be suppressed. Considering the origin of such a blank value, it assumed that there were some differences in the NO concentration in the two lines, which may have been caused by a chromatogram tail as described by Tanner et al. (1998).
3.4. A comparison of the data using an HNO3 analyzer and the denuder method In order to examine the feasibility of the analyzer, a comparative experiment using the denuder method was conducted. For the annular denuder system used, the detection limit was evaluated to be 0.4 ppb, which is about half of the stated value (Waldman, 1987) when taking three standard deviations of the field blank measurement for 1 h sampling and the IC analysis. At levels well above the detection limit, the precision of the measurement is ca. 5%, and overall accuracy is estimated to be 715% for ambient atmospheric measurement of HNO3. Fig. 5 shows all the results of the simultaneous measurement by the HNO3 analyzer and the denuder method. The gray line indicates the sampling period for the denuder method and the concentration of HNO3 measured by the denuder during the period. The solid lines are for a 3-min-running mean HNO3 concentration measured by the HNO3 analyzer and its average values for the periods corresponding to the denuder sampling. In the summer, a high HNO3 concentration above 10–20 ppb was occasionally observed at the sampling site. Therefore, a comparison of such a high concentration range for HNO3 is also needed. We developed an automatic start for the denuder sampling and applied it to collect the sample during a high HNO3 concentration episode, i.e., the sampling starting when the output of the HNO3 analyzer was more than 10 ppb and continued for 1 h. The sporadic data on 6 and 8 September, were thus obtained for the high concentration episode. If the averaged data are plotted against the denuder data, the correlation that is shown in Fig. 6 would be obtained. The result shows a linear relationship between the two methods. The linear regression for all data gives the result, Y ¼ ð0:97370:140ð2sÞÞX þ ð0:07770:370Þ; clearly demonstrating that the analyzer we developed provides data that is almost identical to that of the
Fig. 6. A scatter plot of the HNO3 concentration measured by the fast response HNO3 analyzer against that by a denuder method. The data of the analyzer were averaged during the period corresponding to the denuder sampling.
denuder measurements. It should be noted that the scatter is rather high, the largest deviation from the regression line being ca. 1 ppb. In the low HNO3 concentration range, i.e., 3 ppb or less, the scatter is especially high. This result could be attributed partially to the uncertainty in the HNO3 measurement caused by a long period of denuder sampling of the polluted air, but the extent of the scatter has the same magnitude as the two standard deviations of the measurement for zero-HNO3 ambient air discussed in Fig. 4.
3.5. Atmospheric measurement Atmospheric measurements were conducted for nine days in the summer (4–13 September 2000). The measurement site has frequently experienced elevated levels of NOx. Fig. 7 shows the variation in the HNO3 and NO0y concentrations. The scale of the HNO3 concentration is multiplied by 10. The average NOx and HNO3 concentrations (N ¼ 12950) during the period were 47723 and 2.671.3 ppb, respectively, and the maximum concentrations were 203 and 10.1 ppb, respectively. The concentration of HNO3 had a temporal variation pattern that was similar to NOx except on 11 September, while ca. 10 ppb of the HNO3 concentration was occasionally observed when the NO0y concentration was high, i.e., more than 100 ppb. The average of the ratio HNO3 =NO0y of every instantaneous measurement was
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Fig. 7. An example of temporal variations of HNO3 and NO*y measured by the HNO3 analyzer, measured from 4–13 September 2000, at the Environmental Science Institute of Hyogo Prefecture, Kobe. The gray area indicates the nighttime sampling period.
6.474.0%, and each instantaneous ratio was generally smaller than 10%. When we take into account, the fact that in the urban air, the main component of the NO*y is, in general, NOx, these results agree with those from other studies that were obtained in the polluted urban air (Spicer, 1977).
References Appel, B.R., Povard, V., Kothny, E.L., 1988. Loss of nitric acid within inlet devices intended to exclude coarse particles during atmospheric sampling. Atmospheric Environment 22 (11), 2535–2540. Burkhardt, M.R., Buhr, M.P., Rav, J.D., Stedman, D.H., 1988. A continuous monitor for nitric acid. Atmospheric Environment 22 (8), 1575–1578. Fahey, D.W., Hu. bler, G., Parrish, D.D., Williams, E.J., Norton, R.B., Ridley, B.A., Singh, H.B., Liu, S.C., Fehsenfeld, F.C., 1986. Reactive nitrogen species in the troposphere: measurements of NO, NO2, HNO3, particulate nitrate, peroxyacetyl nitrate (PAN), O3, and total reactive odd nitrogen (NOy) at Niwot Ridge, Colorado. Journal of Geophysical Research 91, 9781–9793. Fehsenfeld, F.C., Dickerson, R.R., Hcbler, G., Luke, W.T., Ninnermacker, L.J., Williams, E.J., Roberts, J.M., Calvert, J.G., Curran, C.M., Delany, A.C., Eubank, C.S., Fahey,
W., Fried, A., Gandrud, B.W., Langford, A.O., Murphy, P.C., Norton, R.B., Pickering, K.E., Ridley, B.A., 1987. A ground-based intercomparison of NO, NO2, and Noy measurement techniques. Journal of Geophysical Research 92, 14710–14722. Goldan, P.D., Kuster, W.C., Albritton, D.L., Fehsenfeld, F.C., Connell, P.S., Norton, R.B., Huebert, B.J., 1983. CalibratioHNO3nd tests of the filter-collection method for measuring clean-air, ambient levels of nitric acid. Atmospheric Environment 17 (7), 1355–1364. Harrison, R.M., Msibi, I.M., 1994. Varidation of techniques fast response measurement of HNO3 and NH3 and determination of the [NH3][HNO3] concentration product. Atmospheric Environment 28 (2), 247–255. Kelly, T.J., Stedman, D.H., Kok, G.L., 1979. Measurements of H2O2 and HNO3 in rural air. Geophysical Research Letters 6 (5), 375–378. Neuman, J.A., Huey, L.G., Ryerson, T.B., Fahey, D.W., 1999. Study of inlet materials for sampling atmospheric nitric acid. Environmental Science and Technology 33, 1133– 1136. Possanzini, M., Febo, A., Liberti, A., 1983. New design of a high-performance denuder for the sampling of atmospheric pollutants. Atmospheric Environment 17 (12), 2605–2610. Spicer, C.W., 1977. Photochemical atmospheric pollutants derived from nitrogen oxides. Atmospheric Environment 11, 1089–1095.
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Tanner, R.L., Valente, R.J., Meagher, J.F., 1998. Measuring inorganic nitrate species with short time resolution from an aircraft platform by dual-chanel ozone chemiluminescence. Journal of Geophysical Research 103, 22387–22395. Waldman, J.M., 1987. Operations manual for the annular denuder system used in the US EPA/RI VM atmospheric acidity study. UMPNJ-Robert Wood Johnson Medical School, Piscataway, NJ.
Williams, E.J., Baumann, K., Robert, J.M., Bertman, S.B., Norton, R.B., Fehsenfeld, F.C., Springton, S.R., Nunnermacker, L.J., Newman, L., Olyszyna, K., Meagher, J., Hartsell, B., Edgerton, E., Pearson, J.R., Rodgers, M.O., 1998. Intercomparison of ground-based NOy measurement techniques. Journal of Geophysical Research 103, 22261– 22280.
HNO3 analyzer by scrubber difference and the NO–ozone chemiluminescence method Masatoshi Yamamotoa,*, Motonori Tamakia, Hiroshi Bandowb, Yasuaki Maedab a
Environmental Science Institute of Hyogo Prefecture, 3-1-27 Yukihira-cho, Suma-ku, Kobe, 654-0037, Japan b Osaka Prefecture University, Graduate School of Engineering, 1-1 Gakuen-cho, Sakai 599-5831, Japan Received 5 February 2001; received in revised form 8 May 2001; accepted 16 May 2001
Abstract A fast response analyzer for HNO3 in highly polluted air is described. The time resolution attainable was 12 s. The method is based on the difference in a technique for HNO3-scrubbed and non-scrubbed air and the reduction of HNO3 to NO with the use of a line of catalytic converters and a method for the subsequent NO-ozone chemiluminescence. A sample air stream, in which particulates are removed with a Teflon filter, is divided into two channels. CH-1 is directly connected to the converter line, and CH-2 contains a HNO3 scrubber packed with a nylon fiber that goes to another converter line. Each converter line is composed of a hot quartz-bead converter (QBC) and a molybdenum converter (MC) in a series. A QBC reduces HNO3 to (NO+NO2), which is called NOx. The MC reduces the NOx to NO. For CH-1, the analyzer detects most compounds that typically comprise NOy (J. Geophys. Res. 91 (1986) 9781). These CH-1 compounds are called NO0y hereafter (NOy-particulate nitrate) because the particulates are removed by the filter. A difference in the detector signal for the two channels indicates HNO3. For a blank test, atmospheric air in which HNO3 was pre-scrubbed by an extra nylon fiber was introduced to the analyzer. Variations in the blank value were 0.3870.42 and 0.3470.55 ppb during the high readings (NO0y -HNO3 ) (called NO*y hereafter) (111712 ppb, N ¼ 180), and low NO*y readings (6278 ppb, N ¼ 180), respectively, indicating that the lowest detection limit of the analyzer is 1.1 ppb (2s). When the data obtained with the analyzer is compared to the data using the denuder method, a linear correlation with the regression of Y ¼ 0:973X þ 0:077 (r2 ¼ 0:916 (N ¼ 20)) in the range of 0–6.5 ppb HNO3 is obtained, which is an excellent agreement. Atmospheric monitoring was carried out at Kobe. Although the average concentration of HNO3 was 2.671.3 ppb, ca.10 ppb for a HNO3 concentration was occasionally observed when the NO*y concentration was high, i.e., more than 100 ppb. r 2001 Elsevier Science Ltd. All rights reserved. Keywords: Fast response HNO3 analyzer; Quartz-bead converter; Molybdenum converter; Nylon filter; NOy
1. Introduction As a component of acid aerosol, HNO3 is one of the most serious atmospheric pollutants and is the key species of nitrogen chemistry in the atmosphere. In order to understand the behavior of the nitrate species in the atmosphere, it will be important to learn more details about the temporal changes of gaseous HNO3. There*Corresponding author. Fax: +81-78-735-7817. E-mail address: [email protected] (M. Yamamoto).
fore, a method that would allow a fast analysis of HNO3 is needed. To speed up the analysis, we have examined the use of a scrubber for different techniques, using a catalytic converter to detect NO or NO2. Tanner et al. (1998) measured inorganic nitrate (HNO3+particulate nitrate) using a method involving a dual-channel converter. Each channel contained an Au–CO converter that reduced NOy (Fahey et al., 1986; Fehsenfeld et al., 1987; Williams et al., 1998): NO+NO2 +HNO3+HONO+HNO2NO2+NO3+2N2O5+PAN +Particulate nitrate(NO 3 )+? to NO; the other channel contained a nylon filter to remove the nitrate
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species. For HNO3 detection, we used a Teflon pre-filter to remove particles prior to splitting the two channels. Each channel contains a QBC and an MC that reduce NO0y to NO (CH-1), and one channel(CH-2) also contains a nylon filter that removes HNO3 prior to the converter. QBC was used for the HNO3 reducing catalyst in place of the Pyrex-bead converter (Kelly et al., 1979; Burkhardt et al., 1988; Harrison and Msibi, 1994). The NO0y detected by the QBC–MC system described here, is supposed to more closely resemble a total reactive nitrogen oxide (NOy). The NO*y (NOy-(HNO3+particulate nitrate)) described by Tanner et al. (1998) may contain the same NO*y compounds as those detected in the CH-2 in this report. In the highly polluted NOy atmosphere, the HNO3 mixing ratio to NOy is too small (only several percentage points) to measure when using the difference technique. In this report, each channel of the apparatus was carefully designed and operated identically to the other. A blank test was used under atmospheric conditions to examine the identity of each channel. In the preliminary experiment, we found that the Pyrex glass tube and beads for the HNO3 converter and the molybdenum oxides for the NO2 converter had deteriorated considerably during the several months that they were monitored at temperatures in excess of 3501C. The QBC is thermally more stable, and, therefore, the present system is expected to be more convenient for long-term monitoring.
2. Experimental 2.1. Instrumentation A schematic view of the measurement system is shown in Fig. 1. A sample air stream, in which particulates are
removed by a Teflon filter, is divided into two pathways. One contains a HNO3 scrubber packed with a nylon fiber sheet followed by the QBC–MC (CH-1, NO0y HNO3). The other is directly connected, i.e., without the HNO3 scrubber, to another QBC–MC (CH-2, NO0y ). CH-1 is connected to a NO-inlet and CH-2 to a NOxinlet on the analyzer (Monitor Lab. Inc., model 9841A). In this report, the part of the valve manifold and NO detection system of the analyzer is called an NO detector. The CH-2 output signal corresponds to NO0y ; and the signal that is electrically subtracted, (CH-1) (CH-2), corresponds to the HNO3 concentration. HNO3 is known to readily adsorb many materials (Goldan et al., 1983; Appel et al., 1988; Neuman et al., 1999). To lower the adsorption on the surface of the inlet line that transports sample air to the HNO3 converter, Teflon materials were used for the inlet parts, which consisted of the following: a particle filter (Sumitomo Electric Co., AF07P, F 47 mm), a filter 1 00 holder (F 47 mm), and a tube and connecting tee (16 00 o.d.). The HNO3 scrubber is a Teflon tube (14 i.d. and 12 cm long) packed with a nylon fiber sheet (Unitica, Spanbond-No. 4, 0.5 10 cm2). The detection system of the instrument provides an alternate measurement of the sample air in CH-1 and CH-2, respectively, by one detection cell. The sample air in each channel flows at a constant rate of 0.32 l/min without stopping during NO detection in one reaction cell by switching valves (as shown in Fig. 1) that are controlled by computer. One channel requires 6 s to detect NO, and both channels require 12 s for the completion of one cycle of measurement. In each cycle, in order to synchronize air samples from each channel, the sample air stream from CH-2 must be delayed by 6 s from CH-1. A Teflon tube 00 (14 o.d.) was inserted as a delay coil behind an HNO3 scrubber in CH-1. It was cut to the calculated size of 122 cm.
Fig. 1. A schematic view of the fast response HNO3 analyzer by the scrubber-difference method followed by NO–O3 chemiluminescence. V and O indicate an air valve and an orifice used to control the flow rate of sample air, respectively.
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In order to lower HNO3 adsorption on the inner surface of the inlet, all the inlet lines between the prefilter and the QB converter were placed in a temperature-controlled (501C) box, except for the two HNO3 scrubbers used to measure atmospheric HNO3 and blanks as shown in Fig. 1. In the summer, water vapor condensation was occasionally observed on the inner surface of the indoor part of the inlet tube when the room temperature where the analyzer was placed and cooled by an air conditioner was considerably lower than the outdoor temperature and the outdoor humidity was high. Under such conditions, heating the indoor part of the inlet effectively prevented the condensation. The HNO3 converter was made up of a quartz tube (7 mm i.d., 24 cm long) filled with quartz beads (F4 mm). Two QBCs, one for each channel, were set together in a heater block 360721C. Two MCs that had the same 00 geometry and material (stainless steel tube, 38 o.d. and 587 mm long) as commercially available ones were placed together in a heater block 315721C. The configuration of the two sets of QBCs and MCs were designed to be identical physically and chemically. The data of the NO0y and HNO3 concentrations were output from the analyzer every 12 s, which implied that the intrinsic time resolution attainable was 12 s. An acquisition of these original data was conducted by a personal computer. A 3-min running average of the original data was used for this paper. 2.2. Standard HNO3 gas
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Institute of Hyogo Prefecture. The building is located in a residential area about 10 km west of the center of Kobe and close to a highway about 150 m west of the building. The sampled air at the site is directly and frequently affected by the traffic exhaust.
3. Results and discussion 3.1. Calibration by the standard HNO3 The standard gases were generated, and the analyzer was calibrated using standard HNO3 gas of 0–88 ppb concentration, which was determined by IC. These tests were performed in dry clean air, which was purified by passing through four columns packed with silica gel, soda lime, charcoal, and molecular sieves. Impurities in the clean air were about 0.5 ppb NO. As can be seen in Fig. 2, linearity was demonstrated for HNO3 concentrations that were relatively high, above 20 ppb, showing a regression of the data which yielded the formula, Y ¼ ð1:0570:06ð1sÞÞX þ ð1:8572:69Þ; and the correlation coefficient of 0.994 (N ¼ 4) for the regression line. In the range between 0 and 20 ppb, no calibration could be performed because it was too difficult to generate stable standard HNO3 concentrations, even though HNO3 was typically measured in that range (Figs. 5 and 7). However, we assumed that linearity existed in that range because the extrapolated regression line nearly intercepted the origin.
To calibrate the response of the analyzer, standard HNO3 was generated by a diffusion tube (4 ml Pyrex glass tube containing 60% HNO3 solution). The concentration of the generated standard gas was determined by ion chromatography (IC) (Dionex DX100) after sampling by two fritted bubblers in a series with an IC eluent solution of 10 ml as an absorbent. 2.3. Annular denuder A denuder technique (Possanzini et al., 1983) was used for simultaneous measurement of HNO3 in ambient air. The annular denuder (URG2000F30 150F3css) was coated with NaCl/glycerol to collect gaseous HNO3. NaCl was prepared as a 1% solution in a 1 : 1 methanol/water ratio that contained 1% glycerol. After sampling, the collected HNO3 was extracted in deionized water and analyzed as nitrate ions using IC. 2.4. Atmospheric measurement Atmospheric measurements were made on the veranda of the second floor of the Environmental Science
Fig. 2. Calibration of the HNO3 analyzer to standard HNO3. Standard gases were generated with the use of a diffusion-tube method, and the concentration was determined by a frittedbubbler sampling and an ion-chromatography. These tests were performed in dry clean air.
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With 20-min sampling periods for each concentration, the precision of the measurement was near 3%, and overall accuracy was estimated to be 710 ppb for the standard HNO3 measurement with a fritted-bubbler-IC method and 75% for the HNO3 analyzer.
3.2. Temperature dependence of the QBC–MC system The response of the analyzer to the temperature of the QBC was examined. The temperature was controlled within a range of 310–3861C for the QBC and kept
Fig. 3. The temperature dependence of the efficiency of a quartz-bead converter for HNO3 conversion. The temperature of a molybdenum converter was constant at 3151C.
Fig. 4. The variation of the NO*y concentration and the blank value of the HNO3 analyzer response for HNO3-scrubbed ambient air. The scale of the blank ordinate is expanded by 50 times for clarity.
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constant at 315721C for the MC. The standard HNO3 of 3671 ppb was constantly generated during the 1-hplus experiment. As shown in Fig. 3, the relative response to the standard HNO3 concentration was obtained at 1.0 within a range of 340–3851C, and 3601C was used as the optimal operating temperature for the QBC. From the result, it can be concluded that HNO3 was completely converted to NO by the QBC– MC system. 3.3. Blank value and detection limit To verify the identity for the two channels under atmospheric conditions and to evaluate the detection limit, HNO3-free ambient air was examined. Two valves immediately after the pre-filter, which are shown in Fig. 1, were switched into a position requiring an extra HNO3 scrubber for the blank experiment. Since HNO3 in the sample air had already been scrubbed by the extra scrubber, the observed signals for the two streams should be equal, and, thus, the output for HNO3, i.e., the difference between the two streams must be 0 ppb. Fig. 4 shows an example of the result for the blank experiment. The scale of the blank is multiplied by 50 of
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the NO*y scale. During periods of high (111712 ppb), middle (9377 ppb), and low (6278 ppb) NO*y regimes, the average blank values for the HNO3 measurement corresponding to the period and the standard deviation associating with the average were (0.3870.42), (0.3370.57), and (0.3470.55) ppb, respectively. These results demonstrate that: (1) although all average blank values are positive, they suggest that the two measurement modes (the NO0y and the NO*y) may have a systematic difference and that the values are all within the standard deviations; (2) the blank values do not associate with the NO0y concentration levels, implying that the blank may not be a plausible artifact resulting from an improper subtraction between the NO0y and the NO*y and (3) although the cause of the positive deviation is not identified yet, it would be rational to regard the blank values as practically zero. From the results shown in Fig. 4, with the consideration made above, the detection limit of this method for HNO3 could be conservatively evaluated to be 1.1 ppb (2s) with a 3-min running average. Generally, HNO3 accounts for only several percentage points of the total nitrogen oxide species in urban
Fig. 5. A comparison of the HNO3 analyzer response with the denuder method. Gray line: The HNO3 concentration measured by the denuder method. The width of the lines corresponds to each denuder sampling period. Solid line: HNO3 concentration measured by the fast response HNO3 analyzer. 3-min-running mean and its average values for the periods corresponding to the denuder sampling.
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air. When measuring HNO3 in highly polluted air in which an NO0y concentration often changes drastically and extensively, a small discrepancy in the stream of both the NO0y and NO*y lines could cause a large variability or high blank values of the HNO3 concentration. To lower the variability and the blank values and to obtain a higher degree of accuracy, a precisely designed delay coil was successfully used. However, the small blank values and the standard deviations could no longer be suppressed. Considering the origin of such a blank value, it assumed that there were some differences in the NO concentration in the two lines, which may have been caused by a chromatogram tail as described by Tanner et al. (1998).
3.4. A comparison of the data using an HNO3 analyzer and the denuder method In order to examine the feasibility of the analyzer, a comparative experiment using the denuder method was conducted. For the annular denuder system used, the detection limit was evaluated to be 0.4 ppb, which is about half of the stated value (Waldman, 1987) when taking three standard deviations of the field blank measurement for 1 h sampling and the IC analysis. At levels well above the detection limit, the precision of the measurement is ca. 5%, and overall accuracy is estimated to be 715% for ambient atmospheric measurement of HNO3. Fig. 5 shows all the results of the simultaneous measurement by the HNO3 analyzer and the denuder method. The gray line indicates the sampling period for the denuder method and the concentration of HNO3 measured by the denuder during the period. The solid lines are for a 3-min-running mean HNO3 concentration measured by the HNO3 analyzer and its average values for the periods corresponding to the denuder sampling. In the summer, a high HNO3 concentration above 10–20 ppb was occasionally observed at the sampling site. Therefore, a comparison of such a high concentration range for HNO3 is also needed. We developed an automatic start for the denuder sampling and applied it to collect the sample during a high HNO3 concentration episode, i.e., the sampling starting when the output of the HNO3 analyzer was more than 10 ppb and continued for 1 h. The sporadic data on 6 and 8 September, were thus obtained for the high concentration episode. If the averaged data are plotted against the denuder data, the correlation that is shown in Fig. 6 would be obtained. The result shows a linear relationship between the two methods. The linear regression for all data gives the result, Y ¼ ð0:97370:140ð2sÞÞX þ ð0:07770:370Þ; clearly demonstrating that the analyzer we developed provides data that is almost identical to that of the
Fig. 6. A scatter plot of the HNO3 concentration measured by the fast response HNO3 analyzer against that by a denuder method. The data of the analyzer were averaged during the period corresponding to the denuder sampling.
denuder measurements. It should be noted that the scatter is rather high, the largest deviation from the regression line being ca. 1 ppb. In the low HNO3 concentration range, i.e., 3 ppb or less, the scatter is especially high. This result could be attributed partially to the uncertainty in the HNO3 measurement caused by a long period of denuder sampling of the polluted air, but the extent of the scatter has the same magnitude as the two standard deviations of the measurement for zero-HNO3 ambient air discussed in Fig. 4.
3.5. Atmospheric measurement Atmospheric measurements were conducted for nine days in the summer (4–13 September 2000). The measurement site has frequently experienced elevated levels of NOx. Fig. 7 shows the variation in the HNO3 and NO0y concentrations. The scale of the HNO3 concentration is multiplied by 10. The average NOx and HNO3 concentrations (N ¼ 12950) during the period were 47723 and 2.671.3 ppb, respectively, and the maximum concentrations were 203 and 10.1 ppb, respectively. The concentration of HNO3 had a temporal variation pattern that was similar to NOx except on 11 September, while ca. 10 ppb of the HNO3 concentration was occasionally observed when the NO0y concentration was high, i.e., more than 100 ppb. The average of the ratio HNO3 =NO0y of every instantaneous measurement was
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Fig. 7. An example of temporal variations of HNO3 and NO*y measured by the HNO3 analyzer, measured from 4–13 September 2000, at the Environmental Science Institute of Hyogo Prefecture, Kobe. The gray area indicates the nighttime sampling period.
6.474.0%, and each instantaneous ratio was generally smaller than 10%. When we take into account, the fact that in the urban air, the main component of the NO*y is, in general, NOx, these results agree with those from other studies that were obtained in the polluted urban air (Spicer, 1977).
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