Simultaneous absolute measurements of gaseous nitrogen species in urban ambient air by long pathlength infrared and ultraviolet-visible spectroscopy

Ammpheric Prinwd

Emirow#wnt

Vol. 22, NO. 8. pp. 1545-1554.

1988.

OOtM-fB981/88 $3.00 + 0.00

0

in Great Britain.

1988 Pergamon Press plc

SIMULTANEOUS ABSOLUTE MEASUREMENTS OF GASEOUS NITROGEN SPECIES IN URBAN AMBIENT AIR BY LONG PATHLENGTH INFRARED AND ULTRAVIOLETVISIBLE SPECTROSCOPY H. W. BIERMANN, E. C. TUAZON, A. M. WINER,* T. J. WALLINGTON~~~ J. N. PIITS, JR Statewide Air Pollution Research Center, University of California, Riverside, CA 92521, U.S.A. (First received 24 November 1986 and in final form 3 April 1987) Abstract-Two new long pathlength spectrometers, utilizing 25-m basepath multiple reflection optical systems, were employed for the first time during an intercomparison of measurement methods for atmospheric nitrogenous species held at Claremont, CA, 11-19 September 1985. Measurement of nitrogenous species using these closed optical path systems, as opposed to single pass systems extending several kilometers, permit the resulting in situ absolute spectroscopic data to serve as benchmark values for point monitors employing denuders or filter packs. The FT-IR spectrometer was operated at a total pathlength of 1150 m and spectral resolution of 0.125 cm-‘, with corresponding detection sensitivities of 160 nmole mm3 for HN03 and 60 nmole m -3 for NH3 (4 and 1.5 ppb, respectively). Concurrent measurements of HONO, NOI and NO3 radicals were conducted with the differential optical absorption spectrometer operated at 800 m total pathlength with detection limits of 24, I60 and 0.8 nmole mm3 (0.6,4 and 0.02 ppb) for HONO, NO1, and NOa radicals, respectively. Key word index: Absolute measurements, ammonia, FT-IR spectroscopy, intercomparison, nitrate radical, nitric acid, nitrogen dioxide, nitrogen species, nitrous acid, ultraviolet-visible spectroscopy, urban air.

I. INTRODUCTION

The gaseous and particulate

species formed in the atmosphere from oxides of nitrogen (NO +N02 = NO,) emissions are critically involved in photochemical air pollution, visibility degradation, acidic deposition and the formation of atmospheric mutagens (California Air Resources Board, 1985; Finlayson-Pitts and Pitts, 1986). Unfortunately, the measurement of species such as HN03, NH,, NO, radicals and HONO at part-per-billion mixing ratios in complex mixtures of primary and secondary pollutants is not a routine matter. For example, research and development efforts to produce reliable monitoring instruments for gaseous HN03 have been under way for nearly a decade (Stevens, 1979; Fellin et al., 1980; Appel et al., 1981; Braman et al., 1982; McClenny et al., 1982; Shaw et al., 1982; Forrest et al., 1982; Spicer et al., 1982; Gailey et al., 1983; Golden et al., 1983: Schiff et al., 1983; Anlaufet al., 1984,1985; Ferm, 1986), but to date no single method has gained acceptance with respect to all the criteria of accuracy, sensitivity, convenience and cost effectiveness. For this reason, the California Air Resources Board (CARB) sponsored a multi-investigator intercomparison study in September 1985, in which all current, viable measurement methods for nitric acid and particulate nitrate were evaluated and compared. * Author to whom correspondence should be addressed.

Over the past 10 years, we have conducted periodic measurements of the diurnal concentrations of HNOJ and NH3 in ambient air in the California South Coast Air Basin (CSCAB) using FT-IR spectrometers interfaced to multiple reflection systems capable of total optical paths of l-l.5 km (Tuazon et al., 1978; Doyle et al., 1979; Tuazon et al., 1980, 1981). In the course of these studies, the long-path infrared absorption method has established a successful record of providing accurate and interference-free measurements of HN03 in complex pollutant mixtures. Although a lower detection limit is highly desirable for this technique, it is still clearly one choice for a reference method, particularly for levels of nitric acid typically encountered in moderate-to-severe smog episodes in the CSCAB. Our previous kilometer pathlength FT-IR system had, in fact, already played the role of a reference method during an EPA-sponsored field study in Claremont, CA, between 27 August and 3 September 1979 (Spicer et al., 1982). For these reasons, our primary role in the ARBsponsored intercomparison study was to provide absolute measurements of gaseous nitric acid and ammonia by kilometer pathlength FT-IR spectroscopy. At the same time, based on our extensive experience (Platt et al., 198Oa,b; Harris et al., 1982; Pitts et al., 1984; Platt et al., 1984) with differential optical absorption spectroscopy (DOAS), and in view of the unique opportunity provided by the Claremont Study for simultaneous measurements of a large number of

1545

H. W. BIERMANN et al.

1546

nitrogenous species, we conducted DOAS measurements for N02, HONO and the NO3 radical using a second 25-m basepath multiple reflection optical system. The unambiguous spectroscopic HONO data, in particular, were important in testing the capability of non-spectroscopic methods such as filter packs and difference denuders for measuring this important species, or alternatively in determining the extent to which HONO may constitute an interference in any of the HN03 and nitrate measurement methods employed in the intercomparison study. We report here detailed data resulting from both our long pathlength FT-IR and DOAS measurements during the 1985 intercomparison of nitrogen species measurement methods.

2. EXPERIMENTAL 2.1. FT-IR Spectroscopy 2.1.1. Kilometer pathlength FT-ZR system. Our present capability in infrared spectroscopic measurement of ambient air pollutants is based on a recently constructed 25-m basepath, open multiple-reflection optical system which is interfaced to a Mattson Instruments, Inc., Sirius 100 FT-IR spectrometer. This spectrometer has a maximum resolution capability of 0.125 cm-i and is equipped with a Motorola 68000 based data system. The long-path optics are of the three-mirror White design (White, 1942),with provisions for adding a corner reflector at the in-focus end (Horn and Pimentel, 1971).to effectively double the system’s pathlength. The mirrors in this new optical system were fabricated from 30 cm diameter, 6 cm thick Pyrex blanks and gold-coated for the best reflectivity ( >99 %) in the infrared. The optimum pathlength for monitoring was determined to be in the l-l.5 km range during routine operation, although shortterm use of optical paths greater than 2 km is possible. The long-path FT-IR spectrometer was operated alongside the long-path DOAS spectrometer (cf. below), as shown in Fig. 1. The sets of mirrors were arrayed in opposite directions but aligned side by side in a compact arrangement which made possible simultaneous analyses of virtually the same air mass. The FT-IR spectrometer was housed in an air conditioned shed while the long-path mirror assembly outside was shaded from direct sunlight by protective shields. The spectrometer system had an initial height of 1.2 m (above ground level) designed into the optical axis, but this was raised to a total of 2.4 m (8 ft), the common sampling height required for all analytical techniques being compared during

25m

BASEPATH

FT -IR

TOTAL OPTICAL

-

the field study. Massive concrete blocks were employed as support structures to attain this sampling height. 2.1.2. Acquisition of spectra. Spectra were recorded at a pathlength of 1150 m and 0.125 cn- ’ resolution (unapodized). Sixty-four scans (interferograms) were added during a 4.5-min period and then transformed (calculation time of 2.5 min) to a single beam spectrum. Each spectrum of 128 K points was truncated and only the spectral region from 400 to 1600 cm- I, which contained the absorption bands of HNO, and NH, used for analysis, was retained and archived. Four to five spectra per hour were collected. This rate of data collection permitted a real-time examination of the quality of spectra being recorded and such checks were consistently made during daytime and early evening hours (0800-2200 PDT). During the ‘noncritical’ period of 2200-0600 hours when nitric acid was expected to be below the detection limit, the FT-IR spectrometer was programmed to conduct automatic monitoring. 2.1.3. Calibration. Figure 2 illustrates the absorption features of HNOs and NH3 in the region accessible to long-path FT-IR measurements. Asterisks mark the peaks which were employed to determine concentrations. For HNOs, the Qbranch at 896.1 cm-’ was the most appropriate one for quantitative measurement because of its favorable half-width and minimal interference by atmospheric H,O. We restricted our measurements to the heights of the sharp absorption (Qbranch) features only, since measurements which included the broad underlying envelope were more susceptible to unknown interferences. The 885.4 cm-’ Q-branch was totally free of interferences but its relatively narrower line shape was more susceptible to distortion by noise, while the strongest Qbranch at 878.9 cm- ’ was severely overlapped by a strong water line. Of the numerous sharp features of the NHs spectrum depicted in Fig. 2, the lines at 1103.4and 867.9 cm-‘, though not the strongest, suffered the least interference by H,O and COs absorption lines and were therefore the ones chosen for analysis. The calibration procedure for NH, was straightforward. Spectra were recorded at 0.125 cm-’ resolution for several NH3 pressures in the range 0.2-l torr which were measured to within + 0.005 torr accuracy with an MKS Baratron capacitance manometer. The NH, samples were transferred to a 25cm cell with KBR windows and pressurized with N, gas to atmospheric pressure. No measurable decay of NH, concentration was observed within the 15-min period that repeat spectra were recorded for each sample. These data yielded absorptivities (base 10) at 23 “C and 740 torr total pressure of 18.2 cm-‘atm-i for the 1103.4 cm-’ peak and 9.7 cm-’ atm-i for the 867.9 cm-’ peak, with uncertainties of + 5 y0 (2~) for both values. A similar calibration procedure was not possible for HNO, because of its significant decay to the wall of the 25-cm cell.

AND DOAS SPECTROMETERS PATHLENGTHS : I - 2 km

25mB

Fig. 1. Schematic diagram of the long-path FT-IR and DOAS spectrometers.

Simultaneous absolute measurements of gaseous nitrogen species in urban ambient air

1547

0.70

0.50

2E

0.40

g

0.38

2 a q

0.28

HNO,

I

1

I

900

I

I 1000

WAVENUMBERS

1100

(cm-‘)

Fig. 2. Reference spectra of gaseous HNOJ (0.61 torr) and NH3 (0.23 torr) at spectral resolution of 0.125 cm- ‘, pathlength of 25 cm, total pressure with N, of 740 torr. The HNO, plot is offset by 0.30 absorbance (base 10) unit for clarity. Asterisks mark the absorption peaks used for analysis.

We determined our high resolution absorptivity values relative to the data of Graham and Johnston (1978), who made an accurate determination of the absorptivity of the HN03 vq (1325 cm- ‘) fundamental band at 2 cm- ’ resolution using two independent methods for generating HNO, (for more details see also Graham, 1975).We applied Graham and Johnston’s (1978) absorptivity value (peak-to-baseline) for the broad vq P-branch at 1315cm- ’ to the same feature in our 0.125 cm- ’ resolution spectrum after smoothing the latter to 2 cm- ’ resolution. The absorptivity for a Q-branch (e.g. 896.1 cm-‘) at 0.125 cm-’ resolution was then determined from the intensity ratio of the unsmoothed Q-branch to the smoothed P-branch. Smoothing removed the fine structure superimposed on the v., band envelope, and in order to test the adequacy of the smoothing function employed, measurements were carried out in which 2 cm ’ and 0.125 cm- ’ resolution spectra were recorded alternately for an HNOj sample. To take into account the HNO, decay in the cell, the 1315cm~’ peak heights at 2 cm _ ’ resolution were plotted against time as were the 0.125 cm- ’ resolution spectra after having been smoothed to 2cm-’ resolution. The two curves were found indistinguishable, indicating the accuracy of the smoothing function applied. The above procedure yielded a value of 5.2 2 0.4 cm-‘atm- ’ for the absorptivity (base 10)of the HNO, peak at 896.1 cm-’ (Q-branch height only) with the 2a error including that reported by Graham (1975).This value is in fact the same as that which we previously determined for this peak under 0.5 cm _ ’ resolution (Tuazon et al., 1980). Following the same method, the absorptivity of the 885.4 cm- ’ Qbranch was found to be 6.1 f0.5cm-‘atm-‘. 2.1.4. Trearment of data. The quantitative analysis of species such as NH,, which possess relatively narrow Qbranch features that are sufficiently resolved from atmospheric H,O absorptions, is normally straightforward. The absorbance (log lo/r) can be derived directly from the single beam spectrum, i.e. a ratio plot against an actual background AE

22:8-c

spectrum is not necessary. Provided that the detector has a linear response, lo and 1 as determined from a single beam spectrum have values on an arbitrary scale in which zero corresponds to zero signal on the detector. The appropriate differential absorption coefficient, such as those determined above, and the absorption path length must be applied to the absorption intensity measured between I, and I in order to convert to a concentration. This was the procedure followed for the 1103.4 and 867.9 cm-’ lines of NH3. Estimates ofthe HNOl concentration can also be obtained from single-beam spectra using the 885.4 cm- ’ Q-branch. However, the HNOa analysis was more reliably carried out using the 986.1 cm-’ Q-branch by ratioing the sample spectrum with a clean background spectrum, a procedure which improved the baseline and resulted in essentially complete cancellation of a very weak Hz0 interference. Background spectra were chosen from the low-noise data recorded around midnight during low pollution days, since previous measurements (Spicer et al., 1982) showed that HNO, levels were much lower (-e 1 ppb) than the FT-IR detection limit under these conditions, and examination of the present background spectra showed no HNO, absorption features. The peak-to-peak noise level in the absorbance specrum was typically 0.0025 absorbance unit (base lo), corresponding to a 250: 1 signal-to-noise (peak-to-peak) ratio of single-beam spectra. The absorption peaks used for the analysis were much wider than the frequency of random noise, such that peak heights as small as the noise level could be detected above the noise itself. Thus the detection sensitivities were 120-200 nmolem-’ (3-5 ppb) for HNO, and 4O-80nmolem-’ (1-2 ppb) for NH, [l ppb = 40 nmolem-3]. The lower detection limit for each compound was realized when turbulence due to temperature gradients and/or wind were minimal (e.g. at night). The largest source of uncertainty in the analysis was the noise level in the spectrum, with other errors being generally negligible. We assigned to each peak height measurement the

H. W.

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BIERMANN et al.

maximum error which was equal to the peak-to-peak noise level. For HNO, this error was k 160 nmole m-3 (z!z4 ppb). The uncertainty arising from the error in the absorptivity (of the 896.1 cm-’ Q-branch in this case) increased with the concentration. However, this error contribution was relatively small for the range of HNOJ concentrations observed here. For example, it increased the f 160nmolem-’ (k 4 ppb) error by only 10% when included in the error calculation for an HNO, concentration of 1000 nmole mm3 (25 ppb). For NH, concentrations $800 nmole mm3 (< 20 ppb), the sum of uncertainties due to noise and error in absorptivity (1103.4 cm _ ’ line) was within + 60 nmole m-j (k 1.5 ppb). At higher concentrations, the contribution from the absorptivity error became relatively more important. A detailed plot of this error contribution as a function of concentration revealed that it was nearly linear in the concentration range 80&4000 nmole (20-100 ppb), permitting the additional error to be calculated from the empirical formula (0.044) ([NH,]-800) nmole m -‘. Thus, for the measurement of 2400 nmole mm3 (60 ppb) NHj, the total error could be calculated as [60 + (0.044)(240&800)] or k 130 nmoleme3

(k 3.2ppb). 2.2.

Ultraviolet-visible

spectroscopy

system. The DOAS system measures the ambient concentrations of trace components which have distinct vibronic structures by their light absorption in the near-u.v. and visible spectral regions. White light from a 75 W high-pressure Xenon arc lamp was transmitted through a 25 m base path optical system essentially identical to that used for the FT-IR system described above. However, to achieve total light paths of about 1km in the u.v.-visible region, the mirrors were coated with a custom 37 layer dielectric coating supplied by Newport Thin Film Corp. The reflectivity curve was designed to have maxima near 99 y0 in the near-u.v. (33&380 nm) and red (> 590 nm) wavelength regions used for the determination of NO,, HONO and NOa radical absorption features. After passing through the multiple reflection system, the light was focused onto the entrance slit of a 0.5 m (SPEX 1870) spectrograph. The light intensity was measured by a photomultiplier and the signal was processed by a (DEC MINC 1l/23) minicomputer. We have calculated that the average light intensity within the multiple reflection system is less than 1 y0of solar noon. In additionan orange filter was inserted in front of the source while scanning the 660 nm region for NO, radicals. In the DOAS system the fixed exit slit of the spectrograph is replaced by a rapid scanning device consisting of a thin metal disk rotating in the focal plane of the dispersed spectrum (Harris et al., 1983; Platt and Perner, 1983). Exit slits which are 100 m wide and spaced 20 mm apart are etched radially on this disk. During the time that each slit moves across an opening between two masks that block out other slits, it sweeps over a spectral range of about 50 nm. A single scan is completed in approximately 10 ms, which minimizes noise in the spectra due to atmospheric turbulence. The output signal was digitized by a high-speed analog-todigital converter and read by the minicomputer. During each scan the digital samples were added to the corresponding channels in the computer, thereby superimposing and signal averaging all scans in memory. In order to preserve the spectrai resolution while superimposing the scans, the rotational speed of the slotted disk was maintained constant to within f 0.1 ‘Z,.Approximately 50,000 scans were added in the computer memory, providing a signal-to-noise ratio sufficiently high that optical densities as low as 10m4 (base 10) could be determined. 2.2.2. Acquisition of spectra. Spectra were acquired with an absorption pathlength of 800m and an averaging time of 12 min. N02, HONO and NO3 radical concentrations were calculated from their absorptions at 365, 354 and 662 nm, 2.2.1. Long-path

DOAS

respectively (DeMore et a/., 1985). During daylight hours, only the wavelength region around 330-380 nm was scanned while at night the region 630-680 nm was monitored alternately with the 33&380 nm region. NO3 radicals have such a high photolysis rate (Magnotta and Johnston, 1980)that they are below the DOAS detection limit of 0.8 nmolem-’ (0.02 ppb) during daylight hours (Platt et al., 1984). The acquired spectra were stored on floppy disks. 2.2.3. Calibration and treatment of DOAS data. In order to determine the differential absorption of NO,, HONO and NOa from the ambient air spectra, the background envelope was first removed. A background spectrum was synthesized from the original spectrum by a sequence of Fourier analysis, truncation after approximately five frequency elements and consecutive Fourier synthesis. After removal of the broadband contour the residual spectra were successively fitted to the reference spectra using a least squares method. Figure 3 gives an example of this data analysis procedure and the system performance. The top trace (a) is an actual, full-scale air spectrum obtained during the night of 15 September [the zero-line overlaps trace (b)]. The broad, largescale structure is due to a superposition of spectral response curves resulting from mirror reflectivity, lamp intensity, photomultiplier sensitivity, and other instrumental characteristics. Trace (b) shows the residue after the synthetic background was subtracted as described above. Trace (c) is the same spectrum magnified 16 times to depict more clearly the low optical densities involved. The predominant structure can be recognized immediately as being due to NO1, a reference spectrum of which is shown in (d). After subtraction ofthe NO2 featuresand further magnification by a factor of4, the residual structure in trace (e) is unambiguously identified as HONO by comparison with its reference spectrum (f). The optical densities were converted to concentrations using recommended literature values (DeMore et a/., 1985) for the differential absorption cross sections for NO2 and

a

/\re d’ I

I

1

I

340

350

360

370

WAVELENGTH (nm) Fig. 3. DOAS spectrum of ambient air covering the wavelength region 340-375 nm from the night of 15-16 September 1985:(a) full-scale air spectrum [the zero-line overlaps trace (b)]; (b) after removal of background curvature; (c) same as (b), magnified 16 x ; (d) NO1 reference; (e) after NO2 subtraction, magnified 4 x ; (f) HNO, reference spectrum.

Simultaneous absolute measurements of gaseous nitrogen species in urban ambient air HONO of 1 x 10-i’ cm2 molecule-i, 365 nm (Bass et al., 1976) and 4.2 x lo-i9 cm’ molecule-‘, 354 nm (Stockwell and Calvert, 1978), respectively. We determined the NO2 differential absorption cross section, using known concentrations of NO2 in 10 cm and 25 cm cells under actual operating conditions of the DOAS system and observed deviations of less than 10% from the literature value cited above. Based on the literature cross sections, an 8OOm absorption pathlength, and the actual noise levels in the spectra, we calculated detection limits of 16Onmolem-’ (4 ppb) for NO1 and 24 nmole mm3 (0.6 ppb) for HONO. The error limits for the NOz data were estimated at + 15 %. For the HONO data the corresponding errors were + 30 %, reflecting the more complex deconvolution. These error limits were based on the observed noise levels, distortion introduced during the substraction procedure, and errors resulting from baseline drift and stray light effects. 2.3. Sampling schedule

The intercomparison study began at 0800 PDT, 11 September and lasted until 0600 PDT, 19 September, 1985. For the majority of the nitrogen species analytical techniques (e.g. denuders and filter packs), there were five sampling time oeriods for each dav: OOOO-0600.080&1200. 120&1600. i600-2000 and 2&2400 PDT, with the period 0600-08od being generally utilized for maintenance and calibration. Both the FT-IR and DOAS techniques, which acquired four to five samples (spectral records) per hour, were considered ‘continuous’ techniques for the purpose of this study.

3. RESULTS AND DISCUSSION

3.1. FT-IR

Measurements

3.1.1. Nitric acid. The field study period was characterized mainly by low pollution days with HNO:, levels mostly below the detection limit of 160 nmolem-3

(4 ppb). The lack of sufficiently high levels of HNOJ for FT-IR detection was compounded by intermittent noise problems (of unknown origin) which plagued the FT-IR measurements during four of the eight days of the study. Fortunately, no severe noise interferences occurred on 14 September when the highest daily maximum oxidant level (CO,] > 0.2 ppm) attained during the study was recorded. Consistent with the strong correlation between HNO, and O:, concentrations (Tuazon et al., 1981; Spicer et al., 1982), the highest levels of HNOs were indeed observed on this day, and HNO, concentrations were above the FT-IR detection limit for most of the daytime hours. The detailed time-concentration data for 14 September are presented in Table 1 and represent the best FT-IR measurements during the study for comparison with other continuous methods (e.g. diode laser spectroscopy). Figure 4 illustrates the detection of HNO:, on 14 September which includes the highest value of 1040nmolem-3 (26 ppb) recorded at 1545, nearly coincident with the peak O3 level. The spectra were ratioed against a common background of ‘clean’ air, in this case a spectrum recorded at 0026 on 18 September when pollution levels were known to be low. Nitric acid concentrations were primarily determined from absorbance spectra such as those illustrated in Fig. 4, primarily using the 896.1 cm- ’ Q-branch. Estimates of concentrations were often possible also with the 885.4cm-’ peak in single-beam spectra, and the 878.9 cm- ’ peak provided qualitative verification when the overlapping Hz0 absorption could be suf-

Table 1. HN03 concentration*,t vs time in Claremont, CA, on 14 September 1985 by long pathlength FT-IR spectroscopy

WWI PDT

(ppb) $

0927 0937 0950 1009 1023 1035 1049 1101 1112 1124 1141 1154 1208 1224 1239 1251 1304 1316 1329 1342

<4 <4 <4 6.3 7.0 6.7 7.5 8.4 8.7 12.0 10.9 14.2 14.0 15.0 13.7 11.7 11.4 13.4 15.9 11.7

CI-RWI

(nmole m- ‘) <160 <160 < 160 250 280 270 300 340 350 480 440 570 560 600 550 470 460 540 640 470

1549

PDT

(ppb)$

(nmolemm3)

1354 1410 1423 1439 1452 1507 1520 1531 1545 1559 1615 1629 1649 1703 1717 1729 1744 1816 1827 1840

10.9 11.7 11.7 13.9 13.4 18.4 20.0 21.7 25.9 21.4 20.0 20.0 16.7 15.5 8.4 8.4 6.7 <4 <4 <4

440

470 470 560 540 740 800 870 1040 860 800 800 670 620 340 340 270 <160 <160 <160

*At 23°C and 740 torr. t Error: f 4 ppb (k 160 nmole m-3). $ Digits beyond two significant figures are retained only to reduce roundoff errors.

H. W. BIERMANNet al.

1550

HNO3 nmole m -3

0927

(160

I049

300

I329

640

1545

898

894

es8

WAVENUMBERS

calculation of the 605nmolem-3 (15.1 ppb) average for this sampling period (Table 2) is straightforward. For the other sampling blocks, for which the number of hourly values above detection limit is at least equal to the number of hourly values below detection, a lower limit and an upper limit average were calculated by substituting 0 and 160 nmolemm3, respectively, for the hourly values that were below detection limit. Thus, for example, a relatively narrow range of 180-220 nmoleme3 (4.5-5.5 ppb) was calculated for the period 1200-1600 hours of 17 September, and its average of 200 nmole mm3 (5 ppb) is noted in Table 2 along with the other values derived in the same manner. The XX entries in Table 2 represent indeterminate values due to the excessive noise encountered during those periods which rendered the recorded spectra unusable. This problem was particularly severe during the evening hours on 13 September that the FT-IR system was shut down until the next morning. Values for the 1200-1600 hours of 13,15 and 16 September are reported in parentheses to represent cases which consisted of ‘normal’ scans as well as moderately noisy spectra. Errors for these values are estimated to be f 260 nmole m - 3 ( + 6.5 ppb). 3.1.2. Ammonia. The measurement of NH3 is important because this compound reacts with gaseous HNO, to form particulate NH4N03. The occurrence of NH, at significant levels in the eastern part of the CSCAB has been postulated as the reason for low observed HN03 levels, even during moderate to severe smog episodes, and corresponding high particulate nitrate levels (Doyle et al., 1979; Tuazon et al., 1980; Stelson and Seinfeld, 1982; Russell et al., 1983). The hourly average ammonia data obtained during the eight-day period are given in Table 3. They indicate that 8&160 nmoles m- 3 (2-4 ppb) was a common background level of NH3 in Claremont during the field study period. Particularly high concentrations of NH3 were observed in the morning hours of 13, 16 and 17 September. However, the highest hourly average concentrations occurred in the early afternoon of 12

the

PDT

I040

902

(cm-‘)

Fig. 4. FT-IR spectroscopic detection of HN03 during the pollution episode of 14 September 1985 in Claremont, CA.

ficiently ratioed out. Measurement errors are well within rt 160 nmole me3 ( f 4 ppb) for these spectra which were not affected by the unusual interference problems mentioned earlier. Hourly average HN03 concentrations were derived from time-concentration data such as those presented in Table 1. To facilitate comparison with time integrated techniques (e.g. filter packs, denuder difference methods, and annular denuders) the average concentration for the designated sampling blocks were derived from the hourly averages (where data are sufficient) and are presented in Table 2. Values below the FT-IR detection limit are represented as < 160 nmolemm3 (~4 ppb) and accounted for the majority of the data for the study period. The data for 14 and 17 September comprise our best measurements since the noise interferences mentioned above were not experienced on those days. Of the sampling blocks for these two days, only the period 12W1600 of 14 September had HNO:, concentrations which were consistently above detection limit. Thus,

Table 2. Average HNO, concentrations (nmolemm3)*,t by long pathlength FT-IR spectroscopy for the designated sampling periodsx Sampling period (EDT) OOOGO600 080&1200 120%1600 16OG2000 200&2400

11 Sept.

< < < <

160 160 160 160

12 Sept.

< 160 < 160 xx < 160

13 Sept.

14 Sept.

<160 < 160 2205 (405) 605 xx 3205 <160

15 Sept.

16 Sept.

17 Sept.

18 Sept.

19 Sept.

< 160 < 160 (340) xx

i 160 < 160 (125)g < 160

< 160 < 160 200§ < 160

< 160 <160
< 160

<160

< 160

clfm

ClM

*At 23 “C and 740 torr (conversion factor: 1 ppb = 40 nmolem-3). t Error: f 160 nmole mm3 ( f 4 ppb); values in parentheses have an estimated error of & 260 nmole mm3 ( k 6.5 ppb) (see text). $ Blank entries fall outside the study period. A dash means incomplete or no data. XX designates an indeterminate value due to the presence of excessive noise levels in a significant number of spectra in the block. 4 Midpoint of a range determined by a lower limit and an upper limit average (see text).

1551

Simultaneous absolute measurements of gaseous nitrogen species in urban ambient air Table 3. Hourly average NH3 concentrations (nmolem-3)*, t by long pathlength FT-IR spectroscopy: Hourly period (PDT) OOG&O100 010&0200 0200-0300 0300-0400 040%0500 050&0600 08OCM900 090&1000 100&1100 1lW1200 120@1300 130&1400 1400-1500 15W1600 16OGl700 17o(t1800 18OG1900 1900-2000 200&2100 21W2200 22W2300 2300-2400

11 seat.

77 110 110 89 61 69 100 95 73 93 110 130 160 110

12 seat.

150 220 250 200 610 1680 2280 (580) (330) 190 190 240 210 260 160 140

13 Sept. 140 140 130 120 120 98 240 260 420 1340 (640) 300 180 180 (:z, xx xx xx xx -

14 Sept. -

170 180 160 140 190 160 190 150 200 160 180 220 170 190 140 120

15 Sept.

16 Sept.

17 Sept.

18 Sept.

19 Sept.

88 140 100 110 100 84 220 230 250 280 290 260 (210) 170 170 (140) (140) 160 190 92 140 120

92 92 120 88 68 48 1260 940 900 1330 710 240 180 130 160 120 140 120 130 120

60

76 88 88 130 110 110 110 81 95 80 74 40 84 120 120 160 170 120 140 100 100 100

75 71 60 54 73 85

68 76 48 210 430 610 690 1850 1590 840 240 140 120 140 110 84 92 140 130 140 100

* At 23 “C and 740 torr (conversion factor: 1 ppb = 40 nmole mea). t Error: i 60 nmolemm3 for [NH,] < 800 nmolemm3; see text for errors which apply to [NH,] > 800 nmole me3. Values in parentheses have an estimated error of f 100 nmole me3. $ Blank entries fall outside the study period. A dash designates no data; XX indicates indeterminate concentrations due to very high noise levels in the spectra.

September when 2280 nmole rnw3 (57 ppb) were measured. These periods all coincided with prevailing winds from the south or southeast, which clearly transported high levels of NH3 to Claremont from the predominantly agricultural areas of Chino and Ontario (15 km away), which contain numerous poultry and dairy farms and feed lots. The average NH, concentrations for the five designated sampling blocks can be calculated from the data in Table 3. Figure 5 presents single-beam spectra recorded on the morning of 16 September in which the NH3 concentration, in nmole m- 3, rapidly changed from below the detection limit ( < 60) at 0819, to 720 at 083 1, and to 3360 at 0856, the latter instantaneous value being the highest NH, concentration recorded by the long-path FT-IR technique during this study. 3.2. DOAS Measurements NO2 is a ubiquitous pollutant in the California South Coast Air Basin. It exhibits a multitude of absorption lines throughout the U.V. and visible regions, and thus its features have to be removed quantitatively from the ambient air spectra before other components, especially HONO, can be analyzed. Throughout the measurement period, NO2 levels were always above the detection limit of 160 nmole mm3 (4 ppb) with a minimum value of 360 nmole mm3 (9 ppb) observed during the early afternoon hours of 15 September, and a maximum of 5400 nmole m- 3

(135 ppb) recorded during the evening of 12 September. A comparison of NOz data obtained with other methods showed good agreement with the DOAS results (see accompanying papers). NO3 radical concentrations never exceeded the detection limit of 0.8 nmole m _ 3 (20 ppt), except on 13 and 14 September for a l-h period on each day around 8 p.m. when the concentration peaked at about 2.8 nmole m 3 (70 ppt). From these NO3 and NO1 data, N205 concentrations can be calculated (Atkinson et al., 1986) from the appropriate equilibrium constant (Kircher et al., 1984), yielding an N,O, concentration of approximately 200 nmole m - 3 (5 ppb) at the time of the NO3 maximum. These equilibrium calculations can be of interest because heterogeneous processes involving N205 and HZ0 are a major source of night-time HNO, and thus a sink for NO, (Tuazon et al., 1983; Finlayson-Pitts and Pitts, 1986; Winer et al., 1987). Data for HONO concentrations as measured by our long pathlength DOAS system are provided in Table 4. These data are hourly averages computed from the original measurements which cover 12-min sampling intervals. When concentration levels were detectable only for part of the hour they are quoted as upper limits. For example, based on a detection limit of 24 nmole m- 3 (0.6 ppb), if during the first 48 min (four 12-min samples) no HONO was detected and for the last 12 min a concentration of 30 nmoleme3 was

H. W. BIERMANNet al.

1552

NH3 mm3 nmole

I! I

i:oa

1090

1 1120

11.10

WAVENUMBERS (cm-

’)

Fig. 5. Single-beam FT-IR spectra illustrating the detection of NH, in Claremont, CA, on 16 September 1985. The arrow indicates the measurement peak of NH3 at 1103.4cm-I.

Table 4. Hourly average HONO concentrations (nmole m- ‘)*, t between 1900 and 0800 by long pathlength DOAS spectroscopy$ Hourly period (PDT)

11 Sept.

000&0100 0100-0200 0200-0300 03OO-0400 O400-0500 0500-0600 0600-0700 07o(M800 190&2000 2000-2100 2100-2200 2200-2300 2300-2400

<24 <24 <24 25 30

12 Sept.

13 Sept.

14 Sept.

:: 29 34 30 40 42 < 33

62 39 30 24 25 <25 <24 <24

:: 35 38 39 41 34 <28

<24 25 34 61 70

<24 <24 < 32 46 57

<24 <24 < 32 51 59

15 Sept. :; 36 35 38 31 26 <26 <24 <24 C 24 38 70

16 Sept.

17 Sept.

18 Sept.

102s 76 55 56 52 55 51 < 32

:: < 25 <24 <24 <24 <24 <24

<24 < 26 <32 <24 <24 <24 <24

124 <24 ~24 < 26 40

<24 <24 <24 124 <24

<25 <32 < 36 < 26 ~24

* At 23 “C and 740 torr (conversion factor: 1 ppb = 40 nmole me3). t See text for a discussion of errors. $ Blank entries fall outside the study period. QDigits beyond two significant figures are retained only to reduce roundoff errors.

measured, an upper limit of [ (4 x 24) +301/S or 25 nmole mm3 would be quoted. A precision of up to three places has been maintained throughout the data for HONO solely to reduce roundoff errors in further calculations. Significant levels of HONO were observed during the nights of 12/13, 13/14, 14115 and 15/16 September. The highest HONO concentration of 107 nmole m - 3

(2.7 ppb) was observed at 0038 on 16 September. In the case of HONO, large discrepancies between the various methods employed in this intercomparison study were evident, with the DOAS system yielding the lowest concentrations by at least a factor of three. For our spectroscopic technique, the only steps between data acquisition and the final calculated values consisted of the determination of the minimum and

Simultaneous absolute measurements of gaseous nitrogen species in urban ambient air adjacent maxima of an absorption band and a subsequent conversion to the equivalent concentration. The only factors which enter this calculation are the pathlength and the absorption cross section. The pathlength can be determined from the basepath by visually counting the number of reflections in the multipass system; the absorption cross sections are taken from the literature. Because the information about NO2 and HONO is contained in the same spectrum, any error occurring during data acquisition and analysis would involve both species. Considering the good agreement between our data and other measurements of NO2 concentrations, there can be only two possible sources for systematic errors in our HONO data: either an interference from another compound or an error in the literature value of the absorption cross section. The former problem always implies an over-estimate of the concentrations, whereas our data are already lower than the results from other methods. For a discussion of the errors of the absorption cross section we refer to the original literature (Stockwell and Calvert, 1978). It should be noted, however, that in simultaneous DOAS and FT-IR measurements of HONO concentrations in our environmental chamber results from the two techniques agreed within 10 %. In view of the above, it is extremely unlikely that the HONO data reported here can be in error by more than 40 % and in fact are more likely to be within 30% of the true ambient concentrations. 4. CONCLUSIONS

In this study, two new long pathlength spectrometers, utilizing 25 m basepath multiple reflection optical systems, have been employed for the first time. Measurement of nitrogenous species using these closed optical paths, as opposed to single pass systems extending several kilometers, allow the resulting in situ absolute spectroscopic data to serve as reference values for point monitors employing denuder or filter pack techniques. Although the detection limits for HN03 and NH3 with the FT-IR system make this technique most suitable for high pollution days, the absence of ‘walls’ and dependence only upon properties such as pathlength and absorption cross section, insures that the data obtained are artifact-free and representative of true ambient concentrations. Hence the nitrogen species data set presented here from both FT-IR and

DOAS measurements may confidently be employed to evaluate the accuracy, precision and utility of other more portable and less expensive analytical methods for important nitrogen-containing atmospheric constituents such as HNOJ, NH3 and HONO. Evaluations of this kind are presented in accompanying papers resulting from the September 1985 intercomparison study. Acknowledgements-The authors thank T. Dinoff, M. Kienitz, W. D. Long, E. Mateer and P. C. Pelzel for assistance in conducting this study and D. R. Lawson, J. R. Holmes, J. K.

1553

Suder, and R. Atkinson for helpful discussions. The support of the California Air Resources Board (Contract Nos A5051-32 and A4-081-32) for this research is also gratefully acknowledged. REFERENCES

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