Evaluation of procedures for measuring atmospheric nitric acid and ammonia

OM+6981/%8 $3.00+0.00 0 1988PcrgamonPrss plc

1988. Armospheric Environment Vol. 22, No. 8. pp. 1565-1573, Printedin GreatBritain.

EVALUATION OF PROCEDURES FOR MEASURING ATMOSPHERIC NITRIC ACID AND AMMONIA B. R. APPEL, Y. TOKIWA,

E. L. KOTHNY,

R. WV

and V.

POVARD

Air and Industrial Hygiene Laboratory, California Department of Health Services, 2151 Berkeley Way, Berkeley, CA 947069980, U.S.A. (First received 24 November 1986 and in jnal form 13 March 1987)

Abstract-A field study to evaluate methods for the measurement of atmospheric HNO, and NH, was performed in parallel with other investigatcrs at Claremont, CA. Our methods for HNO, included the automated, semi-continuous tungstic acid technique (TAT) and the denuder difference method (DDM). Ammonia was measured with the TAT, a filterpack (Teflon prefilter and two oxalic acid on quartz fiber after-filters), and a manual oxalic acid-coated denuder tube collection method. The potential error in HNO, measurement due to HNO,, NO, and NH,NO, retention by the TAT was assessed. The DDM for HNO, appears to be accurate within about 20% by comparison to a spectroscopic method, while the accuracy of the TAT was subject to large variability. Daytime HNO, results generally showed approximate agreement between the TAT and DDM, but TAT results averaged about a factor of six higher at night. The results are consistent with at least partial retention on tungstic acid-coated denuder tubes of NO, species other than HNO, and NO, (e.g. N,O,, NO,). Filter pack NH, results are consistently too high by a factor of 1.5. Ammonia measurement with the TAT is subject to an error of a factor of two. Key word index: Nitric acid, denuder difference method, tungstic acid technique, ammonia, filter pack method, nitrous acid, nitrogen dioxide, ammonium nitrate.

1. INTRODUCTION

Nitric acid is significant as an atmospheric pollutant as a precursor of NH,NO, particles, which cause visibility reduction, as a potential nitrating agent in forming mutagenic nitro-PAH compounds, and as a contributor to the acidity of suspended particles, rain water, lakes and vegetation. Ammonia, while not hazardous at atmospheric levels, participates in aerosol formation with HNO, and can neutralize the acidity of gaseous and particle-phase acids. Knowledge of NH, levels is, therefore, very useful in interpreting visibility data and the spatial variability in atmospheric acidity. Finally, both HNO, and NH, are of concern for their roles in eutrophication of surface waters, especially lakes. In preceding studies (Appel et al., 1980a, b), several filter procedures for nitric acid measurement were evaluated and compared. Ammonia was sampled by an impregnated filter procedure. These methods have in common the need for relatively long collection times (2 2 h), and subsequent laboratory analyses for nitrate and ammonium ions. In addition, methods relying on dual filters (a prefilter to remove particulate matter with retention of the nitric acid or ammonia on an appropriate after-filter), are subject to positive errors caused by dissociation of materials such as NH,NO, on the prefilter. A recently developed procedure offered promise of overcoming many of these difficulties. The procedure employs a diffusion denuder coated with tungstic acid (Braman and Shelley, 1980; Braman et al., 1982; McClenny et al., 1982; Gailey et al., 1983). This coat-

ing provides efficient retention of atmospheric levels of both gaseous nitric acid and ammonia. After sample collection, both components are thermally desorbed. Nitric acid desorbs as NO and is directly measured with a conventional chemiluminescent NO, analyzer. Ammonia desorbs unchanged and is temporarily trapped with a short WO,-coated ‘transfer tube’. Ammonia is subsequently desorbed from the transfer tube, oxidized to NO and measured with the same analyzer. The automated tungstic acid technique (TAT) was evaluated in both laboratory and field trials. The present paper summarizes these efforts with emphasis on method comparisons in atmospheric sampling trials in Claremont, CA during the period 11-19 September 1985, in parallel with other research groups. Limited comparisons to results from other groups are also included in this paper. A companion paper in this volume addresses a comparison of results by most of the methods employed by these groups. In addition to the TAT, the denuder difference method (DDM) measured HNO,, based on the measurement, in parallel, of fine particle NO; and total fine inorganic NO; (Shaw et al., 1982; Appel et al., 1980b). Ammonia was measured by (1) a filter pack method (FP) for gaseous NH, and particulate NH: using a Teflon prefilter and two oxalic acid-glycerol coated quartz fiber after-filters (Richards et al., 1978) and (2) a denuder tube collection procedure (DT) for NH, using oxalic acid-glycerol-coated tubes (Ferm, 1979). A more detailed account of this work is available (Appel et al., 1986).

1565

B. R. Appel et al.

1566 2. EXPERIMENTAL 2.1. Sampling scheme

Table 1 lists the samplers employed. At Claremont, samplers for the DDM were mounted with inlets facing west (i.e. into the prevailing daytime wind), and about 1.5 m above a wooden platform which was about 1 m above a paved surface. Samplers for NH, and NH: faced downward and were located at the same height as the DDM. The TAT was mounted within an air-conditioned mobile laboratory, since efforts to operate it adjacent to the DDM samplers proved unsuccessful. Outside air, about 3 m above the paved surface, was drawn through a 1.5 m length of 10 mm i.d. glass tube employing a squirrel-cage blower. During sampling, the TAT removed a sidestream (at 1 emin-‘) from this inlet through about 75 cm of 6 mm i.d. glass tubing and 15 cm of FEP Teflon tubing. The loss of HNO, within this inlet has not yet been evaluated. 2.2 Tungstic acid technique (TAT) Figure 1 shows a schematic of the TAT system used. It incorporates several of the design elements used by Roberts et al. (1984). Within the system, nitric acid contacts quartz, Pyrex and FEP Teflon tubing employed in minimal amounts as sleeve joints. The 34-cm WO, coating on the preconcentrator tube traps HNO, as well as NH,. In laboratory trials WO,-coated tubes exhibited 93-98% efficiency for HNO,, the efficiency decreasing with increasing loading in the range 340680 ng. After a lo-min sampling period at ld min -‘, a carrier gas, initially He, is introduced while heating the preconcentrator tube. Nitric acid desorbs as NO but NH, desorbs unchanged. The latter is retained on the transfer tube, coated for 12 cm with WO,. The HNO, is measured as NO with a chemiluminescent NO, analyzer (TECO Model 14BE). Following emergence of the peak corresponding to HNO,, the carrier is changed to synthetic air, and the preconcentrator heated for an additional 1 min to enhance preconcentrator tube life by oxidation of collected carbonaceous material. The transfer tube is then heated and the desorbed NH, oxidized to NO over the gold catalyst followed by quantitation as above. The flow rate of the He and synthetic air carrier gas is in excess to the sampling rate of the TECO analyzer. The excess is vented through the sample inlet, effectively sealing it from intrusion of ambient air during the analytical cycle. The operation of all valves and heaters is controlled by a Chrontrol programmable timer. Mass flow controllers are used for controlling the sampling rate and carrier gas flow. Automated data acquisition and diskette storage is provided by an APPLE He microcomputer and an ISAAC System 91A (Dynamic Solutions).

During field trials, the TAT was calibrated daily between 0500 and 0900 hours. The output from a Metronics HNO, permeation tube (containing 68% HNO, in water), maintained at 83.3”C in a Metronics Dynacalibrator, was diluted with ambient air scrubbed through an NaCl-impregnated Whatman 41 (NaClW41) filter to remove HNO,. The levels of NO and NO, from the permeation source were not measured but were of little concern for either Nylon filter collection (Appel et al., 1980a) or, as shown below, the TAT. The total flow was adjusted to provide a C 50 ml mini excess relative to the sampling rate of the TAT. During calibration the TAT was connected to the diluted HNO, source, the excess being vented. The c. 50-cm FEP Teflon line carrying diluted HNO, remained equilibrated with HNO, at all times. The emission rate of the HNO, source, 89.0 +2.8 ngmin-’ (n=3), was measured by sampling for 18-24 h periods with NaCIW41 filters. Dosage to the TAT was altered by varying the sampling time between 0 and 3 min. NaClW41 filters were used in this application because of their low flow resistance, relative to nylon. This eliminated the need for an addition vacuum pump. A previous study showed about equal efficiencies for nylon and NaCI/W41 in HNO, sampling at atmospheric levels (Appel et al., 1980a), but NaCl/W41 had higher capacities for HNO,. At ambient HNO, concentrations, Cl- interference in ion chromatographic analysis of NO; makes nylon filters preferable. For NH, calibration, NH, from a permeation tube containing dilute NH,OH at 4OC, was diluted with ambient air scrubbed through an oxalic acid-impregnated filter, with total flow adjusted to <50 ml excess relative to the TAT sampling rate. The remainder of the calibration strategy is the same as for HNO,. The emission rate of the NH, source, 45.1+ 5.0 ngmin- 1 (n= 3), was measured by collection for 18-24 h on oxalic acid/quartz fiber filters. No significant change in daily calibration of the TAT was observed for either gas. Accordingly, all data were pooled and one regression equation employed for each species. Peak areas were used for HNO, and peak heights, for NH,. 2.3. Denuder difference method (DDM) Sample 2 consisted of a perlluoroalkoxy (PFA) Teflonlined cyclone, 50% cutpoint 2.2 pm at 28 / min- 1(John and Reischl, 1978), a denuder containing 24 tubes, each coated for 30 cm with MgO following initial lo-cm uncoated sections, and a Nuclepore polycarbonate filter holder containing one nylon filter. Sampler 3 differed from 2 only by the absence of the denuder. The cyclone and denuder of sampler 2 and the cyclone and filter holder of sample 3 were connected by a Y-shaped 3 to 3.5 cm i.d. Pyrex pipe. The pipe permitted cyclone operation at 28 1min-i and filter

Table 1. Description of samplers employed at Claremont, CA Sampler No.

Sampler

1 2” 3 4 5

tungstic acid technique (TAT) cyclone, acid-gas denude?, Nylon filter’ cyclone, Nylon filter oxalic acid-glycerine coated denuder tube Teflon prefilterd, 2 oxalic acid impregnated quartz’ after-filters

Sampling rate (LPM) 1.0 20 20 1.5 25

Samples/day 72 5’ 5 2 5

Species measured HNO,, NH, fine particle NO; fine particle NO; + HNO, NH, as NH: particulate NH: NH;

a. Samplers 2 and 3 sampled from separate Teflon-lined AIHL cyclones, each with flow rate 20 emin-’ unit density spheres, 2.2 pm). b. Denuder includes 24,40 cm, 6 mm I.D. tubes each coated for 30 cm with MgO. c. Nylasorb (47 mm dia.) filters, Gelman Inc., Lot 871. d. Two pm pore size Zefluor (47 mm dia.), Gelman Inc. e. Pallflex 2500 QAST (47 mm dia.) quartz fiber filters, Pall Corp. f. Four hour samples, 080&2400 h, OfKlO-0600h, one 6-h sample.

and NH, as

(50% cut-point for

I561

Measuring nitric acid and ammonia

A

1

Vent

To Port

Au

To Port 2-1

Catalyst

3

Transfer Tube

1 Sample Inlet

I

Sample Vat. T

2 Tube A&B=

To Port 9

Needle Valves C = 2-Way Solenoid Valve

MULTIPORT VALVE

MULTIPORT VALVE

Analvtical

Sample Cycle

Cvcle

Open

To Sample Vacuum T

To Vacuum Source

To

Source Syn.

lve

A c B

Air or He

l

Port in use

OPort capped

Fig. 1. Schematic of the automated tungstic acid technique.

sampling at 20 L min _ ’ but provided an additional 25 cm length through which HNO, must pass to reach the nylon filter of sampler 3. Samplers 2 and 3 were employed in parallel to measure fine particle NO; and fine particle NO; plus HNO,, respectively. The latter is referred to as total fine inorganic NO;. (Sampler 3)-(Sampler 2) NO; results provided HNO, measurement by difference. Mass flow controllers on each sampler yielded a precision of about 1% at 20 I min _ ’ (8 / min-’ of the total flow to each sampler was vented directly to the pump).

2.4. Ammonia

samplers

Sampler 4 consisted of a 5O-cm. 4-mm i.d. tube coated for 35 cm with an oxalic acid-glycerine mixture following etching with 50% HF solution. The coating was prepared by drawing up into each tube a methanol solution containing 1.5%~ oxalic acid and 6.3%~ glycerine. Solvent was evag orated by a stream of N,. Sampler 5 consisted of a two section, Nuclepore multiple 47-mm dia. filter holder. A Teflon prefiiter removed particulate NH:, allowing NH, to penetrate to two, oxalic

1568

B. R. Appel et al.

acid-glycerine-impregnated filters contained in the same section of the holder. These filters were prepared by spotting each 47 mm Pallflex QAST quartz fiber disc with 0.7 ml of an ethanol solution containing 5.0%~ oxalic acid and 5.2% w glycerine. Filter spotting and solvent evaporation were done under N,. 2.5.

Analytical

precision

Analytical procedures for NO; and NH; have been previously described (Appel er al., 1980a). The median coefficient of variation (C.V.) was 3.1 and 4.8% for NO; and NH;, respectively, with Claremont samples. The recovery of NO; from spiked nylon filters (Columbia Scientific) was 72% at IO @iher and 2 97% at 3 70 pg/tilter.

3. RESULTS

3.1. Accuracy of the denuder difference HNOJ loss in the cyclone inlet

method

and

Following field trials at Claremont, HNO, from a diffusion tube source was diluted with purified ambient air to provide about 15 fig m -3 concentrations (as NO;). The HNO, in air was adjusted to 50% r.h. and 20°C and sampled in parallel with four samplers: (A) and (B) 47 mm nylon filters (Geiman Lot 871) at 20 t min- t. (C) As in ‘A’ but preceded by one of the Teflon-lined cyclones, still dirty from prior use at Claremont. (D) Same as ‘c’ but with an MgO denuder between a second cyclone and the nylon filter. As in the field trials, the total flow through each cyclone was 28 e min- ’ of which 20 P min _ ’ was sampled through the nylon filter. The denuder had been used for the last four days of the Claremont study (i.e. its efficiency represented a lower limit to that of a fresh denuder). The results for three, 2-h sampling trials are shown in Table 2. Nitric acid concentrations decreased by about 25% between the first and the third trials, which contributed to the l&20% C.V. shown for the mean NO; concentrations by each sampler. Loss of HNO, in the cyclone (plus associated glass pipe) can be assessed by comparing mean total fine NO; results (Sampler C), l5.2+ I.5 pg mm3, with the mean NO; recovered from samplers A and B for all trials, 15.7 f2.1 pg m-3. Alternatively, the mean ratio, Sampler C/Sampler A and B, calculated from individual trials was 0.97f0.07. The results indicate no measurable

loss of HNO, in a cyclone still dirty from 48-h sampling in Claremont. The accuracy of the DDM in the absence of potential interferents may be inferred by comparing the mean results (Total Fine NO; -Fine Particulate NO;), 14.3+ I.5 pg mm3, to the mean of samplers and A and B, 15.7 f 2. I pg m - 3. Alternatively, the results may be determined for each trial separately to eliminate the contribution of the concentration change to the variance. Table 3 indicates, by the latter approach, an accuracy for the DDM of 92 + 6%. The principal cause of the apparent 8% negative error is the NO; measured with sampler D. This NO; represents the sum of HNO, penetrating the denuder (estimated to be about 0.5 pg m - 3 or 3% penetration) and particulate NO; formed from HN03 and NH, not removed by the air purification system. The accuracy of the DDM for atmospheric HNO, measurements was assessed by comparison with the tunable diode laser method (Schiff et al., 1983). The latter sampled ambient air through an inlet consisting of PFA Teflon tubing and a PTFE Teflon filter bringing the filtered air sample into an optical cell at subambient pressure. As shown in Fig. 2, the DDM yielded HNO, concentrations which were highly correlated with those by the tunable diode laser (TDL) method, but averaged about 30% higher. A similar comparison of HNO, measurements between the TDL and Fourier transform infrared results (Tuazon and Winer, priv. comm., 1986) showed the TDL values to be lower by, on average, 15% (Lawson and Hering, 1986). Therefore, the DDM results are inferred to be higher by about 15% compared to those by FTIR. 3.2. Interference

trials with the tungstic acid technique

Potential interferences in HN03 measurements with the TAT were assessed with NO,, an ammonium nitrate aerosol, and nitrous acid (Table 4). With NOz, interference remained below I %, and decreased from 0.8% with NO, in dry air to 0.4% in air at about 50% r.h. The interference with nitrous acid in air at 4&70% r.h. was below detection when allowance was made for the substantial memory effect from the

Table 2. Laboratory assessment of the loss of HNO, in cyclone of samplers the DDM

Trial

Nylon filter (A)b

I 2 3

17.5 15.7 13.5

Mean:

15.6 + 2.0

(pg NO;

Nylon filter(B) 18.9 14.7 14.1 15.9k2.6

Total fine inorganic NO;(C)

Fine particulate

16.4 15.8 13.5

0.8 1.0 0.8

l5.2&

I.5

NO;(D)

0.9 * 0. I

a. All results corrected for a laboratory filter blank, 1.2 &47 mm filter. b. The letter in parenthesis is sampler I.D. as discussed in text. c. Total fine inorganic NO; indicates the sum of fine particulate NO,

HNO,.

for

m -‘)

and

1569

Measuring nitric acid and ammonia

The ratio of TAT/DDM HNO, results are shown by time of day in Table 6 and Fin. 3. Results for the pkriod 18-19 Sept. appear anomalous and have been excluded from mean ratios and from Fig. 4. With this exclusion a significant day-night difference in the TAT/DDM ratio is evident. Average agreement between methods within about 50% is seen during the day with mean ratios about 6 at night. These results suggest that the WO, tubes retain a species other than HNO, which is convertible to NO, and which behaves differently from HNO, in the DDM. The highest l-h average NO, value observed in the study was about 0.1 ppm, with diurnal maxima usually being observed between 2200 and 2400 h (Lawson and Hering, 1986). Assuming 0.5% retention of NO, on WO, tubes would provide an interference of < 1.3 pg mm3 (as NO;). Thus the retention on WO,-coated tubes of additional NO, species is indicated. For example, concentrations of N,O, up to 2 ppb were estimated at Claremont based on measured levels of NO; and NO, during the summer of 1979 (Atkinson et al., 1986). Complete retention of such levels of N,O, and conversion to NO would yield an apparent HNO, value up to about 10 pg rne3. Four or six hour average HNO, values observed with the TAT during the periods 2OO(M600

Table 3. Laboratory assessment of the accuracy of the DDM for HNO, (pg NO; m-‘) Trial

True HNOS,

1 2 3

18.2 15.2 13.8

DDM

HNO,

DDM/True

HNO,

15.6 14.8 12.7

0.86 0.97 0.92

Mean:

0.92 & 0.06

a. True value measured by the NO; collected by a Nylon filter in a open-face filter holder connected directly to the manifold from which dilute HNO, is sampled.

automated TAT previously conditioned for HNO,. Similarly, the fine particulate nitrate exhibited no measurable interference. 3.3. Comparison

of the TAT and DDM atmospheric

nitric acid results

The TAT provided three determinations per hour. Hourly average values were further averaged to permit comparison with four or six hour DDM results (Table 5). Cases in which partial or suspect TAT data were obtained are noted. Only for the period 14 Sept. 1200-1600 h are the TAT results considered inadequate for comparison.

8

47.4,

35.3-

II P =

23.2-

B

6.44

0.00

li.8

19.3

TDL HNO

Fig. 2. Tunable

25.7

3

diode laser (TDL) HNO, against denuder (DDM) HNO, results (pg as NO;).

Table 4. Interference Species NO* 10.2 pm NH,NO, HONO”

difference method

studies with the automated

Dosage

(ng as NO;)

9600 300 or 700 80

32.2

TAT

Interference

(%)

0.4-0.8 none <5

a. Calculated relative to the response expected for an equivalent dosage of HNO, (as NO;). b. Prepared by the technique of Braman and de la Cantera, 1986.

B. R. Appel et al.

1570

Table 6. Nitric acid method comparison by time of day

Table 5. Comparison of the Tungstic Acid Technique (TAT) and Denuder Difference Method (DDM) for nitric acid (rg mm3 as NO;) at Claremont, CA Period (PDT)

TAT”

DDM

08OG1200 1200-1600 16OG2000 200&2400

20.5” 26.6’ 36.9 24.3

25.3 47.4 31.2 6.5

09/15/85

OOOuO600 080&1200 120&1600 16W2000 200%2400

17.4 23.6d 22.5 16.2 7.6

3.0 16.1 18.3 9.6 0.7

09116185

OOOWI600 08W1200 12Wl600 1600-2000 2OOG2400

5.7 5.8b 7.6 6.4 3.2

1.0 5.9 9.1 5.5 1.6

09117185

OoOO-O600 080&1200 120&1600 16OCk2000 200&2400

3.8’ 3.8 7.5 10.5d 6.4

1.9 6.1 10.1 5.5 0.8

09118185

000&0600 0800-1200 120&1600 16W2000 2000-2400

5.9’ 6.0 7.4 10.0 10.3

0.6 0.7 1.9 1.2 0.3

09/19/85

OOOSO600

10.3

0.3

Date

OOOCLO600 09/14/85

Date

TAT/DDM HNO, ratio

9115185 9116 9117 9/18 9119 9114 9115 9116 9117 9,‘18 9114 9115 9116 9117 9,‘18 9114 9115 9116 9117 9/18 9114 9115 9116 9117 9118185

5.8 5.7 2.0 9.8 34.3b 0.81 I .47 0.98 0.62 8.57b 0.56 1.23 0.84 0.74 3.89b 1.18 I .69 I.16 1.91 7.83b 3.74 10.9 2.00 8.00 34.3b

Time period (PDT)

0800-1200

12OG1600

160&2000

200&2400

a. n = 4, each case. b. Excluded from mean.

a. Mean of four or six l-h average values except as noted. b. Mean of two, l-h values. c. Electronics saturated. Minimum value only. d. Mean of three, l-h values. e. Mean of five, l-h values.

10.

T

9.

T I

0 8

10

12

14

16

18

20

22

24

2

Time of Day (PDT) Fig. 3. Ratio of TAT to DDM HNO, results by time of day.

4

6

Mean ratio’

5.8 + 3.2

0.97 +0.36

0.88 kO.34

1.49kO.37

6.2 k 4.0

Measuring

’

0.00

I

I

I

2.40

0.00

nitric acid and ammonia

I

I

I

I

4.80

I

I

7.20

9.60

I 12.0

DENUDER TUBE NH3 0q/M3)

Fig. 4. Filter pack NH, against denuder tube NH,. ranged from 3 to 24 pg rnm3, with median value 7.0~grn-~. The behavior of N,O, in the DDM is unknown. 3.4. Comparison of atmospheric

ammonia results

Ammonia sampling with oxalic acid-coated denuder tubes (DT) was assumed to provide an accurate measure of atmospheric NH,. FP NH, values were averaged to permit comparison with the 10 or 12 h

DT results (Fig. 4). The data sets are highly correlated, but the FP method, based on the ratio of means, averaged 50% higher than the DT. This is consistent with substantial volatilization of particulate NH: (as NH,) from the prefilter in the FP sampler. The ratios of NH, results, TAT/FP, are plotted against mean temperature in Fig. 5. The ratios ranged from 0.3 to 2.1 (median 1.13), decreasing with increasing temperature. TAT ammonia results are similarly

2.50-

II P 5 d

1.25,

6 s t: ;: F

0.63-

0.00

Y = 2.77 - 0.0744 X r = 0.11 n = 18

’ 10.0

I

I 15.0

I

I

I

I

20.0

25.0

Temperature Fig. 5. Ratio of TAT to filter pack NH,

I

I 30.0

('C)

results against

temperature.

I

I 35.0

B. R. Appel et al.

1572 l

3.30

-

1*-

0.0

Y=3.45-.0913x ra0.61

3 \

’

I

10

I 15

I

I 20

I

1 25

I

I

I

30

I 35

Temperature f lCf

Fig. 6. Ratio of TAT to F’TIR NH, against temperature.

compared in Fig. 6 to those by Fourier transform infra-red spectroscopy (FTIR) (Tuazon and Winer, priv. comm., 1986). The median TAT/FTIR ratio was 1.04. Again, lower ratios are observed at higher temperatures, although the correlation with temperature change is poorer. Since FTIR is less likely to vary in response with temperature change Figs 5 and 6 together suggest that the TAT shows much greater temperature sensitivity than the FP method.

4. CONCLUSIONS

At a location with exceptionally high HNO, levels, the denuder difference method for atmospheric HNOS is accurate within about 20%. At a location with exceptionally high HNO, levels, the automated tungstic acid technique (TAT) can measure daytime atmospheric HNO, concentrations within a factor of two but is subject to a large positive error at night. The TAT results are consistent with at least partial retention of NO, species other than HNO,. Ammonia concentrations with the TAT are measured within about a factor of two, both day and night. Within this range, results tend to be higher at night or at lower temperatures.

Acknowledgements-Tungstic acid coated tubes used in this work were prepared by K. M. Hua, visiting scientist from the Southwest Institute for National Min&ities, People’s Republic of China. This work was sunoorted. in Dart. bv the

California Air Resources Board Rkiearch Diiisioh. ‘The statements and conclusion in this paper are those of the authors and not necessarily those of the California Air Resources Board. The mention of commercial products,

their sources, or use in connection with material reported herein is not to be construed as either an actual or implied endorsement of such product.

REFERENCES

Appel B. R., Wall S. M., Tokiwa Y. and Haik M. (198Oa) Simultaneous nitric acid, particulate nitrate and acidity measurements in ambient air. Atmospheric Environment 14, 549-554. Appel B. R., Tokiwa Y. and Haik M. (1980b) Sampling of nitrates in ambient air. Atmosaheric Environment 15. 2833289. Appel B. R., Tokiwa Y., Kothny E. L.: Wu R. and Povard V. (1986) Studies of drv acid denosition in the South Coast Air Basin: intermethod comparison of procedures for nitric acid and ammonia. Final Report to the California Air Resources Board, Contract A-4-147-32. Atkinson R., Winer A. M. and Pitts J. N., Jr. (1986) Estimation of night-time N,O, concentrations from ambient NO, and NO, radical concentrations and the role of N,O, in night-time chemistry. Armospheric ~n~i~o~~enr 20,331-339. Braman R. S. and Shelley T. J. (1980) Gaseous and particulate ammonia and nitric acid concentrations. EPA Report 600/7-80-179. Braman R. S., Shelley T. J. and McClenny W. A. (1982) Tungstic acid for preconcentration and determination of gaseous and particulate ammonia and nitric acid in ambient air. Andyt. Chem. 54, 358-364. Braman R. S. and de la Cantera M. A. (1986) Sublimination sources for nitrous acid and N-compounds in air. Anafyt. Chem. 58, 1533-1537. Ferm M. (1979) Method for determination of atmospheric ammonia. Atmospheric Enuironment 13, 1385-1393. Gailey P. C., McClenny W. A., Braman R. S. and Shelley T. J. (1983) A simple design for automation of the tungsten VI oxide technique for measurement of NH, and HNO,. Atmospheric Environment 17, 1517-1519.

Measuring nitric acid and ammonia John W. and Reischl G. (1980) A cyclone for size-selective sampling of ambient air. Air Pollut. Control Ass. 30, 873-876. Lawson D. and Hering S. V. (1986) Nitrogen species Method Comparison Study-at Claremont, California, September 1985%Overview. Presented at 1986 EPAiAPCA Svmposium on Measurement of Toxic Air Pollutants, Raleigh, N.C. McClenny W. A., Kaneda K., Yanaka T. and Sugai R. (1982) Tungstic acid technique for monitoring nitric acid and ammonia in ambient air. Analyt. Chem. 54, 365-369. Richard L., Johnson W. and Shepard L. S. (1978) Sulfate aerosol study. Final Report to the Coordinating Research Council, Contract No. CAPA-13-76.

1573

Roberts J. M., Hubler G., Norton R. G., Goldan P. D., Fahey D. W., Albritton D. L. and Fehsenfeld F. C. (1984) Measurement of HNO, by the tungsten oxide denuder tube method. Comparison with the nylon filter method. Presented at the American Geophysical Union Meeting, San Francisco. SchitTH. I., Hastie D. R., Mackay G. I., Iguchi T. and Ridley B. A. (1983) Tunable diode laser systems for measuring trace gases in tropospheric air. En&. Sci. Technol. 352A-364A. Shaw R. W., Jr., Stevens R. K., Bowermaster J., Tesch J. W. and Tew E. (1982) Measurements of atmospheric nitrate and nitric acid: the denuder difference experiment. Atmospheric Environment 16,845-853.