Measurement of Atmospheric Ammonia Zhang Genfa,t Purnendu K. Dasgupta," and Shen Dong$
Department of Chemistry and Biochemistry, Texas Tech University, Lubbock, Texas 79409-1 06 1
A diffusion scrubber based instrument is described for the measurement of atmospheric ammonia. Ammonia is collected through a porous membrane into water and the fluorescence due to 1-sulfonatoisoindole,following reaction with sodium sulfite and o-phthaldialdehyde, is measured. A novel electropneumatic slider valve assembly is utilized for the selection of gas streams. The instrument is capable of measuring ammonia at trace levels (limit of detection, 45 pptv) and a short time resolution (5 min). Unless corrected for, a f25% change in relative humidity during sampling can cause a 710% error for low-level NH, measurements. Ambient ammonia levels were measured during different times of the year in Lubbock, TX, with and without a PTFE prefilter ahead of the diffusion scrubber. The mean ammonia concentration is highest in the spring when the local agricultural activity and fertilizer use is maximum. The prefilter consistently reduced the measured concentration and also dampened any shortterm concentration fluctuations. Ammonia is the predominant gas-phase atmospheric base and, as such, plays a paramountly important role in atmospheric chemistry. The most comprehensive available review, with over 1100 citations, describing the environmental aspects of the biogeochemistry of ammonia, is now a decade old ( I ) . However, few changes have occurred in the intervening years that affect our knowledge about ammonia in a profound manner. The importance of ammonia in the atmospheric oxidation of SOzwas first pointed out by Junge and Ryan more than 30 years ago (2) and remains valid today. The promotion of oxidation rates of SO2dissolved in hydrometeors by ammonia occurs primarily as a result of the increase in pH afforded by the presence of ammonia, and thence the enhanced solubility of SO2. The theme has been repeatedly confirmed and expanded on since (3-7). Airborne measurements in the United Kingdom have indicated a 5-25-fold increase in SO2 oxidation rates as SO2-ladenair masses pass over major ammonia emission zones (8, 9). The presence of ammonia increases the washout efficiency of hydrometeors for SO2 (9);the sulfate content of precipitation is much higher in the neighborhood of major ammonia emission sources (10). The major source of ammonia in both the United States and Europe t Permanent address: Shanghai Hygiene and Anti-Epidemic Center, 280 Chang-Su Rd., Shanghai, People's Republic of China. *Permanent address: School of Public Health, Shanghai Medical University, 138 Yi Xue Yuan Road, Shanghai, People's Republic of China.
0013-936X/89/0923-1467$01.50/0
is the decomposition of animal wastes ( I , 11,12). In Europe, the ammonia emission flux is the highest in the Netherlands, and as such, the effects of ammonia emissions and its long-range transport have received particular attention from Dutch scientists (10, 13). A detailed study on the ammonia flux estimates for East Germany has recently been published (14). The deterioration of atmospheric visibility has frequently been shown to be due to ammonium salts. In one investigation in the Teeside area of northeast United Kingdom, the combination of NH, emission from fertilizer factories and SO2emission from the combustion of fossil fuels was found to produce, aided by sea mist producing >80% relative humidity (RH), peak sulfate levels of 830 pg/m3 (15). In another study, the haze over the Great Smoky Mountains in the United States was shown largely to be due to ammonium sulfates (16). The deteriorated visibility in the eastern Los Angeles basin is believed to be mostly due to NH,N03 (17). The formation of a number of nitrogen-containing pollutants in the southern California airshed has been shown to be an acute function of the gaseous ammonia concentration (18,19). Considerable interest exists regarding the equilibrium between gaseous NH, and HNO, and particulate NH4N03(20-23). The need for further studies on the overall cycle of NH3/NH4+,and on the effects of ammonia on acid deposition and aerosol formation, has been emphasized in a recent overview (9). It is obvious that such investigations will be greatly benefited by sensitive, affordable instrumentation capable of measuring gaseous NH, without interference from particulate material containing NH4+. Of direct gas-phase measurement methods, few are capable of the requisite sensitivity (low to sub-ppbv levels) ( I ) . Tunable diode laser spectroscopy (24) is capable of the sensitivity but affordable by few. Several commercial chemiluminescence instruments based on the NO-0, reaction (%) offer an ammonia measurement channel, which catalytically oxidizes NH, to NO and thus measures NH,. In the presence of significant concentrations of NO,, the accuracy of the determination, involving subtraction of two large numbers, is limited. All other methods rely on collecting the gaseous NH, into a suitable solid or liquid phase and then performing the determination. In all such schemes, whether or not to use a prefilter becomes an all-important question. With the exception of diffusionbased collectors, the absence of a prefilter results in at least partial collection of the aerosol ammonium (26). Consequently, the determination suffers from positive errors. On the other hand, when a prefilter is used, even in an otherwise attractive technique (27, 28), both positive and
0 1989 American Chemical Society
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negative errors can, in principle, result (1,22). The first is due to volatilization loss of ammonia from the ammonium salts collected on the prefilter (29), especially at high sampling rates in warm weather (30). The second is due to the potential sorption of ammonia by the filter matrix and/or acidic particulate material collected thereon. A comparable problem exists for the determination of HNO,. There is a paucity of (near-) continuous data to firmly establish how filtration affects the measurement process for either HN03 or NH3 (31, 32). Diffusion-based collectors such as diffusion denuders do not suffer from obligatory collection of aerosols. In its simplest form, a diffusion denuder is a tube (or a multiplicity of tubes) whose inner walls have been coated with a good sorbent for the gas. Stevens et al. were the first to describe the use of a phosphorous acid coated tube for the collection of ammonia (33). Oxalic acid coated tubes have been extensively used by Ferm (34,35)for the same purpose; many others have used this technique as well (see, e.g., ref 22). A highly ingenious, albeit complex, apparatus has been described that coats a tube with citric acid in situ, dries it by blowing NH,-free air, aspirates sample air, removes the coating by washing, analyzes the NH4+in the wash liquid, and then begins the whole cycle anew (36). A thermally reversible sorption process applied to a tungstic acid coated denuder coupled to a chemiluminescence-based NO, monitor has been reported for the simultaneous measurement of NH, and HN03 (37,38).The reliability of this device for accurate measurement of either NH3 or HNO, may be questionable (31,32). The attainable temporal resolution of any thermally cycled method is also generally unattractive. One other automatable method, based on a potentiometric electrode sensor and membrane preconcentration, is in the literature (40). Given a sufficiently long sampling period, the technique appears to be capable of excellent detectabilities; however, it is not clear if the device functions strictly in the diffusive collection mode. It has not been sufficiently characterized for ambient measurements; response times appear to be very long with a very large relative humidity effect. Two wet diffusion-based collectors with discrete collection and analysis steps have been described for the determination of NH3 ( 4 1 , 42). The diffusion scrubber (DS), a membrane-based diffusion denuder, was introduced by Dasgupta (43). The DS provides unusual flexibility in continuous collection and analysis; completely automated, DS-based instrumentation for the measurement of water-soluble gases, e.g., SO,, H,O,, HCHO, etc., and intercomparison studies of these with other instrumentation have been described (44-48). We recently developed a highly sensitive determination method for measuring aqueous ammonium ion (49); the method also shows considerable selectivity over other amines and amino acids. In the present paper, we describe DSbased automated instrumentation capable of measuring atmospheric NH, a t sub-ppbv concentration with short time resolution. The effect of prefilters is addressed based on near-continuous data obtained by measurement with and without prefilters. Experimental Section Diffusion Scrubber. One end of the DS is shown schematically in Figure 1. This is a modified version of the device described in ref 44; step-by-step construction details have been given elsewhere (50). Briefly, a microporous polypropylene membrane tube M (Celgard X-20, 400-pm i.d., 25-pm wall, 0.02-pm mean pore size, 40% surface porosity; Hoechst Celanese, Charlotte, NC) filled with a 300-pm-diameter nylon monofilament N (10-lb1468
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Figure 1. Diffusion scrubber schematic (one end shown). C, PTFE tube (liquid in/out); S,Q, spacer tubes for sealing into the part of tee T (air flows inlout through this tee); G, glass jacket covering PTFE tube F; P, PVC tube segment; M, microporous polypropylene membrane tube; N, nylon monofilament.
strength fishing line, STREN; Du Pont) is deployed centrally within a poly(tetrafluoroethylene)(PTFE) tube F (5-mm id.). The tube F is maintained in a linear configuration by the outer glass tube G (6-mm i.d.). The membrane tube is connected to a PTFE connecting tube C (300-pm i.d., 30 AWG) by a small polyvinyl chloride (PVC) tube segment P (0.015-in. id.; Elkay Products, Shrewsbury, MA), which is covered by PTFE tape. At each end, the DS terminates in a polypropylene tee T (Nalge 6151-0187); tube C is sealed a t the tee exit by spacer tubes S (PTFE, 800-gm i.d., 20 AWG) and Q (PVC, 0.065-in. id.; Elkay). Air is sampled through the open tee ports while the scrubber liquid is pumped countercurrent through tube C. The membrane tube begins at least 5-7 cm from the tee inlet to allow full development of laminar flow. The active membrane length for the DS devices used in this work was 40 cm. The DS was always used in the vertical position to minimize gravitational settling of large particles. Zero Air and Calibration Source. The sensitivity demanded of the technique requires careful removal of ammonia present in the zero and calibrant generation air. Of various alternatives experimented with (cation-exchange resin in H+ form, oxalic acid, NaHS04, etc.) we found acid-impregnated silica gel to be most efficient and convenient. The material is prepared by intimately mixing 50 mL of 6 N H2S04with -200 g of 20-mesh silica gel and drying overnight at 70 "C. As shown in Figure 2, half of a glass column S1 (2.5 X 100 cm) is packed a t the bottom with the acid-impregnated silica gel, followed by standard silica gel on top. The air then passes through a second column S2 containing, in order, granular activated carbon and soda lime. This air is fed on-demand to the instrument as zero air. It is also continuously metered, through needle valve N1 and mass flow meter M1, at 0.5 standard
compressed filtered house zero g a s
Figure 2. Zero/calibrant gas supply. S1, S2, glass columns, containing, in sqeuence, acid-washed silica gel, silica gel, activated carbon, soda lime: N1, N2, needle valves; M1, M2, mass flow controllers; T, thermostated enclosure housing thermal; equilibration coil C, and permeation wafer W.
Figure 4. Analyzer liquid phase schematic. P, peristaltic pump; B, buffered formaldehyde/sulfite; 0, o-phthaldialdehyde, W, water; C, cation-exchange microcolumn; A, A', connections to the diffusion scrubber (DS) in the two configurations shown in the inset; M1, knotted mixing conduit; R, stainless steel reaction coil in heating block H: T, porous membrane tube; F fluorescence detector. Insets: (a) in-line configuration: (b) in-loop configuration; V, six-port rotary valve, W, water.
bG2
T+
compressed air
'
I & - &:'"'
I
Figure 3. Sampling manifold. P, pump; N, needle valve: M, mass flow meter: DS, diffusion scrubber: A, B, pneumatic actuators: B1, B2, slider valve, moving and stationary blocks, respectively; S, sample; C, calibrant: 2, zero; Vl-V3, air solenoid valves: R1, R2, reguhtors, G1, G2, pressure gauges.
liters per minute (slpm) into a permeation chamber containing a wafer-type NH3 permeation device W (VICI Metronics, Santa Clara, CA; gravimetrically calibrated to be emitting at 15.7 ng of NH3/min) via a thermal equilibration coil C. For high-level NH3 experiments (e.g., 1 ppmv), a source with much greater NH3 output was used. The permeation chamber and the thermal equilibration coil are housed in a thermostated enclosure T maintained at 30 "C. The exit gas is diluted by further dilution air, metered by mass flow meter M2, and controlled by needle valve N2. This calibrant stream is sampled by the DS on-demand; otherwise, it is vented. Sampling Manifold. The sampling manifold must maintain sample integrity and minimize particle deposition (44); it is shown schematically in Figure 3. The superiority of slider valves in minimizing particle deposition has been noted (51). Here, sliding block B1 is held against stationary block B2 by a retaining frame, all of Plexiglass. PTFE tubes, 1/4-in.o.d., are push-fitted flush into three apertures in. apart in B2 and connect, e.g., to sample, calibrant, and zero (S, C, and Z). Similarly, a PTFE tube connects the B1 aperture to the DS air inlet port. Actuators A and B (type SDR-05, Cippard Instruments, Cincinnati, OH; 1- and 'l2-in. displacement, respectively) electropneumatically operate the slider via solenoid valves Vl-V3 (Skinner valve, type MBD 002). A is threaded into B1 while B is free-floating. Common ports of V1 and V2 are connected to the ports of A. Air at 7 psi is supplied either to the top port (Vl/V2 off) or to the bottom port ( V l / V 2 on) of A, resulting in the retraction or extension of the A piston. With the B piston retracted (V3 off), this
results in the respective selection of the S and the Z port. V3 provides 17 psi air to actuator B. Thus when Vl-V3 are all on, the A piston is extended but the B piston overrides it, pushing it back halfway, and port C is selected. The valves are programmed with a microprocessor-driven timer (Chrontrol CD-4s; Lindberg Enterprises, San Diego, CA); the slider moves in a subsecond time scale. The selected air stream is sampled through the DS at a rate of 1slpm (needle valve N, mass flow meter M) by a suction pump P. Liquid-Phase Analysis. The chemistry used for analysis is that reported earlier (49). Aqueous NH3/NH4+ reacts in pH 11phosphate buffer with o-phthaldialdehyde and sulfite to produce an intensely fluorescent compound, 1-sulfonatoisoindole (optimum A,, 365 nm; A,, 425 nm). Except for the adaptation for coupling to the DS and the modification of one reagent, the liquid-phase analytical system resembles that described in ref 49. Two separate DS-coupling configurations were utilized in this work and are shown in Figure 4 as insets a and b. A multichannel peristaltic pump P (Model Minipuls 2; Gilson Medical Electronics, Middleton, WI) pumps water (W) at 86 kL/min through a microcolumn C (3 X 50 mm packed with H+-form monosize cation-exchange resin, Bio-Rex MSZ 50; Bio-Rad Laboratories, Richmond, CA) to remove any residual ammonia. Preliminary experiments established that water is sufficiently effective as absorber for measuring ambient levels of ammonia; dilute acids present no significant benefits. The stream W flows into the diffusion scrubber system at A, either in configuration a or b (vide infra) and returns to the analytical system at A'. The first reagent 0 (0-phthaldialdehyde,P 1378; Sigma Chemical Co., St. Louis, MO; 10 mM in 1:3 v/v methanol/water) is added to stream W at 55 pL/min and flows into mixing conduit M1 (0.3 X 250 mm). Reagent B (75.6 mg of Na2S03dissolved in 200 mL of 1.5 mM HCHO in 0.05 M pH 11.0 sodium phosphate) is then added at 55 kL/min, and the stream flows through a heated stainless steel coil R (85 "C, 0.5 X 500 mm) thermostated by heating block H. Bubbles are removed by a short segment of porous tubing T (0.6 x 10 mm, Accurel PP; ENKA AG, Wuppertal, FRG) (52). If NH3 levels of 1 5 ppbv are measured, R can be replaced with an unheated 0.3 X 1000 mm tube. The fluorescence detector F (49) is equipped with a miniature black light tube (General Electric F4T5) powdered by a IS0 28-V power supply (Isodyne Inc., Spring Valley, NY) with an interference-type excitation Environ. Sci. Technol., Vol. 23, No. 12, 1989
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C
0load
E S !
inject
tc-------l 30 m i n Flgure 5. (a) 5 ppbv NH,, 2-min sample, 8-min zero, DS in-line; (b) 5 ppbv NH,, 8min load, 2-min inject, DS in-loop, X - Y scales identical to a; (c) DS in-loop, sample gas 4 min, zero gas 6 min, substitute a sample with calibrant (12 ppbv NH,) every hour: load 3.5 min, inject 1.5 min. (d) timing diagram for c.
filter centered at 360 nm and a long-pass emission filter with 50% cutoff a t 420 nm. Except as otherwise noted, all tubes are PTFE. Reagents are prepared fresh every 3 days. All experiments are conducted with the instrument in a controlled-temperature environment (22 f 1 “C). Configuration a, hereinafter called the in-line configuration, places the DS in-line with the carrier liquid flowing through it at all times, as in all of our previous DS-based instrumentation (see, e.g., ref 44; details of preferred operational protocols are described therein). Configuration b, hereinafter called the in-loop configuration, puts the DS within the loop of a six-port electromechanically actuated rotary valve V (type HVXL 6-6; Hamilton Co., Reno, NV). In the sample/load mode, the carrier bypasses DS while gas is sampled through it. Evaporation through the membrane pores in DS can cause bubble formation within it during such sampling and this is prevented by connecting to it a reservoir of NH,-free water through V. Any evaporation results in aspiration of this liquid. After the chosen sampling period, V is switched to the inject mode long enough to transfer the contents of the DS and the connecting tubes into the analytical system. An injection period of 11 min is needed; 1.5 or 2 min was used. The in-loop configuration allows sample preconcentration; the limits of detection (LOD) can be lowered with increased sampling time (40, 53). In our instrument, the gas and liquid valves (Vl-V3, V) are synchronized; an injection represents a specific gas sample.
Results and Discussion Performance Characteristics. Figure 5 shows examples of system performance in various modes of operation. Figure 5a shows the instrument output in the in-line configuration with the instrument sampling 2 min of a calibrant ( C , 5 ppbv NH,) and then 8 min of zero gas (Z) in a continuous cycle, repeated every 10 min. A 4-min zero period is actually ample for the return of the signal to the base line; a longer period was used here for visual clarity. The lag time (defined to be the interval between a step change at the gas sampling end and the onset of output signal change) of the instrument in this mode is 2.25 min, 10-90% signal rise time (step change, Z to C) is 0.75 min, and 90-10% signal fall time (step change, C to Z) is 1.25 1470
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min. Unlike our general experience with HNO,(g), significant adsorption of NH, to sampling line components (“stickiness”) is not indicated. The detector attenuation setting is lo4 (available range, 1-50 000; 1 being the most sensitive); the absence of base-line noise suggests a subppbv level LOD. The signal height was found to change in the ratio 1:1.42:1.60:1.63 for sampling periods of 1, 2, 4, and 5 min, respectively; a 2-min sampling period appears to be a reasonable optimum. Figure 5b shows the instrument output with the DS in the in-loop configuration, using sampling/hjedion periods of 8/2 min, with 5 ppbv NH, as sample. The detector attenuation is the same as in Figure 5a; the in-loop configuration, given sufficient sampling time, is capable of much higher sensitivity. The relative response (signal height) was 1:1.37:1.63:2.15:2.78 for sampling times of 4, 5, 6, 8, and 10 min, respectively; these data represent a linear correlation coefficient of 0.999 between the sampling period and the signal response. A nonzero intercept (the extrapolated response for zero sampling time is negative) is a consequence of the finite injection period. The return of the signal to an apparent base line does not mean that the base line corresponds to zero ammonia level-the calibration gas is continuously sampled. The intercept approaches zero as the injection period is decreased; past a certain point, however, carryover between successive samples increases due to incomplete flushing of the loop containing the DS. An injection period of
