Determination of ambient aerosol sulfur using a continuous flame photometric detection system. I. Sampling system for aerosol sulfate and sulfuric acid

nnnodpivtic Encir-r Vol. 14, pp. 121427. 6 Paganon Prcw Ltd. 1980. Printed in Chat

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DETERMINATION OF AMBIENT AEROSOL SULFUR USING A CONTINUOUS FLAME PHOTOMETRIC DETECTION SYSTEM. I. SAMPLING SYSTEM FOR AEROSOL SULFATE AND SULFURIC ACID* R. L.

TANNER, T. D’CYITAVIO,

R. GARBERand L. NEWMAN

Environmental Chamistry Division, Department of Energy and Environment, Brookhaven Natknutl Laboratory, Upton, NY i1973, U.S.A. (First received 25 April 1979 and ~~~1 form 19 Juiy 1979) Ahatraet - A system for measurement of ambient levels of aerosol sulfur using a flame photometric sulfur analyxer (FPD) with a heated denuder for SO2 removal has been developed. Cyclic addition of ammonia permits discrimination between sulfuric acid and other sulfate aerosols. The effects of changes in ambient water vapor concentration and barometric pressure on the FPD sulfur response have heen quantitatively evaluated, and it is shown tbat other environmental effects on sulfur response may he eliminated by frequent and proper xeroiug. Calibration plots and ambient data are presented which demonstrate the specificity and sensitivity of the system to x 1.0 ppb aerosol sulfur.

1NTaODUcYlON

Since its invention by Draegerwerk and Drager (1962) and application to gas chro~~~phy by Brody and Chancy (i966), the flame photometric detector (FPD) has been widely appiii for the determination of trace quantities of sulfur-containing species in air. The device, based on measuremen t of the band emission (centered near 400 nm) of excited S, molecules formed from sulfur-containing compounds in a hydrogen-rich flarn& has been applied succesr&lly to the gas chroma&graphic (Cc) determination of traces of sulfur compounds (principally SO2, H,S, CHrSH and CH$CH,) in samples of atmospheric air using the FPD as a GC detector (Stevens et d, 1971, Bruner er al., 1972). The FPD measures flame emission usually at the 394nm band head of the Sy emission spectrum in a burner which partially isolates flame background from the photomultiplier (PM) tube. The sulfur response should theoretically be proportional to the square of sulfur concentration ; that is m = 2 in 1, = K[S]?

(11

Equation (I) where 1, = PM tube response to sulfur, [S] = concentration of sulfur atoms, and m, K = constants. Near quadratic response is observed only under optimum conditions; in fact, m is a function of Ol/H, ratio and total flow conditions in the burner

* This research was nerformed under the ausdces of the United States Department of Energy under Contract No. EY76-C-024016. BY accentance of this article. the Dubhsher and/or recipient acknokledges the U.S. Government’s right to retain a nonexclusive, royalty-free kense in and to any copyright covering this paper.

(Eckhardt et al., 1975, Sugiyama et al., 1973b), the burner block temperature and the chemical structure of the s~fur~n~ining compound (Stevens et of., 1971, Burnett et al., 1978). Details of the game kinetics of Sy formation have been reported by Sugiyama et al. (1973a, 1973b). Application of the FPD to monitoring of the sum of gaseous sulfur compoundsprincipally So2 under most ambient conditions - was suggested by Stevens et uf. (1969) and is now widespread (Farwell and Rasmussen 1976). It is sensitive, selective for sulfur, relatively simpie and involves no wet chemical ap pa&us as do the competitive coulometric (deVeer and Brouwer, 19691, and West-Gaeke (West and Gaeke, 1955) techniques. Scrubbers and filters have been used to prevent interference from H$ and particulate sulfur species, respectively (Baumgartner et of., 1975). Recent advances in flow controi have resulted in commercially-available, FPD-based sulfur monitors sensitive enough (limit of detection, LOD * 2 ppb) for most ambient applications. The FPD was first applied to particulate aerosol sulfur analysis by Crider (l%S) and Crider et af. (1%9) although other workers (DagnaIl et al., 1967) had observed that the optimum flame temperature for Si formation was below the decomposition temperature of many sulfates. Crider et al. (1969) also demonstrated that gaseous SO1 could be removed in an “aerosolpassing gas absorber” by selective diffusion to the coated walls of a tube under conditions in which sulfur-containing aerosol particles (due to their much smaller diffusion coefficient) would pass on to the detector. The FPD response for H2S0, and ammonium sulfate aerosols was similar to gaseous S compounds with exponential factor, m = 1.5-1.9. The performance and theoretical basis for removal by

121

122

R. L. TANNER,T. D’OTTAVIO, R. GARBERand L. NEWMAN

diffusion denuders of gaseous SO2 from aerosol sulfurcontaining gas streams has been reported in detail by Fish and Durham (1971) and Durham et al. (1978). The direct, real-time measurement of aerosol sulfur FPDbased sulfur analyzer with using an diffusion-denuder removal of SO;: has been reported by Cobourn et al. (1978) and by Huntzicker et al. (1978), with ambient particulate sulfur data reported by the former group. An alternate approach to S02aerosol sulfur differentiation based on a pulsed electrostatic precipitator has been introduced by Kittelson er al. (1978). Huntzicker et a/. (1975) and Coboum et al. (1978) introduced the concept of chemical speciation of aerosol sulfuric acid and other aerosol sulfur by thermal pre-treatment of the aerosol at temperatures at which aerosol H2S0, is evaporated and lost in the denuder, but other aerosol sulfates commonly found in ambient air are not volatilized. Huntzicker et al. (1978) suggested the addition of ammonia to avoid the problem of reduced H,SO* response in the FPD under normal operating conditions, which is probably caused by the fact that the heated aluminum burner block behaves as a partial denuder for vaporized sulfuric acid aerosol. Several problem areas in the use of FPD-based

systems for aerosol sulfur measurement at ambient concentrations have been inadequately addressed in the reported systems. These include in decreasing order of significance: (a) zero response current dependence on partial prcasure of Hz0 in the air sampled ; (b) zero response and sensitivity reductions due to decreasing sample air pressure; (c) electronic aberrations in commercial lincarizer circuitry at concentrations c 20 ppb, Ieading to non-linear calibration plots; (d) FPD response to ammonia and CO,; (e) reduced sensitivity of the FPD to aerosol H$O*. We report in this paper the developm~t of a system for measurement of ambient levels of aerosol sulfur (principally sulfate) using a flame photometric sulfur

analyzer with a heated denuder for S02. Cyciic addition of ammonia permits discrimination between sulfuric acid and other sulfate aerosols as well as eliminating the reduced instrumental sensitivity to H,SO, aerosol. The effects of changes in ambient water vapor concentration and barometric pressure on the observed sulfur response of the FPD system are evaluated. It is shown that other environmental effects on sulfur response may be eliminated by careful, frequent zeroing of the system during ambient operations. Calibration plots and ambient data are presented which demonstmte the specificity and sensitivity of the system to < 1.0 ppb aerosol sulfur ( < 4 @g mS3 sulfate) if proper adjustment is made for electronic peculiarities of the commercial linearized amplification system. Detailed description of the instrumental behavior and its dependence on environmental variables will be reported in a separate paper (D’Ottavio et al., 19gO). Suggested modifications of the reported system for use in field operations with minimal operator attendance are described. gXPERlMENTAL The system for calibration of the MeIoy Model 285 Sulfur Analyzer (Mcloy Laboratories, Inc., Sprbgfidd, VA, hereaAetnSandtoasth!FPDffotaerosolammogiltms~~Purd ammonium bisulfate as a function of water vapor concentration (PFzo) in the sample stream is shown in Fig. 1. Compressed asr is scrubbed kee of water vapor, trace basic gases, and gaseous sulfur compounds by successive silica gel, phosphorous acid and charcoaltrapsand dividedinto three streams.One streamis usedto getmate aerosol(NH&SO, or NH,HSO, using a mod&l Model 3076 Constant Output Atomizer (TSI, St. Paul, MN). A second stream passes over a heated water reservoir and condenser to produce a stream of air at 100%RH at the bath temperature. This latter stream is diluted with dry air to praduce air of the desired Pax0 following mixing with the aerosol in a mixing chamber; the final humidity and temperature is mo~tor~ using a wet bulb/dry butb apparatus. The aerosol stream is passed through an aerosol neutralizer (Model 3077, TSI, St. Paul,

i-l PUMP

AIRSCRUBBER

3

A = SILICA

2 WK ga

AERO SOL NEUTRA LIZER

GEL

2

8 = PnOSPnOf?OUS ACID

I

IFI kY MELSY

C f CHARCOAL ,

8

SULFUR ANALYZER

FILTER

i

Fig. 1. Apparatus for aerosol sulfate generation at variable humidity and calibration of sulfur analyzer.

Determination of ambient aerosol sulfur MN) and split between the FPD and a 47 tmn filter assembly. The output concentration from the sulfur analyxer is then compared with the results from analysis of the quartz or Fluoropore (Millipore Corp.) filters by ion chromatography. The diiusion denuder tubewaspmpared by evaporation of a liquid coating of sodium carbonate solution from the walls of a 3 mm ID dia. x IOcm tube of HF-etched borosilicate glass. Sodium carbonate was chosen due to its high hygroscopic transition humidity (-WA) and its stability; Na,CO,-coated tubes are not subject to the reintraimnent (flaking) problems exhibited by PbO,-coated metal or MgQ coated glass tubes. For some experiments a heated copper tube was also used as an H$O,denuder. The denuder was heated in a specially constructed oil bath or, for some experiments, by a heating tape arrangement. The efhciency of the dcnuda was cakuiated from the Gormky-Kennedy equation (Gormky and Kennedy, 1949) for the flow conditions used, and conflrmd by introducing known concentrations of SO1 using a Monitor Model 8508 Calibrator (Monitor Labs., Inc., San Diego, CA) and measuring the SOa concentrations before and after the denuder tube with the FPD. The optimum temperature of the den&r tube - that temperature at which aerosol sulfuric acid is volatilized and removed in the denuder but NH4HSO. and (NH,),SO, aerosols are oot removed - was determined experimentally with the respective pure laboratory aerosols generated at about so”/, RH using the apparatus in Fig. 1 but without filter collection. The xero response function with variable relative humidity (more properly, with changiog partial pressure of water vapor, Pazo) was determined by production of sulfur-free air using the apparatus in Fig. 1 at varying Pa* from 3-24 Torr. The sulfur respoose curves at 3 different Pa* were also determined from data using the same apparatus but with varying SO, concentrations iotroduccd just upstream from the FPD burner block. The xero and sulfur response of the FPD to changes in burner block pressure was measured by artificially reduciog the pmdaure in the air inlet line using an in-line valve, and by placing the entire FPD in a pressure chamber and reducing the pressure therein (while maintaining the calibrator source of xcro and SO+!ontaining air at ambient barometric pressure) to simulate the effects of altitude excursions to 2000 m. Details of these procedures and descriptions of experiments to establish the nature of the electronic aberrations of the lioearixer circuitry used in the Meloy Model 285 FPD are given by D’Ottavio et al. (1980).

123

The system used for ambient measurement is shown in Fig. 2. Air was sampled from atop the laboratory building (about 35 m above mean sea level) at Brookhaven National Laboratory through a Teflon sampling tube by a large pump. A small portion of this flow ( - 250 cm3 min- ’ from a total flow of 10 I mitt - i) was then pulled through the heated denuder, which was kept at 130-135”C, and then into the FPD. Fractional penetrations from inlet to burner block were calculated to be 99.8% for 0.5~ particles and 92% for 0.05ym particles, respectively. During alternate 9 minperiods a few cm3 min-i flow of ammonia was added to the FPD air inlet stream using a timer and solenoid. FPD response to the ~18ppm NH, added was -3 x lo-” A, corresponding to an equivalent sulfur concentration of 0.7 ppb. The purpose of the zeroing system (Fig. 2b) was to remove all gaseous and aerosol sulfur compounds but have no effect on any other parameters affecting flame response. The xeroing system consistul of a particulate filter to remove aerosol sulfur compounds, a valve to exactly match the burner inlet pressure to that used in the sampling step, and the heated denuder. No change in Pa,o is induced between zero and sample air streams hence compensation is provided for the response-reducing effect of water vapor. By correcting for the ammonia response and dilution by the ammonia stream, all known variables signiticantly alkcting the net sulfur response were accounted for, excepting the effect of gaseous sulfur compounds not removed by the denuder. As shown by D’Ottavio er ol. (1980) the calibration curves for sulfur compounds tested (aerosol sulfates or gaseous SOr), although identical to each other, are not linear at concentratioos below about 2Oppb unless the instrument zero recorded corresponds to electronic amplifier zero, i.e., corresponds to mro current input to the logarithmic amp lifkr. The etIects of these environmen tally induced aberrations may be eliminated by direct measnranen tofPMtube currents for zero and sulfttr-containing air and reference to a calibration curve. If, as for the ambient data reported below, a Meloy Model 285 is used, conversion of zero and sulfur signal readings in ppb units back to current output readings and subsequent use of net current vs sulfur concentration calibration curves for concentration determination has been found to remove most of the elfects de&bed above.

RESULTS AND DISCUSSION Denuder temperature optimization The results of the experimental determination of the optimum denudes temperature are shown in Fig. 3. At

(01

AMBIENT

(b)

SAMPLING

ZEROING

SYSTEM

SYSTEM

Fig. 2. Continuous FPD analyxer for ambient measurement of aerosol sulfur and sulfuric acid; (a) ambient sampling system; (b) zeroing system.

130 & 5°C about 95% of the aerosol NH,HSO, and (NH&SO4 penetrates the denuder and reaches the sulfur analyzer but >95% of the H,S04 aerosol is removed in the denuder. Aerosol NH,HSO* and (NH,,)#O, behave identically in the reported system, but meticulous care must be taken to remove ammonia and other basic gases in the air stream or this H,SO,speciated response will not be observed. The So”/, removal temperatures indicated in this work ( - 75°C for HISOb, - 150°C for NH4HS04, (NH&SO,) agree with those reported by Kit&on et al. (1978) and Huntzicker et al. (1978). The values reported by Cobourn et a/. (1978) are substantially higher due to temperature inhomogeneities in the latters’ system which emphasizes that sufficient time must be. allowed in the heated zone for thermal equilibrium in the gas phase to be achieved. It is further recommended that

i24

R. L. TANNER,T. D’Orravto,

loo I20 140 TEMPERA;TURE. t

160

180 200

Fig. 3. Aerosol sulcate mspoaac as a function of heated dutuder temperature.

the denuder itself be heated to avoid possible recondensation of aerosol HISOI prior to diffusional removal. Aerosol calibrrrtion data The FPD was calibrated for aerosol sulfate using the apparatus in Fig 1 by parallel collection of filter sample8 from the aerosol generator at various Pm0 as described above. The resultant calibration plot (corrected as described below) of FPD sulfate vs liltcrcollected sulfate analyzed by ion chro~to~phy is shown in Fig. 4. This plot shows that aerosoi NH,HSO. and (NH*)$O* may be measured quantitatively in the 20-100 ppb range using the FPD and a denuder at 130 f 5°C. Indistinguishably different

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Fig. 4. Calibration of Meioy Mode1285 FPD for ~monium sulfate and ~onium bisulfate aerosols.

R. GARBER and L. NEWMAN

response curves are obtained for the two sulfate aerosols with near-unity slope and very high correlation with the filter results. Since the FPD was originally calibrated for SOI, the results indicate that FPD response to SO1, NH*HSO., and (NH&SO4 are not significantly different. Further segregation of the calibration data for (NH&SO, in humidity ranges above and below WA RH did not si~ifi~ntly change the slope or intercept, indicating that quantitative response was obtained to both liquid droplet and solid aerosols of ammonium sulfate. The original calibration data, when plotted, showed more scatter (especially at lower concentrations) and a significantly negative y-intercept (r = 0.96, y-intercept = - 5.9) compared with the data in Fig. 4. During the experiments which delineated the near-zero electronic aberrations of the FPD log amplifier it became clear that the original FPD data had been taken with instrument zero f; electronic zero. Use of positive zero offsets - the normal experimental approach for avoiding o&scale-negative readings, and the approach used in these experiments - leads to low values for the FPD “linearized” output because of the inherently nonlinear nature of the sulfur concentration vs current relationship. Correction of the data involved conversion of the “linearized” ppb output for zero and aerosol sulfate back to raw current values, determination of corresponding sulfate values from the SOz current-vs-concentration curve, and plotting of these data vs the corresponding filter-collected sulfate values. It is this corrected plot that is shown in Fig 4. No calibration data are shown for aerosol sulfuric acid since a reduced response was observed which was a function of bumer block t~~ature. The 20% reduction in sulfur response (ppb) observed for 60ppb H2SOI aerosol was lower than the &SO% decrease reported (Coboum et al., 19%; Huntzicker er al., 1978) for burner block temperature increase from 75°C to the manufacturer’s recommended value of 160°C. However, there is a strong indication that the cause of this reduction is the volatilization and partial diffusion loss of sulfuric acid aerosol at burner block temperature high enough for rapid volatilization to occur. Use of a high-efficiency denuder at T 2 130°C in fact results in near zero response to HzSO, aerosol as shown in Fig. 3. Use of low burner block temperatures eliminates this variable response problem but introduces commensurate water condensation problems which should be avoided. It was concluded from the previous paragraph that a superior approach to aerosol sulfate measurement with H,SO,, speciation was to add excess ammonia prior to a heated denuder tube when total aerosol sulfur was to be measured. Sufficient time was allowed to convert all of the H,SO, to its ammonium salts as indicated by a response recovery to > the HrSO, value at low denuder temperatures. During the alternate cycle when no ammonia was added, H&?, aerosol was volatilized and removed in the heated denuder, hence no sulfuric acid ever entered the

125

Dctcrmiaation of ambient aerosol sulfur

burner block. Response reduction therefrom was thus eliminated even though sulfuric acid levels could be determined from the difference between NH,-added and NHs-deleted signals. &fleet

Of barometric

pressure

It was established in this and parallel investigations that the response of the FPD is a function of absolute burner block pressure and is thus sensitive to ambient barometric pressure under sampling conditions. The total flow in the FPD is controlled by a thermostatted critical orifice i~~ia~ly downstream from the burner btock. The hydrogen flow is adjustable independently, hence the principal e&et of reducing the burner block pressure is to reduce the mass flow of sulfur-containing air reaching the Aame region. It is thus not surprising that the sulfur response should decrease with decreasing burner pressure since this response depends, in part, on the mass flow of sulfur reaching the flame. A typical FPD response curve for various SO2 concentrations and zero air as a function of absolute pressure (or altitude) is shown in Fig. S(a) in which “linearized” response fppb) is plotted. Both zero and sulfur response decreases with pressure but the near-

zero electronic aberrations of the iog amplifier, which ensue due to the fact that, in these measurements, instrument zero = electronic zero only for P = 760 Torr. are also evident. This compk&y obscures the ABSOLUTE

PRESSURE.

TDW

response-absolute pressure relationship and may be eliminated by plotting the absolute current response of the FPD on the ordinate instead, as shown in Fig. S(b). From the latter plot it can be seen that changes in zero response within the normal barometric pressure range are quite significant for measurements in the < 10 ppb range although the “altitude response” is well defined. The measurement system described herein corrects for the reduction in zero response with absolute pressure by zeroing (and spanning) the instrument at the same used when sampling. Changes in pressure r~~~~n~t~tion curvea are insi~~nt for ground level sampling and were ignored for the data reported below. Such changes are very important in designing an aerosol sulfur measurement device for an aircraft platform, (Garber et af., 1979), or for sampling at altitudes where the absolute pressure varies by more than a few tens of torr from that at which the instrument was calibrated. E@ect of water vapor concentration

Although some indication of a relative humidity effect on FPD zro and sulfur response had informally surfaced, we report here the first q~ti~ti~ demonstration of this effect. The zero response change (plotted in absolute current units) for an FPD with two different burner blocks operating under different flow conditions, is plotted in Fig. 6 as a function of partial pressure of H,O (PHIo) in the incoming air stream. A linear reduction in response with PHa is demonstrated over nearly the entire range of ambient values of Pwzm It may be hypothesized that Hz0 vapor quenches background flame luminescence by 1st order re-

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Fig 5. Response of the FPD to zero air and SO2 as a function of burner block pressure (a) linearized FPD response in ppb; (b) absolute FPD current response in A( x lo-‘); lower abscissa is in equivalent altitude units, ft( x I03); B.P. Range = monthly average range of observed barometric pressure (units = torr) at sampling site.

PARTIAL

PRESSURE

t-1~0.

Torr

Fig. 6. Zero response of the FPD as a function of partial pressure of water (Pa,*) in the inlet air stream; (a) Tefloncoated burner block; (b) standard aluminum burner block.

126

R. L. TANNER, f.

D’OTTAVIO,

actions, analogous to chemiluminescence quenching ofNO: reported by Mathews et al. (1977), but there is no direct experimental ~on~rmation. The water vapor concentration affects principally the zero response - the absolute current response vs concentration calibration curves remain marginally affected. Calibration curves for SOZ at water vapor partial pressures of 3.5 + 1.0 and 12.5 4 1.0 Torr (15% and 51% RH at 25°C) were su~~rn~b~e. The calibration curve at 23.5 f 1.0 Torr (97% RH at 25°C) was negatively displaced by about 5%. The small effect on the sulfur response suggests ST chemiiuminescence is unaffected by H,O, but the background iuminescence is reduced. The data reported below are corrected for the e&et of changing PHLo on the zero current using relative humidity and temperature data from the BNL Meteorological Tower (located 0.5 km distant). Ambient measurements

ofaerasoi sulji~.

HrSO,

Ambient m~urements of aerosol sulfur and WIfuric acid were made using the system in Fig. 2 as described above. Data for the period 19 July-24 July, 1978 are shown in Fig. 7 ; data for filter-collected sulfate analyzed by MTB coiorimetry (Adamaski and Viilard, 1975) are shown for comparison. Real-time aerosol sulfur data have error limits of about 40.5 to 2.0 ppb (compared to + IQ?? for the filter sulfate data) depending on the temporal proximity of the measure ment to that at which the system was zeroed. Valid zeroes are claimed where the measurement was made within two hours of instrument zeroing. An explanation of the corrections applied to recorder output (ppb sulfur) to obtain the data plotted in Fig. 7 is now given: (a) Observed reading, C, (ppb), and nearest zero reading, C, (ppb), are converted to their respective raw current readings, Ii and I,. (b) Zero drift due to barometric pressure change I,, and Pm0 change, iW, are determined. (c) The ammonia FPD response, Al,,, = I (zero air + NH,) - I, is determined.

201

0 = REAL-TIME SULFATE, VALID ZERO l

B 516 _

c

= REAL-TIME

SULFATE, Marginal

ZERO

-

0 = FILTER PACK SULFATE

Lz

2 212 18 s: iz4 2 0 19

20

21 DATE

22 JULY, 1978

23

Fig. 7. Aerosol sulfur measurement in ambient air by the continuous FPD analyzer; sampling at Brookhaven National Laboratory site, 19-24 July, 1978; 0 = real-time sulfate, valid zero; # = reaMme sulfate, marginal zero; 0 = fifter pack sulfate.

R. GARBER and L. NEWMAN (d) The net current due to aerosol sulfur, I,, is calculated from Equation (2) i, = (Ii - Al,,,)

- (I, - I, - I,,).

(2)

(e) From the calibration curve for S02, the concentration of aerosol sulfur is determined directly from the value of I,. (f) Aerosol sulfuric acid is determined from the difference between I, with NHs added and NH, absent as described in (a) through (e); AJmt, = 0 when no NH, is added, AINn, = 0.7 ppb for addition of N 18 ppm NH,. The agreement between continuous FPD data and filter sulfate data is considered to be moderately good despite a few notable exceptions. Principal reasons for negative errors in continuous FPD results are suspected to be: low response in the FPD for refractory aerosol sulfate compounds, e.g., Na,SO, and CaSO, ; loss of coarse particle sulfate in the FPD sample inlet ; and lowered response for particles larger than about 1 pm dia. Positive errors in the filter-coIlected aerosol sulfate measurements due to artifact sulfate formation are considered unlikely for the treated quartz filter medium used. Aerosol sulfuric acid concentrations were calculated from the FPD data as described above. The average of the 32 measured values was 0.5 1 ppb & 20% ( - 2.0 pg m-‘1. The n~rown~s in the spread of corrected H2S04 values during a period in which aerosol sulfur levels varied by at least an order of magnitude suggests that these data should be interpreted as marginally different from zero, within experimental error limits, since the aerosol sulfur data are accurate to no more than f0.5 ppb using this FDP system. Future developments A modified, continuous FPD aerosol sulfur measurement system is being assembled based on the results of the work reported in this paper. This modified system will use an FPD with the advanced, commercial, Row controtied, critical orifice-based burner system but with direct measurement of the output current ; post-measurement processing of current data will yield the desired concentration data. Accurate information on the PxZo for individual measurements will be obtained from a continuous dew point meter operating alongside the FDP. Barometric pressure changes and other zero drift effects will be compensated for by automatic zeroing every 1 or 2 h and spanning at ambient-matched inlet pressure at least daily. This system is expected to have a limit of detection of _ 1.0 ppb for ambient measurements of aerosol sulfur, and to be able to detect -0.3 ppb H$O., by difference when the aerosol sulfur level is _ 5 ppb. This difference in detection limits is apparent, not real, and results from the 1.5-2 power response function of S compounds in the FPD which causes the sensitivity (smallest incremental concentration change detectable) ofthe FPD to increase with concentration. Further developments may be forthcoming from

Dmrmination of ambient aerosol sulfur

studies aimed at increasing the sensitivity of the FPD to low level sulfur aerosols. One approach, previously reported by Crider and Slater (1969) and by Zehner and Simon&is (1976) is to add sulfur species to the air or hydrogen stream. Addition of 20-N ppb SF6 to the H, stream apparently leads to significant increase in the signal-to-noise ratio. Cyclic (-0.01 Hz) fluctuations in burner block and PM tube temperature appear to be principa1 causes of noise in the FPD system. Improved temperature control combined with SF, addition to the Hz stream promise significant improvement in the limit of detection, possibly to 0.1-0.2 ppb of aerosol sulfur routinely in ambient air samples.

127

continuous flame photometric detection system (II). Tbe measurement of low level sulfur concentrations under varying atmospheric conditions. (Submitted to Atmospheric Environment.) Draegerwerk H. and Drager W. (1962) German Patent No. l133918, July 26. Durham 3. L., Wilson W. E. and Bailey E. B. (1978) Appl&ion of an SO#nudtr for continuous measurement of sulfur in submicromctric aerosols. At~s$eric Environment 12,883-886. Eckbardt 1. G., Denton M. B. and Moyers J. L. (1975) Sulfur FPD flow optimization and msponse normalization with a variable exponential function device. 3. chromtagr. Sci. 13, 133-138. Farwell S. 0, and Rasmussen R. A. (1976) Limitations of the FPD and ECD in atmospheric analysis: a review. J. chrotnutogr. Sci. 14,224-234. Fish B. R. and Durham J. L, (1971) Diffusion coefficient of sulfur dioxide in air. Enuir, Left. 2, 13-21. Acknowledgemnt -The authors acknowledge support of this Garber R. W., D’Ottavio T., Doering R. F., Tanner T. 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