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Application of a ring oven technique for determination of atmospheric ammonia gas concentrations
W-6981’79!l?UI-1713
Armusphna Envirmmmt Vol. 13. pp 1713-1714. 0 Pcrgrmon Press Ltd. 1979 Printed in Circa Britm
W2W.I
TECHNICAL NOTE APPLICATION OF A RING OVEN TECHNIQUE FOR DETERMINATION OF ATMOSPHERIC AMMO~A GAS CONCENTRATIONS (First receiwd 21 February
1979 und
infinalfbrm
15 Muy 1979)
Abstract - A technique based on standard ring overt procedures has been developed for the determination of trace quantities of ammonia in the atmosphere. Calibration over the range l-50 kg mm3 STP of ammonia indiites that ammonia is determined q~ti~~vely to an accuracy of + 50”/’ - 3ffk. The detection limit for routine use isO. fig of ammonia so that a sample ftow rate of 83.3 cm3 s-’ ailows 0.05 pgrn- 3 to be detected in 200 min sampling time.
INTRQDUCIION To
date there have been few experimentaI ixwstigations of the spatial and temporal variability of ammonia, although its significance as a trace gas involved in important chemical processes in the atmosphere is undisputed. Lau and Charlson (1977) provide a convenient summary of the work that has been carried out to date and briefly describe our currutt knowledge of the sources and sinks of atmospheric ammonia. They point out that past measurements are sparse bec%use of the difBculty of measuring low ammonia concentrations directly, and that where wet chemkal techniques have been used the uncertainty and detection limits have not been s~~~~~~~~~~~~to~~~~t quantify both the accumcy &d technique.
d&&n
limit of the
EXPEMHENTAL The method is bad on that described by Shendrikar and Lodge (1975) and incorporates the improvement by Cattel and Du Cros (1976). It consists of passing a known volume of sample air through a filter impregnated with oxalic acid, then using the ring oven technique to determine the amount of ammonia collected on the filter. The important features of the present work are the use of an inert prefdter to exclude ammonia-containing aerosols from the determination and the nse of one half the impregnated filter for sample collection and the other half for an in situ blank determination. A Teflon (PTFE) prefilter was used to ensure removal of any ammonium aerosols. This filter was mounted in an ~~~ holder and was connected by a PTFE pipe to a sample filter holder. The sample filter was held between stainless steel masking plates in an aluminium holder. The prefilter used was a 90 mm Fluoropore of 0.2 p nominal pore sixe. This filter was chosen because of the >99.99% collection efficiency reported by Liu and Lee (1976) for submicrometre particles (co&mad by us using a Pollak counter) and because the PTFE filter material would minimixe adsorption of ammonia gas by the pretilter. The possibility of obtaining erratic results as the result of adsorption/desorption of ammonia in sampling lines or in the apparatus itself should not be ignored in any method of ammonia collection. Ammonia is a small, polar, hydrogen-bonding molecule which, just like water vapour, readily bonds to a variety of surfa~. In addition, ammonia will readily exchange with the ubiquitous adsorbed water which occurs on almost any surface exposed to the ambient atmosphere. In the present case some adsorption at h&h ammonia concentrations and
desorption at very low concentrations occurred in the orefilter assembly desnite the use of a PTFE filter. This brobiem was o&corn; by winding a resistance heating tape around the miter assembly and maintaining it at - 315 K (ambient + 25 K) during sampling. Whik this temperature is high enough to reduce adsorption to insignificant levels, the vapour pressure data of Scott and Cattei (1979) indicate that, provided the measurement is restricted to ambient concentrations above about 0.03 M mm3, it is not high enough to add dgniftcantly to the measured ammonia gasconcentration by volntiiization of ammonium sulphate aerosol on the p&liter. For most tropospheric situations we have found ammonium sulphate to be the only ammonium salt important in this mgatvl. However, in polluted urban air significant amounts of more volatiIe ammonium compotmds could be collected by the prefiiter. In this case the apparatus can be used without heating the prefllter if it is first flushed with amm&a-free air whii hot, then allowed to cool and equilibrate with the gaseous ammonia in the ambient air prior to making a measurement. The sample filters consisted of carefully washed Whatman 542 papers 25 mm in diameter which had been soaked in a 5% ethanolic solution of oxalic acid and vacuum dried. Since the filters readily absorb atmospheric ammonia upon standing it is ncc*lsary to keep prepared filters in a stoppered flask or desimtor, as suggested by Sltendrikar and Lodge (1975). Even so it is impossible to completely exclude some contamination during assembly and disassembly. Consequently the sample titer was held between stainiess steel masking plates having a “D” shaped (semicircular) cut-out, so as to allow only a semicircular area of the filter to be exposed to tbe sample air stream. By washing out the filter from the central point on the straight e+e of the D shape, the sample plus contamination is washed to one side of the ring, the contamination alone to tbe other side. It is the.n a simple matter to read both sides of the ring and subtract the blank value from the sample plus blank. Duling this work the blank values never exceeded 35% of the sample plus blank values and most often fell in the range O-200/ Each half of the ring was read individually by five members of the laboratory staff and the quoted values are the average of the five readings.
CALtBRATtON The accuracy with which ammonia could be determined was cbecktd by performing measurements on 48 gas samples which had predetermined ammonia gas concentrations. Concentrations between 1 and 5Opg mm3 STP were pro-
1713
1714
Technical Note
duced in a flow system built around a short length of Pyre? tube through which dry, ammonia-free nitrogen was passed Dilute aqueous ammonia solution made up in freshly distilkd water was introduced into the nitrogen stream through a side arm, the solution being delivered at a constant rate by a motor-driven microsyringe (B. Braun, Melsungen Type 87 1104). Several turns of nichrome wire wound around the Pyrex tube either side of the side arm and heated electrically to B 370 K ensured that the aqueous ammonia solution was completely volatilized into the nitrogen stream. The gas mixture then passed into a length of PTFE tubing in which mixing was completed and the gas temperature returned to ambient prior to delivery to the sampling apparatus. The nitrogen flow rate was controlled by a regulator plus needk valve and measured with Hastings mass flow transducers (Models EA LLlOK and H-1OK). The maximum flow through the sample filter was determined by the area exposed and the filter material. At 80 kPa di&rential pressure Whatman 542 filters typically exhibited a flow rate of 37.5 cm’ s-’ for a semicircular exposed area derived from a circle 10 mm in diameter. Raising the diameter to 15 mm increased the Bow to about 83.3 cm3 s-‘. In Fig. 1 the results of the calibration experiments are displayed as measumd ammonia concentration plotted again&y (l+oratofy:prnduced) amculummn. The ncertamtiesmtberlngovenmchmqueareevident in the spread of meaaurai values obtained at a given calculated value; however, on avenge it is clear that the measurements reproduce the c&u&ted values very well. This was con6rmed by performing a linear, least-squares regresalon analysis on the points as plotted in Fig. 1. The analvais vieldal a done of 0.98f0.04 (95X m) and int&pt-ofo.10 f 0:12whenailthepointswereused,and figures of 1.02 f 0.05 and -0.03 f 0.16 when the points near 1 pg me3 were excluded. The seccmd mgmsaion was cakulated in order to eliminate a poaibk systematic error in the
‘E
-
-I/
s
f
10
s P
*/
” / / ‘/ ’ / /
/
/ ./ / /
/
/
Colculared
i
NH,,
P9
m-’
Fig 1. Measured ammonia gas concentration plotted against calculated (laboratory-produced) concentration. The full line is a linear least-squares fit to all the data and has the form In y =O.lO + 0.98 In x, with sample correlation coeiEcient of0.99. The broken lines are approximate 95% confidence limits.
calculated values near 1 pg m- ’ : limitations in the calibration apparatus precluded production of known concentrations of < 1 pg m-” and caused increased uncertainty at about this level. In fact, neither set of regression coefficients is significantly dilkrent from the values of 1.Oand 0.0 expected if the collection of ammonia by the impregnated filter is quantitative. Approximate 95% confidence limits determined from the first regression represent +50, -3oo/, for a single measurement. We have considered the scatter to be contributed entirely by the measurement technique, since the uncertainties in the calculated ammonia are small. of the order of 3-4% only. Variations in sample flow rate, and of relative humidity in the range 40-95% (obtained by varying the aqueous ammonia concentration and delivery rate), did not affect the collection efBciency of the impregnated filters. Shendrikar and Lodge (1975) similarly found no humidity dependence in the range O-300/, relative humidity.
CONCLUSIONS
In this note we have shown that atmospheric ammonia may be detected quantitatively in the range I-50 pg m -3 STP by the collection of ammonia on acid-impregnated filters. The mass of collected ammonia is subsequently determined using the ring oven technique. Accuracy of a single measurement was determined to be +50, -3o”/, at 95% confidence. calibration at less than 1 pg m-’ could not be carried out with the flow system used to generate known ammonia concentrations; however, we have no reason to believe that the system would be non-linear below 1 pg m-” : in field tests over the Southem PaeifIc Gcenn concentrations of -0.1 pg m-‘hPvebGdlmtMlradoftar.n#detstionlimitissetby the sample ilow rate, sampling time and sensitivity of the spot test for ammonia Our experkacm is that 0.0&j& ammoaia may be deuwmhmd repmdueibly by the method OfCattel and Du Cros (1976X although 0.05 pg is the practical lower limit for routine use. Thus with a flow rate of 83.3 cm3 s-l, 0.05 pg me3 could be determined with a sampling time of 200 min. Division of Cloud Physics. CSIRO, Sydney. Australia
R. W. GILLE~~ G. P. Avxas
REFERENCES
Cattel F. C. R. and Du Cros D. (1976) Microdetermination of ammonia by the ring oven technique and its application to air pollution studies. Atmospheric Enufronment 10, 1141. Lau N. G. and Charlson R. J. (1977) Gn the discrepancy between background atmospheric ammonia gas measurements and the existence of acid sulphates as a dominant :;~~os~eric aerosol. Atmospheric Enuironment 11, Liu B. Y. H. and Lee K. W. ( 1976) Efficiency of membrane and nuckopore filters for submicrometre aerosols. Enair. Sci. Technol. IO, 345-350. Scott W. D. and Cattel F. C. R. (1979) Vapour pressure of ammonium sulphates. Atmospheric Enufronment 13, 307-319. Shendrikar A. D. and Lodge J. P., Jr. (1975) Microdetermination of ammonia by the ring oven technique and its application to air pollution analysis. Atmospkrric &i&onment 9,431-435.
Armusphna Envirmmmt Vol. 13. pp 1713-1714. 0 Pcrgrmon Press Ltd. 1979 Printed in Circa Britm
W2W.I
TECHNICAL NOTE APPLICATION OF A RING OVEN TECHNIQUE FOR DETERMINATION OF ATMOSPHERIC AMMO~A GAS CONCENTRATIONS (First receiwd 21 February
1979 und
infinalfbrm
15 Muy 1979)
Abstract - A technique based on standard ring overt procedures has been developed for the determination of trace quantities of ammonia in the atmosphere. Calibration over the range l-50 kg mm3 STP of ammonia indiites that ammonia is determined q~ti~~vely to an accuracy of + 50”/’ - 3ffk. The detection limit for routine use isO. fig of ammonia so that a sample ftow rate of 83.3 cm3 s-’ ailows 0.05 pgrn- 3 to be detected in 200 min sampling time.
INTRQDUCIION To
date there have been few experimentaI ixwstigations of the spatial and temporal variability of ammonia, although its significance as a trace gas involved in important chemical processes in the atmosphere is undisputed. Lau and Charlson (1977) provide a convenient summary of the work that has been carried out to date and briefly describe our currutt knowledge of the sources and sinks of atmospheric ammonia. They point out that past measurements are sparse bec%use of the difBculty of measuring low ammonia concentrations directly, and that where wet chemkal techniques have been used the uncertainty and detection limits have not been s~~~~~~~~~~~~to~~~~t quantify both the accumcy &d technique.
d&&n
limit of the
EXPEMHENTAL The method is bad on that described by Shendrikar and Lodge (1975) and incorporates the improvement by Cattel and Du Cros (1976). It consists of passing a known volume of sample air through a filter impregnated with oxalic acid, then using the ring oven technique to determine the amount of ammonia collected on the filter. The important features of the present work are the use of an inert prefdter to exclude ammonia-containing aerosols from the determination and the nse of one half the impregnated filter for sample collection and the other half for an in situ blank determination. A Teflon (PTFE) prefilter was used to ensure removal of any ammonium aerosols. This filter was mounted in an ~~~ holder and was connected by a PTFE pipe to a sample filter holder. The sample filter was held between stainless steel masking plates in an aluminium holder. The prefilter used was a 90 mm Fluoropore of 0.2 p nominal pore sixe. This filter was chosen because of the >99.99% collection efficiency reported by Liu and Lee (1976) for submicrometre particles (co&mad by us using a Pollak counter) and because the PTFE filter material would minimixe adsorption of ammonia gas by the pretilter. The possibility of obtaining erratic results as the result of adsorption/desorption of ammonia in sampling lines or in the apparatus itself should not be ignored in any method of ammonia collection. Ammonia is a small, polar, hydrogen-bonding molecule which, just like water vapour, readily bonds to a variety of surfa~. In addition, ammonia will readily exchange with the ubiquitous adsorbed water which occurs on almost any surface exposed to the ambient atmosphere. In the present case some adsorption at h&h ammonia concentrations and
desorption at very low concentrations occurred in the orefilter assembly desnite the use of a PTFE filter. This brobiem was o&corn; by winding a resistance heating tape around the miter assembly and maintaining it at - 315 K (ambient + 25 K) during sampling. Whik this temperature is high enough to reduce adsorption to insignificant levels, the vapour pressure data of Scott and Cattei (1979) indicate that, provided the measurement is restricted to ambient concentrations above about 0.03 M mm3, it is not high enough to add dgniftcantly to the measured ammonia gasconcentration by volntiiization of ammonium sulphate aerosol on the p&liter. For most tropospheric situations we have found ammonium sulphate to be the only ammonium salt important in this mgatvl. However, in polluted urban air significant amounts of more volatiIe ammonium compotmds could be collected by the prefiiter. In this case the apparatus can be used without heating the prefllter if it is first flushed with amm&a-free air whii hot, then allowed to cool and equilibrate with the gaseous ammonia in the ambient air prior to making a measurement. The sample filters consisted of carefully washed Whatman 542 papers 25 mm in diameter which had been soaked in a 5% ethanolic solution of oxalic acid and vacuum dried. Since the filters readily absorb atmospheric ammonia upon standing it is ncc*lsary to keep prepared filters in a stoppered flask or desimtor, as suggested by Sltendrikar and Lodge (1975). Even so it is impossible to completely exclude some contamination during assembly and disassembly. Consequently the sample titer was held between stainiess steel masking plates having a “D” shaped (semicircular) cut-out, so as to allow only a semicircular area of the filter to be exposed to tbe sample air stream. By washing out the filter from the central point on the straight e+e of the D shape, the sample plus contamination is washed to one side of the ring, the contamination alone to tbe other side. It is the.n a simple matter to read both sides of the ring and subtract the blank value from the sample plus blank. Duling this work the blank values never exceeded 35% of the sample plus blank values and most often fell in the range O-200/ Each half of the ring was read individually by five members of the laboratory staff and the quoted values are the average of the five readings.
CALtBRATtON The accuracy with which ammonia could be determined was cbecktd by performing measurements on 48 gas samples which had predetermined ammonia gas concentrations. Concentrations between 1 and 5Opg mm3 STP were pro-
1713
1714
Technical Note
duced in a flow system built around a short length of Pyre? tube through which dry, ammonia-free nitrogen was passed Dilute aqueous ammonia solution made up in freshly distilkd water was introduced into the nitrogen stream through a side arm, the solution being delivered at a constant rate by a motor-driven microsyringe (B. Braun, Melsungen Type 87 1104). Several turns of nichrome wire wound around the Pyrex tube either side of the side arm and heated electrically to B 370 K ensured that the aqueous ammonia solution was completely volatilized into the nitrogen stream. The gas mixture then passed into a length of PTFE tubing in which mixing was completed and the gas temperature returned to ambient prior to delivery to the sampling apparatus. The nitrogen flow rate was controlled by a regulator plus needk valve and measured with Hastings mass flow transducers (Models EA LLlOK and H-1OK). The maximum flow through the sample filter was determined by the area exposed and the filter material. At 80 kPa di&rential pressure Whatman 542 filters typically exhibited a flow rate of 37.5 cm’ s-’ for a semicircular exposed area derived from a circle 10 mm in diameter. Raising the diameter to 15 mm increased the Bow to about 83.3 cm3 s-‘. In Fig. 1 the results of the calibration experiments are displayed as measumd ammonia concentration plotted again&y (l+oratofy:prnduced) amculummn. The ncertamtiesmtberlngovenmchmqueareevident in the spread of meaaurai values obtained at a given calculated value; however, on avenge it is clear that the measurements reproduce the c&u&ted values very well. This was con6rmed by performing a linear, least-squares regresalon analysis on the points as plotted in Fig. 1. The analvais vieldal a done of 0.98f0.04 (95X m) and int&pt-ofo.10 f 0:12whenailthepointswereused,and figures of 1.02 f 0.05 and -0.03 f 0.16 when the points near 1 pg me3 were excluded. The seccmd mgmsaion was cakulated in order to eliminate a poaibk systematic error in the
‘E
-
-I/
s
f
10
s P
*/
” / / ‘/ ’ / /
/
/ ./ / /
/
/
Colculared
i
NH,,
P9
m-’
Fig 1. Measured ammonia gas concentration plotted against calculated (laboratory-produced) concentration. The full line is a linear least-squares fit to all the data and has the form In y =O.lO + 0.98 In x, with sample correlation coeiEcient of0.99. The broken lines are approximate 95% confidence limits.
calculated values near 1 pg m- ’ : limitations in the calibration apparatus precluded production of known concentrations of < 1 pg m-” and caused increased uncertainty at about this level. In fact, neither set of regression coefficients is significantly dilkrent from the values of 1.Oand 0.0 expected if the collection of ammonia by the impregnated filter is quantitative. Approximate 95% confidence limits determined from the first regression represent +50, -3oo/, for a single measurement. We have considered the scatter to be contributed entirely by the measurement technique, since the uncertainties in the calculated ammonia are small. of the order of 3-4% only. Variations in sample flow rate, and of relative humidity in the range 40-95% (obtained by varying the aqueous ammonia concentration and delivery rate), did not affect the collection efBciency of the impregnated filters. Shendrikar and Lodge (1975) similarly found no humidity dependence in the range O-300/, relative humidity.
CONCLUSIONS
In this note we have shown that atmospheric ammonia may be detected quantitatively in the range I-50 pg m -3 STP by the collection of ammonia on acid-impregnated filters. The mass of collected ammonia is subsequently determined using the ring oven technique. Accuracy of a single measurement was determined to be +50, -3o”/, at 95% confidence. calibration at less than 1 pg m-’ could not be carried out with the flow system used to generate known ammonia concentrations; however, we have no reason to believe that the system would be non-linear below 1 pg m-” : in field tests over the Southem PaeifIc Gcenn concentrations of -0.1 pg m-‘hPvebGdlmtMlradoftar.n#detstionlimitissetby the sample ilow rate, sampling time and sensitivity of the spot test for ammonia Our experkacm is that 0.0&j& ammoaia may be deuwmhmd repmdueibly by the method OfCattel and Du Cros (1976X although 0.05 pg is the practical lower limit for routine use. Thus with a flow rate of 83.3 cm3 s-l, 0.05 pg me3 could be determined with a sampling time of 200 min. Division of Cloud Physics. CSIRO, Sydney. Australia
R. W. GILLE~~ G. P. Avxas
REFERENCES
Cattel F. C. R. and Du Cros D. (1976) Microdetermination of ammonia by the ring oven technique and its application to air pollution studies. Atmospheric Enufronment 10, 1141. Lau N. G. and Charlson R. J. (1977) Gn the discrepancy between background atmospheric ammonia gas measurements and the existence of acid sulphates as a dominant :;~~os~eric aerosol. Atmospheric Enuironment 11, Liu B. Y. H. and Lee K. W. ( 1976) Efficiency of membrane and nuckopore filters for submicrometre aerosols. Enair. Sci. Technol. IO, 345-350. Scott W. D. and Cattel F. C. R. (1979) Vapour pressure of ammonium sulphates. Atmospheric Enufronment 13, 307-319. Shendrikar A. D. and Lodge J. P., Jr. (1975) Microdetermination of ammonia by the ring oven technique and its application to air pollution analysis. Atmospkrric &i&onment 9,431-435.