Nitric acid measurement methods: An intercomparison

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NITRIC ACID MEASUREMENT METHODS: AN INTERCOMPARISON* CHESTERW. SPICER,JAMES E. HOMES, JR., THOMAS A. BISHOPand LESLY H. ARNOLD Battelle. Columbus Laboratories. 505 King Avenue. Columbus. OH 43201. U.S.A. and ROBERT K. STEVENS Environmental Sciences Research Laboratory, U.S. Environmental Protection Agency. Research Triangle Park, NC 27709, U.S.A. (First receid

4 May 1981 and in fmal form 14 August 1981)

Abstract-An inter&oratory comparison of methods to measure gaseous nitric acid is described. Field experiments were conducted in Claremont, CA in late August and early September of 1979. Ten nitric acid measurement methods were compared during the 8 day field experiment. Six different research groups participated in the nitric acid measurements. The methods compared include 2 which employed chemilumin~~ principles, I infrared, 2 diffusion denuder and 5 filtration techniques. The 8 study days were divided into 32 sampling intervals of 3,6 and 12 h. Based on the median of the lomethods, the nitric acid concentration ranged from 1.85 to 37.05 pg m -‘. The nitric acid concentration was highly correlated with other photochemical pollutants such as O1 and PAN, and all three were strongly correlated with temperature and solar intensity. On the average, gaseous nitric acid and particulate nitrate contributed equally to the total inorganic nitrate burden at the site. Of the 10 methods investigated, 5 exhibited exatlent agreement with the median nitric acid concentration, 2 yielded slopes significantly exceeding 1.0 and 3 suffered from problems which resulted in relatively poor agreement with nitric acid concentration. A great many chemical and meteorological variables were measured during the study. and the effects of these ancillary variables on the nitric acid measurement methods are discussed.

INTRODUCTION

Atmospheric nitrates are currently receiving a great deal of attention from environmental scientists because of their potential health and ecological effects. Nitrates, both gaseous and particulate, are the end products of nitrogen oxide reactions in the atmosphere. Due to the continuaIfy increasing emissions of NO,, the level of nitrates is expected to increase, so that the effects of nitrates on health, visibility, crops, materials and precipitation chemistry have &ome a major concern. The measurement of nitrate in the atmosphere is not straightforward because of the facile interconversion of gaseous and particulate nitrates. Over the fast five years, a great deaf of effort has been devoted to

*This research was performed in collaboration with J. Forrest,’ 8. Huebert,’ T. Keff ‘.* G. Kok,$ A. Lazru~,~ R. Paur,’ R. Shaw,’ D. Stedman, ? R. Tanner,’ E. Tews and E. Tuazon’. ‘Brookhaven National Laboratory, Upton, NY 11973; 2Colorado College, Colorado Springs, CO 80903; ‘University of Michigan, Ann Arbor, MI 48189; ‘Resent address: ‘above; $Harvey Mudd College, Claremont, CA 91711; 6Nationaf Center for Atmosptic Research, Boulder, CO 80303; ‘U.S. Environmental Protection Agency, Research Triangle park, NC 27711; aNorthrup Services Corp., Research Triangle Park, NC 27709 and- 9Statewide Air Pollution Research Center. University of California. Riverside, CA 92521, U.S.A.

development of specific sampling and analysis techniques. This paper and a companion article (Howes er al., 1981) present the results of a major field experiment designed to intercompare the methods being employed for measuring gaseous and particulate nitrates. This paper will compare the methods used to measure nitric acid; the companion paper discusses the techniques used to measure particulate nitrate. The first successful measurement of nitric acid in the lower atmosphere was reported by Miller and Spicer (1975). They empkryed a coufometric instrument for detection of acidic gases, and a nylon trap for selective removal of nitricacid. The method was compared with a long-path Fourier transform infra-red (i.r.) spectrophotometer under laboratory conditions by Spicer er al. (1978). Earlier attempts to detect nitric acid in polluted ambient air by infrared spectroscopy met with failure (Scott ec af., f957; Hanst et al., 1975). Improvements in inst~mentation and techniques permitted spectroscopic detection of nitric acid in the lower atmosphere (Tuazon et al., 1978). A l-km folded path cell coupled to a Fourier transform infrared spectrometer was employed for these and subsequent m~su~ments in the California south coast air basin (Tuazon et al., 1980, 1981). A chemifuminesccnt method for continuousfy monitoring gaseous nitric acid was developed by Joseph and Spicer (1978). A commercial dual-reaction cell

1487

chrmllumlnescence Instrument isas modtfied so that both sides of the spilt sample stream pass through molybdenum catalytic Converters to reduce ah oxidized nnrogen species. including HN03, to NO. One stream is passed through a nylon filter to remove HN03. The nitric acid is determined as the difference between the two streams. A related concept has been employed by Kelly er 01. (1979), who use a 4-channel system to distinguish the various components of oxidized nitrogen. The instrument uses granular FeSO, to reduce NO1 to NO, and heated Pyrex beads to reduce HN03 to X0?. Either Cotton or nylon fiber is employed to selectively remove HN03 from the sample stream. Okita er rrl. (1976) measured nitric acid concentrations at two locations in Japan using a selective filtration technique which made use of NaCIimpregnated filters. This method has been refined and investigated further by Forrest et al. (1980). Appel et al. (1980) and Crennfett { 1980). Simitar filtration techniques have been employed by other investigators. Huebert and Lazrus (1979) employed filters impregnated with tetra-n-butyiammonium hydroxide to cofleet acidic gases including HN03. Spicer 11977a, 1979) reported the use of nylon filters for collection of nitric acid, preceeded by an inert filter for collection of particulate nitrate. Similar techniques employing nylon filters have been utilized by Huebert and Lazrus ( 1979) and Appel ef ui. I 1980). Hare YI al. ( 1979) and Tesch and Sievers (1979) have measured nitric acid by collection on nylon and cotton fibers, respectively, foIlowed by extraction. conversion of nitrate to nitrobenzene and analysis by electron eapture gas

chromatography. .\ method rrcendy developed hi Braman +~rnl. (1981) makes use of tungstic acid-coated diffusion tubes for preconcentration of nitric acid and ammonia, and separation from ammonium and nitrate. Analysis is by means of thermal desorption and chemiluminescent detection with a conventional NO, monitor. This technique was not sufficiently developed at the time of this study for incorporation in the field inter-comparison. Many of these methodologies were described at a meeting held at Southern Pines, SC in 1978, the proceedings of which are available (Stevens, 1979). The purpose of the program described here was to intercompare, under actual field conditions, all of the methods being used to measure nitric acid in ambient air, in order to determine the relative sensitivity. selectivity, accuracy and precision of the individual methods. A great deal of supplementary data was obtained during the g-day study in order that interfering species or adverse conditions for some of the methods could be identified. The study has yieided one of the richest and most detailed urban air characterizations on record. EXPERIMENTAL The field experiment was conducted on the roof of the Jacobs Science Center of Harvey ‘Mudd College in Claremont, California, 50 km east of Los Angeles. The site is generally downwind of Los Angeles and frequently experiences elevated levels of photochemical pollutants. High concentrations of nitric acid have been measured at this site (Tuazon et al., 1981) and at a nearby site in West Covina (Spicer, 1977b). A l-km folded-path Fourier transform

Fig. 1. West wing roof ofseience building showing Equipment location areas. Key for Fig. I an opposite page.

Nitric acid measurement methods: an intercomparison

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Key for Fig. 1 Location

: 39 4 5 6 : 9 10 11 I2 13 14 15 16 17

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19 20 21 22’

operator

Measurement/equipment Hz Oz and HCHO PAN/GC NH, HNO,/Chemiluminescence Cont. sulfate Cont. sulfate Particulate composition HNO,/FfS-LPIR HNO,/filtcr HNO,/filter HNO,/filtcr HNO,/filter Part. NO;/dichotomous Part. NOf /Hi-vol Part. NO, /Hi-vol HNOJ, Part. NO; /DDE Part. NO; /Hi-vol Part. NO;/dichotomous SCE van HNO,, other species/EPA van HNOs,, NO, NO~/chemilumincsccnce Heated nephelometer

Harvey Mudd College UC-SAPRC EPA Battelle Oregon Grad. Center Brookhaven LBL UC-SAPRC Brookhaven Battelle EPA NCAR EPA Battelle Battelle EPA Battelle EPA SCE EPA U. of Michigan U. of Washington

l The EPA denuder difference experiment was also operated in this location during certain test blocks. ’ Located in 3rd floor laboratory with sampling line extending out the window.

infrared system (FI’IR) operated by the Statewide Air Pollution Research Center of the University of California (UC-SAPRC) was located on the roof of the Science Center. Since the FITR method was to play an important role in the nitric acid meaaurernent study, all of the more portable measurement systems were clustered around the FTIR cell. A schematic of the field site, showing the positions of the FTIR cell. the nearby penthouse, and the locations of the various experiments is shown in Fig. 1. The devices which colkctai integrated samples for nitric acid determination were grouped around the air inlet duct for the infrared cell. High volume and dichotomus samplers were positioned at least l-m apart along the wall next to the i.r. cell. The exhaust from all these integrated samplers was vented through a large diameter tube to the other side of the building. Instruments rquiring a controlled temperature were operated in the penthouse. These instruments took their samples from a 3cm dia. glass manifold which extended through the penthouse wall. The inlet for the gIass manifold was about 2 m from the i.r. cell inlet duct, was at the same height and pointed in the same direction. Inside the penthouse, the manifold split at a tee, with blowers pulling air through the manifold from both sides of the tee. The calcuIatcd residence time for a gas in the manifold was 0.1 s.A number of gas and aerosol monitors were operated by EPA and Southern California Edison (SCE) in two mobile vans located at ground level next to the Science Center building (see Fig 1). The chemiluminescence monitor operated by the University of Michigan was also contained in a mobile van at this same location. Comparisons between 0, concentrations measured sinudtaneously on the roofand in the EPA mobik hb indicated that there was no gradient in 0s concentration over the 5 18m difference in altitude. Recent laboratory (C. W. Spicer, personal communication, 1980) and field (B. J. Huebert. personal communication, 1980) results suggest that HNO, deposits more rapidly than 0,. so the possibility of a gradient in nitric acid concentration can not be completely ruled out. Experiments designed to test this possibility were inconclusive.

The study was performed over a continuous a-day period from 27 August to 3 September 1979. Ten methods for measuring nitric acid were employed during the field experiment. These methods are summarized in Table 1, which includes the names of the organizations performing the measurements, an organization/method code for subsequent reference, sampling and analysis method descriptions, flow rates, and a site code which refers to Fig. 1. All of the methods have been described in detail in the proceedings of the Workshop on Measurement of Atmospheric Nitrates (Stevens, 1979).and the references noted earlier. In particular, the denuder difference experiments operated by EPA and Brookhaven, designated as DDE in the table, have been described by Shaw rr al. (1979) and Forrest et al. ( 1980). In addition to the nitric acid measurements. particulate nitrate was determined by several methods (see Howes er al.. 1981) and many ancillary variables were monitored. These additional variables are listed in Table 2. A compilation of all the study data is available through the National Technical Information Service (Howa et al., 1981a). In order to accommodate the sampling time requirements of the 10 nitric acid measurement methods and to facilitate comparison among the methods, 3, 6 and I2 h sampling intervals were employed. The total of 32 sampling periods were allocated over the 8 day period such that 3, 6 and 12 h sampling intervals would occur during periods of expected high and low HN03 concentrations. This design permits the nitric acid results to be analyzed for the effects of sampling time and concentration on the measurement methods. The test schedule is shown in Table 3. Continuous methods were operated over the entire test period and the data averaged over appropriate sampling intervals for comparison with the integrated procedures. Due to the labor-intensive nature of the FTIR, infrared measurements were made e\ery l5min during the 3 h intervals and every 30min for the 6 and 12 h sampling periods. No FTIR measurements were made during several of the night-time sampling intervals since the HNO, concentration was expected to be less than the instrument’s 6ppb detection limit.

NCAR Univ. California (Riverside) Univ. Michigan

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Nitric acid measurement

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an intercomparison

Table 2. Ancillary measurements performed during the field experiment Organization

Variable

EPA, Univ. Mich. EPA, Univ. Mich.

NO NO*

UC-SAPRC EPA. UC-SAPRC Battelle, EPA Battelle. EPA, Brookhaven W;(2), UC-SAPRC

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EPA Brookhaven. Oregon Grad. Center NCAR

HCHO HsOs Aerosol sixe distribution Light scattering coefficient so:Aerosol Acidity U.V.Intensity i.r. Cell temperature and humidity Ambient temperature and humidity Wind speed and direction

Harvey Mudd College, UC-SAPRC Harvey Mudd College EPA EPA Battelle, Brookhaven, EPA Brookhaven, EPA, Battelle EPA UC-SAPRC Battelle Battelle

RESULTS

Several variables which characterize the air quality and meteorology during the B-day study period are shown in Fig. 2. The diurnal pattern is evident for most of the variables plotted. Solar radiation and temperature both show 8 maxima corresponding to the 8 study

days. Nitrogen dioxide and CO exhibit additional peaks corresponding to daily emissions patterns. Ozone, nitric acid, peroxyacetyl nitrate (PAN) and light scattering aerosol (b,,) show 8 specific peaks for the 8 study days. The peak hourly average OJ concentration varied from a low of 75 ppb on day 3 to a high of 240 ppb on day 8. The nitric acid curve is

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based on the Battelle chemllummescence method I B.4TCHEM) since this method provides continuous data. The nitric acid curve mirrors the diurnal 0, pattern. with the lowest levels of HNO, measured on day 3 and the highest on day 8. Typical peak hourly HNO1 was on the order of 30~9 m-‘. In order to compare nitric acid results from the continuous and integrating procedures, sampling was divided into 32 time blocks of 3,6 and 12 h. The block averages from the continuous instruments can then be compared with the results from the integrated sample collections. If either the FTIR or the chemiluminescence techniques reported data for less than 70”, of any sampling block, the results from that technique were not included for that sampling interval. .4 direct block by block visual comparison of all 10 nitric acid methods is shown in Fig. 3. Asterisks indicate either that data were not reported (e.g. less than 70”, coverage of the sampling interval) or that an experimental problem cast suspicion on the value, causing it to be excluded. Due to its relatively high detection limit, the infra-red procedure (UCR i.r.) was not operated during periods of expected IOU-nitric acid concentrations (unmarked blocks in Fig. 3). and frequentIS reported less than 701, sampling interval coverage. Consequently, the i.r. block data appear rather sparse. even though a great deal of data were obtained. For several sampling blocks, narrow upper and lower concentration limits can be calculated from the i.r. data, thus increasing the number of i.r. blocks available for analysis. These data are not shown in Fig. 3 but are used in the statistical analysis discussed shortly. The EPA denuder difference experiment (EPA DDE) made use of two duplicate sets of equipment. both operating on the Science Center roof. There were some sampling blocks when neither apparatus was operated as well as occasions when both were run simultaneously. The values from both instruments have been combined in Fig. 3. When both experiments reported data, the bar graph is shaded so that both values are shown. Due to problems with sampling and analysis methodology which were pinpointed after the field experiment, the precision of the EPA DDE experiments was much lower than anticipated. leading to the irreproducibility displayed in Fig. 3. DISCLSSIOS

In general, the nitric acid methods track one another reasonably well and are in agreement within rather coarse limits. There are certain exceptions noted in Fig. 3. the most significant being the complete lack of agreement between the UM;CHEM results and all the other methods. This difficulty is addressed in the statistical analysis of the data, discussed later. The nitric acid results were examined for outliers using the criterion that any value within three standard deviations (3a) of its method’s regression line was considered acceptable. This criterion resulted in the exclusion of 8 values, or about 3”, of the data as statistical outliers. Four of the 8 outliers occurred in the data from the first sampling interval, suggesting

Nitric acid measurement methods: an intercomparison

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Fig. 3. Nitric acid measurements by 10 methods.

that difficulties associated with the start-up of the experiment may have played a significant role. The outliers were deleted from the data base used for statistical analysis of the test methods. The approach adopted for the intercomparison of the various nitric acid methods involved the creation of a reference standard. The infrared technique would be a logical choice as a reference, but the high detection limit of the instrument caused the i.r. data to be too sparse to serve as the comparison standard. Thus, we elected to consider the i.r. method in the same manner as the other test methods. Since none of the techniques was regarded as a “true” standard, a decision on a reasonable proxy for the standard had to be made. After examining the data, we decided to use the median of the observations in each sampling block to represent the “true” concentration for the block. The median provides a measure of central tendency which is insensitive to large outlying observations and atypical techniques. Use of the median statistic permits inclusion of data from all the methods without serious risk of biasing the comparison if one method is in error. Consequently, no methods need be excluded a priori. The length of the sampling intervals associated with the sampling blocks varied over 3,6 and 12 h. In order to test whether sampling time influences the results of any of the methods due, for example, to saturation of a collection medium, an initial regression model was employed which regressed each method’s observations

against the median concentration, the length of sampling time, and the interaction of these two variables. From the analysis of the results of these regressions, it was clear that neither sampling time nor the interaction between sampling time and concentration had any significant effect on the observations of any of the methods. In all cases except the UM/CHEM nitric acid measurements, the concentration had a significant effect on the observations for each method. Since the relationship ofeach method’s observations to the median concentration could be adequately described by a straight line regression model, the observations for each method were regressed against the median concentration. Weighted least squares regressions were employed but resulted in no better fits than the standard regressions. Figure 4 shows scattergrams of the observed data vs the median concentration (standard). In the scattergrams, the observed values and the mediin concentration are expressed in pg m - ‘. The solid line is the regression line based on the data with outliers removed. The circled values on the plots are the observations which were removed as outliers. Note that the ordinate scale varies from method to method in Fig 4. The behavior or quality of a particular method can not be characterized by a single quantity associated with the linear regression analysis. The intercept of the fitted regression line measures the constant bias of a method, the slope measures the proportional bias

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(proportional to the concentration level) and the mean squared error (MEUE)measures the precision of the method relative toour choiceofastandetd. AII of these quantitative factors must be considered in evaluating the various techniques. Table 4 lists the descripzive statistics obtained from the recession fits. In addition to the estimated siope and intercept for the regression line and the mean squared error, an indication is given whether the confidence interval for the intercept contains 0 (no evidence of a constant bias) and whether the confidence interval for the slope contains i (no evidence of a proportional bias). Also the percentage of

the 32 blocks where data was obtained for each method is given, as well as the number of observations rejected as outliers. The intercepts of the regression lines for the 10 nitric acid measurement methods ranged from - 1.94 to 5.38 pg m - ‘. The 95 % cadence interval contained O.Oin 8 out of the 10 cases. The slopes ranged from 0.04 to 1.35, with the theoretical sfopt of 1.0 falu outside the confidence interval for 4 of the 10 methods. The mean squared error estimates the variance ~2 of the distribution of errors around the regression. The square root of the MSE estimates the standard de-

1495

Nitric acid measurement methods: an intercomparison Table 4. Descriptive statistics of the regression ftts for nitric acid vs medtan

Method

Intercept, fiBm_’

BAT,‘CHEM BATjLVF BNL/LVF BNL/HVF BNL/DDE EPA:NTR EPA)DDE* NCAR;LVF UCR:i.r. UM/CHEM

0.83(yes)* 0.27 (yes) - 1.94(no) - l.l9(yes) 0.62(yes) 0.95(yes) 0.94(yes) 0.90(yes) 2.60(yes) 5.38(no)

Slope

0.98(yes)t

1.35(no) 1.24(no) 1.05(yes) 099(yes) 1.03(yes) 0.57 (no) 1.02(yes) 0.95 (yes) O.O4(no)

Mean squared error 7.13 9.42 6.17 5.41 13.10 49.29 Il.64 6.66 13.26 3.96

R2 0.93 0.95 0.96 0.95 0.87 0.64 0.73 0.91 0.82 0.03

Percentage data recovery

Number of rejected outliers

97 81 97 97 94 84 81 72

l Paranthesis indicates whether the 95 “/, confidence interval for the intercept contains 0.0. ’ Parenthesis indicates whether the 95 on conlidence interval for the slope contains 1.0. $ Includes 2 negative values.

viation o of the distribution and serves as a measure of precision. The larger the value for the MSE the greater

the scatter about the regression line, i.e. the less precise the measurement system. MSE values were observed between 3.96 and 49.29 bgrn- ‘)*. The multiple correlation coefficient R’ measures the percentage of the total variability observed in the data that can be explained by the regression. A value close to 1 would indicate the method has a strong relationship to the true level of nitric acid as measured by our standard. Seven of the 10 methods reported RZ values greater :han 0.8. The percent data recovery was satisfactory, with all but one of the methods reporting data for at least 807, of the sampling blocks. Most of the missing data are due to human error (e.g. scientists oversleeping the 0300 PDT filter change rather than equipment malfunction). All of the above performance criteria must be considered in evaluating the measurement methods; no one statistic alone can adequately describe a method’s performance. Different methods have different characteristics. Based on the results of this program it should be possible for future studies to select the most appropriate method for a given application. Factors which must be considered in addition to the descriptive statistics include the simplicity of the methods and whether the technique provides continuous or intermittent data. Continuous monitors Based on Fig. 4 and the descriptive statistics in Table 4, it is apparent that the UM/CHEM did not provide a reliable measure of nitric acid during the study. The near zero correlation coefficient and slope, and the high intercept indicate that the instrument was yielding a nearly constant response regardless of the nitric acid concentration. Subsequent to the field study and statistical analysis, it was shown that the UM/CHEM instrument had developed a leak in the vacuum system which was likely present during the Claremont study. The effect of such a leak would be to reduce the response of the instrument, and thereby lower the

slope of the regression line. The leak was apparently large enough to reduce significantly the sample flow rate. It is speculated that HNOJ in the sample air was lost to the surfaces of the sampling tubing because of the long residence times at the lower flow rates (D. H. Stedman, personal communication, 1980). Since the concept of the UM/CHEM instrument is similar to the BATjCHEM monitor, which performed well, it is assumed that the difficulty with the UM:CHEM method during the Claremont study was simply a hardware problem. The BATCHEM instrument results are shown in Fig. 4 and the descriptive statistics are presented in Table 4. The method yielded 97 “/, data recovery, was highly correlated with the median HNOs concentration (R ’ = 0.93), showed high precision (MSE = 7.13), and produced an intercept and slope which were not statistically ditferent from 0 and 1, respectively. During the Claremont study the continuous chemiluminescence minitor provided a detection limit of l-2pg m- 3. This is sufficient for studies in urban areas and in urban plumes, but more sensitive instruments are required for studies in nonurban areas. The UM/CHEM instrument is designed to provide much lower detection limits, and work is underway to lower the limits of detection of the BAT/CHEM monitor. Dualfilrer methods Five of the ten methods evaluated for nitric acid measurement were filtration procedures. The simplicity, high sensitivity and the potential for simultaneously determining nitric acid and particulate nitrate are very desirable features of these methods. Basically, an inert prefilter is used to collect particulate nitrate, as well as other aerosol species, and an active backup filter collects gaseous nitric acid. The potential difficulties with the concept include loss of HN03 on the prelilter or on the collected particulate matter, and volatilization of particulate nitrate from the pre-filter followed by collection on the active filter. The former behavior will yield low nitric acid values, while the latter will result in a positive HNOJ interference. Since

all of the tiltration methods depend on laborator) nitrate analysis for their nitric acid determination. blind nitrate standards of three different concentrations were presented to each of the organizations using filter methods. The purpose was to uncouple the inaccuracies due to sampling and analysis. The standards were handled in the field in the same manner as the samples and were returned to each organizations’ analytical laboratory for analysis along with the samples. The results of the reference nitrate analyses are given in Table 5. The results suggest that. with one or two exceptions caused by improper dilution of the standard. the agreement among the laboratories was very good. with the laboratory results clustering around the reference or design value for the standards. The EPA/NTR dual filtration method suffered from imprecision and a lower correlation with HNO, concentration than desired. The difficulties are attributable to problems in the preparation of the sampling devices and analytical imprecision resulting from high solvent blanks. These problems were identified and corrected based on the results of this study. The Battelle and Brookhaven low volume filter methods, BAT/LVF and BNL LVF, yielded slopes which exceeded 1.0. Both methods were highly correlated with the median nitric acid concentration (R’ = 0.95-0.96) and both had low mean square errors associated with the regression line. The BNL/LVF method yielded an intercept of - 1.9 pg m - 3, which was outside the confidence interval around a zero intercept. The higher slopes might be due to transfer of particulate nitrate from the prefilter to the nitric acid collection filter as mentioned earlier. This has been reported in a recent ambient study by Appel er al. (198 1). The volatilization of aerosol nitrate may occur through chemical reaction or by decomposition of NH,NO, due to variations in the ambient NH, and HNO, concentrations. Loss ofparticulate nitrate from inert filter materials was investigated during the Claremont study and is reported by Forrest et al. (1980) and Howes et al. (1981). Briefly, we observed particulate nitrate losses from four inert filters to range from 16 to 26 y,, averaged over the g-day study. These losses are referenced to the BNL j DDE and have been corrected for estimated nitrate particle losses in the denuder tube. When averaged over 24-h periods, we found that the inert filters collected from 0.77 to 1.13 times as much particulate nitrate as the denuder experiment. For individual sample blocks, the composite Teflon filter losses exceeded 304, four times, with the greatest loss being 417,. The greatest losses were observed during periods of high temperature and low relative humidity ( < 60 “/), as might be expected from equilibrium considerations. In Claremont, the average particulate nitrate and nitric acid concentrations were similar, so a 20°j; average loss of particulate nitrate should result in nitric acid concentrations which average 20 9, high as measured by the filtration methods. In other areas of the country. where the particulate nitrate to nitric acid ratio may be much

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1497

Nitric acid measurement methods: an intercomparison

lower, such losses would have a smaller effect on measured nitric acid concentrations. TWO other HNOJ methods which employed Teflon and quartz prefilters respectively, NCAR/LVF and BNL/HVF, yielded slopes which were very close to 1.0. It is not apparent at this time why the BAT/LVF and BNL/LVF methods should be subject to positive interference due to nitrate loss from the prefllters, while the NCAR/LVF and BNL/HVF are not. Neither is it clear how these methods could be subject to positive error and yet report the highest R2 values and low MSE values. We have not yet been able to identify any environmental factors which might point to the mechanism causing the positive interference. Previous studies have demonstrated that the BAT/LVF and BNL/LVF methods serve as total inorganic nitrate collectors (Forrest et al., 1980; Appel er a[., 1980; Spicer, 1977b) since the sum of nitrate on the prefilter and collector filter represents total inorganic nitrate, independent of any gas/particle interconversion which may occur on the filters. The sum of particulate nitrate and nitric acid from the NCAR/LVF method has been plotted vs the nitrate sum from the BAT/LVF method in Fig. 5. Except for two points, the two methods are in very good agreement for total inorganic nitrate. This result, coupled with the high BAT/LVF slope for HN03, suggests that both methods collect inorganic nitrate quantitatively, but that the gas/particle distribution has changed on the BAT/LVF filters. This same phenomenon apparently accounts for the higher slope of the BNL/LVF curve as well. The major difference between the BAT/LVF and BNL/LVF devices on the one hand, and the NCARJLVF technique on the other, is filter surface area and, consequently, face velocity. The NCAR/LVF method uses a face velocity less than

half of the other two dual filter methods. However, the BNL/HVF method used a much higher face velocity than the BNL/LVF, BATILVF, or NCAR/LVF and its performance, as judged by the statistics in Table 4, was excellent. Thus face velocity does not seem to explain the difference. The filter materials employed for HN03 collection by the BNLiLVF and BNL/HVF methods were the same,as were the HNO, filters in the NCAR/LVF and BAT/LVF devices. Therefore, the filter medium can not explain the differences. At this time we have no conclusive explanation for the higher slopes of the BNL sLVF and BAT/LVF regression lines. As a test of the efficiency of nylon filters for nitric acid collection, a second nylon filter was occasionally employed in the BAT/ LVF method as a backup for the Teflon/nylon filter combination. Under all conditions of nitric acid concentration and sampling time there was never any detectable nitrate on the second nylon filter. This demonstrates that nitric acid breakthrough on nylon filters is not a concern under ambient conditions. Long path infrared

The i-km pathlength Fourier transform infrared instrument generated the results shown in Fig. 4 with the statistical data provided in Table 4. Of the 22 sampling blocks which called for FTIR measurements, 6 were indeterminate due to levels of HNOs which were predominantly below detection limit (less than 6 ppb or 15 pg m - ‘), and 4 were incompletely recorded due to instrument failures. Only 8 sampk periods had sufficient data coverage for calculation of representative block concentrations. Four other periods yielded enough data to allow us to compute a limited

J IO

20

Totol Inorganic

3a

Nitrate,

40

50

60

BAT/LVF,,g/m3

Fig. 5. Total inorganic nitrate by NCAR/LVF and BAT/LVF.

70

concentration range. as noted by the limit bars in Fig. 4. The length of the bar results from assigning the i.r. points below detection limit as either Oppb as a minimum or 6 ppb (just at the detection limit) as a maximum. The actual concentration should fall within this range. We have used the midpoint of the range for all statistical calculations. The plot in Fig. 1 demonstrates good correspondence between the UCR,i.r. HNOJ data and the median HNO, concentrations. The method yielded a slope of 0.95 and a squared correlation coeficient of 0.52. The intercept of 2.6pgm-’ is higher than most of the other methods. This is due to a lack of data at low concentrations and the consequent long extrapolation to obtain an intercept value. The dearth of data at low concentrations is the result of the high (6ppb) limit of detection. Denuder diference

experiments

Measurements based on the diffusion denuding principle were made by two organizations. These experiments, designated EPA/DDE and BNL, DDE, involved the measurement of nitric acid by difference. In these experiments, two devices designed to measure total inorganic nitrate were operated in parallel with one device preceeded by a HNO, denuder. The difference between the two results is a measure of gaseous HNOJ. Unlike the simple dual filter procedure described earlier, the denuder difference experiment should be free from interferences caused by particulate nitrate volatilization from an inert pretilter. The DDE method was first suggested by Shaw er al. (1979). The methods were still in the developmental stage during the Claremont study. The results of the two denuder difference experiments are shown graphically in Fig. 4. The statistical data are reported in Table 4. The EPA/DDE regression yielded a slope considerably less than 1.0 and one of the lower R’ values, at 0.73. The technique suffered from the same difficulties as the EPA NTR method. Problems related to the preparation of the nylon fiber sampling tubes are believed to have caused both positive and negative errors during the study. We have no way of determining whether this source of error was randomly distributed or whether it caused an overall positive or negative bias to the results. Difficulty with flow measurement was also reported during the first few sampling periods. These factors certainly contributed to the mean square error and served to lower the R’ value. A third problem involved the design of the denuder tube. The tube has been shown to have a penetration of greater than 93 O0for particles with aerodynamic diameters up to lpm. If larger nitrate particles are lost in the denuder, an artifically high HNO, concentration will result. Since the slope of the regression line is less than 1.0. it appears that coarse nitrate particle removal by the denuder was not a significant factor in the performance of the method. The BNL/DDE results showed very good correspondence with the median of the other methods, as

judged by the slope of0.99. intercept 0.62 jig m ’ and R’ of 0.57. The performance of the BNL DDE

method demonstrates that the denuder difference concept for nitric acid measurement is sound. Since this method, in theory, should be free from interferences, the excellent agreement with the median gives us confidence in the choice of the median as a reference standard. The Claremont study and subsequent laboratory research have served to identify and correct the difficulties encountered with the EPA!‘DDE method. That the problems have been solved is apparent from recent results with the procedure (Shaw et nl.. 1982).

CONCLUSlONS

Ten methods for measuring atmospheric concentrations of nitric acid were compared during an g-day field experiment in Claremont, CA. The median nitric acid concentration based on all 10 methods ranged from 1.85 to 37.05 pg m- 3. Nitric acid concentration was shown to be highly correlated with other photochemically derived air pollutants such as O3 and PAN. All three gaseous pollutants are, in turn, highly correlated with ambient temperature and solar intensity. On the average, gaseous HNO, and particulate nitrate contributed about equally to the total inorganic nitrate burden. Of the 10 methods evaluated in the field experiment, one(UM/CHEM)experienced hardwaredifficulties to the extent that no valid comparisons with other methods could be made, and two others (EPA/‘NTR and EPA/DDE) suffered precision and accuracy difficulties due to sample device preparation and analytical problems. These latter methods were in the developmental stage during the field comparison and significant improvements in the methods have resulted from the data obtained during this study. Two other methods, BAT/LVF and BNL/LVF, were highly correlated with nitric acid concentration and were quite precise, but yielded regression curves with slopes significantly greater than 1.0 (a similar result has been reported by Appel et al., (1981). Since these two methods would be the simplest and cheapest to implement for routine nitric acid measurements there is a strong impetus to determine the cause of the high responses. Other chemical species, environmental variables and method parameters were examined, but no convincing explanation of the high slopes is available. This is particularly perplexing since two other methods, BNL/HVF and NCAR/LVF, which are very similar to the BAT/LVF and BNL:‘LVF techniques, yielded slopes very near I .O.An analysis of particulate nitrate losses from inert prefilters, conducted as part of this study (Howes ef ~1.. 1981). indicates that losses can reach 40”” during short collection periods with high temperature and low relative humidity, and that average losses of 16-26”. may be expected, In Claremont. where the average

Nitric acid measurement methods: an intercomparison particulate nitrate and nitric acid concentrations were similar, these losses translate into a 16-267” positive error in nitric acid as measured by dual filter techniques. For many areas of the country, the error would likely be lower since particulate nitrate/nitric acid

ratios are lower. Clearly, the dual filter method serves as a total inorganic nitrate collector, and can provide an estimate of the gas-particle nitrate distribution if average errors of up to 267, are tolerable. For some applications, such errors may be acceptable; in these cases the simplicity and the cost effectiveness of the dual filter method recommend it for total inorganic nitrate and nitric acid sampling. For periods of less than 24 h, however, the errors could be much higher, depending on environmental conditions. The other 5 measurement methods all exhibited good agreement on the nitric acid concentration over the full concentration range. None showed either a constant or a proportional bias within the 95% confidence intervals, and all were highly correlated with the median nitric acid concentration and exhibited very reasonable mean squared errors. Indeed, it was a pleasant surprise to most of the researchers involved in the comparison study that such diverse and in many cases developmental methods agreed so well, especially under the limitations imposed by field operation. Clearly, field conditions are necessary in order to truly test the ambient monitoring methods compared in this study. The Claremont study has yielded a rich data base in terms of the number of species and variables measured and the temporal resolution. The data file is now and should in the future be examined for information on pollutant relationships, chemical transformations, gas-particle interactions and atmospheric source and sink information. The data must be used with one important caveat, however; all the results were obtained at one specific location in the Los Angeles basin where pollutant concentrations and environmental conditions are frequently rather different from those encountered in nonurban regions and even other urban areas around the world. For this reason it is recommended that a similar study be conducted in a different geographical area. Such a study, which could have a more limited scope than the one reported here, should be undertaken in an area recognized to have a different pollutant mix and lower pollutant loadings than experienced in Claremont. Important factors which should be considered, in addition to HNO, and particulate nitrate, include the levels of NH, and H,SO,, and temperature and humidity. Such a study would serve to expand and validate the results of the work reported here and permit the conclusions to be generalized over a much broader range of environmental conditions. Acknowledgemenr-The authors would like to acknowledge the assistance of many individuals who contributed significantly to this effort. Scientists who assisted during thi field experiments include: T. D’Ottavio and D. Spandau of Brookhaven National Laboratory, P. Sperry of NCAR, C.

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Lewis and W. McClenny of EPA, R. Hoffman of the Oregon Graduate Center, R. Brown of SAPRC-University of California at Riverside, and C. Ellis and J. Afoug of Southern California Edison. Others who contributed through organizational efforts and/or data analysis include: P. Hanst of EPA,

J. Huntzieker of Oregon Graduate Center, L. Newman of Brookhaven National Laboratory. and J. Pittr Jr. and A. Winer of SAPRC-University of California at Riverside. Special acknowledgement is due to the staff of Harvey Mudd College who served as host during the field experiments. We thank B. R. Appel and co-workers of California’s Air Industrial Hygene Lab for an advance copy of their manuscript. This work was supported by the Environmental Sciences Research Laboratory of the U.S. Environmental Protection Agency under Contract No. 68-02-3228.

REFERENCES Appel B. R., Tokiwa Y. and Haik M. (1981) Sampling of nitrates in ambient air. Atmospheric Encironmenr 15, 283-289. Appel B. R.. Wall S. M., Tokiwa Y. and Haik M. (1980) Simultaneous nitric acid, particulate nitrate and acidity measurements in ambient air. Atmospheric Encironment 14, 549-554. Braman R. S., Sheiley T. J. and McClenny W. A. (1981) Tungstic acid for preconcentration and determination of gaseous and particulate ammonium and nitric acid in ambient air. Analyr. Chem. (submitted.) Forrest J., Tanner R. L.. Spandau D., D’Ottavio T. and Newman L. (1980) Determination of total inorganic nitrate utilizing collection of nitric acid on NaCl-impregnated filters. Atmospheric Encironmenr 14, 137-144. Forrest J., Spandau D., Tanner R. and Newman L. (1982) Determination of atmospheric nitrate and nitric acid by a diffusion denuder and filter pack assembly. .&mospheric Environment (submitted.) Grennfelt P. (1980) Investigation of gaseous nitrates in an urban and a rural area. Atmospheric Encironment 14, 311-316. Hanst P. L., Wilson W. E., Jr., Patterson R. K.. Gay B. W., Chaney L. W. and Burton C. S. (1975) A spectroscopic study of California smog. Report EPA-650/4-75-006, U.S. Environmental Protection Agency, Research Triangle Park, NC. Hare R. J., Wininger M. T. and Ross W. D. (1979) Selective collection and measurement of particulate nitrate and gaseous HNOs in ambient air. In Currenr .tferhods IO Measure Atmospheric Nitric Acid and Nitrare Artifacts (Edited by Stevens R. K.) pp. 63-65. Report EPA-600/279-051, U. S. Environmental Protection Agencv, Research

Triangle Park, NC, March. Howes J. E., Spieer C. W., Bishop T. A.,Arnold L. H., Stevens R. K., et al. (1981) Particulate nitrate measurement methods: an intercomparison. (in preparation) Howes J. E., Spicer C. W., Arnold L. H. and Bishop T. A. (198la) Intercomparison of nitric acid and particulate nitrate measurement methods: field measurement data. Battelle-Columbus final report to EPA (Contract 68-023228). February, 1981. Huebert B. J. and Lazrus A. L. (1979) Tropospheric measurements of nitric acid vapor and particulate nitrate. In Nitrogeneous Air Pollutanrs (Edited by Grosjean D.)Ann Arbor Science, Ann Arbor, MI, U.S.A. Joseph D. W. and Spicer C. W. (1978) Chemiluminescence method for atmospheric monitoring of nitric acid and nitrogen oxides. Analyr. Chem. 50, 1400-1403. Kelly T. J., Stedman D. H. and Kok G. L. (1978) Measurements of H,O, and HNO, in rural air. Geophys. Res. Left. 6, 375-378.

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Miller D. F. and Spicer C. H’. (1975) Measurements of mrr~ acid in smog. J. Air Poiiur. Conrrol dss. 25, 940-942. Okita T., lMorimota S., lzawa M. and Konno S. (1976) Measurement of gaseous and particulate nitrate in the atmosphere. d:morpheric Encironmenr 10, 1085-1089. Scott W. E., Stephens E. R.. Hanst P. L. and Doerr R. C. (1957) Further developments in the chemistry of the atmosphere. Proc. Am. Pwol. Inst. 37, 171. Shaw R. W., Dzubay T. G. and Stevens R. K. (1979) The denuder difference experiment. In Current Methods to Measure Armospheric Xirric Acid and .Virrate Artifacts (Edited by Stevens R. K.) pp. 79-81. Report EPA600 2,79/051, U.S. Environmental Protection Agency, Research Triangle Park, NC, March. Shaw R. W., Jr.. Stevens R. K.. Bowermaster J., Tesch J. W. and Tew E. (1982) Measurements of atmospheric nitrate and nitric acid. .4tmospheric Encironmenr (in press.) Spicer C. W. (1977a) The fate of nitrogen oxides in the atmosphere. Adr. Enrir. Szi. Technol. (EdIted by Pitts J. N.. Jr. and Metcalf R. L.) Vol. 7. John Wile). New York. Spicer C. W. (1977b) Photochemical atmospheric pollutants derived from nitrogen oxides. Armospheric Enrironmenr 11, 1089-1095. Spicer C. W. (1978) Measurement of atmospheric nitrogen compounds. Paper presented at 176th Am. Chem. Sot. meeting, Miami Beach, FL, September. Spicer C. W. (1979) Measurement of gaseous nitric acid by chemiluminescence and electrochemistry. In Current Merhods to Xfeasure Almospheric Sirric Acid and ,Vitrare

.ArrQhcis (Edlted by Stetens R. K.) pp. 17-35. Report EPA600,2-79-051, U.S. Environmental P:otection Agency, Research Triangle Park, NC. March. Spicer C. W., Ward G. F. and Gay B. W.. Jr. (1978) A further evaluation of microcoulometry for atmospheric nitric acid monitoring. Analgr. Leri. All, 85-95. Stevens R. K. (1979) Currsnr .Lferhods ro ,Cfeasure .&morpheric Xirric Acid and .Yitrate Ar;$‘acts. Report EPA600:‘2-79-051, US. Environmental Protection Agency, Research Triangle Park, NC. Tesch J. and Sievers R. E. (1979) Selective collection and measurement of particulate nitrate and gaseous nitric acid in ambient air. In Currenr Methods ro Measure Atmospheric .Virrlc Acid and .l’irrore Arrijhcrs. (Edited by Stevens R. K.) pp. 67-77. Report EPA-600 2-79-051. U.S. Environmental Protection Agency, Research Triangle Park, NC. March. Tuazon E. C.. Graham R. A.. Winer A. S-i.. Easton R. R.. Pitts J. N.. Jr. and Hanst P. L. (1978) A kilometer pathiength Fourier-transform infra-red system for the study of trace pollutants in ambient and synthetic atmospheres. .Atmospheric Enuironmrnr 12, 565-875. Tuazon E. C., Winer A. M.. Graham R. .A. and Pitts J. N. Jr. (1980) Atmospheric measurements of trace pollutants bv kilometer pathiength ft-ir spectroscopy. Adc. Emir. SC;. Technol. 10. 259-300 Tuazon E. C., Winer A. M. and Pitts J. NJ. Jr. (1951) Trace pollutant concentrations in a multi-da)- smog episodein the California south coast air basin by long pathlerqh ft-ir spectroscopy. Encir. Sci. Technol. (submitted.)