Envlron. Sci. Technol. 1985, 19, 862-865
NOTES Response of Chemiluminescence NO, Analyzers and Ultraviolet Ozone Analyzers to Organic Air Pollutants Daniel Grosjean * and Jeffrey Harrison Daniel Grosjean and Associates, Inc., 350 N. Lantana Street, Suite 645, Camarillo, California 93010
A chemiluminescent NO, analyzer and an ultraviolet photometry ozone analyzer were found to respond to a number of organic pollutants and mixtures in purified air. Two types of interferences were observed with the NO, analyzer. A small positive interference 88 NO was observed with organosulfur compounds. Positive interferences as NOz were observed with nitric acid, methyl nitrate, peroxyacetyl nitrate, Nz06(in O,-NOz mixtures in the dark), certain nitro aromatics (nitrocresol), and chlorine-containing compounds (ClNO,) but not with peroxybenzoyl nitrate. The ultraviolet photometry ozone analyzer gave a positive response to ozone-free atmospheres containing aromatic compounds including styrene, methylstyrene, o-cresol, and nitrocresol. Since many aromatic compounds absorb light at -254 nm, methods other than UV photometry should be employed to measure ozone in atmospheres containing these aromatics. H
Introduction The gas-phase chemiluminescent reaction of ozone with nitric oxide serves as the basis for virtually all commercial analyzers now employed for measurements of oxides of nitrogen in the atmosphere. In the same way, many analyzers employed for the measurement of atmospheric ozone now employ ultraviolet photometry. Chemiluminescence NO, analyzers and UV photometry O3analyzers are widely used in, for example, routine monitoring of air quality, urban and regional field studies, and smog chamber and laboratory research in atmospheric chemistry. Chemiluminescence NO, analyzers have been shown to respond to a number of nitrogenous pollutants besides NO and NOz. Thus, nearly quantitative response has been reported for nitric acid ( I ) and for ethyl, n-propyl, and peroxyacetyl nitrate (2,3). Interferences due to sulfides and mercaptans (4) and to chlorinated compounds (5) have also been noted. Interferences in UV photometry ozone measurements have received less attention. However, we have previously noted interferences due to o-cresol(6) and t o pyruvic acid (7). In the course of investigations of hydrocarbon photochemistry and organic aerosol formation (6-g), we have examined the response of chemiluminescent NO, and UV photometry O3 analyzers to a number of organic pollutants. The chemiluminescent NO, analyzer responded to 1 2 of the 28 compounds or pollutant mixtures tested, and the ultraviolet ozone photometer responded to 6 of the 16 compounds or mixtures tested in ozone-free air. These results are presented here along with a brief discussion of their implications for laboratory, smog chamber, computer modeIing, and ambient measurement studies. Experimental Section NO, Analyzer. A Thermo Electron Corp. Model 14 862
Environ. Sci. Technol., Vol. 19, No. 9, 1985
B/E analyzer was employed (serial no. 12053-128). Nitric oxide is measured by monitoring the intensity of the light (A I600 nm) emitted by excited NOz produced by reaction of NO with ozone (0, + NO O2 + NOz*, NO2* NOz + hv). Nitrogen dioxide is measured as NO after reduction at -450 "C by using a molybdenum converter. The total NO (initial + reduced NOz) thus measured is termed NO,, and the difference (NO, - NO, electronic subtraction) is termed NOz. Both NO, and NOz signals may thus include a positive contribution of any pollutant reduced to NO by the catalytic converter. The instrument was equipped with an in-line particulate filter (Teflon; 1-pm pore size) and was operated at a flow rate of 0.6 L min-l. Calibrations involved five-point gasphase titrations in the range 0-1 ppm by using National Bureau of Standards traceable NO and NOz (gas cylinder, 50 ppm in Nz, and permeation tube, respectively). The converter efficiency was measured as part of the calibrations and was 10.98. Periodic zero and span checks were conducted with ultrapure air and certified NO gas cylinders, respectively (Scott Marrin; NO = 430 f 20 ppb). Ozone Analyzer. The instrument employed was a Dasibi Model 1003 AH ultraviolet photometer (serial no. 2601) equipped with an in-line particulate filter and operated at a sampling flow rate of 1.6 L min-l. Calibrations involved test atmospheres of ozone (six concentrations) produced by ultraviolet irradiation of pure air by using a Dasibi Model 1008 PC O3 analyzer with a built-in O3 generator. The 1008 PC unit used as a transfer standard was in turn compared periodically to the reference UV photometer maintained by the California Air Resources Board in their El Monte, CA, laboratories. A second ozone analyzer (McMillan) based on the chemiluminescent reaction of ozone and ethylene was also employed in some of the runs, in addition to the UV photometer unit. The two instruments agreed within f10% when sampling side-by-side test ozone atmospheres in purified air. Test Atmospheres. Test atmospheres were prepared by dilution of the organic compound of interest in 4-m3 chambers constructed from FEP 200A Teflon film and containing purified, dry air (dew point -16 to -20 "C at T = 18-25 "C) provided by an Aadco Model 737-14 pure air generator. The purified matrix air contained low levels of oxides of nitrogen (-6-12 ppb) and no detectable amounts of ozone (1ooc 0-5
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a No response to up to 1 ppm of the following compounds: toluene, PAN, biacetyl, PBzN, methyl nitrate, n-propyl nitrate, n butyl nitrate, methanethiol, methyl sulfide, and ethyl sulfide One experiment may include more than one determination. O3 analyzer off scale on 0-1 ppm range following all hydrocarbon injections.
“Also no response for up to 1 ppm of the following compounds: biacetyl, pyruvic acid, benzaldehyde, o-cresol, styrene, P-methylstyrene, toluene, C1-Cs aliphatic aldehydes, and molecular chlorine in the dark. “ay involve more than one determination during the course of a chamber run (typically 3-8 h). See text. -
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(PAN) and peroxybenzoyl nitrate (PBzN) were prepared in situ by sunlight irradiation of aldehyde-chlorine-NO2 mixtures (11) and by irradiation of appropriate hydrocarbon-NO, mixtures, e.g., propene-NO,, biacetyl-NO,, or pyruvic acid-NO, for PAN (7) and toluene-NO, (12) or styrene-NO, for PBzN ( 1 3 . Test atmospheres containing PAN were also prepared by dilution of the output of a photochemical flow reactor as described before (14). Nitric acid was prepared by passing purified air through aqueous solutions of nitric and sulfuric acids or in situ by sunlight irradiation of appropriate organic-NO, mixtures. All other compounds tested were of commercial origin (purity 199% when available) and were employed without further purification. Measurement Methods. All compounds tested for their possible interference in NO, and O3measurements were also monitored by using one or two independent methods. The aromatic hydrocarbons including 0-cresol, the styrenes, and benzaldehyde were measured by gas chromatography (GC) with photoionization detection (6). Benzaldehyde was also determined off-line by liquid chromatography (LC) quantitation of its 2,4-dinitrophenylhydrazone (15). PAN, methyl nitrate, and biacetyl were measured by GC with electron capture detection (14). PAN was also measured off-line by ion chromatography (IC) as acetate following alkaline hydrolysis (14). The organosulfur compounds were measured by flame photometry as described before (8). Nitric acid was measured by IC as nitrate ion following collection on nylon filters (16). IC with conductivity or ultraviolet detection was also employed for the determination of pyruvic acid following collection in water or in dilute KOH impingers (17),of PBzN as benzoate ion following alkaline hydrolysis (18), and of nitrocresols following collection in dilute KOH (19).
Results and Discussion Experimental results are summarized in Table I (NO, analyzer) and Table I1 (0,analyzer) and are described in more detail below for selected compounds. Chemiluminescent NO, Analyzer. Two types of “interferences” were observed. First, a number of nitrogenous pollutants are obviously reduced to NO by the catalytic converter and are therefore measured as NO, (and NO2). Second, several organosulfur compounds are seen as NO; Le., their reaction with ozone in the analyzer cell
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Figure 1. Concentration-time profiles in sunlight irradiation of a mixture of chlorine, acetaldehyde, and nitrogen dioxide in purified air. The “NO,” and “NO2” curves are from the chemiluminescent NO, analyzer. The PAN curve is from electron capture gas chromatography measurements (data points obtained every 15 mln are omitted for clarity), The dark rectangles are for PAN measurements as acetate following alkaline hydrolysis (length = sampling time; height = concentration f la). The “dip” in the NO, curve is observed when inserting a nylon filter, which removes nitric acid but not PAN.
involves chemiluminescence. Of the compounds “registered” as NO2-NO, by the analyzer, many are converted quantitatively to NO by the catalytic converter. These include nitric acid, PAN, and methyl nitrate. For nitric acid, the quantitative response in the range of concentrations tested was established by comparing, upon inserting a nylon filter upstream of the analyzer, the decrease in apparent NO2 concentration to the nylon filter nitrate concentration determined by IC. Our results for nitric acid are in agreement with those of Joseph and Spicer (1). Methyl nitrate has not been studied prior to this work, but higher alkyl nitrates have been shown to be converted to NO in a near quantitative manner on several substrates (2,3). Our results for PAN also agree with, and expand upon, those obtained in previous studies (1, 2). As for nitric acid, quantitative response of the analyzers to PAN (Figure 1)was verified by inserting a trap upstream of the analyzer, in this case a KOH impinger, since PAN decomposes rapidly to acetate in alkaline solutions (14). Of the other compounds tested for their response as “NO2”,PBzN and one nitrocresol isomer, only the latter gave a small but measurable response, 3-11% of its initid concentration. Since nitrocresols are probably lost to some extent to the chamber walls and sampling lines, the response of the analyzer to nitrocresols may be actually higher than that derived from our measurements. In Environ. Sci. Technol., Vol. 19, No. 9, 1985 863
contrast, PBzN, unlike PAN, yielded no response as NO2 in the range of concentrations tested. In experiments yielding only PBzN (e.g., benzaldehyde-Clz-NO2 or styrene-NO,), no decrease in apparent NO2 was observed when inserting a KOH trap upstream of the analyzer. In experiments yielding both PAN and PBzN (e.g., tolueneNO, or @-methylstyrene-NO,), the apparent NO2 decrease after inserting the KOH trap was equivalent, within experimental precision, to that expected from PAN alone as measured independently by GC and/or as acetate. PBzN was recovered as benzoate ion in the alkaline trap, thus indicating that no substantial loss of PBzN occurred on the walls of the Teflon chamber and/or in the sampling line. PBzN loss in the analyzer lines and cell cannot be ruled out. Organosulfur compounds introduced a small positive interference in the NO mode measurements. In addition, a small negative interference was observed in the NO,-NO, mode, thus indicating that the species responsible for chemiluminescent reaction were in part destroyed by the catalytic converter. Our results are in good agreement, both in “direction” and magnitude, with those obtained by Sickles and Wright ( 4 ) with an earlier version of the NO, analyzer (Thermo Electron Model 14B). These authors speculated on the origin of the observed chemiluminescence and examined literature data for SO2*(from O3+ SO), HSO* (from O3 + SH), and OH* (from O3 H, Meinel band). They concluded that, since reaction of O3 with CH3SH and CH3SCH3is reported to produce H atoms but not SH radicals, OH* was probably responsible for the observed chemiluminescence. The response of the chemiluminescent NO, analyzer to N205was not investigated in detail. However, some information was obtained in the course of chamber studies involving N02-03mixtures. Figure 2 shows selected results for a typical experiment (0, = 500 ppb, NO2 = 470 ppb, 4 h in the dark, dry matrix air with dew point of -17 “C at T = 31 “C). The top curves are NO2,N2O5, and HON02 concentrations calculated by using the model of Leone et al. (12). The calculations included chamber loss terms for O3 and NO2 (determined experimentally in control runs), but not for HON02and N205. The bottom curves labeled NO2 + HON02 (calculated) and NO2 + HON02 + N2O5 (also calculated) are therefore upper limits for the actual concentrations since some HON02 and Nz05are lost to the chamber walls. However, the measured NO2 substantially exceeds, in the early part of the experiment, the upper limit curve expected from its response to NO2 and HON02 alone and, in fact, matches well the NO2 + HONOz f N205curve. Since calculated concentrations of NO3 (6 ppb), HONO (3 ppb), and other nitrogenous species that the NO, analyzer may record as NOz are negligible, the measured values may be rationalized in terms of a quantitative response of the instrument to N2OP The increasing spread between measured NOz and calculated curves later in the run is due mostly to loss of nitric acid on the chamber walls. Although not studied in detail, other interferences were observed in the course of photochemical studies involving irradiation of chlorine-NO, and of organosulfur-chlorine mixtures in air. In the C12-N0,-hu system (NO, = 200-4300 ppb, C12 = 100-400 ppb), the NO2 signal decreased during the course of the irradiation to a constant value, typically -70% of the initial NO, concentration. Expected reaction products in this system (14) are nitryl chloride (ClNOJ, nitrosyl chloride (NOC1, which photolyzes rapidly in sunlight), and perhaps other ClNO, species such as ClONOp Our results indicate that the NO, analyzer response to
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Flgure 2. Concentration-time profiles for an experiment with a mixture of ozone and nitrogen dioxide In purified air in the dark. (Top) Calculated NO,, N,O,, and HONO, concentrations. (Bottom) Experimental curve (“NO,”) vs. calculated values of NO, HONOp and NO, HONO, N205.
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these compounds, while
