Environ. Sci. Technol. 1990,24, 1521-1527
Gross, S. B.; Pfitzer, E. A.; Yeager, D. W.; Kehoe, R. A. Toxicol. Appl. Pharmacol. 1975,32,638-651. Steenhout, A,; Pourtois, M. Br. J. Ind. Med. 1981, 38, 297-303.
Somervaille, L. J.; Chettle, D. R.; Scott, M. C.; Tennant, D. R.; McKiernan, M. J.; Skilbeck,A.; Trethowan, W. N. Br. J. Znd. Med. 1988,45, 174-181. Whittmers, L. E., Jr.; Wallgren, A.; Alich, A.; Aufderheide, A. C.; Rapp, G., Jr. Arch. Environ.Health 1988,43,381-391. Shen, G. T.; Boyle, E. A. Earth Planet. Sci. Lett. 1987,82, 289-304.
Patterson, C. C.; Settle, D. M. Mar. Biol. 1977,39,289-295. Elias, R. W.; Hirao, Y.; Patterson, C. C. Geochim. Cosmochim. Acta 1982, 46, 2561-2580. Nriagu, J. 0.;Pacnya, J. M. Nature (London) 1988,333, 134-139.
Lyons, K. J. M.S. Thesis, University of California, Santa Cruz, 1989.
Estes, J. A,; Jameson, R. J.; Johnson, A. M. In The WorldwideFurbearer ConferenceProceedings; Chapman, J. A., Pursley, D., Eds.; Worldwide Furbearer Conference, Frostburg, MD, 1981; pp 606-641. Simenstad, C. A.; Isakson, J. S.; Nakatani, R. E. In The Environment of Amchitka Island; TID-26712;Merritt, M. L., Fuller, R. G., Eds.; NTIS, U.S. Dept. of Commerce:
Springfield, VA, 1977; pp 451-492. (33) Kay, R. W.; Sun, S. S.; Lee-Hu,C. N. Geochim. Cosmochim. Acta 1978, 42, 263-273. (34) Favorite, F.; Dodimead, A. J.; Nasu, K. Int. North Pacific Fish. Comm. Bull. 1976, 33, 187. (35) Bevington, P. R. Data Reduction and Error Analysis for the Physical Sciences; McGraw-Hill: New York, 1969. (36) Church, S. E. Earth Planet. Sci. Lett. 1976,29, 175-188. (37) Needleman, H. L.; Gunnoe, C.; Leviton, A.; Reed, R.; Peresie, H.; Maher, C.; Barrett, P. N. Engl. J. Med. 1979, 300, 689-695. (38) National Academy of Sciences Lead in the Human Environment;National Academy Press: Washington,DC, 1980. (39) Patterson, C. C.; Shirahata, H.; Ericson, J. E. Sci. Total Environ. 1987, 61, 167-200. (40) Shirahata, H.; Elias, R. W.; Patterson, C. C.; Koide, M. Geochim. Cosmochim. Acta 1980,44, 149-162. (41) Hart, S. R. Nature (London)1984, 309,753-757. Received for review January 16,1990. Revised manuscript received May 7,1990. Accepted May 30,1990. This research was supported by the NSF (OCE-H612113), University of California Institute of Geophysics and Planetary Physics, the University of California Toxics Substances Research and Teaching Rogram, and the Minerals Management Service.
Measurements of Nitrous Acid inside Two Research Houses Michael Brauer," P. Barry Ryan, Helen H. Suh, Petros Koutrakls, and John D. Spengler Department of Environmental Health, Harvard University, School of Public Health, 665 Huntington Avenue, Boston, Massachusetts 02 115
Nell P. Leslie Chamberlain GARD, 7449 North Natchez Avenue, Niles, Illinois 60648-3892
Irwin H. Bllllck Gas Research Institute, 8600 West Bryn Mawr Avenue, Chicago, Illinois 60631
Continuous analyzers for NO, NO2, and HONO were used to study the production and decay of these gases in two indoor air quality research houses, using unvented gas space heaters and ranges as combustion sources. In agreement with previous studies, indoor HONO concentrations were elevated during unvented combustion. Peak (15-min) levels up to 100 ppb HONO and 24-h averages as high as 40 ppb were measured. The observed kinetics suggest the secondary formation of HONO, possibly as a result of heterogeneous reactions involving NOz, in addition to primary production of HONO during combustion. Introduction The production of nitrogen dioxide and other nitrogen oxides in combustion processes has for some time been recognized as a potential indoor air quality problem. Research efforts have indicated that concentrations of NO2 indoors sometimes exceed outdoor concentrations in environments where unvented gas heating the cooking appliances are operating (1-6). Indoor concentrations are a function of both (indoor and outdoor) source and removal processes, such as air exchange, or chemical reactions. Several recent laboratory investigations have identified surface materials commonly present indoors that react with NO2to reduce concentrations (7-10).These studies have primarily been limited to measurements of the gases NOz and nitric oxide, and of nitrite and nitrate ions on the materials. One possible gaseous product of NOz reactivity 0013-936X/90/0924-1521$02.50/0
is nitrous acid. Preliminary results of one of these studies detected significant amounts of nitrous acid, probably resulting from heterogeneous reactions involving NO2and the surface material (9). Although little information exists with respect to HONO toxicity and typical indoor concentrations, HONO has been well studied as a reactant in photochemical smog production (11,12). Photolysis of HONO (310 nm < X < 390 nm) has been recognized as a major source of hydroxyl radical in the early morning hours (13-15). Outdoor (15min average) HONO concentrations of 0.03-15.0 ppb have been measured, with the highest levels measured during predawn hours in heavily polluted urban areas. Typical outdoor concentrations in urban areas peak a t less than 5 ppb (11, 15-18). An important formation pathway is suspected to be the heterogeneous reaction of NO2 with water to produce both HNOBand HONO (19,20). 2N02 + H20 HONO + HN03 (1) Since a heterogeneous reaction mechanism for HONO production was thought to dominate in smog chambers and reaction vessel studies (21-231, Pitts and co-workers conjectured that NO2to HONO conversion would also occur in typical indoor environments, particularly in circumstances where indoor combustion appliances generated significant concentrations of NO2. In a preliminary study in a mobile laboratory, NOz was injected into the laboratory air and HONO concentrations increased with firstorder kinetics with respect to NO2,indicating the potential
0 1990 American Chemical Society
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Environ. Sci. Technol., Vol. 24, No. 10, 1990
1521
for HONO production in homes (24). Several follow-up studies measured peak concentrations of 30-60 ppb HONO in a mobile home when the combustion source was a conventional gas stove producing NO2 concentrations of 0.3-1.2 ppm (16, 25). Although simultaneous outdoor HONO levels were not recorded during these studies, the measured indoor values were well above even the highest previously measured outdoor concentrations. Interestingly, the rate of HONO production appeared to be higher when NOz was generated by combustion than if pure NO2 was injected into the mobile laboratory. These authors suggested that the elevated HONO concentrations found during gas combustion may be associated in part with direct emission of HONO from the combustion flame (25). Evidence for HONO formation in occupied residences comes from a personal monitoring study conducted by our group (26). In these measurements, 1-5 ppb HONO (approximately 12-h average) was detected indoors during the summer in homes without unvented gas combustion sources. Outdoor (24-h average) HONO concentrations were 0.5-1.5 ppb. We hypothesized that the HONO detected in these homes was due to the penetration of NO, species (outdoor NOz concentrations were 5-30 ppb), which reacted indoors to produce HONO. Due to the absence of unvented combustion sources inside these homes, these concentrations may represent a lower limit to indoor HONO concentrations. In a subsequent study, 24-h average HONO concentrations as high as 14 ppb were measured during the winter in homes using gas stoves for cooking (27). In these studies, as well as in the most recent work of Pitts and co-workers (25),nitric acid, a product of reaction 1, was also sampled although rarely detected indoors. The detection limit for HN03 by FTIR (approximately 10-min average) in the study of Pith et al. (25) was 12 ppb, while in the annular denuder studies conducted by our group the detection limits for 24-h integrated sampling were 0.3 (26) and 0.1 (27) ppb. The apparent absence of gas-phase HN03production is likely due to the high reactivity of HN03 with surfaces. In this paper we describe measurements of HONO and NOz in two research homes. Our objective was to measure HONO production in well-characterized indoor environments with a variety of unvented gas combustion appliances. Using the collected data, we then sought to estimate HONO production rates with a single-compartmental box model. An additional objective of this work was to demonstrate the applicability of annular denuder samplers to integrated and continuous HONO measurements in indoor air.
Methods Sampling Conditions. Sampling was conducted during a 7-day period in the spring at an indoor air quality test home in Chicago, IL, operated by Chamberlain GARD. The house, built in 1957, is a one-story, three-bedroom dwelling with an unfinished basement. During sampling, the door to the basement was closed, while all other interior doors remained open. The wall surfaces were painted plaster. Only the living room area of this furnished house was carpeted. Other floor surfaces were linoleum, ceramic tile, or hardwood. The interior house volume was approximately 473 m3. Experiments using both an unvented gas range and two different space heaters were conducted. A second set of measurements were made in a research house operated by GEOMET Technologies in Gaithersburg, MD. This house, built in 1982, is of a bilevel design with an interior volume of approximately 637 m3. Wall surfaces were painted gypsum board drywall. The living room, hallways, and each of the three bedrooms were 1522
Environ. Sci. Technol., Vol. 24, No. 10, 1990
Table I. Test Conditions during Measurements at Chicago and Maryland Research Homes
source expt operation hours B G1 G2 G3 G4
R1-2 C1-2 B1-2 G1 G2 G3 G4
gas combustion source
Chicago Research Home no source: background conditions gas range? one burner, high setting gas ovenb 350 O F gas range: one burner, high setting gas range: one burner, high setting + gas oven 350 O F 0900-1300 radiant space heatere 0900-1300 convective space heaterd no source 0900-1000 1600-1700 0900-1000 1600-1700
Maryland Research Home no source: background conditions 0900-1000 gas range: one burner, high setting 1600-1800 gas range: one burner, high setting 0900-1000 gas range: one burner, high setting 1600-1800 gas range: one burner, high setting
no source
"9200 Btu h-l. Pilotless spark ignition. Range hood not operated during tests. 13800 Btu h-l. Pilotless spark ignition with infrared burner in top of oven. Range hood not operated during tests. 15 100 Btu h-l. Combustion on surface of convectively heated ceramic tile. 11050 Btu h-l. Blue flame with ribbon-type burner.
carpeted in this furnished house. No space heater sources were operated in the Maryland house since the experiments were conducted during the summer. The test conditions are outlined in Table I. In both houses, interior lighting was by diffuse sunlight during the day and by incandescent lights during evening hours. Previous work has suggested that these conditions apparently do not produce any noticeable HONO photolysis (25). For all gas range experiments, a stainless steel pot filled with water was placed on the stove. NO, NO,, and NOz were measured by using chemiluminescent detectors [Thermo Electron 14BE (Chicago home, C), CSI 1600 (Maryland home, M)]. Relative humidity, measured by thin-film capacitance [Vaisala HMP l l l Y (C, M)], and temperature, measured with thermistors [Omega 700 Series (C)] or relative temperature difference [Vaisala HMP l l l Y (M)], were monitored continuously throughout the study. Air-exchange rates were monitored by the measured decay of injected sulfur hexafluoride concentrations. Concentrations of this tracer gas were determined by gas chromatography/electron-capture detection [Thermo Electron 621A (C), S-Cubed 215 (M)]. CO, measured by nondispersive infrared spectroscopy [Horiba APMA 300E (C), Beckmann H65 (M)], was used to determine air-exchange rates in the absence of SF, data. During sampling, the forced air heating/cooling system and all mixing fans were turned off and all windows and outside doors were kept closed. For the Chicago experiments, the house was flushed with outdoor air during the final hour of the 24-h sampling period prior to all space heater experiments. In the Maryland house, internal mixing fans were operated for 0.5 h immediately before the second burn of each day, and again for 0.5 h, 5 h prior the first burn. Outdoor samples (for all pollutants except continuous HONO) were collected throughout the study. Heaters were operated in the living room, while the range and oven were in the adjacent kitchen. All indoor sampling ports for the continuous analyzers and all denuders were located in the living room, with additional indoor sampling inlets in an adjacent bedroom. Except as noted, all data refer to samples collected in the living room. Continuous HONO Measurements. Continuous HONO was monitored by a chemiluminescent NO, mon-
INLET
A
4
TEFLONFILTERS
REACTION CHAMBER
PHOTOMULTIPUER
-
L DATA ACQUISmON
Flgun 1. Schematic of modlRed chemlumlnescence m onm used to measure HONO. Sample llne A collects total NO,. Sample llne B Collects NO. - HONO - HNO,. Subtraction of line B signal from that Of line A provldes measurement of HONO (provided that no HNO, Is present In sample).
itor to which a NaZCO3denuder was attached to one of two colocated sampling inlets. The modified monitor is shown schematically in Figure 1. Similar measurement systems have been utilized by others for HNO, as well as HONO (13, 28-32). The denuder efficiently strips HNO, and HONO from the airstream, while quantitatively transmitting NOz and NO into the chemiluminescent analyzer. The colocated sampling inlet collects all NO, species (NO, NO,, HNO,, and HONO). Assuming negligible interference from HNO,, as is the case for these data (Figure 2), the difference between the measurements of the two samples provides a continuous HONO reading. To allow for appropriate signal averaging and switching from sampling lines, the "continuous" HONO measurement is the difference between the average of two 2.5-min samples (NO, - [NO, - HONO]). Each of these HONO measurements are separated by 15-min intervals. Annular Denuder Measurements. SOz, HNO,, and HONO were sampled with annular denuders (Tefloncoated glass inlet/impactor followed hy two Na2COrcoated denuders, University Research Glassware, 3000 Series) operated at 10 L min-I(33). Ion chromatographic (Dionex 2000i) analysis of the denuder extracts followed sampling. The inlet/denuder system has been shown to collect gaseous acid species efficiently and reliably, with negligible inlet losses (34). Sampling and analysis procedures for the denuders have been described previously (33-36). The denuders, the only noncontinuous samplers used, were operated indoors for sequential 1-, 2-, 4-, 9-, or 16-h periods, depending on the combustion source. Twenty-four hour indoor (living room and adjacent hedrc"/basement) and outdoor annular denuder samples were also collected throughout the study. Outdoors, annular denuder measurements were for 24 h at the Chicago location and for 12 h (0800-2000 and 2000-0800 h) at the Maryland location. Detection limits, based on the sensitivity of the ion chromatographic analysis and the 10 L min" sampling rate, were 1.0, 1.0, and 1.7 ppb.m3 for SOz, HNO,, and
32 30 28 28
24
22 20
01 02 09W-1100
01 02 llW-13M1
D1 M 130015W
D1 D2 15001700
01 D2 17M)-o9W
D1 M NOxl Nor2 G9000900 09000900 OUTDOOR
DENUDER SAMPLE (SAMPLING DuwnoN)
F&a 2. Annular denuder measuemmtsfa ramnt heater expdment R1 in Chicaw researdl home. D1 and D2 Mmote Rrrt and second denuders, r~Specthreiy.In sampling traln. All samples were collected In lblng rwm. except as noted. NOxl was a 2 4 4 indoor sample (single denuder only). N0x2 was a 24-h indoor sample collected in a bedroom. Environ. Sci. Technol., Vol. 24, No. 10, 1990
1523
w ,
1 b
"-1 70
3al CHEMILUMINESCENCE = 0 82 x ANNUUR DENUDER
280 -
R'=086
m240 -
-
HEATER ON
f
I I
._ PREDICTED
40 f
1
0
2
0
3
0
4
0
5
3
5
3
7
0
8
0
9
0
1 2
3
4
5
6
7
8
HONO (PPB) BY ANNULAR DENUDER
HONO, respectively. To determine the detection limit for a given sample, the ppb.m3 values were divided by the sample volume (m3) of interest. For the majority of the overnight (9-12-h duration) samples, HONO (as NOz-) was detected on the second denuder, in some cases, at levels greater than those found on the first denuder. This substantial "breakthrough" of HONO was not found for any of the shorter duration samples, even at much higher HONO concentrations. This behavior suggested a depletion of collection capacity. Subsequent analysis of breakthrough as a function of the total amount (micrograms) of HONO collected on the two denuders, indicated that a strict depletion of denuder capacity by collected HONO could not account for the observed results. Therefore, we believe that collected HONO was displaced from the first denuder, as described by Perrino and colleagues (28). These investigators showed that HONO collected on a Na2C03-coateddenuder may be subsequently released from the denuder (as HONO) by exposing the denuder to purified air. Further, this displacement of collected HONO is increased by exposing the denuder to an acidic gas such as SOz. It also was possible that some other (acidic) species acted to displace collected HONO, or that HONO was formed heterogeneously within the denuders by reactions involving NO2 (37). For these reasons, for the overnight samples only, HONO concentrations were calculated by combining the amount of NO; collected on the first and second denuders. For the measurements conducted in the Maryland house, three NazC03-coateddenuders were connected in series for overnight samples. Although substantial NOz- was found on the second denuder, little was found on the third, justifying the summation of the amount of NOz- collected on the first and second denuders for samples where displacement was evident. NO2 Interference in HONO Denuder Measurements. The observation that no substantial amounts of NO2- (from HONO and NOz) were detected on the second of the two denuders coupled in series (less than 5% of the NO, measured on the corresponding first denuder), except for the overnight samples where displacement apparently occurred, indicated that NO2 was not collected to any significant degree by the NazC03-coateddenuders. Furthermore, even at extremely high levels of NOz (>1ppm), no significant amount of NO2- was detected on the second denuder. These findings are in agreement with published reports evaluating the collection of NO2 and HONO on NazC03-coated denuders (37, 38) and filters (29). Using the formula described by Perrino et al., we estimated the NO2 interference in HONO concentrations to be 1.8% (of 1524
Environ. Sci. Technol., Vol. 24, No. 10, 1990
How
0
0
Flgure 3. Comparison of HONO measurements by chemiluminescence difference and annular denuder integrated samples.
NO2
20
I 0
+
I
i
DATA(N=BO)
0 10 11 12 13 14 15 16 17 18 1Q XI 21 P 23 24 HOUR
Flgure 4. Measured indoor (living room) NO, and HONO concentrations for radiant heater experiment R1 in Chicago research home. The sdid line denotes the 24-h average HONO concentration of 19 ppb. The dashed line denotes the 24-h average NO2 concentration of 69 ppb.
30-1
25 *
gP
15
II
I
-
5 -
MmuredHONO ModeiHONO
1
I 0
1
2
3
4
5
6
7
8
g 10 11 12 13 14 15 16 17 18 19 20 21 2 .
23 24
Hour
Flgure 5. Measured and modeled indoor (bedroom) HONO concentrations for radiant heater experiment R1 in Chicago research home. The model parameters are those listed in Table 111. The heater was operated in the living room 0900-1300 h.
the HONO concentration) for the sampling conditions of this study (28). A comparison between the HONO concentrations determined with the annular denuder integrated average samples and the averages of the chemiluminescence difference readings over the same sampling interval is shown in Figure 3. The high correlation coefficient and proximity of the slope to 1 suggests that both methods were measuring the same compound, presumably HONO. Additionally, the denuder method was recently compared with differential optical absorption spectroscopy (DOAS) measurements of HONO during ambient measurements in Los Angeles (39). Denuder measurements were an average of 7% lower than corresponding DOAS readings, except for several midday readings in which denuder measurements of approximately 2-4 ppb were recorded when DOAS measurements were below the detection limit of 0.8 ppb. The authors postulated that these anomalously high denuder measurements were the result of the collection of an unknown pollutant that yields NOz- in the alkaline denuder extract (39). The response of the chemiluminescent monitor to HONO has also been investigated by Sickles and Hodson, who found good agreement between the chemiluminescence method and a filter pack method (29). The agreement between the denuder and chemiluminescence HONO measurements found in our study also indicates that losses of HONO on Teflon sampling lines are not appreciable,
Table 11. Temperature and Relative Humidity Measurements at Chicago and Maryland Research Homes
location
mean
Chicago Maryland
temp, "C range SD
18.9 f 2.0 27.5 f 1.6
12.3-22.3 24.3-32.2
Table IV. Parameter Fits for One-Compartment Model Using Data Collected in Maryland Research Home"
% RH
mean
* SD
32.3 f 5.8 55.0 f 2.5
23.5-50.9 51.6-64.3
B2
G1
Table 111. Parameter Fits for One-Compartment Model Using Data Collected in Chicago Research Home"
expt
G2 G3 G4
R1 R2
C1 C2
N
source strength SeRf V, ppb h-'
E,,,, ppb
0.30
Background 34
1.31
0.59
burn decay burn decay burn decay burn decay
0.35 0.35 0.35 0.25 0.24 0.25 0.24 0.25
Gas Range 5 23 5 63 5 23 5 63
18.25 2.48 12.71 1.09 19.75 1.20 51.87 1.76
3.19 1.38 0.54 0.93 1.89 1.56 5.33 2.09
burn decay burn decay
0.20 0.20 0.22 0.22
Radiant 17 79 17 77
10.81 2.04 12.75 2.65
1.00 1.94 1.73 3.08
burn decay burn decay
0.24 0.24 0.26 0.27
Convective 17 79 17 43
26.71 3.73 32.47 7.35
9.02 4.91 4.60 4.40
B G1
air-exchange rate R, h-l
" N is the number of observations, after the data are smoothed. E,, is the root mean square error of the model fit. Burn refers to the period during which the combustion source was operated. Decay is the period following source operation until the beginning of the next burn period. as roughly 25 m of Teflon tubing separated the sampling inlets from the chemiluminescent analyzer. Our results indicate that chemiluminescent measurements of NO2 determined by the difference between NO, and NO measurements will be overestimations if HONO (or HN03) is present. After determining that HN03 was not a significant interferent in these environments, we used only NazC03-coateddenuders. For all NO2 measurements we used concentrations determined by chemiluminescenceas [NO, - HONO - HN03] - [NO]. The agreement between the annular denuder measurements and the chemiluminescence difference measurements enabled us to use the continuous HONO and NO2data for the emission strength calculations. Comparison of the chemiluminescent measurements of NO with and without a denuder connected in-line, showed us that the Na2C03-coateddenuders do not collect any NO, as expected.
Results and Discussion Temperature, Relative Humidity, and Air-Exchange Measurements. Temperature and relative humidity measurements are summarized in Table 11. Conditions at the Maryland location were considerablywarmer and more humid than a t the Chicago home. Measured air-exchange rates are shown in Tables I11 and IV. Annular Denuder Measurements. Figure 2 depicts the gas concentrations determined by the annular denuder measurements for one experiment in the Chicago house in which a radiant heater was operated (experiment Rl). Indoor concentrations of SO2were below 1ppb during the
air-exchange rate R, h-'
expt
range
B2
G3 G4
burn decay burn decay burn decay burn decay
N
source strength S,R/V, ppb h-'
0.13
Background 94
0.05
0.74
0.12 0.12 0.13 0.13 0.12 0.12 0.12 0.12
Gas Range 7 22 10 57 5 23 10 48
9.78 0.00 11.56 0.00 17.98 0.00 17.75
2.01 1.69 1.33 2.40 4.85 1.22 5.19 3.71
0.00
" N is the number of observations, after the data were smoothed. E,, is the root mean squared error of the model fit. Burn refers to the period during which the combustion source was operated. Decay is the period following source operation until the beginning of the next burn period. Parameters were not estimated for experiment B1 due to measurement fluctuations. Table V. Measured NOz and HONO Concentrations in Chicago Research Home expt B
G1, G2 G3, G4 R1 R2
c1 c2
NO2 concn, ppb peak 24-hav 29 115 198 291 275 890 1020
17 27
45 69 66 187 231
HONO concn, ppb peak 24-h av 8
26 44 34
5 9 17
40
19 23
105 106
41 42
HONO/NOZ0 0.15-0.4 0.1 0.1 0.1-0.15 0.1-0.15 0.084.1 0.084.1
" Approximate molar HONO/N02 ratio at peak NOz concentration. Equilibrium not achieved. entire study, while outdoor concentrations reached 8 ppb on one day. Likewise, both indoor and outdoor levels of HN03 were low (C1 ppb) or below the detectable limits of 1.0 ppb.m3. Outdoor concentrations of HONO were, in all cases, lower than indoor levels, and ranged from 0.3 to 1.6 ppb HONO. Note that the outdoor measurements were all collected over 24 h. Since HONO is subject to rapid photolysis, it is likely to be detected outdoors only during the night. Assuming 10 h of darkness per day, nighttime 10-h average HONO concentrations as high as 3.5 ppb may have been present outdoors. Indoor HONO 24-h average concentrations were 2-4 ppb on background days when no combustion appliances were operated. Analysis of gas concentrations measured in the Maryland house yielded similar results. Indoor concentrations of SOz were generally below detection limits, while a peak outdoor concentration of 7 ppb was reached for a 12-h daytime period. Both indoor and outdoor levels of HNO, were also low, although outdoor HNO, concentrations above 2.5 ppb were observed in 12-h daytime samples. Indoor HN03 was below the detection limit in most samples, with the highest measured value being 0.8 ppb for a 4-h period. As expected, outdoor HONO concentrations were lower than indoors. Outdoors, HONO samples were collected separately during the day (0.10.22 ppb HONO measured) and at night (0.26-0.90 ppb HONO measured), although levels were low in all cases. Indoor HONO 24-h average concentrations were 2-4 ppb on (background)days when no combustion sources were operated. Continuous Measurements of NO2,NO, and HONO. Tables V and VI report the HONO and NO2 concentrations measured during the experiments using the modified chemiluminescence method. Consistent with results reEnviron. Sci. Technol., Vol. 24, NO. 10, 1990 1525
Table VI. Measured NO2 and HONO Concentrations in Maryland Research Home expt B1 B2 G1, G2 G3, G4
NOz concn, ppb peak 24-h av 5.5 4.5 37 37
HONO concn, ppb peak 24-hav
2.2
4.0
1.4 6.9 9.1
3.0 33 29
3.8 2.9 8.1 11
HONO/NOPn
0.1-0.12 0.1-0.12
Approximate molar HONO/N02 ratio at peak NOz Concentration. Eauilibrium not achieved.
ported elsewhere, HONO concentrations indoors exceed those outdoors during background sampling days, when no unvented combustion sources were operating (26,271. During the background sampling days, both the NO, and HONO levels fluctuated greatly, possibly as a result of infiltration of NO, from outdoors. Figure 4 displays the measured NO, and HONO concentrations during one of the radiant heater experiments (Rl). Both HONO and NO2 concentrations increased on days in which gas ranges or space heaters were operated. Typical of all the combustion experiments, HONO concentrations increased shortly after combustion began. In all of the experiments, the rate of decay of the HONO concentration was less than the air-exchange rate and also less than the NOz decay rate. In several instances, the HONO levels even exceeded the NO, concentrations, 8 h after the source had been turned off. Furthermore, the NOz decay rate was greater than the air-exchange rate. These observations are indicative of NO, reactive decay, which may be related to HONO production. This will be discussed in greater detail in the following section. Comparison of the HONO concentrations when the radiant and convective heaters were operating allowed us to examine the effect of NO on HONO production. When the radiant heater was operated, a relatively constant and low level of NO (peak concentration of 40 ppb during operation) was measured. In contrast, when the convective heater was operated, NO concentrations were only slightly below those of NO,. (Gas range combustion resulted in NO concentrations that were 3-4 times the NOz levels.) Since radiant heater operation produced HONO, under conditions of a low and constant NO concentration, the results suggested that HONO production was not dependent upon NO levels. This observation confirms those of previous investigations in which HONO production was also found to be largely independent of NO concentration (20, 24). HONO/NOZ ratios were calculated for the peak NO, concentration during the different sampling conditions. As a steady state was not achieved, these ratios do not reflect equilibrium conditions. As shown in Tables V and VI, these ratios were generally on the order of 0.1-0.1, which is greater than the reported ambient ratios, but in agreement with the results of Pitts and colleagues (25). The mean outdoor ratio of 24-h averages of HONO and NOz was 0.034 (standard deviation 0.012) for the Chicago data and 0.028 (standard deviation 0.011) for the daytime samples and 0.051 (standard deviation 0.023) for the night samples a t the Maryland location. The decrease in the ratio for the daytime samples can be attributed to HONO photolysis. These outdoor ratios are in good agreement with the nighttime ratios of 0.05 reported for the Los Angeles area (16) and for measurements in West Germany (40).
Estimation of Nitrous Acid Emission Strength. This series of experiments was designed to improve understanding of HONO concentrations in the indoor en1526
Environ. Sci. Technol., Vol. 24, No. 10, 1990
vironment. Accordingly, samples were collected every 15 min, a period of time adequate to ascertain general time trends in HONO concentrations under various appliance-use scenarios. The data allow estimation of the effective source strength associated with combustion appliance operation if certain assumptions are made. In this section we discuss this procedure and present estimates of the source strength. Assuming that the residences can be approximated as a single, well-mixed compartment, the time-dependent pollution concentration ( C ( t ) )is given by
where Seffis the effective source emission strength (ppb h-l), V the mixing volume (m3),R the air-exchange rate (h-l), and C, the initial concentration of pollutant (ppb). Using eq 2, one can estimate the value of SefPSeveral other assumptions are implicit in this equation. First, air-exchange rates are assumed to be constant over the period of interest (up to 9 h). This assumption was found to be valid for the research homes monitored. Second, S,, is assumed constant over the time period appropriate for the estimation. Seflcontains several components including the following: infiltration of HONO from outdoors, direct emission of HONO from the combustion appliance, and secondary production of HONO through heterogeneous and/or homogeneous mechanisms. Of these three components, the first two are likely to be approximately constant during the time periods of measurement. The third, which is likely to depend on other pollutants, such as NOz, may vary during time periods of interest. This will introduce error in the estimation. Finally, reactive losses of HONO are assumed to be zero. This may be a poor assumption. However, if one assumes no reactive loss, the estimate results in a lower bound for SefP Removal of HONO by mechanisms not included in the model must be compensated for by a concomitant increase in SefP The estimation procedure was performed in a stepwise fashion. For each appliance operation cycle, three different estimates of Sefiwere made: prior to the operation of the appliance (from midnight to the beginning of appliance operation), during the appliance operation, and the decay period following appliance operation (until midnight). Different values for air-exchange rates, determined from SF, or CO measurements, were used during each of the three time periods. A typical fit of the data is shown in Figure 5. Estimated source strengths are presented in Tables I11 and IV. Direct emission of HONO is suggested by the large source strengths estimated during the burn periods relative to those estimated for the decay periods. Furthermore, these source emission rates are dependent upon the type of combustion source in operation. Although no corrections have been made for fuel input, these results suggest the influence of appliance characteristics, such as flame temperature, on HONO emission rates. For example, the emission rates determined for convective heater burn periods are greater than those of the radiant heater, gas range, or oven burn periods. Additionally, when the gas range and oven were operated simultaneously (experiment G4),the source rate was higher than when either appliance was operated individually. Secondary emissions can be inferred from the background experiment data in both homes and from the decay data in the Chicago home. While the decay period emission rates are significantly smaller than the burn period emission rates, they are too large to be explained by in-
filtration of outdoor air. Furthermore, the stability of the decay period emission rates between the separate experiments suggests a mechanism of HONO generation that is independent of the combustion source. It is also apparent that the decay period emission rates differ between the two research homes, with the Chicago house rates larger than those estimated from the Maryland house. This result may indicate the importance of indoor surface characteristics in HONO production.
Conclusion Here we report the measurement of HONO in residential buildings in concentrations that exceed concurrently measured outdoor concentrations. Indoors, HONO concentrations were found to be related to combustion although no specific chemical mechanisms are proposed. Through a single-compartmental box model we estimate that indoor HONO production is via both fast (
