Environ. Sci. Techno/, lSS3, 27, 841-845
Nitrous Acid in Albuquerque, New Mexico, Homes John D. Spengler' and Michael Brauert
Department of Environmental Health, Harvard School of Public Health, 665 Huntington Avenue, Boston, Massachusetts 02 115 Jonathan M. Samet and William E. Lambert
Departments of Medicine and of Family and Community Medicine, New Mexico Tumor Registry, Cancer Center, university of New Mexico Medical Center, 900 Camlno d e Salud, N.E., Albuquerque, New Mexico 87131 Experimental studies have shown that nitrogen acid species,particularly nitrous acid, are formed indoors during unvented combustion and by heterogeneous reactions of nitrogen dioxide. Limited measurements support the occurrence of nitrous acid production in occupied homes. We report additional measurements of HONO and NO2 in homes located in Albuquerque, NM, and assess the relationship with housing variables. Indoor HONO concentrations were found to be well correlated with indoor NO2 levels; HONO concentrations ranged from 5 % to 15% of the measured NO2 concentrations. Given the correlation between HONO and NO2 in indoor environments, and the plausibility of HONO respiratory toxicity, investigations of respiratory health effects of unvented combustion should consider HONO, in addition to NOz, as a potentially hazardous indoor pollutant.
As part of an ongoing study of the relationship between residential nitrogen oxide exposure and respiratory infections in over 1000 infants, we monitored the HONO and "03 levels in 10 homes in Albuquerque, NM, with gas cooking stoves and winter average bedroom NO:! concentrations above 36 ppb (9). Our objective was to characterize the residential concentrations of HONO and HN03 acidic species in relation to NO2 concentrations in these southwestern homes. Materials and Methods
t Current address: Department of Medicine, University of British Columbia, 2206 East Mall, 3rdFloor, Vancouver,BC, V5Z 355 Canada.
Selection of Homes. The 10 homes selected for monitoring were chosen from those having gas cooking ranges. All homes had nonsmoking occupants. NO2 monitoring data from the bedrooms of these homes were collected using passive diffusion samplers (10). Consecutive integrated measurements of 2-week duration were performed sequentially in these homes. The measurements made in October, November, and December 1989 were used to select households for the acid monitoring. Homes from the upper decile of the NO2 distribution were selected for monitoring based on availability during the study period. Mean bedroom NO2 levels in the 526 homes, comprising the active cohort of gas range homes at the time, tended to be log normally distributed with a range from 2.5 to 218 ppb and a mean NO2 of 19.8 ppb (SD = 16.0). The upper 10% of the distribution from which the 10 homes were selected for acid monitoring had 3-month averages exceeding 36 ppb. Home Monitoring Protocols. All of the homes were monitored for a single 24-h period during the week of February 5-10, 1990. A filter and denuder system for sampling acid species was set up in the activity room (e.g., family room or living room) of each home. Pairs of passive NO2 diffusion samplers were placed in the bedroom, kitchen, and activity room. Whole-house NO2 concentrations were derived by averaging these measurements (unweighed by room volume). During monitoring, the parents maintained a written log of stove and furnace use. At the beginning of each monitoring period, a "test burn" was conducted. One burner on the stove top was turned to medium-high setting to heat a quart-sized saucepan half filled with water; the burner was operated for 20 min. No other cooking was in progress at the time of these test burns. Families continued to operate their ranges as required during the monitoring period. Acidic aerosols and gases and NH3 were collected with impactor-annular denuder-filter pack sampling systems. A detailed description of the Harvard-EPA annular denuder system (HEADS) and associated analytical techniques is provided elsewhere (11-14). Briefly, the sample enters the system at a flow rate of 10L/min through a glass inlet-impactor, which removes coarse particles ( d , < 2.1 pm). Next, the air passes through a series of
0 1993 American Chemical Society
Environ. Scl. Technol.. Vol. 27, No. 5, 1993 841
Introduction
The formation of nitrogen acid species in indoor environments has been documented ( I ) , but data on their occurrence in homes were not reported until recently (2). Nitrous (HONO) and nitric acid ("03) may be formed in the gas phase during combustion and by heterogeneous hydrolysis of nitrogen dioxide (NO21 (1,3,4). The rate of formation of nitrogen acids, particularly HONO, is expected to vary with the NO2 concentration, humidity and temperature, light intensity, and the various surfaces present in a home. Pitts et al. ( 1 ) estimated that levels of 15 ppb HONO could be present under typical home conditions. Brauer et al. (2) found that HONO concentrations were greater indoors than outdoors for 10 homes in Massachusetts monitored during the summer and winter. Indoor HONO concentrations were greater than outdoor concentrations even in homes without unvented combustion appliances, suggesting heterogeneous conversion of NO2 that has infiltrated from outdoors. Measurements in research houses and in a mobile laboratory show that elevated indoor HONO concentrations result from unvented indoor combustion (5-7). Spicer et al. (8) demonstrated that gas ranges and heaters are sources of HONO(g) emissions. However, their controlled chamber studies suggest that production of HONO by reaction of NO2 with surfaces is an important source of gaseous HONO indoors. Even under the circumstance where the chamber was purged and sources were turned off, the HONO(g) evolved from HONO(aq) (8). Because HONO is an acidic vapor which may be present in high concentrations indoors, HONO should be considered a respiratory irritant. To date, no studies of HONO respiratory toxicity have been performed, and human exposure data are limited.
0013-936X/93/0927-0841$04.00/0
Table I. Descriptive Statistics
mean NOz (ppb) HONO (ppb) 3" (ppb) NOa- (nmol/m3) SO42- (nmol/m?) NH4+ (nmol/m3) range use (min) floor area (ft2)
60.1 4.7 20.1 8.7 11.5 12.8 130.3 940.0
Table 11. Pearson Correlation Coefficients SD 31.8 2.3 4.8 8.7 15.9 16.4 111.1 400.0
min 24.0 1.8 13.9 0.0 0.0 0.5 35.0 348.0
max 115.0 8.1 29.6 23.3 55.9 53.1 426.0 1512.0
HONO NosNOz (ppb) HONO (ppb) 3 " (ppb) Nos- (nmol/m3) SO42- (nmolim3) NH4+ (nmol/m3) range use (min) a
denuders, which selectively remove gases from the sample stream. A sodium carbonate (NazCO+coated denuder collects the acidic gases SOZ, HN03, and nitrous acid (HONO), and a citric acid-coated denuder traps "3. Following sample collection, the denuders are extracted, and the extracts are analyzed by ion chromatography to determine the concentration of gas-phase species. Downstream of the denuders, a Teflon membrane collects the fine particles, including acidic sulfate aerosols. NazC03 and citric acid-coated filters are placed downstream of the Teflon membrane to trap gases that have volatilized or have been displaced from the collected fine particles. All filters are extracted and analyzed by ion chromatography to determine the aerosol concentrations of S042-, NH4+,NO3-, and nitrite (NOz-). An aliquot of the Teflon membrane extract is also analyzed for aerosol strong acidity by a pH method (11). Limits of detection (LOD), based on the sensitivity of the ion chromatographic analysis and a 24-h sample collected at a flow rate of 10 L/min, are 0.2,0.2,0.1, and respectively. 0.3 ppb for S02, HN03, HONO, and "3, Limits of detection for the aerosol species are 6, 1.4, 1.4, 8, and 4 nmol/m3, respectively, for Sod2-, Nos-, NOz-, NH4+, and strong acidity (H+). The measurement precision for concentrations of acid aerosols and gases collected with collocated sampling systems has previously been found to be 2.6,2.6, and 7.6% for SOZ,"03, and HONO, respectively (14). For aerosol Sod2-, Nos-, and NOz-, precision of concentrations determined by collocated samplers is 1.5, 8.2, and 15.6%, respectively (14). Following the exposure of the denuders and filter pack systems, the assemblies were returned to a laboratory at the University of New Mexico. There the filter packs were disassembled and the denuders extracted. The denuders were extracted with 10 mL of ultrapure water (Milli-Q water purification system, Millipore Corp., Bedford, MA). The extracts were stored in Teflon containers at 5 "C and shipped to the Harvard School of Public Health Air Quality Laboratory in insulated packages with ice packs by overnight mail. The extracts were analyzed by ion chromatography (Model 4000i, Dionex Corp., Sunnyvale, CAI. Stove Use Measurement. The parents maintained a written record of the use of range-top burners and ovens, recording the beginning and ending time of each cooking event, the maximum number of burners used, and oven use. Results and Discussion
Table I summarizes the pollution measurements and the house characteristics. The living space area of the homes ranged from 348 to 1512 ft2, indicating that the homes were drawn from the lower range of the distribution 842
Environ. Sci. Technol., Vol. 27, No. 5 , 1993
P
0.77"
-0.21
0.22 0.81"
NH3+
range use
floor
S04'-0.41 -0.32 0.43 0.31
0.72* -0.30 0.39 -0.30 -0.24 0.38 -0.36 0.26 -0.36 0.91" -0.07
-0.15 -0.23 0.45 0.19 0.40 0.24 0.40
area
< 0.01. P < 0.05.
observed in the whole sample of 526 homes (median = 1200 ft2). All of the study homes were detached, singlefamily dwellings and most were of wood frame and stucco construction. The smallest home was an adobe guest house, and one mobile home was sampled. All of the homes were heated by utility-line natural gas. The mean NO2 levels measured by passive diffusion samplers, placed in these homes during the October-December period preceding the acid sampling, ranged from 38 to 220 ppb. The 24-h whole-houseaverages during these acid measurements ranged from 27 to 115 ppb. The number of occupants was similar across the homes-two parents and one or two children in each home. For each of the homes, some use of the gas cooking range was reported. The frequency of range burner use reported in the stove-use log varied from 2 to 10 separate episodes, with one episode being the 20-min test burn. Two homes reported some oven use. The furnace was used in all of the homes during the sampling periods. The Pearson correlations among pollution and home characteristics are presented in Table 11. Home-averaged NO2 was significantly correlated with HONO (0.77) and ammonium. Not shown are the correlations among NO2 measured in other rooms of the home. The between-room correlations for NO2 ranged from 0.73 to 0.90. The room NO2 concentrations were also correlated with HONO and ranged from 0.54 for the activity room to 0.92 for the bedroom. Neither HONO nor NO2 concentrations were significantly correlated to the size of the home or the total amount of time the range operated (oven and burner). Figure 1plots the NO2 concentration (ppb) measured in each of the three rooms of the home and HONO (10-l ppb) concentrations. The typical pattern of NO2 concentrations where the kitchen is higher than the activity room which is higher than the bedroom was not seen in all of these homes. In one case (home 4) the child's crib was at one end of the kitchen. In other cases (homes 3, 9, and lo), the child's bedroom was closer to the kitchen than the activity room. The flow of hot convective combustion gases dynamically responds to the dimensions of the house and the forces that effect infiltration and exfiltration. In some situations the home appears well mixed, such as in home 2, which is a mobile home. In other cases, the gas stove combustion exhaust plume will reach monitors in other rooms before impacting the kitchen monitor. Given the variation of NO2 concentrations within and between homes, we decided to average the room concentrations to create a home-average NO2 value. Subsequent regression analysis involving any of the four NO2 measures and HONO are consistent. Since HONO is generated primarily from reactions on surfaces within a home, we
T 140
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90
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90
80
40
60
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70
80
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NO, (PPb)
Flgure 2. Regression of average 24-h nitrous acid concentratlon on 24-h mean nitrogen dioxide concentratlonfor 10 homes In Albuquerque, NM. The dashed lines represent 95% confidence intervals.
1
2
3
4
5
5
7
8
9
10
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Figure 1. Concentrations (24-h) of HONO (lo-' ppb) and NOp (ppb) for 10 homes in Albuquerque, NM.
decided that analysis using the home-averaged NO2 concentrations was most appropriate. Hydrogen ion and SO2 concentrations were quite low and often below detection; therefore, these data are not included in this report. Nitric acid was detected in very low quantitites in just two homes. Ammonia was detected in every home, ranging in concentration from 13 to 29 ppb. The mean concentration was 20 ppb, similar to the indoor winter concentrations between 19 and 35 ppb reported by Brauer et al. (2). Ammonium ion concentrations indoors showed substantial variation, ranging from 5 to 959 nmol/m3. Brauer et al. (2) reported lower mean concentrations but substantial variation among homes. Sulfate ion concentrations are understandably lower in homes in Albuquerque than in homes in the Northeast since ambient sulfate levels are lower in Albuquerque than in the Northeast. In the winter of 1984-1985, the US. EPA conducted a monitoring study in Albuquerque. Over 39 days in Jan-Feb 1985, the ambient sulfate concentrations averaged 19nmol/m3(15). Earlier studies haveshown moderately strong correlation between indoor and outdoor sulfates with indoor to outdoor ratios between 0.5 and 1, depending on season and other home characteristics (16). These data from Albuquerque show that indoor sulfate concentrations ranged between
