Environ. Sci. Technol. 1999, 33, 217-222
Ozone Measurement with Passive Samplers: Validation and Use for Ozone Pollution Assessment in Montpellier, France NADINE L. BERNARD, MARIETTE J. GERBER,* CECILE M. ASTRE, AND MONIQUE J. SAINTOT Groupe d'Epide´miologie Me´tabolique, Cancer Research Center, INSERM - CRLC, 34298 Montpellier Cedex 5, France
The objective of this pilot study was to determine a way of assessing personal exposure to ozone (O3) for use in a study of O3 effects on health. Passive samplers (Passam, AG) were used to measure pollution levels in Montpellier, France. They were standardized using an O3 analyzer. Blanks and duplicates were tested to evaluate sensitivity (6.6 µg/ m3) and imprecision (2 µg/m3). They were validated by comparing on-site measurements with those of the automatic UV absorption analyzers of the regional air quality network (AMPADI-LR). The correlation coefficient was r ) 0.9, p < 10-3, and the regression coefficient was close to 1. The on-site measurements provided information about local pollution. Distance from NO2 sources (urban traffic) and sunlight intensity were identified as environmental determinants of O3 pollution. Residential microenvironmental concentrations and personal exposure were measured for 110 subjects. The indoor/outdoor ratio is higher than in Mexico City and higher than in Toronto in summer but comparable with that in Toronto in winter. The relationship between personal exposure and indoor home measurements is closer than that between personal exposure and outdoor home environment measurements. This is especially true for the spring and summer months, when the correlation between indoor and outdoor measurements is low (r ) 0.23, p < 0.05). At the workplace, on the other hand, there is a close correlation between indoor and outdoor ozone measurements in summer (r ) 0.80, p < 0.001), as there is between personal exposure and outdoor measurements (r ) 0.60, p < 0.001).
Introduction Tropospheric ozone (O3) is a major pollutant produced by various sources, among them urban traffic through photochemical transformation of nitrogen oxides, carbon monoxide, and volatile organic compounds. It is the major source of O3 pollution in Montpellier, France. O3 pollution is usually assessed by air quality network analyzers, but for studies examining the relationship between O3 exposure and health, it is better to measure personal exposure to O3 (1). Few O3 passive samplers have been described (2-7), and the sampler presented in this study was previously used for microenvironmental measurements, e.g., for fixed-site measurements in a defined environment (8). * Corresponding author phone: (33) 67.61.30.05; fax: (33) 67.52.29.01; e-mail:
[email protected]. 10.1021/es971140k CCC: $18.00 Published on Web 12/03/1998
1999 American Chemical Society
The work reported here is a pilot study for an epidemiological project on the health effects of O3. It had three objectives: (1) to validate O3 passive samplers in closed environment measurements and for the assessment of personal exposure, (2) to assess O3 pollution in the areas of the city where the potential subjects of the epidemiological study live and/or work, and (3) to study O3 concentration and distribution with regard to the main determinants: sunlight and NO2 sources. This paper presents the results of this pilot study.
Materials and Methods Passive Samplers. The passive O3 samplers were provided by PASSAM AG (Switzerland). As supplied by the firm, the tubes are protected from sunlight by an opaque cylindrical box. They cannot be used in this form for personal exposure assessment and were therefore covered with self-adhesive aluminum foil and black poly(vinyl chloride) (PVC) film. The passive samplers consisted of 4.9-cm long calibrated tubes with an inside diameter of 0.9 cm, within which air diffuses by molecular diffusion. The sampling period was 5 days. 1,2-Di(4-pyridyl)ethylene solution (DPE) was deposited on a glass filter, supported by a grid, and fixed the O3. The other end was left open for air diffusion (Figure 1). Addition of MBTH (3-methyl-2-benzothiazolinone hydrazone hydrochloride) reactant (5) produced a colored complex, which was measured in a spectrophotometer at 442 nm after stabilization in a 30 °C water bath for 1 h. The reaction was specific to O3, and there was no interference from NO2 (information from the manufacturer). The diffusion coefficient for O3 is unknown and the O3induced alteration to DPE is not stoichiometric. The samplers were therefore calibrated using a UV photometric ozone analyzer: three samplers protected by aluminum foil and PVC film were placed at the air intake of the analyzer. The cumulated O3 measured by the analyzer over 5 days was used as the reference measurement. The procedure was repeated four times. We determined an F factor expressing the O3 measurement by passive sampler expressed in µg/ m3/h, using the equation:
Ca ) F × Qe where Ca is the mean for hourly O3 concentrations measured in µg/m3/h by analyzer over 5 days and Qe the mean O3 for the three tubes expressed as µg/tube/h (mean of (Q1 + Q2 + Q3)/(exposure time, in hours)). The reliability of the passive sampler equipment was evaluated using blanks (closed passive samplers) on the selected sites for 5 days. In addition, O3 concentration was measured using pairs of passive samplers placed open in the same environmental conditions for the same period of time (5 days). This was repeated 20 times to evaluate imprecision of measurement (Sc) according to the equation: Sc ) SD(diff)/x2 (9). The difference between each pair of values was computed and the standard deviation [SD(diff)] of these differences calculated. Sixteen subjects were asked to wear a pair of O3 passive samplers, to test the reproducibility of personal exposure measurement using passive samplers. A special device was used to avoid the absorption effect of fabrics (10): the samplers were fixed on a black PVC plate (9 × 6 cm) hooked onto the wearer’s belt. Variation and correlation coefficients were computed for the 16 pairs of tubes. NO2 exposure was assessed with Palmes tubes, previously validated in the same conditions of use (11). VOL. 33, NO. 2, 1999 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 1. Scheme of ozone passive sampler. Analyzers. O3 in the urban road network is monitored by the regional agency AMPADI-LR, using O3 41M UV absorption analyzers (Environnement SA). These have a detection limit of 2 µg/m3 (2 µg/m3 ) 1 ppb) and are accurate to (1% in the 0-2000 µg/m3 range. The analyzers are maintained in accordance with the manufacturer’s instructions and checked by AMPADI-LR, which belongs to a national group for the standardization of air pollution measurements set up by the Central Air Quality Control Laboratory. We obtained continuous O3 measurements from four permanent stations and one temporary one. Study Site Characteristics. Meteorology. Montpellier has a subhumid Mediterranean climate: hot, dry summers with a high level of sunlight (2700 h/year) and mild winters. Rainfall is irregular and heavy, with an average of 800 mm/year (12). Wind is a very important feature of the local climate. The annual mean recorded over the last 15 years shows that 37% of the winds are >4 m‚s-1. Prevailing winds come from the northwest, with a speed of >4 m‚s-1, and to a minor extent from the southeast. The latter are humid sea breezes with a speed of 3 km
(N ) 3) 25.0 ( 4.3a-c (N ) 5) 40.5 ( 2.2a,d
(N ) 5) 50.6 ( 24.1f-h (N ) 6) 84.8 ( 9.1f,i,j
(N ) 6) 43.0 ( 6.4b,e
(N ) 9) 103.8 ( 15.3g,i
(N ) 3) 61.9 ( 6.6c-e
(N ) 5) 120.0 ( 9.8h,j
Significance:
f,ip
< 0.05; ap < 0.01;
b,e,jp
< 0.001;
c,d,g,hp
) 0.0001.
for sites 29 and 61 were recorded by AMPADI-LR. There is a strong significant correlation between O3 concentrations and sunlight: r ) 0.96, p < 10-3, site 29; r ) 0.87, p < 0.001, site 61. The correlation between O3 concentration and temperature is weaker: r ) 0.74, p < 0.05, site 29; r ) 0.66, p < 0.05, site 61. When we computed a partial correlation between O3 concentration and temperature taking sunlight intensity into account, the correlation between O3 levels and temperature became nonsignificant. The partial correlation between O3 concentration and sunlight taking temperature into account modified the correlation coefficient only slightly: r ) 0.95, p < 10-3, site 29; r ) 0.77, p < 0.01, site 61. This confirms that temperature has no or very little effect on the O3 production cycle. Temperature is a covariable of sunlight (r ) 0.85, p ) 0.001), and the causal relationship exists only between sunlight and O3 concentration. With regard to the relationship between NO2 concentration and O3 concentration, we first compared NO2 and O3 levels measured with passive samplers at fixed sites and times. We studied the association between NO2 and O3 for the 10 periods between June 1995 and June 1996 on a limited number of sites. There was an inverse correlation between hourly levels of NO2 and O3 at four sites (sites 19, 25, 29, and 61): r ) -0.74, p < 10-3. These sites were selected because there were air quality surveillance network analyzers located there, and a comparison between the two measurement methods was therefore possible. We studied the association between O3 and NO2 for one period only (November 13-20, 1995) on 17 sites. The correlation coefficient between O3 and NO2 measurements was r ) -0.96, p < 0.01. There was likewise an inverse correlation between distance from city center and the NO2 level (r ) -0.74, p < 10-3 during winter 1995; r ) -0.88, p < 10-3 during summer 1995); a positive correlation was observed between distance from city center and the O3 level (r ) 0.77, p < 10-3 during winter 1995; r ) 0.78, p < 10-3 during summer 1995). However, whereas there was an inverse correlation between the minimum distance of the measurement sites from the expressway and the NO2 levels, (r ) -0.63, p < 10-3 in winter 1995; r ) -0.73, p < 10-3 in summer 1995), the positive correlation coefficient between the minimum distance of the measurement sites from the expressway and the O3 levels was not significant, indicating the possible influence of the two NO2 sources, city center and expressway, on O3 generation. We determined the relationship of O3 and NO2 concentrations using a four-class classification of distance from city center (Table 4). The O3 means in summer were higher than those in winter, but both periods showed increasing concentrations as distance from the city center increased. The lowest level was observed at a distance of
