- Email: [email protected]
Measurement of high concentration of nitrous acid inside automobiles
Pergamon
Atmospheric
Environment
Vol. 29, No. 3, pp. 34S351,
1995
Copyright 0 1995 Elsevier Science Ltd Printed in Great Britain. All rights reserved 1352%2310/95 $9.50 + 0.00
1352-2310(94)00260-6
MEASUREMENT
OF HIGH CONCENTRATION ACID INSIDE AUTOMOBILES
OF NITROUS
A. FEBO and C. PERRINO C.N.R. Istituto Inquinamento
Atmosferico, Via Salaria Km 29,300~C.P. 10, Monterotondo (Roma), Italy
(First receioed
10 August 1993 and
injinalform
Stazione
11 August 1994)
Abstract-Nitrous acid concentration levels have been measured inside motor vehicles by using the annular denuder techmque. The results indicate that quite high levels of nitrous acid (lo-25 ppb) occur inside the autos during transit through polluted areas (urban sites) and that high concentrations persist for many hours even in the case where the autos are moved to an unpolluted site. HONO/NO, ratio has been also measured and found to be much higher inside than outside the autos. The influence of indoor temperature on HONO concentration has been studied and found to be very significant. All these findings confirm the heterogeneous nature of HONO formation reaction and the major role played by the adsorption-desorption mechanism in determining the indoor concentration of this pollutant, whose effects on human health still need to be thoroughly evaluated. Key word index: Nitrous acid, indoor pollution, surface reactions, motor vehicles.
INTRODUCTION In the last few years, the role of nitrous acid as initiator of the chain-mechanism leading to photochemical smog occurrence has drawn more and more attention. The photodissociation of HONO, which gives rise to
OH radicals, in fact, constitutes the largest source of this molecule during the hours immediately after sunrise, when other production mechanisms are still inactive. Particular attention has also been paid to HONO formation mechanisms, among which the heterogeneous reaction of NO, with water vapour plays a major role (Sakamaki et al., 1983; Svensson et al., 1987; Jenkin et al., 1988). Due to the heterogeneous nature of this reaction, ambients characterized by the presence of high nitrogen dioxide concentrations and high surface-tovolume ratio (S/V) particularly favour the occurrence of high concentration levels of nitrous acid. This hypothesis has been confirmed by the measurement, carried out by Febo and Perrino (1991), of peak HONO concentrations as high as 50 ppb in indoor environments; Braur et al. (1990) also reported 24-h average concentrations as high as 40 ppb in one of their research houses. These high concentrations, together with the mutagenic activity of this compound, which is known to be a nitrosating agent (Pitts, 1983), raise the problem of a previously unrecognized health risk, and constitute evidence of the primary importance of HONO as an indoor pollutant. Since the interior of vehicles constitutes, by theory, another environment in which
the occurrence of high HONO concentrations is highly probable, measurements have been carried out in order to check the presence and to evaluate the concentration level of this pollutant inside automobiles.
EXPERIMENTAL Annular denuders constitute, up to now, the only method which permits reliable determinations of HONO in very limited spaces. The differential optical absorption technique, which is the other technique successfully used for HONO measurements, in fact, besides being of high cost and complexity, still needs a base path of the order of some meters (2.2m for a HONO detection limit of 24ppb (Pitts et al., 1985)and 10 m for a HONO detection limit of 1 ppb (Pitts et al., 1989)). The simultaneous determination of HONO and NO, was carried out by using an experimental set-up of four aunular denuders set in series. The denuders, made of frosted Pyrex glass, had the following dimensions: length 11 cm, outside diameter 1.4 cm, inside diameter 1.0 cm. Sampling flow rate was 3 emin-i. The set-up consisted of one TCM-coated denuder followed by two Na,CO, + glycerol-coated denuders and one eugenol-coated denuder. The TCM denuder was used for removing sulphur dioxide from the incoming air stream. The presence of high concentrations of SO, and NO*, as in the case of urban environments, in fact, leads to a positive artifact in the determination of HONO on carbonate denuders which is strongly reduced by the insertion of an upstream TCM denuder (Febo et al., 1993). Nitrous acid is efficiently collected on the first Na,CO,-coated denuder (collection efficiency at the flow rate of 3 dmin-’ is higher than 95%). The second Na,CO,-coated denuder permits us to correct the data from the first Na,CO,-coated denuder in order to take into account the small interference possibly due 345
346
A. FEBO and C. PERRINO
to PAN and nitrogen oxides collection (differential technique) (Febo et al., 1990). Nitrogen dioxide is efficiently collected on the eugenol-coated denuder (Possanzini et al., 1993). Coating solutions were as follows: -
-
tetrachloromercurate (TCM) coating: HgCl,, 0.08 M + KCl, 0.16 M in water (solution A), maleic acid, 0.08 M + NaOH, 0.125 M in glycerine-water 0.25: I solution (solution B); the final solution is 25% A, 25% B and 50% methanol; Na,CO, coating: 1% Na,CO, + I % glycerol (w/v) in water-methanol 1: 1 solution; eugenol coating: 5% eugenol + 2.5% NaOH in methanol solution.
The analyses of the extracts were performed by ion chromatograph; (mod. DXlOO, Dionex Corp., Sunnyvale, CA, U.S.A.). The denuders were extracted with 5 ml of distilled water; the analytical detection limit for nitrate ion was 0.01 pgml-‘; the resulting minimum detectable amount was 0.05 pg device- ’; while the minimum quantifiable amount was about 0.15 pgdevice-‘. Denuder samplings were carried out by using battery powered samplers (mod. Chronos, Zambelli srl, Bareggio, MI, Italy; mod. Gemini, Du Pont de Nemours & Co. Inc., PA, U.S.A.). Temperature and relative humidity were continuously measured by means of an automatic digital recorded (mod. Thermos Data, DAS s.r.1. Palombara Sabina, RM, Italy). Some experiments were carried out by means of a modified chemiluminescence analyser. The modified analyser, described by Febo and Perrino (1991), allows a continuous measurement of HONO concentration by taking advantage of the 100% conversion of HONO to NO, assured, at the temperature of 35O”C, by the molybdenum converter. Another chemiluminescence analyser was used, as such, for the determination of NO and NO, concentrations (mod. AC 30M, Environment s.a., Poissy, France). A quality control on the two instruments used for the experiments was carried out by calibrating them by means of NO standard atmospheres generated by using NIST primary reference standards and the gas-phase titration technique (calibrator mod. 9100, Environics Inc., CT, U.S.A.). During the experiments, the two chemiluminescence analysers were exchanged on the two sampling lines, in order to assure dynamically the equivalency of the instrumental responses. In addition, the converter efficiency in yielding NO from HONO was verified by using an improved version of the HONO dynamic generator described by Allegrini et al. (1990). The generator, which relies on the reaction of HCl produced by a low-pressure permeation device with a sodium nitrite fluidized bed, exhibits the following characteristics: HONO mass flow rate range: 0.1-15 pgmin-‘; purity: > 99.5%; 24 h stability: better than 1%. The automobile used for the experiments was a Peugeot mod. 205 XR. During all the experiments the windows were kept closed and the fan was off.
presence of interfering species, and the slope /I, which depends on the overall efficiency of the system, equals the expression p = /I,, + E(B);in optimum conditions we have /?,, = 1 and the indetermination E(/?)-+ 0 (I<< lo-‘, as reported by Febo et al., 1990, 1993). The region of HONO quantitation is given by SM - IR > loo(la) with D standard deviation. When IR can only be indirectly estimated, it is necessary to consider its maximum value. In our case (TCM-Na,CO,-Na,CO, denuder set-up) we have (Febo et al., 1990, 1993) SM = (4‘ - (12)/E*
q1 = E,S + I,
(3)
q2 = E,(l - E,)S + I,
(4)
where E, and E, are the efficiency of the two Na,CO, denuders and I, and I, are the interferent amounts determined on the two devices. By substituting equations (3) and (4) into equation (2) and equating with equation (l), we have El-E,+E,E, p=
I,
p-.
-12
E2
In equation (I), B = 1 (optimum conditions) when the operative efficiency of the two Na,CO, denuders (E, and E2) approach the theoretical value E. I, and I, can be expressed as follows: 1, = L,(SO,, NO,) + I,(NOJ + I,(PAN) + I,(bk) I, = I,(NO,)
(5)
+ I,(PAN) + I,(bk)
(6)
where -
Accuracy of the denuder method
SM = /Is + IR (I) where IR is the apparent nitrous acid amount due to the
p
and
I(N0,) is the small interference due to NO, on the Na,CO, coating layers, which can be expressed as follows (De Santis et al., 1987): I,(NO,)
The measurement of HONO by means of the denuder technique has been specifically addressed by several studies (Febo et al., 1993, and references cited therein); anyway, it can be of help to report here a brief general discussion about the parameters which affect the accuracy of HONO determination by means of the TCM-Na,CO,-Na,CO, denuder sampling line, as well as an evaluation of the resulting minimum detectable concentrations. In this system, the measured amount of HONO SM is linked to the real amount S entering the system by the following relationship:
(2)
where q1 and q2 indicate the sum of nitrite and nitrate molar amounts determined on the first and second Na,CO, denuders, respectively, and E is the collection efficiency for HONO. It is necessary to determine also nitrate amounts in order to take into account the possible small oxidation processes (Perrino et al., 1990);for this reason, ion chromatography has been chosen as detection technique. In equation (2), q1 also includes the small nitrite amount possibly retained on the TCM denuder, which can be either determined by the Griess-Saltzmann calorimetric technique or desorbed from the TCM to the Na,CO, denuder at the end of the sampling. The amounts qr and q2 can be expressed a.s follows:
-
= I,(NO,)
= a[NO,] T
with a<3 x IO-’ m3s-‘, [NO,] air concentration of NO, and T sampling time; I(PAN) is the small interference due to the collection of peroxyacetylnitrate (PAN), which can be expressed as follows: I,(PAN) = ETANFT[PAN] I,(PAN) = E;AN(l - ETAN)FT[PAN]
-
with ET*“’= E!*N z 10-l and F sampling flow rate; I(bk) is the nitrite amount due to the analytical blanks: I,(bk) N I,(bk) 2: 2 x lo-’ pgdevice-’
-
I,(SO1, NO*) is the small residual interference on the TCM denuder due to the contemporary presence of high concentrations of SO, and NO1, which can be
Measurement of high concentration of nitrous acid inside automobiles RESULTS AND
expressed as follows (Febo et al., 1993): WQ,
= YCNO,IFT
NW
where the value of y depends on SOz air concentration. It follows that y[NO,]FZ’+ 1s = -
(EPAN)‘FT[PAN] (7)
E2
From the knowledge of the concentration values [NO,], [SO,] and [PAN] during the sampling period, we can estimate the maximum values of IR and ~‘(1~). If we consider the mixing ratios relative to IR and uZ(IR), by dividing equation (1) by F. T we get CM=j?C+
CR.
In the conditions of the experiment here reported, we have for urban areas
I:SO,l “<30 ppb, WAN] ‘Z2 ppb, [NO,] z 50 ppb and for rural areas
CSW ? 5 ppb, I:PAN] Z 3 ppb, I:NO,] Z 5 ppb. It follows that for y we have the minimum values of about 5 x 10m3 (urban areas) and 1 x 10e3 (rural areas), respectively, and that we can cstimate the values of CR and u(CR) as follows: urban areas:
CR < 0.25 ppb,
u(CR) < 0.07 ppb rural areas:
CR < 0.05 ppb,
u(CR) < 0.025 ppb. Thus, we can calculate the following quantitation limits for HONO, which only derive from the use of this sampling method: urban areas:
C > 0.75 ppb,
rural areas:
C > 0.25 ppb.
If we take into consideration also the limits of the analytical technique (IC), we have no variations in the value of C for urban areas, while the quantitation limit for rural areas increases to a value of about 0.5 ppb. In addition to these theoretical considerations, the technique gives the opportunity of directly testing the quality of the data gathered in the field. From the experimental data, in fact, we can calculate the value of the two parameters d = q1 - q2 and s = q1 + q2. From equations (3) and (4) we get s - d = (1 - /9E2)S + 21,. In case the system maintained its efficiency during the whole sampling period (B = l), the above relationship allows us to obtain an experimental value I;:
z* = (s - 4 - (1 2
@IS
?
which must be in agreement with the estimated value I, obtained from the air concentration of NO,, SO, and PAN (equation (6)). The observation of a possible disagreement between the values of I!~and 1; allows any malfunctioning of the sampling line during the samplings to be clearly detected.
341
DISCUSSION
Automobiles constitute an experimental environment of great interest, since they can be easily transferred from very polluted to unpolluted environments, and since the temperature of the interior can be easily changed by simply exposing the car to Sun rays; that is to say, some of the parameters involved in the generation and adsorption-desorption mechanisms (concentration of nitrogen dioxide, temperature) can be easily modified while remaining in field conditions. Nitrous acid and nitrogen dioxide concentrations, simultaneously measured inside and outside the automobile during its transit through the centre and the suburbs of the city of Rome, are reported in Table 1, together with the range of temperatures observed during the experiments. The results show that quite high concentration levels of HONO, ranging from 7 to 29 ppb, could be measured inside the car, in the presence of low outside HONO concentrations (below 2 ppb). NO, values measured inside (21-43 ppb), instead, did not differ very much from those measured outside (26-52 ppb). The average ratio between inside and outside concentrations (a parameter which does not suffer from the possible poor precision of the sampled volume) was 14.8 in the case of HONO (range 5-30) and 0.85 in the case of NO, (range 0.71-0.97). This is in agreement with the model of HONO heterogeneous formation on the interior surfaces of the vehicle, and subsequent release in the gaseous phase. A possible explanation for the slight negative difference between NOz concentration levels measured inside and outside the vehicle is given by the hypothesis of NO, consumption due to the heterogeneous reaction forming HONO. The data of Table 1 also show, as expected, a slight difference between the concentration levels measured during the run across the centre of the city and those observed during the runs across the suburbs, for both NO, and HONO. The data of Table 1 also give a first indication that the highest HONO indoor concentration values occur together with the highest values of inside temperature. A second set of experiments was deviced in order to evaluate the persistency of HONO in the environment under study, and to investigate the effect of temperature variations. The persistency of HONO had been already verified during the experiments carried out inside homes by Febo and Perrino (1991), and constitutes a good indication of the significance of the adsorption-desorption phenomena, which are governed by the temperature. The experiments were planned in the following way: (1) the car was kept for many hours in an urban area (city of Rome and suburbs); (2) then it was placed for 2 h in a sheltered parking lot located in a rural area characterized by very low HONO and NO, concentration levels; (3) then it was moved in the Sun and left there for some hours.
348
A. FEBO and C. PERRINO Table 1. Measurement
of HONO and NO, concentration values inside and outside an automobile in a polluted area
HONO (ppb)
NO, (ppb)
Inside
Outside
Inside
Outside
Temp. range (“C)
* * * * * * * *
9.9 11.2 9.1 10.0 6.8 18.7 9.2 10.2
0.9 1.5 0.6 1.5 1.2 0.6 0.5 0.7
32.4 25.0 34.4 35.8 22.9 36.7 21.4 33.5
40.7 32.4 36.4 38.8 26.5 40.1 30.2 36.4
24-28 22-25 21-29 27-31 25-30 30-40 22-25 22-27
Avg.
10.6
0.9
30.3
35.2
# # # # # # # #
13.7 10.8 12.1 19.3 27.0 11.7 10.6 29.2
0.7 1.5 1.1 1.0 1.9 0.8 0.6 1.4
37.6 40.9 42.1 25.1 37.5 36.9 42.9 30.6
47.0 46.2 43.3 31.5 40.8 46.5 51.6 41.2
Avg.
16.8
1.1
36.7
43.5
24-29 17-18 23-29 28-33 30-41 23-27 22-27 35-39
* Suburbs; # Centre.
Denuder measurements were carried out during the last hour of phase 1 and during phases 2 and 3. The results, reported in Table 2, indicate that quite high concentrations of HONO persisted in the examined
air volume far after the removal of the car from the polluted site (traffic); this underlines the importance of the adsorption-desorption phenomena, which uncouple, from a temporal point of view, the occurrence of the formation reaction from the release of the reaction product in the gaseous phase. The ratio between indoor HONO and outdoor NO* concentrations increased, in fact, from the values in the range 0.2-0.5 determined in the urban area (phases 1) to the values in the range 6-60 determined in the case of the rural area (phases 2 and 3). It is worth noting that in outdoor atmospheres the ratio between HONO and NO, does not usually exceed the values 0.1-0.2. The results also show that HONO concentration in the third phase of the experiments exceeded by far the concentration measured during the second phase, showing the dependence of the gas-phase concentration of HONO on indoor temperature, in agreement with the proposed desorption mechanism. For a better description of the time trend of HONO release in the gaseous phase, some experiments were carried out by measuring HONO concentration during phases 2 and 3, also by means of a modified chemiluminescence analyser. The time trend of NO,, NO and HONO concentrations, as measured by the modified chemiluminescence detector during the last one of the experiments reported in Table 2, is shown in Fig. 1. A very good agreement between the data yielded by the two techniques was obtained (per cent
difference between the results yielded by the two methods ranged from 0.6 to 3.3%). Before the experiment of Fig. 1, the car was kept for the whole night in an urban site, then it was moved to the rural site. NO and NO* concentrations at the rural site were below 1 and 2 ppb, respectively; HONO concentration was below 1 ppb. From the beginning of the experiment until time t, the temperature inside the car was approximately constant (about 27S”C). At time t, the car was placed in a position partially exposed to Sun rays; at time t, it was moved again and exposed to Sun rays completely. An air conditioning system, with introduction of outside air, was set on at time t, and off at time t,. The data of Fig. 1 clearly show that a relationship between the temperature and the concentration of HONO occurs; the obvious dependence on the ventilation factor is also evident. At the end of the ventilation phase (time t4), a new increase can be observed in the HONO concentration, which tends to reach the new equilibrium value (which depends on the temperature and on the amount of HONO in the adsorbed phase at that time). Similar experiments showed that a brief opening of the windows also causes a temporary decrease of HONO concentration to outside levels; however, as soon as the windows are closed, HONO concentration increases again to the equilibrium value. It is worth noting that NO and NO, concentration levels inside the car increase together with the increase of HONO concentration. This observation, which confirms the data of Table 2, can be justified by the hypothesis of the following dissociation reaction of
Measurement of high concentration of nitrous acid inside automobiles
349
Table 2. Temporal trend of nitrous acid concentration inside an automobile
HONO(mb) Date
NO, (ppb)
Phase
Inside
Outside
Inside
Outside
156.92
1 2 2 3 3
16.7 20.5 21.1 38.2 52.0
1.0 0.6 0.7 0.6 0.5
28.2 5.4 6.4 10.1 13.5
36.9 3.2 3.5 3.7 3.5
28-32 30-32 32-33 33-48 48-55
16.6.92
1 2 2 3 3
19.0 21.8 22.7 32.5 42.7
1.1 0.7 0.5 b.q.1. b.q.1.
36.3 3.9 4.2 5.1 7.4
41.7 2.1 2.2 2.6 2.5
30-33 30-34 34-35 35-41 41-45
17.6.92
1 2 2
15.5 19.7 19.8
0.9 0.6
36.7 3.7
42.3 3.1
28-31 30-30
b.q.1.
3.6
2.7
30-31
3
24.3
3
47.7
b.q.1. b.q.1.
7.3 12.1
3.2 2.6
31-38 38-41
23.6.92
1 2 2 3 3
14.0 15.6 17.4 30.8 49.2
0.7 b.q.1. b.q.1. b.q.1. b.q.1.
27.9 1.2 1.4 7.4 12.5
37.1 1.0 1.0 0.7 0.8
26-29 29-30 30-30 31-46 46-52
24.6.92
1 2 2
12.3 13.0 21.9
0.5 b.q.1.
31.6 b.q.1.
38.6 1.4
26-28 25-31*
3 3
35.8 37.7
b.q.1. b.q.1. b.q.1.
0.5 3.4 5.5
1.2 1.3 1.4
- -
Temp. range (“C)
31-36* 36-46 44-47 #
* Partial exposure to Sun rays. # Air conditioning on. b.q.1. Below quantitation limit.
HONO: HONO,,,
+ HONO,,,
+ NO + NO* + H,O .
The difference in the concentration levels of NO and NO2 could be due to a different removal rate of the
two compounds; NOz, for example, may start the HONO formation cycle again. To summarize, for a given autovehicle, the following parameters govern the occurrence of indoor HONO concentration: -
-
concentration of nitrogen dioxide and exposure time, which determine the HONO amount produced in the adsorbed phase; ambient temperature, which determines the rate of release in the gaseous phase; airing of the vehicle, which, obviously, influences the ventilation factor and thus the equilibrium concentration of HONO.
In the scientific literature, HONO formation constants ranging frolm 10m4 to 10m6 ms-’ (relative humidity content: about 25,000 ppm) have been reported, the values of the constant depending on the characteristics of the generation surfaces (Lammel et al., 1990; Febo and Perrino, 1991). Before being used for the experiment of Fig. 1, the car was kept for about 18 h in an urban site where the concentration level of
nitrogen dioxide was of the order of 50 ppb; thus, the formation of HONO on the interior surfaces of the car (having an area of about 10 m’) which took place during this period may reasonably lead to a HONO amount of the order of a few milligrams. During this loading time, the desorption of HONO in the gaseous phase also occurred, with a desorption rate which can be assumed to be of the order of 10m4 s-l. As a consequence, at the beginning of the experiment (phase 2) a HONO amount of the order of a few hundreds of micrograms can be reasonably estimated to still exist in the adsorbed phase. In this phase of the experiment, an air volume of about 2 m3 was available for the ripartition between the adsorbed and the gaseous phase; by taking into account the air exchanges during the experiment, the occurrence of a HONO air concentration of about 50 ppb is thus plausible, and in agreement with the above reported order of magnitude of the formation constants.
CONCLUSIONS
The preliminary results reported in this paper indicate that quite high and persistent concentrations of nitrous acid can be measured inside automobiles. The interior of automobiles, in fact, can be regarded as
350
A. FEBO and C. PERRINO
60
1
3
2
4
Time (h) -
HONO-
NO2 ----NO
Fig. 1. Relationship between indoor temperature and the concentration HONO, NO, and NO inside an automobile.
both a reaction chamber (where the nitrogen dioxide produces nitrous acid) and as a reservoir, when the produced amount of nitrous acid is slowly released into the gaseous phase, the extent of the release depending on the temperature. As a consequence, drivers and occupants are subjected to a notable exposure to this pollutant, that is a health risk the consequences of which still need to be evaluated. From an environmental point of view, automobiles constitute a series of spot sources of nitrous acid, which (as well as other indoor environments) contribute to urban pollution. This study, which indicates the significance of the surface reaction of NO, as a source of HONO and of the adsorption-desorption processes, can also be of help in the development of models intended to describe HONO behaviour in the atmospheric environment.
REFERENCES
Allegrini I., Cortiello M., Febo A. and Perrino C. (1990) Generation of standard atmospheres of nitrous acid. In Physic+Chemical
Behaviour of Atmospheric
Pollutants
(edited by Restelli J. and Angeletti G.), pp. 140-144. Kluwer, Dordrecht. Brauer M., Barry Ryan P., Suh H. H., Koutrakis P. and Spengler J. D. (1990) Measurement of nitrous acid inside two research houses. Envir. Sci. Technol. 24, 1521-1527.
of
De Santis F., Febo A. and Perrino C. (1987) Nitrite and nitrate formation on a sodium carbonate layer in the presence of nitrogen dioxide. Anna/i di Chimica 20, 763-769. Febo A. and Perrino C. (1991) Prediction and experimental evidence for high air concentration of nitrous acid in indoor environments. Atmospheric Environment 25A, 1055-1061.
Febo A., De Santis F., Perrino C. and Giusto M. (1990) Evaluation of laboratory and field performance of denuder tubes: a theoretical approach. Atmospheric Environment 19,1517-1530.
Febo A., Perrino C. and Cortiello M. (1993) A denuder technique for the measurement of nitrous acid in urban environments. Atmospheric Environment 27A, 1721-1728. Jenkin M. E., Cox R. A. and Williams D. J. (1988) Laboratory studies of the kinetics of formation of nitrous acid from the thermal reaction of nitrogen dioxide and water vapour. Atmospheric Environment 22,487-498.
Lammel G., Perner D. and Warneck P. (1990) Nitrous acid at Mainz: observation and implication for its formation mechanism. In Physico-Chemical Behauiour of Atmospheric Pollutants (edited by Restelli J. and Angeletti G.), pp. 469-476. Kluwer, Dordrecht. Perrino C., De Santis F. and Febo A. (1990) Criteria for the choice of a denuder sampling technique devoted to the measurement of atmospheric nitrous and nitric acids. Atmospheric Environment 24A, 617-626. Pitts J. N. Jr (1983) Formation and fate of gaseous and particulate mutagens and carcinogens in real and simulated atmospheres. Enoir. Hlth Perspect. 47, 115-140. Pitts J. N. Jr, Wallington T. J., Biermann H. W. and Winer A. M. (1985) Identification and measurement of nitrous acid in an indoor environment. Atmospheric Environment 19, 763-767.
Measurement of high concentration of nitrous acid inside automobiles Pitts J. N. Jr, Biermann H. W., Tuazon E. C., Green M., Long W. D. and Winer A. IM. (1989) Time-resolved identification and measurement of indoor air pollutants by spectroscopic techniques: gaseous nitrous acid, methanol, formaldeyde and formic acid. JAPCA 39. 1344-1347. Possanzini M., Di Palo V. and Masia P. (1993) Evaluation of a denuder method for ambient NO, measurement at ppb levels. Int. J. envir. tmalyt. Chem. 49, 139-147.
351
Sakamaki F., Hatkeyama S. and Akimoto H. (1983) Formation of nitrous acid and nitric oxide in the heterogeneous dark reaction of nitrogen dioxide and water vapour in a smog chamber. Int. J.-c/rem. Kinet. 15, 1013-1620. Svensson R..._Liunastrom E. and Lindauist 0. (1987) _ ~ I Kinetics of the reaction between nitrogen dioxide and water vapour. Atmospheric Environment 21, 1529-1539.
Atmospheric
Environment
Vol. 29, No. 3, pp. 34S351,
1995
Copyright 0 1995 Elsevier Science Ltd Printed in Great Britain. All rights reserved 1352%2310/95 $9.50 + 0.00
1352-2310(94)00260-6
MEASUREMENT
OF HIGH CONCENTRATION ACID INSIDE AUTOMOBILES
OF NITROUS
A. FEBO and C. PERRINO C.N.R. Istituto Inquinamento
Atmosferico, Via Salaria Km 29,300~C.P. 10, Monterotondo (Roma), Italy
(First receioed
10 August 1993 and
injinalform
Stazione
11 August 1994)
Abstract-Nitrous acid concentration levels have been measured inside motor vehicles by using the annular denuder techmque. The results indicate that quite high levels of nitrous acid (lo-25 ppb) occur inside the autos during transit through polluted areas (urban sites) and that high concentrations persist for many hours even in the case where the autos are moved to an unpolluted site. HONO/NO, ratio has been also measured and found to be much higher inside than outside the autos. The influence of indoor temperature on HONO concentration has been studied and found to be very significant. All these findings confirm the heterogeneous nature of HONO formation reaction and the major role played by the adsorption-desorption mechanism in determining the indoor concentration of this pollutant, whose effects on human health still need to be thoroughly evaluated. Key word index: Nitrous acid, indoor pollution, surface reactions, motor vehicles.
INTRODUCTION In the last few years, the role of nitrous acid as initiator of the chain-mechanism leading to photochemical smog occurrence has drawn more and more attention. The photodissociation of HONO, which gives rise to
OH radicals, in fact, constitutes the largest source of this molecule during the hours immediately after sunrise, when other production mechanisms are still inactive. Particular attention has also been paid to HONO formation mechanisms, among which the heterogeneous reaction of NO, with water vapour plays a major role (Sakamaki et al., 1983; Svensson et al., 1987; Jenkin et al., 1988). Due to the heterogeneous nature of this reaction, ambients characterized by the presence of high nitrogen dioxide concentrations and high surface-tovolume ratio (S/V) particularly favour the occurrence of high concentration levels of nitrous acid. This hypothesis has been confirmed by the measurement, carried out by Febo and Perrino (1991), of peak HONO concentrations as high as 50 ppb in indoor environments; Braur et al. (1990) also reported 24-h average concentrations as high as 40 ppb in one of their research houses. These high concentrations, together with the mutagenic activity of this compound, which is known to be a nitrosating agent (Pitts, 1983), raise the problem of a previously unrecognized health risk, and constitute evidence of the primary importance of HONO as an indoor pollutant. Since the interior of vehicles constitutes, by theory, another environment in which
the occurrence of high HONO concentrations is highly probable, measurements have been carried out in order to check the presence and to evaluate the concentration level of this pollutant inside automobiles.
EXPERIMENTAL Annular denuders constitute, up to now, the only method which permits reliable determinations of HONO in very limited spaces. The differential optical absorption technique, which is the other technique successfully used for HONO measurements, in fact, besides being of high cost and complexity, still needs a base path of the order of some meters (2.2m for a HONO detection limit of 24ppb (Pitts et al., 1985)and 10 m for a HONO detection limit of 1 ppb (Pitts et al., 1989)). The simultaneous determination of HONO and NO, was carried out by using an experimental set-up of four aunular denuders set in series. The denuders, made of frosted Pyrex glass, had the following dimensions: length 11 cm, outside diameter 1.4 cm, inside diameter 1.0 cm. Sampling flow rate was 3 emin-i. The set-up consisted of one TCM-coated denuder followed by two Na,CO, + glycerol-coated denuders and one eugenol-coated denuder. The TCM denuder was used for removing sulphur dioxide from the incoming air stream. The presence of high concentrations of SO, and NO*, as in the case of urban environments, in fact, leads to a positive artifact in the determination of HONO on carbonate denuders which is strongly reduced by the insertion of an upstream TCM denuder (Febo et al., 1993). Nitrous acid is efficiently collected on the first Na,CO,-coated denuder (collection efficiency at the flow rate of 3 dmin-’ is higher than 95%). The second Na,CO,-coated denuder permits us to correct the data from the first Na,CO,-coated denuder in order to take into account the small interference possibly due 345
346
A. FEBO and C. PERRINO
to PAN and nitrogen oxides collection (differential technique) (Febo et al., 1990). Nitrogen dioxide is efficiently collected on the eugenol-coated denuder (Possanzini et al., 1993). Coating solutions were as follows: -
-
tetrachloromercurate (TCM) coating: HgCl,, 0.08 M + KCl, 0.16 M in water (solution A), maleic acid, 0.08 M + NaOH, 0.125 M in glycerine-water 0.25: I solution (solution B); the final solution is 25% A, 25% B and 50% methanol; Na,CO, coating: 1% Na,CO, + I % glycerol (w/v) in water-methanol 1: 1 solution; eugenol coating: 5% eugenol + 2.5% NaOH in methanol solution.
The analyses of the extracts were performed by ion chromatograph; (mod. DXlOO, Dionex Corp., Sunnyvale, CA, U.S.A.). The denuders were extracted with 5 ml of distilled water; the analytical detection limit for nitrate ion was 0.01 pgml-‘; the resulting minimum detectable amount was 0.05 pg device- ’; while the minimum quantifiable amount was about 0.15 pgdevice-‘. Denuder samplings were carried out by using battery powered samplers (mod. Chronos, Zambelli srl, Bareggio, MI, Italy; mod. Gemini, Du Pont de Nemours & Co. Inc., PA, U.S.A.). Temperature and relative humidity were continuously measured by means of an automatic digital recorded (mod. Thermos Data, DAS s.r.1. Palombara Sabina, RM, Italy). Some experiments were carried out by means of a modified chemiluminescence analyser. The modified analyser, described by Febo and Perrino (1991), allows a continuous measurement of HONO concentration by taking advantage of the 100% conversion of HONO to NO, assured, at the temperature of 35O”C, by the molybdenum converter. Another chemiluminescence analyser was used, as such, for the determination of NO and NO, concentrations (mod. AC 30M, Environment s.a., Poissy, France). A quality control on the two instruments used for the experiments was carried out by calibrating them by means of NO standard atmospheres generated by using NIST primary reference standards and the gas-phase titration technique (calibrator mod. 9100, Environics Inc., CT, U.S.A.). During the experiments, the two chemiluminescence analysers were exchanged on the two sampling lines, in order to assure dynamically the equivalency of the instrumental responses. In addition, the converter efficiency in yielding NO from HONO was verified by using an improved version of the HONO dynamic generator described by Allegrini et al. (1990). The generator, which relies on the reaction of HCl produced by a low-pressure permeation device with a sodium nitrite fluidized bed, exhibits the following characteristics: HONO mass flow rate range: 0.1-15 pgmin-‘; purity: > 99.5%; 24 h stability: better than 1%. The automobile used for the experiments was a Peugeot mod. 205 XR. During all the experiments the windows were kept closed and the fan was off.
presence of interfering species, and the slope /I, which depends on the overall efficiency of the system, equals the expression p = /I,, + E(B);in optimum conditions we have /?,, = 1 and the indetermination E(/?)-+ 0 (I<< lo-‘, as reported by Febo et al., 1990, 1993). The region of HONO quantitation is given by SM - IR > loo(la) with D standard deviation. When IR can only be indirectly estimated, it is necessary to consider its maximum value. In our case (TCM-Na,CO,-Na,CO, denuder set-up) we have (Febo et al., 1990, 1993) SM = (4‘ - (12)/E*
q1 = E,S + I,
(3)
q2 = E,(l - E,)S + I,
(4)
where E, and E, are the efficiency of the two Na,CO, denuders and I, and I, are the interferent amounts determined on the two devices. By substituting equations (3) and (4) into equation (2) and equating with equation (l), we have El-E,+E,E, p=
I,
p-.
-12
E2
In equation (I), B = 1 (optimum conditions) when the operative efficiency of the two Na,CO, denuders (E, and E2) approach the theoretical value E. I, and I, can be expressed as follows: 1, = L,(SO,, NO,) + I,(NOJ + I,(PAN) + I,(bk) I, = I,(NO,)
(5)
+ I,(PAN) + I,(bk)
(6)
where -
Accuracy of the denuder method
SM = /Is + IR (I) where IR is the apparent nitrous acid amount due to the
p
and
I(N0,) is the small interference due to NO, on the Na,CO, coating layers, which can be expressed as follows (De Santis et al., 1987): I,(NO,)
The measurement of HONO by means of the denuder technique has been specifically addressed by several studies (Febo et al., 1993, and references cited therein); anyway, it can be of help to report here a brief general discussion about the parameters which affect the accuracy of HONO determination by means of the TCM-Na,CO,-Na,CO, denuder sampling line, as well as an evaluation of the resulting minimum detectable concentrations. In this system, the measured amount of HONO SM is linked to the real amount S entering the system by the following relationship:
(2)
where q1 and q2 indicate the sum of nitrite and nitrate molar amounts determined on the first and second Na,CO, denuders, respectively, and E is the collection efficiency for HONO. It is necessary to determine also nitrate amounts in order to take into account the possible small oxidation processes (Perrino et al., 1990);for this reason, ion chromatography has been chosen as detection technique. In equation (2), q1 also includes the small nitrite amount possibly retained on the TCM denuder, which can be either determined by the Griess-Saltzmann calorimetric technique or desorbed from the TCM to the Na,CO, denuder at the end of the sampling. The amounts qr and q2 can be expressed a.s follows:
-
= I,(NO,)
= a[NO,] T
with a<3 x IO-’ m3s-‘, [NO,] air concentration of NO, and T sampling time; I(PAN) is the small interference due to the collection of peroxyacetylnitrate (PAN), which can be expressed as follows: I,(PAN) = ETANFT[PAN] I,(PAN) = E;AN(l - ETAN)FT[PAN]
-
with ET*“’= E!*N z 10-l and F sampling flow rate; I(bk) is the nitrite amount due to the analytical blanks: I,(bk) N I,(bk) 2: 2 x lo-’ pgdevice-’
-
I,(SO1, NO*) is the small residual interference on the TCM denuder due to the contemporary presence of high concentrations of SO, and NO1, which can be
Measurement of high concentration of nitrous acid inside automobiles RESULTS AND
expressed as follows (Febo et al., 1993): WQ,
= YCNO,IFT
NW
where the value of y depends on SOz air concentration. It follows that y[NO,]FZ’+ 1s = -
(EPAN)‘FT[PAN] (7)
E2
From the knowledge of the concentration values [NO,], [SO,] and [PAN] during the sampling period, we can estimate the maximum values of IR and ~‘(1~). If we consider the mixing ratios relative to IR and uZ(IR), by dividing equation (1) by F. T we get CM=j?C+
CR.
In the conditions of the experiment here reported, we have for urban areas
I:SO,l “<30 ppb, WAN] ‘Z2 ppb, [NO,] z 50 ppb and for rural areas
CSW ? 5 ppb, I:PAN] Z 3 ppb, I:NO,] Z 5 ppb. It follows that for y we have the minimum values of about 5 x 10m3 (urban areas) and 1 x 10e3 (rural areas), respectively, and that we can cstimate the values of CR and u(CR) as follows: urban areas:
CR < 0.25 ppb,
u(CR) < 0.07 ppb rural areas:
CR < 0.05 ppb,
u(CR) < 0.025 ppb. Thus, we can calculate the following quantitation limits for HONO, which only derive from the use of this sampling method: urban areas:
C > 0.75 ppb,
rural areas:
C > 0.25 ppb.
If we take into consideration also the limits of the analytical technique (IC), we have no variations in the value of C for urban areas, while the quantitation limit for rural areas increases to a value of about 0.5 ppb. In addition to these theoretical considerations, the technique gives the opportunity of directly testing the quality of the data gathered in the field. From the experimental data, in fact, we can calculate the value of the two parameters d = q1 - q2 and s = q1 + q2. From equations (3) and (4) we get s - d = (1 - /9E2)S + 21,. In case the system maintained its efficiency during the whole sampling period (B = l), the above relationship allows us to obtain an experimental value I;:
z* = (s - 4 - (1 2
@IS
?
which must be in agreement with the estimated value I, obtained from the air concentration of NO,, SO, and PAN (equation (6)). The observation of a possible disagreement between the values of I!~and 1; allows any malfunctioning of the sampling line during the samplings to be clearly detected.
341
DISCUSSION
Automobiles constitute an experimental environment of great interest, since they can be easily transferred from very polluted to unpolluted environments, and since the temperature of the interior can be easily changed by simply exposing the car to Sun rays; that is to say, some of the parameters involved in the generation and adsorption-desorption mechanisms (concentration of nitrogen dioxide, temperature) can be easily modified while remaining in field conditions. Nitrous acid and nitrogen dioxide concentrations, simultaneously measured inside and outside the automobile during its transit through the centre and the suburbs of the city of Rome, are reported in Table 1, together with the range of temperatures observed during the experiments. The results show that quite high concentration levels of HONO, ranging from 7 to 29 ppb, could be measured inside the car, in the presence of low outside HONO concentrations (below 2 ppb). NO, values measured inside (21-43 ppb), instead, did not differ very much from those measured outside (26-52 ppb). The average ratio between inside and outside concentrations (a parameter which does not suffer from the possible poor precision of the sampled volume) was 14.8 in the case of HONO (range 5-30) and 0.85 in the case of NO, (range 0.71-0.97). This is in agreement with the model of HONO heterogeneous formation on the interior surfaces of the vehicle, and subsequent release in the gaseous phase. A possible explanation for the slight negative difference between NOz concentration levels measured inside and outside the vehicle is given by the hypothesis of NO, consumption due to the heterogeneous reaction forming HONO. The data of Table 1 also show, as expected, a slight difference between the concentration levels measured during the run across the centre of the city and those observed during the runs across the suburbs, for both NO, and HONO. The data of Table 1 also give a first indication that the highest HONO indoor concentration values occur together with the highest values of inside temperature. A second set of experiments was deviced in order to evaluate the persistency of HONO in the environment under study, and to investigate the effect of temperature variations. The persistency of HONO had been already verified during the experiments carried out inside homes by Febo and Perrino (1991), and constitutes a good indication of the significance of the adsorption-desorption phenomena, which are governed by the temperature. The experiments were planned in the following way: (1) the car was kept for many hours in an urban area (city of Rome and suburbs); (2) then it was placed for 2 h in a sheltered parking lot located in a rural area characterized by very low HONO and NO, concentration levels; (3) then it was moved in the Sun and left there for some hours.
348
A. FEBO and C. PERRINO Table 1. Measurement
of HONO and NO, concentration values inside and outside an automobile in a polluted area
HONO (ppb)
NO, (ppb)
Inside
Outside
Inside
Outside
Temp. range (“C)
* * * * * * * *
9.9 11.2 9.1 10.0 6.8 18.7 9.2 10.2
0.9 1.5 0.6 1.5 1.2 0.6 0.5 0.7
32.4 25.0 34.4 35.8 22.9 36.7 21.4 33.5
40.7 32.4 36.4 38.8 26.5 40.1 30.2 36.4
24-28 22-25 21-29 27-31 25-30 30-40 22-25 22-27
Avg.
10.6
0.9
30.3
35.2
# # # # # # # #
13.7 10.8 12.1 19.3 27.0 11.7 10.6 29.2
0.7 1.5 1.1 1.0 1.9 0.8 0.6 1.4
37.6 40.9 42.1 25.1 37.5 36.9 42.9 30.6
47.0 46.2 43.3 31.5 40.8 46.5 51.6 41.2
Avg.
16.8
1.1
36.7
43.5
24-29 17-18 23-29 28-33 30-41 23-27 22-27 35-39
* Suburbs; # Centre.
Denuder measurements were carried out during the last hour of phase 1 and during phases 2 and 3. The results, reported in Table 2, indicate that quite high concentrations of HONO persisted in the examined
air volume far after the removal of the car from the polluted site (traffic); this underlines the importance of the adsorption-desorption phenomena, which uncouple, from a temporal point of view, the occurrence of the formation reaction from the release of the reaction product in the gaseous phase. The ratio between indoor HONO and outdoor NO* concentrations increased, in fact, from the values in the range 0.2-0.5 determined in the urban area (phases 1) to the values in the range 6-60 determined in the case of the rural area (phases 2 and 3). It is worth noting that in outdoor atmospheres the ratio between HONO and NO, does not usually exceed the values 0.1-0.2. The results also show that HONO concentration in the third phase of the experiments exceeded by far the concentration measured during the second phase, showing the dependence of the gas-phase concentration of HONO on indoor temperature, in agreement with the proposed desorption mechanism. For a better description of the time trend of HONO release in the gaseous phase, some experiments were carried out by measuring HONO concentration during phases 2 and 3, also by means of a modified chemiluminescence analyser. The time trend of NO,, NO and HONO concentrations, as measured by the modified chemiluminescence detector during the last one of the experiments reported in Table 2, is shown in Fig. 1. A very good agreement between the data yielded by the two techniques was obtained (per cent
difference between the results yielded by the two methods ranged from 0.6 to 3.3%). Before the experiment of Fig. 1, the car was kept for the whole night in an urban site, then it was moved to the rural site. NO and NO* concentrations at the rural site were below 1 and 2 ppb, respectively; HONO concentration was below 1 ppb. From the beginning of the experiment until time t, the temperature inside the car was approximately constant (about 27S”C). At time t, the car was placed in a position partially exposed to Sun rays; at time t, it was moved again and exposed to Sun rays completely. An air conditioning system, with introduction of outside air, was set on at time t, and off at time t,. The data of Fig. 1 clearly show that a relationship between the temperature and the concentration of HONO occurs; the obvious dependence on the ventilation factor is also evident. At the end of the ventilation phase (time t4), a new increase can be observed in the HONO concentration, which tends to reach the new equilibrium value (which depends on the temperature and on the amount of HONO in the adsorbed phase at that time). Similar experiments showed that a brief opening of the windows also causes a temporary decrease of HONO concentration to outside levels; however, as soon as the windows are closed, HONO concentration increases again to the equilibrium value. It is worth noting that NO and NO, concentration levels inside the car increase together with the increase of HONO concentration. This observation, which confirms the data of Table 2, can be justified by the hypothesis of the following dissociation reaction of
Measurement of high concentration of nitrous acid inside automobiles
349
Table 2. Temporal trend of nitrous acid concentration inside an automobile
HONO(mb) Date
NO, (ppb)
Phase
Inside
Outside
Inside
Outside
156.92
1 2 2 3 3
16.7 20.5 21.1 38.2 52.0
1.0 0.6 0.7 0.6 0.5
28.2 5.4 6.4 10.1 13.5
36.9 3.2 3.5 3.7 3.5
28-32 30-32 32-33 33-48 48-55
16.6.92
1 2 2 3 3
19.0 21.8 22.7 32.5 42.7
1.1 0.7 0.5 b.q.1. b.q.1.
36.3 3.9 4.2 5.1 7.4
41.7 2.1 2.2 2.6 2.5
30-33 30-34 34-35 35-41 41-45
17.6.92
1 2 2
15.5 19.7 19.8
0.9 0.6
36.7 3.7
42.3 3.1
28-31 30-30
b.q.1.
3.6
2.7
30-31
3
24.3
3
47.7
b.q.1. b.q.1.
7.3 12.1
3.2 2.6
31-38 38-41
23.6.92
1 2 2 3 3
14.0 15.6 17.4 30.8 49.2
0.7 b.q.1. b.q.1. b.q.1. b.q.1.
27.9 1.2 1.4 7.4 12.5
37.1 1.0 1.0 0.7 0.8
26-29 29-30 30-30 31-46 46-52
24.6.92
1 2 2
12.3 13.0 21.9
0.5 b.q.1.
31.6 b.q.1.
38.6 1.4
26-28 25-31*
3 3
35.8 37.7
b.q.1. b.q.1. b.q.1.
0.5 3.4 5.5
1.2 1.3 1.4
- -
Temp. range (“C)
31-36* 36-46 44-47 #
* Partial exposure to Sun rays. # Air conditioning on. b.q.1. Below quantitation limit.
HONO: HONO,,,
+ HONO,,,
+ NO + NO* + H,O .
The difference in the concentration levels of NO and NO2 could be due to a different removal rate of the
two compounds; NOz, for example, may start the HONO formation cycle again. To summarize, for a given autovehicle, the following parameters govern the occurrence of indoor HONO concentration: -
-
concentration of nitrogen dioxide and exposure time, which determine the HONO amount produced in the adsorbed phase; ambient temperature, which determines the rate of release in the gaseous phase; airing of the vehicle, which, obviously, influences the ventilation factor and thus the equilibrium concentration of HONO.
In the scientific literature, HONO formation constants ranging frolm 10m4 to 10m6 ms-’ (relative humidity content: about 25,000 ppm) have been reported, the values of the constant depending on the characteristics of the generation surfaces (Lammel et al., 1990; Febo and Perrino, 1991). Before being used for the experiment of Fig. 1, the car was kept for about 18 h in an urban site where the concentration level of
nitrogen dioxide was of the order of 50 ppb; thus, the formation of HONO on the interior surfaces of the car (having an area of about 10 m’) which took place during this period may reasonably lead to a HONO amount of the order of a few milligrams. During this loading time, the desorption of HONO in the gaseous phase also occurred, with a desorption rate which can be assumed to be of the order of 10m4 s-l. As a consequence, at the beginning of the experiment (phase 2) a HONO amount of the order of a few hundreds of micrograms can be reasonably estimated to still exist in the adsorbed phase. In this phase of the experiment, an air volume of about 2 m3 was available for the ripartition between the adsorbed and the gaseous phase; by taking into account the air exchanges during the experiment, the occurrence of a HONO air concentration of about 50 ppb is thus plausible, and in agreement with the above reported order of magnitude of the formation constants.
CONCLUSIONS
The preliminary results reported in this paper indicate that quite high and persistent concentrations of nitrous acid can be measured inside automobiles. The interior of automobiles, in fact, can be regarded as
350
A. FEBO and C. PERRINO
60
1
3
2
4
Time (h) -
HONO-
NO2 ----NO
Fig. 1. Relationship between indoor temperature and the concentration HONO, NO, and NO inside an automobile.
both a reaction chamber (where the nitrogen dioxide produces nitrous acid) and as a reservoir, when the produced amount of nitrous acid is slowly released into the gaseous phase, the extent of the release depending on the temperature. As a consequence, drivers and occupants are subjected to a notable exposure to this pollutant, that is a health risk the consequences of which still need to be evaluated. From an environmental point of view, automobiles constitute a series of spot sources of nitrous acid, which (as well as other indoor environments) contribute to urban pollution. This study, which indicates the significance of the surface reaction of NO, as a source of HONO and of the adsorption-desorption processes, can also be of help in the development of models intended to describe HONO behaviour in the atmospheric environment.
REFERENCES
Allegrini I., Cortiello M., Febo A. and Perrino C. (1990) Generation of standard atmospheres of nitrous acid. In Physic+Chemical
Behaviour of Atmospheric
Pollutants
(edited by Restelli J. and Angeletti G.), pp. 140-144. Kluwer, Dordrecht. Brauer M., Barry Ryan P., Suh H. H., Koutrakis P. and Spengler J. D. (1990) Measurement of nitrous acid inside two research houses. Envir. Sci. Technol. 24, 1521-1527.
of
De Santis F., Febo A. and Perrino C. (1987) Nitrite and nitrate formation on a sodium carbonate layer in the presence of nitrogen dioxide. Anna/i di Chimica 20, 763-769. Febo A. and Perrino C. (1991) Prediction and experimental evidence for high air concentration of nitrous acid in indoor environments. Atmospheric Environment 25A, 1055-1061.
Febo A., De Santis F., Perrino C. and Giusto M. (1990) Evaluation of laboratory and field performance of denuder tubes: a theoretical approach. Atmospheric Environment 19,1517-1530.
Febo A., Perrino C. and Cortiello M. (1993) A denuder technique for the measurement of nitrous acid in urban environments. Atmospheric Environment 27A, 1721-1728. Jenkin M. E., Cox R. A. and Williams D. J. (1988) Laboratory studies of the kinetics of formation of nitrous acid from the thermal reaction of nitrogen dioxide and water vapour. Atmospheric Environment 22,487-498.
Lammel G., Perner D. and Warneck P. (1990) Nitrous acid at Mainz: observation and implication for its formation mechanism. In Physico-Chemical Behauiour of Atmospheric Pollutants (edited by Restelli J. and Angeletti G.), pp. 469-476. Kluwer, Dordrecht. Perrino C., De Santis F. and Febo A. (1990) Criteria for the choice of a denuder sampling technique devoted to the measurement of atmospheric nitrous and nitric acids. Atmospheric Environment 24A, 617-626. Pitts J. N. Jr (1983) Formation and fate of gaseous and particulate mutagens and carcinogens in real and simulated atmospheres. Enoir. Hlth Perspect. 47, 115-140. Pitts J. N. Jr, Wallington T. J., Biermann H. W. and Winer A. M. (1985) Identification and measurement of nitrous acid in an indoor environment. Atmospheric Environment 19, 763-767.
Measurement of high concentration of nitrous acid inside automobiles Pitts J. N. Jr, Biermann H. W., Tuazon E. C., Green M., Long W. D. and Winer A. IM. (1989) Time-resolved identification and measurement of indoor air pollutants by spectroscopic techniques: gaseous nitrous acid, methanol, formaldeyde and formic acid. JAPCA 39. 1344-1347. Possanzini M., Di Palo V. and Masia P. (1993) Evaluation of a denuder method for ambient NO, measurement at ppb levels. Int. J. envir. tmalyt. Chem. 49, 139-147.
351
Sakamaki F., Hatkeyama S. and Akimoto H. (1983) Formation of nitrous acid and nitric oxide in the heterogeneous dark reaction of nitrogen dioxide and water vapour in a smog chamber. Int. J.-c/rem. Kinet. 15, 1013-1620. Svensson R..._Liunastrom E. and Lindauist 0. (1987) _ ~ I Kinetics of the reaction between nitrogen dioxide and water vapour. Atmospheric Environment 21, 1529-1539.