Envlron. Sci. Technol, 1084, 78, 82-87
Street Canyon Concentrations of Nitrogen Dioxide in Oslo. Measurements and Model Calculations Bystein Hov" and Steinar Larssen Norweglan Institute for Air Research, N-200 1 Llllestr0m, Norway
rn Concentrations of NO2 in excess of the frequently quoted exposure limit of 190-320 pg of N02/m3 (95-160 ppb) as hourly mean have repeatedly been recorded in a street in Oslo during the winter. In Jan 1981, the hourly mean concentrations exceeded 200 ppb on 7 days; the maximum was 270 ppb. Summertime concentrations were much lower. With a simple model of the combined effect of chemistry and dilution of the exhaust gas plume, it was shown that NO2 generation through the reaction NO + NO + 02 NO2 + NO2 +
may explain a small fraction of the NO2 formation. The study indicated that typically 5-10% of street level NO, can be taken as NO2 formed through this reaction. To account for the NO2measured, it is suggested that the NO2 fraction of NO, in car exhaust may be higher in the driving conditions found in Oslo during the winter than what is recorded in the standard cycles for car exhaust emissions testing.
Introduction A nitrogen dioxide concentration of 940 pg/m3 (470 ppb) has been selected as an estimate of the lowest level at which adverse health effects due to short-term exposure to nitrogen dioxide can be expected to occur ( I ) . When a minimum safety factor of 3-5 is adopted, a maximum 1-h exposure of 190-320 pg of N02/m3(95-160 ppb) should be consistent with the protection of public health. This exposure should not be exceeded more than once per month ( I ) . This exposure limit has been frequently exceeded during the winter in street canyons in Oslo, as well as in other cities in North Europe. As examples, an hourly mean concentration of 270 ppb of NO2was recorded in St. Olavs street in Oslo at 0900 h on Jan 13,1981 (see Figure l), and 390 ppb of NO2 was measured in Gothenburg on Jan 16, 1980, on a downtown sampling site, 20 m above the street level (2). At this site, more than 500 ppb of NO2as hourly mean has been recorded on a number of occasions during the last winters (3). Curbside air pollution measurements in London during the years 1972-1978 showed maximum hourly mean concentrations of NO2of -300 ppb, with the highest levels during the winter season (4). Measurements in Frankfurt/M on a few winter days indicated average NO2 concentrations as high as 240 ppb in the middle of the street for some categories of streets, with a substantial fall off toward the pavement (more than a factor 3) (5). The monitoring of NO and NO2 concentrations in a street canyon (St. Olavs street) and on a nearby rooftop in Oslo has been carried out during selected winter and summer months for a few years. The highest NO2 concentrations have been found during the winter. In this study observations taken in Jan 1981 were analyzed, and a simple numerical model of the chemistry and turbulent mixing of the exhaust gases from vehicles was developed.
NO, Measurements The concentrations of NO and NO2 were monitored continuously during the time period Dec 1980-Feb 1981 82
Environ. Sci. Technol., Vol. 18, No. 2, 1984
on the pavement at a street level site in the center of Oslo and at a nearby roof top station assumed to be representative of the urban atmosphere away from the street canyons. The location of the sampling inlet is shown in Figure 2. The measurements were made by cyclic chemiluminescence, where one detector operated in a cyclic mode in which the air alternatively passed through a NO2 to NO converter or a bypass. The cycling time was 30 s, short enough to exclude the possibility of significant bias in measured NO2 concentrations due to the separation in time between NO and NO, measurement (6). The effect of the inlet residence time on the analysis as described in ref 7 was thought to be negligible, in particular during the winter when light intensities are low and the characteristic time for NO2 dissociation is rather long compared to the residence time of the air sample in the inlet of the instrument (5-10 s). The instruments were calibrated in the laboratory and in the field by means of a NO2 permeation tube system available commercially. The instrument output (NO, and NO) was integrated to give hourly mean values by means of electronic integration, and the concentration of NO2 was found by the difference. Measurement errors in the NO2 concentration due to instrument drift and less than 100% efficiency of the converter were thought to be less than 5%. The diurnal variation of the concentration of NO2 and the ratio N02/N0, (volume) for all days in Jan 1981 when the NO2 concentration exceeded 200 ppb as an average over 1 h in St. Olavs street in Oslo are shown in Figure 1. This occurred on 7 days, and the highest concentration was 270 ppb (Jan 13 at 0900 h). The hourly average NO2concentration exceeded 200 ppb on 27 occasions during Jan 1981, during the morning or afternoon rush hours, or both. The average street canyon concentration for the 27 h when 200 ppb was exceeded was 230 ppb for NO2 and 850 ppb for NO, with a peak hourly NO concentration of 1195 ppb (NO2was then 265 ppb). The highest hourly average rooftop concentration during the 27 h when the hourly average street canyon concentration of NO2exceeded 200 ppb was 150 ppb for NO2with 65 ppb as an average, and for NO the maximum was 560 ppb, with 280 ppb as an average. Hourly average NO and NO2 concentrations were measured successfully 98.4% of the time (732 out of 744 h) in the street canyon during Jan 1981 and 95.6% of the time on the rooftop. There were no simultaneous ozone measurements taken. Boundary layer ozone is rarely monitored during winter in Scandinavia. Local buildup of ozone of any significance is considered to be an unlikely event during the dark winter months from November to February. Ozone was measured within the urban area of Gothenburg, about 2 km south of the downtown area, 15 m above the ground, during the period Jan 1972-Aug 1973 (8). The maximum hourly mean ozone concentration during Jan 17-Feb 10,1972, was 42 ppb, Nov 1-30,1972,49 ppb, Dec 1-31,1972,43 ppb, Jan 11-31,1973,39 ppb, and Feb 1-28, 1973,39 ppb. These numbers also indicate the maximum ozone concentrations to be expected at rooftop in Oslo during the winter months.
0013-936X/84/0918-0082$01.50/0
0 1984 American Chemical Society
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2
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8
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Figure 1. Street canyon measurements of NOpand N02/NOxin St. Olavs street in Oslo, Jan 1981, for ail days with hourly NOpmaximum exceeding 200 ppb. On the diagram is indicated date, maximum and minimum temperature in O C together with time of occurrence, wind maximum and time of occurrence, and diurnal mean wind in meters per second, with cloud cover in fractions of 8 at 0700, 1300, and 1900 h. Also indicated are monthly mean maximum and minimum temperature and maximum and diurnal mean wind. /
/
/
sampling Inlet
Figure 2. Location of the sampling site in the street canyon. Neighboring rooftops are indicated.
The daily maximum and minimum temperatures are also indicated in Figure 1 at their times of occurrence, together with maximum and mean diurnal wind in meters per second and degree of cloudiness expressed in fractions of 8, at 0700,1300, and 1900 h. All the meteorological data were collected at The Norwegian Meteorological Institute approximately 2 km away from the site of measurement. Several conclusions may be drawn from Figure 1: The NO2 concentration peaks were fairly well correlated with the peak in traffic density through the street. Temperature and cloudiness did not seem to influence the level of NO2 in a straightforward manner, because high NO2 concentrations were found both on fairly warm (Jan 30) and cold days (Jan 26) and in cloudy weather (Jan 21) and fairly clear weather (Jan 26). The winds were low on all 7 days shown in Figure 1, however, indicating that ventilation usually is poor on days with high NO2concentrations. The mean daily wind for the 7 days shown was less than 1ms-l, compared to 2.3 ms-l for all of January. The ratio N02/N0, stayed between 0.2 and 0.3 by volume when the NO2 concentration was high on all the days shown in
Figure 1. This is a useful result, which is further illustrated in Figure 3, where this ratio was plotted as a function of the NO, concentration for Jan 1981 (upper curve). In the lower part of Figure 3, the simultaneous NO2 and NO, concentrations obtained at rooftop were subtracted from street level NO2and NO,, and the resulting NO,/NO, ratio was plotted against NO,. These concentrations were then believed to represent the immediate street contribution. It is fairly obvious that in this street -25% is a fair estimate of the NO2fraction of NO, by volume for medium to high NO, concentrations during Jan 1981.
Formation of NOP in Vehicle Exhaust A fairly simple model of the simultaneous effect of chemical conversion of NO to NO2 and turbulent dilution of the vehicle exhaust plume was developed. It is rather similar to the power plant plume model published by Schurath and Ruffing (9). The model was used to find out if the following reaction mechanism, together with reasonable assumptions about exhaust gas dilution in the air, could provide an explanation for the high NO2concentrations occasionally observed in the street: NO
+ NO + 0 2 L N 0 2+ NO2 NO + O3 -!% NO2 + O2 NO2 + hv 2 NO + 0 0 + 0 2 M -% 0 3 4- M
(R1) (R2) 033)
034) The rate coefficients Itl, It2, and k4 as recommended by the National Bureau of Standards (11)are (6.6 X exp(530/T) cm6molecule" s-l, (2.1 X 10-l2)exp(-1480/T) cm3 (molecule s)-l, and (1.1X exp(510/T) cm6molecule-2 s-l, respectively, where T is the temperature in kelvin. Swedish studies (12) of reaction R1 in polluted air, summarized in (13), indicated that there was a significant contribution to the oxidation of NO to NO2 of reactions catalyzed on street surface material, which gave an overall Environ. Sci. Technoi., Voi. 18, No. 2, 1984
83
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Figure 3. Street canyon measurements of the ratio NO,/NO, (by volume) as a function of the NO, concentration for St. Oiavs street in Oslo, Jan 1981. The numbers on the diagram indicate the number of observations with coordinates which coincide (upper part). In the lower part, NO,/NO, is shown as a function of NO, where the measured rooftop concentrations of NO2and NO, were subtracted to give what was thought to be the net contribution to the NO, and NO, concentrations from vehicle exhaust in the street.
Table I. Values of k , at Various Temperaturesa temp, O C k,,aNBS ( 1 1 ) k,,a catalyzed (13) 12 4.2 x 1 0 - 3 8 7.7 x 10-38 0 4.6 x 1 0 - 3 8 1.1 x 10-37 -13 5.1 x 1 0 - 3 8 1.4 x 10-37 a Unit: cm6 molecule-2 s-l.
expression k1 = (1.50 X lo4) exp(1780/T) cm6molecule-2 ~ ~ ( 1 3The ) . NBS value and the catalyzed value of kl are given in Table I for three different temperatures. There was a factor of 2 increase in k l in the catalyzed case when the temperature was lowered from 12 to -13 OC, while the difference was only about 20% for the values recommended by NBS. The following system of equations was set up to describe the combined effect of chemistry and dilution with time in the exhaust gas plume: 84
Envlron. Sci. Technoi., Vol. 18, No. 2, 1984
where brackets denote concentration. D denotes the Lagrangian derivative along the trajectory of the exhaust gas puff. DV/dt is the Lagrangian derivative of the puff volume along its trajectory. The measured hourly average NOz concentration represented the average of a large number of puffs of various age and dilution, of exhaust from the flow of cars passing slowly by the point where NO and NOz were monitored. It is difficult to do a realistic model simulation by following each individual puff reaching the instrument inlet and then averaging over each hour. The assumption was made that during the hours when the NO2concentration in the street was at a maximum, there was continuous traffic through the street, and on the average the concentration of NO, in the exhaust puffs reaching the inlet to the instrument was diluted to a level typical of a polluted situation in the street canyon. The initial conditions were [NO], = 1500 ppm, [NOz], = 0, [03], = 35 ppb, and [O2lO= 5%. Seinfeld (ref 10, p 359) quoted 1500 ppm as a typical level of NO, in car exhaust. Swedish investigations (14) indicated that car exhaust generally contained more than 2000 ppm of NO, when driving at speeds around 80 km h-l, 1200 ppm at 50 km h-l, and 500 ppm at 25 km h-l. The choice of 1500 ppm as the initial NO, concentration may therefore be an overestimate for the street canyon studied. In the calculations, the background (rooftop) concentrations were taken as [NO], = 225 ppb, [No& = 75 ppb, [03]b = 35 ppb, and [0&= 21%. It was assumed that the amount of initial NO, was diluted 2500 times at the time the model puff reached the monitoring point, and this would take 100 s. It is evident that the length of the time period before the NO concentration drops significantly is very important for the NOz formation, since the square of the NO concentration enters the rate equation for NO> To investigate how the NOz generation depended on the change of dilution with time, a number of dilution functions were introduced. These are shown in Figure 4. For function 1, the dilution was very efficient initially, resulting in a rapid decline in the NO concentration. For higher function numbers, the length of time with a substantial NO concentration in the exhaust gas plume increased. A realistic measure for the oxygen content of exhaust is probably in the range 3-570 (ref 10, p 360). The unburned hydrocarbons were assumed not to play any role in the NO to NO2 conversion in the street canyon. On a time scale of 1 h or more in the sunlit atmosphere, conversion of NO to NO2 through photochemical smog reactions is important. On the temporal scale of interest here, a few minutes only, the photochemical smog reactions are unimportant for the formation of NOz within the street canyon but may indirectly influence the street canyon
NO, NO
b
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Figure 4. Dilution as a function of time, for elght cases of exhaust puff volume development.
processes through the establishment of the rooftop level of NO2. The following factors were thought to influence the NOz formation in vehicle exhaust in a street canyon and were investigated further: (a) choice of kl (NBS recommendations or catalyzed value), (b) oxygen deficit initially in the exhaust gas, (c) the degree of dilution and the intensity of mixing with time, (d) background concentrations of NO, NOz, and 03, (e) the ratio of N02/N0, in the exhaust emissions, (f) initial concentration of NO, (g) characteristic reaction time of the exhaust gases, and (h) air temperature. With respect to point e, the NOz/NO, ratio in vehicle exhaust emissions seems to be quite variable, between 0 and 0.54 by volume from various mobile sources according to ref 15-18. The ratio seems to be very close to zero in the exhaust from vehicles equipped with internal gasoline combustion engines and variable with respect to driving mode and make of car for diesel engines, with values up to 0.50 or more. NO,/NO, in diesel exhaust is low for full load driving conditions, higher at low load, and reaching 0.56 for one make of cars during idle (16). This is an extreme value, however, with N02/N0, ratios up to 0.05 or 0.10 by volume most commonly occurring in diesel exhaust. Equations i-iv were solved by a QSSA (quasi-steadystate approximation) technique described in ref 19 and independently by using a Gear routine (20,21). The numerical error associated with the computations was negligible.
Results and Discussion Figure 5 shows the development with time of the concentrations of NO and NOz in the exhaust gas puff for various dilution functions, in the case with temperature 0 "C, and by using the catalyzed value for K1. In all cases the NO, concentration was the same (0.90 ppm) after 100 s of integration. The NO2 fraction varied with the development with time of the dilution, however, as shown in Figure 6. Three processes contributed to the NOz concentration in the exhaust gas: (I)mixing with ambient air where the NOz concentration was 75 ppb; (11)reaction between NO and O3where, in a small fraction of a second, ozone (the ambient ozone level was 35 ppb) was depleted and an equivalent amount of NO2was generated (dilution and reaction between NO and O3consequently contributed
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Figure 5. Calculated NOpand NO concentrations as a function of time for dilution functions 1, 3, 5, and 8 in a case with 1500 ppm of NO, zero NOz, and 5 % of Oz present initially. The vehicle exhaust was diluted 2500 times in 100 s. The ambient levels of NO, NOz, and O3 were 225, 75, and 35 ppb, respectively. The temperature was 0 OC, and the catalyzed value for k , was applied.
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Figure 6. NOz/NOx(by volume) for dilution functions 1, 5, and 8 for the same case as shown in Figure 5.
Table 11. Amount of NO, Generated through Reaction R1 after 100 sa dilution function 1 2 3 4 5 6 7 8 amount ofNO, 3.5 4.4 26 33 40 53 56 94 formed, ppb a T = 273 K. The catalyzed value of k , was used. k , = J = 0. Compare reactions R 2 and R3.
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110 ppb of NO2 in the runs shown in Figures 5 and 6); (111)the amount of NO2generated after 100 s through the reaction between NO, NO, and Oz for a case where reactions R2-R4 were put to zero (shown in Table 11). It can be seen from Figures 5 and 6 and Table I1 that the choice of dilution function was critical for the formation of NOz. Significant quantities were formed only when the initial dilution was slow enough to leave some time for reaction R1 to act efficiently. Figure 5 shows how a fairly constant level of N02/N0, was reached quite rapidly. The ratio was typically 0.10-0.20, and the NO2 concentration after 100 s was 110-200 ppb (Figure 5). These numbers were lower than the measured NO2 concentrations and NOz/NO, ratios shown in Figure 1, indicating that the processes modeled only to a certain degree could contribute to the NO2 conEnvlron. Sci. Technol., Voi. 18, No. 2, 1984
85
Table 111. Net Formation of NO, (in ppb) through Reaction NO + NO + 0, NO, + NO, ( R l ) , after 100 s of Integration and 2500 Times Dilution of the Initial Mixture (1500 ppm of NO, Zero of NO,, 5%of 0,)of Vehicle Exhaust for Two Temperatures and Two Choices of k , a --f
net NO, formed, ppb (NO,/NO, (volume)) 273 K
dilution function no.
k l (NBS)
2 3 5 7 8
0 (0.12) 8.1 (0.13) 14 (0.14) 21 (0.15) 37 (0.16)
260 K k (catalyzed)
k , (NBS)
1.9 (0.12) 0 (0.12) 22 (0.15) 8.2 (0.13) 35 (0.16) 1 5 (0.14) 50 (0.18) 23 (0.15) 8 3 (0.21) 39 (0.17) a Ambient air concentrations were 225 ppb of NO, 75 ppb of NO,, and 35 ppb of 0,. Also ratio (volume) in the exhaust after 100 s.
k , (catalyzed)
2.8 (0.13) 30 (0.16) 48 (0.18) 67 (0.20) 1.1X lO’(O.24) shown is the NOJNO,
Table IV. Net NO, Formation (in ppb) through Reaction R1 for Various Levels of Molecular Oxygen Initially at 0, 5, and 21%a net NO formation, ppb, at initial oxygen of dilution function 0% 5% 21% 2 1.7 1.9 2.3 3 17 22 37 35 89 5 16 7 34 50 97 83 1.3 X 10’ 8 68 a T = 273 K, catalyzed values of k , ; otherwise the same case as shown in Table 111. -171915)
Figure 7. Rate of production of NOp through reaction R 1 in ppm per second, as a function of time, for various dilution functions (indicated on the graphs) and for the same case as in Figure 5.
centrations measured in St. Olavs street in Oslo. The production of NO, in ppm per second through reaction R1 for various dilution functions is shown in Figure 7. A marked change occurred when moving from function 2 to 3. For function 2 and lower, there was a slight NO, formation initially, declining rapidly. For functions 3 and higher, the high initial NO concentration was maintained for a few seconds, leading to NO2formation. As the mixing was intensified, the oxygen deficit was removed, leading to a brief peak in the NOz production rate. Factors Influencing the NO, Formation. The influence of factors a-h, discussed under Formation of NO, in Vehicle Exhaust, on the NO, formation in the vehicle exhaust will now be addressed. (a) The NO, formation was much larger when the catalyzed value for k, was applied. There was typically a threefold increase in net NO, formation at 260 K for the catalyzed case, compared to the case where the NBS value for klwas used (Table 111). The NO,/NO, ratio went up as well, but it should be remembered that the total NO, concentration was determined by the ambient level of NOz, the amount of NO oxidized through reaction with 03,and the contribution from reaction R1. As long as the two former processes dominated, a sizable increase in net NO2 formation through reaction R1 did not give rise to much higher N02/N0, ratios. (b) The initial molecular oxygen deficit in the vehicle exhaust exerted a controlling power on the intensity of NOz formation. In Table IV is shown net NO, formation through reaction R1 in the case of zero oxygen initially, 5% which probably is fairly realistic, and ambient level. (c) The choice of dilution factor affected the NO, formation significantly, as is shown in Table V. The NO,/NO, ratio rose as the dilution factor increased, but it should be kept in mind that a sizable part of NO, was 86
Environ. Scl. Technol., Vol. 18, No. 2, 1984
Table V. Influence of the Dilution Factor on the Formation of NOZa dilution factor dilution 1000 2500 5000 function 2 1 3 (0.08) 1.9 (0.12) 0 (0.18) 3 69 (0.12) 22 (0.15) 3.4 (0.19) 5 94 (0.14) 35 (0.16) 10 (0.20) 1.3 X 102(0.16) 50 (0.18) 1 5 (0.21) 7 8 2.0 X 10’ (0.20) 83 (0.21) 30 (0.23) kl= 0 O(O.07) 0 (0.12) 0 (0.18) a Otherwise the same case as Table IV. Also shown is the NOJNO, ratio (volume) in the exhaust after 100 s (in parentheses).
made up by the ambient air level (75 ppb) and NO, formed through reaction R2. The ratio between NOz generated through reaction R1 and total NO, at the end of the integration decreased as the dilution factor increased. (d) The amount of NO, formed through reaction R1 was not significantly affected by the choice of ambient air As long as the initial NO concentration of NO, NO2,or 03. concentration in the exhaust was much larger than the ambient level (e.g., by a factor of loo), ambient air NO would not influence the NOz formation through reaction R1 in the exhaust. (e) When the initial NO,/NO, ratio in the exhaust was different from zero, less than 1500 ppm of NO was available initially. This had a more than linear impact on the NO, formation through reaction R1, since the NO concentration enters the rate expression in the square power. The total NOz concentration at the end of the integration increased with increasing initial NO,/NO, ratio, however. If NO, was zero initially, the concentration was 75 ppb (ambient NO,) + 35 ppb (NO2formed through reaction R2) net generated through reaction R1, after 100 s. If NO,/NO, was 0.15 initially, the sum was 75 ppb 35 ppb 90 ppb (initial NO2) + R1, while if NO,/NO, was 0.30 initially, the sum was 75 ppb + 35 ppb + 180 ppb
+
+
+
+ R1 after 100 s of integration.
( f ) This factor addresses the relationship between the
initial NO concentration and the NO2 formation through reaction R1. As the initial mixing became very slow, the ratio between the amounts of NO, formed through reaction R1 approached the ratio between the squares of the initial NO concentrations. (g) If the time required to dilute the vehicle exhaust to the street canyon concentration level was reduced, the NO2 generation through R l was reduced since the time period during which the NO concentration remained high became shorter. (h) A lowering of the air temperature increased the value of izl, and the formation of NO2through reaction R1 was consequently speeded up. This effect was discussed in point a above.
Conclusions This model study showed that reaction R1 may contribute somewhat to the concentrations of NO2 found in street canyons, in particular, at low winter temperatures. The intensity and time dependence of the mixing of the vehicle exhaust puffs had a critical influence on the NO2 formation through reaction R1, and only when the mixing during the initial few seconds was extremely slow did reaction R1 contribute significantly to the NO2formation. The effect of a decrease in temperature and the catalytic effect of street surface material may increase the net NO2 formation through reaction R1 -3 times at -13 “C compared to the situation in clean air at 0 “C. This effect was even more pronounced if the temperature was lowered further. The initial concentration of NO was important for the NO2 formation through R1 due to the quadratic dependency on the NO concentration in the formation rate expression for NO2. It had a less than additive effect on the NO2 concentration if a fraction of NO, initially was made up of NO2 On the other hand, changes in the ambient levels of NO, NO,, or O3had only an additive effect on the NO2 concentration. The magnitude of the molecular oxygen deficit initially in the vehicIe exhaust controlled the NO2 formation through reaction R1. A lower limit of the amount of NO2 in a street canyon can be found To imby just adding the rooftop levels of NO2 and 03. prove this estimate, a small fraction of the NO, found in the street canyon can be added, thought to represent the contribution from reaction R1. Depending on the geometry and the mixing conditions in the street canyon, this study indicated that typically 5 1 0 % of street level NO, can be taken as NO2 formed through reaction R1. The time resolution of the recorded NO2 concentrations indicated that the hourly mean values consisted of a number of peaks, representing exhaust puffs of various age and degree of dilution. The large spatial and temporal variability in the NO2 concentrations points a t a NOz production mechanism active before, or in the first seconds after, the release of the exhaust gas. It was concluded above that NO2 formation through reaction R1 may contribute somewhat to the NO2 levels found in the street canyon, but only under extreme conditions with respect
to initial mixing of the exhaust gas plume would this contribution be substantial. It is therefore suggested that the NOz fraction of NO, in car exhaust may be higher in the driving conditions found in Oslo during the winter than what is recorded in the standard cycles for car exhaust emissions testing. The results of a recent Swedish investigation of the N02/N0, ratio in emissions from gasoline-powered cars seem to support this suggestion (22). At idle, the NOz/NO, ratio went as high as 0.3 for a warm engine. Registry No. NOz, 10102-44-0; NO, 10102-43-9.
Literature Cited World Health Organization, Environmental Health Criteria 4. Oxides of Nitrogen, World Health Organization, Geneva, 1977. Gothenburg Health Authorities, Air Pollution in Gothenburg, Measurements (in Swedish), 1980. Greenfelt, P. Gothenburg, private communication, IVL, 1981. Apling, A. J.; Rogers, F. S. M.; Sullivan, E. J.; Turner, A. C. Stevenage, U.K., 1979, Warren Spring Laboratory Report LR 338 (AP). Rudolf, W. Staub-Reinhalt. Luft 1980, 40, 485-490. van de Wiel, H. J. Atmos. Environ. 1977, 11, 93-94. Butcher, S. S.; Ruff, R. E. Anal. Chem. 1971,43,1890-1892. Grennfelt, P. Gothenburg, 1975, Report B221, IVL. Schurath, U.; Ruffing, K. Staub-Reinhalt. Luft 1981,41, 277-281. Seinfeld, J. H. ”Fundamentals of Air Pollution”; McGrawHill: New York, 1975. Hampson, R. F.; Garvin, D. NBS Spec. Publ. (U.S.) 1978, No. 513. Lindqvist, 0.;Ljungstrerm, E.; Svensson, R. Atmos. Environ. 1982, 16, 1957-1972. Greenfelt, P.; Sjerdin, A., literature review of dark chemistry transformation reactions for NO, in the atmosphere and in precipitation, Gothenburg, 1981, preliminary report (in Swedish), IVL. Egeback, K. E. investigation of methods for collection and analysis of samples of vehicles exhaust (in Swedish), Studsvik, 61182 Nykoping, Sweden, 1972, internal note. Braddock, J. N.; Bradow, R. L. SAE [Tech.Pap.] 1975, No. 750682. Springer, K. J.; Stahman, R. C. SAE [Tech.Pap.] 1977, No. 770254. Springer, K. J.; Stahman, R. C. SAE [Tech.Pap.] 1977, No. 770258. Wimmer, D. B.; McReynolds, L. A. SAE Trans. 1962. Hesstvedt, E.; Hov, 0.;Isaksen, I. S. A. Int. J. Chem. Kinet. 1978,10,971-994. Gear, C. W. Commun. ACM 1971, 14, 176-179. Hindmarsh, A. C.; Byme, G. D. “Episode: An Experimental Package for the Integration of Systems of Ordinary Differential Equations”; Lawrence Livermore Laboratories: University of California, 1975. Lenner, M.; Lindqvist, 0.;Rosh, A. “The NO,/NO, Ratio in Emissions from Gasoline-Powered Cars: High NOP Percentage in Idle Engine Measurements”; Chalmers University of Technology: Gothenburg, 1981 (preprint).
Received for review September 10, 1982. Revised manuscript received May 18, 1983. Accepted August 9, 1983.
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