Effect of Nitrogen Oxide Emissions on Ozone Levels in Metropolitan Regions William B. lnnes Purad Inc., 724 Kilbourne Drive, Upland, California 91786
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Since ozone reacts rapidly with nitric oxide (NO O:< NO2 0 2 1 , NO controls would be expected to be counterproductive in controlling ozone levels. This study of the photooxidation process lends support to this view with respect to metropolitan areas. In the process of examining the relationships, the following subjects are considered: nature of the photooxidation process and calculation of NO oxidation; reactivity coefficients of organics; calculations of ozone levels after NO depletion; calculation of peak ozone levels; Los Angeles basin ozone levels; ozone levels a t high HC/NO,; emissions, ozone levels, and meteorology in metropolitan areas other than Los Angeles; the ambient air standard for NO2 vs. NO? levels: effects due to nitrates from NO, emissions.
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The high cost of NO, controls and their effect on fuel usage, together with the rapid reaction of nitric oxide with ozone, make it imperative to better understand the consequences of controlling NO,. Maximum ozone concentrations in the Los Angeles basin, instead of decreasing with the advent of catalytic converters, have increased from ca. 0.35 to 0.45 ppm from 1975 to 1979. This was reported in a 1980 conference on Los Angeles air quality ( I ) .The current strategy of reducing both HC and NO, emissions appears to have been ineffective. Some of the factors other than NO, controls discussed a t this conference that might account for this included the following: growth, meteorology, measurement error, and catalytic device failures. None of these or even a combination appears to offer a good answer. Heuss et al. (Los Angeles) ( 2 ) ,Dervent and Hov London) ( 3 ) , and Cleveland and Graedel (New York) ( 4 ) concluded that NO, controls increased ozone levels on the basis of modeling methodology. The objective of the present work is to see whether NO, controls offer an explanation for ozone trends as well as attainment of a simplified understanding of the effects of NO, emissions in a general context. The term NO, is used in the usual sense to be NO + NO:! because data are often lacking on individual components. About 90% of NO, emissions after air dilution are NO. Smog-chamber studies and conclusions herein are based on this fraction. The term HC refers to total volatile organics excluding methane on a carbon basis, while (HC) is their concentration in ppm C. H refers to any compound that can react with OH while (H) is its mole concentration in ppm.
Photostationary-State Equilibria between Oxides of Nitrogen and Ozone In considering the effect of NO, emissions on ozone, one must keep in mind the photostationary state that is established a t normal concentrations as a result of the following reactions: 0 2 + NO2 hu NO 0 3 and 0 3 NO NO2 02.This relationship has been examined and validated by Stedman et al. ( 5 ) ,Shen et al. ( 6 ) , etc., and can be written as
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Environmental Science 8 Technology
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(03)
= Ji(NOz)/[k’(NO)]
0.02(NO2)/(NO)
(1)
for noon in Los Angeles during the fall smog season where J l = rate of photolysis of NO2 (min-l), h’ = rate of reaction of 0:1with NO (ppm-I min-l), and (NO), (NOz), and ( 0 3 ) = concentrations in ppm of the indicated compounds. The relationship should apply to metropolitan areas with adequate mixing because other reactions involving 0 3 , NO, and NO2 are relatively slow.
Photooxidation of NO-HC Since ozone is determined by the concentration ratio (N02)/(NO),the photooxidation of NO to NO2 determines ozone buildup. The photooxidation of organics in the presence of NO, leading to photochemical smog formation appears to proceed by a series of reactions such as the following for propylene (7,8): C:jHe + OH CsH70 (2)
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-
(3)
+ NO2 C3H702 + 0 2 CH:$HO + HCHO + HOz HO2 + NO OH + NO2 C3H703 + NO
C3H702
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net:
+
C ~ H G 202
+ 2N0
-
CHsCHO
+ HCHO + 2N02
(4) (5) (6)
(7)
Similar reactions may be written for other organics although the stoichiometry with respect to NO oxidized per organic molecule reacted with OH can vary from 1 to 3 (e.g., 1 for HCHO and 3 for CH4 ( 7 ) . The photooxidation process, illustrated by Figure 1,appears to be initiated by the reaction of oxidizable compounds with OH radicals ( 7 ) , and rate constants for these reactions have been published (9).However, to compute NO oxidation, one must know the rate and stoichiometry of various steps as well as the concentration of the OH radicals. The latter concentration can be determined from the rate of HC disappearance. Results of such calculations of OH level lead to the conclusion that OH is essentially constant a t typical smog-chamber irradiation levels (1.1 X lo-’ ppm OH) (7). Dervent and Hov ( 2 ) and Singh et al. ( 1 0 ) give OH values for typical daylight atmospheric conditions that are in line with this assumption. The further postulate is made that the reaction of organics with OH (eq 2) is the rate-determining step. Since secondary products such as aldehydes also undergo photooxidation, they also contribute to NO oxidation, and this contribution must be considered. The major problem in the calculation of NO oxidation rate, assuming constant (OH), is to properly take into account the NO oxidation related to reactions of products. This becomes complicated where several oxidation stages are involved. However, a step-by-step procedure is suitable for single cal-
0013-936X/81/0915-0904$01.25/0 @ 1981 American Chemical Society
culations if the stoichiometry is known. Consider a hypothetical case involving the following series of reactions: K2
Kl
K3
2
compd 1+compd 2 -+ compd 3 -+compd 4
CO ("stable") dhlldt = -Klhl
- ~2h2 dhaldt = ~ 2 -h~ 3~h 3 dhrldt = ~ 3 h 3 ~4h4
dhzldt = Klhl
Solution of these equations gives an expression in the form
+ Bie-KZt + Cie-K3t + Die-K4t
hi = & - K l t
(8)
0
m
60
*
?a
303
24
180 IPdDIATlaY T I E IN M l W E S
Figure 1. Nature of photochemical smog formation from Los Angeles basin surrogate in terms of typical component concentrations in a smog chamber under static conditions (data of Pitts et al. ( 7 7) for (HC)o = 2.0 PPmh
are first calculated. Then values of An are determined from the expression An = So - hlS1- h2S2 - h3S3 - h4S4 = -A(NO)/(Ho) (13) where Si values refer to the total NO oxidized if all of the ith components reacted to end-point states (normally CO for 360 min). Values of Si for an organic molecule correspond to the number of oxygen atoms required to effect complete oxidation to a CO end point. For a hydrocarbon:
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S ( 0 ) C,H,
hi = concentration of compound i in the series relative to the initial concentration of the primary reactant (Hi)/(Ho); ~i = rate ponstant for reactant i in min-l if (OH) is assumed to be 1.1X loR7ppm; t = time in minutes. For a longer sequence, the calculation process can simply be extended. Values of hi
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xCO
+ (y/2)H20
S =x
+ y/2
(14)
The rate constants of Pitts et al. (9) for reaction with OH radicals were employed where available with minor exception (7). With the assumption of (OH) = 1.1X ppm, values expressed as krel (rate constant relative to methane) can be converted to units of min-l by the expression K(min-l) = 11.7 X 1.1 X 10-7k,,~ = 12.9 X 10-7k,,l
(15)
Table 1. Calculation of Coefflcients for h, and An for Following Assumed Reaction
2m-2-butene
/
acetone
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4acetaldehyde 3 formaldehyde +carbon monoxide
acetaldehyde
compd
(1) 2m-2-butene
S
A
branch no. 1 8 C
D
A
I 4 3 formaldehyde + carbon monoxlde
branch no. 2 8 C
D
10
(2) acetone
5
(3) acetaldehyde
3
(4) formaldehyde
1
A
lolal (branch 1 f branch 2) 8 C D
1 -1.040 0.048 -0.016
1.040 0.346
-0.394
0.106
-0.158
0.067
- 1.040
1.040
0.356
0.416
-0.774
produd 01 W a I X S 8S CS
DS
10
-1.04
1.04
-0.992
0.346
0.646
0.106
0.258
0.342
AS
-5.20
5.20
-2.976
1.038
1.938
-0.707
0.342 0.106
0.256
-0.707
total
2.166 6.344
2.196
-0.707
Steps in the calculation are as follows: (1) Determine rate constants from other work (for this case kl = 0.0126, k2 = 0.0005, k3 = 0.002, k4 = 0.007 min-I). (2) Consider branches separately and compute coefficients of exponential terms A-D from rate constants by using eq 9-12. (3) Add terms for branches to obtain coefficients for calculation of h, (e.g., Nacetaldehyde) = -0.992e-0.0'28' t 0.346e-0 Ooo5' t 0.646e-0.0023. (4) Multiply coefficients by S for each compound. (5) Sum up the resultant products for each coefficient as shown. (6) Then An = SO - ZKfS,e-Klt = 10 - 2.17e-"Tt - 6.344e-X*' - 2 . 1 9 6 e - ~ ' t 0.707e-x4'where K, = A-D.
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Since Pitts et al. (9) provide little information on aldehydes, these have been empirically estimated from other data on aldehyde photooxidation rate as described elsewhere (7). Values, assuming (OH) = 1.1 X ppm, are K(min-1) = 0.007 (HCHO), 0.002 (CHsCHO),and 0.0035 (C2H&HO). Higher olefins may split into two aldehydes, and the higher paraffins may be oxidized to ketones or aldehydes. However, the approach described above for calculating An vs. time then involves separate consideration of the various branches as illustrated in Table I. The calculation result is plotted in Figure 2 along with illustrative plots for other compounds. This approach has made it possible to calculate the time required for NO depletion for a wide variety of compounds and mixtures ( 7 ) . This enables calculation of the NO requirement for prevention of ozone buildup in a given time period. Typical agreement with available smog-chamber data is shown in Table 11.
I
I
I
I
I
1
Reactivity Coefficients The relative effects of various organic emissions, termed reactivity coefficients, r , can be calculated for compounds or mixtures (assuming additivity) from (NO) values for pertinent time periods (180-360 min) ( 7 ) .These were computed from the A (NO) relative to a base-line mixture (Pitts’ Los Angeles air basin surrogate, 1974,at same organic carbon content (11)). Values are listed in Table 111.They are useful in considering relative effects of various organic emissions on the time required to reach the NO end point (time when (NO) = (03)) (start of ozone buildup) (see Figure 1). Ozone Levels After NO “Depletion” Only after nitric oxide “depletion” do ozone levels rise significantly. The term NO “depletion” is used in a relative sense, since NO cannot be entirely depleted as long as it is being regenerated by NO2 photolysis. Reactions such as C3H6
1
+ 2N0 + 202
4
CH3CHO
+ HCHO + 2N02
(16)
can still proceed even though the NO level is low. However, to obtain the overall reaction, it should be combined with the net photolysis reaction 202
+ 2 N 0 2 + 2hu
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2N0
+ 203
(17)
This gives C3H6
Figure 2. Calculated values of A(No)/(H~) = An for various hydrocarbons. See ref 7and Table I for details.
Table II. Some Calculated vs. Experimental NO EndPoint Times a Initial (NO), ppm
end-polnt tlmes, mln calcd measdb
1.0
0.45
102
83
25a
propylene
0.45 4.00
0.32
90
90
26a
1.oo
30
30
27b
1-butene
+ 2hv NO,
CHsCHO
+ HCHO + 2 0 3
(18)
The expected net rate of ozone formation that might be expected is
ref 7 page no.
ethylene
402
If ozone did not undergo reaction, the rate of O3 formation would equal the prior rate of NO depletion if (OH) remained constant. However, ozone reacts with NO2 (12) and olefins so that lower ozone levels would be expected.
TINE IN MINUTES
reactant concn, nature ppm
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where s; = stoichiometric ratio for the ith component (ozone formed per mole of organic reacted with OH = 2 for olefins as shown in eq 18),(Hi) = concentration of the ith component (ppm) reactive with OH, and ~i = rate constant for the ith
a See ref 7for details of calculations and for comparisons of other compounds and mixtures. Defined as time (t,) to point where (NO) = (03).
Table 111. Summary of NO Oxidation Reactivity Values for Pure Compounds source
low-reactivity HCs
moderate-reactivity HCs
high-reactivity HCs
a
906
C no.
calcd -A(NO)/(HC)o, 100 mln
ppm/ppm C 360 min
reaclivlty coeff for 360 mln ( I )a
0.06
acetylene
2
0.01
0.015
benzene
6
0.01
0.06
0.25
ethane
2
0.014
0.05
0.21
propane monoalkyl benzenes
3
0.023
0.08
0.33
7+
0.03
0.10
0.42
C4+ paraffins
6
0.05
0.16
0.67
pxylene
8
0.05
0.155
0.65
o-xylene mxylene
8 8
0.08 0.10
0.25 0.21
0.88
trimethylbenzene
9
0.16
0.36
1.50
ethylene
2
0.15
0.62
2.58
1.oo
C3+ 1- and isoolefins
3+
0.25
0.80
3.33
2-butenes
4
0.38
0.92
3.83
A(No)/(Hc),, relative to that for Pitts et ai. (1975) Los Angeies basin surrogate mixture. See ref
Environmental Science & Technology
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component for reaction with OH for (OH) = 1.1 X in ppm-I, However, the contribution of the ozone-olefin reaction to ozone decrease in the Los Angeles atmosphere as well as the basic surrogate appears small relative to ozone reaction with NO2 (
