EPA-600/3-77-114
October 1977
INTERNATIONAL CONFERENCE ON OXIDANTS, 1976 -
ANALYSIS OF EVIDENCE AND VIEWPOINTS
Part II. The Issue of Reactivity
J.G. Calvert
Ohio State University
Columbus, Ohio
Contract No. DA-7-1840A
H.E. Jeffries
University of North Carolina
Chapel Hill, North Carolina
Contract No. DA-7-2261J
. Project Officer
Basil Dimitriades
Environmental Sciences Research Laboratory
Research Triangle Park, North Carolina 27711
ENVIRONMENTAL SCIENCES RESEARCH LABORATORY
OFFICE OF RESEARCH AND DEVELOPMENT
U.S. ENVIRONMENTAL PROTECTION AGENCY
RESEARCH TRIANGLE PARK, NORTH CAROLINA 27711
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DISCLAIMER
This report has been reviewed by the Environmental Sciences Research
Laboratory, U.S. Environmental Protection Agency, and approved for publica-
tion. Approval does not signify that the contents necessarily reflect the
views and policies of the U.S. Environmental Protection Agency, nor does
mention of trade names or commercial products constitute endorsement or
recommendation for use.
In general, the texts of papers included in this report have been repro-
duced in the form submitted by the authors.
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ABSTRACT
In recognition of the important and somewhat controversial nature of the
oxidant control problem, the U.S. Environmental Protection Agency (EPA)
organized and conducted a 5-day International Conference in September 1976.
The more than one hundred presentations and discussions at the Conference
revealed the existence of several issues and prompted the EPA to sponsor a
follow-up review/analysis effort. The follow-up effort was designed to review
carefully and impartially, to analyze relevant evidence and viewpoints report-
ed at the International Conference (and elsewhere), and to attempt to resolve
some of the oxidant-related scientific issues. The review/analysis was con-
ducted by experts (who did not work for the EPA or for industry) of widely
recognized competence and experience in the area of photochemical pollution
occurrence and control.
J.G. Calvert, Ohio State University, Columbus, Ohio, and H.E. Jeffries,
University of North Carolina, Chapel Hill, N.C., reviewed the papers presented
at the 1976 International Conference on Oxidants related to the issue of
reactivity, and offered their views on the current status of research in the
field, resolutions of the issue, and the need for additional research.
111
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CONTENTS
ABSTRACT iii
FIGURES V
TABLES vi
INTRODUCTION 1
B. Dimitriades and A.P. Altshuller
THE ISSUE OF REACTIVITY 3
B. Dimitriades and A.P. Altshuller
REVIEW AND ANALYSIS 5
J.G. Calvert
Introduction 5
Review of the International Conference Papers
Related to the Issue of Reactivity 6
General Conclusions Concerning the Issue of Reactivity .... 29
Comments by H.E. Jeffries 33
REVIEW AND ANALYSIS 35
H.E. Jeffries
Introduction 35
Discussion 38
Recommendations for Future Research . 70
Comments by J.G. Calvert 70
REFERENCES 71
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Number
FIGURES
REVIEW AND ANALYSIS - H.E. Jeffries
"Ultra Clean" air irradiation in cylindrical
400-liter glass/teflon EPA chamber. . .
Page
. 47
Number
TABLES
REVIEW AND ANALYSIS - J.G. Calvert
Photolytic Rate Constants Estimated from
the Formulations of Chang and Weinstock
Page
. 9
1
2 Comparison of Relative Photolytic Rate
Constants Employed by Chang and Weinstock
and Other Workers .10
3 Estimated HO from EPA Chamber Studies of
Organic Reactivities 25
REVIEW AND ANALYSIS - H.E. Jeffries
Number Page
1 Comparison of Smog Chamber and Computer
Simulation Studies 43
2 Computer Simulation Results for the
September 8, 1976 RTI Outdoor Smog Chamber
propane and NO Experiment 57
X
3 Computer Simulation Results at Initial
Conditions near Rural Conditions 58
4 Comparisons of Inorganic Portions of Chang
and Weinstock Mechanisms with Dodge
Mechanism for Propane 59
5 Comparison of Organic Portion of Che ng and
Weinstock Mechanism with D'.dge
Mechanism for Propane 60
vi
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ACKNOWLEDGMENTS
These contracts were jointly funded by the Office of Research and Devel-
opment (Environmental Sciences Research Laboratory) and the Office of Air
Quality Planning and Standards.
The assistance of the technical editorial staff of Northrop Services,
Inc. (under contract 68-02-2566) in preparing these reports is gratefully
acknowledged.
vii
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INTRODUCTION
Basil Dimitriades and A. Paul Altshuller
In recognition of the important and somewhat controversial nature of the
oxidant control problem, the U.S. Environmental Protection Agency (EPA)
organized and conducted a 5-day International Conference in September 1976.
The one hundred or so presentations and discussions at the Conference revealed
the existence of several issues and prompted EPA to sponsor a followup review/
analysis effort. Specifically, this followup effort is to review carefully
and impartially and analyze relevant evidence and viewpoints reported at the
International Conference (and elsewhere) and to attempt to resolve some of the
oxidant-related scientific issues. This review/analysis effort has been
contracted out by EPA to scientists (who do not work for EPA or industry) with
extensive experience and expertise in the area of photochemical pollution
occurrence and control. The first part of the overall effort, performed by
the EPA Project Officer and reported in a scientific journal (1), was an
explanatory analysis of the problem and definition of key issues, as viewed
within the research component of EPA. The reports of the contractor expert/
reviewer groups offering either resolutions of those issues or recommendations
for additional research needed to achieve such resolutions are presented in
the volumes composing this series.
This report presents the reviews/analyses prepared by the contractor
experts on the issue of natural organic emissions. In the interest of com-
pleteness the report will include also an introductory discussion of the
issue, taken from Part I. The reviews/analyses prepared by the contractor
experts follow, along with the experts' comments on each other's reports.
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THE ISSUE OF REACTIVITY
Basil Dimitriades and A. Paul Altshuller
In regard to reactivity, the questions most urgent and directly related
to the oxidant control problem pertain (a) to the effectiveness of "substitu-
tion" (of less reactive for more reactive organic emissions) as an approach to
oxidant control and (b) to the identification of those organics that are
essentially of no concern insofar as the oxidant problem is concerned. The
more specific questions that need to be answered follow.
• Does the scientific evidence alone justify formulation and enforce-
ment of interim substitution rules more stringent than Rule 66?
• Considering all relevant factors, e.g., impact upon urban air qual-
ity, -impact upon rural air quality, cost, technological feasibility,
etc., would it be preferable to abandon altogether the idea of devel-
oping interim improved substitution rules and devote instead and
immediately all attention and resources to development of methods and
practices for "nearly indiscriminate" control of organics?
• Are there any organics so little reactive that they would neither
cause nor contribute significantly to oxidant buildup at problem
levels under any circumstances?
There are also the relatively less important questions regarding definition of
reactivity and validity of the data and procedure used to classify organics
based on their relative abilities to contribute to the urban oxidant problem.
Of these questions, the one on the merits of substitution has been dis-
cussed both internally in EPA and informally at an open meeting (EPA's Forum
on Solvent Substitution, Chicago, 111., Oct 13-14, 1976); there was a consen-
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sus that substitution will have a small but possibly significant benefit upon
urban air quality — more precisely, the air quality in the vicinity of the
source area — but will have less or no benefit upon distant downwind areas.
Although not quantitatively answered, the question was nevertheless treated
adequately so that further discussion here is not warranted. Also, the ques-
tion on merits of "nearly indiscriminate" control is outside the scope of this
review, since it calls for judgments on cost, technological feasibility, etc.
The question that is offered as the specific subject of this review is
the one regarding the possible existence and identities of organics incapable
of causing oxidant problems. This question was discussed at the International
Conference and drew conflicting answers. The specific issue here is centered
around the method used for reactivity-rating the various organics and for
defining the borderline separating the reactive ones from those of virtually
no concern with respect to the oxidant problem. In general, two distinctly
different approaches were proposed: The smog chamber approach applicable to
all organics (4), and the mathematical modeling approach (5) applicable, at
present, to certain organics only, namely, paraffinic and olefinic hydrocar-
bons and aliphatic aldehydes. To facilitate the process of judging these two
and/or any other approaches, it would perhaps be useful to break the issue
down to two parts: One pertaining to the reactivity-rating of the organics,
especially of those of low reactivity, and one pertaining to the positioning
of the borderline separating the almost totally unreactive ones from the
reactives. Judgments that must be made are on:
• whether the two proposed approaches agree or disagree in results and
to what extent,
• whether one or the other or the two approaches combined in some way
or any other approach yields the most reliable results, and
• the specific additional research needed to substantiate or refute
these first judgments.
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REVIEW AND ANALYSIS
J. G. Calvert
INTRODUCTION
In this report a critical examination will be made of the specific
papers related to reactivity presented at the International Conference on
Photochemical Oxidant Pollution and Its Control (Raleigh, N.C., September 12-
17, 1976). Furthermore, reference to other pertinent scientific information
will be made in an attempt to examine the results and conclusions related to
oxidant control strategy and the reactivity issue. As charged by the Officers
of the Environmental Sciences Research Laboratory, U.S. Environmental Pro-
tection Agency (1), an effort has been made to (a) examine the reported
evidence and viewpoints for conflicts; (b) make judgments on strengths and
weaknesses of opposing viewpoints or evidence, and based on such judgments,
attempt to reconcile conflicting viewpoints and evidence; and (c) derive a
factual or judgmental conclusion regarding resolution of the status of the
issue, and offer recommendations for additional research.
This report has been organized to present first a critical review of each
of the conference scientific papers related to the reactivity issue. In the
second part of this report an attempt is made to face those questions that the
EPA has pinpointed as the most urgent and directly related to the oxidant
control problem. A major question of interest to the agency is the possible
existence and identities of organics incapable of causing oxidant problems.
We are concerned with the reactivity rating of the organics, especially those
of low reactivity, and the positioning of the borderline separating the "al-
most totally unreactive" ones from the reactives.
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In our considerations of these issues we must compare the conclusions
based upon smog chamber experiments and those based upon chemical modeling of
the atmospheric changes. Finally we must answer the questions: (a) To what
extent do the two approaches (chamber verf is modeling) agree or disagree? (b)
Does one or the other of the two approaches combined in some way yield the
most reliable results? (c) What additional research is needed to substantiate
or refute these first judgments?
A REVIEW OF THE INTERNATIONAL OXIDANT CONFERENCE PAPERS RELATED TO THE ISSUE
OF REACTIVITY
"Net Ozone Formation in Rural Atmospheres" by Tai Yup Chang and Bernard
Weinstock.
Chang and Weinstock have developed a simple chemical reaction scheme to
rationalize the results observed in the EPA and General Motors Corporation
smog chamber experiments utilizing "unreactive" hydrocarbons, C_H_, C,H0, and
t, £• JO
n-C H . An effort is made to introduce into the homogeneous mechanism,
certain heterogeneous steps peculiar to the smog chamber. After identifying
such smog chamber peculiarities, those not expected to occur in the atmosphere
were removed from the mechanism, and simulations were made of the chemistry of
the "unreactive" hydrocarbons in a rural atmosphere. The authors conclude
from their study that "unreactive" hydrocarbons released in the urban atmo-
spheres contribute little either to the generation of elevated rural ozone
levels or to the increase of elevated levels already present. They conclude
that the major cause of elevated rural O levels is the transport of high 0
concentrations generated in urban areas and additional O produced by reac-
tions of fresh reactive hydrocarbon (RH) and NO emissions from local rural
X
sources, both natural and man-made. The authors' analysis is at variance with
the new EPA Policy Statement that severe control of all hydrocarbons, without
regard to reactivity, will be necessary to reduce elevated rural 0^ levels.
There are some major questions that shoul d be considered here relating to
the chemical modeling of Chang and Wernstock. First the formulation of the
model as given appears to provide an artificial source of organic radicals of
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unlimited supply. Note in the following sequence that the RO radicals formed
by HO-radical attack on HC2 (Reaction 35), react in Reaction 46 to create a
fraction $ of the time, an aldehyde (RCHO), which has gained a carbon atom:
HC2 + HO -> RO + H20 (35)
RO + NO -> RO + NO (42)
RO + O + 6RCHO + (1 - 0)CH O -I- HO (46)
£, £. £•
RCHO + HO -*• RC03 + H20 (38)
RCHO + hv -*• RO + HO + CO (9)
RCO + RCO + 2RO +0 (54)
+ NO ->• RO + N0 (43)
The RCHO species in turn provides a reactive oxidizing agent for NO through
the radicals RCO , RO , and HO . The RCO radical may reform RO in Reactions
42 and 43, and may in principle provide an unlimited new supply of these
radicals that is never depleted. It seems to me that the use of less general
reactions that do not allow an unrealistic source of RO would be more suit-
able and very few additional reactions need be employed. Thus the degradation
of C_H by the way of real chemical entities is important to maintain the mass
3 o
balance: C H ->• iso-C H O -> iso-C_H_O -> CH_COCH. and CH + CH.CHO; C.H0 +
jo j / 2. J / oj j o oo
n-C H?02; n-C3H7O -> C2H5CHO + CH2O + C2H5O2; CH302 -»• CH3O -»• CH2O •* HC03 ^
HCO -> CO , CO; C H O -> C H O -> CH CHO + CH + CH O, etc.
^ ^ ^O^-bO J -J £•
The rates of all of the reactions that are initiated by sunlight absorp-
tion employed by Chang and Weinstock appear to be unusual. Specifically the
equation given for k (NO + hv ->• NO + O) must be reported here incorrectly.
If the authors really used this to determine the values of k , and in turn
used these estimates of k to establish the other photochemical rates as they
outline in their table on pages 13 and 14 of their Appendix, then practically
no chemical change would occur or would be expected to occur in their Simula-
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tions. See Table 1 of this review. The values estimated from the Chang and
Weinstock Equation 1 may be compared with those estimated in previous studies.
At the maximum solar intensity Chang and Weinstock's value for k = 0.0149
-1 -1
min , compared to the value of 0.46 min estimated by Calvert for Los An-
geles on November 5, 1973 (4). Thus the Equation 1 presented by Chang and
Weinstock gives values about a factor of 31 too low. As stated previously,
this must be an error introduced in the typing or preparation of the manu-
script and probably does not reflect actual values used in che simulations.
If they are the values employed, then the results of the simulations are
meaningless. This point must be cleared up, of course, if the conclusions of
Chang and Weinstock are to be accepted.
Whatever the value of k chosen by Chang and Weinstock, the other photo-
chemical rate constants for HONO, CH O, RONO, etc, are derived from this k
value for the given time. In Table 1 reference to a few of the other values
should be made to illustrate the magnitude of the photochemical rates employed.
The estimated k (HONO + hv -»• HO + NO) from Chang and Weinstock's work reaches
a maximum of 1.0 x 10 min ; this is a factor of 100 less than that esti-
mated in our previous studies (5). The most recent work of Derwent and Cox
(6) requires that the estimate of Johnston and Graham (7) of the extinction
coefficients of HONO be increased significantly. We find that the use of the
new data together with a reasonable solar distribution function leads to a
value as high as 0.20 min for k . Also the extinction data for CH ONO do
not support the value picked for k (RONO + hv -»• RO + NO); it is 70-times
smaller than the value I would choose. As I stated previously, the k values
calculated from the Chang and Weinstock Equation 1 may not be those actually
employed because of some mechanical error.
If some error in the manuscript for the k equation exists, then the
relative values of these photolytic processes should give us some feel for the
presumed importance of each step. In Table 2 shown here, I have compared the
relative values for these constants chosen by Char , and Weinstock and others,
taking the k for NO photolysis = 1.0. The
------- An error occurred while trying to OCR this image.
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estimates suggest that a value about 3.7-times larger than that chosen would
be more appropriate. In the case of RONO I feel that a value about 71-times
larger should be used.
TABLE 2. COMPARISON OF RELATIVE PHOTOLYTIC RATE CONSTANTS EMPLOYED BY CHANG
AND WEINSTOCK AND OTHER WORKERS
Compound
N02
HONO
H,,CL
2 2
2
T>f*r\ u
RoO_H
3
0 (ZD)
CH20(a)
CH20(b)
RCHO
RONO
Chang-Weinstock
1.0
0.068
0.0035
f\ f\ A O C
0.0035
0.0047
0.011
0.0012
0.0035
Demerjian, Korr,
Calvert (4)
1.0
0.25
0.0063
0.0071
0.0042
0.011
0.0052-0.0009
0.25
S.A.I. (7)
1.0
0.21
0.0031
0.0133
0.0025
0.0081
0.0016
HONO and RONO are termination products that stop the oxidation chains
unless they photolyze to regenerate the HO and RO radicals. A choice of
values for the photolysis constants for these compounds that is too small
leads to an underestimation of the rates of chemical change in the simulation.
The effect of a more realistic choice for these constants should be investigated.
In one phase of the Chang and Weinstock work they have utilized the
modeling approach of Pitts et al. (8), who have assumed a constant HO-source
in their smog chamber as a possible technique to employ for atmospheric simu-
lations. This approach is not one I favor. If the chain sequence is initi-
ated by an unknown reactant forming HO-radicals, then detailed simulations
lose their meaning, and the utility for extrapolation to the real atmosphere
is lost. All such an approach does is to hide our ignorance and avoid identi-
fication of the important chain initiating steps. There is not reason to
10
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believe this source to be of infinite capacity to supply radicals at a con-
stant rate throughout the run.
The choice of NO + NO + H?O ^ 2HONO rate constants by Chang and Wein-
stock are those found by Wayne and Yost (9) for a capillary tube reactor; we
also employed these numbers in treating data from a number of early chamber
data (5). These numbers seem to be unusually high for use in a large chamber
today. The alternative use of k (NO + NO + HO -> 2HONO) = 1.0 x 10~
-2 -1
ppm min for the atmospheric simulations is also unusual in my opinion; the
number is 50-times smaller than that observed recently in a chamber study by
Chan et al. (10); in this work a significant effort was made to minimize the
heterogeneous reaction pathways for these reactions (very low surface/volume
ratio employed), but the authors recognized that these estimates were at best
upper limits to the true homogeneous rate constants. The, as yet unpublished,
work of Kaiser and Wu from which Chang and Weinstock pick their number, should
be compared with that of Chan et al., and reasons for the choice given. In
any case, for most atmospheric conditions involving the dilute NO mixtures,
X
the formation of HONO through Reaction 20 should be slow.
On page 6 of the Chang and Weinstock paper, the point is made that rural
NO values are probably below the 5-10 ppb reported in the 1974 Midwest Study
X
(19). When one considers the relatively large impact of NO from auto traf-
X
fie, power plants, homes, chemical plants, and other stationary sources located
near the "rural" area, then levels of NO as high as 10 ppb are not unexpected
X
in my view.
It is important to learn more detail about the method used by Chang and
Weinstock to keep the NO constant in their simulations shown in their Table
x
2. Was new NO (not NO) added as HONO , RONO , etc, depleted the NO 1 If NO
^ ^ ^ X
was added to keep NO constant in these simulations then it is hardly a fair
test for O buildup, since the ozone will be titrated in part by the rapid
reaction, NO + O -*• NO + O . This point should be clarified by the authors.
In spite of the several questions that appear to require some clarifica-
tion before the results can be considered definitive, it is interesting to see
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in the data of Table 3 of Chang and Weinstock that the mechanism outlined by
them does predict that the presence of only 0.1 ppm of C H in air of a typi-
cal 0.04 ppm background O level, increases the [0 ] to 0.074 ppm on the first
day of the irradiation. Also during the "irst day slight increases (0.10 to
0.114 and 0.15 to 0.153) in [o ] occur with 0.1 ppm C H insertion into an air
-j 3 o
mass with higher preexisting [Oj, With alternative choices in the mechanism
o
as outlined above, it would be .interesting to see what changes, if any, would
be found.
The authors contend that the hydrocarbon analyses made in the 1974 Mid-
west Study show no evidence of accumulation of less reactive species that
would be characteristic of an aged air mass that has had no significant new
impurity input. The presence of alkenes in the analysis certainly suggests
that new RH impurity has been added. However the levels of acetaldehyde
reported suggest that some significant chemistry involving RH oxidation has
occurred in these air masses as well.
The potential role of C H or other "unreactive" RH compounds in photo-
3 o
chemical smog development in urban air masses remains unresolved in my mind.
It is unlikely that any old air mass will contain only "unreactive" hydro-
carbons; it is most reasonable that certain oxidation products (CH O, CH CHO,
etc.) should build up to significant levels. Although these compounds are
reactive both photochemically and toward HO- radicals, they will continue to
form in an aged air mass as the RH oxidations proceed. A significant level of
these species should persist for several days. Chang and Weinstock do not
consider this issue and start their simulations with only C H (0.10 ppm), CO
J o
(0.20 ppm), CH (2 ppm), and NO (10 ppb) . Photolytic generation of radicals
in their system initially is restricted to those derived from O photolysis
largely:
03 + hv •> C (D) + 02
O^D) + HO -> 2HO
This rate is very low. In my opinion a more realistic model would assume an
initial [CH 0] and [CH CHO] carryover as products of the RH oxidation reac-
12
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tions that lead to the C H -rich air mass they choose. Even 0.01 ppm of each
J O
of these aldehydes would provide a more significant rate of radical generation
than the O photolysis. Thus an artificially low :
be present in the Chang and Weinstock simulations.
than the O photolysis. Thus an artificially low rate of chemical change may
The acceptance of the Chang and Weinstock conclusion that "unreactive"
hydrocarbons will not enhance rural ozone formation may be correct, but this
has not been proven by the study. Acceptance of this or some other conclusion
should await the resolution of the several problem areas that seem to exist in
this work.
"Multiday Irradiation of NO -Organic Mixtures" by W. A. Glasson and P. H.
X
Wendschuh.
Glasson and Wendschuh have simulated experimentally the photochemical
smog formation in single day and multiday irradiations of different polluted
air masses. The experiments were carried out in the General Motors Corpora-
tion smog chamber. Irradiations of a typical urban hydrocarbon mixture and
several automotive paint solvents of different "reactivity" were made in
dilute mixtures with NO in- air. In the case of the most reactive of the
x
three paint solvents, the [0 ] peaked much higher than with the other solvents
(0.45 ppm) at 6 hours irradiation, but after 24 hours all solvent mixtures had
similar [O ] levels (0.15 ppm). In transport simulations (light and dark
periods) with a variety of reactive and unreactive hydrocarbons, the ozone
maximum during the "second day" irradiation remained higher for the more
reactive RHs; on the "third day" the highest maximum [O_]s, 0.12 and 0.13 ppm,
also came from the reactive species cis-2-butene and auto exhaust, respectively.
In another series of runs the initial [NO ] was varied while the initial
X
concentration of the typical urban hydrocarbon mixture was held constant at
0.935 ppm C. Typical NO -inhibition curves for [0 ] were found. However the
X 3
[NO ] required for maximum [0 ] increased with each succeeding day. The
X j
authors conclude from these results that reductions in urban [NO ] will in-
crease [O,] on the first day (urban air mass), slightly reduce O on the
second day (rural), and have little effect on O on the third day (rural).
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Thus they feel that the result of reducing urban NO concentrations may be to
trade-off higher [C>3] exposures in the urban population centers for small
effects in the sparsely populated rural environment.
One important feature of the Glasson and Wendschuh study is the deter-
mination of the concentration dependence of the CH 0 product on time. It is
important to recognize that the CH O that formed and survived photochemical
decomposition and HO-radical attack in the multiday irradiacion is an appreci-
able fraction of the orginial RH used. See Table IV of the more complete
paper (GMR-2236) (11). From these data we may estimate (taking into account
the dilution rate of 0.0015% sec ) that about 1 ppm of CH is formed for each
ppm of hydrocarbon reacted. Without a knowledge of the complete product
distribution as a function of time, it is not possible to accurately assess
the RH to CH O conversion efficiency here, but it is clear that a large amount
of CH 0 (and presumably other aldehydes) are formed during these oxidations.
These compounds provide an excellent radical source, and even the unreactive
RH species are stimulated to oxidize and provide for 0 formation, provided
that sufficient NO is present to supplement the RH-RCHO mixture. This con-
X
sideration is particularly important in formulating models to simulate long-
range transport transport of urban air masses.
In the multiday irradiations of the NO -organic solvent mixtures the
ozone levels eventually approached the same value; this suggests to me that
there is some reasonable reactivity of the ultimate "unreactive" mixture of
the A and B solvents as well as the "reactive" C solvent. The formation of
the light-absorbing aldehydes in all mixtures may ultimately control the
reactivity of the final "unreactive" mixtures that result in the later days of
the irradiation.
One might question whether the square wave pulse of "sunlight" employed
in the General Motors Corporation experiments provides a realistic simulation
of the true solar day irradiations. The somewhat low value of the k (NO- + hv
-1
-> NO + O) = 0.25 min used in the chamber sh' uid give a depressed O compared
to the real atmosphere. It is not clear from the present data given that this
«. 1
value of 0.25 min" represents the actual best value of k.^ in these recent
14
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chamber experiments. Simulations of chamber experiments suggest that at any
given time t during chamber irradiations of dilute NO mixtures as employed
X-l
here, the product ratio [NO] [O ] (23)/[NO ] - k (min ). The data for 2.1
and 2.6 min (times when NO, NO , and O are all measurable), taken from Figure
-1
2 of the Glasson and Wendschuh paper, give k = 0.13 nun . In any case the
somewhat low k value will alter the [O ]-time profile from that which would
be observed in the real atmosphere. It is not clear that this effect would
alter the results and conclusions significantly, but the difference remains as
an uncertainty.
Another factor that may lead to chamber results that are somewhat unique
compared to those expected in the true atmosphere is the wavelength distribu-
tion of the lamps in the chamber. The transmission of the Pyrex glass tubes
in which the lamps were housed may restrict from the chamber the 3000-3100 A
region uriduely, and hence the true impact on the CH^O product and the induced
^
rate of oxidation of the RH mixtures may not be representative of those ex-
p^cifc.i in tli: atmosphere. However the comparison of the expected chamber
distribution to the solar distribution suggests that this problem may not be
important here (j2). However actual measurements in the chamber should be
madt to rest tnis ptopeily.
'^irtco Cr..\a±>c..i f imulaticn of L,o.~ Anqeies Pollatant Transport" by William A.
Gla^son.
This papec presents the results of the 24-hour irradiation (equivalent to
12-bear soia^" day; of cypical urban Los Angeles RH-HC impurity mixtures chat
X
are i:; '_ei.rled to simulate pollutant transport during the day from Los Angel- .
to 'c? vo^-;-; de and Sar beinadino. The effects of variations of [NO ] emissions
x
or. U'C generation of 0 within the air masses were studied. The author con-
cluded that the downwind oxidant levels are only slightly affected by large
changes Jn NO emissions, while reduced NO emissions in the Los Angeles area
x .JC
v.'iJ i lead to an increased oxidant level in downtown Los Angeles.
The f-iublems of chamber intensities below tnose of solar intensities,
discussed in the preceding section of this review, remain as does the possible
15
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problem of differences in wavelength distributions of the chamber and the
sunlight. It is apparent from [O ] versus time data given in Figure 3 of this
paper that a significant delay in O formation is seen at higher [NO ]°.
-^ X
Thus at 0.85 ppm = [NO ]°, 7 hours of irradiation are necessary to reach the
X
national air quality standard (1 hour maximum) of 0.08 ppm. While at 0.099
ppm NO , this standard is exceeded in somewhat over 1 hour of irradiation.
X
However also note that 0.77 ppm of NO leads to about twice the [O,] after 24
X j
hours (0.165 ppm) that 0.099 ppm = [NO ]° gives at this tJne (0.085 ppm).
X
Thus it appears to me that the effect of ozone levels in transported air
masses can be significantly elevated downwind when larger NO levels are
X
employed. The effect of this delay on population exposure to ozone is not
clear however. Perhaps a more realistic criterion for ozone control strategy
should be designed in the future in terms of minimization of the population
exposure. Thus assume the city is divided into n different blocks of equal
area, and N. is the population in the ith area. Then perhaps we should at-
tempt to minimize the total O dosage of the total population over each 24-
hour period:
24 hr .
„ / [O.J [N.] dt = Ozone-Population Dosage
i=z1 0 3 t 1 t
When the density of the population over which the air mass is transported is
fairly uniform, then the / [0 ]dt function is a proper measure of the function
to be minimized. The integral ozone is not altered much over the range of
[NO] employed by Glasson. If the density of population is weighted toward
downtown LA, where the Riverside air mass may originate in the early morning
hours, then the total function above will favor the use of higher [NO ]
X
values on the basis of O exposure alone.
However, control stategy should take a more educated view of the exposure
of the population to the many insults from impurities present in the atmos-
phere, and focus of O exposure alone should not be made. One should in
principle minimize the t Dtal impurity insult to the population to optimize the
control strategy:
n 2t+ hr . n 2<4hr *
Total 24-hr insult I / a[O0l [N.] dt + Z / b[NO.]. [N.] dt . .
16
-------
The quantities a, b, etc., are proportional to the relative intensities of the
health effects induced by each impurity at the same concentration. The com-
plete function for the 24-hour insult would include terms for PAN, HONO ,
CH O, HONO, RONO , etc. The best choice of NO in their early morning hours
^ £, X
will depend upon the magnitude of the NO , PAN, HONO , CH 0, HONO, and other
terms as well as that of the O term. If the penalty paid for the high NO
o X
levels in the early morning, as determined by the sum of all of the terms
other than that of O , is greater than the benefit achieved by the lower O
term, then obviously the strategy of high NO is not sound. Control stra-
X
tegies of sufficient sophistication to attempt such analyses as outlined above
should be considered in planning if reasonable trade-offs in pollutant emis-
sions are to be attempted. Obviously it is not enough to consider only the
effect of lowered [O ] at early times through increased [NO ] in the morning
j X
hours.
It would be instructive to have multiday exposure experiments as shown in
Figure 4 for NO -RH experiments with new insertions of impurities, since this
X
would represent much better the multiday exposure case than allowing the drift
of an air mass that is unaltered by insertions. If such a case does not
occur, it is relatively unimportant in health effects considerations, since no
human activity exists in the area if no emissions are added.
"Hydrocarbon Reactivity and the Role of Hydrocarbons, Oxides of Nitrogen, and
Aged Smog in the Production of Photochemical Oxidants" by James N. Pitts, Jr.,
Arthur M. Winer, Karen R. Darnall, Alan C. Lloyd, and George J. Doyle.
The study presents a reactivity classification for a variety of atmos-
pheric impurities (alkanes, alkenes, aromatics, oxygenates, natural hydro-
carbons, etc.) based upon the rates of the reactions with the HO-radical. The
work is built upon the premise that the HO-radical is the dominant chain
carrier in photochemical smog; it is also implicitly assumed that the HO-
reactions are rate determining steps in the subsequent oxidation of NO to NO
and 0 production. The reactivity of the organics is divided into a five-
class reactivity scale in which each class spans a factor of 10 in magnitude
of the reaction rate constant. The authors suggest that a comparison of HO-
17
-------
rate constants for different compounds will provide a good insight into the 0
forming potential for this species in the atmosphere.
This study on the evaluation of the influence of various RHs on smog
formation is an important guide to determine the relative removal rates of RHs
in the atmosphere. However, if cur major concern in the development of
control strategy remains focused upon the maximum ozone lev.-l that is reached
in an air mass, then the relative rates of HO-attack on hydrocarbons are only
one part of the necessary input for prediction. The O l'-vel during daylight
hours is controlled by the existing [NO ]/[NO] ratio and the values of k (NO
+ hv -> NO + 0) for the particular solar intensity present, when, as is often
the case in urban air masses, the rate of O reaction with NO (NO + O -> NO
+ 0 ) is the dominant O loss pathway. Thus, for these conditions at a given
NO level, the important fundamental factors controlling ozone are the con-
X
centrations of HO and RO radicals that increase the [NO ]/[NO] ratio and
hence control the [O ]:
HO + NO -> HO + NO
RO + NO -> RO + NO
These RO and HO concentrations at any instant are in turn directly related
^ £•
to the [HO] levels since each radical is formed following HO-radical attack on
RHs:
HO + RH •> HO + R
R + 02 -> R02
HO + CO -»• H + C02
H + O + M -*- HO + M
£, £
Note that the HO-radical level is established by tv••> balance between the rates
of the primary reactions that form it and thos .. chat destroy it. To consider
the potential of a mixture for O formation in the atmosphere then requires a
knowledge of the primary HO-formation rates. Demerjian, Kerr, and Calvert
18
-------
(5), and Calvert and McQuigg (13) have concluded that the aldehydes will
probably be the dominant primary source of HO radicals for high smog con-
ditions, while the reaction HO + NO ->• HO + NO is the likely dominant source
of HO-radicals for these conditions. Thus it can be seen that if predictions
concerning the ozone-forming potential of a mixture are to be made realis-
tically, not only the HO-radical rates with RH are important, but equally
important are the primary routes that form the radicals and the rates of their
generation. What may be a very reactive alkane will not produce ozone unless
the NO-NO levels are sufficiently high and some primary source of HO
and/or HO radicals is present to generate these radicals at a significant
rate.
The observations of Pitts et al. that propane and n-butane can react to
produce substantial amounts of ozone, based upon smog chamber results, confirms
the observations of many others who have done smog chamber experiments.
However this result should be considered in some detail. The reactivity of
RHs of low reactivity in smog chambers is in part a consequence of seemingly
unique radical sources (HONO, CH 0,...) present in the chamber. Unless the
RHs are in a suitable reactive mixture of alkenes, aromatics, aldehydes, etc.,
the induction period for their reaction will likely be much more extensive
than that seen in the chambers. However in the real atmosphere it is unlikely
that any such radical source will be entirely missing. 0 , CH , CH CHO, etc.,
are expected to be present in the old air masses. Thus the RHs of low re-
activity can participate in smog formation. Note the estimates made by
Calvert for Los Angeles and a typical RH distribution (4). The rate of HO-
attack on alkanes amounted to 32.5%; alkenes, 35.1%; aromatics, 20.2%; CO,
12.1% for a typical early morning mixture.
The use of [HO] = 10 molecules cc by Pitts et al. in estimating
lifetimes of the different RHs is questionable. Probably this is much too
high an estimate for [HO] in rural ambient air. With this concentration of HO
the halflife of NO is only 1.5 hours. This seems much too short to fit the
existing data. The direct estimate of Wang et al. (14), [HO] - 5 x 10
appears to be much higher than that observed by Perner et al., [HO] = (4-7) x
10 molecules cc (15). The use of [HO] = (2.5 ± 2.0) x 10 molecules cc ,
19
-------
which I estimated from the LARPP data for Los Angeles (Nov. 5, 1973) (16),
leads to an estimated NO lifetime in the range of 6 hours. So lifetimes
shown by Pitts et al., in Table 1 of their paper, may actually be much shorter
than the true atmospheric lifetimes in rur"l environments. A factor of 4 or
so greater lifetimes would be my estimate. This does not detract from the
utility of the Pitts et al. model as a qualitative indicator of smog forming
potential for RHs in a reactive environment. It is the use of these data to
predict effects in a rural atmosphere that could be misleading.
The statement of Altshuller and Bufalini quoted in this paper (".. almost
every hydrocarbon except methane can produce some oxidant when photooxidized
in the presence of high enough ratios of hydrocarbons to oxides of nitrogen")
applied to smog chambear data. It is not clear what will be the case in the
real atmoshphere if RJi is present with much less abundant radical sources.
The authors state that demonstration by Niki et al. (17), that O and
alkenes lead to few radical products, infers that ozone-alkene reactions may
not influence the oxidant level. Of course CH O, CH CHO, CH?OO, CH CHOO e
other species are formed in their reactions, and it is highly likely that
CH OO, Cl
£
readily:
not influence the oxidant level. Of course CH O, CH CHO, CH?OO, CH CHOO and
md it is highly likely
CH OO, CH CHOO, and other such species that may be formed will oxidize NO
£ J
CH OO + NO -> CH 0 + NO ,
and hence increase the O levels. Of course the alehyde formed in these and
similar reactions, as well as in the original ozonide cleavage reaction, will
be reasonably good radical sources and will influence the oxidant level.
It is pointed out by Pitts et al. that the direct formation of RONO2 by
RO -NO interaction, RO + NO -> RONO-, with the C -C radicals (R) is an
important limitation of HO rate use in predicting oxidant formation. If these
reactions are of great signi f icance then the high ^r alkanes will be poorer
oxidant formers. Thus the RO s formed by HO-af ;-ok on them will not pump NO
£
to NO as HO and the smaller RO radicals do. The net result of an HO
attack on a higher hydrocarbon will be the removal of a potential NO and O
20
-------
molecule: RO + NO -> RONO . It seems to me if this reaction is important, as
£, b
these authors suggest, then the pentanes and hexanes should not be as effec-
tive as the lighter hydrocarbons in O generation in chamber experiments. This
does not seem to be the case however, as illustrated in the smog chamber data
summarized by the Western Oil and Gas Association paper (18). Here the C
reactions forming O. are more efficient than those with C , and the C
J 4 D
efficiency is about the same as the C,.. The point that Pitts et al. make is
an important one, and further work should be done to clarify this apparent
problem.
The use of HO rate constant data as one criterion for smog generating
ability is a meaningful approach. To use these data to suggest anything
quantitative about the ozone levels expected in rural areas is a meaningless
exercise unless it is coupled with rates of radical generation and other
mechanistic features which control ozone formation.
"Photochemical Reactivity Classification of Hydrocarbons and Organic Com-
pounds" by F.F. Parley (the Western Oil and Gas Association).
A review has been made of available smog chamber data related to hydro-
carbon reactivity as measured by [0 ] maxima, and a photochemical reactivity
classification of hydrocarbons and organic compounds has been developed. The
authors suggest that the wide range of reactivities observed argues for a
multiclass reactivity scale.
There are several important points that must be remembered in attempting
to evaluate oxidant-forming potential in the atmosphere. Not only smog
chamber data should be considered but also the concentrations of the various
RH species and the nature of the other components occurring with the given
compound. Obviously the rate of reaction of a given hydrocarbon, RH, with the
HO-radical (RH + HO -> R + HO) is given by:
-d[RH]/dt = [RH] [H0]k
Three factors, the HO-radical concentration, the specific RH concen-
21
-------
tration, and the rate constant K for the given reaction are involved. The
detailed nature of the sunlight-irradiated mixture, that is, the effective
rate of generation and destruction of the HO-radical, will establish the [HO].
The relatively unreactive RH may become a major reactant in a given HO-
containing environment if its concentration is sufficiently high. Thus it is
instructive to note that the estimated rate of reaction of the HO-radical with
the most unreactive of the hydrocarbons, methane, in the Los Angeles atmos-
phere on Nov. 5, 1973, was somewhat greater than that for C H , C H , cyclo-
2. o 2. £
pentane, 2,2,3-, 2,3,3-, and 2,2,4-trimethylpentanes and several other "re-
latively reactive" alkanes (16). This seemingly improbable happening resulted
solely from the relatively large concentration of CH. relative to the other
more reactive hydrocarbons cited. The extent to which CH., C_H_, C_H_, and
4 2. b £ £
the other hydrocarbons contributed to the 0 generation for this day, however,
was insignificant. Nevertheless, the example was cited to remind us that not
only must we be concerned with the HO-rate constants and smog chamber reac-
tivities, but obviously the amount of the RH present as well.
The point is made in this W.O.G.A. paper that there are no data showing
that the slower reacting compounds are the prime sources of rural oxidant in
transported, aged, urban air masses, and not the many other compounds present
in the air mass, such as organic reaction products of the faster reacting
compounds. Evidence that exists from the 1974 Midwest Study (19) and smog
chamber results indeed show that aldehydes are a major product of the smog
reactions. These species are known to initiate radical formation and can be
important reactants for HO-radicals in aged air masses. To what extent the
"unreactive" RHs and the aldehydes and the other compounds successfully
compete for HO-radicals and help drive the NO to NO conversion leading to
higher ozone production, depends upon the relative magnitude of the concentra-
tion-HO-radical rate constant products: k[CH20], k'[CH3CHO], k"[RH], . . .
It is possible that the aldehyde terms outweigh those for many of the un-
reactive hydrocarbons. So the point of this paper is not a trivial one, but
can only be answered as better product analysis of the aged air masses are
obtained. Each mixture needs to be considered in view of the composition.
Generalizations from existing limited data are dangerous and unwarranted.
22
-------
Major sources of a given light hydrocarbon mixture may result in the signifi-
cant involvement of a given alkane. For this to ocrur, however, some primary
radical source, not inherent in the presence of the alkane alone, must be
present as well.
The W.O.G.A. placement of acetone in their "unreactive" Class I is
surprising to me. The absence of acetone in the tables of Pitts et al. is
also a surprise. I would imagine that it is at least as reactive as C H and
hence in Class II of the W.O.G.A. list. The new EPA chamber data (compare
Table 3) show the acetone reactivity similar to that of butane and isobutyl
acetate, therefore, in the W.O.G.A. reactivity Class III. This situation
should be clarified by W.O.G.A.
"Application of Reactivity Criteria in Oxidant-Related Emission Control in the
USA" by Basil Dimitriades and S. B. Joshi.
The authors discuss the occurrence of pollutant transport and smog
chamber oxidant studies using various "unreactive" organics. They interpret
the buildup of oxidant in old air masses in terms of the participation of
"unreactive" hydrocarbons and conclude that there is a need for a new two-
class reactivity classification of organic emissions.
It appears to be realistic to assume that if enough "unreactive" RH
species is added to the atmosphere, it can participate in smog formation.
When the product of HO-rate constant for the reaction of the "unreactive"
hydrocarbon times the concentration of the "unreactive" hydrocarbon is greater
than the same product for the reactive hydrocarbons, i.e.,
1»- r RT-? 1 ^ if F RT-T1
unreact RH unreact react RH react'
the significant participation of the unreactive hydrocarbon is assured, pro-
vided, of course, that there is some significant free radical source to ini-
tiate HO-formation. However the amounts of [RH] required to cause
unreact
comparable effects are large, in the ratio, k /k . For pro-
^ ^ ' react RH unreact RH ^
pane and propylene, for example, 15.1 x 10 /I.3 x 10 - 11.6-Limes as much
C H is required as C H to have equal HO-attack rates in the atmosphere.
JO 8 O
23
-------
The authors are seemingly convinced that the direct atmospheric obser-
vation of reactivity-related control measures upon air quality is impossible.
I am not so sure that we should come to this conclusion. Certainly smog
chamber results are of very great value ir controlling the variables and
developing quantitative theories and reaction mechanisms of simple RH-NO
X
dilute mixtures in air, but the ultimate test of these theories must include
atmospheric testing. I believe that experimentation involving RH-NO mixture
A
insertions into the real atmosphere can be designed to te.et current ideas
related to oxidant formation. Since rules and regulations resulting from all
of the studies will be applied to the real atmosphere and not to the control
of smog chamber atmospheres, we must be careful to test the conclusions formu-
lated from chamber experiments in the atmosphere before accepting these as
completely applicable. This is particularly true in dealing with the un-
reactive RH compounds.
The real danger in using smog chamber results to assess the reactivity of
the less reactive species is the unpredictable influence of the chamber walls
on the laboratory experiments. It is not clear that the radical source
attributed to the wall reactions has an equivalent counterpart in the real
atmosphere. We know that the unreactive species do react slowly in the
chambers, and it appears that by some unexplained mechanism HO-radicals have
been produced to initiate this reaction. It is very important to recognize
that the magnitude of this radical source may be much above that of atmos-
7 8
pheric sources of HO-radical. The values of 1.5 x 10 to 1.7 x 10 molecules
cc , estimated from CH removal experiments in the EPA smog chamber, are
somewhat higher than those estimated in the Detroit atmosphere by Wang et al.
(14) using resonance fluorescence measurements of HO (5 x 10 molecules cc ).
However, they seem considerably higher than those estimated by absorption
6 6
spectroscopy by Perner et al. (15) in Germany (4 x 10 to 7 x 10 molecules
cc ), indirectly by Calvert (16) from relative RH removal rates in the LARPP
experiments (4.5 x 10 tc 0.5 x 10 molecules cc ), and estimated theoreti-
cally by our group (3 x 10 to 1 x 10 molecules c ) (13) and by Crutzen and
Fishman for the ambient air in the tropospher* (0.8 x 10 to 1.5 x 10 mole-
cules cc'1) (20) .
24
-------
Note that if one makes the reasonable assumption that the rate deter-
mining step in the removal of the relatively unreactive species qiven in Table
1 of the Dimitriades and Joshi paper is in each case the attack of HO-radical
on the species, then we can take the reported average rate of organic dis-
appearance reported in the Table (% per hr), the estimated rate constant for
HO reaction with the given RH (k_tl) and derive the average [HO] in the chamber:
RH
[HO], molec/cc
(Rate RH removal, %/hr) (4.10 x 10 )
RH
-1 . -1
Values calculated in this fashion for [HO] for all species for which the rate
constant is known are shown in Table 3 (last column).
TABLE 3. ESTIMATED [HO] FROM EPA CHAMBER STUDIES OF ORGANIC REACTIVITIES
Compound Rate (%/hr)
CK4
CHC1
3
C H
6 6
CH Cl
2 2
Ethane
Propane
Acetylene
n-Butane
Methanol
Methylethyl Ketone
Isopropanol
0.05
0.1
0.8
3.1
5.7
0.5
2.0
6.3
1.4
1.3
1.5
3.3
k (ppm min ) [HO] , molecules/cc
RH
11.8
22
61
2083
206
417
3185
242
4410
1397
4900
10500
1.7 x 10?
1.9 x 10?
5.4 x 10
7
0.6 x 10
7
11.3 x 10
7
0.5 x 10
7
0.3 x 10
10.7 x 107
7
0.13 x 10
7
0.38 x 10
7
0.13 x 10
7
0.12 x 10
Several features of these data should be noted. The [HO] estimated
Q
varies over a factor of one hundred, from 1.1 x 10 using CH Cl to [HO]
= 1.2 x 10 molecules/cc in experiments with isopropanol. The highest of
these estimates which appear with CH Cl and C H and the large variations
25
-------
between compounds are not clearly understandable in terms of elementary rate
data. It seems to me that the charter data may not correspond to atmospheric
rates, but the rates may be as much as a factor of 100 too high in some cases.
Certainly one should exercise caution in the use of these chamber data, to
predict atmospheric happenings with the x-._ast reactive compounds,
A study should also be made in the chamber of the reactivity of carbon
monoxide gas as well as the organic compounds. It is clerr from previous
studies (21-24) that CO may be classified as a "reactive" species in O_
generation under some conditions.
In considering the uniqueness of the chamber walls for possible ini-
tiation reactions, one must include the reaction of HONO formation: H~O + NO
+ NO -> 2HONO. Heterogeneous generation of HONO at chamber walls may allow a
unique source of HO radicals (HONO + hv -»• HO + NO) in the chamber experiments.
Although HONO may form homogeneously in the chamber and in the atmosphere, its
rate through the homogeneous reaction, HO + NO + NO -> 2HONO, is very low for
the levels of NO, NO , and HO employed. Turbulent mixing in the chamber can
be aided by thermal gradients within the operating chamber, and reactant
contact with the walls can accentuate HONO formation and subsequent HO-
radical generation. The analogous reactions may occur in the real atmosphere
at ground level or on aerosols, but their importance is probably much less
than in the smog chamber. Most HONO in the atmosphere probably arises from
HO + NO + M -»• HONO + M, and HO + NO -»• HONO + O .
A further consideration is the possible significant deposition of CH 0
polymer on the chamber walls during these runs; it is a major product of the
oxidation of most RH species. As the chamber wall is heated through the
action of the lights, CH O evolution from the wall may occur to initiate
radical generation and HO-radical attack on the RH:
26
-------
CH O + hv -»• CHO + H
H + O + M -> HO + M
CHO + O -»• HO + CO
£t £
HO + NO •> NO2 + HO
HO + RH -> HO + R
In any case the nature of the initial source of the HO-radicals remains un-
clear, and it appears to me that the rates of removal of some of the unre-
active RH compounds may be very much higher in the chamber than they would be
in the atmosphere.
It is stated that the amounts of desorbed organics in the chamber are at
immeasurably low concentrations at most. Immeasurable by what technique? Is
the cell heated and evacuated between runs? Are blanks with clean air run
periodically for extended periods? My experience with chambers suggests that
the memory of the chamber for the previous chemicals employed can be very
good, and it is very difficult to return to a clean chamber again without
thorough heating, pumping, wall washing, etc.
The subtraction of O created by irradiation of background air need not
correct for the impurity influence, since the radicals formed from the initial
impurity in an experiment with added unreactive hydrocarbon, can initiate long
chains involving the "unreactive" hydrocarbon. These synergistic effects may
be unique to the "dirty" chambers.
Of course reaction products such as CH O, CH CHO, HONO, etc., form in the
atmosphere as RH oxidation occurs, and I would expect these species, as well
as 0 , to initiate chain oxidation of the RH species in the atmosphere. The
question that must be answered in the comparison of chamber and real atmo-
sphere is whether the amounts of these compounds in the atmosphere are compar-
able to those released from the wall or formed at the wall in the chamber.
The conclusion that the existing classification of organics should be
27
-------
revised tc reclassify most of the "unreactive" organics into a single class of
reactives appears to me to be premature. Certainly we must understand the
smog chamber and its apparent ill-defined sources of HO-radical before we can
extrapolate its results to the atmosphere in any scientifically meaningful
way. we must know the ambient levels of CH 0, CH CHO, HONO, H C> , 0 and
other potential photochemical sources of radicals in the atmosphere before we
can make reasonable predictions concerning the reactivity of various RHs
toward C development in the atmosphere.
It is entirely reasonable that photochemically active radical sources
exist in the atmosphere containing aged air masses. However the rate of ozone
generation by these mixtures will remain unclear until the quantitative analy-
tical information concerning their detailed chemical compositon is established
and 0 development in equivalent mixtures can be studied without the presence
of unique radical sources peculiar to the smog chambers.
"Control Regulation for Stationary Sources of Hydrocarbons in the United
States" by Robert T. Walsh.
The paper gives an accounting of the sources of the volatile organic
compounds released to the atmosphere of the United States. The increase in
the emissions from stationary sources, now at 60% of the total released into
the atmosphere from all anthropogenic sources, is a concern in view of the
possible influence of these compounds on photochemical oxidant formation.
It is stated on page 6 of the paper, "For oxidant control purposes, the
program is aimed at reducing emissions of all volatile organic pollutants
regardless of their photochemical reactivity." It appears to me that this is
an extremely cautious approach that may involve considerable overkill. If
excellent control is possible with little economic hardship, then the plan to
eliminate all RH emissions is a very good one. I personally feel that the
scientific evidence of rural oxidant formation ."oes not warrant laws that
eliminate all "unreactive" as well as "reacti/e" hydrocarbons at this time.
The expenditure of some years of well planned research is necessary to estab-
28
-------
lish the impact which such indiscriminant controls will have on oxidant
formation. There is a good chance that little influence from the complete
removal of "unreactive" hydrocarbons will be seen.
GENERAL CONCLUSIONS CONCERNING THE ISSUE OF REACTIVITY
The Question of the Borderline Separating Reactive Species from Those
of "Virtually" No Concern in Oxidant Generation
The answers to the questions given in this section should be supplemented
by reference to the discussion of the previous sections. The present labora-
tory and computer methods for evaluating oxidant-forming potential of organic
compounds in the atmosphere all appear to have rather serious unevaluated
problems related to their use. This conclusion is evident in the varied
conclusions at which different workers arrive using these methods. It is, I
hope, an accidential correlation that can be seen between the conclusions of
the industrial researchers who find little influence from "unreactive" hydro-
carbons on rural oxidant formation and the government supported researchers
who feel there is a significant impact from these compounds. It appears to me
that the judgmental flexibility and uncertainty in the evaluation methods now
available make these conflicting conclusions inevitable.
It is highly unlikely that any one pure hydrocarbon will be the dominant
ingredient in an RH-HO -polluted atmosphere, so the relevance of the ozone-
X
forming potential in initially pure hydrocarbons in NO mixtures in smog
X
chambers is not entirely clear to me. I believe we should be more concerned
with the synergistic effects of added unreactive hydrocarbons on the common
atmospheric contaminants present in rural and urban air masses. What emerges
from the vast quantity of chamber data from NO -"unreactive" RH mixture
irradiations are several rather quantitative measures of oxidant-forming
potential of these mixtures in smog chambers. The applicability of these data
to many cases in the real atmosphere remains unclear. Even the radical ini-
tiation steps as well as the HO levels present in chambers may not match well
those present in rural and urban air masses.
29
-------
To extrapolate the chamber data for computer models to the conditions
present in the atmosphere, both EPA-supported scientists and industrial
scientists have had to make certain questionable assumptions. Foremost among
the problems is that of the seemingly artificially high [HO]-levels present in
many of the smog chambers. The identification of the unknown initial driving
force for the oxidation of pure CH , C H , C H , or n-C H, , etc., in NO
'i £ O >3 O ft -Lv X
mixtures in chamber experiments is a scientific problem of large magnitude;
its solution is of vital importance to the value of our extrapolations to the
real atmosphere. It is possible that HONO and possibly some other species are
contributors to this initial radical source, as many of the current workers
assume. If this is the case, then we are quite sure that chamber data for the
"unreactive" hydrocarbons will not relate well to their behavior in the real
atmosphere. This is a consequence of the fact that HONO homogeneous develop-
ment in the atmosphere, through the reaction HO + NO + NO. •+ 2HONO, is ex-
tremely slow for the ambient levels of NO present, Although HONO must be
X
formed in the atmosphere, it is probable that the reactions, HO + NO + M -*•
HONO + M, and HO + NO -»• HONO + Oo, are its major sources here. If this is
the case, the heterogeneous development of HONO, which may occur readily in a,
chamber, provides an unusual and atmospherically unrelated boost to the re-
actions of the "unreactive" species.
In the previous discussions of the papers in this review, I have indi-
cated that two experimental estimates and several theoretical estimates of
[HO] levels in the troposphere suggest that these are a factor of l/10th to
l/100th of the levels estimated in present chambers. Another serious concern
I have about chamber data is the undetermined effect of the NO -removal re-
actions, some of which are unique to the chamber. Thus N20_ + H-0 -> 2HONO
reaction is probably only significant on the walls of the reactor. It is
relatively unimportant in the atmosphere. When the [HO]-levels in chambers do
not mimic well those in the atmosphere, then the formation of HONO through
the reaction, HO + NO + M •* HONO + M, and the attendant removal of NO will
not correspond to the atmospheric case, and obviously the 0_ formation pattern
will be disturbed. In this regard it is interesting that the [0 ]-time pro-
file seen in the Los Angeles Reactive Pollutant Program (LARPP) atmospheric
study of the single air parcels shows a continuous rise of the [O ] during the
•J
30
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entire day of Nov. 5, 1973; there is no maximum in [O ] that is so charac-
teristic of most smog chamber irradiations of auto exhaust. It is indeed a
very risky business to use chamber data to extrapolate to the atmospheric
conditions as they might apply to the "unreactive" hydrocarbons.
There appears to be a real uncertainty in view of the existing data as to
whether the high rural oxidant is really developed significantly by the action
of the unreactive hydrocarbons or whether new reactive hydrocarbon and NO
X
insertions from natural and anthropogenic sources are the main stimulus to
further O development in aged air masses.
There is no question that working in consort with reactive hydrocarbons
and oxidation products, the "unreactive" organics can participate in reactions
leading to NO to NO conversion and 0 generation in the atmosphere. However,
the important question that must be answered is: To what extent do these
compounds increase the ozone levels in the aged mass ? The present data do
not prove that the very high rural O levels, sometimes observed, arise to any
appreciable extent as a result of this involvement. It is equally true that
the present data do not disprove this contention.
The results of chamber experiments and modeling available to me today do
not lead to unambiguous answers as to which compounds are reactive and which
are totally unreactive in the atmosphere. The classification of propane,
butane, and all higher alkanes as class III compounds through their [HO]
rate constants in the reactivity scale of Pitts et al. should not be taken as
an indication that these compounds contribute significantly to O. development
^ r
in aged air masses. If the [HO] level in the troposphere is 2 x 10 mole-
cules/cc, rather than the 1 x 10 molecules/cc assumed by Pitts et al., then
the halflives of these alkane species will be in the range of 5 to 0.5 days,
and not 1 to 0.1 days as they suggest. Such relatively slow oxidations may
not significantly increase the HO and RO radical levels above those created
alone by the reactive oxidation products in the aged air mass, and hence they
may not change the rate of NO to NO conversion much and alter O little.
Although the chamber and computer simulation experiments have given us a
31
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great deal of useful information on the mechanism of smog formation, it is
equally clear that there are significant differences between the chamber and
the atmosphere. Thus, I feel that it is very important that we go back to the
real atmosphere to answer questions that relate the subtle effects of "un-
reactive" hydrocarbons. Well planned atmospheric studies will answer ques-
tions about the atmosphere. If we wanted to control ozone levels in a given
smog chamber, then the chamber data alone would suffice. If we knew well all
of the critical rate constants and the precise composition of the urban and
rural air masses, then the computer simulations would be of great value in
formulating our strategy concerning reactive and unreactive hydrocarbons. AS
it is now, neither of these situations exists, although we seem to forget this
now and then.
The Question Whether One Method or the Other or the Two Approaches Combined
in Some Way or Some Other Approach Yields the Most Reliable Results
As outlined in the previous section, both chamber and modeling methods
have serious problems related to their predictive value for the "unreactive"
hydrocarbons. Obviously both techniques are valuable in principle, but exist-
ing problems and new unforeseen complications in their use are bound to appear
as we look closely at these methods, making the unambiguous answers from
either method impossible now. I honestly believe that EPA would be well
advised to consider a third alternative, while the needed improvement in the
other two methods is continuing. This alternative is the use of new direct
atmospheric studies of the chemical changes in rural and urban air masses.
Specific Additional Research Needed
It appears to me that we need a great deal more information concerning
the chemical composition of urban and rural air masses as a function of time
before we can determine the reliability of the chamber data and computer
studies in simulating the atmospheric problems. The cost of such studies need
not be prohibitive if other federal agencies such as NASA, NBS, ERDA, NCAR,
etc., would share their expertise and their financial assets in the project.
If such experiments are to be meaningful they must be planned well to yield
32
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the necessary kinetic and analytical data. Such plans will include unam-
biguous lifetime determinations through labeled compound injection, [HO]
estimates in the atmosphere, etc.
We must identify the radical sources that stimulate the HO-chains and the
oxidation of "unreactive" hydrocarbons in chambers. Good spectroscopic
methods now exist that would seem to solve this problem easily.
It would be particularly embarrassing to us all and destructive to the
EPA scientific efforts to the future if we proceed with the development of
highly restrictive RH control measures from theory and chamber experiments at
this time, and then learn that the real atmosphere is very different from what
our simple picture had assumed. When it comes to the evaluation of the "unre-
active" hydrocarbons, the present information of which I am aware is too
contradictory and incomplete to reach a sound scientific judgment. I recom-
mend strongly that we add an effort utilizing well planned field studies aimed
at the role of "unreactive" hydrocarbons.
COMMENTS BY H.E. JEFFRIES
Dr. Calvert took a much broader view of the issues than I did in my
review and came to a slightly different conclusion. We both question the
relevance of ozone-forming potential of high concentrations of initially pure
hydrocarbons in NO mixtures in smog chambers.
j\
Although I agree with Dimitriades and Joshi that "it is not possible to
directly assess the impact of reactivity-related solvent control by examina-
tion of air quality data ...", I do not disagree with Calvert1s suggestion
that "well planned atmospheric studies will answer questions about the atmos-
phere." Simple, fixed-state, ground-level air quality monitoring-type experi-
ments are probably not what Calvert had in mind.
I suggested that smog chamber work and photochemical modeling must be
used in combination if a clear understanding of the chemistry is to be ob-
33
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tained; Calvert has reminded us that these are probably not sufficient to give
unambiguous answers as to what the atmosphere does without actual atmospheric
data to support the conclusions.
I have no major disagreements with Dr. Calvert"s analysis.
34
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REVIEW AND ANALYSIS
H. E. Jeffries
INTRODUCTION
Since 1971 the Environmental Protection Agency (EPA) has pursued an
oxidant control strategy that focuses mainly on the control of organic emis-
sions from mobile and stationary sources. The alternatives for hydrocarbon
(EC) control are direct reduction of emissions, substitution of less "re-
active" HCs for more reactive HCs, or some combination of both.
The- concept of HC reactivity grew out of early smog chamber experiments
in which different HCs were irradiated with oxides of nitrogen (NO ) for fixed
X
time periods. Different HCs gave rise to different ozone (0) or oxidant (O )
3 x
levels at the end of the irradiation period. Thus, hydrocarbons were ranked
according to their ability to produce O . Experiments carried out in dif-
ferent laboratories resulted in different absolute O concentrations, but the
relative rankings of HCs were in fairly good agreement. The experiments were
primarily conducted at a HC to NO molar ratio of two, which was considered
X
typical of urban centers, and for a duration of 6 hours. These data were
reviewed and summarized at the International Conference on Photochemical
Oxidant Pollution and Its Control by F. F. Farley (18),
In 1966 the Los Angeles County Air Pollution Control District implemented
Rule 66 as a control measure for volatile organic solvents substitution based
on photochemical reactivity as a means of reducing ambient oxidant levels.
Similar regulations have been adopted by many jurisdictions, and industry has
accommodated itself to these reductions and substitutions.
In its 1976 "Policy Statement on Use of the Concept of Photochemical
35
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Reactivity of Organic Compounds in State Implementation Plans for Oxidant
Control," EPA supported the positive reduction aspects of Rule 66 regulation,
but questioned the utility of solvent substitution strategies based on more
recent information on photochemical reactivity. This information grew out of
investigations related to the so-called "rural oxidant problem," the finding
of elevated O (> 0.10 ppm) in areas relatively remote from urban centers in
the Eastern part of the U.S. Thus "pollutant and oxidant transport problems"
were introduced into the oxidant control strategy. Because of potential
transport problems, the reactivity concept was reexamined and new smog chamber
experiments were performed to measure reactivities under long irradiation
times and higher organic-to-NO ratios (2).
X
These experiments suggested that almost all volatile organic compounds
eventually reacted in the atmosphere to form some oxidant. Thus EPA's current
policy changes tend toward positive reduction of all organic emissions except
those that are shown to be so little reactive that they would neither cause
nor contribute significantly to oxidant buildup at problem levels under any
circumstances. This approach leads to a two-class reactivity scale: unre-
active and reactive. Therefore, from EPA's viewpoint, the reactivity issue
has become one of defining unreactive organic compounds. Dimitriades and
Joshi (2) argue that the only method for identifying unreactive compounds is
to conduct careful smog chamber experiments.
At the International Conference several papers dealt with the issue of
reactivity. The EPA viewpoint was strongly challenged by Ford Motor Co.
research scientists (3) on the basis of computer simulations of photochemical
models and an analysis of aerometric data from a rural site. In addition, the
General Motors Research Laboratories, on the basis of "multiday" smog chamber
experiments, suggest that, in transport simulation experiments, HC reactivity
is "only moderately altered on the second and third days of irradiation and,
thus, hydrocarbon reactivity is still an important consideration in control-
ling organic emissions even in situations involving long-distance transport"
(11).
Farley (18), representing the Western Oil and Gas Association, in his
36
-------
review of previous smog chamber reactivity studies, recognized the possibility
of slower reacting compounds participating in oxidant formation under trans-
port conditions, but was not convinced by existing evidence that these caused
much increase in oxidant compared to the organic reaction products of faster
reacting compounds. * He also said that if the slower reacting compounds are
significant precursors to oxidant, then the utility of a reactivity scale
based on O formation in (older) smog chamber studies would be diminished.
Pitts et al. (25) suggested an alternative co smog chamber experiments as
a means of assessing reactivity. Smog chamber experiments, they noted "would
be extremely time consuming and expensive and would inevitably suffer to some
degree from the problems noted in previous chamber studies of this type."
Instead, they propose a "supplementary (rather than a substitute) approach in
obtaining the required data." This approach consists of a hydrocarbon re-
activity scale based not on secondary smog manifestation criteria' (that is, 0
formation), but rather on the primary chemical act of hydroxyl radical (OH)
attack on organic species. Two reasons were given: OH is the most important
reactive intermediate, and this approach can identify those compounds that
particpate at "significant" rates in atmospheric reactions. Chang and Wein-
stock also took exception to this OH reactivity scale on the basis that it
over predicts the reactivity of less reactive HCs compared to more reactive
HCs.
The purpose of this review is to critically examine the reported evidence
and viewpoints for conflicts, to make judgments on the strengths and weak-
nesses of opposing viewpoints or evidence, to attempt a reconciliation of
these conflicts, to derive factual or judgmental (referee) conclusions re-
garding resolution or status of the issue, and finally, to offer recommenda-
tions for additional research.
The central aspects of this issue are embodied in the evidence and
arguments presented in the Dimitriades and Joshi paper and in the Chang and
Weinstock paper. Therefore, these two papers will be reviewed extensively.
Other sources of information will be drawn upon as needed.
37
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DISCUSSION
Areas of Conflict
Dimitriades and Joshi make several arguments of a philosophical nature.
I is not possible to directly assess the impact of reactivity-related solvent
Control by examination of air quality data because any impact is masked by the
overwhelming effects of other factors such as auto exhaust emission reduction
and other effects. This impact must therefore be assessed indirectly based
on laboratory evidence; direct atmospheric observation is inconclusive.
The relative roles of "reactive" and unreactive" organics, when they are
subjected to pollutant transport conditions, likewise, can only be examined
indirectly based on laboratory evidence. Relative contributions of the reac-
tive and unreactive organics to ambient oxidant cannot be determined from
aerometric data.
Chang and Weinstock, in support of their argument that the less reactive,
"leftover" HCs were not responsible for rural O , but that "fresh" emissions
of both NO and HC from local sources were responsible, carried out an analy-
sis of aerometric data from the summer rural area study of 1974. They stated
that the importance of "leftover" HCs compared to "fresh" HCs can be deduced
from detailed measurements of rural HC, in that, if "leftover" HCs from urban
areas are present in significant quantities, this should be evident by an
accumulation of less reactive species compared to more reactive ones (i.e., a
fractionation process). They did not find any evidence of such fractionation
and thus assumed that local sources rather than transport was the main source.
They also interpreted HC and O vertical profiles at three times during
a single day at Wilmington, Ohio, to support their theory that the 0 observed
at this "rural" site was the result of buildup of new nonmethane hydrocarbon
(NMHC) under the inversion layer. Thus they contend that rural O is the
result of reactions of "fresh" HCs and NO emitted into the mixing layer and
X
"leftover" HCs play only a minor role in augmenting this increase.
38
-------
Hence, Dimitriades and Joshi claim that aero-metric data cannot be used to
assess the role of "reactive" and "unreactive" organics and Chang and Wein-
stock claim to have done exactly this.
Dimitriades and Joshi continue their argument as follows. Laboratory
evidence was interpreted to mean smog chamber evidence. It was recognized
that there are uncertainties arising from the indirect nature of smog chamber
experiments, but this approach was considered to be the only basis for re-
assessing the use and utility of the reactivity concept.
Chang and Weinstock express a viewpoint that smog chamber data is mis-
leading with respect to oxidant reactivity. Some of the basis for this
conclusion was presented in their Conference paper, but more information
appeared in Weinstock and Chang (26) and in Niki and Weinstock (27). The
details of this argument will be discussed later; the following is a brief
summary of their position:
Chang and Weinstock believe that smog chamber studies generally make less
reactive compounds appear to have an anomalously high photochemical
reactivity compared with more reactive compounds because of the existence
of a substantial hydroxyl (OH) radical background in smog chambers. It
is this OH background that drives these unreactive systems and not the
inherent reactivity of the HC. Therefore, the behavior of HC in smog
chambers is not representative of their atmospheric behavior. No speci-
fic cause of the OH background was given, but it was implied that it
arises from material that is not removed during the usual cleanup of the
chamber.
In their paper, Dimitriades and Joshi stated that although the above
claim (that there may have been an OH background in the EPA chamber of com-
parable or slightly higher magnitude than that observed in the ambient atmos-
phere at Ford research laboratories) might have been qualitatively correct,
this did not invalidate the original interpretation of the smog chamber data.
That is, under optimum conditions many of the thought-to-be unreactive or-
ganics can, in fact, cause significant O buildup. They argue that:
39
-------
• OH formed from photolysis of nitrogen compounds desorbed off walls is
unlikely to be important relative to the source created by the NO
X
reactant itself.
• OH could form from photochemical reactions of wall-desorbed organics
and the NO reactant, but such organics are at such almost immeasur-
X
ably low concentrations that this source must also unavoidably be
present in the real atmosphere as a natural background contamination.
® Any bias caused by a background OH source can be reduced — but not
eliminated — by subtracting from measured reactivity values the
background air reactivity.
• Irrespective of chamber background OH problems, it has been shown
that conditions of prolonged irradiation and an optimum organic-to-
NO ratio do enhance the reactivities of the less reactive organics.
They also proposed and illustrated a procedure for determining the
borderline betT> ?en unreactive and reactive organic compounds that consisted of
obtaining smog chamber data in an appropriate chamber under optimum irradia-
tion time and organic-to-NO ratio conditions. A compound that would riot
produce more than 0.08 ypia. c,, would be declared unreactive. Only the least
reactive organic would have to be tested. No definition of appropriate was
given. The illustrative data, which were obtained at 4.0 ppm organic and 0.2
ppm NO initial conditions in constant light intensity (Pk for NO0 — 0.33
X di 2
min ) irradiations lasting up to nearly 14 hours, suggested that the border-
line was at propane. It was indicated that the test conditions might not have
been optimum.
Chang and Weinstock used computer simulation of a Ford Research Staff
derived photochemical model (FPM) to challenge Dimitriades and Joshi smog
chamber results. The following approach was used. Chang and Weinstock
discounted the results obtained by Dimitriades and Joshi for the propane/NO
X
system (EPA Run 194) because, first, the results were not in agreement with
those from the FPM (2 pphm for the model versus 11 pphm for the chamber), and
second, major modifications involving nitrous acid (HONO) productions, in-
creased O heterogeneous loss, and NO heterogeneous conversion to nitric acid
40
-------
(HNO ) were required to obtain a reasonable agreement between the FPM and the
smog chamber data. Chang and Weinstock indicate that similar results were
also found for the ethane smog chamber data of Dimitriades and Joshi. In
using the modified FPM, Chang and Weinstock found general agreement with smog
chamber data by Heuss (28) for ethane (0.08 ppm O ), propane (0.13 ppm O ),
and butane (0.20 ppm 0 ) thus implying that most smog chambers share the same
problems and that the FPM model had captured the essence of this problem.
They applied the unmodified model to simulate a rural situation having various
low levels of propane and NO and based on the results suggested that propane
X
would not generate elevated levels of rural 0 .
The Chang and Weinstock approach is essentially a proposal to substitute
photochemical modeling results for smog chamber results in that an assumption
was implicitly made that the Ford photochemical model had greater validity
than the smog chamber data.
Thus, there are several specific areas of conflict presented in the two
papers with supporting data frequently cited. These areas all touch upon the
utility of smog chamber data. Dimitriades and Joshi's position is that, even
though smog chambers have some problems, they are the only method available.
Chang and Weinstock believe that, based primarily on photochemical model
predictions, the technical basis of proposed reactivity policy changes is
highly questionable.
Comments on Strengths and Weaknesses
Use of Aerometric Data
The first viewpoint of Dimitriades and Joshi is essentially obvious. So
many important factors have changed in Los Angeles since Rule 66 was imple-
mented that it is not possible to assess the benefits that may have occurred.
Their second proposition is considerably more difficult to judge in light of
Chang and Weinstock1s suggestion that relative distributions of HCs in rural
areas be examined. Pitts et al. (25) in their paper suggest that the half-
life for the less reactive alkanes is between 0.1 and 1 day with most of them
41
-------
falling nearer the 0.1 side of the interval. Thus significant consumption of
even the less reactive HCs might be expected as opposed to accumulation as
suggested by Chang and Weinstock.
Lonneman (29) in his analysis of detailed HCs samples from Wilmington,
Ohio, suggested that 74% of the total NMHC was due to vehicular tailpipe
emissions. In addition, he presented data to show that the Wilmington, Ohio,
sample generally indicated reduced olefin and aeromatic content. He concluded
that the samples taken at Wilmington represented diluted urban HC mix with
associated photochemical loss of the more reactive compounds during the
transport process. This conclusion directly contradicts that of Chang and
Weinstock.
With respect to the vertical profile data for Wilmington, Ohio, I find
the arguments advanced by Chang and Weinstock weak in certain aspects. First,
whether the higher NMHC in the lower levels is due to "fresh" sources or not
can only be determined by an examination of the detailed HC distribution and
this was not given. Second, since significant time elapsed between soundings
and no wind speed information was given, it is not possible to assume that the
morning conditions gave rise to the afternoon 0 observed unless spatial
uniformity is also assumed.
Overall, the Dimitriades and Joshi viewpoint concerning the utility of
aerometric data are probably correct; however, supporting information may
possibly be obtained by careful analysis of detailed HC data; it probably will
not be conclusive.
Use of Smog Chamber Data
The smog chamber has played a central role in developing an understanding
of photochemical smog; it is not without its problems, however. Table 1 gives
some of the characteristics of smog chambers as an investigative tool. The
smog chamber's greatest attribute is that it is capable of producing observa-
tions of actual chemical events, limited only by availability of suitable and
accurate analytical methods. The smog chamber's greatest weakness is that it
has walls that can potentially influence the outcome of the chemistry (see
42
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Table 1, item 5). It is to this area that Chang and Weinstock direct their
strongest criticism.
The term "dirty chamber effect" was coined by Bufalini et al. (30) to
describe the significant NO-to-N02 conversion rates obtained in smog chamber
irradiations without the addition of HC to the system. They were able to
prevent this conversion by washing the walls of their glass reactor between
runs. This term is also used to describe the formation of 0, in irradiation
of ultra-high purity air. An example of this process is shown in Figure 1
(this is the chamber used by Dimitriades and Joshi).
Chang and Weinstock make the claim that the "dirty chamber effect" is due
to generation of OH and hydroperoxy (HC-) radicals from background contamina-
tion during irradiation and that this source is of overwhelming magnitude
compared to the hydrocarbon related sources. They base this argument on the
following evidence.
In experiments conducted in their chamber ("dry glass chamber"), NO at
0.95 ppm decayed very slowly when irradiated (t . = 512 mins), thus illus-
trating the purity of the background air, but when CO was added at high con- .
centrations (110 and 665 ppm) the NO decay was greatly increased (t. ,_ ~ 89
and 22 mins) thus illustrating that OH was involved because CO is an efficient
converter of OH to HO?:
°2
OH + CO -*• CO2 + H02
HO- + NO -> N02 + OH
No explanation was offered as to the actual source of this OH other than small
contaminants in the system.
Ford research scientists had measured OH in ambient air outside the
Dearborn laboratory by a pulsed laser fluorescence method. The uncertainty of
OH concentration by this technique was given as about a factor of 3 and the
461
-------
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47
-------
minimum detectable concentration was about 2 x 10 ppm (14). The atmospheric
measurements showed significant diurnal variation with maximum values near 2.4
-6 -6
x 10 and average values of about 1.2 x 10 ppm. Ozone measured simulta-
neously, however, was only 30 to 70 ppb indicating relatively clean air.
A check on the laser calibration was performed by irradiating propy-
lene/NO mixtures at high NO-to-NO and NO-to-HC ratios so as to suppress 0
XX j
formation. Hydroxyl concentration was deduced by calculation from propylene
concentrations and the known rate constant for propylene + OH reaction. These
calculations gave a value of 5.26 x 10 ppm OH while the laser method gave
6.1 x 10 ppm OH. A photochemical model of this system, however, predicted
OH concentrations an order of magnitude lower. It was concluded, therefore,
that there was a large background source of OH.
Long path infrared/FTS spectra of the Ford chamber, when filled with
clean air, showed [HNO ] = 0.14 ppm, [formic acid] = 0.08 ppm, [HO] = 8 ppm,
and [CO] = 0.14 ppm. It was stated that HONO had been observed in other
studies, but they did not state the circumstances nor the magnitudes. These
were considered potential sources of OH, but a mechanistic explanation was not
offered.
An estimate of the average OH during methane (CH )/NO experiments in
T X
Dimitriades and Joshi's glass smog chamber was made from the disappearance of
CH during the 1500-1900 minute irradiation time. Methane concentrations
decayed from 4.0 to 3.45 ppm (1880 mins) and from 3.6 to 2.8 (1500 mins).
These values included sampling loss and CH was measured with a Beckman 6800
environmental chromatograph. Stating that they had corrected the final CH
values for sampling losses, Weinstock and Chang (26) calculated an average OH
-7 -6
concentration of 6.07 x 10 ppm for the 1880 min, 4.0 ppm run and 6.88 x 10
ppm for the 1500 min, 3.6 ppm run. They indicated that the first value is
near that measured in the atmosphere, but the second is an order of magnitude
greater. Thus, they concluded that there is an OH background source in the
EPA chamber that drives the system and therefore makes less reactive HCs
appear to be more reactive.
48
-------
Finally, there was major disagreement between Dimitriades and Joshi's
chamber results and a photochemical model derived by Chang and Weinstock.
This point will be discussed in detail later.
Although there certainly is some type of "dirty chamber effect," and
although Chang and Weinstock may be correct in their hypothesis that there is
a chamber background of OH, there are several weaknesses in the evidence
presented. For example: in the NO decay experiments, although it is clear
that OH was present, it was not clear that the source was the chamber walls.
The irradiation of NO in air alone does not demonstrate the purity of the
background air in that, if there is no material to serve as an OH to HO
converter, the dominant reactions, given an existing gas phase OH source,
would be:
4 -1 -1
OH + NO ^ HONO k = 1.2 x 10 ppra -min
HONO -> NO + OH k = 0.18 (f>Ka for NO
4-11
OH + NO -> HNO k = 1.5 x 10 ppm -min
Thus, there could have been a source of OH in the background air, and it would
not have been detected. Since no details were given, it is difficult to
assess the homogeneous and heterogeneous process
NO + NO + HO ^ HONO,
but it was stated that the reactor was "dry." Acceleration of NO conversion
on addition of CO indicates that OH is present; it does not indicate where the
OH originates.
It has been suggested that the OH concentrations measured by Wang et al.
(14) are too high because too broad a laser pulse was used (Whitten and Hogo,
(43), who cited a personal communication between D.D. Davis and M. Dodge in
1976). Davis et al. (31) have also measured much lower OH concentrations in
the troposphere than those reported by Wang et al. The OH concentrations
reported do seem to be quite high in that associated NMHC were below 10 ppb,
49
-------
CH was at its background level of 1.55 ± 0.05 ppm, Cu was approximately 0.5
ppm, NO was 3 ppb, and NO was 20 ppb. Using an average peak value of 2.4 x
— 6
10 ppm OH, which lasted from 1.5 to 3 hours, the half-life for propylene
would only be 7.5 minutes (using average OH values, approximately 15). The CO
half-life would be 648 minutes, and cis-2-butene would have a half-life to
only 4 minutes. These are extremely short times and cast doubts on the repre-
sentativeness of the measurements.
Chang and Weinstock report good agreement between the laser measurements
and estimates calculated from the rate constant for OH attack on propylene,
but these were an order of magnitude higher than those predicted by a photo-
chemical model. The rate constant for OH + propylene has recently been re-
4 -i -i
vised upward by a factor of 1.5 to 3.8 x 10 ppm -min (31,25,32). Esti-
mating a half-life for propylene of 80 minutes from the graph of this experi-
ment and using the new rate constant, I obtained an average [OH] of 2.28 x 10
ppm, almost a factor of three less than the measured OH. With respect to
the disagreement between the OH predictions of the photochemical model, the
question that must be asked is: did the model predict the propylene concen-
tration-time profile? If it did, then either there is something wrong with
the model or there is something wrong with estimating OH from HC decay. If it
did not predict the propylene decay then there is clearly something wrong with
the model. Other modelers (43) have had little difficulty obtaining excellent
agreement between photochemical model propylene predicted values and actual
chamber propylene data. Their OH predicted values, however, are less than
those measured by Wang et al. (14) . It should be noted, that aldehydes formed
from products of the OH attack photolyze to produce HO / which is converted to
OH by HO + NO ->- NO + OH. The quantum yields for aldehyde photolysis are
strongly dependent on wavelength and are also uncertain, leading to modeling
difficulties. This source may also account for the calculated OH concentra-
tion in this system.
The photolysis of HNO in the gas phase is very slow making this a poor
-4 -1
source of OH. ($Ka - 3 x 10 min in fluorescent lamp illuminated chamber
having a OKa of NO =0.3 min , Bufalini et al., (33).) Bufalini et al.,
however, suggested that HNO may undergo a bathochromic shift in the absorp-
50
-------
tion region due to its absorbed state, leading to a highar rate of photolysis.
Formic acid does not absorb UV light at wavelengths greater than 240 nm (34)
making it a poor source of OH. On the other hand, Figure 1 makes it clear
that there is some nitrogen compound absorbed on the walls that is capable of
giving a response to the chemiluminescent NO meter and leading to O pro-
X ~>
duction. Therefore, unless Bufalini's hypothesis is correct, Chang and
Weinstock's observations are not conclusive in explaining the cause of the
"dirty chamber effect."
In the CH data reported by Dimitriades and uosni, the ratio of final-to-
initial CH was 0.862 and 0.778. These values included sampling losses over
the 1500 to 1900 minute exposure period. Chang and Weinstock state that they
corrected these values for sampling losses. I back calculated from the rate
constant for OH + CH , the time, and the calculated OH values of Chang and
Weinstock to obtain the loss due to OH. These values were 1.33% loss in the
1830 minute run and 11.43% loss in the 1500 minute run. The environmental
chromatograph has at least ± 0.05 ppm noise on the signal (1.25% of 4.0 ppm)
and the sampling represents part of the loss (factor of 10 in first case, and
factor of 2 in the second case). Thus, a small error in reporting the sam-
pling rate or in the total time of sampling could lead to substantial errors
in estimating CH loss due to chemistry. Therefore, the accuracy of the
estimated OH is probably very poor (perhaps by a factor of 10).
Thus, it appears that much of Chang and Weinstock's evidence for an
overwhelming OH background source is of an uncertain nature primarily because
they can not offer a reasonable explanation for the origin (Bufalini may have,
but this remains to be tested), nor are their measurements of OH concentration
consistent with other data; therefore, it is difficult to conclude that OH
concentrations in chambers are greatly different from those in the atmosphere.
Dimitriades and Joshi's counterarguments, however, are also somewhat weak
and based more on intuition than on factual evidence. For example, the
argument that OH formed from nitrogen material desorbed off the walls is
unlikely to be important relative to the source created by the NO reactant
X
itself can only be true if substantial initiating sources are included in the
51
-------
gaseous chamber charge, such as a few tenths of a ppb of HONO or a few ppb of
aldehyde. It may well be that NO cannot be injected into chambers without
X
forming some HONO, but since the levels required cannot be measured, one
cannot tell whether the material is in the air or comes off the walls. If
Bufalini's spectral shift for HNO is correct, then it would be difficult to
distinguish between 0.1 ppb gas phase HONO and OH + NO from photolysis of
HNO on the walls.
The initial source of OH is very important, because, once OH is formed,
propagation and chain branching steps can rapidly increase the concentration.
Alkanes, for example, (according to current theory) are converted into alde-
hydes subsequent to OH attack while preserving the original radical (as HO ).
Photolysis of the aldehydes then introduces two new radicals that, in the
presence of NO , leads to more OH. Without some initial source, the system
X
may be so rate-limited in generating OH or HO sources that no significant
conversion takes place.
It is clear from background runs such as Figure 1 that some nitrogen
containing source capable of providing NO and some source of radicals capable
of converting NO to NO is probably associated with smog chamber walls. The
entity that is oxidized in converting OH back to HO is a major unknown. The
question that must be answered is: what is the relative importance of these
wall-associated processes with reasonable NO concentrations and organic
X
material? If it is not possible to inject NO into a chamber without forming
X
0.1 ppb of HONO, then it seems unlikely that atmospheric emissions of NO
X
would be completely free of HONO (10).
The presence of two initial sources of OH radicals is not additive in
their impact. For example, Demerjian et al. (5) in their modeling study
compared two simulations of HC/NO system having initial aldehydes. In one
simulation the rate constants for HONO equilibrium with NO, NO,,, and HO were
set to zero thereby removing an initial source of HONO. Also the rate con-
stant for HNO formation by NO and HO was made zero. In the second simula-
tion, these rate constants were assigned what are now considered to be large
values, thereby providing a relatively large source of HONO almost immediately.
52
-------
There was almost no difference in time to NO or NO maximum, but 0 was 0.03
ppm lower when HNO formation from N_O was allowed. The second simulation
was repeated with HONO pre-equilibration assumed (i.e., initial HONO was
present at 5 ppb). Again there were only small differences; 22 minutes to
NO maximum versus 24 minutes for simulation 2 and 31 minutes for simulation
1. An examination of radical sources showed that the OH and H09 flux had been
approximately doubled between simulation 1 and 3. The aldehyde photolysis
reactions were major radical sources in all three cases.
Thus the applicability of results obtained during clean air irradiations
to regular run results with HC and NO present may depend upon whether NO and
X
NO can be injected into a chamber without forming small concentrations of
HONO (< 1 ppb). Thus, the need of the modeler to add a few tenths of a ppb of
HONO initially in his model is not necessarily an indication of a "dirty
chamber effect."
Finally, the real atmosphere is not free of OH sources, and the emission
of the compounds of interest are not likely to occur in a total pure state;
therefore, as suggested by Dimitriades and Joshi any "boost" given to the
chemistry in a smog chamber with as low a background reactivity as the EPA
chamber is likely to be less than what would occur in the atmosphere.
/
Use of Photochemical Models
Computer simulation of photochemical models (modeling) has become a major
research tool providing a kind of understanding and insight that is almost
impossible to obtain otherwise. The application of models and the generation
of the fundamental kinetic information necessary to support the models are
very active research areas, and, consequently, the mechanisms used in the
simulations are constantly in a state of flux. The trend of this flux has
generally been toward improved predictions and greater understanding. Un-
fortunately, this constant revising and updating has led some to be overly
suspicious of modeling efforts, and it has made it difficult for everyone to
keep up with the latest information.
53
-------
Table 1 gives some of the attributes of computer simulations of photo-
chemical mechanisms. The strongest attribute is its ability to combine a
great deal of theoretical information into a unified result. It is ideal for
testing various hypotheses about mechanisms. The greatest shortcoming of
modeling is in the data base: precise and accurate values of rate constants,
product identity and yields, and the uncertainty that all reactions of impor-
tance have been found and quantified.
Modeling and smog chamber experimenting are highly complementary. When
one models a particular run, the question being asked is: are the observa-
tions made during one experimental run consistent with known theory as ex-
pressed by the model mechanism used?
If there is close agreement between experimental observation and model
predictions it simply means that there is agreement for this particular run.
This does not mean that the observations and the model are correct. Since
there are many choices in constructing a mechanism (different interpretations
of theory), the agreement may be coincidental. Agreement at other conditions
may be very poor.
If there are substantial differences between experimental observation and
model (theory) predictions the source of disagreement may be in the observa-
tions, in the theory, or in both. The observations may include the influence
of a process that was not intended to be present (and therefore, not included
in the model, e.g., wall-related processes), or operational parameters (such
as spectral distribution) may actually have been different from what they were
thought to be (and from the representation used in the model). Analytical
errors could have contributed significant differences ([OJ high by 35% due to
calibration errors).
Alternatively, the theory used to establish the mechanism could be
substantially wrong: rate constants have been revised by more than a, factor
of three, a reaction was written with wrong products, or the representation of
the theory used in the model introduced a model artifact (use of "lumping" or
steady-state assumptions where not appropriate).
54
-------
Ultimately, a disagreement between experimental observation (assuming the
absence of significant analytical errors) and theory as expressed by the
mechanism simulation results must mean that the theory (as expressed) is
wrong, since the chemical events actually occurred. Not being able to account
for the experimental outcome, however, is a very uncomfortable position, since
it may mean that the observations do not represent what was intended.
A major component of the argument presented by Chang and Weinstock at the
International Conference was based on a comparison of their photochemical
modeling results with Dimitriades and Joshi's chamber results. Because there
was substantial disagreement between their initial model and the EPA chamber
results and because substantial modifications in the form of heterogeneous
type changes were required to obtain agreement, they concluded that Dimitriades
and Joshi's, and Heuss' smog chambers had important "idiosyncrasies."
Although it may be possible that Chang and Weinstock are correct in their
conclusions about the EPA chamber, the discussion given above suggests other
alternative interpretations. This is especially true in light of other smog
chamber data and modeling results for propane.
On September 8, 1976, the Research Triangle Institute (RTI) carried out a
4.0 ppm propane, 0.16 NO , 0.032 ppm NO run in one of their 1000 ft outdoor
X £
Teflon chambers. These chambers were described in a paper by Sickles et al.
at the Conference (35). Following the outcome in this experiment, six simul-
taneous similar propane experiments were performed on September 20, 1976,
involving the four 1000 ft RTI outdoor chambers and
side) University of North Carolina outdoor chambers.
involving the four 1000 ft RTI outdoor chambers and the dual 5500 ft (each
September 8, 1976, was the hottest day of the month (maximum air tempera-
ture > 90°F while September 20 was cooler (air temperature 82°F) with in-
creasing cloud cover all day (6/10 by 1600 EDT). The September 8 run had a
maximum O of 0.61 ppm while the six runs on September 20 had an average value
of 0.211 ppm O with a standard deviation of 0.027 ppm (these values include
the effects of not having identical initial conditions in all chambers but
55
-------
include O calibration corrections). The maximum occurred at the same time in
all six chambers (36).
To investigate the high O yield in the RTI September 8 run, Dr. Marcia
Do ige, Environmental Protection Agency, Research Triangle Park, modeled the
rua (memorandum from M. Dodge to J. Bufalini, October 15, 1976). In these
simulations, an effort was made to duplicate the actual conditions closely:
diurnal values of the photolytic rate constants corresponding to September 15
for 40°N latitude (RTI is at 36°N) were used; rate constants for reactions
having significant activation energies were computed at 85°F; the observed
-4 -1
dark decay rate of 4.5 x 10 min for O was used; lastly, the rate con-
stants for processes normally associated with heterogeneous loss processes
were assigned low rate constants (homogeneous limits). Two factors were
considered "unknown": the levels of impurities in the "clean air" and the
rate constant for OH + propane. Therefore, some initial HONO was assumed to
be present due to reactions of NO, NO , and HO (injections occurred several
hours before sunrise). Values of 0.1 and 0.2 ppb HONO were chosen. Also
formaldehyde and acetaldehyde were assumed to be present in equal amounts at
total concentrations of 1 or 4 ppb. Two values of the rate constant had been
reported at the time of this work: Greiner reported 2.2 x 10 ppm -min ,
and Volman more recently reported 3.2 x 10 ppm min . Pitts et al. (25)
give a value that agrees with 3.2 x 10 ppm -min
The comparison results are given in Table 2. Using 0.2 ppb [HONO]
and 4 ppb [aldehyde] and Volman1s rate constant, a value of 0.61 ppm O peak
occurred at 4:00 compared to the chamber time of 4:18, and the chamber NO-NO-
crossover occurred between 10:00 and 12:00 compared with the model value of
11:30. Dodge states, "Thus, it is not necessary to invoke chamber contamina-
tion effects to explain the results, if one assumes that the higher propane +
OH rate constant is correct and if one assumes that the NO loss is a minimal
X
as was used in the model" (memorandum from M. Dodge to J. Bufalini),
Dodge also applied her model to a "rural" situation; using LA summer
solstice photolytic rate constants and removing the O. dark loss reaction, she
repeated the simulation for one tenth the RTI values. The results are given
56
-------
TABLE 2. COMPUTER SIMULATION RESULTS FOR THE SEPTEMBER 8, 1976
CHAMBER PROPANE AN
(FROM DODGE, 1976)
RTI OUTDOOR SMOG CHAMBER PROPANE AND NO EXPERIMENT
X
[N°x]0 =
No.
e
1
2
3
4
0.16 ppm, [NO2
[HONO]QC
ppb
.
0.
0.
0.
0.
1
2
1
2
10 = 0.032
ppm,
[C3H6]0
= 4.
[RCHO] ° KOH
v. -i . -i
ppb ppm -mm
_
1
4
1
4
2
2
3
3
.2
.2
.2
.2
x 10
x 10
x 10
x 10
3
3
3
3
0 ppm
vb
[°3]max
ppm
0.
0.
0.
0.
0.
61
32
46
55
61
Max. time
EST
4
>5
>5
>5
4
:18
:00
:00
:00
:00
Sept 15 diurnal photolytic rate constants assumed and a constant temperature
85°F.
b
Initial conditions in experiment and simulations
Q
Initial conditions specified in simulations
Rate constant specified for OH + propane
Q
Observed conditions in experiment
in Table 3. Either the high or low choice of initial conditions gave sub-
stantial amounts of O , and the modeled situation included the conversion of
NO to NO before O formation could begin.
Again, one should avoid the conclusion that because it was possible to
show agreement between chamber data and a model that either is "correct." It
simply means that the RTI observations are not in disagreement with known
theory as expressed in Dodge's model. Much more extensive testing at other
conditions would be required before assuming either was a reasonable repre-
sentation of actual events.
57
-------
TABLE 3. COMPUTER SIMULATION RESULTS AT INITIAL CONDITIONS
NEAR RURAL CONDITIONS3
[ro
N<
i
2
,,
3
JQ = 0.016 ppm, [N02]Q
[HONO]Q
ppb
0.1
0.2
0.2
= 0.0032 ppm,
[RCHO]Q
ppb
1
4
4
[C3H8]0 = °
KOH
ppm
2.2 x
3.2 x
3.2 x
. 4 ppm V
-min
io3
io3
3
10
[°3]max
ppin
0.152
0.179
h
0.135
a
LA sxammer solstice diurnal photolytic rate constants assumed and a constant
temperature of 85 F.
Same as 2 but PPN and PAN decomposition reactions (R57, R69 Table 5) omitted.
It is insts:'.: -tive to examine the Dodge model in comparison with the Cha,ng
and Weinst ck mi'del and in comparison with. newt.r informatior. Tables 4 and ~-
give the reactions ar.'i their rate constants used in ""he Inciganic and organic
portion?- of the mechanisms, 7 he re?u-tioriS have t ~>en collected .i.nto sequences,
and ^ nee the iuorganc portions were so sin±lat -~' I the rea,ction3 were ool--
] ected together, a notation in the \c. r.t. constant: co.i'
-------
TABLE 4. COMPARISONS OF INORGANIC PORTIONS OF CHANG AND WEINSTOCK
MECHANISM WITH DODGE MECHANISM FOR PROPANE
No
1
2 0 +
3 03 +
4 0 +
5 03 +
6 NO3 +
7 N03 +
8
9
10
11
12
13
14 NO +
15 HONO +
16
17 OH +
18 OH +
19 OH +
20 OH +
21 OH +
22 OH +
23 OH +
24 HONO +
25 NO2 +
26 HO2 +
27 HO2 +
28 HO2 +
29 H02 +
30
31 H02 +
Reaction
N03 ->-
(02) -*•
NO +
NO2 ->
NO2 ->•
NO -»•
NO2 ->•
N205 ->
N205 -*•
03 -»•
03 -»•
01D -».
DID ->.
N02 +
HONO ->-
HONO ->.
NO -).
NO2 +
03 ->-
H202 +
HONO ->-
HN03 +
CO ->-
NO3 ->
N02 +
NO -*•
NO2 ->•
03 ->•
H02 •*
H202 ->•
OH -»•
NO
03
NO2
NO
NO3
2NO2
N205
NO 3
2HNO3
01D
0
0
20H
2HONO
NO
NO
HONO
HNO3
HO2
H20
N02
N03
H02
2NO2
HONO
NO2
HONO
OH
H202
20H
(H20)
+ 0
+ (02)
+ (02)
+ (02)
+ N02
+ (02)
+ (02)
+ N02 + (H20)
+ OH
+ (02)
+ (02)
+ (H20)
+ (H20)
+ CO2
+ (H20)
+ HNO3
+ OH
+ (02)
+ (202)
+ (02)
Chang
Rate Constant
kl
2.1E-5[02][M]
25
not used
4.8E-3
1 . 5E+4
4.4E+3
14
1.5E-6tH20]
combined
3.5E-3 -kl
4.7E+4[M]
3.1E+5£H201
1.9E-11[H20]
1.8E-5
6.8E-2 -kl
1.2E+4
1.5E+4
83
1.2E+3
3 . 1E+3
1.9E+2
210
2.2E-2
8.7E-9JH20J
700
35
22
4.9E+3
3.5E-3 kl
1.5E+5
Dodge
Rate Constant
kl
4.4E+6
25
1.3E+4
5E-2
1.3E+4
5.6E+3
24
2E-2
4.9E-3 wAl=0.52
2.8E-2 wAl=0.52
8.7E+10
l.OE+10
2.0E-5 *
l.OE-3 *
3.0E-2 wAl=0.52
1.2E+4
1.5E+4
87
not used
not used
not used
not used
not used
not used
2000 *
not used *
2.3
4.0E+3
1.6E-3 wAl=0.52
not used
59
-------
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62
-------
aldehyde photolysis for example. The Dodge photolytic rates are quoted for
noon.
Dodge used the most recent findings by Chan et al.(10) for HONO equilib-
rium with NO , NO, and HO as opposed to the values given by NBS 866. There
was, therefore, a factor of 100 in the rates used by the two models.
Dodge used the most recent confirmed rate for R26, which is almost a
factor of three higher than the value used by Chang and Weinstock. (See notes
after Table 5 for explanations of new rate constants.) This difference is
significant.
Dodge did not use R27. (See later discussion concerning the significance
of this reaction.)
In short, the Dodge inorganic mechanism should be slightly more reactive
than the Chang and Weinstock mechanism.
In Table 5, the organic mechanism portions, substantial differences
between the Dodge and Chang and Weinstock mechanism are revealed. These arise
partly because a "lumping" scheme was employed by Chang and Weinstock while
Dodge used an explicit treatment. All peroxyalky (RO2) radicals are treated
alike by Chang and Weinstock . To account for the decomposition of the RO
radicals (R45) formed in the NO oxidation (R39), they introduced a parameter
that is supposed to represent the fraction of total aldehydes produced that
are not formaldehyde. They gave values of 0, 1/2, 2/3, and 2/4 for methane,
ethane, propane, and butane. No justification was given for these values.
In Dodge's model, 75% of the propane consumed formed acetone with no
further reaction; 25% of the propane lost was available to be converted into
propionaldehyde; formaldehyde arises as a consequence of the degradation of
propionaldehyde. Dodge's model also has a greater potential for oxidizing NO
to NO . Chang and Weinstock's model has more losses for RO and RO and for
NO . Other comments on rate constants are given in the notes following Table
5.
63
-------
Dodge commented, "It should be pointed out that the handling of the NO
chemistry in this system is extremely important. For example, in light of
Hendry's recent findings, two reactions were included in this modeling:
PPN ->- CH CH CO + NO
•J ** .3 ff
PAN -> CH CO + NO
«J J £*
A rate constant of 0.04 min , which is the rate of PAN decomposition at 85°F,
was assigned to these reactions. If these two reactions are eliminated from
the model, the 0 max of 0.179 ppm, obtained in the previous example, drops to
a value of only 0.135 ppm. ...Similar effects on 0 formation can be achieved
by varying rate constants for a number of other reactions involving NO
X
chemistry."
The Hendry data referred to by Dodge was the finding that PAN chemistry
is very similar to N^O,. chemistry (37) .
NO + NO -> 2 NO2 1.29 x 10 ppnT^-min
3 -1 -1
NO + NO -> NO 5.7 x 10 ppm -min
NO ->- NO + NO 3.42 x 1016exp(-10,600/T)min~1
and
CH C(0)02 + NO -> NO2 + CH3C(0)0 4.9 x 1Q3 "
CH C(O)02 + NO2 -> PAN 1.5 x 102
PAN -> CH3C(0)02 + N02 1.2 x 1018exp (-13,537/T)min~1
Thus giving the decomposition of PAN a strong temperature dependence.
A more significant possibility, initially suggested by Hendry, is (with
rate constants estimated by Jeffries):
64
-------
HO + NO -> NO + OH
HONO
2.1 x 10 ppm -min
2 -i -i*
7.2 x 10 ppm -min
3 x 1017exp(-ll,685/T)min~1
0.94 min"1*
*estimated
Although not fully tested by kinetic measurements, this formulation is con-
sistent with data obtained by Niki, Gay, Cox and Derwent, Simonaitis and
Heicklen, Calvert and co-workers, and Hendry (38,39,6,40,46,37). It nearly
explains the temperature dependence of O formation in the UNC outdoor cham-
ber, and with slight modifications of rate constants (increase in the A factor
for HO NO decomposition and a decrease in rearrangement rate) it accounts for
the temperature dependence of O in the UCR chamber (42). Furthermore, there
may be metastable intermediates formed by most RO and NO,, reactions that
would have similar chemistry to that suggested for HO above. Thus, it may be
that even the Dodge mechanism, which was quite complete at the time of its
formulation, may not adequately represent the actual chemistry occurring in
propane/NO systems.
X
The influence of the above reaction sequences on the hot day outdoor
model was either accounted for in Dodge's mechanism (NO and PAN chemistry),
^ O
or their influence on the O chemistry would be minimized by the high tempera-
ture existing during the run. Recall that Dodge omitted R27, the overall
effective reaction for the HO + NO sequence, thus assuming total HO NO
decomposition and no HONO formation by this path. Under cooler temperature
conditions, such as those on early mornings, the HO + NO sequence could form
significant HONO (recall that Dodge used 0.2 ppb initial HONO). If the
temperature does not increase, however, this reaction sequence would limit the
NO -to-NO ratio (and therefore O ) to values near 10 or 15 to 1.
The Dimitriades and Joshi chamber runs were performed at a reported air
temperature of 85°F. This measurement was made, however, by a stainless-
steel-encased thermocouple that is permanently installed through one of the
65
-------
Teflon endplates. It projects 6 to 8 inches into the chamber and is illu-
minated by the chamber light sources (personal communication from S. Joshi to
H. Jeffries, February 1977). Standard meteorological air temperature sensors
are always shielded from all radiation sources and aspirated to correctly
measure air temperature since the air is relatively transparent to radiant
he. it.
In the UNC outdoor chamber, a directly exposed temperature sensor inside
the chamber showed 10-15°F higher temperatures than the same sensor mounted
under the chamber (shielded from the direct sun) and aspirated at high veloc-
ities with chamber air. (My digital thermometer when exposed to two 15-watt
fluorescent lamps at a distance of 12 inches showed ^ 6°F temperature rise in
one hour.) Thus, it may be reasonable to hypothesize that the reported air
temperature in the EPA chamber may be too high by 10-15°F. This hypothesis
combined with the HO NO chemistry described above may offer partial explana-
tion for the results described below.
Dodge applied her model to the Dimitriades and Joshi propane run that had
been modeled by Chang and Weinstock. In this run, [propane] =4.0 ppm], [NO]
-? C
= 0.18 ppm, [NO ] = 0.02 ppm, and $Ka for NO was 0.33 min . Except
for the light intensity and the temperature, these were essentially the condi-
tions used in the RTI run. Using the conditions for simulation number 4 in
Table 2, Dodge's model predicted only 0.04 ppm O at 450 minutes instead of
the 0.11 ppm O at 200 minutes obtained in the actual run. NO,, had just
reached its maximum at 450 minutes in the model, and O was rising. In the
chamber, NO maximum occurred at about 70 minutes after irradiation started.
Numerous manipulations of model conditions were tried; all were unsuccessful.
If, as hypothesized above, the chamber was actually at 70"-72PF, then the
temperature dependence of the peroxyacylnitrate decomposition reactions, the
potential temperature dependence of HO NO decomposition reactions and, if
they occur, other RO NO decomposition reactions could have exerted a major
effect. Dodge has already demonstrated the importance of PPN and PAN decom-
position in this model (see No. 3 in Table 3). Cooler chamber temperatures
would lead to higher HO NO concentrations and thus greater production of
66
-------
HONO, which would serve as an efficient OH source without substantial 0
production because the NO -to-NO ratio would be limited to 10 or 12 to 1.
Thus rapid NO to NO could possibly occur without much O formation.
It is therefore recommended that the chamber air temperature at run
conditions be verified, and that further modeling studies be performed to
determine if the above suppositions have the hypothesized impact in this
system.
Thus, there are alternatives to the Chang and Weinstock hypothesis for
the Dimitriades and Joshi smog chamber results and, although computer simula-
tion of photochemical mechanisms is a very useful investigative tool of great
potential, it does not necessarily represent the "truth." Clearly, a model
can be wrong. That a model representation has general validity requires
extensive comparisons with actual data obtained under a wide range of con-
ditions. Chang and Weinstock presented no evidence that this was done for
their model; therefore disagreements between Dimitriades and Joshi's chamber
results and Chang and Weinstock1s model have little meaning.
An Approach for Testing for Unreactive Organics
As illustrated by the entries in Table 1, neither smog chamber experi-
menting nor modeling used alone is likely to provide highly reliable answers
to the question of whether a particular organic compound is likely to form
more than 0.08 ppm 0 under some reasonable set of atmospheric conditions.
Obviously, if this question is to be pursued in a serious and rigorous fash-
ion, the two methods must be used as a check on each other. This would re-
quire a much larger effort than just performing the smog chamber experiments.
Extensive effort should be devoted to understanding the background reactivity
problem. Special experiments would have to be designed to provide insight
into the processes that are occurring. Bufalini et al. (33) have started a
modeling effort for data such as that shown in Figure 1, but their work was
data limited. They did not actually try to reproduce the results shown in
Figure 1. What is the compound being oxidized? Why does the "NO " not
X
disappear?
67
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A large number of experiments must be directed at determining the repro-
ducibility of the smog chamber under various conditions of operation. This
may give more insight into the impact of wall-related processes than just
clean air irradiations. For example, in Figure 1, the measured NO is ap-
-7 X
preaching a value of 16 ppb or a mass of 2.6 x 10 moles of NO . If the
-6 X
p: avious run had 0.2 ppm of NO (3.28 x 10~ moles), then 8% of this NO had
X X
t i be retained on the walls after the 38 hours of cleanup treatment. If the
clean air run is repeated without an intermediate NO /HC run, what are the
X
results? What is the magnitude of the wall source? How long can it keep up?
What is the repeatability of a run that, say, just does produce 0.1 ppm 0 ?
How does this repeatability vary with cleanup procedures, e.g., cleanup with
lights on versus cleanup with lights off or no cleanup? In other words, what
is the impact of potential wall effects on oxidant production if drastically
different cleanup procedures are used (sensitivity analysis),
In addition to dealing with potential wall effects, the smog chamber must
be shown to be consistent with general knowledge about reasonable initial
concentration HC and NO systems. This is where the modeling effort can play
X
its greatest role. Can the system be modeled at a wide range of initial
conditions with our current understanding of chemistry? The chamber depend-
ence of the model must be tested by applying it to another chamber. In other
words, I would hope that one consistent model could be found that would rea-
sonably predict both an outdoor chamber and an indoor chamber. This would
require quality assurance programs for independently calibrated equipment.
Initially this modeling effort and data generation for the model would occupy
a great deal of time, but once a reasonable agreement had been obtained at a
wide range of conditions, only random check runs and remodeling would be
required throughout the experimental program. Recognition must be given,
however, to the possibility that some unusual organic compounds will be tested
that may drastically alter the wall conditions compared to those used in the
wall test runs. Therefore, when unusual outcomes are observed, rather de-
tailed tests may be required to ascertain if the chamber has changed its
characteristics.
68
-------
It is my opinion that a combined approach as described above is currently
the most reliable method for determining if a given organic compound, when
considered by itself with NO , is capable of forming O above a given value.
X o
A broader, more philosophical issue, however, is the question of whether
the irradiation of single compounds, starting with mostly NO, is an effective
test of a compound's ability to participate in atmospheric chemistry and to
generate O . It may be more realistic, for example, to test the ability of an
organic compound, at some upper limit of concentration, to influence the O
generation in a hydrocarbon or time limited photochemical system that would
normally produce, say, 0.1 ppm of 0_ at a temperature and light intensity
reasonably representative of the current high oxidant region of the U.S. The
hydrocarbon in the base mixture could be a simple mixture anticipatory of
expected urban conditions at some future time or a model auto exhaust/urban
emissions mixture. The base system might be, for example, using the O
isopleth diagram for the adjusted and unadjusted Bureau of Mines dilute auto
exhaust model, an [NO ] of 0.2 ppm and an [HC] of 0.5 ppmC. Any compound
that, when added to this mixture at levels up to 5 ppmC, produced 0 at the
end of a fixed time period exceeding the base systems low mean 0 value by
more than two standard deviations of the base system reproducibility would be
declared reactive in the urban environment.
To deal with transport or "rural" reactivity, the base system would be
one that had low [HO ] and probably mostly in the form of NO , say 0.05 ppm,
X O £
but ample [HC] , say 1.0 ppmC, primarily alkane. Such a system, under long
irradiation, might produce 0.10 ppm O . Again the O resulting from the
addition of 5 ppmC of the test organic to the base system would be compared
with the mean O of the base system plus two standard deviations of the base
system's O . Any compound that compared high would be declared reactive in
the rural environment.
These types of tests are considerably more representative of actual
atmospheric situations and address the practical problems of conducting smog
chamber experiments in that many of the issues related to so-called wall
problems are irrelevant and the inherent precision of the method is built into
69
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the test method. Considerably less chamber testing and modeling would be
required with this approach.
RECOMMENDATIONS FOR FURTHER RESEARCH
• Expand upon the work started by Bufalini et al. (33) and develop the
information on chamber reproducibility discussed in the preceding
section. In particular, experimental evidence related to a possibly
enhanced photolysis rate for HNO should be developed. If necessary,
the entire 400 liters of the glass/Teflon chamber should be collected
by freeze-out at, e.g., 1200 minutes in a run such as that shown in
Figure 1, and the trapped materials subjected to gas chromatograph/
mass spectrometer analysis.
• Test the proposed HO NO mechanism and rate constants as a function
of temperature. If supported, determine the most realistic tempera-
ture at which to conduct smog chamber experiments.
• Verify glass/Teflon chamber temperature. If it is found to be cooler
than thought, remodel chamber results for propane using most recent
version of mechanism. If the temperature measurements were correct,
repeat propane experiments using different cleanup procedures between
runs.
• Investigate methods to determine product yields and stoichiometrics
of organic reactions important in photochemical mechanisms.
COMMENTS BY J.G. CALVERT
I have read Jeffries' comments and find few things to fault. Some
matters are those of interpretation, which are the best Jefferies (or I) ca,n
come up with — who is correct, is a matter one cannot prove.
One final comment refers to p. 20 middle page — he means: "H02 + NO2 +
HO NO " not "HO + NO •*• HO NO ." It seems that we all have a share in the
£, £, £ «3 £ £
interesting new area of HO NO . Our work on this is continuing and should
£, £
shed some light on mechanism alternatives.
70
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REFERENCES
1. Dimitriades, B., and A.p. Altshuller. International Conference on
Oxidant Problems: Analysis of the Evidence/Viewpoints Presented. Part
I: Definition of Key Issues. JAPCA, 27(4):299-307, 1977.
2. Dimitriades, B., and S.B. Joshi. Applications of Reactivity Criteria in
Oxidant-Related Emission Control in the U.S.A. International Conference
on Photochemical Oxidant Pollution and Its Control, Proceedings. 2:705-
711. EPA-600/3-77-001b. Environmental Protection Agency, Research
Triangle Park, N.C., 1977.
3. Chang, T.Y., and B. Weinstock. Net Ozone Formation in Rural Atmospheres.
International Conference on Photochemical Oxidant Pollution and Its
Control, Proceedings. 1:451-466. EPA-600/3-77-001a. Environmental
Protection Agency, Research Triangle Park, N.C., 1977.
4. Calvert, J.G. Test of the Theory of Ozone Generation in Los Angeles
Atmosphere. Environ. Sci. Technol., 10(3):248-256, 1976.
5. Demerjian, K.L., J.A. Kerr, and J.G. Calvert. The Mechanism of Photo-
chemical Smog Formation. Adv. Environ. Sci. Technol., 4:1-262. John
Wiley-Interscience, New York, 1974.
6. Cox, R.A., and R.G. Derwent. The Ultraviolet Absorption Spectrum of
Gaseous Nitrous Acid. J. Photochem., 6(l):23-34, 1976/77.
7. Johnston, H.S., and R. Graham. Photochemistry of Nitrogen Oxide (NO )
and Nitrogen Oxyacid (HNO ) Compounds. Can. J. Chem., 52(8pt2):1415-
X
1423, 1974.
71
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8. Pitts, J.N., Jr., G.J. Doyle, A.C. Lloyd, and A.M. Winer. Chemical
Transformations in Photochemical Smog and Their Applications to Air
Pollution Control Strategies. 2nd Annual Report on NSF RA.NN Grant AEN
73-02904A02. University of California, Statewide Air Pollution Research
Center, Riverside, California, 1975.
9. Wayne, L.G., and D.M. Yost. Kinetics of Rapid Gas-Phase Reaction between
NO, NO , and HO. J. Chem. Physics, 19:41-47, 1951.
10. Chan, W.H., R.J. Nordstrom, J.G. Calvert, and J.H. Shaw. Kinetic Study
of MONO Formation and Decay Reactions in Gaseous Mixtures of HONO, NO,
NO , HO, and N . Environ. Sci. Technol., 10(7):674-682, 1976.
11. Glasson, W.A., and P.H. Wendschuh. Multiday Irradiation of Nitrogen
Oxide/Organic Mixtures. Research Publication GMR-2236. Environmental
Science Dept., Research Laboratories, General Motors Corp., Warren,
Michigan, 1976. 17 pgs.
12. Tuesday, C.S., B.A. D'Alleva, J.M. Heuss, and G.J. Nebel. The General
Motors Smog Chamber. Research Publication GMR-490. Research Labora-
tories, General Motors Corp., Warren, Michigan, 1965. 59 pgs.
13. Calvert, J.G., and R.D. McQuigg. The Computer Simulation of the Rates
and Mechanisms of Photochemical Smog Formation. Int. J. Chem. Kinetics,
Symp. No. 1, p. 113-154. John Wiley and Sons, New York, 1975.
14. Wang, C.C., L.I. Davis, C.H. Wu, S. Japar, H. Niki, and B. Weinstock.
Hydroxyl Radical Concentrations Measured in Ambient Air. Science, 189-
(4205):797-800, 1975.
15. Perner, D., D.H. Enhalt, H.W. Patz, U. Platt, E.P, R6th, and A. Volz.
Hydroxyl Radicals in the Lower Troposphere. Geophys. Res. Letters,
3(8):466-468, 1976.
72
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16. Calvert, J.G. Hydrocarbon Involvement in Photochemical Smog Formation in
Los Angeles Atmosphere. Environ. Sci. Technol., 10(3):256-262, 1976.
17. Niki, H. , P. Maker, C. Savage, and L. Breitenbach. Fourier-Transform
Spectroscopic Studies of Organic Species Participating in Photochemical
Smog Formation. Paper 24-4. Proceedings of the Int. Conf. Environ.
Sensing and Assessment, 1975.
18. Farley, F.F. Photochemical Reactivity Classification of Hydrocarbons and
Other Organic Compounds. International Conference on Photochemical
Oxidant Pollution and Its Control, Proceedings. 2:713-726. EPA-600/3-
77-OOlb. Environmental Protection Agency, Research Triangle Park, N.C.,
1977.
19. Research Triangle Institute. Investigation of Rural Oxidant Levels as
Related to Urban Hydrocarbon Control Strategies. EPA-450/3-75-036.
Environmental Protection Agency, Research Triangle Park, N.C., 1975. 385
pgs.
20. Crutzen, P., and J. Fishman. Paper presented at EPA Research Conference
on Assessment of the Freon Involvement in Stratospheric Ozone Reduction,
Research Triangle Park, N.C., February 8-9, 1977.
21. Calvert, J.G., K.L. Demerjian, and J.A. Kerr. The Effect of Carbon
Monoxide on the Chemistry of Photochemical Smog Systems. Environ.
Letters, 4(4):281-295, 1973.
22. Dodge, M.C., and J.J. Bufalini. The Role of Carbon Monoxide in Polluted
Atmospheres. Advances in Chem. Series, No. 113, p. 232-245. American
Chemical Society, 1972.
23. Westberg, K., N. Cohen, and K.W. Wilson. Carbon Monoxide: Its Role in
Photochemical Smog Formation. Science, 171(3975):1013-1015, 1971.
73
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24. Glasson, W.A. Effect of Carbon Monoxide on Atmospheric Photooxidation of
Nitric Oxide-Hydrocarbon Mixtures. Environ. Sci. Technol., 9(4): 343-
347, 1975.
25. Pitts, J.N., Jr., A.M. Winer, K.R. Darnall, A.C. Lloyd, and G.J. Doyle.
Hydrocarbon Reactivity and the Role of Hydrocarbons, Oxides of Nitrogen,
and Aged Smog in the Production of Photochemical Oxidants. International
Conference on Photochemical Oxidant Pollution and Its Control, Proceed-
ings. 2:687-704, EPA-600/3-77--001b, Environmental Protection Agency,
Research Triangle Park, N.C., 1977.
26. Weinstock, B., and T.Y. Chang. Methane and Nonurban Ozone. Presented at
the 69th Annual Meeting of the Air Pollution Control Association, Portland,
Oregon, June, 1976.
27. Niki, H., and B. Weinstock. Recent Advances in Smog Chemistry. EPA
Scientific Seminar on Automotive Pollutants, EPA-600/9-75-003. February
11, 1975. 24 pgs.
28. Heuss, J.M. Photochemical Reactivity of Mixtures Simulating Present and
Future Concentrations in the Los Angeles Atmosphere. 68th Annual Meeting
of the Air Pollution Control Assoc., 2:1-25 (paper 75-16.1), 1975.
29. Lonneman, W.A. Ozone and Hydrocarbon Measurements in Recent Oxidant
Transport Studies. International Conference on Photochemical Oxidant
Pollution and Its Control. 1:211-223, EPA-600/3-77-001a. Environmental
Protection Agency, Research Triangle Park, N.C., 1977.
30. Bufalini, J.J., S.L. Kopczynski, and M.C. Dodge. Contaminated Smog
Chambers in Air Pollution Research. Environ. Letters, 3(2):101-109,
1972.
31. Davis, D.D., W. Heaps, and T. McGree. Direct Measurements of Natura.1
Tropospheric Levels of OH via an Aircraft Borne Tunable Dye Laser.
Geophys. Res. Letters, 3 (6):331-333, 1976.
74
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32. Atkinson, R., and J.N. Pitts, Jr. Data Constants for the Reaction of OH
Radicals with Propylene and the Butenes over the Temperature Range 297-
425°K. J. Chem. Phys., 63(8):3591-3595, 1975.
33. Bufalini, J.J., T.A. Walter, and M.M. Bufalini. Contamination Effects on
Ozone Formation in Smog Chambers. (Ozone Formation Potential of Organic
Compounds). Environ. Sci. Technol., 10(9):908-912, 1976.
34. Calvert, J.G., and J.N. Pitts, Jr. Photochemistry. John Wiley and Sons,
New York, 1966. 899 pgs.
35. Sickles, J.E., II, L.A. Ripperton, and W.C. Eaton. Oxidant and Precursor
Transport Simulation Studies in the Research Triangle Institute Smog
Chambers. International Conference on Photochemical Oxidant Pollution
and Its Control, Proceedings. 1:319-328. Environmental Protection
Agency, Research Triangle Park, N.C., 1977.
36. Wright, R., J.E. Sickles, W.C. Eaton, R.K. Kamens, H.E. Jeffries.
Comparison of Outdoor Smog Chamber Results in Two Different Smog Chambers.
Presented at Annual Meeting of Air Pollution Control Assoc., Toronto,
Ontario, Canada, June, 1977.
37. Hendry, D.G., and R.A. Kenley. Generation of Peroxy Radicals from
Pernitrates. Decomposition of Peracylnitrates. Stanford Research Insti-
tute, Menlo Park, California, 1976.
38. Niki, H., P. Maker, C. Savage, and L. Breitenbach. IR-Fourier-Transform
Spectroscopic Studies of Atmospheric Reactions. 12th Informal Conference
on Photochemistry, U.S. Department of Commerce, National Bureau of
Standards, Extended Abstracts. N2-1-4, 1976.
39. Gay, B.W., Jr., R.C. Noonan, J.J. Bufalini, and P.L. Hanst. Photochemi-
cal Synthesis of Peroxyacyl Nitrates in Gas Phase via Chlorine-Aldehyde
Reaction. Environ. Sci. Technol., 10(1):82-85, 1976.
75
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40. Simonaitis, R. , and J. Heicklen. Reactions of HO with NO and N02 and
of OH with NO. J. Phys. Chem. , 80(1): 1-7, 1976.
41. Calvert, J.G. , S.Z. Levine, W.M. Uselman, W.H. Chen, and J,H. Shaw. The
Kinetics and Mechanism of the HO -NO Reactions: The Significance of
Peroxynitric Acid Formation in Photochemical Smog. (Submitted for
publication in Chem. Physics Letters, 1977.).
42. Hogo, H. , G.Z. Whitten, and P. Bekowies. Mathematical Modeling of
Simulated Photochemical Smog. Monthly Technical Progress Report No. 4,
EPA Contract No. 68-02-2428, 1976. 62 pgs.
43. Whitten, G.Z. , and H.H. Hogo, (Systems Applications, Snc.), Mathematical
Modeling of Simulated Photochemical Smog. EPA-600/3-77-011, Environ-
mental Protection Agency, Research Triangle Park, N.C,, 1977. 306 pgs.
76
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TECHNICAL REPORT DATA
(Please read Instructions on the reverse before completing)
1. REPORT NO.
EPA-600/3-77-114
2.
3. RECIPIENT'S ACCESSI Of* NO.
4. TITLE AND SUBTITLE
INTERNATIONAL CONFERENCE ON OXIDANTS, 1976 -
ANALYSIS OF EVIDENCE AND VIEWPOINTS
Part II. The Issue of Reactivity
5. REPORT DATE
October 1977
6. PERFORMING ORGANIZATION CODE
7. AUTHOR(S)
1. J.G. Calvert
2. H.E. Jeffries
8. PERFORMING ORGANIZATION REPORT NO
9. PERFORMING ORGANIZATION NAME AND ADDRESS
1. Ohio State Univ., Columbus, OH
2. Univ. of North Carolina, Chapel Hill, NC
10. PROGRAM ELEMENT NO.
1AA603 AJ-13 (FY-76)
11. CONTRACT/GRANT NO.
1. DA-7-1840A
2. DA-7-2261J
12. SPONSORING AGENCY NAME AND ADDRESS
Environmental Sciences Research Laboratory - RTF, NC
Office of Research and Development
U.S. Environmental Protection Agency
Research Triangle Park, NC 27711
13. TYPE OF REPORT AND PERIOD COVERED
Final
14. SPONSORING AGENCY CODE
EPS/600/09
15. SUPPLEMENTARY NOTES
Partially funded by the Office of Air Quality Planning and Standards.
16. ABSTRACT
In recognition of the important and somewhat controversial nature of the
oxidant control problem, the U.S. Environmental Protection Agency (EPA)
organized and conducted a 5-day International Conference in September 1976.
The more than one hundred presentations and discussions at the Conference
revealed the existence of several issues and prompted the EPA to sponsor a
follow-up review/analysis effort. The follow-up effort was designed to review
carefully and impartially, to analyze relevant evidence and viewpoints report-
ed at the International Conference (and elsewhere), and to attempt to resolve
some of the oxidant-related scientific issues. The review/analysis was con-
ducted by experts (who did not work for the EPA or for industry) of widely
recognized competence and experience in the area of photochemical pollution
occurrence and control.
J.G. Calvert, Ohio State University, Columbus, Ohio, and H.E. Jeffries,
University of North Carolina, Chapel Hill, N.C., reviewed the papers presented
at the 1976 International Conference on Oxidants related to the issue of
reactivity, and offered their views on the current status of research in the
field, resolutions of the issue, and the need for additional research.
17.
KEY WORDS AND DOCUMENT ANALYSIS
DESCRIPTORS
b.lDENTIFIERS/OPEN ENDED TERMS
c. COSATI Field/Group
* Air pollution
* Ozone
* Photochemistry
* Chemical reactivity
13B
07B
07E
07D
13. DISTRIBUTION STATEMENT
RELEASE TO PUBLIC
19. SECURITY CLASS (ThisReport)
UNCLASSIFIED
21. NO. OF PAGES
83
20. SECURITY CLASS (This page)
UNCTARSIFIFD
22. PRICE
EPA Form 2220-1 (9-73)
77
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