United States
Environmental Protection
Agency
Environmental Sciences Research
Laboratory
Research Triangle Park NC 27711
Research and Development
EPA-600/S3-83-028 Sept. 1983
SEPA Project Summary
Nitrogen Oxides Reactions
Within Urban Plumes
Transported Over the Ocean
Chester W. Spicer, John R. Koetz, G. William Keigley, George M. Sverdrop,
and Gerald F. Ward
The rate of removal or conversion of
nitrogen oxides was determined from
airborne measurements in the urban
plume of Boston. The mean pseudo-
first-order rate constant for removal
was 0.18 per hour, with a range of 0.14
to 0.24 per hour under daylight con-
ditions for four study days. The re-
moval process is dominated by chem-
ical conversion to nitric acid and or-
ganic nitrates. The removal rate sug-
gests an atmospheric lifetime for nitro-
gen oxides of about five to six hours in
urban air.
This Project Summary was developed
by EPA's Environmental Sciences Re-
search Laboratory, Research Triangle
Park. NC, to announce key findings of
the research project that is fully docu-
mented in a separate report of the
same title (see Project Report ordering
information at back).
Introduction
Since the early 1970's interest in the
atmospheric reactions of sulfur and nitro-
gen pollutants has continually increased.
To date, sulfur compounds have been
studied more thoroughly, especially in
terms of the rate of ambient-air trans-
formations; knowledge of the ambient-air
reactions of nitrogen oxides (NOX; defined
as NO + N02) has lagged.
The aspect of atmospheric NOX chem-
istry that is least understood is the rate at
which NOX are transformed to products.
Few atmospheric studies have addressed
this problem because of the technical
difficulties involved. Breeding and co-
workers (1) investigated the transforma-
tions of several pollutants around St. Louis.
Their results suggest a 2- to 4-h half-life
for NO in the St Louis plume, but their
Nİ2 data are inconclusive. Thus, little was
learned about the important conversion of
NOX to products.,
Spicer et al. (2) investigated the fate of
NOX in the St Louis and Los Angeles
urban areas in 1973 to identify the major
NOX reaction products and, to the extent
possible, to define the nitrogen mass
balance. The experiment was not designed
to obtain data on conversion rates, but if
certain assumptions are accepted, a trans-
formation rate of 0.10 ħ 0.05 h'1 is
consistent with the Los Angeles data. (The
quoted value is in the form of a rate
constant Since it is calculated from an
average percent conversion divided by a
time interval, it should not be taken as a
true rate constant and does not imply first-
order kinetics.)
Calvert (3)'used (LARP) data to estimate
the rate of NOX removal from Los Angeles
air. He calculated a (3.0 ħ 2.4) x 10'4 ppm
min'1 rate of conversion to products that
were not detected by the LARP chemi-
luminescence monitors. This rate is some-
what higher than our measurements sug-
gest for conversion to all products under
moderate Los Angeles smog conditions.
In experiments downwind of Los Angeles
in 1 976, we observed lower-limit NOX
conversion rates of 0.02 to 0.16 h'1 in
ground-level and aircraft sampling. Typ-
ical rates were 0.05 to 0.10 h"1. Trans-
formation to peroxyacetyl nitrate (PAN)
ranged from < 0.01 to 0.08 rr1.
The Los Angeles conversion rates are
generally higher than those observed in
Phoenix, Arizona, in 1977. Phoenix was
selected for study because it is an isolated
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source; thus, the NOX reactions could be
followed downwind of the city, over the
desert, where complications due to fresh
IMOX and tracer emissions were minimal.
The Phoenix study was the first experi-
ment designed principally to .determine
NOX conversion rates. Based on the NOX
and tracer concentration levels and in-
strument sensitivities, NOX conversion
rates of 0.05 h"1 should have been de-
tectable around Phoenix. This conversion
rate was rarely exceeded, a puzzling result,
since it is inconsistent with the earlierfield
results in Los Angeles and the expecta-
tions derived from modeling and smog-
chamber studies.
While much has been learned of the
distribution, balance, and fate of oxidized
nitrogen in urban atmospheres, this brief
review clearly indicates that much un-
certainty remains regarding the rates of
NOX transformations in ambient air. The
few measurements available of this con-
version rate cover a broad range and seem
to be city-specific. To better understand
and model atmospheric NOX reactions and
to assess their impact on health, visibility,
and precipitation chemistry, much more
exact NOX conversion rates must be known,
and the factors that affect these rates must
be identified.
We were able to accurately estimate
NOX lifetime in polluted urban air. The
results of the experimental phase of the
program and the details of the experi-
mental methods are presented m this
project report.
The aim of the Boston urban plume
study from July 27 to August 30, 1978
was a better understanding of NOX reac-
tions and especially the NOX reaction rate
in polluted urban air.
Aircraft field measurements were ob-
tained in the Boston plume as it was
transported eastward over the ocean. A
mobile laboratory located at the Beverly,
Massachusetts airport provided ground
support for the flight program and ob-
tained ground-level data on air quality and
meteorological conditions.
Project Description
To better define the rates of NOX trans-
formations within urban plumes in the
least ambiguous way, it was desirable that
the urban source be isolated so that down-
wind emissions into the urban plume are
minimal. The city must also be a strong
source of NOX, and the downwind terrain
should be relatively smooth. Based on
these considerations, Boston was selected
for this study. The predominant summer
winds from the west and southwest take
the city's polluted air eastward over the
ocean, where smooth terrain and minimal
fresh emissions are the rule. As a north-
eastern coastal city, Boston also contrasts
sharply with the hot, dry Phoenix site
studied earlier.
The field experiment was conducted
fromjuly27 to August 30,1978. Atwin-
engirTe~research aircraft was used for con-
tinuous monitoring and sample collection
in the Boston urban plume. A mobile
laboratory served as ground support for
the aircraft program and continuously
monitored ground-level air quality and
meteorological conditions. The mobile lab
and aircraft were based at Beverly Airport,
in Beverly, MA, approximately 25 km
northeast of Boston. Hourly meteorologi-
cal data from several eastern Massachu-
setts stations were obtained by telephone
several times each day and were used to
construct working trajectories for Boston's
polluted morning air mass. The morning
air mass was followed throughout the day
by three or more monitoring flights along
these trajectories. Data were collected
during multiple traverses of the urban
plume on each flight to bracket the morning
air parcel. Thus, data on pollutant con-
centrations within the air parcel were
obtained from early morning through late
afternoon. Transport out to ^150 km
from the source was investigated. Back-
ground pollutant concentrations were also
obtained. The variables measured during
aircraft operations included NOX, ozone
(O3), nitric acid (HON02), PAN, carbon
monoxide (CO), nonmethane hydrocarbons
(NMHC), fluorotrichloromethane (F-11),
to G! to GS hydrocarbons, condensation
nuclei, nitrate (I\I03~), sulfate (S04=), tem-
perature and dew point. The ground sta-
tion measured these and a number of
other variables.
Results and Conclusions
Important findings of the study can be
summarized as follows.
The weather during the Boston plume
study was generally rainy and over-
cast, interspersed with a few sunny
clear days. Pollutant levels at the
ground station in Beverly were usu-
ally low. On a few days, southwesterly
winds transported Boston's polluted
air to Beverly. One very clear case of
pollutant transport occurred on Au-
gust 16, when 03, PAN, and light-
scattering aerosol reached very high
levels just before dawn.
Table 1 gives the 24-h distribution
of oxidized nitrogen for a subset of
14 days during which all the perti-
nent species were measured. The
usual oxidized-nitrogen burden at
Beverly Airport was low because of
its location (the emissions density to
the west, north, and east was quite
low). On average, about 14% of the
oxidized nitrogen was present in the
form of reaction products. The ratio
of the average PAN and HON02
concentrations was 1.8.
The concentration of nitrate in the
aerosol phase was extremely low at
Beverly Airport, with gaseous nitrates
accounting for 98% of the total
atmospheric nitrate. For comparison,
gaseous nitrate as a proportion of
total nitrate for several geographical
locations is given in Table 2. For
many of the locations, gaseous ni-
trate dominated the total nitrate bur-
den, as observed at Beverly.
Based on four days of flights for
which detailed analysis is presented,
the removal of NOX from the trans-
ported Boston plume followed first-
order kinetics reasonably well. The
pseudo-first-order rate constant for
NOX removal ranged from 0.14 h"1 to
0.24 rr1 with a mean of 0.18 hr"1.
Maximum 03 concentrations in the
plume on these four days ranged
from 0.095 to 0.160 ppm.
On the days when all the necessary
data were available, the nitrogen
balance in the plume was accounted
for with PAN and HON02 account-
ing for nearly all the reacted NOX. For
example, during three transects of
the urban plume on August 30,
1978, the nitrogen mass balance
averaged 88%.
The pseudo-first-order rate constants
obtained in these experiments provide a
basis for estimating the lifetime of NOX (as
NO + N02) in transported urban air under
photochemically active conditions. The
lifetime, r, can be defined as the time
required for chemical and physical removal
Table 1. 24-Hour Distribution of Oxidized Nitrogen
Concentration (ppm)
Fraction of Total Oxidized
Nitrogen (%J
NOX (NO + NO2)
PAN
HON02
N03-
0.018
0.0018
0.0010
0.00004
86
9
5
1
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Table 2
Gaseous Nitrate Measured at
Several Locations
Gaseous Nitrate
Location Total Nitrate
St. Louis. MO (197 3)
WestCovma, CA(1973]
Phoenix, AZ (1977)
Temple City, CA (1976)
Up/and, CA(1976)
Rubidoux, CA (1976)
Beverly, MA (1978)
95
98
50
75
.96
.45
.98
processes to reduce the NOX concentra-
tion to 1/e of its initial value (i.e. when
(NOJp=
(N0x)t
Substituting into the integrated form of
the rate equation,
. (NOx)o ,
1 n - = kt
(N0x)t
1 n e = kr
For the four available experiments, r
ranges from 7.1 to 4. 2 hr. The average of
the four rate constants yields a lifetime of
5.5 h. The NOX half-life defined as
0.69
Chester W. Spicer, John R. Koetz, G. William Keigley, George M. Sverdrop, and
Gerald F. Ward are with Battelle-Columbus Laboratories, Columbus, OH 43201,
William A. Lonneman is the EPA Project Officer (see below).
The complete report, entitled "Nitrogen Oxides Reactions Within Urban Plumes
Transported Over the Ocean," (Order No. PB 83-196 378; Cost: $17.50, subject
to change) will be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Environmental Sciences Research Laboratory
U. S. Environmental Protection Agency
Research Triangle Park, NC 27711
*US GOVERNMENT POINTING OFFICE 1983-659-017/7169
averages 4.1 h.
To the best of our knowledge, this is the
first determination of NOX lifetime in pol-
luted urban air. The reported values are
applicable only to daylight hours under
sunny, photochemically active conditions.
Further research will be required before
these conclusions can be extended to
other areas with different meteorological
conditions or emissions factors.
References
1. Breeding, R. J., Klonix, H. B., Lodge,
J P., Jr., Pate, J. B., Sheesley, D. C.,
Englert, T. R. and Sears, D. R., Atmos
Environ 10, 181 (1976).
2. Spicer, C. W., "The Fate of Nitrogen
Oxides in the Atmosphere", in
Advances in Environmental
Sciences, V 7, Pitts, J. N. and Metcalf,
R L, eds., John Wiley, New York,
New York, (1977).
3 Calvert, J. G , Environ Sci. Technol.
10, 256 (1976).
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Information
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