United States
Environmental Protection
Agency
Atmospheric Sciences
Research Laboratory
Research Triangle Park NC 27711
i\
Research and Development
EPA/600/S3-87/045 Feb. 1988
SEPA Project Summary
Nonmethane Organic Carbon
Concentrations in Air Masses
Advected Into Urban Areas in
the United States
Hal Westberg and Lome MacGregor
The purpose of this study was to
measure non-methane organic
carbon (NMOC) levels aloft in the
upwind vicinity of several U.S. cities.
An instrumented aircraft was
employed to collect samples during
the months of July and August in
1985 and 1986. NMOC, ozone and
oxides of nitrogen concentrations
were measured during the 0400 to
0900 period in background air
masses advected into Dallas-Ft.
Worth, Tulsa, Atlanta, and
Birmingham in 1985 and Philadelphia
and New York City in 1986.
Measurements were concentrated in
the layer above the morning surface
inversion but below the normal
afternoon mixing height. This is the
air mass that would eventually mix
with the surface layer within the
urban area after the break-up of the
morning inversion. Special emphasis
was placed on NMOC speciation.
Individual hydrocarbons were
identified by gas chromatography
(GC) and derivatized carbonyl
compounds were measured by high
performance liquid chromatography
(HPLC). Details concerning the
experimental design and quality
assurance aspects of the program
are provided within the report
A tabulation of airborne data
collected during the two year study
period is provided. A data summary
is presented for each day which
includes a flight map, table of ozone
and NOX concentrations, a listing of
NMOC information, and data recorded
during aircraft ascents and/or
descents. A brief comparison of the
NMOC results obtained for the
various cities investigated is also
provided.
This Project Summary was
developed by EPA's Atmospheric
Sciences Research Laboratory,
Research Triangle Park, NC, to
announce key findings of the research
project that is fully documented in a
separate report of the same title (see
Project Report ordering information at
back).
Introduction
Characterization of NMOC levels in
transported air masses is currently of
interest in terms of oxidant production
and the formation of acidic species in the
atmosphere. Organic species in the layer
above a morning surface inversion and
below the afternoon mixing level are of
interest because oxidant precursors in
this layer mix with urban plumes
following breakup of the surface
inversion. Most photochemical models
that are designed for regulatory purposes
incorporate oxidant precursors from aloft.
Recent modeling studies have indicated
that control requirements are quite
sensitive to the concentration and
composition of organics entrained from
aloft. Therefore, it is important that
NMOC concentrations in this layer be
defined.
NMOC concentrations aloft have been
determined in several oxidant field study
-------
programs, however, the emphasis in
these previous studies was centered
around urban plumes. Prior to the field
work described herein, there had not
been a study with the primary objective
being to define NMOC levels transported
into urban areas from aloft. Organic
species were measured in air masses
advected into Dallas, Tulsa, Atlanta, and
Birmingham during the summer of 1985;
Philadelphia and New York City during
the summer of 1986. In this report we
will summarize the types and
concentrations of organic species
measured aloft. Relationships between
the organics and other pollutant and
meteorological parameters will be
described as well. The areas
investigated in 1985 were isolated urban
areas whereas those investigated in 1985
were part of a complex urban area where
major upwind sources may exist under
certain meteorological conditions.
Experimental Procedures
The aircraft flew arcs upwind of the
urban areas during the early morning
hours. On most days, there were three
arcs flown at two different altitudes above
the morning surface inversion but below
the normal afternoon mixing height.
Figure 1 provides a schematic illustration
of the flight pattern. Sample collection
involved transferring air from a ram-air
manifold inside the aircraft to the
collection medium. For hydrocarbons,
this entailed filling a metal canister to a
positive pressure with a metal bellows
pump. Carbonyl compounds were
trapped by passing air through adsorbent
cartridges impregnated with dinitrophenyl
hydrazine. The hydrazones that formed
were analyzed by HPLC. An attempt
was made to measure PAN levels aloft
by collecting air samples in Tedlar bags
and measuring the PAN content by EC-
GC immediately after the aircraft landed.
Analytical difficulties along with the
absence of a reliable field calibration
standard compromised the quality of the
PAN data and consequently it is not
included in this report.
Ozone, NOX, temperature and relative
humidity were measured continually in
the aircraft. These parameters were
employed to define the morning mixing
level as well as to provide additional
information for modeling the urban
photochemistry.
A complete description of the air
quality instrumentation used, calibration
methods data processing and validation
procedures are included in Appendix G
of the report.
Winds reported at 3000 feet were
obtained from the local flight service
station. This information was used to
plan the location of upwind flight paths
each morning.
Results and Discussion
Table 1 provides a summary of the
average non-methane hydrocarbon
(NMHC) and aldehyde concentrations
measured aloft over the six cities. Mean
hydrocarbon concentrations varied
between 14 and 39 ppbC with the
highest average concentrations recorded
upwind of Tulsa. The low mean value
determined in Birmingham may not be
truly representative of that particular
urban area. The morning flights were
plagued with bad flying weather while in
Birmingham and consequently fewer
samples were collected. The mean
NMHC concentration for all cities
combined was 22 ppbC with a coefficient
of variance of approximately 40%.
Paraffinic species were by far the major
class of hydrocarbons measured aloft.
These compounds comprised
approximately 78% of the identified
hydrocarbons while the mean aromatic
contribution was about 17% and the
olefins averaged 3%.
Formaldehyde and acetaldehyde
levels in Atlanta and Birmingham were
generally below the detection limit of the
analytical procedure. In Dallas, Tulsa,
Philadelphia and New York City the two
carbonyls accounted for about 10% of
NMOC that was identified. A positive
correlation between NMHC and
aldehyde concentrations was generally
observed. For example, NMHC levels
upwind of Dallas on the morning of July
6, 1985 averaged 30 ppbC with a
corresponding mean aldehyde
concentration of about 2 ppb On July
14, Dallas NMHC levels were down to
about 4 ppbC and the aldehyde
concentrations ranged from below the
detection limit (-0.1 ppb) up to 0.7 ppb
(see Table 2).
The data base makes it possible to
examine the variability in NMHC
concentrations aloft during the morning
hours since sampling arcs were flown at
different distances and altitudes upwind
of the urban area. Generally, one or
more samples were collected along each
of three upwind arcs. Figure 2
graphically illustrates the daily variations
in NMHC levels observed in each of the
six areas. The horizontal lines in Figure
2 represent measured NMHC levels in
individual samples. The vertical lines
connect maximum and minimum levels
each day. On most days, the high NMHC
measurement exceeds the low value b
at least a factor of two and on severe
occasions by as much as a factor of ter
As can be seen in Figure 2, on day
when the NMHC concentration range wa
large, one sample exhibited a muc
higher hydrocarbon level than others
The reason for this anomalous behavic
is not immediately obvious. The hig
readings showed no consisten
correlation with altitude or distance fror
the city.
NMHC compositions were generall
consistent from city to city; howevei
influences from selected source type
were periodically observed. For exampl
in Tulsa occasional elevated levels of th
£2 io £4 paraffinic hydrocarbons wer
observed, possibly from the refmerie
located upwind of the city. In Atlant
relatively large concentrations of toluen
were occasionally observed. The upwin
source for this compound was nc
identified.
Also of interest is the correlation (
NMOC aloft with other pollutant!
Generally speaking, there appeared to b
a direct relationship between NMO'
concentrations with ozone and NC
levels. For example, Table
summarizes NMOC, 03 and NOX dal
collected upwind of Dallas on th
mornings of July 6 and 14, 1985. It
evident that elevated NMOC levels o
July 6 are associated with ozone an
NOX concentrations which ar
considerably higher than those recorde
on July 14.
Philadelphia and New York City wei
chosen for a preliminary examination i
possible correlations between weath<
patterns and oxidant precursor levels
the air advected from aloft. Becaus
they are geographically close, it
expected that they would receive a
masses from the same source area
under westerly and northwesterly flo
conditions
Two sets of days with similar mornir
wind speed and direction are compare
in Table 3. In the first group, whic
includes two days with westerly wine
exceeding 20 kts, oxidant precursi
levels are nearly identical. The maximu
ozone levels recorded at surface statior
in each of the urban areas on July 14 ar
August 8 were very comparable, as we
A similar pattern exists for the two da]
listed with westerly winds less than 1
kts. Precursor levels are vei
comparable as are the surface ozor
concentrations which in this case a
elevated when compared to the hi<;
wind conditions of the first group. Tl
fact that higher surface ozone levels a
-------
Wind Direction
Afternoon
Mixing
Height
Figure 1. Schematic diagram of aircraft flight pattern.
Table 1. Mean NMHC Concentrations Measured Aloft
City
Sampling Dates
Identified NMHC Formaldehyde Acetaldehyde
(ppbC) (ppb) (ppb)
Dallas
Tulsa
Atlanta
Birmingham
Philadelphia
New York City
July 5-14, 1985
July 16-24, 1985
Aug. 13-23, 1985
Aug. 26-29, 1985
July8-Aug. 14, 1986
JulyS-Aug. 14, 1986
23
34
25
14
16
19
1.7
3.5
<0.7
<0.7
1.8
1.9
0.2
0.9
<0.3
<0.3
0.7
0.7
recorded under more stagnant
meteorological conditions agrees with
expectations.
Figure 3 shows the average NMOC
concentrations measured aloft during this
study and compares them to the
concentrations measured at a rural,
surface site near State College,
Pennsylvania and on the ground in
Philadelphia. As is readily apparent,
NMOC concentrations at the rural site
are comparable to those found aloft and
much less than those measured in
Philadelphia. The NMHC levels recorded
in the urban area are on the order of
twenty times those observed in the
background air while the aldehyde levels
are three to four times that found aloft or
at the rural site. Since the rural surface
NMOC levels are comparable to those
collected aloft, it appears that surface
data could be utilized as boundary
condition values for modeling urban
oxidant production when aircraft data is
not available.
Summary and Conclusions
The NMOC results reported for the six
cities studied indicate that the air above
urban areas for the most part represents
an aged air mass that is depleted in the
more reactive hydrocarbons. The
average NMHC composition contains
78% paraftinic, 17% aromatic and 3%
olefmic compounds. The organic
carbonyl levels in 4 of the 6 urban areas
contributed on the average of 10% to the
total NMOC levels. The carbonyl levels
at the other two sites were below
detectable limits of the analytical
procedure. On some flights the NMOC
levels and compositions were quite
consistent on each of the three sample
arcs flown. At other times, however,
NMOC levels and composition changed
significantly. A positive correlation
between NMHC and organic carbonyl
levels was generally observed. The mean
NMHC concentration for all cities was 22
ppbC with a coefficient of variance of
40%. Synoptic meteorological conditions
are likely to have an impact on the
NMOC concentration level observed in
the background air. From the limited data
available, it appears that the NMOC
levels measured aloft agree reasonably
well with those measured at remote
ground level locations. More data are
required to confirm this relationship.
-------
0
<3
5
0
a:
5
100 -
90 ~
80 ~
70 ~
60 ~
50 ~
40 ~
30 ~
20 ~
10 ~
n
I .
•
I
I .
•
Atlanta
m
: I
\ \
0
-Q
CX
EX
Cj
3:
|
100 -1
90 ~
80 ~
70 ~
60 ~
50 ~
40 ~
30 ~
20 -
10 ~
Birmingham
• •
I
I
1 I
1 1 \ \
co ^ QQ oj
CN
-------
o
-Q
&
Cj
1
700~
90-
80-
70-
60 ^
50-
40-
30~
20~
10~
Tulsa
1 -
* ~
-,-
i I -
I
\ \ \ \
Date
700
90 ~
80 ~
70~
% 60-
|5°J
| 40~\
30
20 H
10 ~\
0
/Vew York
T i
!•
CM f
•X
rs r-
i
•»
M O
x \
s 00
\
§
\
oo
Date
\
§
\
00
\
o
N
00
l^
»*,
\
00
\
^
\
oo
\
«»
»-.
X
CO
Figure 2.
Continued.
-------
100-
90-
80-
70-
O
•Q
Philadelphia
§: 50-
o
| 40-
^ 30-
20-
w-
I
\
§
\
I
\
<3
rs
I
\
(S
1 I
* 1 ;
' I .
1 1 1 ' 1 1 ^
s;?10^^^^?
v v^\\\\ \
^^^K^ri^^
Date
Figure 2. Continued.
Table 2. Summary of Airborne Data Collected Upwind of Dallas on July 6 and 14, 1985
Date Arc (mi) Alt (ft)
NMHC
ppbC
Formaldehyde Acetaldehyde
(ppb) (ppb; 03 (ppbj NO* (Ppb)
July
My 14
15
20
25
25
20
15
2000
3000
2000
2000
3000
2000
22.4
36.7
15.3
3.8
4.7
1.5
1 6
1.4
0.1
0.3
0.7
0.5
07
<02
<0.2
<0.2
0.7
-50
-20
15
~5
-------
Table 3. Oxidant Precursors and Wind Data
Wind
Date
07/14
08/11
07/16
07/22
City
P
NY
P
NY
Speed
(Ms)
25
22
8
7
Direc-
tion (deg)
290
270
290
290
Ident. NMHC
(PpbC)
20 1
21 8
15.5
15.6
Aldehyde
(ppb)
2.8
2.7
50
3.3
NO, (ppb)
3
4
3
3
Peak Surface
03 (ppb)-
62
64
114
107
P = Philadelphia.
NY = New York.
The O3 data was taken from SAROAD sites 310720003F01 for 7/14, from 330280002F01 for 8/11, from
311760002F01 for 7/16, and from 313260001F01 for 7/22.
500 -
400 •<
300 -
200 -
700 -
n -
ppbC
X/>TvN.\
~i^S/.rs
r~ss, r
'/•'ft'/ ^
Ip
l^
£/
'%//,
NMHC
Aloft
ppb
Formaldehyde
Rural
Acetaldehyde
Urban
Figure 3. Comparison of average NMOC concentrations measured at 11) a rural surface
site in Pennsylvania, /2) urban Philadelphia, and (3) aloft in the vicinity of Atlanta.
Birmingham, Dallas, Tulsa, New York City and Philadelphia.
------- |