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JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 104, NO. D5, PAGES 5623-5634, MARCH 20, 1999
Influence of biomass combustion emissions on the distribution
of acidic trace gases over the southern Pacific
basin during austral springtime
R. W. Talbot, • J. E. Dibb, • E. M. Scheuer, • D. R. Blake, 2 N.J. Blake, 2 G. L. Gregory, 3
G. W. Sachse, 3 J. D. Bradshaw, 4,s S. T. Sandholm, 4 and H. B. S ingh 6
Abstract. This paper describes the large-scale distributions of HNO3, HCOOH, and CH3COOH
over the central and South Pacific basins during the Pacific Exploratory Mission-Tropics (PEM-
Tropics) in austral springtime. Because of the remoteness of this region from continental areas, low
part per trillion by volume (pptv) mixing ratios of acidic gases were anticipated to be pervasive
over the South Pacific basin. However, at altitudes of 2-12 km over the South Pacific, air parcels
were encountered frequently with significantly enhanced mixing ratios (up to 1200 pptv) of acidic
gases. Most of these air parcels were centered in the 3-7 km altitude range and occurred within the
15 ø-65 øS latitudinal band. The acidic gases exhibited an overall general correlation with CH3C1,
PAN, and 03, suggestive of photochemical and biomass burning sources. There was no correlation
or trend of acidic gases with common industrial tracer compounds (e.g., C2C14 or CH3CC13). The
combustion emissions sampled over the South Pacific basin were relatively aged exhibiting
C2H2/CO ratios in the range of 0.2-2.2 pptv/ppbv. The relationships between acidic gases and this
ratio were similar to what was observed in aged air parcels (i.e., >3-5 days since they were over a
continental area) over the western North Pacific during the Pacific Exploratory Mission-West
Phases A and B (PEM-West A and B). In the South Pacific marine boundary layer a median
C2H2/CO ratio of 0.6 suggested that this region was generally not influenced by direct inputs of
biomass combustion emissions. Here we observed the lowest mixing ratios of acidic gases, with
median values of 14 pptv for HNO3, 19 pptv for HCOOH, and 18 pptv for CH3COOH. These
values were coincident with low mixing ratios of NOx (<10 pptv), CO (•50 pans per billion by
volume (ppbv)), 03 (< 20 ppbv), and long-lived hydrocarbons (e.g., C2H 6 <300 pptv). Overall, the
PEM-Tropics data suggest an important influence of aged biomass combustion emissions on the
distributions of acidic gases over the South Pacific basin in austral springtime.
1. Introduction
Acidic gases are important participants in tropospheric chemical
processes. They are major end products of oxidative cycles, with
wet and dry removal of HNO3 and H2SO 4 from the atmosphere
principal sinks for tropospheric NOx (NO + NO2) and SO2 [Logan,
1983; Hales and Dana, 1979]. In remote regions the
monocarboxylic acids HCOOH and CH3COOH are often the
dominant acidic gases and acidity components of cloud water and
precipitation [Keene et al., 1983; Andreae et al., 1988, 1990].
Formic acid is also a major sink for OH radicals in cloudwater
[Jacob, 1986].
Formic acid may be produced by aqueous phase OH oxidation
of hydrated formaldehyde (H2C(OH)2) in cloudwater and subse-
mInstitute for the Study of Earth, Oceans, and Space, University of New
Hampshire, Durham.
2Department of Chemistry, University of California, Irvine.
'•NASA Langley Research Center, Hampton, Virginia.
4Department of Earth and Atmospheric Science, Georgia Institute of
Technology, Atlanta.
5Deceased June 16, 1997.
6NASA Ames Research Center, Moffett Field, California.
Copyright 1999 by the American Geophysical Union.
Paper number 98JD00879.
0148-0227/99/98JD-00879509.00
quently provides an important source of gas phase HCOOH in the
remote troposphere [Chameides and Davis, 1983; Jacob, 1986].
Aqueous phase production mechanisms for CH3COOH appear to be
quite slow and probably are a negligible source of this species to the
troposphere [Jacob and WoJ•y, 1988]. The major sources of
HCOOH and CH3COOH to the global troposphere appear to be
emissions from combustion [Kawamura et al., 1985; Talbot et al.,
1988; Helas et al., 1992; Lefer et al., 1994], vegetation [Keene and
Galloway, 1986; Talbot et al., 1988, 1990], and possibly soils
[Sanhueza and Andreae, 1991; Talbot et al., 1995]. Permutation
reactions of peroxy radicals have been proposed as potentially
important sources of carboxylic acids [Madronich and Calvert,
1990; Madronich et al., 1990], but recent measurements at a
continental site indicate that this pathway may be relatively
unimportant [Talbot et al., 1995].
There are potentially numerous production mechanisms for
HNO3 in the troposphere including, NO2 + OH, recycling of
reactive nitrogen reservoir species, and evaporation of NO3' in
aerosol and aqueous phases [Roberts, 1995]. Many of these
processes are thought to very slow in the upper tropical troposphere
due to low 03 mixing ratios and cold temperatures retaining most
of the reactive nitrogen in the form of NO during the daytime
[Folkins et al., 1995].
Measurements conducted in winter 1992 at 10-12 km altitude
between Tahiti and California showed an abrupt decrease in the
mixing ratios of 03 and NOy (i.e., the sum of reactive nitrogen
species) at the southern edge of the Intertropical Convergence Zone
5623
5624 TALBOT ET AL.' ACIDIC GASES OVER THE SOUTHERN PACIFIC BASIN
(ITCZ) [Folkins et al., 1995]. Owing to the remoteness of the South
Pacific basin from continental areas, the observed trend in O3 and
NOy is not surprising. In fact, low mixing ratios would be expected
to be pervasive over the South Pacific basin for most tropospheric
trace species, including acidic gases.
In this paper we present the large-scale distributions of HNO•,
HCOOH, and CH•COOH over the central and South Pacific basins
during the NASA Global Tropospheric Experiment/Pacific
Exploratory Mission-Tropics (GTE/PEM-Tropics) in Septem-
ber/October 1996. Objectives of PEM-Tropics included obtaining
baseline data for important tropospheric gases, evaluating the
oxidizing capacity of the troposphere and factors influencing it, and
improving our understanding of the natural sulfur cycle over the
South Pacific basin.
The first part of this paper focuses on the distributions of acidic
gases in the 2-12 knit, altitude range which were apparently heavily
impacted by aged biomass combustion emissions. Supporting
evidence for this source is provided by coincident distributions of
selected hydrocarbon compounds. The distribution of acidic gases
is examined in the marine boundary layer and overlying transition
region in the second part of this paper. Here there was little
evidence for a direct influence of biomass combustion inputs on the
chemistry, in stark contrast to the middle and upper troposphere.
Overall, the PEM-Tropics measurements provide unique informa-
tion of the chemistry of this extensive remote region during austral
springtime.
2. Experimental Methods
2.1. Study Area
The PEM-Tropics airborne expedition was conducted using the
NASA Ames DC-8 research aircraft. Transit and intensive site
science missions composed 18 flights, averaging 8-10 hours in
duration and covering the altitude range of 0.3 to 12.5 km. The base
of operations for these missions progressed as follows: (1) Tahiti
(three missions), (2) Easter Island (two missions), (3) Tahiti (one
mission), (4) New Zealand (one mission), and (5) Fiji (three
missions). The data used in this paper were obtained in the geo-
graphic grid approximately bounded by 60øN-75øS latitude and
165øE-105øW longitude. Data obtained on transit flights (eight
missions) were also utilized in this paper. A geographic map of the
study region is shown in several companion papers [e.g., Gregory
et al., this issue; Hoell et al., this issue].
The overall scientific rationale and description of individual
aircraft missions is described in the PEM-Tropics overview paper
[Hoell et al., this issue]. The features of the large-scale meteorologi-
cal regime and associated air mass trajectory analyses for the
September-October 1996 time period are presented by Fuelberg et
al. [this issue].
2.2. Sampling and Analytical Methodology
Acidic gases were subsampled from a high-volume (500-1500
standard liters per minute (sLpm)) flow of ambient air using the
mist chamber technique [Talbot et al., 1988, 1990, 1997a]. The
subsample flow rate was always <10% of the primary manifold total
flow. Sample collection intervals were typically 4 min in the
boundary layer, 6 min at 2-9 km altitude, and 8 min above 9 km
altitude, reflecting decreased pumping rates in the middle and upper
troposphere. The inlet manifold consisted of a 0.9 m length of 41
mm ID glass coated stainless steel pipe. The pipe extended from the
DC-8 fuselage to provide a 90 ø orientation to the ambient air
streamline flow. To facilitate pumping of the high-volume manifold
flow, a diffuser was mounted over the end of the inlet pipe parallel
to the DC-8 fuselage. This device provided a "shroud" effect,
slowing the flow of ambient air through it slightly below the true air
speed of the DC-8 and adding 50-100 hPa of pressurization to the
sampling manifold. This effectively eliminated the reverse venturi
effect (•40 hPa) on the sampling manifold. An additional feature of
the diffuse was a curved step around the manifold pipe which
provided the streamline effects of a backward facing inlet. Its
function was to facilitate exclusion of aerosol particles greater than
• 2 •m in diameter from the sampling manifold. Aerosols smaller
than this were removed from the sampled air stream using a 1
pore-sized Zefluor teflon filter that was readily changeable every 5-
10 min to minimize aerosol loading on the filter and gas/aerosol
phase partitioning from ambient conditions.
In addition to the features described above, the inlet manifold
was equipped with the capability for conducting a standard addition
of HNO• into the manifold ambient air stream. This spike was
added • 10 cm downstream inside the manifold pipe through a 6.5
mm OD glass coated stainless steel tube mounted perpendicular to
the air flow. This tube was •20 mm long and maintained at 40øC to
facilitate passing of calibration gas through it. Through a tee, this
injection length of tubing was connected to about 1.5 m of heated
tubing that was directly linked to the perm oven output. This design
effectively tested the passing efficiency of the entire manifold
system, which was indistinguishable from 100 + 15%. The calibra-
tion system for HNO• consisted of a permeation oven held at 50 øC
and a dilution flow of ultra zero air (1.5 sLpm) which swept the
oven outflow to either a nylon filter for output quantification or the
sampling manifold for standard addition on ambient air. The heated
tubing through which the HNO• stream passed was kept equili-
brated by a flow design that allowed the calibration gas to con-
stantly pass to near the point of injection into the manifold flow
before being dumped to waste through a retfirn line. The mixing
ratio of HNO3 in the 1.5 sLpm flow was typically 200 parts per
billion by volume (ppbv). This spike was then diluted several
hundred times by the high flow rate of ambient air in the sampling
manifold, producing standard additions of 100-1200 pptv. Previ-
ously we have studied the passing efficiency of carboxylic acids
through our inlet manifold, and found it to be > 95% [Talbot et al.,
1992]. Thus we focused our attention on HNO3 due to its
importance to the reactive nitrogen cycle and tropospheric chemis-
try.
The permeation oven output of HNO3 was monitored on the
ground and in the air in near-real time. We fabricated a new
calibration system for PEM-Tropics which maintained the perme-
ation tube to 50.0 + 0.1 øC and 1850 + 1 hPa pressure at all altitudes.
The permeation source was constant to + 8.5% over the course of
the expedition, with no equilibration time required at any altitude,
even with rapid changes such as during spiral maneuvers. This new
design utilized upstream pressure control (i.e., before the perme-
ation tube) so that there were no fittings, valves, or flow/pressure
controllers in-line between the tube and the injection point into our
sampling manifold. The flow through the oven varied from 20-25
cm 3 min '• depending on the ambient pressure and was diluted into
the 1.5 sLpm flow described above. Standard additions were
conducted with and without a teflon filter in-line to verify that the
filter did not influence the passing efficiency of the sampling
manifold.
Computer controlled syringe pumps were used to move sample
solutions in and out of the mist chamber samplers and our sample
containers. This essentially provided a closed system of liquid
handling which greatly simplified contamination control. The
concentrations of acidic gases in our samples were quantified using
TALBOT ET AL.: ACIDIC GASES OVER THE SOUTHERN PACIFIC BASIN 5625
a custom built dual ion chromatography system equipped with a
computer interface for data acquisition. The system was composed
primarily of Dionex components with the detectors and flow system
thermostated to 40 ø C. Eluants were constantly purged with He gas.
Nitric acid was measured using a fast anion column while the
carboxylic acids were determined using an AS4 column. Concentra-
tor columns and electronic suppression was used in both chroma-
tography systems. Calibration curves generated on the ground and
in the air agreed within ñ2%. We thus were able to determine
atmospheric mixing ratios of acidic gases in near-real time.
In addition to data for acidic gases, we present selected informa-
tion on several important trace gases including ozone (03), carbon
monoxide (CO), ethyne (C2H2), perchloroethylene (C2C14) , and
peroxyacetylnitrate (PAN). Aerosol NO 3' was measured on bulk
filter samples collected with a forward facing isokinectic probe
housed in a shroud to ensure isoaxial flow [Dibb et al., 1996a].
Ninety millimeter diameter 2 •m pore-sized Zefluor teflon filters
were used as the collection substrate. Specific details regarding the
measurement of various other species used in this paper are
presented in companion papers [Blake et al., this issue; Dibb et al.,
this issue; Gregory et al., this issue; Vay et al., this issue]. The
measurements of these species were averaged to provide mean
values that corresponded directly to the acidic gas sampling times.
This merged data product was generated at Harvard University, and
it is used exclusively in this paper.
3. Results
In the data presented in this paper, obvious stratospherically
impacted values have been removed based on coincident measure-
ments of 03, CO, dew point, and selected hydrocarbons and
halocarbons. This amounted to removing a total of about 25 data
points obtained on three different flights.
The large-scale latitudinal distribution of acidic gases over the
central and South Pacific basins is presented in Figure 1. It is
evident from these distributions that numerous air parcels were
encountered between 15 ø and 60øS latitude which contained large
mixing ratios of acidic gases. The northern border of the impacted
Pacific troposphere appears to be controlled by the presence and
location of the South Pacific Convergence Zone (SPCZ) [Gregory
et al., this issue]. Nitric acid mixing ratios, for example, typically
decreased by a factor of 2-5 in crossing the SPCZ region from south
to north. This trend was also apparent in other trace gases such as
CO, C2H2, C2H6, 03, and PAN [Gregory et al., this issue]. Thus
polluted air parcels appeared to be present south of the SPCZ with
"clean" air north of it fed by an easterly flow regime along the
southern edge of the ITCZ.
Mixing ratios of acidic gases over the South Pacific were
generally less than 200 pptv but approached or exceeded 1000 pptv
in some air parcels. These air parcels (i.e., plumes) were observed
mainly between 2 and 7 km altitude (Figures 2a-2c). Because of the
strong trade wind inversion over this region, the marine boundary
layer exhibited very small mixing ratios of acidic gases. The
inversion appeared to be a very effective barrier to downward
mixing of acidic gases from aloft. Indeed, the most processed (i.e.,
aging and mixing influences) air parcels were sampled in the
marine boundary layer. This feature of the data is illustrated using
the ratio C2H2/CO which had a median value of 0.6 below 1 km
altitude but showed significantly larger values in the rest of the
tropospheric column (Figure 3). Values of this ratio less than 1.0
are typical of photochemically aged and well mixed (diluted) air
parcels [Smyth et al., 1998; Talbot et al., 1997b].
As an example of the detailed vertical structure over the South
Pacific selected data from a slow spiral (80 m min 4) conducted east
of Fiji is shown in Figure 4. An apparent combustion plume was
sampled near 5 km, with corresponding large increases in HNO3,
C2H2, and C2H2/CO but not C2C14. Notice the very rapid vertical
changes in the mixing ratios and generally good correspondence
between HNO3 and C2H2. While the plumes with large mixing
ratios of many trace gases clearly stand out, the PEM-Tropics data
in general support the idea that much of the tropospheric column
from 2-10 km altitude was fumigated with varying degrees of
combustion emissions. The smooth shape of the vertical distribu-
tion of C2C14 is typical of what was observed over the South Pacific
(Figure 1), and it suggests minimal influence on the chemistry from
industrial emissions. The distribution of CH3CC13 and other
halocarbons also supports this ascertain (N. Blake, personal
communication, 1998).
To provide a detailed description of the distribution of acidic
gases over the central and South Pacific basins, this information is
presented in a regional summary format (Table 1) consistent with
that used in companion papers [Gregory et al., this issue; Dibb et
al., this issue]. Information on the distribution of many other trace
gases can be found in these papers, so it is not duplicated here. The
regional breakdown was developed to provide data summaries that
correspond to logical latitudinal and longitudinal areas (e.g., the
ITCZ, and the eastern, central, and western Pacific basins). In some
regions the sampling was quite sparse, so interregional comparisons
need to be conducted with caution. On the basis of the vertical
measurement density of acidic gases, the data were broken into four
altitude bins: (1) the marine boundary layer (<1 km), (2) the
transition or cloud layer (1-2 km), (3) the middle (2-8 km), and (4)
upper (8-12 km) troposphere.
As shown in the large-scale vertical distributions (Table 1 and
Figure 2), the smallest mixing ratios of acidic gases were found in
the marine boundary layer. Here median mixing ratios were 14 pptv
for HNO3, 19 pptv for HCOOH, and 18 pptv for CH3COOH. The
very small mixing ratios of HNO3 are consistent with the observed
NOx values of only a few or sub (i.e, <1) pptv in the boundary layer
(Georgia Institute of Technology NO.• data are available from the
Distributed Active Archive Center (DAAC) at NASA Langley
Research Center, Hampton, Virginia). There was no significant
regional difference in the mixing ratio of HNO3 in the marine
boundary layer, but the carboxylic acids exhibited values 2-3 times
larger in the central Pacific region. In the middle troposphere the
mixing ratios of acidic gases showed the largest values in the
western and central regions. This is consistent with the generally
westerly flow of air at these altitudes over the South Pacific basin,
implying that the least processed air parcels would be found in
these regions [Fuelberg et al., this issue]. Most of the plumes that
we sampled were, in fact, encountered over the western and central
Pacific areas. The eastern Pacific regions were dominated by
relatively "clean" air parcels. This longitudinal difference seem-
ingly reflects chemical and physical losses of acidic gases as air
parcels transverse the Pacific basin in a westerly flow regime.
The mixing ratios of the carboxylic acids HCOOH and
CH3COOH are generally found to be highly correlated in the gas
and liquid phases in the troposphere [Keene and Galloway, 1986].
Over continental areas the ratio HCOOH/CH3COOH usually has a
value near 2.0 with a correlation coefficient between these two
species near 0.9 [Keene and Galloway, 1986; Talbot et al., 1988].
Although we observed linear correlations between HCOOH and
CH3COOH over the Pacific basin (Figures 5a and 5b), they were
less robust than what we observed during the Pacific Exploratory
Mission-West Phases A and B (PEM-West A and B) [Talbot et al.,
5626 TALBOT ET AL.' ACIDIC GASES OVER THE SOUTHERN PACIFIC BASIN
O
12oo
800
400
o
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lOOO
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' I ' I ' I ' I ' I ' I '
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Latitude
Figure 1. Latitudinal distribution of acidic gases at altitudes of 2-12 km over the central and southern Pacific
basins.
1997a]. In some of the plumes, HCOOH was highly enhanced with
regard to CH3COOH, and the ratio HCOOH/CH3COOH had values
as large as 5.0 (plume median equal to 1.6). This suggests the
possibility of substantial photochemical production of HCOOH
compared to CH3COOH (or more efficient loss of CH3COOH) in
some of the plumes that we sampled over the South Pacific. This
point is further explored in later sections of this paper.
4. Discussion
4.1. Altitude Range of 2-12 km
The large-scale impact of pollution over much of the western and
central Pacific basins is a significant feature of the PEM-Tropics
data set. Backward trajectories indicate that many of the air parcels
we sampled had not been over continental areas for 10-20 days
[Fuelberg et al., this issue]. This is consistent with the chemical
measurements which suggest that the air parcels were photochemi-
cally aged and physically processed for a week or two since the last
injection of combustion emissions. Many of the trajectories follow
a path that implies that the last continental areas that the air parcels
passed over were Brazil and Africa. Since biomass burning occurs
on both of these continental areas during austral spring [Cahoon et
al., 1992], this is likely to be a major source of combustion
emissions over the Pacific basin at this time of year.
Methyl chloride is a reasonably good chemical tracer of biomass
burning emissions [Blake et al., 1996]. The relationship between
TALBOT ET AL.' ACIDIC GASES OVER THE SOUTHERN PACIFIC BASIN 5627
E
12
10 eee ß
e eme ß
ß ß ß
ß
ee ß
ß ß
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ee e
ß
ß
ß ß
ß ß ß
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ß
ß
ß ß
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o 3oo
ee ß •
ß ß
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600 900
HNO3, pptv
(a.)
1200 1500
12
10
E
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2
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.•.-ooo
e•, ß
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0 300 600
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HCOOH, pptv
(b.)
I
1200 1500
12
10
(c.)
_ ß ß
e•me--- ß ß
ß ß
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ß ,•, .'.. .'
.•;-_ .......
L :.;J .
ß .
.... .
1•' , I , , I
0 250 500
CHsCOOH, pptv
0 I ,
750 1000
Figure 2. Vertical distribution of acidic gases over the central and southern Pacific basins. These distributions
show that plume encounters with enhanced mixing ratios of acidic gases commonly occurred in the 3-7 km
altitude region.
the mixing ratios of CH3CI and acidic gases is depicted in Figure 6.
These plots indicate a general relationship between acidic gases and
CH3CI (r 2 = 0.4). The enhancements of CH3CI in the plumes are
small due to the significant dilution these well aged air parcels have
undergone. Plots of C2H2 and C2H6 versus CH3C1 (not shown) show
similar relationships to those in Figure 6, again reflecting the
substantial processing of the air parcels over the Indian and Pacific
basins.
One feature of the plumes is the absence of enhancements in
aerosol or aerosol associated species [Dibb et al., this issue], even
for ammonium which is released in large quantities from biomass
combustion [Lobert et al., 1991 ]. This indicates that the air parcels
over the Pacific basin have been effectively scavenged by clouds
and precipitation. It also suggests, since most acidic gases are
highly water-soluble, that their large mixing ratios in some of the
plumes may be due to photochemical production since the last
scavenging event. Evidence for a photochemical source of acidic
gases is provided in Figures 7 and 8, where the relationships
between these species and 03 and PAN are presented. As with
CH3CI, the trends are only general (r 2 near 0.4) but suggestive of
photochemical production of acidic gases. The break in the
relationships at <5 pptv of PAN is presumably due to thermal
decomposition of PAN to NOx at lower altitudes [Roberts, 1995 ].
The data corresponding to <5 pptv PAN was obtained in the 2-4 km
altitude band where air temperatures were typically 280-285øK.
Plotting an individual species as a function of the ratio C2H2/CO
gives insight on the effect of air parcel processing on its mixing
ratio. These relationships for acidic gases are shown in Figure 9. It
is evident that the relationship is much tighter for the carboxylic
acids compared to HNO3, but it is unclear as to why this is the case.
5628 TALBOT ET AL.' ACIDIC GASES OVER THE SOUTHERN PACIFIC BASIN
E
12
10
• 8
a. 4
ß
ß
eeee e•ee ee • ß e e ß
ß • e•--- • e•edl• ß ß
ß ß ee
-,,,.... ,•..,..•.,,. ß . .- ß
' ß
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e• ß el' ß e•e,•e ee e e% ß ß ß
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e'•ee ee • ß eee4e ß
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ß e e e e ß ß ß ,.
0.5 1.0 1.5 2.0
C2H 2 / CO, pptv / ppbv
, I
2.5 3.0
Figure 3. Vertical distribution of the ratio C2H2/CO over the
central and southern Pacific basins.
Clearly, the largest mixing ratios of acidic gases were contained in
the least processed air parcels (C2H2/CO >1). On the basis of the
correlations shown in this paper, it follows that these same air
parcels also contained the largest mixing ratios of CH3CI, 03, and
PAN. It appears that the chemical composition of these air parcels
reflects photochemical activity of biomass burning emissions aged
over a minimum of a one week time frame. This is based largely on
the absence of reactive hydrocarbons (i.e., C4 and higher) in these
air parcels. It appears that mixing processes (i.e., dilution) are
responsible for much of the variation in individual species mixing
ratios and inter-relationships between various compounds. Thus, air
parcels can be quite photochemically aged with significant mixing
ratios of secondary species but still appear much younger (e.g.,
C2H2/CO > 1) due to less mixing with background air.
To examine the potential combustion source of HNO3, only
mixing ratios greater than 100 pptv are plotted versus CO and C2H 2
(Figure 10). Only in a few plumes does there appear to be a direct
relationship between HNO3 and CO or C2H 2. The largest mixing
ratios of HNO3 occurred at relatively low values of CO and C2H 2
and correspond to a C2H2/CO ratio near I (Figure 9). In general,
there was very substantial amounts of HNO3 in air parcels with CO
of 50-100 ppbv and C2H 2 <150 pptv. Together these results point
to significant photochemical production of HNO3 (since the last
scavenging event) during long-range transport of air parcels over
the South Pacific. Similar arguments can be made for photochemi-
cal generation of carboxylic acids in these air parcels, especially
HCOOH. Previous measurements of the ratio HCOOH/CH3COOH
in biomass burning plumes transported long distances in the middle
troposphere show values of 1.5-3 [Helas et al., 1992; Lefer et al.,
1994; Dibb et al., 1996b].
Additional modeling studies are needed to enhance our under-
standing of photochemical processes occurring within plumes over
the South Pacific basin. Limited insight as to whether we observed
loss of CH3COOH in these plumes can be gleamed by examination
of the CH3COOH and CH3OOH data. Plotting various subsets of
these data obtained over the South Pacific (not shown) revealed no
correspondence between the two species, as would be expected if
CH3COOH were a significant decomposition source of CH3OOH
12
E 10
-o 8
•: 6
= 4
a_ 2
o
70
12
E lO
'o 8 -
•: 6
'-, 4
a. 2
I ' I ' I ' I ' I ' I
I , I , I , I • I , I
80 90 100 110 120 130
C2H 2, pptv
I ' I ' I ' I ' I
140
12
o
0.9
12
' I ' I ' I ' I ' I '
ß
, I , I , I , I , I
1.0 1.1 1.2 1.3 1.4
C2H2/CO, pptv / ppbv
1.5
0 , I , I , I , I , I , 0 , I , I I ,
0 100 200 300 400 500 600 0.5 1.0 1.5 2.0 2.5
HNO 3, pptv C2CI 4, pptv
Figure 4. Vertical distribution of selected trace gases during a slow spiral conducted during mission 17 just east
of Fiji.
TALBOT ET AL.' ACIDIC GASES OVER THE SOUTHERN PACIFIC BASIN 5629
Table 1. Regional Summary of Acidic Gases Over the Central and
Pacific Basins
Altitude, HNO3 HCOOH, CH3COOH N
km pptv
15 ø-45 øN, 120o-170øW
< 1 13 q- 8.5 (10) 45 +_ 18 (50) 55 q- 23 (60) 17
1-2 46 + 10 (50) 39 + 30 (30) 44 + 35 (36) 6
2-8 87 + 43 (83) 72 + 49 (55) 72 + 45 (59) 42
8-12 63 + 53 (42) 55 + 26 (52) 54 + 30 (55) 29
0o-15 øN, 120o-170oW
< 1 20 ___ 11 (15) 35 ___ 30 (27) 34 q- 23 (20) 19
1-2 52 q- 14 (49) 33 q- 24 (18) 36 q- 26 (22) 10
2-8 67 +_ 35 (51) 31 +_ 14 (33) 33 +_ 14 (35) 31
8-12 150 q- 107 (107) 27 q- 26(14) 23 q- 19(13) 16
0o-35 øS, 120o-170øW
< 1 18 q- 10 (17) 66 q- 216 (23) 84 q- 293 (27) 40
1-2 32 q- 14 (30) 44 q- 20 (43) 43 q- 345 (39) 21
2-8 139 q- 145 (84) 124 q- 181 (49) 84 q- 81 (54) 192
8-12 63 q- 63 (37) 61 q- 54 (46) 56 q- 46 (41) 122
0o-35 øS, > 170 øW
< 1 15 q- 7.7 (13) 18 q- 7.4 (17) 17 q- 8.1 (15) 12
1-2 40 q- 3.0 (40) 50 q- 5.0 (50) 46 q- 8.5 (46) 2
2-8 160 q- 124 (97) 71 q- 54 (47) 84 q- 70 (53) 46
8-12 100 q- 61 (81) 66 q- 32 (52) 92 q- 27 (94) 19
0o-35 øS, 80o-120øW
< 1 13 q- 7.1 (14) 18 q- 5.9 (18) 16 q- 13 (18) 27
1-2 28 q- 8.2 (30) 33 q- 8.5 (32) 67 q- 47 (47) 5
2-8 43 q- 37 (31) 43 q- 28 (36) 52 q- 43 (36) 50
8-12 45 q- 30 (45) 54 q- 41 (39) 42 q- 26 (43) 28
35 o-72 øS, > 170 øW
< 1 14 q- 4.6 (14) 11 +__ 4.1 (10) 13 q- 3.9 (12) 23
1-2 25 q- 9.9 (28) 38 q- 26 (33) 29 q- 16 (26) 13
2-8 180 q- 262 (80) 162 q- 176(90) 114 q- 12 (57) 70
8-12 54 +_ 71 (25) 54 q- 39 (40) 45 q- 25 (37) 23
35 o- 72 øS, 80 ø-I 20 øW
< 1 12 +_ 2.3 (11) 16 q- 4.4 (18) 18 q- 4.6 (19) 7
1-2 31 q- NA (31) 22 +_ NA (22) 20 q- NA (20) 1
2-8 26 + 16 (23) 35 q- 19 (36) 33 q- 18 (26) 16
8-12 38 q- 60 (9.0) 18 q- 5.0 (19) 16 q- 4.8 (17) 5
Values are stated as mean -4- one standard deviation (median). N represents
the number of data in altitude •in. NA means not applicable.
>
o
o
lOO
lO
(a) <2 km altitude
ß •//•!::: •COOH = 0.88 x CH3COOH + 3.1 '
ß r2 = 0.61
I I I I • , I I I I I I I • I I
10 100
1 ooo
lOO
lO
(b) 2 - 12 km altitude ß
,,.f.;,
,
CH3COOH, pptv CH3COOH, pptv
Figure 5. Relationship between mixing ratios of HCOOH and CH3COOH over the central and southern Pacific
basins; (a) <2 km and (b) 2-12 km altitude.
5630 TALBOT ET AL.' ACIDIC GASES OVER THE SOUTHERN PACIFIC BASIN
1 ooo
lOO
lO
I
I ß ß • =* .:* . ß ß ß
ß * * . * .'. * * I ** ,% .. ø*
- ... ß a,... ß f .....•,q,l; • • .....
ß ß , ...<;. •"•l,,j•&•_ ".•,..*.. * ß *
ß. •, •,• •' •.• '.,* ..
.... :..'. -.. '.?.;i"g'--' •71.•.-.. ,. :.
'. '.-*;:;,5:_'--':=_•:•:.'.:.'" ß ß
/ ** _ •*•.. ,•***e,. •,,t .. * , ß
.. ..... ß ,• _,.•.... ** *. ....
ß ß.. ß ß ..-.q, I.ß.ß ß ß ß
ß ' . .:•.... ,.... :'?. .
ß .ß ß
ß ß ß
ß
HNO 3 = 10.0 x CH3CI- 26
burning plumes, there is clearly much uncertainty surrounding the
production and decomposition of carboxylic acids in such air
parcels.
4.2 Altitude Range of 1-2 km
The altitude band <2 km was broken into the marine boundary
layer (<1 km) and the transition or cloud layer from 1-2 km. The
mixing ratio of acidic gases in these two layers arc shown as a
function of latitude in Figures 11 a-11 c. Nitric acid mixing ratios
wcrc smaller at < 1 km compared to 1-2 km altitude, except for the
most southerly data where they wcrc about equal. There does not
1 ooo
>
• 100
o
o
co 10
ß ß . .;.'..._..,•.•.....:
ß ;,.•: <,.:.•..••,.' .. •
ß" :•' "••.37.•. ';'": ß '
ß _.....: • ;%•: .':..'
, ... :
ß
ß
HCOOH = 12.0 x CH3CI- 31
1 ooo
>
•.. 100
O
• 10
' ' ' ' I ' ' ' ' ' " ' ' I
ß e ß
ß '. ß '.
.. .;•,..: :•:'_••;C:.•, -
,._:. <';• '.. ' • ;. ,:'•'i•,•iK•t•' ß .'
ß % ß 4'. ß
• ß ee ß ß ß ee
ß ß ß
ß
HNO 3 = 1.2 x 0 3 - 0.25
: i i i i I
lO
i i i i i i i i I
lOO
>
o
o
i
1 ooo
lOO
CH3COOH = 9.0 x CH3Cl - 23
i
6OO
CH3CI, pptv
Figure 6. Relationships between mixing ratios of acidic gases and
CH3C1 in the altitude range 2-12 km. The r 2 values for these
correlations were •0.35. Particularly for HCOOH and CH3COOH,
there is a general trend of enhanced mixing ratios at the largest O
values of CH3CI. These correlations potentially indicate an
important biomass burning source for acidic gases over the South co
Pacific basin. Note that the abscissas are also a logarithmic scale,
ranging from 500 to 650 pptv.
[Madronich and Calvert, 1990]. This result appears to support a
significant photochemical source of HCOOH rather than a predomi-
nance of decomposition of CH3COOH in aged biomass burning
plumes over the South Pacific. We can not rule out, however, some
photochemical production of CH3COOH as well. Because of
potentially complex (and unknown) chemistry in these biomass
1 ooo
>
• 100
o
o
co 10
HCOOH = 1.0 x 03 + 0.02
lO lOO
1 ooo
lOO
ß . .., &4. ...
. ' • , '.I, &.•v.•.•A._•; "'• J
ß .- ,._' ::_'.. •>.';'%--•d'• •'.•r;.._ ß •- ,
: '""-' ..,'" l '.IhT.,,t•Ji. llS•__"t•i•.'": .":[/
ß ..v.r-r,• ...•..t'.',p..,•_:__t-;.,ß'_, '. .
ß ß t'•;..' '
ß
CH•COOH = 0.84 x • + 0.30
... .
:
, , , , I • , , • , • , , I
'10 '100
03, ppbv
Figure 7. Relationships between mixing ratios of acidic gases and
O3 in the altitude range 2-12 km. The r 2 values for these correla-
tions were •0.40. These general correlations potentially indicate a
photochemical source for HCOOH and CH3COOH.
TALBOT ET AL.' ACIDIC GASES OVER THE SOUTHERN PACIFIC BASIN 5631
1 ooo
lOO
lO
, , , , , , ,,1 , , i , , i ill ' ' i i i i iii
10 100 1000
1000
100
10
_
ß ß ee
ß eeee el e ß ß
ß e ß ß e ß eel•L ß ß
ß ' .• .. ;2;.•;.}'."i' :,. •
ß ' ß ß . •..•.-.-•.;.,..•.•a,.j., •
._..: ;. •.•.. ß, ..'.... '••.,.3,,'•..,'. ß
i ..
ß '' '_.•.'7". ' ..
ß
HCOOH = 0.39 x PAN + 1.1
els ß ß ß
:
1
, , , , ,,,,I , , , , ,,,,i i , i i i 1111
10 100 1000
o
o
(D
(D
1 ooo
lOO
lO
PAN, pptv
Figure 8. Relationships between mixing ratios of acidic gases and
PAN in the altitude range 2-12 km. The r 2 values for these
correlations were •0.40. These general correlations potentially
indicate a photochemical source for HCOOH and CH3COOH.
Mixing ratios of PAN below • 5 pptv were observed in the altitude
range 2-4 km, where thermal decomposition of PAN was apparently
significant.
appear to be any systematic variation of HNO3 mixing ratios at <1
km altitude with latitude. Although the data are somewhat scattered,
HNO3 mixing ratios appear to increase in the transition layer going
from south to north latitude. This apparent trend is driven to a large
extent by the low values near 60øS. At midlatitudes and in the
tropics there was 2-3 times more HNO3 in the transition layer than
at <1 km altitude. This observation could be related to evaporation
of cloud droplets releasing HNO3 to the gas phase in the transition
layer. This process would be most active near the ITCZ, which is
where the largest mixing ratios of HNO3 were observed at this
altitude. In both layers, aerosol NO3' mixing ratios were about a
factor of 2 greater than those of HNO3 [Dibb et al., this issue],
presumably due to uptake of HNO3 onto sea-salt particles in the
marine boundary layer [Huebert, 1980] and possibly production of
aerosol-NO3' from cloud processing in the transition layer.
The mixing ratios of carboxylic acids in the marine boundary
layer over the South Pacific were about an order of magnitude less
than those previously determined from shipboard sampling in the
central North Pacific region [Ariander et al., 1990]. This probably
is due to the remoteness of the South Pacific basin from continental
areas and restricted downward mixing across the trade wind
inversion. Formic and acetic acid did not exhibit a difference in
their mixing ratios between the marine boundary and transition
layers. In the marine boundary layer they had the largest mixing
ratios noah of the ITCZ. This may reflect the closer proximity of
1 ooo
100
lO
i i i i i i i i I i
1
o
o
1 ooo
lOO
lO
1
o.1
' ' ' ' ' ' ' ' I '
ß
.ß e. e ß
.'
ß *At ' _•/I '1'• ß ß ß
ß .' ..•g,qi.' ß
ß .' ,,..'.:•.• ;•
" ß '.• . • ....
.... .' ,• •'.5 ,-....
e
e
I I I I I I I I I I
1
o
o
o
(D
1 ooo
lOO
lO
1
.lie
ß ß ß . v . I,..:•;'•=: ' .
ß ' .. l ;. "%". ' ß ß •,' .
,:. v' ...
.... •'.': .'. '•' ' .•;2 •:7,:. ' '"
ß . •'½•,.• r•.'• e• • '
ß ß ,.e ß ß ß ß ß ß ß .i.e
ß .'......'.•.•-• . ß
ß
ß
i i i i i i i i I i
1
C2H 2 / GO, pptv / ppbv
Figure 9. Distribution of the mixing ratios of acidic gases as a
function of the ratio C2H2/CO.
5632 TALBOT ET AL.' ACIDIC GASES OVER THE SOUTHERN PACIFIC BASIN
1400 1400 , , , , ,
05
0
1200
1000
8OO
6OO
4OO
2OO
ee ß
ß
e e ß
ß
e e
ß eel, ß ß
ß ß •
ee• ee e e• ß
'*
ß ½•o.•..,.... •.'
I , I ,
50 IO0
ß
e ß e e
I I
150 2(
1200
1000
>
o_ 800
6OO
400
200
)0 0
ßß
ß ß
ß
e e ß
ß •e e ß
• e •e•.•. • ß
..•.
, I , I , I ,
100 2oo 3oo
co, ppbv C2H2, pptv
Figure 10. Mixing ratio of HNO3 as a function of CO and C2H 2 for HNO3 >100 pptv.
I i
4OO 5OO
lOO
o
-8o
lOO
o
-8o
' I ' I ' I ' I ' I
Transition Layer (1-2 km) .
ß ß ß
•e ß e e' ß ß
ß .?
ß
, I , I , I , I , I
-60 -40 -20 0 20
' I ' I ' I ' I ' I
Marine Boundary Layer (<1 krn)
I '
(a.)
I i
40 60
•e
ß ß ß
e, : ß eee •
ß I e ß e ee ß e•e
I , I , I , I , I , I
-60 -40 -20 0 20 40
15O
120
>
0 60
c)
-i-
o
-8o
15o
12o
' I ' I ' I
Transition Layer (1-2 km)
, I , I , I , I , I , I
-60 -40 -20 0 20 40
i '
(b.)
' I ' I ' I ' I ' I ' I
ß
Marine Boundary Layer (<1 km)
ß ß
0 ' , I , I , I , I , I
-80 -60 -40 -20 0 20 40
Latitude Latitude
Figure 11. Latitudinal distribution of acidic gases in the marine boundary layer (<1 kin) and the overlying
transition layer (1-2 km). The solid lines represent a plot of the median mixing ratio value as a function of
latitude.
TALBOT ET AL.' ACIDIC GASES OVER THE SOUTHERN PACIFIC BASIN 5633
150
120
0
-80
150 ,
120
60
30
' I ' I ' el
Transition Layer (1-2 km)
I '
(c.)
' ß 4
• ß ß ß ß ß
, I , I , I , I , I , I
-60 -40 -20 0 20 40
,
' I ' I ' I ' I ' I ' I
Marine Boundary Layer (< 1 km)
0 I ,
-80 40 60
Latitude
Figure 11. (continued)
continental areas to this region leading to enhanced primary or
secondary production of these species. One difference between the
vertical distribution of HNO3 and the carboxylic acids in the two
surface layers is the similarity 'of the mixing ratios of HCOOH and
CH3COOH in these two altitude bands but not those of HNO3. This
observation may be explained by more extensive uptake of HNO3
onto sea salt aerosols compared to the carboxylic acids. Detailed gas
phase, cloud droplet, and aerosol measurements over the remote
oceans are needed to better understand this issue.
5. Conclusion
The distribution of acidic gases over the South Pacific basin in
austral springtime appears to be strongly influenced by emissions
from biomass burning, most likely occurring in Africa and Brazil.
Owing to the generally westerly flow of air over this area in the
middle and upper troposphere, elevated mixing ratios of acidic
gases and the presence of pollution plumes were concentrated in the
western and central South Pacific. The eastern Pacific basin was
relatively "clean" presumably due to chemical and physical removal
of these species during the long transit across the South Pacific.
The enhanced mixing ratios of acidic gases in pollution plumes
were coincident with relatively low mixing ratios of the combustion
tracers CO and C2H 2. This observation and their general correla-
tions with 03 and PAN suggest that the mixing ratios of acidic gases
may have been sustained by photochemical production in the
pollution plumes. Most likely this generation of acidic gases
occurred after the last scavenging event the air parcels encountered
since other soluble species such as aerosols were not enhanced in
these same plumes. The PEM-Tropics data document the
hemispheric-scale pollution of the southem troposphere by biomass
buming in austral springtime. The impact of these emissions on the
chemistry of the southern hemisphere troposphere appears to be
much greater than previously recognized.
Acknowledgments: We honor the outstanding unselfish contributions of
our colleague and friend John Bradshaw (deceased) to the overall success and
accomplishments of the GTE/PEM-Tropics airborne expedition. Excellent
support was provided by the ground and flight crews of the NASA Ames DC-
8 aircraft This research was supported by the NASA Global Tropospheric
Chemistry program.
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D. R. Blake and N.J. Blake, Department of Chemistry, University of
California, Irvine, CA 92717.
J. E. Dibb, E. M. Scheuer, and R. W. Talbot, Institute for the Study of
Earth, Oceans, and Space, University of New Hampshire, Durham, NH
03824. (e-mail: rwt•christa. unh.edu)
G. L. Gregory and G. W. Sachse, NASA Langley Research Center,
Hampton, VA.
S. T. Sundholm, Georgia Institute of Technology, Atlanta, GA 30332.
H. B. Singh, NASA Ames Research Center, Moffett Field, CA 94035.
(Received October 10, 1997; revised March 2, 1998;
accepted March 10, 1998.)
