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JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 104, NO. D5, PAGES 556%5583, MARCH 20, 1999
Pacific Exploratory Mission in the tropical Pacific:
PEM-Tropics A, August-September 1996
J. M. Hoell, 1 D. D. Davis, 2 D. J. Jacob, 3 M. O. Rodgers, 2 R. E. Newell, 4 H. E. Fuelberg, 5
R. J. McNeal, 6 J. L. Raper, 1 and R. J. Bendura 1
Abstract. The NASA Pacific Exploratory Mission to the Pacific tropics (PEM-Tropics) is the
third major field campaign of NASA' s Global Tropospheric Experiment (GTE) to study the
impact of human and natural processes on the chemistry of the troposphere over the Pacific basin.
The first two campaigns, PEM-West A and B were conducted over the northwestern regions of the
Pacific and focused on the impact of emissions from the Asian continent. The broad objectives of
PEM-Tropics included improving our understanding of the oxidizing power of the tropical
atmosphere as well as investigating oceanic sulfur compounds and their conversion to aerosols.
Phase A of the PEM-Tropics program, conducted between August-September 1996, involved the
NASA DC-8 and P-3B aircraft. Phase B of this program is scheduled for March/April 1999.
During PEM-Tropics A, the flight tracks of the two aircraft extended zonally across the entire
Pacific Basin and meridionally from Hawaii to south of New Zealand. Both aircraft were
instrumented for airborne measurements of trace gases and aerosols and meteorological
parameters. The DC-8, given its long-range and high-altitude capabilities coupled with the lidar
insmnnent in its payload, focused on transport issues and ozone photochemistry, while the P-3B,
with its sulfur-oriented instrument payload and more limited range, focused on detailed sulfur
process studies. Among its accomplishments, the PEM-Tropics A field campaign has provided a
unique set of atmospheric measurements in a heretofore data sparse region; demonstrated the
capability of several new or improved instruments for measuring OH, H2SO 4, NO, NO 2 , and
actinic fluxes; and conducted experiments which tested our understanding of HO x and NO x
photochemistry, as well as sulfur oxidation and aerosol formation processes. In addition, PEM-
Tropics A documented for the first time the considerable and widespread influence of biomass
burning pollution over the South Pacific, and identified the South Pacific Convergence Zone as a
major barrier for atmospheric transport in the southern hemisphere.
1. Introduction
During the early part of this decade, NASA through its Earth
Sciences Program initiated the Pacific Exploratory Missions
(PEM) to improve scientific understanding of human influences
on the tropospheric chemistry over the Pacific Ocean. The PEM
studies were conducted as part of NASA's Global Tropospheric
Experiment (GTE) and, to date, have consisted of three airborne
campaigns. PEM-West A and B were conducted over the north-
western Pacific during September-October 1992 (PEM-West A
[Hoell et al., 1996]) and February-March 1994 (PEM-West B
[Hoell et al., 1997]) to study Asian outflow during contrasting
meteorological conditions. The third, PEM-Tropics A, was con-
ducted during August-September 1996 and focused on the
southern tropical regions of the Pacific Ocean, extending zonally
across the entire Pacific basin and meridionally from Hawaii to
INASA Langley Research Center, Hampton, Virginia.
2Georgia Institute of Technology, Atlanta.
3Harvard University, Cambridge, Massachussetts.
4Massachusetts Institute of Technology, Cambridge, Massachussetts.
5Florida State University, Tallahassee.
6NASA Headquarters, Washington, D.C.
Copyright 1999 by the American Geophysical Union.
Paper number 1998JD 100074.
0148-0227/99/1998JD100074509.00
south of New Zealand. A fourth campaign, PEM-Tropics B, is
scheduled for March/April 1999. Like PEM-West A and B, the
combined PEM-Tropics A and B campaigns will provide obser-
vations during contrasting seasons (i.e., dry versus wet). This
paper describes the experimental design of the PEM-Tropics A
campaign and summarizes some of the results given in the
companion papers in this issue.
One of the most important issues in global tropospheric chem-
istry is the sensitivity of the oxidizing power of the troposphere to
human influence. From the perspective of global tropospheric
chemistry, the Pacific basin is a very large chemical reaction
vessel. From Peru to Borneo, it stretches 17,700 km in the east-
west direction; the distance from the Antarctic ice shelf to Alaska
is 13,300 kin. The Pacific basin covers 35% of the total surface
area of the Earth, and 50% of the ocean surface. Since much of
the Pacific basin is far removed from continental influences,
observations in this region can provide sensitive indicators of the
global-scale impact of human activity on the oxidizing power of
the troposphere.
There also is a need to improve our understanding of atmos-
pheric sulfur chemistry over the Pacific. Sulfate aerosols affect
the Earth's radiative balance through direct backscattering of solar
radiation and indirectly as cloud condensation nuclei (CCN).
CCN, themselves products of aerosol growth processes, are be-
lieved to originate through nucleation processes involving gas
phase H2SO 4 that is produced from the oxidation of SO 2 by OH.
Sulfate and SO 2 over the Pacific may originate from a number of
5567
5568 HOELL ET AL.: PACIFIC EXPLORATORY MISSION IN THE TROPICAL PACIFIC
sources including long-range transport of anthropogenic pollution,
marine biogenic releases of dimethlysulfide (DMS), and volcanic
emissions. The relative contributions of these sources over differ-
ent regions of the Pacific are still poorly known, representing a
serious limitation in our ability to evaluate the role of sulfur in
global climate change.
Prior to the PEM campaigns, there were little chemical data for
the southern tropical Pacific region. The Global Atmospheric
Measurements Experiment on Tropospheric Aerosols and Gases
(GAMETAG) aircraft missions in 1977 and 1978 [Davis, 1980]
provided some early data over the western part of the Pacific.
However, these campaigns were restricted by the low ceiling and
limited endurance of the aircraft, and also by the instrumentation
available at the time. The more recent STRATOZ Ill [Drummond
et al., 1988] and PEM-West A and B missions have provided de-
tailed data along the South American and Asian rims of the
southern Pacific basin, respectively and ozonesonde and CO
measurements have been made from island sites during the Sea-
Air Exchange (SEAREX) program. Even so, there were virtually
no data for the southeast quadrant of the Pacific basin extending
from the international dateline to the South American coast. The
PEM-Tropics A observations (and those from the forthcoming
PEM-Tropics B campaign) provide an extensive set of
atmospheric measurements in a heretofore data sparse region.
Data from this mission (e.g., airborne chemical measurements,
meteorological and ozonesonde obsen, ations, and model
products), along with data from all previous GTE field campaigns,
have been archived in the Langley Distributed Active Archive
Center (http://eosweb.larc.nasa. gov) and/or on the GTE Home
Page at http://www-gte.larc.nasa.gov.
2. Implementation of the PEM-Tropics Campaign
The major objectives of PEM-Tropics (phases A and B) are to
provide baseline data over the southern Pacific Ocean for gases
important in controlling the oxidizing power of the atmosphere,
including 0 3, H20, NO, CO,.. and hydrocarbons, to improve
scientific understanding of the factors controlling the concentra-
tions of these gases, and to assess the resulting sensitivity of the
oxidizing power of the atmosphere to anthropogenic and natural
perturbations. In addition, PEM-Tropics A had three secondary
objectives: (1) to survey the concentrations of aerosol precursors
and ultrafine aerosol particles over the southern Pacific basin;
(2) to improve our understanding of sulfur gas-to-particle
formation over the region; and (3) to provide detailed latitude-
altitude transects of long-lived gases for the evaluation of global
tropospheric models.
To address these objectives, NASA, through a competitive
process (Announcement of Opportunity), selected investigators
who provided measurements and/or model analyses during PEM-
Tropics A. The PEM-Tropics Science Team consisted of the
Principal Investigator(s) from each investigative group. The
Mission Scientists and Mission Meteorologists, also competitively
selected, led the science team in developing the detailed design of
the PEM-Tropics A campaign. Tables 1 a and lb list the DC-8 and
P-3B aircraft investigations, respectively, with the salient mea-
surement characteristics for each instrument. Table 2 lists the
meteorological and modeling investigations, along with the
Mission Meteorologists and Scientists. The aircraft instrumenta-
tion layout is included in Figures l a and lb for the DC-8 and
P-3B, respectively.
The NASA DC-8 aircraft has a nominal ceiling of 12 km, a
cruising speed of 800 kin/h, and, as configured during PEM-
Tropics A, a 10 hour flight endurance. The P-3B aircraft has a
nominal ceiling of 8 km, a cruising speed of 500 kin/h, and, as
configured in PEM-Tropics A, a nominal 8 hour endurance.
These differing characteristics, coupled with the instrument pay-
load of each aircraft, favored use of the DC-8 for long-range
transport studies and high-altitude observations and favored the
P-3B for lower-altitude, process-oriented studies. The differential
absorption lidar (DIAL) aboard the DC-8 provided vertical pro-
files of ozone and aerosol above and below the aircraft [Browell
et al., 1996], thereby enhancing transport studies conducted by the
DC-8. The DIAL profiles provided a two-dimensional perspec-
tive of the structure of the troposphere in which the in situ mea-
surements aboard the DC-8, and in some cases the P-3B, were
recorded. The DIAL profiles also provided real-time guidance
for adjusting the DC-8 flight tracks to exploit interesting
measurement opportunities encountered in-flight.
The P-3B instrument payload included measurements of a
unique suite of sulfur species [DMS, SO 2, methane sulfonic acid
(MSA) [gas], H2SO 4 [gas], non-sea-salt sulfate (NSS), and meth-
ane sulfonate (MS)], along with aerosol composition and size
distributions, including ultrafine particles. These measurements,
combined with the capability for reliable observations of OH, via
the Chemical Ionization Mass Spectrometry (CIMS) technique
[Eisele and Tanner, 1991], provided a unique opportunity for the
focused sulfur process studies that were conducted by the P-3B.
The broad design of the PEM-Tropics A campaign employed a
series of flights from remote operational sites in the South Pacific
basin. Figures 2a and 2b show these sites, along with flight tracks
of the DC-8 (Figure 2a) and the P-3B (Figure 2b) aircraft. The
flight numbers in Figure 2 are keyed to Tables 3a and 3b in which
the major focus of each flight is summarized. As part of the over-
all design of PEM-Tropics, measurements obtained from the
NASA DC-8 and P-3B during the intensive deployment period
were augmented by ozonesonde observations from operational
launch sites at Easter Island, and Lauder, New Zealand (also
shown in Figure 2), and by stations established by the GTE
Project at Papeete, Tahiti, and American Samoa. Figure 2 also
shows the ozonesonde release site on Fiji which was established
by the GTE project in August 1997 to support the PEM-Tropics B
mission. Ozonesondes at all the sites, except Fiji, were released at
a rate of one per week, beginning in August 1995, approximately
1 year prior to the aircraft campaign in 1996. Releases at all sites
are scheduled to continue through October 1999, to encompass the
upcoming PEM-Tropics B mission, and a final survey of the dry
season period. During the aircraft deployment periods, the sites
established by GTE and the Lauder station increased their launch
rate to two per week. It is anticipated that the ozonesonde data
from Fiji and Samoa will provide an indicator of the seasonal
changes in the gradients of trace gases that were observed to exist
across the South Pacific Convergence Zone (SPCZ) during PEM-
Tropics A [Gregory et al., this issue].
Meteorological support for real-time flight planning and post-
mission analyses to assist in coupling air mass transport and
chemistry is a critical element of GTE field campaigns. Meteoro-
logical support for in-field flight planning during PEM-Tropics A
was provided by a Mission Meteorologist stationed with each air-
craft, with supporting activities at the Massachusetts Institute of
Technology (MIT) and Florida State University (FSU). Meteo
France also provided significant support by allowing direct access
to European Centre for Medium-Range Weather Forecasts
(ECMWF) gridded data and by providing valuable consultations
at their Papeete, Tahiti, facilities. Satellite images received and
stored at MIT, along with derived products from FSU and MIT
HOELL ET AL.: PACIFIC EXPLORATORY MISSION IN THE TROPICAL PACIFIC
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5570 HOELL ET AL.' PACIFIC EXPLORATORY MISSION IN THE TROPICAL PACIFIC
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HOELL ET AL.: PACIFIC EXPLORATORY MISSION IN THE TROPICAL PACIFIC 5575
Table 2. PEM-Tropics Modeling and Meteorological Studies
Principal Investigator (s) Institution Study
S. C. Liu Georgia Institute
of Technology
J. Rodriguez and Atmospheric Environmental
D. S ze Research, Inc.
D. D. Davis Georgia Institute
of Technology
R. E. Newell Massachusetts Institute
of Technology
Henry Fuelberg Florida State University
Daniel Jacob Harvard University
T. Krishnamufti
Don Lenschow
Florida State University
National Center for
Atmospheric Research
three-dimensional transport,
photochemical model
point-by-point photochemical model;
steady state diurnal model and trajectory
photochemical process model
instantaneous photostationary state and
time dependent box model/
comission scientist/P-3B
chemical and meteorological analysis
comission meteorologist/P-3B
real time and postmission trajectory
model analysis;
comission meteorologist/DC-8
photochemical point model;
comissoin scientist/DC-8
Florida State University global
spectral model
analysis of trace as flux measurements
(i.e., streamline and trajectory analyses at various pressure levels,
potential vorticity, etc.), were transmitted to the field operations
sites at Christmas Island and Easter Island using a portable high-
speed satellite-telephone data link. For operations at Christmas
Island, and to a lesser extent at Easter Island, this data link pro-
vided the only access to meteorological data. A combination of
the satellite data link, faxed data products, and the use of local
meteorological facilities were used during operations at Papeete,
Tahiti, Christchurch, New Zealand, Nadi, Fiji, and Guayaquil,
Ecuador.
Integration of instruments aboard the P-3B and DC-8 aircraft
occurred at the NASA Wallops Flight Facility, Wallops Island,
Virginia, and the NASA Ames Research Center (ARC), Moffett
Field, California, respectively. The P-3B deployed approximately
1 week prior to the DC-8's departure, with transit flights to
Christmas Island via ARC and Hawaii (see Table 3b). The origi-
nal flight plans called for two local P-3B flights at Christmas
Island; however, the P-3B experienced aircraft problems after the
first local flight and returned to Hawaii for repairs. After com-
pleting integration and test flights, the DC-8 departed ARC for
Tahiti via Hawaii. The DC-8 and P-3B arrived in Tahiti on the
same day. During this transit flight to Tahiti, both aircraft con-
ducted substantial vertical profiling to document interhemispheric
gradients of gases, along with a coordinated low-altitude flyby of
Christmas Island to document concentrations of several key
constituents measured several days earlier during the P-3B's local
flight from Christmas Island.
Flight operations from Tahiti included three local flights by
each aircraft, followed by transit flights by both the DC-8 and the
P-3B to Easter Island. Each aircraft conducted two local flights
from Easter Island after which the P-3B traveled to Guayaquil,
Ecuador, and the DC-8 traveled to Christchurch, New Zealand,
with a stopover and local flight from Tahiti. While the DC-8 op-
erated out of New Zealand, the P-3B conducted three local flights
from Guayaquil to examine interhemispheric exchange and the
primary productivity rich oceanic region near the equator prior to
returning to the NASA Wallops Flight Facility. The DC-8's local
flight out of Christchurch extended south to the Antarctic coast to
investigate meridional gradients and the influence of transport
from high latitudes on the tropical troposphere. From
Christchurch, the DC-8 flew to Fiji, where three local flights were
conducted, followed by the return flight to ARC via Tahiti.
3. Overview of PEM-Tropics Results
This section provides a brief synopsis of the salient results
from PEM-Tropics A that are discussed in detail in the companion
papers of this issue. Among its accomplishments, the PEM-
Tropics A field campaign has provided a unique set of atmos-
pheric measurements in a heretofore data sparse region; demon-
strated the capability of several new or improved instruments for
measuring OH, H2SO 4, NO, and NO 2, and actinic fluxes; and
conducted experiments which tested our understanding of HO x
and NO x photochemistry, as well as for sulfur oxidation and
aerosol formation processes. In addition, PEM-Tropics A docu-
mented for the first time the considerable and widespread influ-
ence of biomass burning pollution over the South Pacific, and
identified the South Pacific Convergence Zone (SPCZ) as a major
barrier for atmospheric transport in the southern hemisphere.
3.1. New/Improved Measurements
The DC-8 instrument payload included a new generation
photo-fragmentation two-photon laser-induced fluorescence (PF-
TP-LIF) instrument for measuring NO and NO 2 [Sandholm et al.,
1997; J. Bradshaw etal., 1999]. The improved sensitivity of this
instrument provided, for the first time, accurate NO measurements
at sub-pptv concentrations. Further, a high flow rate sample inlet
system was employed to minimize the possibility that complex
nitrogen oxides would decompose in the inlet system, which had
been speculated to be a problem in previous measurements of NO 2
by this group [ Crawford etal., 1996]. The PEM-Tropics A data
showed that comparisons between measured NO/NO 2
concentration ratios and those computed with a photochemical
steady state model agreed to within 30% [Schultz et al., this
issue]. This finding is in sharp contrast to results from the GTE
PEM-West A mission, where the deviation between predicted and
observed NO 2 was nearly a factor of 4 [Crawford etal., 1996].
The agreement between modeled and measured NO/NO 2 is
attributed to the improved sensitivity and the high flow rate inlet.
Moreover, measurements by the PF-TP-LIF instrument revealed
that NO concentrations in the marine boundary layer (MBL) were
frequently below 1 pptv.
Also aboard the DC-8 was a new airborne actinic flux spec-
troradiometer [Shetter and M•iller, this issue]. This system
consisted of nadir and zenith 2 pi str radiometers, and provided
5576 HOELL ET AL.: PACIFIC EXPLORATORY MISSION IN THE TROPICAL PACIFIC
PAN, PPN, CaCI4 Acetone,
Methanol, Ethanol, Acetaldehyde, Nitrates
(NASA, Ames)
NMHC/Organic Nitrates
(UCl/NCAR)
Storage and Weather Receiver
NO, NO2
(Ga. Tech.)
Mission Director Console
and Power Distribution
Navigation Station
and Housekeeping
Electronics
Exit
Exit
03, Aerosol Profiles
(NASA, LaRC)
Crew Sleeper
Exit
il
il
Experiment
Storage
Exit
Exit Exit
Exit
H202, CH3OOH, CH20
(Univ. of RI)
Photolysis Rates
(NCAR)
Storage Rack
H20, CO, CH4, N20, CO2
(NASA, LaRC)
03, CN, H20, Stormscope
(NASA, LaRC)
Aerosols, Organic Acids, HNO3
(Univ. of NH)
SO2, DMS
(Drexell U.)
DADS
(NASA, Ames)
GTE Data Distribution
43 Seats Available
Experiment
Storage
Figure la. Instrument layout on the NASA DC-8 aircraft during the PEM-Tropics A mission.
NSS, MSA, HNO3
(Huebert U HI.)
• Aerosols
(Univ. HI.) ' NMHC/Organic Nitrates
(Univ. of CA., Irvine/NCAR)
-- Exper,ment
Storage
H2SO4, MSA,
CO, CO2, CH4
DMSO (Univ. Mich) Project Data (GA. Inst. Tech.)
OH, DMSO2 (NASA, LaRC) SO2, DMS
(NCAR/GA Inst. Tech) (Drexel, Univ.)
NO, 03, j(O'd)j(NO2)
(NASA, LaRC) H202, CH300H, CH20
TAMMS (Univ. RI)
(NASA, LaRC)
Figure lb. Instrument layout on the NASA P-3B aircraft during PEM-Tropics A.
HOELL ET AL.: PACIFIC EXPLORATORY MISSION IN THE TROPICAL PACIFIC 5577
50N
40N
30N
20N
10N
EQ
10S
20S
30S
40S
50S
60S
, ,
, ,
HaWaii
......................... _, ........ ,,_ ....... ,• .......
,
, ,,
,
......
l
l
,
,
Ne,iw
ze-a-la-tid--
' ,
: 17,D '
....... !-1-6D .... '• ....... '-
Samoa
!
!
!
t4D
19D
11D
i
...........
,
! ,,
i i
i i
--r ...............
,
,
: ,
, ,
, ,
! ,
Easter IS.
,
9D'
,
:,•t• ::Ozone So,' nde !Launch Site
, , , , , ,
150W 130W 110W 90W
150E 170E 170W 70W 50W
Figure 2a. Flight tracks for the DC-8 aircraft during the PEM-Tropics A campaign.
50N
'0•' Ami•s Resear•ch ,
40N i-Cen•er- !;2•3P_
30N ................................................................. _5__• ...... ' '
, : Flight '
, , , FaCility
20N .................................. i ........ !- , -Hawai• ..................... , ........... ! ..............
, , , ',
, , , ,
10N .................................. i ........ i---81•,'- ' ' '
: ' : ; :19
se ........................... -C--h--r!-s-t--m--•-s--!-S'-• - , ' , -•[7P '
10S i Am.i Samoa 1 P
,
' ' , T,,ahiti I ! i ! 1
20S F!ji ' •:'•'•- .............. ? ....... ! ....... ! ........ :: ....... 16P
30S ............................... [ ...............
'
4os i :: _, ___,_
60S. i i i i i ......
150E 170E 170W 150W 130W 110W 90W 70W
50W
Figure 2b. Flight tracks for the P-3B aircraft during the PEM-Tropics A campaign.
5578 HOELL ET AL.' PACIFIC EXPLORATORY MISSION IN THE TROPICAL PACIFIC
Table 3a. Summary of PEM-Tropics DC-8 Aircraft Flights
Flight State Date, Start Time, Stop Time,
UT UT UT
3D Aug. 30, 1996 1607:36 2350:00
4D Aug. 31, 1996 1908:59 0435:30
5D Sept. 3, 1996 1851:08 0320:01
6D Sept. 5, 1996 1954:11 0325:11
7D Sept. 7, 1996 1702:38 2312:00
8D Sept. 10, 1996 1231:01 1953:28
9D Sept. 11, 1996 1603:58 2335:38
10D Sept. 14, 1996 1758:40 0011:52
lid Sept. 16, 1996 1905:21 0257:06
12D Sept. 18, 1996 1907:20 0448:48
13D Sept. 21, 1996 2010:45 0617:14
14D Sept. 23, 1996 2237:55 0628:05
15D Sept. 26, 1996 1916:38 0504:12
16D Sept. 28, 1996 2119:33 0456:16
17D Oct. 1, 1996 2107:48 0458:41
18D Oct. 3, 1996 1951:44 0327:54
19D Oct. 5, 1996 1854:38 0341:42
Major Focus of Flight
NASA ARC to Hickam, Hawaii:
latitudinal profile
Hickam, HI to Tahiti:
latitudinal profile and ITCZ
Tahiti Local Flight number 1'
equatorial photochemistry
Tahiti Local Flight number 2:
biomass burning profile
Tahiti to Easter Island:
longitudinal profile
Easter Island local flight number 1'
latitudinal profile (north)
Easter Island local flight number 2:
latitudinal profile (south)
Easter Island to Tahiti:
longitudinal profile
Tahiti Local Flight number 3'
equatorial photochemistry
Tahiti to Christchurch, New Zealand:
latitudinal profile and SPCZ
Christchurch local flight:
latitudinal profile (high latitudes)
Christchurch to Fiji:
biomass burning and SPCZ
Fiji local flight number 1:
tropical warm pool
Fiji local flight number 2:
SPCZ
Fiji local flight number 3:
SPCZ
Fiji to Tahiti:
biomass burning/Samoa profile
Tahiti to NASA ARC:
latitudinal profile
,
spectral radiance in 1 nm steps from 282 nm to 330 nm (UV-B
region) and in 2 nm steps from 330 nm to 420 nm (UV-A region)
at 30 seconds intervals. The accuracy of the actinic flux was
stated as approximately +11.5% in the UV-B and +8% in the
UV-A range. Shetter and Miiller [this issue] report that
uncertainties in the resulting photolysis frequencies for 0 3, NO2,
HONO, CH20 , H202, CH3OOH , HNO3, PAN, CH3NO3,
CH3CH2NO 3, and acetone vary between +15% and +20%.
Aboard the P-3B, a new airborne Chemical Ionization Mass
Spectrometer (CIMS) was fielded to measure OH, H2SO4, and
MSA. Measurements reported by Mauldin et al. [this issue (a,b)]
throughout PEM-Tropics A, and particularly for the local P-3B
flight from Christmas Island (discussed in the next section), pro-
vide convincing evidence of the capability of CIMS for airborne
measurement of OH and H2SO 4.
3.2. Sulfur Photochemisty
One of the reasons for selecting Christmas Island as an opera-
tional site during PEM-Tropics A (and for the 1999 PEM-Tropics
B campaign) is the almost ideal environmental conditions that
exist in this region for an atmospheric sulfur photochemistry ex-
periment. Specifically, Christmas Island is located where the
prevailing meteorological conditions are relatively uniform with
little, if any, anthropogenic contributions to the local sulfur
budget. These conditions were highlighted in results from earlier
ground-based observations [Bandy et al., 1996] which showed a
persistent anticorrelation between SO 2 and DMS over several di-
urnal cycles. The local flight from Christmas Island (P-3B flight
7P) was designed to exploit these conditions and the unique set of
sulfur and photochemistry instruments aboard the P-3B aircraft.
The specific objective of the Christmas Island flights was to
study the evolution of DMS oxidation chemistry in a common air
mass from before sunrise through early afternoon. This was
achieved using a Lagrangian sampling pattern within, and just
above, the boundary layer. Simultaneous measurements of OH,
DMS, SO2, MSA (gas), H2SO4(gas), US (mSA-particle), NSS,
and aerosol size number distribution, as well as critical
meteorological parameters, permitted one of the most intensive
examinations yet of the de tailed chemical processes involved in
the oxidation of DMS via hydroxyl radicals. Results from this
flight [Davis et al., this issue] revealed distinct presunrise minima
in the concentrations of OH, H2SO4, and SO 2 that increase to a
maxima at the end of the flight in the early afternoon. Concurrent
with the increase in these species was a decrease in DMS
beginning near sunrise. Concentrations of OH were observed to
range from sunrise values near 105/cm 3 to noon time maximum
values of 8x106/cm 3. Davis et al. [this issue] placed the overall
efficiency for conversion of DMS to SO 2 for the conditions of the
Christmas Island flight at 75%. This efficiency strongly supports
the notion that SO 2 was the dominant precursor to H2SO 4 in this
marine environment. Equally important, these investigations
showed that most of the SO 2 is converted to sulfate via
heterogeneous processes as was found to be true also for the
formation of MS. Using concurrent measurements of the key
controlling species to constrain their model calculations, Davis et
al. [this issue] report that the agreement between model
simulations and observations for OH ranges from 5 to 20%. This
HOELL ET AL.: PACIFIC EXPLORATORY MISSION IN THE TROPICAL PACIFIC 5579
Table 3b. Summary of PEM-Tropics P-3B Aircraft Flights
Flight State Date, Start Time, Stop Time, Major Focus of Flight
UT UT UT
4P Aug. 15, 1996 0724:19 0119:55
5P Aug. 18, 1996 1817:04 0231:02
6P Aug. 21, 1996 2124:12 0447:29
7P Aug. 24, 1996 1434:28 2357:18
8P Aug. 26, 1996 1816:40 2306:46
9P Aug. 30, 1996 0328:59 0746:05
10P Aug. 31, 1996 2015:47 0525:38
11P Sept. 3, 1996 1929:35 0359:04
12P Sept. 5, 1996 1929:11 0318:13
13P Sept. 7, 1996 1902:33 0248:15
14P Sept. 10, 1996 1236:18 2013:53
15P Sept. 11, 1996 1806:23 0125:52
16P Sept. 14, 1996 1453:05 2313:33
17P Sept. 18, 1996 1353:05 2151:18
18P Sept. 22, 1996 1428:38 2243:20
19P Sept. 23, 1996 1559:51 2314:16
20P Sept. 25, 1996 1428:11 2221:12
21P Sept. 26, 1996 1501:44 1902:41
NASA Wallops to NASA ARC
NASA-ARC to Hickam, Hawaii
stratocumulus photochemistry
Hickham, HI to Christmas Island
latitudinal survey and TICZ gradient
Christmas Island local flight
0 3 photochemistry and sulfur
oxidation cycle
Christimas Island to Hickam, HI
latitudinal survey of trace gases
Hawaii local test flight
Hickam, Hawaii to Tahiti
marine upwelling and latitudinal
survey trace gases
Tahiti local flight number 1:
marine upwelling and latitudinal
survey trace gases
Tahiti local flight number 2:
ozone photochemistry and latitudinal
survey trace gases
Tahiti to Easter Island
longitudinal survey of trace gases
Easter Island local flight number 1:
latitudinal survey of trace gases
Easter Island local flight number 2:
longitudinal survey and continental
outflow
Easter Island to Guayaquil, Ecudaor
longitudinal survey and continental
outflow
Guayaquil local flight number 1:
marine upwelling and continental
outflow
Guayaquil local flight number 2:
marine upwelling and continental
outflow
Guayaquil local flight number 3:
sulfur oxidation cycle and continental
outflow
Guayaquil to New Orleans
ITCS profiles
New Orleans to NASA Wallops
suggests that for the tropical MBL, the photochemical
mechanisms in current models represent those operating in the
real atmosphere.
Directly related to the sulfur/photochemical study conducted
on Christmas Island are the observations of new particle formation
reported by Clarke et al. [this issue] during three distinctly
different environmental conditions. One of these, cloud outflow,
is consistent with previous observations suggesting cloud outflow
as a major source for aerosol nucleation. The other two
environmental conditions included (1) aged and scavenged air ex-
hibiting characteristics of long-range transport and the influence
of combustion and (2) well scavenged air within the boundary
layer over productive waters high in DMS. Clarke et al. [this
issue] report that all three environments exhibited similar charac-
teristics: low aerosol surface areas, elevated sulfuric acid, and
enhanced water vapor concentrations. In the case of the MBL,
Clarke et al. [1998] report results for a tropical MBL setting in
which a nucleation event, forming new ultrafine particles was for
the first time directly linked to DMS oxidation via OH through the
direct observation of the intermediates SO 2 and H2SO4(g ). Clarke
et al. [this issue] note that while the nucleation observed in the
marine boundary layer may have occurred under rare conditions, it
nevertheless demonstrates that nucleation can occur when surface
areas are sufficiently small and concentrations of water vapor and
sulfuric acid are sufficiently large.
3.3. Sulfur Distribution/Sources in the Pacific
Thornton et al. [this issue] combined measurements of SO 2
obtained during the PEM-Tropics A, the PEM-West A and B cam-
paigns, and the First Aerosol Characterization Experiment
(ACE 1) to produce a data set containing 4679 observations of
SO 2 at altitudes ranging from 50 m to 12 km and covering a geo-
graphicalregion from 60 ø N to 72 ø S and 110 ø E to 80 ø W. This
combined data set showed that in the northwestern Pacific, an-
thropogenic sources from eastern Asia dominate the sulfur chem-
istry in the lower troposphere out to distances of about 1500 kin,
and much farther at mid to upper troposphere altitudes, resulting
in a significant gradient between the northern and southern hemi-
spheres. Because of the absence of significant anthropogenic
sources of SO 2 in the southern hemisphere, Thornton et al. [this
issue] notes that volcanic sources from east Asia may be the
dominant source of SO 2 in the mid and upper troposphere of the
southern hemisphere, while DMS is a significant source of SO 2
only in the tropical marine boundary layer.
5580 HOELL ET AL.: PACIFIC EXPLORATORY MISSION IN THE TROPICAL PACIFIC
3.4. Trace Gas Distributions and Biomass Burning
An important aspect of PEM-Tropics A was to determine the
extent of biomass burning influence over the remote South Pacific
during the dry season of the austral tropics. The Transport and
Atmoshperic Chemistry Near the Equatorial Atlantic (TRACE A)
mission conducted in the same season had previously demon-
strated a strong biomass burning influence over the South
Atlantic, downwind of Brazil and southern Africa [Fishman etal.
1996]. A remarkable finding of PEM-Tropics A was the
pervasiveness of biomass burning plumes and their impact on
trace gases throughout the southern Pacific region. Flights from
Fiji, New Zealand, Tahiti, Easter Island, and Guayaquil frequently
encountered layers of biomass burning pollution in the 2-12 km
column [Gregory etal., this issue; Schultz etal., this issue; Talbot
etal., this issue; Fuelberg etal., this issue]. Ozone mixing ratios
in these layers frequently exceeded 80 ppbv and were associated
with high mixing ratios of CO and other tracers of biomass burn-
ing (C2H2,C2Ht,CH3C1, CH3Br ). Urban pollution tracers
(e.g., C2C14) were not enhanced, and hydrocarbon data indicated
that these pollution layers were 1-3 weeks old. The
enhancement ratio typically was greater than 1, consistent with
chemical production of O 3 and chemical decay of CO during
aging. Back trajectory analyses suggest that most of these layers
originated from fires in Africa and South America and were trans-
ported to the South Pacific by strong westerly flow at subtropical
latitudes [Fuelberg etal., this issue], although analyses of
advanced very high resolution radiometer (AVHRR) satellite
images indicate that fires in Indonesia and Australia may also be
sources of some of the layers [Olson etal., this issue]. Of particu-
lar interest is the analysis of climatological data from 1986 to
1996 by Fuelberg et aL [this issue] indicating that the PEM-
Tropics mission period was representative of the previous
11 years. Ozonesonde data for 1995-1997 at the PEM-Tropics
sites, and earlier ozonesonde data at Samoa and New Zealand,
also confirm that 1996 was not anomalous (S. J. Oltmans and
J. A. Logan, private communication, 1998)o
Analysis of acidic gases (e.g., HNO 3, HCOOH, and
CH3COOH ) measured on the DC-8 aircraft showed that over the
altitude range of 2-12 km, but particularly in the 3-7 km range,
air parcels were frequently encountered within 15ø-65 ø S latitude
with mixing ratios up to 1200 pptv [Talbot etal., this issue].
Talbot etal. [this issue] suggest the correlation of the acidic gases
with CH3C1, PAN, and O 3 and the absence of correlation with
common industrial tracer compounds such as C2C1 or CH3CC13
indicate a photochemical and biomass burning source for the
plumes. Talbot etal. [this issue] also report that the ratio of
C2H2/CO was typically in the 0.2-2.2 parts per trillion by volume
(pptv)/ppbv range indicating relatively aged air masses in the
plumes, consistent with the trajectory analysis reported by
Fuelberg etal. [this issue].
Dibb etal. [this issue] report on the distribution of aerosol-
associated soluble ions measured aboard the NASA DC-8. The
authors found low mixing ratios of all ionic species throughout the
free troposphere, suggesting that the soluble ions that might have
been expected in air masses influenced by biomass burning had
been scavenged by precipitation. They note, however, that the
activity of 7Be frequently exceeded 1000 fCi/m 3 throughout the
troposphere, indicating that the scavenging of soluble ions had oc-
curred far upwind of the DC-8 sampling region. These observa-
tions are again consistent with the trajectory analysis presented by
Fuelberg etal. [this issue] and the chemical analysis by Talbot
etal. [this issue] suggesting that the plumes originated from
sources well west of the DC-8 sampling region. Dibb et al. also
report decreasing mixing ratios of NH 4+ with increasing altitude
through out the PEM-Tropics A study area, consistent with
shipboard sampling indicating strong NH 3 emissions from the
equatorial Pacific. It interesting to note that observations during
the P-3B flights along the west coast of South America indicated
that some fresh biomass burning plumes originated from the south
American continent and were transported westward by the trade
winds.
An important question relative to the long-range transport of
biomass burning emissions into the tropical Pacific is the impact
on the general background concentrations of trace gases. Statisti-
cal analysis of the PEM-Tropics data showed that the influence of
biomass burning extended beyond the plumes and pervaded the
regional atmosphere, as seen, for example, in the strong positive
correlation of ozone with CO found for the ensemble of PEM-
Tropics data [see McNeal etal., 1998, Figure 9]. A further indi-
cation of the level of enhancement for 03 comes from the model-
ing analysis by Schultz etal. [this issue] showing that advection of
ozone associated with biomass burning increases the average
ozone concentration throughout the troposphere by about
7-8 ppbv. Further, during both TRACE A [Singh etal., 1996]
and PEM-Tropics A [Schultz etal., this issue], biomass burning
was found to be the dominant source of atmospheric PAN.
An interesting consequence of the layering associated with the
biomass burning plumes observed in PEM-Tropics A was dis-
cussed by $toller etal. [this issue]. By analyzing in situ airborne
measurements obtained during PEM-Tropics A, PEM-West A and
B [Newell etal., 1996; Wu etal., 1997], and TRACE A [Collins
etal., 1996], Stoller etal. [this issue] concluded that within the
troposphere "Atmospheric layers are ubiquitous," occupying
approximately one fifth of the atmospheric vertical extent sam-
pled. During PEM-Tropics A, the most commonly observed layers
exhibited characteristics of air originating from biomass burning
sources. The authors note that the extensive layering observed
during these GTE field missions can have an important impact on
atmospheric heating, and that the rates of photochemical processes
in these layers will be different from those calculated using
"average" atmospheric mixing ratios.
3.5. Trace Gas Distributions
The analysis by Gregory etal. [this issue] illustrates the influ-
ence that convergence associated with the ITCZ and SPCZ have
on chemical characteristics of their respective geographical re-
gions. The authors analyzed in situ measurements obtained north
and south of the ITCZ and the SPCZ, respectively. They report
that in each region the air north and south of the respective con-
vergence zones has distinctively different signatures indicative of
the source regions. For example, the air north of the 1TCZ exhib-
its a modest urban/industrial signature, while air south of the
ITCZ and north of the SPCZ is relatively clean. Consistent with
other observations reported above, the chemical signature of the
air south of the SPCZ was noted to be dominated by combustion
products associated with biomass burning. Gregory etal. [this
issue] noted that the resulting chemical gradients across each zone
was more pronounced below 5 km in consistent with the strong
low-level convergence that is characteristic of each zone,
becoming much less pronounced at higher altitudes. Back
trajectory analysis by the FSU group [Fuelberg etal., this issue]
also showed that much of the tropical air having low ozone mix-
ing ratios originated east of the observation region and had not
passed over land masses for at least 10 days.
Measurements of CO 2 on the DC-8 and P-3B during PEM-
Tropics A resulted in the most extensive aerial CO 2 data set re-
HOELL ET AL.: PACIFIC EXPLORATORY MISSION IN THE TROPICAL PACIFIC 5581
corded over the South Pacific basin. Vay et al. [this issue] ana-
lyzed PEM-Tropics A flight data combined with CO 2 surface
measurements from NOAA/CMDL and NIWA to establish verti-
cal and meridional gradients for the region. They conclude that the
observed CO 2 distributions in the south tropical Pacific were no-
ticeably affected by interhemispheric transport with northern air
masses depleted of CO 2 frequently observed south of the ITCZ.
Vay et al. [this issue] note that regional processes also modulated
background concentrations as large-scale plumes from biomass
burning activities produced enhanced CO 2 mixing ratios within
the lower to mid troposphere over portions of the remote Pacific.
Of particular interest was a shift in the location of an apparent
equatorial CO 2 source observed in the surface data between 15 ø N
and 15 ø S, but realized in the airborne data between 8 ø N and
8.5 ø S, demonstrating the importance of vertical trace gas profiles
in potential source/sink regions as they provide an additional
constraint for global scale trace gas budget models.
Measurements of gas phase hydrogen peroxide (I-I202)and
methylhydroperoxide (CH3OOH) aboard the DC-8 and P-3B air-
craft have also provided an extensive data set covering the region
from 70 ø S to 60 ø N and 110 ø E to 80 ø W in the Pacific and 40 ø S
to 15 ø N and 45 ø W to 70 ø E in the South Atlantic over an altitude
range from 76 m to 13 kin. O'Sullivan et al. [this issue] reports
that both of these compounds exhibited a maximum concentration
at a given altitude along the equator and decreasing in concentra-
tion with increasing latitude in the southern and northern
hemisphere. Similar to the chemical gradients reported by
Gregory et al. [this issue], O'Sullivan et al. [this issue] notes that
the latitude gradient above 4 km is substantially reduced and at
altitudes above 8 km there is no latitudinal dependency.
3.6. Photolysis Frequences
Calculated photolysis rate coefficients, that is, J values, are key
parameters in photochemical models. In past GTE campaigns,
clear-sky model-calculated J values have been adjusted for cloud
effects based on differences between calculated J(NO2) and
J(NO2) derived from Eppley radiometers. However, a continuing
cause of concern has been the large systematic changes in the
clear-sky baseline for the Eppley radiometers between previous
campaigns (e.g., as great as a factor of 1.5). This prompted a de-
tailed comparison of several radiometric determinations of J(NO2)
during PEM-Tropics A, discussed in detail by Crawford et al.
[this issue]. As noted earlier, a new actinic flux spectroradiometer
system was flown aboard the DC-8 aircraft [Shetter and Miiller,
this issue] to measure the actinic flux values needed to calculate
J values. Also aboard the DC-8 were zenith and nadir viewing
J(NO2) filter radiometers (see Table la). Aboard the P-3B, solar
flux measurements were obtained using zenith and nadir viewing
Eppley radiometers (see Table lb).
The only period during PEM-Tropics A where the two aircraft
were sufficiently close geographically and temporally to permit
comparison of all three radiometric techniques was during flights
4D (DC-8) and 10P (P-3B) in the marine boundary layer near
Christmas Island. During this period, the three radiometers ex-
hibited trends consistent with each other and with model calcula-
tions. However, they disagreed in magnitude with the J(NO2)
filter radiometers being approximately 30% greater than the spec-
troradiometers and the Eppley radiometers falling between the
J(NO2) and the spectroradiometers. Across all DC-8 flights,
agreement between the J(NO2) filter radiometers and
spectroradiometers was exceptional with regard to the variation in
J(NO2) due to clouds (i.e., R2=0.98). The J(NO2) filter radiome-
ters, however, continued to be consistently higher than the
spectroradiometers by 30%. Crawford et al. [this issue] note that
while model calculations agreed best with values from the spec-
troradiometer, the accuracy of J(NO2) cannot be assured to better
than within 30%.
Since each technique appears to accurately capture the
variability in J(NO2) due to clouds, the recommendation of these
findings has been to normalize the variations to the clear-sky
baseline of the model calculations. Although, this does not im-
prove the accuracy of photochemical model calculations, it does
ensure a consistent baseline for J values that permits comparisons
of photochemical calculations between GTE campaigns that are
free of any systematic biases introduced by changes in the
radiometer baselines.
A secondary finding based on spectroradiometer measurements
of J(NO2) and j(O1D) suggests that the two J values exhibit
roughly equivalent responses to clouds. This confirms that
correcting all model calculated J values based on the cloud re-
sponse of J(NO2) should not be considered a large source of
uncertainty in model calculations.
3.7. Ozone Photochemistry
A major objective of the PEM and TRACE-A missions has
been to improve understanding of 03 production and loss in the
remote troposphere. As noted above, comparisons by Schultz
et al. [this issue] using the PEM-Tropics A measurements of NO
and NO 2 reported from the PF-TP-LIF system reproduced the
modeled ratio to within 30%, with similar success in simulating
concentrations of peroxides. These results are important since
they indicate that key aspects of photochemical processes in the
troposphere are understood. Model evaluation of the
photochemical ozone budget over the South Pacific [Schultz et al.,
this issue] revealed that the tropical Pacific is a net photochemical
sink. Specifically, chemical production was found to balance only
half of photochemical loss. On the basis of the ratio (delta 03)
/(delta CO), 0 3 transport from biomass burning regions was found
to be significant in balancing the remaining photochemical loss.
Photochemical production in the lower troposphere was also
found to be largely driven by NO x derived from the
decomposition of PAN that was transported from biomass burning
regions.
Photochemical model analysis of the PEM-Tropics A data has
reinforced a scenario that has emerged from previous GTE mis-
sions about the factors controlling 0 3 in the tropical troposphere.
In this picture, which is markedly different from the prevailing
view of 20 years ago, tropospheric NO x levels are sufficiently
high that 0 3 concentrations in the tropical troposphere are deter-
mined by a balance between in situ photochemical production and
loss. As a result, photochemical production of ozone dominates
the stratospheric flux in controlling column 03 density. Strato-
spheric intrusions are dramatic local events that strongly impact
the local tropospheric ozone column density. However, the photo-
chemical production of 0 3 on a global scale, driven by NO x from
natural and anthropogenic sources, ultimately dominates the tro-
pospheric 0 3 budget [Schultz et al., this issue; Crawford et al.,
1997a, b; Davis et al., 1996; Jacob et al., 1996]. Changes in NO x
emissions in the tropics as a result of industrialization and changes
in biomass burning practices would have a major impact on 03 in
the tropical troposphere and, hence, the global oxidizing power of
the atmosphere.
4. Concluding Remarks
The PEM-Tropics A mission provided the first detailed survey
of tropospheric ozone and sulfur chemistry over the South Pacific.
5582 HOELL ET AL.: PACIFIC EXPLORATORY MISSION IN THE TROPICAL PACIFIC
It complements previous GTE missions conducted in other
regions of the tropics (ABLE 2A and 2B, CITE 3, PEM-West A
and B, TRACE A). The observations from PEM-Tropics A
showed that seasonal biomass burning in the southern tropics
causes major enhancements in background concentrations of
ozone and other gases over the most remote regions of the South
Pacific atmosphere. From this result it appears that the biomass
burning perturbation to atmospheric composition is global in
scale, with major implications for the global oxidizing power of
the atmosphere. Another important finding of PEM-Tropics A
was that the South Pacific Convergence Zone (SPCZ) represents a
major barrier to atmospheric transport, and acts in combination
with the ITCZ to define boundaries for air masses over the
tropical Pacific. Focused studies of sulfur chemistry conducted in
PEM-Tropics A demonstrated a diurnal evolution of DMS-SO 2-
H2SO4-OH consistent with results from photochemical models,
and observed for the first time episodes of new particle formation
in the marine boundary layer. Finally, the PEM-Tropics A
mission featured many improvements in critical instrumentation
for tropospheric chemistry including sub-pptv measurement of
NO, high-quality measurements of NO 2, OH, and I-I2SO 4, and
spectroradiometer measurements.
The PEM-Tropics B mission to be conducted in March-April
1999 will conclude the PEM mission series by surveying the
atmosphere over the South Pacific during the wet season of the
southern tropics. The objectives of PEM-Tropics B extend
beyond those of PEM-Tropics A to include focused studies of
HO x chemistry, of the large-scale ozone minimum in the western
equatorial Pacific, and of vertical transport by deep convection in
the ITCZ and the SPCZ. Biomass burning influence on ozone
should be near its seasonal minimum during the PEM-Tropics B
period, while lightning influence should be near its seasonal
maximum. Data for ozone over the South Pacific indicate
particularly low concentrations in March-April [Fishman et al.,
1990; Johnson et al., 1990; Oltrnans and Levy, 1994]. The
continuous ozonesonde network operated as part of PEM-Tropics
indicates low ozone concentrations throughout the tropospheric
column in March-April, with none of the high-ozone layers found
in September-October. Some anthropogenic pollution could still
be transported to the South Pacific during PEM-Tropics B by the
circulation of Asian outflow around the Pacific High. Some
interhemispheric transport of biomass burning pollution from the
northern tropics is also possible. In any case, considerable
contrast should be found with the conditions observed in PEM-
Tropics A.
Acknowledgments. The PEM-Tropics A expedition would not have
been successfully conducted without the cooperation, support, and
collaboration of personnel and colleagues from organizations both foreign
and domestic. Many exhibited a personal interest in our endeavors and
performed well beyond the normal call of duty. We give special thanks to
Patrick Simon, Isabelle Leleu, and colleagues at Meteo France in Tahiti.
Without their dedicated efforts prior to our arrival in Tahiti, and patience
during our operations in Tahiti, the PEM-Tropics A mission would not
have been possible. In addition, we owe a special thanks to the DC-8 and
P-3B personnel. Their dedication and patience were a critical element in
translating the, often, unrealistic desires of the PEM-Tropics A science
team into reality. Finally, we express thanks to Mike Cadena and Fred
Reisinger in the GTE contractor Project office for their dedicated support
in the "care and feeding" of the science team during the field deployment.
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of acidic trace gases over the southern Pacific basin during austral
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R. J. Bendura, J. M. Hoell, and J. L. Raper, NASA Langley Research
Center, Hampton, VA 23681-0001. (e-mail: j.m.hoel@larc.nasa.gov)
D. D. Davis and M. O. Rodgers, Georgia Institute of Technology,
Atlanta, GA 30332.
H. E. Fuelberg, Florida State University, Tallahassee, FL 32306.
D. J. Jacob, Harvard University, Cambridge, MA 02138.
R. J. McNeal, NASA Headquarters, Washington, DC 20546.
R. E. Newell, Massachusetts Institute of Technology, Cambridge, MA
02139.
(Received July 1, 1998; revised October 13, 1998;
accepted November 10, 1998.)
