GTE: PEM Tropics B - Scientific Objectives
TECHNICAL DESCRIPTION OF THE SECOND PACIFIC
EXPLORATORY MISSION IN THE TROPICS PEM-TROPICS B
For global tropospheric chemistry, the Pacific Basin troposphere is
a very large chemical reaction vessel. From Peru to Borneo it is
17,700 km in the east-west direction, and from the southern ice shelf
to Alaska it is 13,300 km. It covers 35% of the total surface area of
the earth and 50% of the ocean surface. Much of the Pacific Basin is
remote from continental influence, and hence provides a particularly
sensitive indicator of the global-scale impact of human activity on
the chemistry of the troposphere.
One of the most important issues in global tropospheric chemistry
is the sensitivity of the oxidizing power of the troposphere to human
influence. Oxidation by the hydroxyl radical (OH) in the troposphere
is the main sink for a number of gases important for climate change
and stratospheric O3 depletion, including
hydrochlorofluorocarbons, methane, methyl bromide, and methyl
chloroform. A decrease in tropospheric OH concentrations would
increase the tropospheric concentrations of these gases and increase
their fluxes into the stratosphere. It would also reduce the global
atmosphereís ability to cleanse itself of reactive pollutants
and maintain its current chemistry.
Concentrations of tropospheric OH are determined by a number of
photochemical reactions involving O3 , nitrogen oxides (NOx),
carbon monoxide (CO), methane (CH4), and non-methane
hydrocarbons (NMHC's). Ozone, the primary precursor of OH, is supplied
to the troposphere by transport from the stratosphere, and is also
produced within the troposphere by photochemical oxidation of methane
and NMHCís in the presence of NOx. Anthropogenic emissions of
NOx, CH4, NMHC's, and CO could have a large effect on the
oxidizing power of the troposphere, directly by affecting OH and
indirectly by providing a source of O3 . Tropical regions
play an especially critical role in determining the global oxidizing
power of the atmosphere because of the high UV and humidity, which
promote the formation of OH from reactions that follow the photolysis
of O3.
Another major issue in global tropospheric chemistry is the role of
atmospheric sulfur chemistry in aerosol formation Sulfate aerosols
affect the earth's radiative balance both through direct back
scattering of solar radiation and indirectly as cloud condensation
nuclei (CCN). CCN, themselves products of aerosol growth processes,
are believed to have their origin in nucleation processes involving
gas phase H2SO4, the latter species being
produced from the oxidation of SO2 by OH. Sulfate and SO2
over the Pacific may originate from a number of sources including
long-range transport of anthropogenic pollution, marine biogenic
releases of dimethlysulfide (DMS), and volcanic emissions.
The GTE PEM-Tropics A mission, a two-aircraft mission conducted in
1996, provided the first detailed survey of tropospheric chemistry
over the South Pacific Basin. Previous data from this region were
sparse, reflecting the difficulty of access. The GAMETAG aircraft
missions in 1977 and 1978 provided some early data over the western
part of the Basin; they were, however, restricted by the low ceiling
and limited endurance of the aircraft used, and also by the
state-of-the-science of the instruments available at the time. The
more recent STRATOZ III and PEM-West A and B missions provided
detailed data along the South American and Asian rims of the South
Pacific Basin, respectively. Ozonesonde and CO measurements
have been made at Samoa for a number of years. Additional observations
have been made from island sites (SEAREX program) and from ships. Even
so, there were virtually no data for the southeast quadrant of the
Basin, extending from the international dateline to the South American
coast prior to
PEM-Tropics A.
PEM-Tropics A was conducted during August-October 1996. The two
aircraft platforms used were the NASA DC-8 and P3-B. They covered an
area extending zonally across the entire Pacific Basin and
meridionally from Hawaii to south of New Zealand (flight
track map). Significant coverage of the Walker circulation cell
over the Pacific Basin was achieved, including the upwelling region
over the western equatorial Pacific, the subsiding region offshore of
South America, and the connecting atmosphere in between. Much emphasis
was placed on vertical profiling to obtain as complete a
three-dimensional picture of trace gas concentrations as possible.
The PEM-Tropics A mission was very successful. It provided an
excellent atmospheric chemical survey of the region that was enhanced
by significant breakthroughs in instrumental capability to measure OH,
NO at extremely low levels, and many sulfur compounds, some for the
first time . PEM-Tropics A also revealed significant influence of
biomass burning emissions over the South Pacific. Air trajectory
analyses point strongly toward these emissions as having been
transported from South America, Africa, and Oceania.
The PEM-Tropics A Science Team is currently analyzing the data from
the mission and plans to submit a coordinated group of papers for
publication in October 1997. A list of papers currently in preparation
and preprints where available and approved for preprint release by the
authors may be obtained from the GTE Project Office at Langley
Research Center.
PEM-Tropics A data can be accessed directly from GTE anonymous ftp
site (http://www-air.larc.nasa.gov/pub/PEMTROPICSA)
or through the GTE Web Site (http://www-gte.larc.nasa.gov)
using the "Data Archive" hot link. PEM-Tropics A data can
also be accessed from the DAAC Web Site (http://eosweb.larc.nasa.gov)
through the "Access Data" then "Data Accessible from
the Web" hot links. The GTE ftp and GTE web sites provide data
from individual investigations, on a flight-by-flight basis, while the
DAAC site provides data from all investigators for each flight. Data
from ozonesonde launches at Easter Island, Tahiti, American Samoa, and
Lauder New Zealand beginning approximately one year prior to the
PEM-Tropics mission are also available on the GTE ftp and web sites.
PEM-Tropics B will investigate tropospheric chemistry over the
north and south tropical Pacific Oceans. It will be an airborne study
that will complement the PEM-Tropics A mission. It will be carried out
in February-April, 1999, thereby providing an opportunity to
investigate different chemical and transport properties of the
tropical troposphere. PEM-Tropics B will take place in the
southern-tropical wet season, when the influence from biomass burning
observed in PEM-Tropics A should be minimal. The nominal study region
will range from 165W to 80E longitude with major deployment sites at
Hawaii, Christmas Island, Tahiti, and Fiji.
As in PEM-Tropics A, the goals of PEM-Tropics B are to improve our
understanding of the oxidizing power of the atmosphere and the
processes controlling sulfur aerosol formation. Like PEM-Tropics A, it
will also continue the effort to establish baseline values for
chemical species that are directly coupled to the oxidizing power and
aerosol loading of the troposphere.
PEM-Tropics B will encompass observations over a wide range of
geographical locations within the tropical Pacific and will sample a
wide range of meteorological conditions. These observations will form
the basis for defining current baseline values for chemical
composition in the tropical Pacific and establish the range of
variability in this composition. Supported by transport-chemistry
models, they also will be used to improve our understanding of the
importance of different source types, and hence, their roles in
controlling the oxidizing power and aerosol loading of the tropical
troposphere.
OBJECTIVES
Reflecting our current state-of-knowledge of the tropical
Pacific, two general objectives are defined for PEM-Tropics B:
- A. TO PROVIDE BASELINE DATA FOR CHEMICAL SPECIES THAT DETERMINE
THE OXIDIZING POWER AND AEROSOL LOADING OF THE TROPICAL PACIFIC.
- B. TO EVALUATE THE CHEMICAL AND DYNAMIC FACTORS CONTROLLING
OZONE, OH, AND AEROSOL LEVELS OVER THIS REMOTE REGION.
Within the framework of these general objectives, five specific
tasks are identified:
-
QUANTIFY THE FAST PHOTOCHEMICAL PROCESSES CONTROLLING OH
CONCENTRATIONS.
Of critical importance to our assessment of changes in the
oxidizing power of the troposphere is an improved understanding of the
fast photochemical processes producing and consuming OH, and the
cycling of OH with other members of the HOx family. Currently, this
understanding is still sketchy, in part because instrumentation for
measuring HOx species has only recently come available. Some of the
first OH measurements ever recorded at equatorial latitudes were those
reported during PEM-Tropics A. A major priority in PEM-Tropics B will
be to expand on this initial effort by including both upper and lower
tropospheric measurements of OH as well as peroxy radicals. In
addition, observations of the ensemble of species that determine the
production, loss, and cycling of HOx will be recorded. The PEM-Tropics
B data base will thus provide one of the best opportunities yet to
quantitatively evaluate current photochemical models over a wide range
of tropical conditions.
INVESTIGATE THE FACTORS RESPONSIBLE FOR LARGE-SCALE LOW
CONCENTRATIONS OF TROPOSPHERIC OZONE OVER THE EQUATORIAL PACIFIC.
Tropospheric ozone column densities over the equatorial Pacific are
the lowest in the world, with values as low as one third those found
in other regions of the tropics. It has not yet been quantitatively
determined what combination of dynamical and chemical factors is
responsible for this large-scale depletion. The chemical measurements
in PEM-Tropics B will allow direct computation of ozone production and
loss rates over the equatorial Pacific and surrounding regions. These
data, combined with detailed meteorological information and supported
by simulations with chemistry-transport models, will provide a
quantitative assessment of the factors contributing to the very low
levels of ozone in this region.
STUDY THE ROLE OF THE ITCZ AND SPCZ AS BARRIERS TO ATMOSPHERIC
TRANSPORT BETWEEN THE NORTHERN AND SOUTHERN HEMISPHERES AND WITHIN THE
SOUTH PACIFIC.
Observations from PEM-Tropics A have shown that the SPCZ (South
Pacific Convergence Zone) constitutes an important barrier for
transport between the tropical and subtropical latitudes of the
western South Pacific. In particular, it was found that the SPCZ
represented the northern boundary for the influence of biomass burning
advected by westerly winds in the subtropical atmosphere. The
unusually low ozone concentrations in the equatorial Pacific
troposphere could quite possibly be related to the presence of the
SPCZ. PEM-Tropics B will take place at a time of year (February-April)
when the SPCZ is particularly strong and thus will include targeted
investigations of the SPCZ as a barrier to atmospheric transport.
Also, there will be at least six flights crossing the ITCZ
(Intertropical Convergence Zone) south of Hawaii, making possible a
further investigation of the interhemispheric transport
characteristics for this region.
INVESTIGATE THE SCAVENGING OF GASES/AEROSOLS ASSOCIATED WITH DEEP
CONVECTION AND GAS-TO-AEROSOL CONVERSION TAKING PLACE IN CONVECTIVE
OUTFLOWS.
Deep convection is thought to provide the dominant mechanism for
ventilation of the tropical upper troposphere. There is a need to
better understand how this mechanism affects oxidants and aerosol
concentrations as well as their respective precursors in the upper
troposphere. Specific questions relate to the extent of convective
influence, the mass fluxes of different species in convection, the
scavenging of soluble gases and aerosols in the convective
precipitation, and the chemistry and aerosol formation taking place in
convective outflows. Several cases of deep convection were identified
during some of the tropical flights of the earlier NASA field program
PEM-West B; however, instrument limitations at that time prevented a
detailed characterization of these events. During PEM-Tropics B
coordinated flights of the P-3B and DC-8 aircraft, with augmented
instrumentation, will specifically target this issue.
ELUCIDATE THE PROCESSES CONTROLLING PHOTOCHEMISTRY AND AEROSOL
FORMATION BENEATH THE TRADE WIND INVERSION.
The lower troposphere over the tropical oceans, including the
marine boundary layer and the transition layers below the trade-wind
inversion, is an important region for chemical loss of ozone and for
growth of sulfate aerosols. Aerosol nucleation can also take place as
evidenced from low altitude studies conducted during PEM-Tropics A. At
this time there is a need to better understand ozone photochemistry
and sulfur oxidation in this region, and the role of ocean-atmosphere
exchange processes. PEM-Tropics B will provide data on atmospheric
composition, aerosol microphysics, and ocean-atmosphere fluxes for a
range of marine productivity and meteorological conditions over the
tropical Pacific.
Measurement requirements for experiments aboard the DC-8 and the
P-3B are given in Tables 1a and 1b,
respectively. Priority ratings (1-5) for each measurement are based on
our current understanding of the importance of the respective
measurement for addressing the mission objectives, as well as some
reflection on the current state-of-the-science of available
instrumentation. The definition of the priority ratings are given at
the end of Table 1.
The preliminary flight plans for each aircraft are given in
Table 2 and the flight tracks are
shown in the flight track map
After integration and test flights at the respective integration
sites, both aircraft will begin the PEM-Tropics B deployment with a
transit flight from the west coast to Hawaii. During the initial
phase, the DC-8 will be based at Hilo, Hawaii and the P-3B at
Christmas Island. The two aircraft will conduct three coordinated
lights from these bases of operation, after which the DC-8 will
transit to Fiji for focused studies around the SPCZ and north west of
Fiji. The DC-8 and P-3B will each transit to Tahiti where additional
coordinated studies will be conducted. After completion of 4 local
flight from Tahiti, the P-3B will return to the Wallops Flight
Facility via Hawaii. The DC-8 will conduct a total of 5 local flights
from Tahiti before returning to the Dryden Research Flight Center via
Easter Island where it will carry out one local sortie.
The information presented in this Appendix is for broad guidance
only. The selected investigators will be organized into a Science
Team, which will develop the final detailed mission plans that would
best achieve the primary and secondary objectives of this mission as
given above.