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 (NO_x), 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 (https://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.