Overview of PEM-West A Results

September-October 1991 

J. M. Hoell¹, D. D. Davis², S. C. Liu³, R. Newell⁴, M. Shipham¹, H. Akimoto⁵, R. J. McNeal⁶,
R. J. Bendura¹, and J. W. Drewry¹

Abstract

The NASA Pacific Exploratory Mission-West (PEM-West) was a major component of the East Asia/North Pacific Regional Study (APARE), a project within the International Global Atmospheric Chemistry (IGAC) program. The broad objective of the PEM-West/APARE initiative was to study chemical processes and long-range transport over the northwestern Pacific Ocean and to estimate the magnitude of the human impact on the oceanic atmosphere over this region particularly for tropospheric ozone and its precursors as well as for sulfur species. The first phase of this mission, PEM-West A, was conducted during September-October 1991. The PEM-West A included intensive airborne measurements of trace gases from the NASA DC-8 aircraft coordinated with measurements at PEM-West A surface sites as well as with measurements obtained from collaborating APARE ground and airborne platforms. This paper reports the experimental design for PEM-West A with a brief summary of the general content and focus of companion papers in this special issue.

Introduction

The National Aeronautics and Space Administration’s Pacific Exploratory Mission-West A (PEM-West A) is a major component of the East Asia/North Pacific Regional Study (APARE) [Akimoto et al., 1994], a project within the International Geosphere-Biosphere Programme (IGBP). The broad objectives of the PEM West/APARE initiative was to study chemical processes and long-range transport over the northwestern Pacific Ocean and to estimate the magnitude of the human impact on the oceanic atmosphere over this region. More specifically, the goals of this initiative were to study the budgets of ozone and sulfur species. The Pacific Ocean is the only major region in the northern hemisphere that is "relatively" free from direct anthropogenic influences. In the remote regions of the northern Pacific and in most of the southern Pacific, it should be possible to study the biogeochemical cycles of carbon, nitrogen, ozone, sulfur, and aerosols in an environment which, from a global perspective, is the least perturbed by anthropogenic activities. On the other hand, there is little doubt that long-range transport of air pollutants from Asia and, to a lesser extent, North America is beginning to have a significant impact on the atmosphere over a large part of the Pacific. The results from these studies will provide an extensive set of baseline data from which the anthropogenic impact on this region can be reliably assessed for decades to come.

The overall experiment design for the PEM-West/APARE program has encompassed two field studies positioned in time such that contrasting meteorological regimes in the northwestern Pacific could be sampled. The first phase of PEM-West A was conducted during September and October 1991. A significant characteristic of the lower tropospheric airflow during this time period is the predominance of flow from mid-Pacific regions. Phase B of PEM-West was conducted during the late winter/early spring of 1994, a period characterized by maximum out flow from the Asian continent. The experimental design of PEM-West was centered on conducting intensive airborne studies over the northwestern Pacific Ocean using the NASA DC-8 aircraft. The NASA airborne component was coordinated with studies conducted at PEM-West and other collaborating APARE surface sites and with APARE airborne sampling activities. 
The PEM-West study is a component of the NASA Global Tropospheric Experiment (GTE) sponsored by the Tropospheric Chemistry Program [McNeal et al., 1983; Hoell et al., 1990a]. The PEM-West program represents the first GTE program to specifically focus on large-scale, long-range transport. Previous GTE missions, such as the Atmospheric Boundary Layer Experiments (ABLE) [Talbot et al., 1986; Ferek et al., 1986; Gregory et al., 1986; Harriss et al., 1988, 1990, 1992, 1994], have focused on the study of major ecosystems with emphasis on understanding the rate of exchange of material between the Earth’s surface, the atmospheric boundary layer, and the free troposphere. Another series of GTE field expeditions designated as Chemical Instrumentation Test and Evaluation (CITE) [Beck et al., 1987; Gregory et al., 1985, 1986; Hoell et al., 1990b, 1993] have focused on the evaluation of instrumentation for making measurements of important trace tropospheric species.
The international collaboration, coordinated through the APARE, included the NASA-sponsored PEM-West measurements (airborne and ground based); the NSF, NCAR, and NOAA sponsored MLOPEX II ground-based measurements at Mauna Loa, Hawaii; the Japanese National Institute of Environmental Science (NIES) sponsored study, perturbation by East Asian Continental Air Mass to Pacific Oceanic Troposphere (PEACAMPOT-airborne and ground-based measurements); the Taiwan-sponsored Climate and Air Quality Taiwan Station (CATS) (ground-based measurements); and ground-based stations sponsored by the Peoples Republic of China and Korea. Important meteorological support was also provided by the Hong Kong Royal Observatory. This paper reports on the overall experimental design for PEM-West A and provides a brief summary of the focus and content of papers appearing in the PEM-West A Special Section of the Journal of Geophysical Research, vol. 101, No. D1, January 20, 1996. A more detailed list of publications describing results from PEM-West A is available in the Publications section on this CD-ROM.  
Approach

The objectives of PEM-West were (1) to investigate the atmospheric chemistry of ozone (O3) and its precursors over the northwestern Pacific, including examination of their natural budgets as well as the impact of anthropogenic sources, and (2) to investigate the atmospheric sulfur cycle over the northwestern Pacific with emphasis on the relative importance and influence of continental versus marine sulfur sources. To address these objectives, the PEM-West A mission plan was developed through joint meetings of the PEM-West A Science Team and representatives of the collaborating science teams within APARE.

The mission design provided for coordinated measurements from fixed surface sites and aircraft platforms. Many of the ground-based sites were located at remote islands and were instrumented with automated filter collectors for measurement of particulate species. Several of these "automated" sites, as well as, additional "intensive" surface sites were instrumented to provide a more extensive set of particulate and trace gas species during the time period centered on the PEM-West A mission. The airborne measurements during the PEM-West/APARE included studies from the NASA DC-8 aircraft and a Cessna aircraft sponsored by the< NIES/PEACAMPOT. Airborne studies were specifically designed to characterize vertical and horizontal spatial variability over the western Pacific Ocean.
Table 1 list the locations of the ground sampling sites and Figure 1 shows the DC-8 flight tracks during PEM-West A. A more detailed overview of the APARE mission, results from the PEACAMPOT airborne studies, and the results from the other APARE collaborating science teams during the first phase of APARE are given by Akimoto et al. [1994] and by companion papers in the .
The PEM-West A/APARE experimental design also included extensive modeling activities explicitly coordinated with the airborne and surface measurements. Principal investigators involved with these activities participated in all phases of the PEM-West A/APARE experiment. As part of the GTE PEM-West A/APARE data protocol, all measurements obtained by the collaborating science teams are available through the GTE data archive at NASA’s Langley Research Center. Modeling products, analogous to the measurement products provided by the instrument investigators, are also available through the data archive.

Aircraft Experiments

The primary measurement platform during PEM-West A was the NASA DC-8 aircraft based at the NASA Ames Research Center in Moffett Field, California. Figure2 illustrates the instrument layout aboard the DC-8 during PEM-West A, and Table 2 lists the respective measurements along with the associated accuracy, precision, and limit of detection (LOD) for each species. The characteristics given in Table 2 are nominal. More detailed descriptions are included in the companion papers and in the PEM West A data archive.
As seen from Table 2, the parameters measured aboard the DC-8 were extensive, particularly with respect to questions associated with the photochemical production/destruction of O3 and the distributions of precursor species. Of particular importance is the temporal resolution afforded by the instrumentation aboard the DC-8. Many measurements were submitted to the GTE data archive with averaging times of <1 min (Table 2 column 4). In the case of the grab sample measurements (investigator codes I and K, Table 2), typically more than 140 samples were taken per flight with a nominal acquisition time of 15 s per sample at lower altitudes to 75 s per sample at the higher altitudes.
In the case of O3 and aerosol measurements the in situ aircraft data were augmented by vertical profiles obtained above and below the DC-8 by the onboard differential absorption lidar (investigator code D, Table 2). The lidar profiles extend vertically upward into the stratosphere and downward to the ocean surface. The lidar measurements along with selected in situ measurements (e.g., O3, CO, H2O) were displayed in real time aboard the DC-8 and used for in-flight "fine tuning" of each flight plan.
Figure1 illustrates the PEM-West A study area and the DC-8 flight tracks. The flight number shown for each mission is keyed to Table 3 which gives the major purpose, date, and nominal takeoff and landing time for each mission. Over the time period from September 16 to October 21, 1991, a total of 18 missions encompassing approximately 120 flight hours were conducted. 
In general, the PEM-West A flights can be characterized as either of the "survey" or "intensive" type. Eight survey flights (e.g., 4, 5, 10, 11, 14, 18, 19, and 21), and ten intensives (e.g., 6, 7, 8, 9, 12, 13, 15, 16, 17, and 20) were conducted. All intensives involved flights that originated and returned to the same location. (Flights 1-3 were instrument and engineering test flights based at Ames.) While the prime objective of the survey flights was to move the aircraft to a new base of operation, the flight plans were designed to permit full operation of all instruments and, in general, provided as much information on the vertical and/or horizontal distributions of trace gases as possible. The three intensive operational bases were Yokota Air Force Base, Japan, Hong Kong, and Guam. Intensive flights were designed to take advantage of the geographical location of the site and local meteorological conditions in addressing science objectives. As part of two intensive flights, the DC-8 flight plan incorporated a "flyby" of the Taiwan CATS ground station (flight 12) and the PEACAMPOT aircraft (flight 13). Flight 20 was designed as a dedicated intercomparison of selected measurements aboard the DC-8 and similar measurements at the Mauna Loa, MLOPEX II surface site. Flight 9 is also noteworthy in that it represents the first airborne study of trace gases within a typhoon. Typhoon Mireille, a category 4 typhoon, struck the western side of Japan on September 27, 1991. As Mireille approached the coast of Japan, the DC-8 conducted a flight to study the role of typhoons in the transport of trace gases. The flight track for flight 9, was designed to sample the inflow to the typhoon at low altitudes and the out flow within the eye at high altitudes. Survey flight 14 also provided an opportunity to study the high-altitude out flow associated with Typhoon Orchid, although in much less detail than for Mireille. A more detailed description of flight 9 and the resulting analysis of the transport associated with Typhoon Mireille is given by Newell et al. [1996] and Thornton et al. [this issue].

Ground-Based Experiments

Table 4 lists the collaborating PEM-West A surface sites which were active during the intensive measurement period. Also given are the parameters measured, the principal investigator(s), and the responsible organization(s). The location of each site is listed in Table 1. The long-term time series data obtained at the automated site are particularly useful in providing a broader perspective in which to view the measurements obtained during the intensive period. Data from all the sites listed in Table 4 have been submitted to the GTE Data Archive in a format similar to the DC-8 measurements listed in Table 2.


Modeling Activities

Modeling activities were included as part of PEM-West A as well as the other collaborating science teams within APARE. These activities were conducted not only as part of the post mission analysis but also as part of the premission planning and the field work at the intensive sites. Table 5 summarizes the various types of modeling activity as well as the modeling products that have been submitted to the GTE data archive.
Results from a trajectory model provided source information which was invaluable for several analyses described in the companion papers of this issue. Further, back trajectory profiles were utilized during planning the flight tracks and sampling strategies of the intensive flights. The two box models listed in Table 5 include an extensive set of trace gases and photochemical reactions. By fixing the key species and parameters to observed values, these models have provided a convenient means of calculating the photochemical production and loss of ozone (Davis et al., 1996) and the concentration of OH, HO2, and other reactive species (J. Rodriguez et al., unpublished material, PEM-West A archive, 1995). The 3-dimensional regional transport/photochemical model encompassed a domain that included eastern Asia and the western one third of the North Pacific. It is used for studying the continental out flow of anthropogenic pollutants (Liu et al., 1996). In addition, it has been shown to be an effective tool for studying the transport and photochemistry of nonmethane hydrocarbons and other trace gases (McKeen et al., 1996; Smyth et al., this issue). Detailed discussions of the modeling products and results are given in companion papers (Liu et al., 1996; Crawford et al., 1996; Davis et al., 1996; McKeen et al.).


Meteorological Measurements

Meteorological support for flight planning was provided by a team of meteorologists stationed with the aircraft and at the Hong Kong Royal Observatory. The Royal Observatory and the GTE weather center at NASA’s Langley Research Center served as the prime forecasting facilities during the mission. Satellite observations received at the Royal Observatory were archived for post mission analysis. The Langley GTE weather center served as the central collection point for the NMC-grided data, Asian sounding and the Pacific surface reporting stations. Derived products consisting of streamline analysis and wind fields at various pressure levels, were automatically faxed (twice daily) to the mission meteorologists traveling with the DC-8. In addition to these two central data centers the local forecasting offices at each intensive site provided invaluable forecasting support for flight planning. Flight-planning meteorological data were archived for post mission analysis. A meteorological analysis of the PEM-West A program has been provided in companion papers by Bachmeier et al., [1996] and Merrill [1996].

NO and NOy Measurements

During PEM-West A a de facto intercomparison of NO and NOy measurements was conducted from measurements obtained by investigators from the Nagoya University (investigator code H, Table 2) and Georgia Institute of Technology (investigator code C, Table 2). This section and the paper by Crosley [1996] provide a brief summary of the intercomparison.
The Nagoya instrument [Kondo et al., 1996] is a two-channel device designed to provide simultaneous measurements of NO and NOy via detection of chemiluminescence from the reaction of NO with O3. The measurement of NOy is obtained by catalytic conversion of odd nitrogen compounds to NO on the surface of a heated gold tube, followed by chemiluminescence detection of NO. The Georgia Tech instrument is a three-channel device designed to provide simultaneous measurements of NO, NO2, and NOy. In each channel the species detected is NO using a two-photon, laser-induced fluorescence technique [Sandholm et al., 1990]. In the NO2 channel, NO is generated via photolytic conversion of NO2. In the NOy channel, NO is generated by means of a catalytic converter similar, in principle, to that used by Nagoya University. Each NOy converter is based on a design by Fahey et al. [1985].
Comparison of the NO and NOy data resulted in two major conclusions. (1) With the exception of a few periods of time encompassing a very small fraction of the temporally overlapping data, the NO measurements reported by each instrument were in excellent agreement over a wide range of mixing ratios and environmental conditions. (2) In contrast, there was substantial disagreement between the NOy measurements. In general, the values reported by Georgia Tech tended to be higher that those reported by Nagoya University, although on the whole, the differences between the two data sets could best be described as "random" rather than "systematic." For both NOy measurements, there was often found to be a significant deficiency in the odd nitrogen budget in that the measured NOy values were typically greater than the sum of the components measured during PEM-West A, e.g., PAN, HNO3, NO, and NO2 [see Singh et al., 1996 (a)].
Because of the importance of the NOy measurements to both the PEM-West A and the B studies as well as future atmospheric studies, NASA convened a Blue Ribbon Panel to critically review the PEM-West A NOy measurements, and to examine the question of "missing NOy". A synopsis of the panel’s conclusions is provided by Crosley [1996].

Overview of PEM-West A Papers

In this section we briefly note the general content and focus of many of the companion papers included in this special issue. Discussions, herein, will allow readers to quickly find those papers most related to their field of interest.
From a meteorological perspective, PEM-West A provided an opportunity to examine the chemical characteristics of several different air mass types over the northwestern Pacific region (e.g., midlatitude maritime, subtropical maritime, tropical maritime, and continental out flow). Meteorological papers by Merrill [1996] and Bachmeier et al. [1996] provide discussions of the meteorological settings encountered during PEM-West A. The discussion by Merrill describes the meteorological regimes from the perspective of event-specific trajectories and also includes climatological trajectories for the years 1988-1990. Bachmeier et al. describe the synoptic meteorological conditions on a flight-by-flight basis and place the various PEM-West A meteorological settings in perspective relative to the large-scale climatology for the northwestern Pacific region.
A number of the PEM-West A papers address the topic of air mass characterization. Trajectory analyses served as the primary basis for the air mass classification described by Gregory et al. [1996 (a)] and Talbot et al. [1996]. The analysis by Gregory et al. centers on aged-marine air masses and notes, in particular, that air masses exposed to the marine environment for periods as long as 10 days may still exhibit significant influences of continental source(s). The analysis by Talbot et al. defines the chemical signatures for air masses identified (via trajectories) as having significant and recent (few days) continental influences. Their analysis illustrates the importance of transport from the Asian continent, particularly at altitudes below 2 km, and even suggests that long-range transport, not only from Asia but also from continental sources upwind of Asia, may have a significant impact on the chemical characteristics of high-altitude free tropospheric Pacific air. This observation is consistent with independent analysis by Bradshaw et al. [1996] and Liu et al. [1996]. The analysis by Browell et al. [1996] describes a unique air classification scheme based on the large-scale distributions of ozone and aerosols as measured remotely via the lidar technique. The deviation of ozone and aerosol profiles from a reference ozone and aerosol profile are quantified and coupled to air mass trajectories and potential vorticity. These analyses quantify the relative contributions of different ozone sources (e.g., stratospheric, convective cloud out flow, continental out flow, and marine boundary layer advection). The analysis by Bradshaw et al. [1996] describes an air mass classification scheme based on the relative abundance of combustion products, namely, C2H2 and CO. Their results, when combined with the trajectory and lidar classification schemes, provide an additional measure of insight into the degree of atmospheric processing (e.g., photochemistry plus transport/mixing) for the different air mass types. The analysis described by Davis et al. [1996] provides an additional level of classification in terms of the photochemical O3 tendency (production minus loss) of different air masses.
In addition to the papers just noted above which tend to focus on air mass characterization, several PEM-West A papers address issues related to out flow from the Asian continent. The analyses by Dibb et al. [1996], using radioactive tracers, reach similar conclusions, as Talbot et al. [1996], showing that high-altitude out flow from the Asian continent exhibits characteristics of emission from upwind of the Asian continent. The analysis by Liu et al. [1996] utilizes a 3-dimensional mesocale transport/photochemical model to study the transport and chemical transformation of the emission from the Asian continent into the Pacific Ocean. Comparison of model and measurement results by Liu et al. provides new estimates for sources of NOy in the upper tropospheric.
Further insight into the chemical characteristics of air mass types, but with emphasis on the nitrogen species is provided by Kondo et al. [this issue], Koike et al. [1996], Crawford et al. [1996], Singh et al. [1996 (a)], and Davis et al. [1996]. The analysis by Kondo et al. describes the distribution of NO and NOy in marine (aged and fresh) and continental air masses, while Koike et al. examine the changes in the partitioning of reactive nitrogen species with air mass age. Both authors utilize trajectory analysis as the basis for air mass classification. Results from their analyses offer information relative to the sources and distribution of NO and NOy. One measure of our understanding of the chemistry of reactive nitrogen species is afforded by how well model estimates compare to measured results, and the degree to which closure is obtained in the partitioning of the reactive nitrogen species. These questions are explored in the papers by Crawford et al. and Singh et al. Crawford et al. provide a comparison of the measured and calculated NO2 concentrations and conclude that on average the PEM-West NO2 measurements significantly exceed those NO2 levels calculated on the basis of measured NO and other photochemical parameters used as input to a photochemical model. The available data suggest that the most likely source of this difference is associated with interferences in the measurement of NO2. Singh et al. examine the PEM-West A measurements with regard to the total nitrogen budget and partitioning of the measured reactive nitrogen species as indicated by comparisons of the sum of individual nitrogen species (e.g., NO, NO2, HNO3, and PAN) relative to the total reactive nitrogen, NOy, as well as comparison of observations with predictions obtained from a global 3-dimensional model. Their analyses show a substantial "short fall" under many flight conditions between the sum of the individual measured nitrogen species and NOy. Comparison of model and measured values provides some insight on the relative importance of the various sources of reactive nitrogens as well as discrepancies between measured and predicted HNO3. The analyses of Crawford et al. and Singh et al. are particularly noteworthy. They indicate that for many of the conditions encountered during PEM-West A, our ability to measure certain nitrogen species and NOy [Crosley, 1996] and/or our understanding of the basic processes controlling these species may still be lacking. It is important to note, however, that the PEM-West A results reaffirm our ability to measure NO, a pivotal component in the photochemistry of ozone. In the latter regard, Davis et al. [1996] discuss how the photochemical ozone tendency (i.e., the difference between photochemical formation and destruction is a strong function of the level and distribution of NO in the western North Pacific. Their analysis further points toward lightning as being one of the major high-altitude sources of NO during the PEM-West A sampling period.
Issues related to the understanding of the distribution and sources of ozone in the western Pacific, a primary objective of the PEM-West/APARE study, are addressed in several of the PEM-West A papers. The previously noted paper by Browell et al. [1996] utilizes the large scale distributions of ozone afforded by the lidar observations to evaluate the latitudinal distribution and the relative contribution from different ozone sources. Gregory et. al. [1996 (b)] describe in situ ozone measurements with particular emphases on the extent to which the variability of lower tropospheric ozone can influence total atmospheric column ozone (e.g., observations from the Total Ozone Monitoring Satellite, or TOMS), and unique airborne observations of low ozone within the "ozone trough" along the Pacific equatorial region are discussed by Singh et al. [1996 (b)]. Photochemical models are employed by Davis et al. [1996 (a), (b)] to assess the northwestern Pacific region in terms of the relative production or destruction of ozone and the oxidative capacity of this region. Davis et al. [1996] point out that the ozone tendency in the western North Pacific is strongly dependent on both geographical location and altitude. For more remote regions the tendency is one of net destruction of ozone at nearly all altitudes. By contrast, for regions closer to the continental Pacific rim, the ozone tendency is negative at low altitudes but switches to become< positive at altitudes above 6 km. As noted earlier in the text, this behavior is believed to reflect the important role played by convection/lighting as a high-altitude source of NOx. Davis et al. [1996 (b)] have also explored the possible influence of halogen and iodine chemistry on tropospheric levels of O3 as well as on the radical ratio HO2/OH. They conclude that during the time period of PEM-West A, because of low marine productivity, the effects from iodine were probably quite small.
PEM-West A observations represent the first significant enhancement in sulfur measurements in the western Pacific region since the GAMETAG project in 1978 [Maroulis et al., 1980]. The observations reported by Thornton et al. [1996 (b)] of the vertical distribution SO2 observed in the remote marine environment are similar to those reported by Maroulis et al. [1980]. Both sets of observations exhibit higher concentrations of SO2 in the free troposphere than the boundary layer. Analysis by Thornton et al. suggests new insight into the importance of volcanic emissions (e.g., Pinatubo) as sources for SO2 in the upper troposphere. In addition, observations of ocean color scanner (OCS) described by Thornton et al. [1996 (a)] give new estimates of the lifetime of OCS in the troposphere.
The ground-based sites described in Tables 1 & 4 form an extensive sampling network providing a measure of the extent to which local and distant sources impact the chemistry in the western Pacific basin region. Moreover, the extensive time series measurements more readily acquired at ground-based sites provide a context in which the more intensive airborne measurements obtained during the PEM-West A study period can be evaluated. With the exception of the Lin-An site, results from the sites along the Asian continent suggest that during the PEM-West A period the typical condition was one of relatively clean conditions interrupted with pollution episodes. Analysis of measurements obtained at the PEACAMPOT ground sites are reported by Akimoto et al. [1996] and Jaffee et al. [1996]. The surface measurement of O3 at the coastal site Oki, Okinawa, Japan, and Kenting, Taiwan reported by Akimoto et al. [1996] shows similar ranges of O3 mixing ratio (about 4-65 parts per billion by volume (ppbv)). The more extensive set of measurements reported at the Oki Island site by Jaffee et al. [1996] were used to evaluate the sources of pollutants at this site and to assess the O3 photochemical tendency.. It was noted, in particular, by Jaffee et al. [1996] that the average NO concentration (e.g., 55 parts per trillion by volume (pptv)) at the Oki island site was larger than that required to support net O3 production. The observations reported by Zhou et al. (unpublished material, 1995) suggest that the Lin-An site is significantly influenced by human activities. The observations reported during the measurement period at the CATS site in Kenting, Taiwan [Buhr et al., 1996], indicated that Taiwan was influenced by three different air mass types: marine boundary layer, urban plume, and continental out flow. Interestingly, the relationship between O3 and peroxyacetyl nitrate (PAN) at the Taiwan site was shown to be very similar to that observed under similar physical conditions in April and May 1985 [Parrish et al., 1992] at a coastal site in California some 11,000 km across the Pacific Ocean. The observations reported by Arimoto et al. [1996] provide a contrast between the measurements obtained at the Asian coastal sites and the more remote Pacific island sites and therefore are more indicative of the impact of long-range transport into the western Pacific basin. In the paper by Arimoto et al. [1996] the relationships between MSA and nss sulfate, between aerosol sulfate and nitrate, as well as the relationships between trace elements and sulfate and nitrate are examined to provide a measure of the relative contributions of the anthropogenic versus natural sources.
A particularly noteworthy characteristic of the Pacific environment during PEM-West A mission study period was the frequent occurrence of typhoons. During PEM-West A a target of opportunity to investigate the impact of typhoons on the transport and redistribution of trace gases was exploited during flight 9. During this flight inflow and out flow regions of Mireille, a level IV typhoon were examined. Analysis by Newell et al. [1996] of observations made during this flight indicate that substantial amounts of boundary layer trace species were entrained by the typhoon and then transported into the upper troposphere, particularly in the eye-wall region of the typhoon. On the other hand no evidence was found for any downward intrusion of stratospheric air near the eye region.

Concluding Remarks

Measurements obtained during PEM-West A have provided a significant enhancement in the data available over the western North Pacific region. These data, in themselves, will undoubtedly prove to be invaluable as a baseline from which to evaluate the impact of the growing influence of the Asian continent on the chemistry of the troposphere in this region over the next decade. The detailed analyses of these measurements are presented in the companion papers of 1996. The finding reported by the PEM and collaborating APARE investigators provide important new insights into the chemical and transport processes that impact the tropospheric over a large portion of the Pacific Ocean. Equally important are new questions that are posed as a result of the first phase of the PEM-West/APARE study.
Acknowledgments The PEM-West A project was made possible through the cooperation and collaboration of many international colleagues, agencies, and citizens. We give special thanks to the Gary Marston of the USAF, who gave well beyond the call of duty during our operations at the Yokota Air Force Base. We also especially thank Patrick Sham, C. Y. Lam, Elaine Koo, W. L. Chang, and the staff at the Hong Kong Royal Observatory who provided invaluable assistance both logistically and scientifically. In Guam we received considerable help from Charles P. Guard and Frank Wells of the Joint Typhoon Warning Center; we thank them for sharing their knowledge of the meteorological systems of the region and for arranging for us to be provided with satellite images and synoptic maps. In addition, we owe special thanks to the DC-8 personnel. Their dedication and patience were critical elements in translating the often unrealistic desires of the PEM-West science team into reality; and finally we express thanks to Jackie Johnson and Dennis Owen 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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¹NASA Langley Research Center, Hampton, Virginia.
²Georgia Institute of Technology, Atlanta.
³NOAA Aeronomy Laboratory, Boulder, Colorado.
⁴Massachusetts Institute of Technology, Cambridge.
⁵Now at University of Tokyo.
⁶NASA Headquarters, Washington, D.C. .