TRANSPORT AND CHEMICAL EVOLUTION OVER THE PACIFIC (TRACE-P): A NASA/GTE AIRCRAFT MISSION
by D.J. Jacob, D.D. Davis, S.C. Liu, R.E. Newell, B.J. Huebert, B.E. Anderson, E.L. Atlas, D.R. Blake, E.V. Browell, W.L. Chameides, S. Elliott, V. Kasputin, E.S. Saltzman, H.B. Singh, and N.D. Sze.
Contact information: email@example.com
24 June 1999
This document describes TRACE-P (TRAnsport and Chemical Evolution over the Pacific), a two-aircraft mission over the western Pacific to be conducted by the Global Tropospheric Experiment (GTE) of the National Aeronautics and Space Administration (NASA) in March-April 2001. TRACE-P is motivated by the need to better understand how outflow from the Asian continent affects the composition of the global atmosphere. The mission has two objectives:
TRACE-P will use two NASA aircraft, the DC-8 (ceiling 12 km) and the P-3B (ceiling 7 km) operating out of Yokota Air Force Base (near Tokyo, Japan) and Hong Kong. The aircraft payloads will include a suite of long-lived greenhouse gases, photochemical oxidants, aerosols, and their precursors.
TRACE-P is part of a long series of GTE aircraft missions aimed at better understanding of global tropospheric chemistry [McNeal et al., 1998]. Over the past two decades, GTE has conducted missions in several remote regions of the world (Amazonia, the Arctic, the tropical Atlantic, the Pacific) to characterize the natural processes determining the composition of the global troposphere and to assess the degree of human perturbation. The rapid industrialization now taking place in Asia is of compelling interest. Energy use in eastern Asia has increased by 5% yr-1 over the past decade and this rate of increase is expected to continue for the next two decades [U.S. Dept. of Energy, 1997]. Combustion of fossil fuels is the main source of energy. Emission of NOx in eastern Asia is expected to increase almost 5-fold from 1990 to 2020 [van Aardenne et al., 1999]. There is a unique opportunity to observe the time-dependent atmospheric impact of a major industrial revolution. Long-term observations of from ground sites and satellites can provide continuous monitoring of the temporal trend of atmospheric composition but are limited in terms of spatial coverage (ground sites) or the suite of species measurable (satellites). Aircraft missions can complement surface and satellite observations by providing a detailed investigation of the dynamical and chemical processes affecting atmospheric composition over broad geographical regions. .
The first objective of TRACE-P is to identify the major pathways for Asian outflow over the western Pacific, and to chemically characterize the outflow in a way that provides a basis for quantitative model analysis. A number of three-dimensional chemical tracer models have been used in recent years to examine Asian influence on global atmospheric composition [Berntsen et al., 1996; Mauzerall et al., 1997; Bey et al., 1999; Carmichael et al., 1998; Jacob et al., 1999]. TRACE-P will provide the information needed to test these models. We expect the Asian chemical outflow over the western Pacific to represent a complicated superimposition of contributions from different Asian source regions and from long-range transport of European and North American pollution. The Asian emissions themselves represent a mix of contributions from fossil fuel combustion, other industrial activities, biomass burning, vegetation sources, and soil dust. Scavenging of soluble aerosols and gases during wet convective transport out of the boundary layer modifies the composition of the outflow. Unusually strong stratosphere-troposphere exchange around the Japan jet [Austin and Midgley, 1994] further complicates the interpretation of the outflow by adding a stratospheric component [Carmichael et al., 1998]. The use of two aircraft in TRACE-P will allow sampling of a range of Asian outflow pathways in different regions and at different altitudes, as is needed for quantitative analysis of the impact of this outflow on global atmospheric composition.
The second objective of TRACE-P is to better understand the chemical and dynamical evolution of the Asian outflow over the west Pacific, focusing on tropospheric O3 and aerosols. The processes involved in this evolution include photochemistry, heterogeneous chemistry, gas-to-particle conversion, aerosol growth, scavenging, and subsidence to the marine boundary layer followed by rapid removal of some species by deposition. Different patterns of evolution are expected depending on the direction of outflow (tropics vs. high latitudes); the altitude (boundary layer vs. free troposphere); the presence of soil dust, soot, or other chemically active aerosols in the outflow; and the contributions from natural sources including lightning and stratospheric intrusions. Previous studies have pointed to the potential importance of strong UV radiation [Crawford et al., 1997] and heterogeneous chemistry involving dust aerosols [Dentener et al., 1996] in modifying the chemical composition of the Asian outflow over the western Pacific.
The selection of a March-April flight period for TRACE-P is guided by several factors. Spring is the season of maximum Asian outflow over the Pacific, due to a combination of active convection over the continent and strong westerlies [Merrill. 1989]. In summer, deep convection exports Asian air to the upper troposphere [Kritz et al., 1990; Balkanski et al., 1992] and a significant fraction of the outflow may travel above the DC-8 ceiling of 12 km. Early spring also affords the opportunity to sample the biomass burning outflow from southeast Asia apparent in the Hong Kong ozonesonde data [Liu et al., 1999] as well as dust outbreaks over central Asia. Photochemistry over the western Pacific is already active in early spring [Crawford et al., 1998]. Of course, results from a spring mission may not be generalizable to other seasons because of differences in dynamical and chemical environments as well as differences in emissions. We expect that sequel missions in other seasons will be necessary. As discussed in section 3 , a January-February mission would be of particular interest to isolate the Asian contribution to the outflow from that due to long-range transport of pollution from Europe and North America.
TRACE-P will build on the heritage of the previous GTE Pacific Exploratory Missions - West (PEM-West A and B) conducted over the western Pacific in August-September 1991 and February-March 1994 [Hoell et al., 1996, 1997]. Key findings of the PEM-West missions related to the Asian outflow are summarized in section 2 . The PEM-West missions were exploratory, with multiple objectives achieved from a single aircraft. TRACE-P will provide a considerably more extensive characterization of the Asian outflow to allow for quantitative interpretation. In addition, TRACE-P will take advantage of numerous developments in aircraft instrumentation over the past decade including in particular measurements of HOx, NOx, sulfur, species, aerosols, and UV actinic fluxes. Ten years will have elapsed between PEM-West A and TRACE-P, during which Asian emissions will have grown considerably (70% for NOx; van Aardenne et al. ). Secular change in the composition of the Asian outflow should be apparent between the PEM-West and TRACE-P missions.
The GTE PEM-West A and B missions examined the impact of natural and human activities on the chemistry of the troposphere over the northwestern Pacific Ocean from 10oN to 50oN. PEM-West A was conducted in August-September 1991 and PEM-West B in February-March 1994. Important meteorological differences between these two seasons include the position and strength of the Japan Jet, and the location of the Pacific High [Merrill et al., 1997]. During August-September (PEM-West A), the Japan Jet is weaker and shifted north compared to February-March (PEM-West B). The Pacific High is at its northernmost and easternmost position during August-September, impeding continental outflow and enhancing inflow of marine air to the western Pacific from the south particularly at low altitudes. In PEM-West A, this southerly flow was accompanied by extensive vertical mixing along a typhoon storm track oriented parallel to the Asian coast; continental outflow was largely confined to north of 40oN. PEM-West B experienced stronger and faster continental outflow over an extended range of latitudes, principally below 5 km due to weak convection over eastern Asia in winter. Blake et al.  found higher mixing ratios of continental hydrocarbons and halocarbons during PEM-West B than A, especially at low altitudes, and similar observations were made for acidic gases [Talbot et al., 1997]. The composition of the hydrocarbon mix indicated a more recent origin for the continental outflow in PEM-West B.
The strong Asian outflow during PEM-West B had a major influence on the regional ozone budget over the western Pacific. Photochemical model calculations by Crawford et al.  showed net ozone production taking place at all altitudes, in contrast to PEM-West A where net loss at low altitudes balanced net production at higher altitudes. PEM-West B marked the first time that net ozone production has been found to take place in the lower marine troposphere. That this condition was observed in late winter/early spring further emphasizes the critical role of fast transport of ozone precursors from the Asian continent. Calculated rates of increase in the tropospheric ozone column during PEM-West B were as large as 2% per day south of 30oN and 1% per day to the north. An important implication of the rapid transport observed during PEM-West B is that the photochemical activity of the continental outflow remained strong even after several days of travel time over the ocean.
The first 2-3 months of the year come under the influence of the winter monsoon in east Asia, characterized by intense Siberian high pressure systems and strong outflow over the western Pacific, with maximum sea level pressures occurring in December-February [Yihui, 1994]. The percentage of days when the high pressure exceeds 1050 hPa is as follows: November (25), December (45), January (51), February (38), March (13), and April (3) [Yihui, 1994]. During the peak of the monsoon in December-February there is strong subsidence in the major part of the east Asian continent, pollution is trapped in the boundary layer, the middle free troposphere is often cloudless in association with the high pressure systems and rainfall is practically non-existent (Figure 1 ). Sandstorms have maxima in Kantze (30oN, 103oE) in December-February and in Hami (43oN, 93oE) in March-May [Watts, 1969]. They depend on the occurrence of strong winds and the absence of snow. Precipitation is very low over China in January but is substantial over Japan. As the year progresses the precipitation belt moves westward towards the coast, gradually increasing until it covers the entire area of interest by May.
Another feature of these high pressure systems is the occurrence of cold air outbreaks. Some of these outbreaks, which themselves will contain pollution trapped in layers by subsidence, bring cold air into southern China and sweep pollution southwards over the South China Sea. The fact that the Siberian anticyclone dominates the circulation in December-February implies that much of the pollution originating in continental east Asia will flow clockwise out to sea then return to the southwest and move southwards into the South China Sea. Some pollution injected at higher altitudes within the continent may be caught in the westerlies and head directly out to sea. This domination by the anticyclonic subsidence keeps local pollution at low altitudes permitting pollution entering China's airspace from the west to be partly distinguished from that which originates in China. If measurements are made off the coast, it is not correct to attribute this pollution measured solely to China. Even the pollution that heads southward over the South China Sea, as noted above, is likely to be substantially oxidized in its passage towards the ITCZ (at about 10oS in the previous PEM-West B mission). After pollution has being raised in the ITCZ there is a flow back towards China (at 200 hPa) before the air turns eastwards and moves into the westerly wind global circulation [Newell et al., 1997]. Hence the impact of pollution from China itself on the global atmosphere is not easy to measure.
Figure 1. Monthly mean precipitation, January-March
In the upper troposphere the main meteorological feature is the westerly jet stream with December-February mean speeds of 65 ms-1 south of Japan [Newell et al., 1972]. This phenomenon brings pollution from further west as will be illustrated later. As convection starts in late March pollution from China itself is mixed with the pollution arriving from the west from other longitudes before it can be measured offshore. Instabilities in the jet stream are often associated with the transfer of air from the stratosphere to the troposphere, and these form another major factor influencing the chemistry of the region.
Figure 2. Climatological flow streamlines
Mean streamlines for January-April 1997 are shown in Figure 2 for levels of 1000, 850, 700, 500, 300 and 200 hPa. In January the clockwise flow at 1000 and 850 hPa associated with the continental anticyclone carries boundary layer air out over the ocean north of Taiwan and then back westwards over the South China sea, the Philippines and the region north of New Guinea. At 700 hPa the flow moves eastwards from the continent in the 20-50oN region. There is some recirculation back towards the west south of 15oN around the subtropical anticyclone. This provides more opportunity to measure the chemical evolution of pollution. At 300 hPa winds reach 70 m s-1 near Japan, as in the climatology, yielding a transit time of only a few days between Asia and North America. The maximum speeds diminish to 48 m s-1 and 31 m s-1 by March and April respectively. The flow pattern is quite similar in February, although in the 10-year precipitation climatology (1988-1998) there is some precipitation along the coast, east of Hong Kong and south of Shanghai (Figure 1 ).
By March the flow is onshore at 1000 hPa and 850 hPa for China south of about 30oN but is still offshore further north and in the upper troposphere. The lower layer continental anticyclone is much weaker by March and disappears by April.
The differences between sampling in the January-February period and sampling in the March-April period can be illustrated by trajectories. Three sets for January and March are shown in Figure 3 . The first set originates from 5 polluted regions of China with one trajectory per day starting at 7 points near each city for the days 1-26, 1997, of each indicated month. The color changes along the trajectory indicates the changes in pressure of the trajectory. Trajectories are divided into two groups depending on the pressure at the end, after five days, being > or < 700 hPa. As expected from the wind maps and analysis of PEM-West B data [Newell et al., 1997] considerable low-level flow heads to the south in January, some reaching the SPCZ after a period greater than 5 days. In March most of the air heads out eastwards and a significant fraction ends in the upper troposphere.
The second set shows air parcels arriving at a wall along 100oE at 20-40oN in western China. Seven pressure levels were used with 41 points spaced along each pressure level. Monthly dates 17-22 were used for the calculation for each month shown. Pressure levels are shown at the beginning of each 5 day trajectory to the wall. In both months it seems that more of the air parcels arrive at the wall from the upper troposphere than from the lower troposphere. In January air arrives from central Africa, north Africa, even the north of Greenland and the west of the United States. The flow converges laterally in the upper troposphere from two main streams and one subsidiary stream. In the lower troposphere there are two main source regions: the Middle East and Europe. In March 1997 the sources are not so distant from the wall because of the lower speeds and do not span quite such a large range of latitude.
Figure 3. Air flow trajectories
The third set shows forward trajectories for air leaving the same wall. The spread in latitude is again large in January in the upper troposphere, with two main plumes. The spread into the tropics is minimal but there is an extensive spread to higher latitudes in January. On the contrary there is a downstream convergence in the lower troposphere towards the central Pacific in both months. Again little air moves south of the southern boundary of the wall at 20oN. Thus assuming the arriving wall air and the surface layer derived air are combined when sampled, it seems that in January they could be identified because much of the former head to the south (as we suggested in the discussion of the winter anticyclones). On the other hand, sorting the local and distant sources in March would seem to be practically impossible as they will be well mixed. There are very few if any days of continental anticyclonic flow in April hence the possibility of differentiating sources is very low.
In summary then there are two possible approaches:
Nominal flight tracks for the two TRACE-P aircraft are shown in Figure 4 . The aircraft will operate out of two sites: Yokota Air Force Base (near Tokyo, Japan) and Hong Kong. As shown in Figure 4 , these two bases of operations are well situated to sample Asian outflow over the full range of latitudes from 10oN to 50oN. Specific targets for the flights out of Hong Kong will include biomass burning pollution from southeast Asia [Liu et al., 1999], tropical inflow and outflow, and industrial outflow from the Pearl River Delta inland of Hong Kong which is one of China's fastest growing regions. Specific targets for the flights out of Yokota AFB will include outflow of pollution from northern China. Korea, and Japan [Akimoto and Narita, 1994; Carmichael et al., 1998], long-range transport of European and North American pollution in the westerlies, dust outbreaks, and stratospheric influence combined with continental outflow in the Japan jet [Wakamatsu et al., 1989; Murao et al., 1990; Austin and Midgley, 1994; Carmichael et al., 1998].
The sampling of outflow in flights from Yokota AFB and Hong Kong will use a wall pattern (Figure 5 ) with the aircraft flying stacked patterns of horizontal legs perpendicular to the outflow and separated by a few km altitude. Regions of outflow will be identified on a day-to-day basis using meteorological and chemical tracer model forecasts. The length of a typical wall will be several hundred km, and the wall pattern may be repeated over the duration of the flight, in order to assess photochemical aging of reactive species as part of our process studies and also to obtain the representative sampling of the outflow needed for testing 3-dimensional chemical tracer models. The two aircraft will be used to sample different ouflow regions on any particular day; typically the P-3B will focus on low altitudes and the DC-8 on high altitudes. Since outflow at different altitudes may be geographically and temporally separated, the DC-8 and the P-3B will in general cover different horizontal flight tracks and may not fly on the same days or out of the same operational base.
Figure 4. Nominal TRACE-P flight tracks.
Chemical aging of the Asian outflow over the western Pacific will be examined with flights extending east from Hong Kong and Yokota AFB, and most specifically with DC-8 flights using Guam as an overnight stop (Figure 4 ). These flights will sample Asian outflow having traveled a few days over the western Pacific. Under conditions of steady westerly outflow, transects between Yokota AFB and Guam may be used to revisit air previously sampled close to the China coast on flights south of Yokota AFB or north of Hong Kong (Figure 4 ). A generic pattern for the chemical aging flights is shown in Figure 6 . Specific patterns will be guided by meteorological and chemical forecasts in the field. Near-Lagrangian sampling will be attempted if meteorological conditions are favorable.
Figure 5. Typical wall flight patterns for the DC-8 and P-3B in TRACE-P
Figure 6. Typical chemical aging flight pattern for the DC-8 and P-3B in TRACE-P
It is expected that 160 and 171 flight hours will be allocated to the DC-8 and P-3B aircraft respectively for this mission, including test and transit flights. More hours will be allocated for the P-3B to account for the longer transit time to the study region. Sorties out of Hong Kong and Yokota AFB will include both 8-hour and 10-hour flights. A nominal breakdown of flight hours is shown in Table 1 . The DC-8 will conduct 4 sorties out of Hong Kong and 7 out of Yokota AFB, while the P-3B will conduct 4 sorties out of Hong Kong and 6 out of Yokota AFB. The DC-8 sorties will include one return flight to Guam (to be counted as two sorties).
Priority measurements for the DC-8 and the P-3B are listed in Table 2. The priorities reflect the focus of the mission on radiatively important species, photochemical oxidants, sulfur, and aerosols. Chemical tracers of air masses are also included in the list. The priority ratings 1-4 in Table 2 indicate a decreasing level of importance of the measurement for meeting the mission objectives. Priority 1 measurements are of highest importance and a failure of one of these measurements prior to the mission or in the field could alter mission plans. It is expected that the aircraft will include all measurements of priority 1 and 2 plus some measurements of priority 3. Measurements of priority 5 ("new-technology") would enhance the mission but are considered not yet technically established in terms of airborne sampling. It is expected that at least one such instrument will be included in the payload.
The instrument detection limits and time resolutions quoted in Table 2 are minimum requirements below which the instrument will be considered not responsive to the objectives of the mission. Performance beyond these minimum requirements in terms of speed, precision, accuracy, and specificity will be an important consideration in the selection of the aircraft payload. The size of instrumentation will also be an important consideration.
* redundancy will be provided where feasible.
+ will be provided by the GTE Project Office
Supporting measurements for TRACE-P are intended to place the aircraft observations in a broader temporal and spatial framework. Ozonesondes have proven to be particularly valuable for that purpose in past GTE missions. TRACE-P will include a program of intensified launches at three established ozonesonde sites: Hong Kong [Chan et al., 1998], a southern Japan island site such as Naha, and a Japan mainland site such as Tateno. Ozonesondes will be launched once a week from March 2000 to March 2002 (one year before to one year after the mission) and twice a week during the mission.
Measurements at coastal sites, islands (Cheju, Lanyu, Oki...), and ships will also be important for extending the aircraft observations in TRACE-P. Key species to be measured include ozone, aerosols, CO, and hydrocarbons. It is expected in the framework of APARE that the Asian partners to NASA will play a leading role in the operation of these surface measurements.
Space-based observations from the Measurement Of Pollution In The Troposphere (MOPPITT) and the Global Ozone Monitoring Experiment (GOME) instruments should be of considerable value for interpretation of the TRACE-P data. MOPPITT (to be launched in polar orbit in summer 1999) will provide global distributions of CO vertical profiles including 4 levels in the troposphere. GOME (in polar orbit since 1995) is expected to provide operational data for tropospheric ozone columns by the time of the TRACE-P mission.
Day-to-day flight planning in the field will require high-quality meteorological forecasts and back-trajectory analyses. Chemical and aerosol forecasts using 3-D model simulations with forecast weather would be of considerable value for guiding the aircraft towards outflow regions and for planning chemical aging flights. These 3-D models can provide an integrated analysis of the outflow from the Asian continent that includes the effects of emissions, boundary layer dynamics and chemistry, convective pumping, and long-range transport from Europe and North America. Both mesoscale and global models should be engaged in this role. Considering that a major goal of TRACE-P is to provide the observations needed for testing the simulation of Asian outflow in 3-D chemical tracer models, use of these models in the flight planning stage both before and during the mission is of great importance. Additional modeling support will be needed in the field for quick analysis of the aircraft observations using a combination of statistical approaches, 0-D photochemical box models, and aerosol models. This modeling support is of great value for monitoring the achievement of the mission objectives and for guiding flight planning.
Itis expected that the Aerosol Characterization Experiment - Asia (ACE-Asia) aircraft mission will be in the field concurrently with TRACE-P (B.J. Huebert is the ACE-Asia mission scientist). ACE-Asia will study the outflow of aerosols and aerosol precursors from Eastern Asia to the Pacific. Its objectives are to characterize the physical, chemical, and radiative properties of Asian aerosols that impact the Pacific atmosphere and to quantify the processes needed to model these properties. ACE-Asia will involve two years of observations from a surface network, in addition to springtime intensive observations with aircraft and ships in 2000 and 2001. Since the goals of TRACE-P and ACE-Asia are complementary, collaboration will take place to the extent possible while maintaining the integrity and independence of each mission. The collaboration may take several forms: allocating a fraction of the P-3B payload to aerosol-related measurements, reciprocal representation at planning meetings, conducting joint flight operations, and sharing some portion of the infrastructure support when the aircraft are operating from common airfields.
There are tentative plans to conduct an APARE/BIBLE aircraft campaign in 2001 in complement of TRACE-P (Y. Kondo is the BIBLE mission scientist). Previous BIBLE campaigns using a Japanese Gulfstream 2 aircraft have investigated Asian outflow and biomass burning in southeast Asia in different seasons. The most effective use of a BIBLE mission in support of TRACE-P would be to extend the temporal range of TRACE-P with flights in other seasons.
The recently conducted Photochemical Ozone Budget of the Eastern North Pacific Atmosphere (PHOBEA) aircraft campaign off the northwest coast of the United States in April-May 1999 (http://weber.u.washington.edu/~djaffe/phobea/) revealed layers of high ozone and aerosols transported across the north Pacific from the Asian continent (D. Jaffe is the PHOBEA mission scientist). A second PHOBEA mission conducted concurrently with TRACE-P would be of great value for investigating the long-range transport and chemical evolution of the Asian outflow sampled with the TRACE-P aircraft.
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