Overview of PEM-West B Results
In this section, we briefly highlight some of the salient observations of the PEM-West campaigns, with particular emphasis on the difference/similarities between phases A and B. More detailed discussions and additional results are available in the companion papers found in this special section. As already noted above, the broad goals of the PEM-West campaigns were to characterize the transport and chemistry of natural and anthropogenic emissions to the northwestern Pacific region. From previous studies [Duce et al., 1980; Prospero et al., 1985; Merrill, 1989a, b], one expects an enhanced transport of Asian emissions into the North Pacific Ocean during the winter and early springtime. Indeed, the time periods for the two PEM-West studies were selected to maximize the seasonal contrast based upon these expectations. PEM-West A was conducted in August/September and represents the northern hemisphere summertime scenarios, while PEM-West B, conducted in February/March, represents the winter.
Results from the PEM-West campaigns identify the impact of the major meteorological features within the region on the quality of tropospheric air over the northwestern Pacific Ocean region. For example, the PEM-West data provide seasonal information on the relative importance of the position/strength of the Japan Jet; the location of the Pacific High; the influx of cleaner, southern hemisphere air; and convection and lightning over the Asian continent on the chemical signature of troposphere air at locations just off the coast of Asia to mid-Pacific Ocean sites. As a result of the PEM-West campaigns, data are now available to begin to assess the regional and global implications of the observations. These data include (1) a complete suite of chemical measurements at all tropospheric altitudes and locations throughout the northwestern Pacific for two distinctive seasons, (2) extensive ground-based longer-term measurements, and (3) extensive meteorological products. In addition, the PEM-West database is a baseline tool by which future assessments of the impact of growing emissions from the Asian continent on remote Pacific regions can be referenced.
From a large-scale perspective, the meteorological differences between PEM-West A and B can be characterized by the position and strength of the Japan Jet and the location of the Pacific High [Merrill et al., 1997]. During the northern hemisphere late summer/early fall (i.e., PEM-West A), the Japan Jet tends to be weaker and positioned more northerly than during the late winter to early spring (i.e., PEM-West B). For example, during PEM-West A, the maximum speed of the jet, located at about 12 km altitude, 35·N latitude, was about 40 m/s, while during PEM-West B, the minimum speed was observed to be about 70 m/s located at 32·N latitude and at a lower altitude [Merrill et al., 1997]. In concert with the lower speed and more northerly position of the Japan Jet, the Pacific High tends to be positioned more eastward and northerly in the summer to fall period, serving to impede continental outflow and to enhance inflow of marine air from the south particularly at the lower altitudes. As a result, the PEM-West A study period was characterized by more inflow of marine and southern hemisphere air into the midtropical latitudes, accompanied by extensive vertical mixing along a typhoon storm track running roughly parallel with the Asian coast. During PEM-West A, the continental outflow tended to be limited to north of about 40·N latitude. In contrast, the PEM-West B period was characterized by enhanced continental outflow throughout the study region, with a pronounced continental enhancement north of about 20·N latitude. It is particularly noteworthy that during the PEM-West B season at latitudes of >20·N, velocities of strong westerlies resulted in Asian pollutants being transported to sites thousands of kilometers from the coast in 1 to 2 days compared to more than 5 days for PEM-West A. An extremely important implication of quick transport is that Asian outflow, enhanced in anthropogenic pollutants, arrives at remote ocean sites relatively fresh in terms of potential for photochemistry [Merrill et al., 1997; Talbot et al., 1997 (a, b); Crawford et al., 1997 (b); Blake et al., 1997].
Superimposed upon seasonal differences attributed to the position and strength of the Japan Jet and the Pacific High, are other seasonal influences associated with the position and strength of Asian highs, convection and lightning over Asia, and vertical mixing along typhoon tracks.
Detailed analysis of the chemical signatures of trace gases observed during PEM-West B are discussed by Gregory et al. , Talbot et al. [this issue (a, b)], Blake et al. , Kondo et al. [1997 (a, b)], and Dibb et al. . It is significant to note that the methodologies employed to classify the outflow regions varied from the use of backward trajectories to selected hydrocarbon ratios, yet the result presents a consistent perspective of the PEM-West B outflow regions. In general, their conclusions are that (1) the PEM-West B period exhibited a greater degree of fresh continental influence throughout the northwestern Pacific region than during PEM-West A and (2) that the southern boundary of the region having the more pronounced continental influence observed during each season moved southward from about 40·N in PEM-West A to about 20·N during PEM-West B. Blake et al. , for example, notes that during PEM-West B, significantly higher mixing ratios for the NMHCs and C2Cl4 were observed for a larger portion of the PEM-West B region as compared to the PEM-West A, particularly in the low and midtroposphere at latitudes greater than about 20·N latitude. [S. Smyth et al. (unpublished manuscript), 1997] also notes that larger values of C2H2/CO were observed over the western North Pacific during PEM-West B compared to PEM-West A, indicative of less processed anthropogenic emissions during PEM-West B. Kondo et al. [1997 (b)], observes that in the continental air mass below 4 km, the levels of reactive nitrogen, as well as other continental species were found to be significantly higher during PEM-West B than those of PEM-West A. However, above 7 km, Kondo et al. [1997 (b)] note that the converse was true because convective activities were much weaker during early spring than in the late summer to early fall time period. The above cited studies also conclude that during PEM-West B, unlike PEM-West A, the chemical signatures of trace gases indicative of aged marine air were infrequently observed, and then only at latitudes below about 20·N. The influence of clean or aged marine air or regional chemical budgets is illustrated by the acidic gas data (HNO3, HCOOH, and CH3COOH) of Talbot et al., [1997 (a, b)] showing mixing ratios <100 parts per trillion by volume (pptv) for aged marine air contrasted to 10 parts per billion by volume (ppbv) in discrete plumes measured off the coast of Asia.
An indicator of the strong influence that transport from markedly different sources, such as aged marine air and Asian outflow, can have on the photochemistry in the tropics is discussed by Crawford et al. [1997 (a, b)]. Their analysis suggest three distinct photochemical/geographical regions during PEM-West B (1) tropical, less than 20·N latitude; (2) extratropical between 20· and 30·N latitude and (3) extratropical, greater than 30·N latitude. They further divide the tropical region into a "high NOx" and a "low NOx" regime based upon a shift in the air mass source into the tropics, which resulted in a dramatic shift in the observed NOx mixing ratio.
The implications of the ozone photochemisty associated with the high/low NOx regimes in the tropical region are discussed by Crawford et al. [1997 (a)]. During the high NOx regime, encompassing the transit flight to Guam and the first three local flights from Guam, NO mixing ratios at high altitudes, often exceeded 100 pptv. In contrast, during the low NOx regime, encompassing the final local Guam flight and the transit flight to Hong Kong, NO values never exceeded 20 pptv. Crawford et al. [1997 (a)] note that the low NOx regime was found to be composed of air from the remote north/south Pacific that had experienced significant convective processing as evidenced by elevated CH3I levels at high altitude and uniform ozone levels from the surface to 10 km. By contrast, the regime designated as high NOx showed elevated levels of C3H8 and Pb210, suggesting air transported from the outflow of deep convection over the continent. Interestingly, Crawford et al. [1997 (a)], and also Kawakami et al. , suggest that DMSP satellite observations recorded during the PEM-West B sampling period point strongly toward lightning as contributing to the continental source of NOx during the high NOx period. The photochemical analysis by Crawford et al. [1997 (a)] suggest that the net effect of photochemistry on ozone in the tropics was that of net destruction at all altitudes within the low NOx regime. Their analysis also indicates the tropospheric ozone column decreasing by nearly 4% per day. For the high NOx regime, net production of ozone for altitudes above 4 km approximately offset net destruction at the lower altitudes. By comparison, the PEM-West A tropical data was found to most resemble the low NOx regime.
The geographical division of the tropical and extratropical component of the PEM-West B study region by Crawford et al. [1997 (b)] along the 20·N latitude is consistent with the results from the air mass characterization papers noted in the previous section. The additional subdivision of the extratropical component along 30·N latitude by Crawford et al. [1997 (b)] was based on the observation of an abrupt drop in the tropopause height from 16 to 89 km, and a corresponding increase in the total O3 column of about 150 Dobson units. While significant outflow of ozone and its precursors were observed both to the north and south of 30·N, the dramatic increase in overhead O3 north of 30·N led to a significant decrease in photochemical activity. In spite of these decreases, calculations by Crawford et al. [1997 (b)] showed that both extratropical regions produced net ozone at all altitudes. This finding is especially important in that it marks the first time that net ozone production has been observed in the lower marine troposphere. That this condition is observed in the late winter/early spring also emphasizes the critical role of meteorology in transporting ozone precursors into the western, North Pacific. While the calculated rates of increase for the tropospheric O3 column appeared to be unusually large (e.g., 2% per day south of 30·N and 1% per day to the north), they serve as a potential source for offsetting the large net destruction observed in some regions of the tropics.
Crawford et al. [1997 (b)] note that the importance of meteorological flow patterns to the photochemical environment of the extratropical western North Pacific becomes even clearer when comparing PEM-West A with PEM-West B. While differences between the two for altitudes above 4 km were small, the impact of continental outflow for the lower troposphere during PEM-West B was enough that if similar levels had been present during PEM-West A it would have doubled the rate of tropospheric O3 column formation during PEM-West A. Given the actual levels of precursors for both field studies and the fact that they had similar rates of O3 destruction, the enhanced formation for PEM-West B was sufficient to create net column ozone production in PEM-West B; whereas, for PEM-West A an approximate balance was observed.
An additional important focus of the PEM-West campaigns was evaluation of the sources of tropospheric sulfur species. During PEM-West A, Thornton et al.  reported that SO2 exhibited a marked increase in mixing ratio with altitude. This strong dependence of SO2 on altitude was attributed to (1) long-range transport of anthropogenic emissions injected into the free troposphere via continental convection, (2) the influence of direct emissions of SO2 from the Pinatubo eruption of June 1991 into the troposphere, and (3) inputs of stratospheric air affected by Pinatubo into the free troposphere. In contrast to PEM-West A, no clear gradient in SO2 was observed in the upper troposphere during PEM-West B, and furthermore, the mixing ratios of SO2 were significantly lower [Thornton et al., 1997 (a)]. Thornton et al. [1997 (a)] also conclude that during PEM-West A and B the oxidation of dimethylsulfide (DMS) was a relatively insignificant source of free troposphere SO2. Of particular interest is the observation by Thornton et al. [1997 (a, b)] and Talbot et al. [1997 (a, b)] of the coincidence of high mixing ratios of SO2, NO, and ultrafine CN observed above 9 km in the tropical convergence zone during PEM-West B. These observations support those of Clarke [1992, 1993] relative to the production of new particles at high altitude.
Ground-based observations were incorporated in the design of the PEM-West campaigns to provide a longer temporal record of the Asian outflow than possible with the short intensive airborne measurements from the DC-8. Analyses reported by Arimoto et al. , comparing chemical data from ground stations in Asia and the North Pacific with the data from DC-8, provide important insight into the conclusions emerging from the ground and airborne measurements. Aerosol sampling from ground stations on Hong Kong, Taiwan, Okinawa, and Cheju; and at three islands in the North Pacific (Shemya, Midway, and Oahu) are compared with aircraft samples obtained during flights over the western Pacific. Interestingly, Arimoto et al.  note that conditions representative of a strong, or even moderate dust storms, were never encountered by the DC-8. Observations at the ground sites did indicate a significant stronger continental outflow during PEM-West B than PEM-West A which is consistent with the aircraft observations [Kajii et al., 1997; Liu et al., 1997; Wang et al., 1997 ; K. S. Lam et al., unpublished manuscript, 1997]. In addition, a study of the transport of O3, CO, HNO3, SO2, and HCl through trajectory analysis at Oki Island, Japan, showed substantially elevated levels of all species other than O3 for air masses from the west compared to those from the north [Kajii et al., 1997]. The corresponding enhancement of O3 was only about 20%, which was attributed to relatively slow photochemical production in the fast moving air masses. This is substantiated by observations in Hong Kong and Taiwan, which showed similar levels of O3 as Oki Island [Liu et al., 1997; Wang et al., 1997; K. S. Lam et al., unpublished manuscript, 1997].
The observations of surface O3, CO, NO, and SO2 at the southern tip of Hong Kong Island indicate that the local pollution episodes can be identified by pollutant tracers such as CO and NO and thus the regional characteristics of these species can be obtained from the observations by filtering out the local episodes [Wang et al., 1997; K. S. Lam et al., unpublished manuscript, 1997]. An interesting finding from their observations is that the seasonal variation of O3 in Hong Kong is controlled strongly by the Asian monsoon system which is characterized by strong northeasterlies in winter and tropical marine winds in summer. As a result, the median O3 level in winter was found to be about 50% higher than that in summer. Furthermore, photochemically derived high mixing ratios of O3 occurred primarily in the fall when the combination of continental outflow of O3 precursors and prevailing fair weather provided conditions conducive to photochemical production of O3.
During PEM-West (A and B) measurements of NO and NOy aboard the DC-8 by investigators from the Nagoya University (NU) and the Georgia Institute of Technology (GIT) provided an opportunity for the first airborne intercomparison of NOy measurements in the troposphere, as well as additional intercomparisons of NO measurements [Gregory et al., 1985, 1987]. The NU measurements were obtained using a two-channel chemiluminescence instrument [Kondo et al., 1997 (a)] for simultaneous detection of atmospheric NO, and the NO resulting from the catalytic conversion of odd nitrogen compounds (i.e., NOy) to NO on the surface of a heated gold tube. The GIT measurements were obtained using a three-channel system for simultaneous measurement of NO, NO2, and NOy. In each channel, the species detected was NO by using a two-photon, laser-induced-fluorescence technique [Sandholm et al., 1990]. In the NO2 channel, NO was generated through photolysis of NO2 to NO. (Because of concerns over inlet artifacts [J. D. Bradshaw et al., unpublished manuscript, 1997], the GIT NO2 data were not archived for the PEM-West B campaign.) In the NOy channel, NO was generated through a catalytic converter. The NOy converters used by NU and GIT were based on a design by Fahey et al. , although important aspects, such as size, mass flow rate, flow stream surface characteristics, etc. differed in each instrument.
During both the PEM-West A and B missions, the NO measurements from each instrument agreed very well. During PEM-West A, the two sets of NOy measurements exhibited virtually no correlation [Crosley, 1996]. While recommendations from the Crosley report were implemented during PEM-West B by investigators from NU [Kondo et al., 1997 (a)] and GIT [J. D. Bradshaw et al., unpublished manuscript, 1997] and the correlation between the two sets of measurements was improved somewhat, there continued to be substantial disagreement between the two sets of NOy measurements. Investigators from NU, reporting on post PEM-West B laboratory studies to quantify potential interference and/or artifacts during the PEM-West B mission, concluded that their inlet system and application of the techniques for converting odd nitrogen species to NO is substantially free of inlet losses (e.g., HNO3) and/or interference or artifacts [Kondo et al., 1997 (a)]. Investigators from GIT included a statement with their archived NOy data (http://www-gte.larc.nasa.gov/gte_arch.htm: readmeni.git and readmeny.git ) describing tests from which they conclude that their inlet system is also free of loss of HNO3, but their measurements of NOy are likely to have contributions from non- NOy forms of fixed nitrogen (e.g., HCN, NH3, etc.) of unknown and variable magnitudes.
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