Three-dimensional distribution of nonmenthane hydrocarbons and halocarbons over the northwestern Pacific during the 1991 Pacific Exploratory Mission (PEM-West A)

. A total of 1667 whole air samples were collected onboard the NASA DC-8 aircraft during the 6-week Pacific Exploratory Mission over the western Pacific (PEM-West A) in September and October 1991. The samples were assayed for 15 C2-C7 hydrocarbons and six halocarbons. Latitudinal (0.5øS to 59.5øN) and longitudinal (114øE to 122øW) profiles were obtained from samples collected between ground level and 12.7 km. Thirteen of the 18 missions exhibited at least one vertical profile where the hydrocarbon mixing ratios increased with altitude. Longitude-latitude color patch plots at three altitude levels and three-dimensional color latitude-altitude and longitude-altitude contour plots exhibit a significant number of middle-upper tropospheric pollution events. These and several lower tropospheric pollution plumes were characterized by comparison with urban data from Tokyo and Hong Kong, as well as with natural gas and the products from incomplete combustion. Elevated levels of nonmethane hydrocarbons (NMHC) and other trace gases in the upper-middle free troposphere were attributed to deep convection over the Asian continent and to typhoon-driven convection near the western Pacific coast of Asia. In addition, NMHCs and CH3CC13 were found to be useful tracers with which to distinguish hydrocarbon and halocarbon augmented plumes emiued from coastal Asian cities into the northwestern Pacific.


Introduction
Nomethane hydrocarbons (NMHCs) and halocarbons affect both tropospheric and stratospheric photochemistry. The sources of these atmospheric trace species are predominantly land based and the main mechanism for the removal of those hydrocarbons and halocarbons with abstractable hydrogen atoms is through oxidative reaction with hydroxyl radicals (OH) as in (1). RH + HO.
--> R. + H20 (1) Nonmethane hydrocarbon emission patterns can be used to characterize anthropogenic sources such as fossil fuel leakage and incomplete combustion [Ehhalt and Rudolph, 1984;Blake et al., 1992Blake et al., , 1994Blake et al., , 1995Smith, 1993]. Because all of the halocarbons reported here are anthropogenic and released predominantly in cities, their measurement can be used to estimate recent urban influence on an air mass. For example, an air parcel which has been augmented by biomass burning would be expected to contain elevated levels of ethane, ethene, and ethyne and many The operation of a field laboratory at Yokota Air Base, Japan, one of the main staging sites for this mission, reduced the time required for sample canister transportation between aircraft landing sites and the analytical laboratory. For staging sites other than Yokota the filled sample canister sets were shipped by air cargo on commercial flights to Tokyo and then transported to Yokota. On average, approximately 90 air samples were collected on each flight and analyzed within 7 days. The total number of sample canisters available for the mission was approximately 700, requiring multiple use of canisters to maintain this high sampling frequency. After the air samples from earlier flights had been satisfactorily assayed, the canisters were evacuated to a pressure of 10 -2 torr and shipped back to the DC-8 for subsequent missions.

Chemical Analysis
Each sample was analyzed for NMHCs and halocarbons using two Hewlett-Packard 5890 series II gas chromatographs (GCs). One gas chromatograph (GC-1), was equipped with two flame ionization detectors (FIDs), while GC-2 was equipped with an electron capture detector (ECD). One of the FIDs monitored the output of a 30 m x 0.53 mm A1203/KC1 PLOT column (J & W Scientific) with a hydrogen carrier flow of 9.5 mL/min and provided data on C2-C5 NMHC compounds. A 60 m x 0.25 mm, 0.25-1xm film thickness DB-1 column (J & W Scientific) with a hydrogen carrier flow of 2.0 mL/min was installed in the second FID for the purtmse of detecting C3-C10 NMHCs. Coupled to the ECD, GC-2 utilized a 50 m x 0.2 mm, 0.5-gm film thickness PONA column (Hewlett-Packard) with a hydrogen carrier flow of 1.2 mL/min for halocarbon separation and detection.
A precise volume of air (1309 mL STP) from each sample canister was trapped in a preconcentration loop (8 inch x 1/4-inch OD, stainless steel) filled with 1-mm-diameter glass beads and immersed in liquid nitrogen. The volatile contents of the sample were pumped off while the higher boiling gases remained on the glass beads. The contents of the loop were voporized by heating with warm water and flushed with the hydrogen carrier gas through a splitter which divided the sample into three separate flow streams. Precisely reproducible percentages of the total flow were injected onto the various columns: 75% to the PLOT column, 15% to the DB-1, and 10% to the PONA. It was found that the sample split was very reproducible if the specific humidity of the sample was greater than 2.0 g H20/kg air. Thus to raise the specific humidity of the driest samples, as much as 2.0 tort of purified water was added to the 2119 mL STP volume of the analytical vacuum line prior to sample introduction. Upon sample injection the initial temperature of GC-2 was held at -60øC for 30 s, and then ramped up to 240øC at 20ø/min, while the initial temperature of-20øC for GC-1 was ramped up to 200øC at 20ø/min. A complete analysis cycle, including return to the cooled initial conditions, required 20 min. The gas chromatographs were interfaced to three Spectra Physics 4400 computing integrators and a computer, using LABNET software (Spectra Physics) for data capture and storage. To monitor the consistency and reproducibility of the analytical system, a secondary standard (dry ambient air collected at Niwot Ridge, Colorado, NOAA CMDL by Paul Steele) was analyzed after every eight ambient samples. Throughout the project a second cylinder of whole air was assayed every other day. These mixing ratios were compared to those obtained from the working standard to ascertain that the working standard mixing ratios did not change over the period of the experiment. Approximately 60 air samples could be analyzed during a 24-hour period. The chromatographic apparatus was run continuously (24 hours/d, 7 days/week) from September 17 to November 2, 1991. Detailed descriptions of the analytical setup, standards and calibrations have been given previously [Blake et al., 1992[Blake et al., , 1994. While data from the integrators provided preliminary concentration values for the reported compounds, after returning to our home laboratory, the chromatograms were examined to conf'mn that the baselines were correctly drawn and that the peaks showed no unusual characteristics.
The precision of the air sample measurements, based on 10 air samples collected during a constant altitude leg of mission 15, was 3% or 3 pptv (whichever was larger) for the alkanes and ethyne and 5% or 5 pptv (whichever is larger) for the olef'ms. Measurement precision for CFC-12, CFC-11, CFC-113, CH3CC13, CC14, and C2C14 was 0.7%, 0.8%, 1.4%, 3.8%, 1.7%, and 4.2%, respectively. The precision estimates represent upper limit values because these successive samples may not have had identical mixing ratios. The limit of detection for benzene and toluene was 5 pptv, while that for the remaining NMHCs was 3 pptv. The reported halocarbons were always present at concentrations well above their detection limits.

Results
The flight paths and locations of the 1667 whole air samples collected onboard the DC-8 during missions 4 to 21 (missions 1 to 3 were test flights) are displayed in Figure 1. Figure 2 shows the grab-sampling distribution as a function of altitude and latitude and altitude and longitude. The figures reflect the good altitudinal, latitudinal, and longitudinal sampling coverage by the DC-8 but also show that in accord with the major goal of the Pacific Exploratory Mission-West the most densely sampled region was concentrated in the western Pacific between 12 ø and 32øN latitude and between 120 ø and 150 ø E longitude.
While attempts were made to sample many different types of air masses, the atmosphere was not randomly sampled so may bias the air mass descriptions discussed below. In addition, the very complex and diverse meteorological conditions encountered during the 6-week sampling period mean that the averaged data should not be conslxued to represent a particular ouffiow scenario. Rather, they are intended to present an averaged trace gas  distribution for the sampled areas of the northern Pacific region during September and October 1991. By contrast, the week-long intensive missions flown out of Japan, Hong Kong, and Guam allow the regional data to be considered more as a "snapshot." The meteorological aspects of the project are described in an overview by Bachmeier et al. [this issue], but some general features are summarized here. The mean flows averaged for the PEM-West A time frame include a high-pressure system over the western North Pacific which, at lower tropospheric altitudes, brought marine air from the east across the Guam region and toward Japan from the south to southwest. High pressure at lower altitudes over central and eastern China tended to bring continental air across the Sea of Japan and over the East China and South China Seas. The middle and upper troposphere north of 25ø-30øN was dominated by westerly flow off the Asian continent. Long-range transport was also affected by the several tropical cyclones that occurred during the mission which tended to track northeastward close to the east coast of Asia.

Fingerprint Characterization
Samples collected near large accessible NMHC and halocarbon sources such as Tokyo, Hong Kong, and San Jose and in more remote regions exhibiting enhanced mixing ratios were characterized or "fingerprinted," as was incomplete combustion from vehicle exhaust [Blake and Rowland, 1995]. This fingerprinting was achieved by subtracting appropriate "background" mixing ratios from each hydrocarbon and halocarbon measurement. Ideally, air of the same composition as that which originally diluted the source material would provide the background mixing ratio values. Thus, the average mixing ratios of several samples collected in close proximity immediately prior to or after encountering a "plume" exhibiting relatively enhanced trace gas mixing ratios were usually employed. After background subtraction, these residual or "excess" concentrations were added together according to their classification as NMHC or halocarbon, then the percentage contribution made to the category respectively. This suggests that CH3CC13 is likely to be an excellent tracer for polluted air parcels originating fxom industrial cities around the Pacific region. However, these results should be interpreted with caution as each city. f'mgerprint was calculated employing samples collected on single aircraft approaches, and as such should serve as a guide rather than as being representative of the typical average. For example, on the approach to San Jose (October 22, 1991) a very large brush/suburban fare was taking place in the Oakland area. In addition, no samples were available from cities deep in the continent of Asia. Table lb shows that incomplete combustion, mainly from vehicle exhaust, is characterized by high alkene and ethyne, and low ethane and propane contributions. Table 2 shows that the alkenes are relatively short-lived in the atmosphere (t < 2 days) and so are expected to persist only long enough to be observed close to their source regions or as indicators of rapid air mass transport. However, ethyne has a relatively long lifetime of approximately 23 days (Table 2), allowing it to persist long enough in the atmosphere to act as a tracer for air masses that have been transported over long distances. Table lb shows

1976]. All Compounds exhibit higher mixing ratios in the PBL
due to the proximity of surface sources. However, the NMHCs (Table 3b) typically show higher median concentrations in the middle-upper and PBL altitude sections than in the lower free troposphere. This was a general feature of the NMHCs and will be discussed in more detail below. Several of the very short lived hydrocarbons (t < 1 day, see Table 2  apparent. However, consistent with the high median middleupper tropospheric ethane mixing ratios shown in Table 3  Except when near urban sources, hydrocarbons such as ethane (Plate 3) have significantly more small scale structure than the much longer-lived gases such as CFC-11 (see Plate 10). This enhanced hydrocaxbon structure is in addition to source locations and regional scale transport features and occurs because photochemical removal by hydroxyl is extremely important in determining the distribution of the relatively short lived NMHC species. Contributing to the relatively small scale variability in the concentrations of many of the trace gases (as seen in Plates 1-10), tropical storm Luke (mission 6) and Typhoons Mireille (mission 9) and Nat (mission 10) transported clean Pacific air north to the Japanese region. Therefore the highly structured features of the data reflect the influence of both transport and photochemistry on the complicated mix of sources and their distributions. However, mixing ratio data for C2C14 and CH3CC13 were not available for most of mission 5 because of analytical difficulties encountered at the beginning of the field mission. These problems also meant that CFC-11 data were affected for both missions 4 and 5. In addition, water vapor levels were extremely low in the stratospherically influenced air masses of missions 4 and 5, causing wall losses to occur for some of the assayed halocarbons. Subsequently, it has been found that the addition of small aliquots of p.urified water vapor to each sample canister immediately prior to its deployment in the field effectively eliminates such losses.

Rapid Vertical and Long Range Transport
The previous discussions of Table 3b and Plates 1 to 7 revealed that a strong feature of the averaged PEM-West A data was a free-tropospheric mixing ratio enhancement, typically above 7 km. In addition, 13 out of the 18 missions show at least one vertical profile where the trace gas concentrations increase with altitude.
Such observations are consistent with the earlier STRATOZ II and STRATOZ lIl campaigns which took place in the Atlantic [Ehhalt et al., 1985;Rudolph, 1988;Ehhalt, 1992] and with the Atmospheric Boundary Layer Expedition (ABLE) 3A, and ABLE 3B projects in Alaska and eastern Canada [Blake et al., 1992[Blake et al., , 1994. this latitude (see Figure 2) the feature appears to be decoupled from the sources below. Also, Plate 3 includes samples from mission 4 which were collected only a few degrees south of the enhanced mission 5 samples, but were separated by more than 40 ø in longitude from those from mission 5, and which have lower mixing ratios. The difference between the two missions is illustrated in Figure 4 which shows two vertical profiles of ethane. One profile, showing significantly lower ethane mixing ratios at all altitudes (0-8 km), was made during mission 4 in the Gulf of Alaska (46.8ø-51.1øN and 131.7ø-139.4øW), the other took place during mission 5 along the Aleutian Islands (52.6øN  and 177.7øW-175.1øE). Thus the mission 5 enhancement is likely to have been a more general free tropospheric feature than it appears in the latitude-altitude plot of Plate 3, demonstrating the importance of using Figure 2 Table 2  The potential for a typhoon to drive rapid vertical transport is illustrated for NMHCs as a large hydrocarbon-enhanced plume centered at about 22øN and 120øE over the Pacific at 12.5 km and shown for ethane, ethyne, propane, and benzene in Plates 3 to 6, respectively. This air mass was sampled at the end of mission 10 east of Taiwan and southwest of Okinawa (19.0 ø to 24.0øN and 120.9 ø to 125.7øE). Ten samples were collected in the plume and their halocarbon and hydrocarbon concentrations are shown in Tables la and lb, respectively. It can be seen that methane, ethane, ethyne, propane and other nonmethane hydrocarbons were significantly enhanced above the very low concentrations associated with the air surrounding it. However, the halocarbon and hydrocarbon composition of this high-altitude plume are very similar to the background boundary layer levels employed for the Hong Kong (mission 13) and South Korea (mission 13) plumes (Tables la and lb). The halocarbon concentrations associated with this plume were not enhanced and confh'med that the air mass sampled in mission 10 had not come directly from a heavily industrialized urban source area such as Tokyo or Hong Kong. This is consistent with meteorological analysis which suggests that fast convection associated with nearby Typhoon Nat, was likely to have swept surface air up to the upper troposphere [Bachmeier et al., this issue]. Thus the aged free tropospheric air at high altitude probably had a very different origin from this plume, precluding its use to represent appropriate plume background conditions. Further evidence of typhoon activity affecting the high-altitude atmospheric trace gas composition was observed during mission 9 and can be located on the NMHC contour plots (Plates 3 to 7) as two enhanced regions between 10 and 12.5 km, centered at approximately 33 øN and 130øE. The entire high-altitude portion of this mission, which took place along the perimeters of Kyushu and Shikoku, the southernmost of the four main Japanese islands (32. 0'-35.2'N, 129.1'-137.0'E), exhibited enhanced NMHC mixing ratios. Transport of these gases above 10 km is att•ibutecl to the fast convective transport observed near the top of Typhoon Mireille [Newell et al., this issue] which, during mission 9, was centered approximately 50 km to the southwest of Nagasaki, Kyushu Island, Japan (32.2'N, 129.0'E). Plate 7 shows that the short-lived NMHC i-pentane (lifetime about 5 days) exhibited a greater enhancement than for the mission 10 plume, which was sampled in the same altitude range approximately 10' to the south. Dimethyl sulfide, which is a useful marker for oceanic boundary layer air, was also elevated above the free tropospheric background levels [Newell, et al. this issue]. The presence of these gases suggests that the air in this mission 10 plume was more recently in contact with the surface. The ethane enhancement seen in Figure 5 for mission 9 is relatively small in comparison to that for mission 10 and the halocarbons were not significantly enhanced; however, the NMHC concentrations more similar to those of Tokyo than Hong Kong. By contrast, the South Korea plume had lower ethyne and the highest propane contribution, which may indicate greater. use of bottled propane for cooking and/or heating in its region of origin. However, further characterization of these two NMHC fingerprints is very difficult in the absence of source data from Korea and China.

A typical vertical profile of ethyne up to 8 km is shown in
If the two mission 13 halocarbon fingerprints are compared with those for Tokyo and Hong Kong, it can be seen that CH3CC13 is the dominant halocarbon species, accounting for 50% of total halocarbons over background in the first South Korea plume and 30% in the second eastern China plume. However, the CC14 contribution of 3% to the Tokyo and Hong Kong fingerprints is very small, whereas the CC14 contribution from the South Korea and eastern China plumes was much higher at 12% and 15%, respectively. With the exception of CC14, the eastern China halocarbon fmgerprint was very similar to that for Hong Kong, and the South Korea halocarbon fingerprint was more similar to that of Tokyo, probably due to the different types of industry and consumption in each region of origin.
Another plume, encountered during mission 14 at an altitude of 430 m and at a location about 500 km northeast of the Philippines, was quite different in character. As shown in  Table lb shows that the percentage NMHC composition follows the order of atmospheric lifetime for each NMHC (Table 2), with the alkenes and shorter-lived allcanes having decayed to or near zero concentrations over background, suggesting that the air parcel had undergone substantial photochemical removal and dilution with pristine background air. This is a likely scenario because the encounter took place on the western outskirts of Typhoon Orchid. Thus, the air parcel had probably spent 2 to 3 days over the Pacific Ocean prior to being

Conclusions
The results presented here cover a suite of trace gases with lifetimes ranging from approximately 50 years to as short as a few hours (see Table 2), providing a valuable too1 with which to study transport processes and the impact of Asian pollutants on the Pacific region. Data from the PEM-West A mission appear to confirm that the occurrence of enhanced mixing ratios of NMHC and other trace gases in the middle-upper free troposphere is caused by convective processes, including convective outflow from the Asian continent. Cloud-driven convective activity over land and typhoon-driven convection over the ocean strongly influenced the distribution of trace gases over the northern Pacific. The collection of urban samples has established CH3CC13 as a good tracer for coastal Asian induslxial activity. This study also underlined the need to identify and characterize different pollution sources in Asia. This project established a three-dimensional dislxibution for a large suite of Ixace gases in a season with expected m'mimum continental outflow and set a good foundation for comparison with the maximum continental outflow events expected during the PEM-West B project flown in early 1994.