Aircraft measurements of the latitudinal, vertical, and seasonal variations of NMHCs, methyl nitrate, methyl halides, and DMS during the First Aerosol Characterization Experiment (ACE 1)

. Canister sampling for the determination of atmospheric mixing ratios of nonmethane hydrocarbons (NMHCs), selected halocarbons, and methyl nitrate was conducted aboard the National Center for Atmospheric Research (NCAR) C-130 aircraft over the Pacific and Southern Oceans as part of the First Aerosol Characterization Experiment (ACE 1) during November and December 1995. A latitudinal profile, flown from 76øN to 60øS, revealed latitudinal gradients for most trace gases. NMHC and halocarbon gases with predominantly anthropogenic sources, including ethane, ethyne, and tetrachloroethene, exhibited significantly higher mixing ratios in the northern hemisphere at all altitudes. Methyl chloride exhibited its lowest mixing ratios at the highest northern hemisphere latitudes, and the distributions of methyl nitrate and methyl iodide were consistent with tropical and subtropical oceanic sources. Layers containing continental air characteristic of aged biomass burning emissions were observed above about 3 km over the remote southern Pacific and near New Zealand between approximately 19øS and 43øS. These plumes originated from the west, possibly from fires in southern Africa. The month-long intensive investigation of the clean marine southern midlatitude troposphere south of Australia revealed decreases in the mixing ratios of ethane, ethyne, propane, and tetrachloroethene, consistent with their seasonal mixing ratio cycle. By contrast, increases in the average marine boundary layer concentrations of methyl iodide, methyl nitrate, and dimethyl sulfide (DMS) were observed

heavier alkyl nitrates (>C l) are known to form photochemically in the atmosphere during the hydroxyl radical initiated oxidation of alkanes in the presence of NOx [Darnall et al., 1976;Atkinson et al., 1982].
Methyl iodide is a metabolic product of many species of marine algae [Manley and de la Cuesta, 1997]; however, the details of exactly which species represent the most important producers are not well characterized. Previous studies have reported elevated atmospheric mixing ratios over coastal waters [e.g., Orarn and Penkett, 1994], with macroalgae being a well-defined source [Manley and Dastoor, 1988;Nightingale et al., 1995;Laturnus et al., 1998]. Manley and de la Cuesta [1997] have demonstrated that certain phytoplankton are sources; however, no sufficiently prolific organism has yet been found to explain measurements in pelagic surface waters [e.g., Singh et al., 1983;Nightingale, 1991;Moore and Tokarczyk, 1993]. Photochemical production of methyl iodide in seawater has been observed [Moore and Zafiriou, 1994] and may be a source of methyl iodide in the open ocean [Happell and Wallace, 1996]. Methyl iodide has a photolysis lifetime of about 3-4 days in the MBL at low latitudes, increasing to 2 weeks or more at higher latitudes.
Dimethylsulfide (DMS) is one component of an active seawater sulfur cycle and is secondarily produced by many phytoplankton species [Bates et al., 1994]. DMS can be lost from the water column by air-sea exchange, microbial consumption, or photochemical oxidation. The factors controlling the rate of DMS cycling in surface seawater, and hence its seawater concentration, strongly affect the amount of DMS that is released to the atmosphere. In addition to the concentration of DMS in seawater, atmospheric mixing ratios of DMS depend on chemical removal rates, boundary layer height, exchange with the free troposphere, and wind speed. The lifetime of DMS in the atmosphere is short, ranging from <1 day in the tropical MBL to several days at higher latitudes.
In this paper, we present atmospheric measurements of the concentrations of selected anthropogenic and biogenic halocarbons, light NMHCs, methyl nitrate, and DMS collected aboard the National Center for Atmospheric Research (NCAR) C-130 aircraft during the International Global Atmospheric Chemistry (IGAC) Project's First Aerosol Characterization Experiment (ACE 1). A complete rationale of the experiment and description of the research effort involving scientists

Experiment
A complete description of sampling procedures is given by Blake et al. [1992,1994], and the experimental apparatus employed during ACE 1 is described by D. R. Blake et al. [1996], Sive [1998], and Sive et al. (manuscript in preparation, 1999). Both operations are summarized below.
A metal bellows pump was used to fill evacuated sample canisters to a pressure of 40 psi, and the canisters were returned to the University of California, Irvine laboratory for analysis. Samples were usually analyzed within 1 week of collection and always in less than 2 weeks. A 1520 cm 3 (STP) aliquot from each canister was preconcentrated cryogenically, split reproducibly into five portions, and the trace gas components in each aliquot were separated by gas chromatography. Selected C1-C2 halocarbons were separated using a 60 m, 0.25 mm ID, DB-1 column with a 1 •m filrd for the determination of C1-C2 halocarbons. The separation characteristics of each column were optimal for a particular subset of NMHC or halocarbon gases.

Latitudinal Transect
The NCAR C-130 is based at the Research Aviation Facility in Broomfield, Colorado. In order to make maximum scientific use of the transit flights to Australia, the aircraft flew an extended latitudinal survey between 76øN and 60øS along the middle of the Pacific Basin following the route shown in Figure la. Figure 2 displays the latitude-altitude distribution of the 565 canister samples collected during these flights, which were made between November 2 and 13, 1995. Three-dimensional color-coded concentration contour plots were generated from the ACE 1 hydrocarbon and halocarbon southbound latitudinal transit using the Stanford Graphics (SPC Software) statistical package. Regions where no samples were collected (see Figure 2) have been removed from the plot. The plots for ethane, ethyne, methyl chloride, tetrachloroethene, methyl iodide, and methyl nitrate are shown in Plates 1-6, respectively. The wide range of mixing ratios observed for these gases during the transit flights prescribed the use of logarithmic mixing ratio scales.
The trace gas distributions displayed in Plates 1-6 provide a useful perspective for characterizing large-scale regional features encountered during the ACE 1 transect. However, the atmospheric sampling was neither uniform nor completely random. Therefore, when these plates are being interpreted, reference should be made to the canister sampling distribution illustrated in Figure 2. Data collected downwind of Hawaii on flights 4 and 5 during studies of the Kilauea volcanic plume were not included as they contained significant enhancements in fresh pollutants from local Hawaiian urban areas. During one of the two approaches into Christchurch, New Zealand, we again sampled freshly emitted urban pollutants, leading to the removal of three additional samples from the database employed for Plates 1-6 (but which remain in the archive version of the data set).

Latitudinal Gradients
The color contour plots in Plates 1, 2, and 4 illustrate the strong latitudinal gradient in mixing ratios of the gases ethane, ethyne, and tetrachloroethene, with their highest concentrations at high latitudes in the northern hemisphere and relatively low concentrations in equatorial regions and the southern hemisphere. This is consistent with our knowledge of the urban/industrial sources of these gases and with previous measurements made at the surface [Blake and Rowland, 1986 ] during long-range transport, and indicate that the air was significantly aged. Ozone mixing ratios as high as 70 ppbv imply that the observed pollution was associated with in situ ozone production. Our 10-day back trajectory calculations suggest that the air mass originated at very high northern latitudes, including northern parts of Asia and possibly northern Europe, but had spent many days over the North Pacific and Arctic Oceans before sampling. At 6-7 km altitude between 60øN and 20øN, a gradual decrease in the mixing ratios of ethane, ethyne (Plates 1 and 2), and the industrial tracers tetrachloroethene (Plate 4) and methyl chloroform (not shown) was observed. The mixing ratios of benzene, ethene, and toluene decreased to 10 pptv or less as a result of the advanced photochemical age of the air mass. By contrast, relatively high levels of the biomass burning tracer methyl chloride (Plate 3) were observed. The increased methyl chloride, and decreasing tetrachloroethene and methyl chloroform, was associated with a shift to a more southerly continental Asian origin for the sampled air mass. This trace gas signature is consistent with a regional source similar to that observed previously for southern Asia [Blake et

al., 1997].
South of about 10øN, the mixing ratios of the gases in Plates 1-4 decrease even further at all altitudes to very low levels just south of the equator. Average mixing ratios between 0 ø and 13øS were approximately 305 pptv for ethane, 16 pptv for ethyne, 555 pptv for methyl chloride (Figure 3), and 5 pptv for benzene. The shorter-lived gases, including ethene and toluene, were below the detection limit. These low values correspond with a transition to slow moving equatorial clean marine air, frequently originating in the southern hemisphere. ]. In addition, the elevated ozone levels, which were more than 200% above background levels, are consistent with photochemical aging accompanied by in situ ozone production.

Biomass
The emission ratio of methyl chloride from individual fires is controlled by the amount of chloride present in the fuel [Reinhardt, 1987]. This might help explain the lower methyl chloride in the more southerly plume (Figure 3), even though this air mass appears to have been somewhat less aged by transport and dilution during transport.
Several 10-day back trajectories have been calculated for different points during the flight track and are shown in  Plates 5 and 6 also reveal slight enhancements in methyl nitrate, but not in methyl iodide, in the free troposphere near the equator up to the maximum aircraft cruise altitude. The fact that methyl nitrate, but not methyl iodide, is enhanced may be associated with the actively convective equatorial location of the low-altitude methyl nitrate enhancements. Methyl nitrate also has a substantially longer atmospheric lifetime of about 1 month compared with about 3 days for methyl iodide at the equator. Thus methyl nitrate would be expected to persist in the free troposphere much longer than methyl iodide after any convective event.

Continental plumes with trace gas characteristics similar to those encountered during the ACE 1 transit flights were frequently observed over the southwestern Pacific during NASA's Pacific Exploratory Mission-Tropics A (PEM-Tropics
The fact that methyl iodide and methyl nitrate both clearly originate from the ocean, while exhibiting markedly different distributions, serves to illustrate the complexity of oceanic production mechanisms and air-sea exchange processes. Plate 6  The only NMHC gases that were usually observed at mixing ratios above the limit of detection of our instrumental method were ethane, ethyne, and propane. Benzene mixing ratios were lower than the detection limit in nearly 20% of samples, particularly during the later part of the intensive period. Greater than 80% of samples contained ethene levels below the detection limit, and ethene was less than 10 pptv in all but seven samples. Levels of the gases propene, i-butane, nbutane, i-pentane, n-pentane, and toluene were below their detection limits in more than 95% of the samples. The remaining C3-Ci0 saturated and unsaturated hydrocarbons were always less the 3 pptv detection limit. Model calculations using these low NMHC levels are in excellent agreement with measured OH concentrations [Suhre et al., 1998' Sire, 1998 Wingenter, 1998; Wingenter et al., this issue]. For gases with principally anthropogenic sources, such as ethane, ethyne, propane, and C2CI 4, the low variability in their mixing ratios at the 95% confidence level of the mean ( Figure 5) indicates that local and regional pollution sources made very little impact at altitudes less than 4 km. Fewer samples were collected at altitudes above 4 km, resulting in larger error bars for many gases. Some of these high-altitude samples contained relatively high levels of propane and ethane, possibly from long-range transport from sources such as natural gas and liquefied petroleum gas, contributing to the variability shown in Figure 5 [D. R. Blake et al., 1996; Blake and Rowland, 1995]. Although distant biomass burning emissions may have had an impact on the relatively longlived gases methyl chloride and methyl bromide at lower altitudes (see below), transit times were too long to have influenced the NMHCs. In general, the air masses encountered south of Hobart during the ACE 1 intensive period were highly aged. For example, ethyne/CO ratios were less than 0.7 pptv/ppbv for all free tropospheric samples, and 80% were less than 0.4 pptv/ppbv. Typical MBL ethyne/CO ratios were 0.2-0.4 pptv/ppbv.
The average vertical profiles shown in Figure 5  The mixing ratio of methyl chloride decreased by nearly 5 pptv between late November and early December ( Figure 5). This is only about half of the expected decline if we were to assume that the region was completely isolated from methyl chloride sources. Figure 5 also illustrates a significant positive vertical gradient for methyl chloride, which persisted for the entire Hobart intensive period. This gradient is most likely caused by the long range transport of air containing enhanced levels of long-lived methyl chloride, possibly from distant biomass burning sources, in combination with a relative lack of sources in the study region.
Methyl bromide also has a positive vertical gradient (

Vertical and Spatial Distribution of Oceanic Gases
The average vertical profiles for methyl iodide, methyl nitrate, and the corresponding in situ DMS measurements are shown in Figure 6. In November, DMS mixing ratios were enhanced in the MBL, methyl nitrate is only slightly enhanced, and methyl iodide shows no significant MBL enhancements. However, in December, all three gases reveal enhancements in their MBL mixing ratios, reflecting their oceanic sources. For the samples collected at altitudes less than 0.5 km. the mixing ratios of both methyl iodide and DMS nearly doubled during this short period. The average methyl iodide mixing ratio in the MBL in late November was 0.30 pptv, compared with a value of 0.56 pptv in early December. Average mixing ratios of methyl nitrate increased by about 50% from approximately 15 pptv to nearly 23 pptv. The corresponding DMS averages were 50 pptv and 95 pptv.
The seasonal mixing ratio increase for methyl iodide, methyl nitrate, and DMS within the MBL corresponds to the onset of summer in the southern hemisphere. Increased sunlight and surface water temperatures are two of the many factors that may influence marine production/emission of these gases.  (Figure 1 a) To construct these plots, 1ø latitude by 1 ø longitude mixing ratio averages were calculated including all samples collected at altitudes less than 0.5 km during the flights as shown in Figure lb.
Plates 7-9 reveal no strong spatial trend in MBL mixing ratios of methyl iodide, methyl nitrate, and DMS during November except for somewhat higher levels of DMS to the north, particularly near Cape Grim. By contrast, during December generally greater, and more variable, mixing ratios were observed for methyl iodide, methyl nitrate, and DMS. A pronounced latitudinal trend, with an abrupt increase in mixing ratio south of about 44øS, was observed for all three gases. No latitudinal gradient was observed for any of the other halocarbon or NMHC gases that we measured.
To investigate the latitudinal trend observed for methyl iodide, methyl nitrate, and DMS, all data collected in December were separated by latitude into subsets of <44øS (40ø-44øS) and >44øS (44ø-55øS) and plotted versus altitude in Figure 8. Figure 8 shows that at altitudes less than about 0.5 km the mixing ratios of methyl iodide, methyl nitrate, and DMS were consistently lower in the <44øS region compared with the region to the south. Figure 8 also illustrates that much of the variability in the December mixing ratio seen in both Figure 6 and Plates 7-9 can be accounted for by the latitudinal transition at 44øS.
In the <44øS region, relatively high mixing ratios of methyl nitrate and DMS were observed between 2 and 4 km compared with this same altitudes at higher latitudes (   observed for methyl iodide, methyl nitrate, and DMS at latitudes >44øS (Figure 8). The atmospheric mixing ratio transition at 44øS closely corresponds to a major water mass boundary change from oceanic subtropical convergence zone water to colder subantarctic water [Bates et al., 1998a, b]. Bates et al. [1998b] defines the surface water masses by salinity, with subtropical convergence zone salinities >34.8 and subantarctic water salinities in the range 34.2-34,8. Even colder and fresher polar waters were located south of about 50øS (salinity <34.2). Oceanographic parameters other than salinity also changed at the 44øS transition. Nitrate levels were higher in the subantarctic (11-16 gM) and polar (>20 gM) surface water compared with the subtropical convergence zone (<12 !.tM), but chlorophyll a was lower [Jones et al., 1998]. week in subtropical convergence zone, subantarctic, and polar waters. Bates et al. [1998b] report that average atmospheric mixing ratios of DMS over subantarctic waters increased from approximately 80 pptv during the early part of the intensive ACE 1 experiment to approximately 150 pptv for the latter part. These values roughly correspond to atmospheric DMS mixing ratios at altitudes below 0.5 km and latitudes >44øS of 42 pptv (lo range, 26-58 pptv) for the November measurements, and 115 pptv (62-170 pptv) for December. The fact that the aircraft mixing ratio averages are lower may be the result of sampling at higher altitudes and of natural spatial variability.
Mean atmospheric DMS mixing ratios of 53 pptv and 120 pptv were measured for November and December, respectively, at Cape Grim from 1988 to 1992 [Ayers et al., 1995]. By comparison, the average aircraft DMS values at altitudes <0.5 km near Cape Grim were 95 pptv for November and 110 pptv for December. Hainsworth et al. [1998] report that DMS measurements at Cape Grim during the ACE 1 intensive period were mostly typical of those from previous years. However, DMS concentrations increased quite sharply on November 21, 1995, and were substantially higher than the monthly average on November 23 (Julian Day 331) [Hainsworth et al., 1998], when aircraft samples were collected close to Cape Grim ( Figure lb).
Thus it appears that the atmospheric distribution of methyl nitrate, methyl iodide, and DMS in the MBL during November and December was related to the different water masses found, but exactly what is causing the differences is not clear. It seems reasonable to assume that atmospheric mixing ratios should roughly be controlled by the geographic distribution of the oceanic organisms or other precursors generating these gases. Griffiths et al. [this issue] report increasing concentrations of chlorophyll a between late November and early-mid December in the subantarctic region, corresponding to the timing of the observed increases in methyl iodide, methyl nitrate, and DMS south of 44øS. They also observed chlorophyll increases in the Cape Grim area during late November, corresponding to increases in atmospheric DMS concentrations measured at the Cape Grim station. However, we have seen earlier that methyl iodide and methyl nitrate can exhibit very different distributions at tropical and subtropical latitudes.

Summary and Conclusions
The extensive latitudinal profile sampled by aircraft in early November revealed strong latitudinal gradients for many NMHC and halocarbon gases, including ethane, ethyne, and tetrachloroethene. High mixing ratio enhancements in the northern hemisphere extended up to the maximum altitude of 8 km. These enhancements were the result of the proximity of strong anthropogenic source regions for these gases, principally on the Asian continent. By contrast, methyl chloride exhibited low concentrations in the lower troposphere in the high-latitude northern hemisphere, but significantly elevated levels at midlatitudes, suggesting a high-latitude oceanic sink and a low-latitude Asian source for this gas. Very low mixing ratios were observed •n the remote equatorial regions (between about 10øN to 20øS) for all gases except those with oceanic sources. Methyl nitrate appears to have a significant equatorial oceanic source; however, the different distributions of methyl iodide and of methyl nitrate indicate that the two gases have distinct oceanic production mechanisms and/or air-sea exchange processes.
Continental plumes containing material characteristic of biomass burning were observed at mid to high altitudes between approximately 19øS and 43øS. These plumes represented a significant perturbation to the levels of many trace gases, including ozone. They were considerably aged, suggesting that the emissions may have originated from the South Atlantic region.
The 24-day intensive investigation of the remote marine troposphere at midlatitudes south of Hobart revealed a significant increase in average mixing ratios of methyl iodide, methyl nitrate, and DMS in the MBL between late November and early December. This increase was associated with the rapid transition to summer conditions in the region. The region most affected was to the south of 44øS over waters characterized as subantarctic and polar. The onset of summer was also responsible for seasonal decreases in the mixing ratios of ethane, ethyne, propane, and tetrachloroethene.