AASE-II OBSERVATIONS OF TRACE CARBON SPECIES DISTRIBUTIONS IN THE MID TO UPPER TROPOSPHERE

. We report tropospheric (altitudes > 5 kin) observa- tions of CO2, CO, CH4, and light hydrocarbons (C2 - C4) over the latitude range from 90øN to 23øS recorded onboard the NASA DC-8 aircraft during the winter 1992 Second Airborne Arctic Stratospheric Expedition (AASE-II). Mixing ratios for these species exhibited significant north-south gradients with maximum values in subpolar and arctic regions and minima over the southern tropics. At latitudes > 40øN, the mixing ratios of most species increased significantly over the course of the 3-month measurement period. Also at high northern latitudes, the variations of all relatively long-lived reactive carbon species were linearly correlated with fluctuations of CO2 with CO, CH4, C2H6, C2H2, C3H8, and n-C4H(cid:127)o exhibiting average enhancement ratios in terms of ppbv(X)/ppmv(CO2) of 13.8, 8.4, 0.21, 0.075, 0.085, and 0.037, respectively.


Introduction
Carbon dioxide and reactive trace carbon gases play importat roles in regulating global tropospheric composition and climate [i.e., Logan, 1981;Liu et al., 1987]. As examples, the oxidation of CO is a major global-scale sink for OH [i.e., Logan,i981]; CO2 and CH4 are noted greenhouse gases; and non-methane hydrocarbons (NMHCs) are potentially significant sources of atmospheric CO [Logan, 1981] and tend to dominate OH radical chemistry in re•ions close to their emission sources [Liu et al.,i987]. Because of their varying reactivity and widely distributed sources and sinks, background tropospheric mixing ratios of these species, including CO2, vary considerably with latitude, season, and altitude [i.e., Novelli et al., 1992; Blake and Rowland, 1986;Singh et al., 1988;Blake et al., 1992;. Mixing ratios generally decrease with increasing altitude near source regions [Singh et al., 1988;Blake et al., 1992] and, except for CO2 in summer, are greater over continental regions than ocean surfaces [Singh et al., 1988]. Recent studies suggest that the carbon species with combustion-related origins exhibit maximum values in the lower troposphere over the northern mid to polar latitudes during winter [Blake and Rowland, 1986;Steele et al., 1987;Novelli et al., 1992] owing to a greater abundance of anthropogenic sources in the northern midlatitudes, a greater consumption of fossil fuels in winter, and the reduced efficiency of tropospheric removal processes at this time of year.
During AASE-II, in situ CO2, CO, CH4, and NMHC measurements were obtained onboard the NASA DC-8 aircraft within the mid to upper troposphere over large spatial regions (23øS to 90øN) during the time period from January through March 1992 Blake et al., 1992], these data significantly augment the data base for detailing the distributions and secular trends of the more abundant carbon species. The following text describes the observed latitudinal distributions of CO2, CO, CH4, and C2-C4 hydrocarbons in the mid to upper troposphere (altitudes above 5 km) and temporal trends observed over the 3-month experiment period. The relationships (correlations) between the reactive carbon species are also examined. The reader is referred to Anderson et al. [this issue] for an overview of the AASE-I! mission.

Experimental
The NASA DC-8 was deployed each month from January to March !992 to underfly regions of the arctic polar vortex. A minimum of five flights were conducted during these monthly deployments including transits from San Jose, California, to Anchorage, Alaska; Anchorage to Stavanger, Norway; Stavanger to Bangor, Maine; and Bangor to San Jose. In addition, survey flights were conducted during January and February to Tahiti (from San Jose) and Puerto Rico (from Bangor), respectively, to investigate the spatial distribution and atmospheric effects of Pinatubo aerosols. Individual flights were 10 to 12 hrs in duration and included sampling both within the troposphere and lower stratosphere (to 12 km altitude).
The TDL instrument was composed of a 20-m folded path White Cell and individual, cryogenically cooled tunable diode laser/infrared detector pairs for each species of interest. The system provided measurement precisions (2 sigma) of 2% for CO and 7 ppbv (0.5%) for CH4; the system time constant, determined by the White Cell flushing rate, was 5 to 10 s. A modified Licor model 6252 nondispersive infrared monitor was used to determine CO2 mixing ratios. This dual cell instrument achieves high precision by measuring the differential absorption between sample air and a calibrated reference gas. (e.g., mixing ratios were generally independent of altitude within the mid to upper troposphere). Next, stratospheric data were eliminated by rejecting points with corresponding N20 values < 309 ppbv (the observed N20 average within the lower troposphere minus 1 standard deviation). The effects of stratospheric intrusions were reduced by excluding all points with corresponding Os values > 90 ppbv. Finally, measurements from any fresh pollution plumes (as judged by enhancements in short-lived hydrocarbon values) were also deleted. The final tropospheric data set includes over 1600, 1-min averaged C02, CH4, and CO observations and about 260 individual hydrocarbon grab sample measurements; the geographic area represented by this data set is shown in Figure 1. flight. The canisters were subsequently shipped to the University of California at Irvine, and samples were cryogenically preconcentrated and analyzed for NMHCs using gas chromatography with flame ionization detection. Mixing ratios were referenced to Scott Specialty Gases (Plumsteadville, PA) and NIST standards. The system has a detection limit of 5 pptv, a precision (_if_. 1 sigma) of 2 %, and an accuracy of about 5 %. Samples were generally analyzed within 1 to 2 weeks after collection.
Tropospheric data were extracted from the overall DC-8 data set using the following procedure. First, all data recorded  comprise over 90% of the data within this bin, but contribute a maximum of < 45 % to any other latitude bin (Figure 2) or grouping (Table 1) north of 40øN. Because, as discussed below, the carbon species mixing ratios increased significantly between the January and later flight series over high northern latitudes, this temporal sampling bias creates an artificial minimum at 50ø-60øN in the Figure 2 plots. This temporal bias does not substantially affect the 40 ø to 60 ø grouping of Table 1 because this band is dominated by points from between 40øN and 50øN which have a more uniform temporal spread. Figure 2 and Table 1 (40øN to 60øN). This is apparently not caused by temporal sampling biases or the fact that a larger fraction of the arctic measurements were acquired at a lower average altitude (6.8 krn versus 8.6 km for the 40øN to 60øN band) where greater values might be expected. Temporal sampling biases are discussed above, and we note that the criteria used to create the tropospheric data set eliminates measurements from the lower troposphere where steep vertical concentration gradients were observed. However, to confirm the lack of altitude bias, we examined data from narrow height intervals and still found equivalent or larger values in the arctic. For example, CO mixing ratios between 6-and 7-km altitude were 152.7+14.6 (+ 1 sigma) above 70øN as opposed to 141.7+14.5 between 40øN and 60øN. Novelli et al. [1992] also report larger wintertime CO values at surface sites in the arctic than at midnorthern latitudes. This is possibly caused by a lack of sinks at high northern latitudes coupled with more frequent influx of clean subtropical air masses to the subpolar region.
In terms of individual species, Figure 2 indicates that CO2 increased about 1% between the Equator and North Pole. The wintertime northern hemisphere CO2 maximum is caused by reduced vegetative uptake of CO2 coupled with emissions from northern midlatitude combustion sources. Carbon monoxide also exhibited a substantial increase, rising almost a factor of 2 between the southern tropics and polar regions. This increase is attributable both to the dominating effect of northern midlatitude pollution and the latitudinal gradient in photochemical destruction rates. Novelli et al. [1992] observed a similar CO gradient in surface measurements in the Pacific region and reported an annually averaged concentration difference between 71øN and 14øS of about 90 ppbv.
Methane increased 3 to 4% in going from southern to arctic latitudes. Because continental biogenic sources of CH 4 are relatively inactive during winter, this difference is caused not only by differences in photochemical sink rates, but possibly by midlatitude anthropogenic sources of CH4 [Conway and Steele, 1989]; this latter speculation is supported by the observed high positive correlations of CH4 with CO and CO2 at northern latitudes (see Table 2). Hansen et al. [1989]

also reported positive correlations between CH 4 and combustion tracers during winter and suggested that the enhanced levels of C['I 4 seen in the arctic during pollution events arise from anthropogenic activities.
The light hydrocarbons, because of their reactivity, exhibited much more pronounced inter-hemispheric gradients than the monocarbon trace species. In going from the Equator to the Arctic, C2H 6 decreased a factor of 4 whereas C3H8 dropped by more than a factor of 25. The shorter-lived species (e.g., C2H4 and C4H10), while present in significant amounts at high northern latitudes, were generally at or below detection limits near the Equator and south. These differences are consistent with previous measurements [i.e., Blake and Rowland, 1986;Singh et al., 1988] [Tanaka et al., 1987]. Carbon monoxide also apparently accumulated over the winter at the northern latitudes, increasing by about 30% over the 66-day experiment period. Methane values rose by about 13 pbbv which is comparable to its current globally averaged annual increase. The light hydrocarbons also exhibited substantial temporal increases, consistent with their reported late winter/early spring seasonal maxima [e.g., Blake and Rowland, 1986].
The late winter carbon species mixing ratios above 40øN were also greatly enhanced relative to comparable summertime measurements. For example, our March CO values (Table 3) were about 50% larger than those reported by Talbot et al.

[!993] for summertime polar air masses over Canada and
Alaska. In addition, the March C2H6 and C3H8 mixing ratios (see also Table 3) are 2 and 4 times greater, respectively, than those measured by Blake et al. [1993] within summertime background air over Canada. In fact, the March hydrocarbon concentrations in Table 3 are more comparable to values recorded within relatively fresh biomass burning emissions than to summertime background measurements. For example, the March C2H6, C2H2, and C3H8 values are similar to the peak mixing ratios of 1573 pptv, 444 pptv, and 262 pptv, respectively, observed by Blake et al. [1993] in a prominent, 12-to 18hour old smoke plume over central Canada.
The high positive correlations of Table 2 suggest common causes for the northern latitude wintertime concentration enhancements. The longer-lived species--CO2, CO, CH4, and C2H6--tend to be well correlated which implies that their enhancements arise from combustion processes (industrial or biomass). The more reactive species--C3I-Is and n-C4H•o--are extremely well correlated with each other and with C2H 6 of the group referenced above, which reflects not only their combustion-related sources but similar atmospheric residence times. The enhancement ratios (e.g., regression slopes shown in 1989] indicate long-range transport of combustion products from Eurasia, in particular from the former U.S.S.R., significantly contributed to the observed trace carbon enhancements.

Conclusion
The AASE-II observations substantially enhance the available base of information on the latitudinal distributions of CO2 and reactive trace carbon gas distributions in the mid to upper troposphere. Specific results indicate that concentrations are much greater and more variable over the mid to high northern latitudes than within the tropics or subtropics. This reflects both the seasonally reduced rates of photochemical destruction and the greater abundance of biogenic and anthropogenic sources in northern regions. Concentrations within polar and arctic air were also observed to increase between the January and March flight series implying that, because of the reduced efficiency of removal processes, pollutants tend to accumulate in these air masses over the course of the winter. Finally, the fluctuations of most species over mid to high northern latitudes were correlated with those of CO2 and CO which suggests their enhancements may be caused by combustion-related sources.