Introduction

The Earth Observation Research Center of the National Space Development Agency of Japan (NASDA) is implementing its scientific activities in association with the NASDA Earth observing satellites. The program for atmospheric chemistry-Global Atmospheric Chemistry Experiment (GLACE)-covers tropospheric and stratospheric chemistry, and is designed to be carried out in close collaboration with international projects such as IGAC and SPARC. One of the core activities of GLACE is the Biomass Burning and Lightning Experiment (BIBLE). Started in 1997, BIBLE is a tropospheric chemistry research project which uses aircraft as a sampling platform to study important natural and anthropogenic processes in the tropical Asia/Pacific region.

Objectives of BIBLE

Tropical Asia, in particular the region stretching from Indonesia to northern Australia, is a place where trace gas and aerosol distributions in the troposphere are strongly influenced by deep convection, frequent lightning and biomass burning. Quantitative studies of these processes in the region have been limited because of the lack of simultaneous measurements of key species. The BIBLE activity is comprised of in situ observations aboard an aircraft supplemented with simultaneous satellite observations and numerical modeling efforts. BIBLE uses a Gulfstream II (G-II) jet plane chartered by NASDA. Coordinated ground-based measurements of trace gases and aerosols, including balloon-borne ozonesonde measurements, are also carried out at strategic times. Instruments used for the BIBLE aircraft measurements conducted in 1998 are listed in Table 1.

Table 1. Instruments aboard the GII Aircraft during the BIBLE Experimenmts in 1998.

The NO concentration in the tropical maritime troposphere is usually quite low, and as a result, ozone tends to be destroyed there rather than produced (ozone transported into the region is photochemically removed) [Crawford et al., 1997]. However, very different conditions occur in the presence of bio-mass burning products when lightning occurs and during periods of strong convective transport. The BIBLE campaign was planned to evaluate these three influences.

During the dry season, biomass burning is an important source of ozone precursors and aerosols for the tropical troposphere, and ozone formation can occur in biomass burning plumes originating in Indonesia and tropical Australia. Late in the dry season (September-October) in the El Niño years of 1994 and 1997, large scale biomass burning occurred in Indonesia, leading to significant increases in tropospheric ozone [Fujiwara et al., 1999; Kita et al., 2000]. Biomass burning has been reported to occur regularly in northern Australia in the dry season. Major scientific objectives involving biomass burning for BIBLE include the following:

• Estimate the amounts of trace gases and aerosols emitted by biomass burning
• Study the vertical transport of biomass burning plumes
• Study the long-range transport and chemical transformation of the biomass burning plumes
• Evaluate the impact of production of ozone precursors on ozone photochemistry

During the rainy reason, lightning and convective transport play important roles in the distributions of trace gases in tropical Asia [Kawakami et al., 1997; Koike et al., 1997]. In particular, it has been shown that convective activity associated with the Walker circulation influences trace gas distributions in subtropical regions of the western Pacific [Koike et al., 1997]. Major scientific objectives involving lightning and convective transport to be studied during the 2000 BIBLE mission are to:

Measurements over the western Pacific Ocean

The data obtained over the mid-latitude and subtropical Pacific Ocean are most relevant to APARE. These data were obtained during BIBLE-T and transit flights during BIBLE-A and B. Some of the interesting features obtained by these measurements are described below.

BIBLE-T: Influence from surface sources in Northeastern China

Five flights were made from Nagoya between April 17 and 24, 1998. The region covered by these flights was 26 to 44°N and 136 to 144°E. Profiles of O₃ and NO_Y are shown in Figure 1. For reference, the median values obtained in the continental air masses during PEM-West B conducted in March 1994 [Kondo et al., 1997] are also shown. For further comparison, median O₃ profiles from the Japanese ozonesonde stations at Naha (26°N), Kagoshima (32°N), Tateno (36°N), and Sapporo (43°N) were derived. The median O₃ values at 2-12 km in April of 1993-97 were 60-80 ppbv, which were close to the BIBLE-T values. The median ozonesonde values in February-March were about 10 ppbv lower than April. The median BIBLE-T NO and NOY values were 20-80 and 400±100 pptv, respectively, similar to those for PEM-West B at 4-11 km. However, the NO, NO_Y, and NO/NO_Y values during flight 5 made on April 24 were much higher than the average BIBLE-T values in the upper troposphere.

Figure 1: Ozone (a) and NOY (b) profiles obtained in April 1998 during BIBLE-T. The data for flight 5 are shown as crosses. The median values for all BIBLE-T observations (closed circles) and PEM-West B continental air (open circles) are shown for comparison.

A detailed meteorological analysis indicates that the air mass sampled during this flight was strongly influenced by convection over northeastern China 1 to 2 days prior to the sampling. The convection, which reached up to 12 km, was associated with cyclonic activity. The NMHC and CO concentrations suggest that high O₃ and NO_Y values were anthropogenic in origin. Strong convection, such as that encountered during BIBLE-T, can transport pollutants far over the Pacific Ocean in conjunction with the strong westerly flow. In April, solar UV intensity is higher than February-March. If the mixing ratios of key precursors of ozone, such as NO, CO, and NMHC, remain similar or increase from March to April, the net photochemical ozone production rate should also increase over this time period. However, more systematic measurement and modeling studies are needed to evaluate the importance of convective transport of precursor gases during the whole spring. An intensive aircraft sampling of trace gases and aerosols to be conducted in April-May, 2001 will significantly enhance the understanding of the points mentioned above.

BIBLE-A, BIBLE-B: ITCZ

Latitudinal variations in the mixing ratios of ozone and precursor gases above 8 km were obtained during the transit flights from Japan to Australia along the 140 ± 5°E meridian in September 1998, as shown in Figure 2. Both O₃ and NO_Y showed abrupt decreases at 23°N, corresponding to the changes from midlatitude to subtropical air masses, according to trajectory analysis. The O₃ and NO_Y reached minimum values of 20 ppbv and 30 pptv, respectively, at 0-7°N. TRMM and meteorological data indicate that the ITCZ was located in this region. Large- scale convection over the clean tropical ocean transported boundary layer air poor in O₃ and NO_Y to the upper troposphere. Because the dominant component of NO_Y in the maritime lower troposphere should be HNO₃ [Kondo et al., 1997], heterogeneous removal of HNO₃ during upward transport might have further decreased NOY. Similarly, low NO_Y and O₃ values were observed near the ITCZ and SPCZ in February during PEM-West B [Kawakami et al., 1997]. The data from the other transit flights from Indonesia to Japan basically showed similar features, although the boundary between different air masses shifted somewhat.

Figure 2: Latitudinal distribution of O3 (a) and NOY (b) obtained during transit from Japan to Darwin in September 1998 during BIBLE-A.

Similar transit flights were made in August-September 1999 during BIBLE-B. Distributions of trace gases in the mid-latitude and subtropical regions above 8 km were quite different during this period as compared with 1998 mainly due to the differences in meteorological conditions. In addition, lightning activity over the western Pacific strongly influenced the level of reactive nitrogen in subtropical air masses. These results indicate significant variability in trace gases and aerosols in the upper troposphere over the western Pacific, even during the same season. The data obtained during BIBLE will improve our understanding of the transport of Asian continental air to the western and central Pacific Ocean.

Biomass burning in Indonesia and Africa/South America

Biomass burning activities over Indonesia were lower than average in October 1998. In 1997, biomass burning activity was quite high [Fujiwara et al., 1999; Kita et al., 2000]. This was because the regrowth of burned biomass was comparatively slow in 1998, and therefore the amount of fuel in 1998 was much smaller than that of 1997. In addition, La Niña prevailed during this period causing higher convective activity leading to higher humidity and a higher precipitation rate. Although there was some influence of biomass burning or urban pollution on CO, NMHCs, and reactive nitrogen, the enhancements were much lower compared with those in 1997. Lightning activity seems to have elevated concentrations of reactive nitrogen in the upper troposphere. However, the production of reactive nitrogen by lightning was sometimes mixed with the effect of biomass burning or urban pollution, judging from the CO and NMHC data. Enhanced reactive nitrogen either from lightning or biomass burning did not lead to significant increases in ozone in the vicinity of Indonesian islands because of the limited time for photochemical ozone production. By contrast, trajectory analyses showed that in the absence of intense convection, ozone was produced photochemically at a rate of about 2 ppbv/day in air masses with moderately high ozone precursors during the transport from the Indonesian region to the Indian Ocean. Considering that BIBLE-A was conducted in a year of very low biomass burning, the obtained data will provide a good baseline for assessing the impact of biomass burning in future studies.

In the upper troposphere in the subtropical region over northern Australia, O₃ and NO_Y mixing ratios were enhanced, reaching as high as 100 ppbv and 1000 pptv, respectively (Figure 2). Trajectories show that these air masses were from Africa/South America or Indonesia. During PEM-Tropics, enhanced values of O₃, HNO₃, CO, NMHC were observed mostly between 3 and 7 km over the subtropical central Pacific Ocean in September [e.g., Gregory et al., 1999]. The present observations indicate that the biomass burning plumes over Africa/South America can be transported to above 8 km.

Further information, including highlights of the BIBLE campaigns, can be obtained at the BIBLE website.

References

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