Biomass-Burning Influence on
TOMS-derived Tropospheric Ozone in the Tropics

presented at the
AGU Fall meeting
San Francisco, CA
December, 1997

Abstract

The seasonal variation of lower-tropospheric ozone over New Guinea shows a distinguishable annual cycle with a maximum in July-September and a minimum in January-February. Because the ozone monthly variation is well anti-correlated with monthly rainfall, biomass burning appears to be a likely precursor of the elevated ozone in a mechanism similar to the mechanism observed over South America and Africa. The tropospheric-ozone linear trend derived from a regression of the deseasonalized monthly averaged lower-tropospheric ozone amount versus time east of New Guinea (upwind of the biomass-burning areas) shows no significant trend; however, west of New Guinea (downwind of the biomass burning regions) we find a statistically significant increase. The magnitude of this trend is similar to the magnitude of the trend in the lower-troposphere ozone in western South America. The tropospheric-ozone linear trend derived from a regression of the deseasonalized monthly averaged lower-tropospheric ozone amount versus time east of New Guinea (upwind of the biomass-burning areas) shows no significant trend; however, west of New Guinea (downwind of the biomass burning regions) we find a statistically significant increase. The magnitude of this trend is similar to the magnitude of the trend in the lower-troposphere ozone in western South America.

Method

• Monthly averaged NIMBUS-7 version-7 level-3 TOMS total ozone data on a 1^o latitude by 1.25^o longitude grid from January 1979 to April 1993. Lower tropospheric ozone derived from the difference between total column ozone amounts over two nearby regions with a topgraphic contrast.

• High-density (level-2) TOMS data contains height information over the TOMS field-of-view (Figure 1).

• Plate 1 shows version-7 level-3 TOMS total ozone over the western Pacific ocean (upper panel) and over South America (lower panel) in August 1983, as an example of the total atmospheric ozone field surrounding our points of study. Regional minimums in total ozone result from the fact that the depth of the troposphere is less over the mountain locations relative to the sea-level locations.

• The important assumption made by the JY method is that both stratospheric ozone and upper-tropospheric ozone are the same over both the mountain and the adjacent land (or ocean).

• The vertical location of the ozone in the troposphere will be somewhat more uncertain in the New Guinea case than in the Andes case (Figure 1, bottom panel). In this fashion, the lower tropospheric ozone signal over western New Guinea can be amplified during the biomass burning season.

• Cloud error suffered by the version-6 TOMS algorithm is much less of a factor because the version-6 inversion algorithm uses the ISCCP (International Satellite Cloud Climatology Project) climatology as a function of month, latitude, and longitude [McPeters and Labow, 1996].

Results

• Figure 2 shows the seasonal variations of lower-tropospheric ozones east of the Andes and west of the Andes along with the tropospheric-column ozone at Natal measured from ozonesondes [Olson et al., 1996].

• The annual variation at Natal agrees remarkably well in phase and relative annual variation with lower-tropospheric ozone derived by the JY method over the East Andes. This agreement strengthens our confidence in the JY method.

• A possible explanation for the lower minimum over the western Pacific Ocean may be that O(1D) from photolysis of ozone with sufficient UV radiation reacts with water vapor to produce hydroxyl radicals. In a low NOx, high-H2O environment, the hydroxyl radical will destroy ozone, so this pathway represents a net sink for ozone [Crutzen 1988; Johnson et al., 1990; Davis et al, 1996].

• Figure 3 shows the comparison of lower-tropospheric ozone over western New Guinea with lower-tropospheric ozone over eastern New Guinea. Because the maximum ozone episode occurs during the dry season, biomass burning appears to be a more likely precursor of the elevated ozone over western New Guinea in a mechanism similar to the mechanism observed over South America and Africa [Lindesay et al., 1996, and references therein].

• The lack of an in-situ biomass-burning source over eastern New Guinea results in no significant increase of ozone during the dry season. Transportation of polluted air from western New Guinea to eastern New Guinea is not likely because the prevailing wind throughout the troposphere during the dry season is easterly or south [Kalnay et al., 1996]. Therefore, eastern New Guinea is under the influence of clean maritime air [Kley et al., 1996] coming from the central Pacific Ocean during the dry season.

• It is possible that Austrailian air affected by biomass-burning activity can be transported to New Guinea with southeasterly winds during the dry season. However, this polluted air is unlikely to be transported to eastern New Guinea because of the high mountains on the eastern side of New Guinea in the face of prevailing easterlies.

• Agreements in the amplitude of the seasonal variation suggest that the TOR map should show the seasonality over western New Guinea as it does from South America to Africa.

• Possible explanations for why the TOR maps do not show seasonality over the western Pacific:

                SAGE data do not accurately represent the stratospheric ozone field because of its low spatial and                   temporal resolution in the tropics.

                Because the overall derived lower-tropopspheric ozone over New Guinea is less than that over                   eastern South America [Kim and Newchurch, 1996], the tropospheric column ozone amounts (surface                   to tropopause) varying from non-biomass to biomass burning seasons over the western Pacific may                   be smaller than the sensitivity of the TOR method.

• Figure 4 shows the time series of deseasonalized tropospheric ozone over western New Guinea for the 14.5-year TOMS mission.

• During the El Niņo period, a positive sea-level pressure anomaly associated with reduced convective activity leads to droughts covering the western Pacific and eastern Australia [Philander, 1990; USDC, 1993].

• During the La Niņa period, a negative sea level pressure anomaly associated with intensified convective activity was observed over the western Pacific [USDC, 1993].

• Peridiogram analysis, which is consistent with the El Niņo period (Figure 4), suggests that vertical wind velocity in the middle troposphere plays an important role in controlling the ozone amount in the lower troposphere over the eastern Pacific Ocean.

• Figure 5, the seasonal variation of vertical pressure velocity at 500 mb, suggests that persistent rising motion throughout the entire year over N57 prevents biomass-burning-induced ozone from sinking to enhance lower-tropospheric ozone amounts.

• Photochemical loss in the marine boundary layer would contribute to the summer ozone minimum while photochemical producion from biomass-burning precursors would contribute to enhance ozone during the burning season.

• The tropospheric ozone linear trend derived from a regression of the deseasonalized monthly averaged ozone amounts versus time over eastern New Guinea shows no significant trend; however, the trend over western New Guinea (Figure 6) shows a statistically significant increase.

• This trend is of the same order found by Jiang and Yung [1996] and by Kim and Newchurch [1996] in the latitude range 10^o-23^oS in the eastern Pacific Ocean immediately west of South America; but it is larger than the zero trend found in the latitude range 0-10^oS.

• Because intensive burning occurs in July through November and the dominant zonal wind in the middle and upper troposphere is easterly [Kalnay et al., 1996], the seasonal maximum in ozone (Figure 7) provides strong evidence for the influence of biomass burning.

Conclusions

• Lower-tropospheric ozone derived from the difference between TOMS total ozone columns over two nearby regions with a topographic contrast contains significant information about seasonal variations and long-term trends.

• Seasonal variations of the derived lower-tropospheric ozone both east and west of the Andes show good agreement with tropospheric ozone seasonal variation from ozonesondes at Natal and suggest that biomass burning plays a crucial role in the formation of tropospheric ozone over equatorial South America.

• The study found strong evidence linking biomass burning to enhanced production of tropospheric ozone in the western Pacific Ocean near New Guinea.

• The de-seasonalized time series of the tropospheric ozone column over western New Guinea shows clear responses in the expected sense to the strong El Niņo event in 1982-83 (increased ozone) and to the strong La Niņa in 1988 (decreased ozone).

• The latitudinal variation of tropospheric ozone over the eastern Pacific Ocean shows three domains that have different characteristics.

• Persistent rising motion over the norheastern Pacific Ocean prevents any westward-transported ozone from sinking to enhance lower tropospheric ozone amounts.

• Persistent sinking motion over latitudes 2^oN-22^oS coupled with strong sesonal variation that is well correlated with the biomass-burning season over South America suggests that a significant amount of tropospheric ozone is transported from South America over the Andes to the eastern Pacific Ocean with the prevailing easterly wind during the biomass-burning season to enhance ozone in the lower troposphere.

• Vertical motions between neutral and rising over latitudes 23^o-36^oS coupled with no annual signal in biomass burning indicate tropospheric ozone in these latitudes experience little influence from biomass burning.