Effects of the 2006 El Niño on tropospheric composition as revealed by TES
There were severe drought conditions over Indonesia during the moderate El Niño in the fall of 2006. The drought resulted in huge fires in southern Borneo in September and October, the largest since the major El Nino of 1997. These fires produced large quantities of tropospheric gases (such as carbon monoxide (CO), nitrogen oxides (NOx)), and aerosols. TES made unique coincident profiles of the distribution of tropospheric CO, ozone (O3) and water vapor (H2O). Of these, CO and O3 were highly elevated during this period (relative to 2005) and water vapor was significantly lowered. TES data also reveal that the largest differences in CO were contained within the mid-to-lower troposphere.
Mid-tropospheric O3 mixing ratios were found to be 30–75% higher in October to November of 2006 compared to the same months in 2005, and mid-tropospheric CO levels peaked at nearly twice those in 2005, considered to be a neutral year in terms of ENSO. Average CO profiles during October 2006 in the mid-troposphere exceeded 200 ppb, compared with about 100 ppb for October 2005. Ozone profiles were 45–55 ppb in 2006, and 25–35 ppb in 2005. The O3 profiles from TES have a vertical resolution of ~6 km, thereby providing a new perspective on how ENSO can perturb ozone.
The image above shows differences in the distribution of tropospheric ozone between 2006 and 2005 for the months of October (left) and November (right), between zero to 12 degrees south latitude. The CO peak occurred in October and November 2006, in the lower half of the troposphere. In October the O3 anomaly in the lower troposphere is slightly higher than in the upper troposphere; whereas in November the O3 anomaly in the lower troposphere is slightly lower than the O3 anomaly in the upper troposphere.
TES data for the 2006 El Niño reveal the 3-dimensional distribution of co-located CO and O3 for this event, and the reduced water vapor associated with the higher O3. By December 2006, the high CO anomaly has decreased to less than ~30 ppb, and the CO anomaly is gone by January, which is consistent with the fires ending in by mid-November. However, the O3 anomaly is almost as large in December as it in November.
Ozone is formed in the presence of CO and NOx. The high CO and O3 result from the increased fire emissions (associated with the lowest rainfall since 1997), the reduced convection during the El Niño, and the reduced photochemical loss due to lower H2O. The persistence of the O3 difference into December may also be partly due to the higher NOx emissions from the increase in lightning in November and December (by a factor of 2-3) in 2006 compared to 2005. Lightning provides a source of NOx in addition to that from fire emissions, and plays an important role in ozone formation.
Improving our understanding of the distribution and movement of tropospheric O3, and of the interactions between O3 and other gases, is important for evaluating models of tropospheric chemistry and dynamics. These TES retrievals were compared with data from other instruments and with other years (1997, 2004 and 2005), and are being used to test a model of atmospheric chemistry and dynamics (GEOS-Chem). In this way, vertically resolved profiles of CO, O3 and H2O from TES help to better characterize the mechanisms responsible for the differences in tropical O3 and the effects of El Niño on the atmosphere.
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Biomass burning is the widespread practice, especially in the tropics, of using fire to clear forests and grassland for agriculture and to dispose of crop residue. Among its environmental impacts, biomass burning releases large amounts of carbon monoxide (CO) into the atmosphere, where it reacts with other chemicals to produce ozone. Other instruments observe and measure CO, but only TES measures both CO and ozone — at the same time and at various altitudes. This enables scientists to see the extent to which biomass burning contributes to ozone in the troposphere.
